CO-CULTURE OF HUMAN INDUCED PLURIPOTENT STEM CELL (HIPSC) – DERIVED MACROPHAGES AND HEPATOCYTES FOR GENERATION OF HIPSC-KUPFFER CELLS FIELD OF THE INVENTION [0001] The invention relates to a method of generating Kupffer cells from pluripotent stem cells. Specifically, the invention relates to a method of co-culturing pluripotent stem cell derived macrophages and pluripotent stem cell derived hepatocytes to generate Kupffer cells. BACKGROUND OF THE INVENTION [0002] Liver-resident Kupffer cells (KCs) form a major population of hepatic macrophages. They play key roles in hepatotoxicity, liver injury and repxyxy KCs are involved in maintaining liver homeostasis and contributing to progression of hepatic injury in the absence and presence of external stimuli respectively. For example, stimuli from foreign particles/bacterial endotoxins are endocytosed by KCs; consecutively, cell damage or soluble stress signals leads to KC activation and production of cell signaling and stress pathway modulators, including reactive oxygen species and cytokines. KC activation also regulates parenchymal hepatic and non-parenchymal cell death by apoptosis and metabolic activity of hepatocytes. Such crosstalk between liver parenchymal cells (hepatocytes) and non- parenchymal modulators (KCs) is critical in modelling liver injury, especially immune- mediated drug-induced responses. [0003] Animal models have been used to model immune-mediated liver injury. However, these animal models have often not been able to recapitulate clinical findings. Consistent liver injury was not observed upon treatment with drugs known to cause immune response in humans, even at high doses. In cases where injury was observed, the characteristics were different from humans and possibly manifested through different mechanisms. Such interspecies variations, contradictory findings as well as cost and ethical issues have spurred the development of in vitro models to supplement in vivo animal models. [0004] In vitro models, possibly due to the lack of appropriate human immune cell sources, have focused on soluble factor-driven single hepatocyte cultures in combination with inflammatory mediators or animal cell co-cultures. However, mono-cultures lack cell-cell interactions and animal in vitro models often fail to recapitulate in vivo and clinical findings. Another main challenge in studying human KCs in in vitro models is the cell source. The source of commercial primary human KCs (PhKCs) is limited and costly and high donor-to- donor variability is often observed. Alternative human cell sources include THP-1 cells and peripheral monocyte-derived macrophages; however their cytokine profiles are dramatically different from human KCs, and they lack liver-specific imprinting. Induced pluripotent stem cell (iPSC) derived KCs (iKCs) have been developed from using conditioned media from primary hepatocytes (PHCM) to drive iPSC-derived macrophages (iMacs) towards iKCs. Although these iKCs functionally resembled PhKCs, PHCM is expensive due to the cost of primary cell culture and exhibit differences depending on donor variability of primary hepatocytes. [0005] Therefore, there is a need to provide a method to generate Kupffer cells that overcomes, or at least ameliorates, one or more of the disadvantages described above to predict drug-induced cytokine responses and investigate new drugs that cause cytokine imbalance. SUMMARY [0006] In one aspect, provided herein is a method of generating an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC) comprising the steps of: (a) providing an iPSC- derived macrophage (iMac); (b) providing an iPSC-derived hepatocyte (iHep); (c) contacting and incubating the iMac from step (a) with the iHep from step (b) to generate the iKC. [0007] In another aspect, provided herein is a kit when used in the method as described herein, for generating an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC) comprising: (a) an iPSC-derived macrophage (iMac); (b) an iPSC-derived hepatocyte (iHep); and (c) a premixed cell culture media comprising about 1% Penicillin/Streptomycin, about 2% ITS, about 4mM GlutaMAX ^TM and about 30mM HEPES buffer with instructions for use. [0008] In another aspect, provided herein is an induced pluripotent stem cell (iPSC)- derived Kupffer cell (iKC) obtained by the method as described herein. [0009] In another aspect, provided herein is a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep). [0010] In another aspect, provided herein is a method of detecting an immune-mediated response to an agent comprising the steps of: (a) providing an iPSC-derived macrophage (iMac); (b) providing an iPSC-derived hepatocyte (iHep); (c) contacting and incubating the iMac from step (a) with the iHep from step (b) to generate a combined cell population comprising an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC); (d) contacting and incubating the combined cell population from step (c) with the agent; (e) detecting the presence or absence of a marker or measuring a level of said marker to detect the immune-mediated response to the agent. [0011] In another aspect, provided herein is a method of treating a liver disease in a subject in need thereof, wherein an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC) as described herein or a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep) as described herein is to be administered to said subject, wherein the disease is selected from the group consisting of: liver injury, drug- induced liver injury (DILI), liver disease, steatohepatitis, cholestasis, liver fibrosis, viral hepatitis, liver cancer, primary hepatocellular carcinoma, relapse hepatocellular carcinoma and colorectal metastasis in the liver. [0012] In another aspect, provided herein is a use of an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC) as described herein or a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep) as described herein in the manufacture of a medicament for the treatment of a liver disease selected from the group consisting of: liver injury, drug-induced liver injury (DILI), liver disease, steatohepatitis, cholestasis, liver fibrosis, viral hepatitis, liver cancer, primary hepatocellular carcinoma, relapse hepatocellular carcinoma and colorectal metastasis in the liver. [0013] In yet another aspect, provided herein is an induced pluripotent stem cell (iPSC)- derived Kupffer cell (iKC) as described herein or a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep) as described herein, for use in treating a liver disease selected from the group consisting of: liver injury, drug- induced liver injury (DILI), liver disease, steatohepatitis, cholestasis, liver fibrosis, viral hepatitis, liver cancer, primary hepatocellular carcinoma, relapse hepatocellular carcinoma and colorectal metastasis in the liver. [0014] In yet another aspect, provided herein is an induced pluripotent stem cell (iPSC)- derived Kupffer cell (iKC) as described herein or a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep) as described herein, for use in therapy. DEFINITIONS [0015] As used herein, the term “induced pluripotent stem cells” or “iPSCs” refers to pluripotent stem cells that are derived from a non-pluripotent cell such as a somatic cell that has been reprogrammed to a pluripotent state. The process of generating iPSCs typically involves introducing products of specific sets of pluripotency-associated genes, or "reprogramming factors". The somatic cells used to derive the stem cells may include but are not limited to liver macrophages, Kupffer cells, fibroblasts, myoblasts, keratinocytes, melanocytes, hepatocytes, β-cells, dental pulp cells, blood cells and urine-derived renal epithelial cells. The stem cells as used herein may include but are not limited to human, non- human primate, murine and avian stem cells. [0016] As used herein, the term “differentiate” refers to the developmental process by which a cell has progressed further down a developmental pathway than its immediate precursor cell. A differentiated cell is a cell of a more specialized cell type derived from a cell of a less specialized cell type in a cellular differentiation process. A differentiated cell is one that has taken on a more committed position within the lineage of the cell. [0017] As used herein, the term “macrophage” refers to a mononuclear phagocyte that recognizes, ingests and degrades cellular debris, microbes or any other foreign materials in a process termed phagocytosis. Macrophages are the most abundant liver immune cells and play a key role in tissue homeostasis and immunity. Embryonically-derived Kupffer cells (KCs) are the main population of macrophages of the liver and express liver specific markers. Monocyte-derived macrophages (MoMϕs) exist in the liver but represent a minor population of macrophages that increase during inflammation. [0018] As used herein, the term “hepatocyte” refers to a primary liver cell or hepatic parenchymal cell that is involved in metabolic processes. Hepatocytes comprise up to 80% of the total cell population of the human liver. Some of the metabolic processes that hepatocytes are involved in include carbohydrate metabolism, for example glycogen synthesis, glycolysis, and drug metabolism. [0019] As used herein, the term “Kupffer cells” refers to the self-renewing, resident and principally non-migratory phagocyte population in the liver. Kupffer cells originate from yolk sac-derived specific progenitor cells that seed the liver during embryogenesis. Kupffer cells are innate immune cells in the liver. Some of the important roles that Kupffer cells are involved in include scavenging for foreign materials and phagocytosis of cellular debris, foreign material or pathogens and maintenance of iron homeostasis via phagocytosis of red blood cells and recycling of iron. As such, they are regarded as critical sentinels that ensure liver homeostasis and eliminate antibodies, debris or dead cells. In healthy livers, Kupffer cells are exclusively located in the intravascular compartment (mainly within the hepatic sinusoids). [0020] As used herein, the term “iMac” refers to induced pluripotent stem cell (iPSC)- derived macrophage. iMacs are derived by differentiation of iPSCs according to the method described in Takata et al., (2017). Specifically, the iPSCs were cultured to 80% confluency, then passaged using ReLESR. As ReLESR releases the cells as small cell clumps, the passage conditions were optimised to result in roughly 1 x 10 ⁵ cells in each well of a 6 well plate (Day -1). Starting from the next day (Day 0) to day 16, the cells were cultured in Stempro Media, consisting of Stempro-34 SFM (GIBCO, 10639-011), supplemented with 200 ug/mL Human Transferrin (Roche, 10-652-202-001), 1x L-Glut, 1x Pen/Strep, 0.5 mM Ascorbic Acid (Sigma, A4544) and 0.45mM MTG (Sigma, M6145). A full media change was done every other day for the next 16 days, supplemented with the following cytokines: Differentiation Day 0 (5 ng/mL BMP4, 50 ng/mL VEGF, and 2 µM CHIR99021), Differentiation Day 2 (5 ng/mL BMP4, 50 ng/mL VEGF, and 20 ng/mL FGF2), Differentiation Day 4 (15 ng/mL VEGF and 5 ng/mL FGF2), Differentiation Day 6 to 10 (10 ng/mL VEGF, 10 ng/mL FGF2, 50 ng/mL SCF, 30 ng/mL DKK-1 (RnD, 5439-DK), 10 ng/mL IL-6 (RnD, 206-IL), and 20 ng/mL IL-3), Differentiation Day 12 and 14 (10 ng/mL FGF2, 50 ng/mL SCF, 10 ng/mL IL-6, and 20 ng/mL IL-3). From day 16, the cells were cultured in 75% IMDM with GlutaMAX ^TM (GIBCO, 31980-030), 25% F12 (GIBCO, 11765- 047), 1x N2 supplement (GIBCO, 17502-048), 1x B27 Supplement (GIBCO, 17504-001), 0.05% BSA (GE Healthcare, SH30574) and 1x Pen/Strep, supplemented with 50ng/ml of CSF-1 (RnD, 216-MC), with a full media change every 3 days. In addition, for the first 8 days of differentiation, the cells were kept in a hypoxic incubator (5% O ₂ and 5% C O ₂ ), before being transferred to a standard cell culture incubator for the next 18 days. After transfer to a standard cell culture incubator, floating cells were collected every media change and resuspended back into the culture to retain the hematopoietic progenitors. After 26 days, the floating cells were collected and the purity of the iMacs was determined by flow cytometry. [0021] As used herein, the term “iHep” refers to an induced pluripotent stem cell (iPSC)- derived hepatocyte. According to the method as described in Cai et al., (2015), iHeps are derived by differentiation of iPSCs. Specifically, the iPSCs were cultured to 80% confluency, then made into single cells using Cell Dissociation Buffer (Gibco, 13151014) and seeded at 1.5 x 10 ⁵ cells/cm ² on the desired well plate format (i.e.1.5 x 10 ⁶ cells for a 6 well plate) and placed under hypoxic conditions (5% O ₂ and 5% CO ₂ ). The cells are seeded onto appropriate plate and cultured overnight under the hypoxic conditions as described herein. The day after the culture overnight is referred to as differentiation day 1. On differentiation day 1, culture medium was removed and replaced with RPMI/B-27 (minus Insulin) (Invitrogen/Gibco) supplemented with 100 ng/ml Activin A (R&D systems), 10 ng/mL BMP4 (Peprotech) and 20 ng/mL FGF2 (Invitrogen). Culture medium was changed daily and cultured for 2 days at 37°C in ambient O ₂ /5% CO _2. On differentiation days 3 to 5, culture medium was changed to RPMI/B-27 (minus Insulin) supplemented with 100 ng/ml Activin A and continued to culture for an additional 3 days at 37°C in ambient O ₂ /5% CO ₂ with daily medium changes. On differentiation day 6, culture medium was changed to RPMI/B27 (complete with Insulin) supplemented with 20 ng/ml BMP-4 and 10 ng/ml FGF2. Culture was incubated for 5 days at 37°C in 4% O ₂ /5% CO ₂ and culture medium was replaced daily for a total of 5 days. On differentiation day 11, culture medium was changed to RPMI/B27 (complete with Insulin) supplemented with 20 ng/ml Hepatocyte Growth Factor (HGF) (Peprotech). Culture was incubated for 5 days at 37°C in 4% O ₂ /5% CO ₂ and medium was replaced daily. On differentiation day 16, culture medium was changed to Clonetics® HCM™ Hepatocyte Culture Medium (Lonza CC-3198) containing ‘Singlequots’ (see HCM medium below) omitting the EGF from the Lonza HCM bullet kit. In addition to the kit supplements, 20 ng/ml Oncostatin-M (R&D Systems) was added. Culture was incubated for 5 days at 37°C in ambient O ₂ /5% CO ₂ and culture medium was replaced daily. After 20 days of differentiation, iPSC-derived macrophages (iMacs) were directly added to the iHeps. [0022] As used herein, the term “subject” or “patient” refers to all animals classified as mammals and includes but is not limited to primates, humans, cows, horses, pigs, sheep, goats, dogs, cats, or rodents. Preferably, the subject is a human man or woman of any age or race. [0023] As used herein, the term “treatment”, refers to any type of therapy, which aims at terminating, preventing, ameliorating or reducing the susceptibility to a clinical condition as described herein. The effect may be prophylactic in terms of completely or partially preventing a disorder or symptom thereof and/or may be therapeutic in terms of a partial or complete cure for a disorder and/or adverse effect attributable to the disorder. As such, “treatment” includes preventing the disorder from occurring or recurring in a subject, inhibiting the disorder, such as arresting its development, stopping or terminating the disorder or at least symptoms associated therewith, so that the host no longer suffers from the disorder or its symptoms, such as causing regression of the disorder or its symptoms, for example, by restoring or repairing a lost, missing or defective function, or stimulating an inefficient process, or relieving, alleviating, or ameliorating the disorder, or symptoms associated with the disorder. [0024] As used herein, the term “about”, is used in the context of, but not limited to, concentrations of components and percentages of compounds. Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub- ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range. BRIEF DESCRIPTION OF THE DRAWINGS [0025] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which: [0026] Figure 1 depicts the model development and shows a schematic of the experimental layout. [0027] Figure 2 shows the assessment of hepatocyte performance as a function of media supplementation. Figure 2A shows gene expression of hepatic markers upon treatment with media supplemented 2X and media changed performed every day, every other day or every two days. Figure 2B shows hepatic marker expression with 2X media supplementation and 2X media volume to account for the infrequent media change. Gene expression was compared as fold change to standard hepatocyte culture conditions (daily media change with 1X supplement and standard media volume). Increasing supplements alone could maintain the expression of ALB (albumin), AAT (alpha-1-antitrypsin), cytochrome P450 (CYP)- 1A2, 3A4, 2B6 and UDP-glucuronosyltransferase (UGT)1A3, but not CYP2C19 (36 - 68% decrease), glutathione S-transferase (GST)A2 (35-66% decrease) and multidrug resistance protein-2 (MRP2; 7 to 18% decrease) (Figure 2A). In contrast, combining the increase in supplements with increase in culture volume resulted in maintenance of all hepatic markers (Figure 2B). Figure 2C shows albumin and urea production when hepatocytes were harvested and reseeded for experimental set up compared to unharvested hepatocytes (in situ), demonstrating that hepatocytes harvesting does not compromise cell functionality. [0028] Figure 3 shows the effects of iHeps on iMacs. Figure 3A shows the Principal Component Analysis of bulk RNAseq data from Day 0 iMacs, Day 3 co-iMacs, Day 7 co- iMacs, Day 7 iMacs and Day 7 cond-iMacs. Figure 3B shows the heatmap showing the top 5 differentially and uniquely expressed genes from culturing iMacs alone, with conditioned media (cond-iMacs) and after co-culture with iHepatocytes (Day 7 co-iMacs). Figure 3C shows the volcano plot showing differentially-expressed genes between Day 0 iMacs and Day 7 co-iMacs. Figure 3D shows the differentially expressed upregulated pathways between Day 0 iMacs and Day 7 co-iMacs. Figure 3E shows the volcano plot showing differentially-expressed genes between Day 7 cond-iMacs and Day 7 co-iMacs. Figure 3F shows the differentially expressed upregulated pathways between Day 7 cond-iMacs and Day 7 co-iMacs. Figure 3G shows the Gene expression levels of LYVE1, ID1, ID3, RXRA, HBEGF and OSM in Day 0 iMacs, Day 3 co-iMacs, Day 7 co-iMacs, Day 7 iMacs and Day 7 cond-iMacs. [0029] Figure 4 shows the gene expression of iMac and iHep markers in co-cultures. Figure 4A shows the expression of macrophage markers (top panel) and KC-specific markers (bottom panel) on day 3 and day 7 of co-culture. Figure 4B shows the expression of macrophage markers (top panel) and KC-specific markers (bottom panel) on day 7 of co- culture compared to iMac cultured in PHCM. Figure 4C shows the expression of hepatic markers on day 3 and day 7 of co-culture. * p<0.05. [0030] Figure 5 shows the changes in IL-6 level upon treatment with 7 paradigm compounds when iMac-derived KCs were used. Cytokine production was assessed in iMac- derived iKC and iHep co-culture treated with the drug stated in the plot title. Data is expressed as the percentage of the LPS-treated vehicle control. Error bars represented S.D., n=3. One-way ANOVA was applied. *p<0.05, **p<0.01 between treatment and vehicle control. [0031] Figure 6 shows the changes in IL-6 level upon treatment with 7 paradigm compounds without iMac-derived KCs. Cytokine production was assessed in blood monocyte-derived macrophages and iHep co-culture treated with the drug stated in the plot title. Data is expressed as the percentage of the LPS-treated vehicle control. Error bars represented S.D., n=3. One-way ANOVA was applied. *p<0.05, **p<0.01 between treatment and vehicle control. DETAILED DESCRIPTION OF THE PRESENT INVENTION [0032] In a first aspect the present invention refers to a method of generating an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC) comprising the steps of (a) providing an iPSC-derived macrophage (iMac); (b) providing an iPSC-derived hepatocyte (iHep); (c) contacting and incubating the iMac from step (a) with the iHep from step (b) to generate the iKC. [0033] In some examples, the iKC may demonstrate one or more biological characteristics or phenotypes of a Kupffer cell (KC). In some examples, the biological characteristic of a KC may be an expression of a macrophage marker, an expression of a KC marker, an expression of a Liver X Receptor/Farnesoid X Receptor (LXR/RXR) signaling pathway regulator, an expression of a marker for liver regeneration and/or development, or combinations thereof. It will be appreciated by a person skilled in the art that where the iKC demonstrates one or more biological characteristics of a KC, these can be one or more biological characteristics from different groups of markers. For example, the iKC may express one or more macrophage markers and one or more KC markers. In another example, the iKC may express one or more macrophage markers and one or more LXR/RXR signal pathway regulators. In another example, the iKC may express one or more macrophage markers and one or more markers for liver regeneration and/or development. In another example, the iKC may express one or more KC markers and one or more LXR/RXR signal pathway regulators. In another example, the IKC may express one or more KC markers and one or more markers for liver regeneration and/or development. In another example, the iKC may express one or more LXR/RXR signal pathway regulators and one or more markers for liver regeneration and/or development. [0034] In some examples, the macrophage marker may be but is not limited to CD68, CD163, CD11b, CD32 and CD14. It will be appreciated by a person skilled in the art that iKC may express one or more macrophage markers. In some examples, a person skilled in the art will understand that iKC may express combinations of macrophage markers. [0035] In some examples, the KC marker may be but is not limited to Lymphatic Vessel Endothelial Hyaluronan Receptor 1 (LYVE1), DNA-binding protein inhibitor ID-1 (ID1), and DNA-binding protein inhibitor ID-3 (ID3). It will be appreciated by a person skilled in the art that iKC may express one or more KC markers. In some examples, a person skilled in the art will understand that iKC may express combinations of KC markers. [0036] In one example, the Liver X Receptor/Farnesoid X Receptor (LXR/RXR) signalling pathway regulator may be but is not limited to Retinoid X Receptor Alpha (RXRA). [0037] In some examples, the marker for liver regeneration and/or development may be but is not limited to Heparin Binding EGF Like Growth Factor (HBEGF) and Oncostatin M (OSM). It will be appreciated by a person skilled in the art that iKC may express one or more markers for liver regeneration and/or development. In some examples, a person skilled in the art will understand that iKC may express combinations of markers for liver regeneration and/or development. [0038] In some examples, the number of iHeps to iMacs may be cultured at a ratio of about 1:1: to about 10:1. For example, the ratio of iHeps to iMacs may be about 1:1, about 1.5:1, about 2:1, about 2.5:1, about 3:1, about 3.5:1, about 4:1, about 4.5:1, about 5:1, about 5.5:1, about 6:1, about 6.5:1, about 7:1, about 7.5:1, about 8:1, about 8.5:1, about 9:1, about 9.5:1, about 10:1. In a preferred example, the ratio of iHeps to iMacs is about 2.5:1. [0039] In one example, the iHeps and iMacs may be incubated on a cell culture plate. The culture plate may be a 6-well plate, 12-well plate, 24-well plate, 48-well plate or 96-well plate. In another example, the iHeps and iMacs may be incubated on a culture dish. The culture dish may be a 35mm dish, 60mm dish, 100mm dish or 150mm culture dish. In another example, the iHeps and iMacs may be incubated in a culture flask. The culture flask may be a T25 flask, T75 flask, T175 flask or T225 flask. [0040] The iHeps and iMacs of the present invention may be cultured in the presence of an extracellular matrix (ECM), for example, Matrigel. [0041] In one example, the iHeps and iMacs may be cultured on a 6-well plate. In one example, the iHeps and iMacs may be cultured on a Matrigel-coated 6-well plate or an extracellular matrix (ECM)-coated 6-well plate. [0042] In one example, the number of iHeps and iMacs cultured on a 6-well plate is about 5 x 10 ⁵ and about 2 x 10 ⁵ respectively. It will be appreciated by a person skilled in the art that the cell density of iHeps and iMacs to be cultured can be scaled proportionately based on the surface area of the culture vessel used. [0043] In one example, the total number of iHeps and iMacs cultured on a 6-well plate is up to 1.5 x 10 ⁶ . It will be appreciated by a person skilled in the art that the cell density of iHeps and iMacs to be cultured can be scaled proportionately based on the surface area of the culture vessel used. [0044] In some examples, the concentration of the Penicillin/Streptomycin in the culture medium may be between 0.1% and about 3%. For example, the concentration of the Penicillin/Streptomycin may be about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2.0%, about 2.1%, about 2.2%, about 2.3%, about 2.4%, about 2.5%, about 2.6%, about 2.7%, about 2.8%, about 2.9%, and about 3.0%. In a preferred example, the concentration of Penicillin/Streptomycin is about 1.0%. [0045] In some examples, the concentration of the ITS in the culture medium may be between 0.1% and about 4%. For example, the concentration of the ITS may be about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2.0%, about 2.1%, about 2.2%, about 2.3%, about 2.4%, about 2.5%, about 2.6%, about 2.7%, about 2.8%, about 2.9%, about 3.0%, about 3.1%, about 3.2%, about 3.3%, about 3.4%, about 3.5%, about 3.6%, about 3.7%, about 3.8%, about 3.9% and about 4.0%. In a preferred example, the concentration of ITS is about 2.0%. [0046] In some examples, the concentration of the GlutaMAX ^TM in the culture medium may be between 1mM and about 7mM. For example, the concentration of the GlutaMAX ^TM may be about 1.0mM, about 1.1mM, about 1.2mM, about 1.3mM, about 1.4mM, about 1.5mM, about 1.6mM, about 1.7mM, about 1.8mM, about 1.9mM, about 2.0mM, about 2.1mM, about 2.2mM, about 2.3mM, about 2.4mM, about 2.5mM, about 2.6mM, about 2.7mM, about 2.8mM, about 2.9mM, about 3.0mM, about 3.1mM, about 3.2mM, about 3.3mM, about 3.4mM, about 3.5mM, about 3.6mM, about 3.7mM, about 3.8mM, about 3.9mM, about 4.0mM, about 4.1mM, about 4.2mM, about 4.3mM, about 4.4mM, about 4.5mM, about 4.6mM, about 4.7mM, about 4.8mM, about 4.9mM, about 5.0mM, about 5.1mM, about 5.2mM, about 5.3mM, about 5.4mM, about 5.5mM, about 5.6mM, about 5.7mM, about 5.8mM, about 5.9mM, about 6.0mM, about 6.1mM, about 6.2mM, about 6.3mM, about 6.4mM, about 6.5mM, about 6.6mM, about 6.7mM, about 6.8mM, about 6.9mM, and about 7.0mM. In a preferred example, the concentration of GlutaMAX ^TM is about 4.0mM. [0047] In some examples, the concentration of the HEPES buffer in the culture medium may be between 5mM and 50mM. For example, the concentration of the HEPES buffer may be about 5mM, about 6mM, about 7mM, about 8mM, about 9mM, about 10mM, about 11mM, about 12mM, about 13mM, about 14mM, about 15mM, about 16mM, about 17mM, about 18mM, about 19mM, about 20mM, about 21mM, about 22mM, about 23mM, about 24mM, about 25mM, about 26mM, about 27mM, about 28mM, about 29mM, about 30mM, about 31mM, about 32mM, about 33mM, about 34mM, about 35mM, about 36mM, about 37mM, about 38mM, about 39mM, about 40mM, about 41mM, about 42mM, about 43mM, about 44mM, about 45mM, about 46mM, about 47mM, about 48mM, about 49mM, and about 50mM. In a preferred example, the concentration of HEPES buffer is about 30mM. [0048] In a preferred example, the iHep and iMac are incubated with culture media comprising about 1% Penicillin/Streptomycin, about 2% ITS, about 4mM GlutaMAX ^TM and about 30mM HEPES buffer. [0049] In one example, the culture medium may contain CSF-1. [0050] In one example, the culture medium does not contain dexamethasone. [0051] In some examples, the concentration of the CSF-1 in the culture medium may be between 25ng/mL to 75ng/mL. For example, the concentration of the CSF-1 may be about 25ng/mL, about 26ng/mL, about 27ng/mL, about 28ng/mL, about 29ng/mL, about 30ng/mL, about 31ng/mL, about 32ng/mL, about 33ng/mL, about 34ng/mL, about 35ng/mL, about 36ng/mL, about 37ng/mL, about 38ng/mL, about 39ng/mL, about 40ng/mL, about 41ng/mL, about 42ng/mL, about 43ng/mL, about 44ng/mL, about 45/mL, about 46ng/mL, about 47ng/mL, about 48ng/mL, about 49ng/mL, about 50ng/mL, about 51ng/mL, about 52ng/mL, about 53ng/mL, about 54ng/mL, about 55ng/mL, about 56ng/mL, about 57ng/mL, about 58ng/mL, about 59ng/mL, about 60ng/mL, about 61ng/mL, about 62ng/mL, about 63ng/mL, about 64ng/mL, about 65ng/mL, about 66ng/mL, about 67ng/mL, about 68ng/mL, about 69ng/mL, about 70ng/mL, about 71ng/mL, about 72ng/mL, about 73ng/mL, about 74ng/mL, and about 75ng/mL. In a preferred example, the concentration of CSF-1 is about 50ng/mL. [0052] In one example, the culture medium does not contain primary hepatocyte conditioned media. A person skilled in the art will understand that the primary hepatocyte conditioned media refers to culture media used in culturing primary hepatocytes. [0053] In some examples, the iPSCs may be human iPSCs. In one example, the iPSCs may be but is not limited to IMR90-F4, IMR90-iPSCs, Kyou-DXR0109B, PCi-ASI and PCi- CAU. [0054] In another example, the iPSC may be derived or reprogrammed from a somatic cell obtained from a subject or patient. [0055] In some examples, the volume of culture media per well of a 6-well plate may be between 2.0mL to 6.0mL. For example, the volume of culture media per well of a 6-well plate may be about 2.0mL, about 2.1mL, about 2.2mL, about 2.3mL, about 2.4mL, about 2.5mL, about 2.6mL, about 2.7mL, about 2.8mL, about 2.9mL, about 3.0mL, about 3.1mL, about 3.2mL, about 3.3mL, about 3.4mL, about 3.5mL, about 3.6mL, about 3.7mL, about 3.8mL, about 3.9mL, about 4.0mL, about 4.1mL, about 4.2mL, about 4.3mL, about 4.4mL, about 4.5mL, about 4.6mL, about 4.7mL, about 4.8mL, about 4.9mL, about 5.0mL, about 5.1mL, about 5.2mL, about 5.3mL, about 5.4mL, about 5.5mL, about 5.6mL, about 5.7mL, about 5.8mL, about 5.9mL and about 6.0mL. In a preferred example, the volume of culture media per well of a 6-well plate is 4.0mL. [0056] In some examples, the iHep and iMac may be cultured for 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 days, wherein day 0 refers to the day iHep and iMac are in contact and cultured in a single culture vessel. In a preferred example, the iHep and iMac are cultured for 7 days. [0057] In some examples, about 2mL of culture media is aspirated from a well in the 6- well culture plate and replaced with about 2mL of newly prepared culture media of the same on or after day 4. It will be appreciated by a person skilled in the art that the volume of culture media aspirated can be scaled proportionately based on the volumetric size of the culture vessel used. A person skilled in the art will understand to replace the aspirated culture media with an equal volume of freshly prepared culture media of the same. In one example, 2mL of culture media is aspirated from the 6-well culture plate and replaced with 2mL of newly prepared culture media of the same on day 4. [0058] In another aspect, provided herein is a kit when used in the method as described herein, for generating an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC), comprising: (a) an iPSC-derived macrophage (iMac); (b) an iPSC-derived hepatocyte (iHep); and (c) a premixed cell culture media comprising about 1% Penicillin/Streptomycin, about 2% ITS, about 4mM GlutaMAX ^TM and about 30mM HEPES buffer with instructions for use. [0059] In another aspect, provided herein is an induced pluripotent stem cell (iPSC)- derived Kupffer cell (iKC) obtained by the method as described herein. [0060] In one example, the iKC may express higher levels of KC-specific markers compared to an iMac cultured in primary hepatocyte conditioned media. [0061] In some examples, the iKC may express one or more biological characteristics of a Kupffer cell (KC). In some examples, the biological characteristic of a KC may be an expression of a macrophage marker, an expression of a KC marker, an expression of a Liver X Receptor/Farnesoid X Receptor (LXR/RXR) signaling pathway regulator, an expression of a marker for liver regeneration and/or development, or combinations thereof. It will be appreciated by a person skilled in the art that where the iKC demonstrates one or more biological characteristics of a KC, these can be one or more biological characteristics from different groups of markers. For example, the iKC may express one or more macrophage markers and one or more KC markers. In another example, the iKC may express one or more macrophage markers and one or more LXR/RXR signal pathway regulators. In another example, the iKC may express one or more macrophage markers and one or more markers for liver regeneration and/or development. In another example, the iKC may express one or more KC markers and one or more LXR/RXR signal pathway regulators. In another example, the IKC may express one or more KC markers and one or more markers for liver regeneration and/or development. In another example, the iKC may express one or more LXR/RXR signal pathway regulators and one or more markers for liver regeneration and/or development [0062] In some examples, the macrophage marker may be but is not limited to CD68, CD163, CD11b, CD32 and CD14. It will be appreciated by a person skilled in the art that iKC may express one or more macrophage markers. In some examples, a person skilled in the art will understand that iKC may express combinations of macrophage markers. [0063] In some examples, the KC marker may be but is not limited to Lymphatic Vessel Endothelial Hyaluronan Receptor 1 (LYVE1), DNA-binding protein inhibitor ID-1 (ID1), and DNA-binding protein inhibitor ID-3 (ID3). It will be appreciated by a person skilled in the art that iKC may express one or more KC markers. In some examples, a person skilled in the art will understand that iKC may express combinations of KC markers. [0064] In one example, the Liver X Receptor/Farnesoid X Receptor (LXR/RXR) signalling pathway regulator may be but is not limited to Retinoid X Receptor Alpha (RXRA). [0065] In some examples, the marker for liver regeneration and/or development may be but is not limited to Heparin Binding EGF Like Growth Factor (HBEGF) and Oncostatin M (OSM). It will be appreciated by a person skilled in the art that iKC may express one or more markers for liver regeneration and/or development. In some examples, a person skilled in the art will understand that iKC may express combinations of markers for liver regeneration and/or development. [0066] In another aspect, provided herein is a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep). [0067] In one example, the iKC and iHep of the combined cell population as described herein are derived from a single stem cell source. It will generally be understood that any somatic cell may be used to derive the stem cells of the present invention. For example, the somatic cell used to derive the stem cells as used herein may include but are not limited to liver macrophages, Kupffer cells, fibroblasts, myoblasts, keratinocytes, melanocytes, hepatocytes, β-cells, dental pulp cells, blood cells and urine-derived renal epithelial cells. The stem cells as used herein may include but are not limited to human, non-human primate, murine and avian stem cells. It will be appreciated by a person skilled in the art that by obtaining both iHep and iKC from a single stem cell source, both iHep and iKC are thus, derived from the same subject. As such, iHep and iKC may be donor-matched. In a preferred example, the iKC and iHep are derived from the single stem cell source of human IMR90- iPSCs. [0068] In one example, the combined cell population as described herein is for use as a model of a disease. In one example, the disease may be but is not limited to liver injury, drug- induced liver injury (DILI), liver disease, steatohepatitis, cholestasis, liver fibrosis, viral hepatitis, liver cancer, primary hepatocellular carcinoma, relapse hepatocellular carcinoma and colorectal metastasis in the liver. [0069] In another aspect, provided herein is the method of detecting an immune-mediated response to an agent comprising the steps of: (a) providing an iPSC-derived macrophage (iMac); (b) providing an iPSC-derived hepatocyte (iHep); (c) contacting and incubating the iMac from step (a) with the iHep from step (b) to generate a combined cell population comprising an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC); (d) contacting and incubating the combined cell population from step (c) with the agent; (e) detecting the presence or absence of a marker or measuring a level of said marker to detect the immune-mediated response to the agent. [0070] The presence or absence of a marker or the level of a marker may be detected or measured using conventional means in the art. [0071] As described herein, it would be understood by a person skilled in the art that the detection or measurement of the marker be an absolute measurement or a relative measurement. An absolute measurement of the expression of a marker may be determined by the presence or absence of said marker, or a characteristic of said marker, such as a concentration. A relative measurement of a marker may be obtained by comparing a level of expression or characteristic of said marker with a reference to determine a relative expression level of said marker, or relative concentration of said marker. The reference control may be but is not limited to an untreated combined cell population comprising an iKC, a combined cell population comprising an iKC generated from other methods and a primary KC. [0072] As described herein, the agent may be but is not limited to a drug, a prodrug, a toxicant, a virus, a chimeric antigen receptor (CAR) T-cell, a protein, a phosphomimetic, a peptide, an antibody, an antibody-drug conjugate, a recombinant DNA, a short hairpin RNA, a microRNA, a short-interfering RNA, an agro-/environmental toxin, food, a consumer and cosmetic product. [0073] In one example, the agent is a hepatotoxic drug. The hepatotoxic drug may be but is not limited to diclofenac (DIC), sulindac (SLD) leflunomide (LFM); amodiaquine (AQ), lamotrigine (LTG), allopurinol, carbamazepine, celecoxib, clavulanate, deferasirox, erythromycin, flucloxacillin, nevirapine, phenytoin, posaconazole, propylthiouracil, rifampicin, roxithromycin, sulfasalazine, dihydralazine, halothane, methyldopa, minocycline, nitrofurantoin. [0074] Markers include but are not limited to genes or proteins. As described herein, the marker may be but is not limited to an inflammatory mediator, a growth factor and a reactive oxygen species. [0075] In one example, the inflammatory mediator is a cytokine. The cytokine may be but is not limited to interferon-γ (IFN-γ), interleukin (IL) IL-1β, IL-6, IL-10, IL-12, IL-17, IL-18 and tumor necrosis factor-α (TNF-α). In another example, the inflammatory mediator is a chemokine. The chemokine may be but is not limited to CCL2, CCL3, CCL4, CCL5, CCL11. [0076] In one example, the marker is a growth factor. The growth factor may be but is not limited to platelet-derived growth factor (PDGF), insulin-like growth factor (IGF-1), vascular endothelial growth factor (VEGF), transforming growth factor-beta 1 (TGF-β1), growth factors from the tumor-necrosis factor (TNF), transforming growth factor (TGF), vascular endothelial growth factor (VEGF) and bone morphogenetic protein (BMP) families. [0077] In one example, the marker is a reactive oxygen species. The reactive oxygen species may be but is not limited to superoxide anion (O ₂ -), hydrogen peroxide (H ₂ O ₂ ), hydroxyl radical and singlet oxygen. [0078] In another aspect, provided herein is a method of treating a liver disease in a subject in need thereof, wherein an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC) as described herein or a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep) as described herein is to be administered to said subject, wherein the disease may be but is not limited to liver injury, drug-induced liver injury (DILI), liver disease, steatohepatitis, cholestasis, liver fibrosis, viral hepatitis, liver cancer, primary hepatocellular carcinoma, relapse hepatocellular carcinoma and colorectal metastasis in the liver. [0079] In another aspect, provided herein is a use of an induced pluripotent stem cell (iPSC)-derived Kupffer cell (iKC) as described herein or a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep) as described herein in the manufacture of a medicament for the treatment of a liver disease, wherein the liver disease may be but is not limited to liver injury, drug-induced liver injury (DILI), liver disease, steatohepatitis, cholestasis, liver fibrosis, viral hepatitis, liver cancer, primary hepatocellular carcinoma, relapse hepatocellular carcinoma and colorectal metastasis in the liver. [0080] In yet another aspect, provided herein is an induced pluripotent stem cell (iPSC)- derived Kupffer cell (iKC) as described herein or a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep) as described herein, for use in treating a liver disease, wherein the liver disease may be but is not limited to liver injury, drug-induced liver injury (DILI), liver disease, steatohepatitis, cholestasis, liver fibrosis, viral hepatitis, liver cancer, primary hepatocellular carcinoma, relapse hepatocellular carcinoma and colorectal metastasis in the liver. [0081] In yet another aspect, provided herein is an induced pluripotent stem cell (iPSC)- derived Kupffer cell (iKC) as described herein or a combined cell population comprising an iPSC-derived Kupffer cell (iKC) and an iPSC-derived hepatocyte (iHep) as described herein, for use in therapy. [0082] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including”, “expressing”, etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention. [0083] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein. [0084] Other embodiments are within the following claims and non-limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group. EXPERIMENTAL SECTION [0085] Non-limiting examples of the invention and comparative examples will be further described in greater detail by reference to specific examples, which should not be construed as in any way limiting the scope of the invention. [0086] Materials and Methods [0087] Maintenance of iPSCs [0088] Human IMR90-iPSCs were obtained from WiCell Research Institute (Madison, WI). The cells were maintained under antibiotic-free conditions in mTESR media (StemCell Technologies, 85850) on Matrigel (Corning, 354234)-coated plates. Cells were passaged using ReLESR (StemCell Technologies, 05872) or Dispase (StemCell Technologies, 07913) as per manufacturer’s protocol whenever they reached 80% confluency. [0089] Differentiation of iPSCs to iHeps [0090] IMR90-iPSCs were differentiated into iPSC-hepatocytes (iHeps) as previously described by Cai et al., (2015). Briefly, the iPSCs were cultured to 80% confluency, then made into single cells using Cell Dissociation Buffer (Gibco, 13151014) and seeded at 1.5 x 10 ⁵ cells/cm ² on the desired well plate format (i.e. 1.5 x 10 ⁶ cells for a 6 well plate) and placed under hypoxic conditions (5% O ₂ and 5% CO ₂ ). Growth factors and media used starting from the next day to day 20 are detailed in Cai et al., (2015). Media was changed every day and hypoxic conditions were used during days 5 to 15 of differentiation. After 20 days of differentiation, iPSC-derived macrophages (iMacs) were either directly added to the iHeps or iHeps were harvested for seeding into smaller well formats for drug testing. The harvesting protocol has previously been detailed in Tasnim et al., (2016). [0091] Differentiation of iPSCs to iMacs [0092] IMR90-iPSCs were differentiated into iMacs as previously described by Takata et al., (2017). Briefly, the iPSCs were cultured to 80% confluency, then passaged using ReLESR. As ReLESR releases the cells as small cell clumps, the passage conditions were optimised to result in roughly 1 x 10 ⁵ cells in each well of a 6 well plate (Day -1). Starting from the next day (Day 0) to day 16, the cells were cultured in Stempro Media, consisting of Stempro-34 SFM (GIBCO, 10639-011), supplemented with 200 ug/mL Human Transferrin (Roche, 10-652-202-001), 1x L-Glut, 1x Pen/Strep, 0.5 mM Ascorbic Acid (Sigma, A4544) and 0.45mM MTG (Sigma, M6145). A full media change was done every other day for the next 16 days, supplemented with the following cytokines: Differentiation Day 0 (5 ng/mL BMP4, 50 ng/mL VEGF, and 2 µM CHIR99021), Differentiation Day 2 (5 ng/mL BMP4, 50 ng/mL VEGF, and 20 ng/mL FGF2), Differentiation Day 4 (15 ng/mL VEGF and 5 ng/mL FGF2), Differentiation Day 6 to 10 (10 ng/mL VEGF, 10 ng/mL FGF2, 50 ng/mL SCF, 30 ng/mL DKK-1 (RnD, 5439-DK), 10 ng/mL IL-6 (RnD, 206-IL), and 20 ng/mL IL-3), Differentiation Day 12 and 14 (10 ng/mL FGF2, 50 ng/mL SCF, 10 ng/mL IL-6, and 20 ng/mL IL-3). From day 16, the cells were cultured in 75% IMDM with GlutaMAX ^TM (GIBCO, 31980-030), 25% F12 (GIBCO, 11765-047), 1x N2 supplement (GIBCO, 17502- 048), 1x B27 Supplement (GIBCO, 17504-001), 0.05% BSA (GE Healthcare, SH30574) and 1x Pen/Strep, supplemented with 50ng/ml of CSF-1 (RnD, 216-MC), with a full media change every 3 days. In addition, for the first 8 days of differentiation, the cells were kept in a hypoxic incubator (5% O2 and 5% CO2), before being transferred to a standard cell culture incubator for the next 18 days. After transfer to a standard cell culture incubator, floating cells were collected every media change and resuspended back into the culture to retain the hematopoietic progenitors. After 26 days, the floating cells were collected and the purity of the iMacs was determined by flow cytometry. [0093] Differentiation of human peripheral blood monocytes (PBMCs) into monocyte- derived macrophage. [0094] Human PBMCs were purified from blood apheresis cones by ficoll gradientCytiva, 17-1440-02), before being magnetically sorted with CD14+ microbeads (Miltenyi, 130-050- 021). 1 x 10 ⁶ CD14+ cells were then seeded per well of a 6 well plate in RPMI 1640 (Hyclone, SH30027.01) with 10% FBS (Biowest, S1810-500), 1x Pen/Strep, 1x Sodium Pyruvate, 1x NEAA and 1x L-Glut with 50ng/ml CSF-1 and cultured for 7 days. The purity of the monocyte-derived macrophages was then determined by flow cytometry. [0095] Co-culture of iMacs and monocyte-derived macrophages with iHeps [0096] 2 x 10 ⁵ iMacs or monocyte-derived macrophages were added to 5 x 10 ⁵ iHeps during a full media change (4ml) per well of a 6 well plate on Day 0. This media was comprised of Williams’ Medium E containing the following supplements: Penicillin/Streptomycin (10,000 U/mL / (10,000 μg/mL) – final conc = 1%, ITS+ - final conc= 2%, 4mM GlutaMAX ^TM , 30mM HEPES buffer. The concentrations of the supplements were double that of standard conditions used due to the infrequent media change. In addition, 50ng/ml of CSF-1 was added to each well. The cells were cultured for 7 days, with a half-media change (2ml) on Day 4 consisting of the same media as described above with 100ng/ml of CSF-1, resulting in a final concentration of 50ng/ml CSF-1 in the well. The cells were collected by digesting with Accutase (StemCell, 07920) for flow cytometry, RNAseq analysis and qPCR. For pharmacological testing, the number of iHeps and iMacs were downscaled to 96-well plates while maintaining the same iHep:iMac ratio (2.5:1). [0097] Flow cytometry [0098] Standard staining procedures were used to prepare the cells for flow cytometry analysis. Briefly, cells were dissociated into single cells using the different methods described above for the various tissues, before being incubated with 100µl of antibody containing FACS buffer (1% BSA and 4mM EDTA in PBS) per 5 million cells for 20 minutes at 4°C. The cell suspension was then washed with 5ml of FACS buffer, centrifuged at 400g, and the supernatant was removed. The cells were finally resuspended in PBS containing 3uM DAPI (Invitrogen, D1306). Data was acquired by LSRII (BD Bioscience) and analyzed by Flow Jo (Tree Star, Inc.). For cell sorting, cells were sorted using FACS Aria II (BD Bioscience) or FACS Aria III (BD Bioscience). The following antibodies were used: FITC-conjugated anti- human CD14 (Biolegend, 325604), APC/Cy7-conjugated anti-human CD45 (Biolegend, 368516), PE-conjugated anti-human CD163 (R&D Systems, FAB1607P-100), Alexa Fluor 647-conjugated anti-human CX3CR1 (Biolegend, 341608, PE/Cy7-conjugated anti-human CD11b (eBioscience, 25-0118-42). [0099] Bulk RNAseq [00100] Total RNA was extracted using Arcturus PicoPure RNA Isolation kit (Thermofisher, KIT0204) according to manufacturer’s protocol. All RNAs were analyzed on Labchip GX system (Perkin Elmer, United States) for quality assessment with median RNA Quality Score of 9.15. cDNA libraries were prepared using 2ng of total RNA and 1µl of a 1:50,000 dilution of Ambion ERCC RNA Spike-in Controls (Thermofisher, 4456740) with the following modifications: 1. Use of 20mM TSO, 2. Use of 250pg of cDNA with 1/5 reaction of Nextera XT kit (Illumina, FC-131). The length distribution of the cDNA libraries was monitored using DNA High Sensitivity Reagent Kit (Perkin Elmer, CLS60672) on the Labchip GX system (Perkin Elmer). All samples were subjected to an indexed PE sequencing run of 2x51 cycles on HiSeq 2000 (Illumina) at 16 samples per lane. [00101] Bulk RNAseq analysis [00102] Paired-end raw reads were mapped to GRCh38 human genome build. Genes were counted for reads that were mapped to genes based on GENCODEv29 gene annotation. Log2 transformed counts per million mapped read (log2CPM) and log2 transformed reads per kilobase per million mapped reads (log2RPKM) were computed. Across all samples, genes with log2CPM inter-quartile range (IQR) less than 0.5 were filtered out from subsequent differential expression gene (DEG) analysis. DEG analyses for culture condition comparisons were all done using edgeR. Selection of DEGs was done with Benjamini-Hochberg which adjusted P-values <0.05. R function ‘prcomp’ was used to perform Principal Component Analysis (PCA) on log2RPKM values. [00103] Quantitative real time PCR (qPCR) [00104] Total RNA extracted using RNeasy Plus Micro-kit (Qiagen, 74034) was quantified using a NanoDropTM ND-1000 Spectrophotometer and converted to cDNA using iScript cDNA synthesis kit (Bio-Rad Laboratories, 1708890). qPCR was performed in 7000 Fast Real-Time PCR System (Applied Biosystems, Foster City, USA) with FastStart Universal SYBR Green Master (Rox) (Roche, 04913 850 001) and primers from GeneCopoeia, Inc. (Rockville, MD, USA). Gyceraldehyde-3-phosphate dehydrogenase (GAPDH) served as internal control. Accession numbers of tested genes are listed in Table 1. [00105] Table 1. Accession numbers of tested genes. Target Gene Gene Name Accession GAPDH glyceraldehyde-3-phosphate dehydrogenase NM_001256799.2 AFP Alpha fetoprotein NM_000295 AAT Alpha-1 antitrypsin NM_001134 ASGPR Asialoglycoprotein Receptor NM_001671 ALB albumin NM_000477.3 CYP1A2 Cytochrome P4501A2 NM_000761.5 CYP3A4 Cytochrome P4503A4 NM_017460.5 CYP2B6 Cytochrome P4502B6 NM_000767.4 CYP2C9 Cytochrome P4502C9 NM_000771.3 UGT1A3 UDP-glucuroosyltransferase 1A3 NM_019093.2 GST1A2 Glutathione S-Transferase NM_000846.3 MRP2 Multidrug resistance-associated protein 2 NM_000392 CD14 Cluster of differentiation 14 NM_001174105.1 CD32 Cluster of differentiation 32 NM_001136219.1 CD68 Cluster of differentiation 68 NM_001251.2 CD163 Cluster of differentiation 163 NM_203416.3 ID1 Inhibitor of DNA-binding protein 1 NM_002165.3 ID3 Inhibitor of DNA-binding protein 3 NM_002167.4 [00106] Enzyme-linked immunosorbent assay (ELISA) for measurement of cytokines [00107] Interleukin-6 (IL-6) levels in the media were measured using the human IL-6 ELISA kit (Abcam, ab178013) according to manufacturer’s instructions. The cytokine production from dosed cells was normalized with the viability of cells measured from the same well. This ensures that changes in cytokines are not due to changes in cell numbers that might arise upon drug treatment. The normalized cytokine level was expressed as percentage of DMSO+LPS which allowed better comparison between different batches of experiments. [00108] Cell viability assay [00109] Cell viability was examined with AlamarBlue ^TM cell viability assay (Thermo Fisher Scientific Inc., Dal1025) according to manufacturer’s instructions. Briefly, AlamarBlue was diluted 10-fold using PBS containing 2 mg/mL of glucose and this working solution was added to the cells and incubated for 1 hour. Fluorescence (Ex: 530nm, Em: 590nm) readings were obtained using Tecan Microplate Reader M1000 PRO. [00110] Drug administration [00111] Following set up of co-culture, the model was subjected to treatments including 100 ng/ml LPS (Sigma Aldrich, L2654), and drugs (Sigma Aldrich) (with/without LPS) for 48hr. Stock solutions of drugs were prepared in dimethyl sulfoxide (DMSO) and diluted in medium prior to use. Medium containing 0.1% DMSO was used as vehicle control except. After 48 hr, supernatant was collected cytokine measurement and cell viability in the same wells were measured. [00112] Statistical analysis [00113] Mean and standard deviation were obtained from at least three independent batches of cells. Unpaired, paired student’s y-test and one-way or two-way analysis of variance (ANOVA) were performed accordingly at an overall confidence level of 95% using Prism software (GraphPad, San Diego, CA, USA) and indicated below each figure. [00114] Results [00115] Example 1 [00116] Optimization of culture conditions allow survival and function of iHeps and iMacs [00117] In order to investigate the effects of co-culturing iHeps with iMacs, a co-culture utilising IMR90 iPSC-derived iHeps and iMacs was established and optimized (Figure 1). To ensure survival and functionality of both cell types in the desired culture period (at least one week), certain key factors had to be accounted for: Hepatocytes are high in metabolically activity and often require frequent media change for replenishment of nutrients and removal of waste. On the other hand, it would be ideal to allow the iHeps and iMacs to interact through direct cross-talk as well as through secreted soluble factors without removing these factors from the system via media exchange. To assess the performance of hepatocytes with more infrequent media changes, expression of key hepatic markers was assessed when media was changed every other day, every two days or every three days and compared to a daily media change (Figure 2). To compensate for the lower frequency of media change, the effects of 2X supplements; standard media volume (Figure 2A) and 2X supplements with 2X standard culture volume (Figure 2B) were tested. Gene expression was compared as fold change to standard hepatocyte culture conditions (daily media change with 1X supplement and standard media volume). Increasing supplements alone could maintain the expression of ALB (albumin), AAT (alpha-1-antitrypsin), cytochrome P450 (CYP)- 1A2, 3A4, 2B6 and UDP-glucuronosyltransferase (UGT)1A3, but not CYP2C19 (36 - 68% decrease), glutathione S-transferase (GST)A2 (35-66% decrease) and multidrug resistance protein-2 (MRP2; 7 to 18% decrease) (Figure 2A). In contrast, combining the increase in supplements with increase in culture volume resulted in maintenance of all hepatic markers (Figure 2B). Albumin and urea production were also maintained to 1-1.4 pg/cell/48 hrs and 126-146 pg/cell/48 hrs (Figure 2C) which is in the range of albumin and urea production. Once iHep marker and functionality were confirmed, the cells in the co-culture system were sorted via flow cytometry on Day 3 and Day 7. Flow cytometric analysis showed the presence of CD45 ⁺ CD11b ⁺ CD14 ⁺ iMacs and CD45-CD11b- iHeps at Day 3 and Day 7 of co-culture (Figure 2D). [00118] Example 2 [00119] Co-culture with iHeps imparts KC-like identity to iMacs [00120] Next, the attention was turned towards the iMacs within the co-culture system. PCA analysis showed that the co-iMacs clustered much tighter together, while iMacs that were cultured alone or in hepatocyte conditioned media (cond-iMac) were much more scattered (Figure 3A). This suggests that co-culturing iMacs in direct contact with iHeps helps maintain the identity of the iMacs better, mirroring how ex-vivo resident tissue macrophages (RTMs) rapidly lose their identity when removed from their tissue environment. DEG analysis of all the iMac groups showed that both the Day 7 iMacs and the Day 7 cond- iMacs shared an activated and inflammatory profile, upregulating genes such as TNFSF18 and ACP5 (Figure 3B). Day7 iMacs had a more migratory profile, uniquely expressing Chemokine Receptor 7 (CCR7) and Chemokine Ligand 5 (CCL5), while Day 7 cond-iMacs uniquely expressed Dishevelled Associated Activator of Morphogenesis 2 (DAAM2), a developmental regulator of the WNT pathway. The co-iMacs were much more similar to each other (Figure 2), and expressed genes relating to liver homeostasis and function such as TTR and Albumin (ALB). [00121] As there was interest in how the tissue environment might educate and impart Kupffer cell-like identity to the iMacs, the DEGs between Day 0 iMac and Day 7 co-iMacs were looked at. Day 0 iMacs expressed the granulocytic and mast cell markers Proteoglycan 2 Pro Eosinophil Major Basic Protein 2/3 (PRG2/PRG3) and Carboxypeptidase A3 (CPA3), indicating that they might be immature cells that have been newly derived from hematopoietic myeloid progenitors (Figure 3C). In contrast, Day 7 co-iMacs upregulated the angiogenic Angiopoietin-like 4 (ANGPTL4), matching the angiogenic signature observed in the Day 7 co-iHeps. Ingenuity pathway analysis revealed that the top pathway in Day 7 co- iMacs is the Liver X Receptor/Farnesoid X Receptor (LXR/RXR) signalling pathway (Table 2), a key regulator of Kupffer cell identity, while the top pathway in Day 0 iMacs is the axonal guidance pathway, consisting of chemokines like C-X-C Motif Chemokine Ligand 12 (CXCL12) and C-X-C Motif Chemokine Receptor 4 (CXCR4), metalloproteases like ADAM Metallopeptidase With Thrombospondin Type 1 Motif 15 (ADAMTS15), and semaphorins like Semaphorin 3C (SEMA3C) (Table 3). This suggests that co-culturing the iMacs with iHeps activated a KC-specific programme, while the Day 0 iMacs had a more general and unspecified identity. In addition, Day 7 co-iMacs also upregulated pathways associated with tissue repair and remodelling, as well as tissue growth factors, while migratory and immune signalling related pathways were upregulated in the Day 0 iMacs (Figure 3D). These data suggest that co-culturing iMacs with iHeps is sufficient to impart a more tissue supportive and Kupffer cell-like identity to the iMacs. [00122] Table 2. Ingenuity Pathway Analysis for genes upregulated in Day 7 co-iMacs vs Day 0 iMacs. [00123] Table 3. Ingenuity Pathway Analysis for genes upregulated in Day 0 iMacs vs Day 7 co-iMacs.

[00124] The use of conditioned media as a surrogate of the in vivo tissue microenvironment is a popular approach, iMacs cultured in direct co-culture with iHeps may be different from iMacs cultured in PHCM. Comparing the DEGs between Day 7 cond-iMacs and Day 7 co- iMacs, the Day 7 cond-iMacs upregulated immune-related genes such as Toll-Like Receptor 4 (TLR4) and CPA3 while the Day 7 co-iMacs more highly expressed tissue residency-related Fc Gamma Receptor IIIa (FCGR3A) (Figure 3E). Pathway analysis showed that the top upregulated pathway in the Day 7 cond-iMacs was autophagy, which might indicate that conditioned media alone was insufficient to maintain proper macrophage biology in the absence of contact with other cells, although some hepatic related pathways were also upregulated as well (Figure 3F, Table 4). On the other hand, the most highly upregulated pathway in the Day 7 co-iMacs was the FXR/RXR pathway, which along with the upregulation of other anabolic pathways related to hepatic health (Table 5), suggest that co- culturing rather than conditioned media is better at inducing Kupffer cell-like identity in macrophages. [00125] Table 4. Ingenuity Pathway Analysis for genes upregulated in Day 7 cond-iMacs vs Day 7 co-iMacs.

[00126] Table 5. Ingenuity Pathway Analysis for genes upregulated in Day 7 co-iMacs vs Day 7 cond-iMacs.

[00127] Lastly, the expression of selected key Kupffer cell genes was looked at. Lymphatic Vessel Endothelial Hyaluronan Receptor 1 (LYVE1), DNA-binding protein inhibitor ID-1 (ID1) and DNA-binding protein inhibitor ID-3 (ID3), which are important markers of KC identity, were all upregulated in the co-iMacs but not in the cond-iMacs (Figure 3G). Retinoid X Receptor Alpha (RXRA), the obligate binding partner of LXR, was also upregulated in only the co-iMacs. Interestingly, the co-iMacs were also found to upregulate the expression of Heparin Binding EGF Like Growth Factor (HBEGF) and Oncostatin M (OSM), which are potent effectors of liver regeneration and development. [00128] Altogether, the data strongly supports the hypothesis that co-culturing iMacs with iHeps induces KC-like identity in the iMacs, which in turn supports the maturation and development of the iHeps. [00129] Example 3 [00130] Gene expression confirms KC-like identity of iMacs and maturation of iHeps in co-culture [00131] The observations from the RNA sequencing analysis were confirmed by analysing gene expression of macrophage and KC-specific markers in Day 3 co-iMacs and Day 7 co- iMacs. Macrophage markers CD68, CD163, CD11b, CD32 and CD14 were all upregulated at Day 3 and Day 7 by 1.8 to 7.6 fold (Figure 4A; top panel) compared to iMac mono-culture control. Importantly, KC-specific markers ID1 and ID3 were upregulated by 3.8 fold and 3.9 fold respectively at Day 3 and by 6.3 fold and 6.5 fold respectively at Day 7 (Figure 4A; bottom panel). This suggests that macrophage phenotype is maintained during the 7 days of co-culture but KC-specific development increases in a time-dependent manner. When Day 7 co-iMacs were compared to iMacs cultured in PHCM (Day 7 cond-iMacs; without the presence of iHeps), macrophage marker upregulation compared to iMac mono-culture control were similar in both conditions (Figure 4B; top panel) but KC-specific ID1 and ID3 were upregulated to a lower extent in Day 7 cond-iMacs (2.6 fold and 2.5 fold respectively) compared to Day 7 co-iMacs (6.3 fold and 6.5 fold respectively; Figure 4B; bottom panel). [00132] Next, the RNA sequence analysis of iHep was confirmed via gene expression. With the exception of Glutathione-S transferase1A2 (GST1A2) and Multidrug Resistance Associated Protein 2 (MRP2), co-culture with iMacs improved expression of several hepatic marker genes including ALB, ^-1 antitrypsin (AAT), cytochrome P450 enzymes (CYP1A2, CYP3A4, CYP2C9, CYP2B6) and UDP-glucuronosyltransferases (UGT1A3) by 1.8-53.1 fold at Day 3 and Day 7 of co-culture (Figure 4C). Importantly, AFP, a marker that is expressed at high levels in fetal hepatocytes and increases with hepatocyte maturation, had 37% lower expression at Day 3 and 37% lower expression in Day 7 iHeps as compared to Day 0 iHeps (Figure 4C). This is consistent with RNA sequencing data that showed a downregulation of AFP. [00133] Overall, the gene expression data confirms the RNA sequencing analysis findings that culturing iMacs with iHeps imparts KC-like phenotype in iMacs and supports maturation and function of the iHeps. [00134] Example 4 [00135] Co-culture model accurately mimics clinical/in-vivo immune responses of drugs [00136] To test the application of the co-culture in detecting immune-mediated response to hepatotoxic drugs, a group of 7 drugs was carefully selected based on the following criteria: 1) confirmed reports of immune/inflammation-associated effects according to 3 databases; 2) available clinical data, or in vivo responses if the former is unavailable; and 3) known Cmax (maximum serum concentration of drug) to guide the concentration of drug used in in the in vitro model (Drug concentrations used in the system was no more than 20-fold of its Cmax. Details of selected drugs are summarized in Table 6. The drugs included: diclofenac (DIC), sulindac (SLD) leflunomide (LFM); amodiaquine (AQ), lamotrigine (LTG), penicillin (PEN), pyrazinamide (PZA). DIC, SLD and LFM have been reported to cause a decrease in interleukin-6 (IL-6); whereas AQ and LTG have been reported to cause an increase in IL-6 in patients with drug-induced liver injury. PEN and PZA were selected as negative controls as no immune-mediated effects of these drugs have been reported. The co-culture model could correctly recapitulate IL-6 decrease in DIC, SLD and LFM, IL-6 increase in AQ and LTG and no change in IL-6 in PEN and PZA-treated samples (Figure 5). Interestingly, LFM did not show a dose-dependent IL-6 decrease when iKCs were generated from iMacs using conditioned media and co-cultured with iHeps (data not shown). When iMac-derived iKC- like cells were replaced with blood Monocytes/Macrophages, none of these cytokine changes were recapitulated with the exception of SLD treatment (Figure 6). This shows, from a functional/application perspective, how iMac-iKCs are indispensable for in liver vitro models that can detect immune-mediated changes of hepatotoxicants. [00137] Table 6. Details of drugs used for testing.

[00138] In vivo, KCs have been known to release mediators that are early features of inflammatory responses. They are activated in response to inflammatory stimuli such as bacterial lipopolysaccharide (LPS) that can damage the liver at large doses in a KC- dependent manner. At smaller doses, LPS activates KCs (without causing liver injury) but sensitizes the liver to a variety of other xenobiotics. In the model, when the co-culture was co-treated with paradigm hepatotoxicants and LPS, drug-dose-dependent changes in cytokine production were observed, suggesting that the system can indeed mimic physiological responses. In contrast, such observations have not been reported in monocyte-derived macrophages in vivo, and hence, the work demonstrates the importance of liver-specific imprinting of macrophages in recapitulating responses in an in vitro liver model. [00139] Equivalents The foregoing examples are presented for the purpose of illustrating the invention and should not be construed as imposing any limitation on the scope of the invention. It will readily be apparent that numerous modifications and alterations may be made to the specific embodiments of the invention described above and illustrated in the examples without departing from the principles underlying the invention. All such modifications and alterations are intended to be embraced by this application. [00140] References [00141] K. Takata, T. Kozaki, C.Z.W. Lee, M.S. Thion, M. Otsuka, S. Lim, K.H. Utami, K. Fidan, D.S. Park, B. Malleret, Induced-pluripotent-stem-cell-derived primitive macrophages provide a platform for modeling tissue-resident macrophage differentiation and function, Immunity 47(1) (2017) 183-198. E6. [00142] F. Tasnim, Y.C. Toh, Y. Qu, H. Li, D. Phan, B.C. Narmada, A. Ananthanarayanan, N. Mittal, R.Q. Meng, H. Yu, Functionally enhanced human stem cell derived hepatocytes in galactosylated cellulosic sponges for hepatotoxicity testing, Molecular pharmaceutics 13(6) (2016) 1947-1957. [00143] J. Cai, J. Fisher, A. Urick, T. Wagner, K.T.M. Cayo, M. Nagaoka, S.A. Duncan, Protocol for directed differentiation of human pluripotent stem cells toward a hepatocyte fate, African Scientist 16(1) (2015) 13-26.