PROCESS FOR RECOMBINANT PRODUCTION AND PURIFICATION OF SARS- COV-2 SPIKE PROTEIN FIELD OF THE INVENTION The present invention relates to a process for the production and purification of recombinant SARS-CoV-2 spike protein. The present invention discloses the process for the generation of stable cells for spike protein production, purification process which yields more the 95% pure spike protein. The present invention relates to a process that yields in 150-500 mg/l titer. BACKGROUND OF THE INVENTION Covid-19 is an infectious disease caused by the SARS-CoV-2 virus. It is an unprecedented global healthcare challenge. The spike glycoprotein on the surface of the virus is highly immunogenic and is the main target of various neutralizing antibodies (Hussain et al., 2021). It plays a key role in recognizing the ACE2 receptor and host cell membrane fusion (Duan et al. 2020). The stabilized variants of the glycoprotein have been used in the detection of neutralizing antibodies using serological assays on patients’ sera samples (Patent No: US 11020474). The Spike protein is also the main immunogen being used as a vaccine candidate (Ong et al., 2021; Leach et al., 2021; Sarkar et al., 2020; Rihn et al., 2021; Patent No: US 10953089B1). The demand for stabilized full-length spike glycoprotein is huge as it is being used in serological testing, as a vaccine candidate, and also in research. However, the spike glycoprotein is being produced in a very low titer, thus codon optimization is an urgent need (Alonso et al., 2020; Toh et al., 2021; Hou et al., 2020; Dilucca et al., 2020; Calcagnile et al., 2021). Here, we disclose the process for production and purification of the spike glycoprotein using mammalian cell culture. This document discloses the process of producing high titres of recombinant spike protein using stable single-cell clone. Spike glycoprotein is the key component in recognizing and binding of the SARS-CoV-2 to the host cell receptor and is also the most important antigenic determinant site on the surface of the coronavirus. The stabilized prefusion spike protein can be used to generate an immune response (Malik et al., 2021). Many neutralizing antibodies have been found to target the spike protein (Yuan et al., 2020). The neutralizing antibodies against the virus bind to the spike protein and hence by using the spike protein an infection can be identified in an individual. The immunogenic nature of the spike protein makes it an excellent vaccine candidate against COVID-19 (Patent No.: WO2021178623A1). But, the expression of the spike protein in the mammalian cells is very low. The yield of recombinant spike protein can be remarkably improved by making stable cells for spike production (Jitender et al, 2023; Patent No.: IN202211030140). The expression can be improved by developing an optimized process for production. To recover the produced protein an efficient downstream process can help in an increased overall process yield. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a process flow of production of recombinant spike glycoprotein using stable CHO-S cell line/clone, and its purification. (A) is the production process from initial passages to prepare the cell for fermentation. (B) describes the different purification strategies for the purification of the recombinant, untagged spike glycoprotein. Fig. 2 represents the parameters monitored during the production phase of the 4 stable cell- pools (C20-1a, C20-1b, C20-2a, and C20-2b) generated. (i) is the viable cell density of the 4 stable cell pools; (ii) is the cell viability; (iii) is the glucose consumption level; (iv) is the level of production of lactic acid; (v) is the day-wise concentration of recombinant SARS-CoV-2 spike protein; and (vi) is the gel showing the concentration of the recombinant trimeric, untagged spike protein ectodomain on production days 5 and 6. Fig. 3 is the optimization of media and temperature conditions to enhance the expression of recombinant spike ectodomain. In (A), Runs 1-5 represent the different feeding strategies 5% feed on alternate days in Run 1, 1% feed daily in Run 2, 2% feed daily in Run 3, 2.5% feed daily in Run 4, and 3% feed daily in Run 5, respectively. The viability of cells dropped drastically in Runs 2 and 3, hence they were not continued further. In Runs 6-8, the feed conditions of Runs 1, 4, and 5 (5% on alternate days, 2.5% every day and 3% everyday, respectively) were applied and additionally, on day 5 the incubation temperature was lowered (from 37°C to 32°C), as shown in (B). The samples collected daily, throughout the duration of the fed-batch, were analyzed by SDS-PAGE, and the protein concentration was calculated using densitometric analyses. Run 6, as shown in (C) having 5% feed on alternate days and a shift to 32°C on day 5, resulted in high expression with cell viability maintained at >85%, was hence selected for further scale-up production in bioreactor. Fig.4 is the parameters monitored during the production in bioreactor. (i) is the cell viability, (ii) is the glucose consumption and lactic acid production, (iii) is the total protein produced, and (iv) is the SDS-PAGE of the final day crude broth. Fig.5 is the strategies for the purification of recombinant, untagged SARS-CoV-2 spike protein ectodomain. (A) is the two-step chromatographic purification process involving first cation- exchange chromatography using CaptoQ resin after buffer exchange via TFF, followed by size- exclusion chromatography. This is done in case the viability at the time of harvest is <80%. (B) is the one-step chromatographic purification of recombinant, untagged SARS-CoV-2 spike ectodomain, involving only one round of size-exclusion chromatography after buffer exchange via TFF. This one-step purification can be done when the viability of cells at the time of harvest is >85%. Fig. 6 is the characterization of the untagged SARS-CoV-2 spike protein ectodomain. (A) is the SDS-PAGE of the purified spike protein. (B) is the LC/MS analysis of the purified and reduced spike protein. This represents the molecular weight of the monomer. (C) is the TEM analysis of the intact (trimeric), pure recombinant spike. Fig.7 is the ELISA result from one of the SARS-CoV-2 S protein induced antibody responses in mice. Each dataset represents one group containing five mice injected with different adjuvant formulations. Fig. 8 is the neutralization potential of the antisera generated in mice against the purified recombinant trimeric spike ectodomain immunogen. The anti-sera generated against the recombinant trimeric spike ectodomain has a broadly neutralizing potency. The pseudoviral neutralization assay was performed against the Wild-Type, Alpha, Beta and Delta pseudoviruses. Percent neutralization was determined with reference to the control and neutralization titres (Median infective dose; ID50) were determined as the serum dilution at which infectivity was reduced by 50% by using GraphPad Prism™ software. Fig. 9 is the thermostability analyses of the recombinant spike protein. The spike used here was purified and stored at -80°C almost a year ago, indicating that this recombinant, untagged spike ectodomain is stable at -80°C for at-least year. (A) is the SDS-PAGE of pure spike ectodomain subjected to multiple freeze-thaw cycles. Samples were collected every thawing; (B) is the SDS-PAGE of pure spike ectodomain incubated at room temperature for 24 hours. Samples were collected every 4 hours; and (C) is the SDS-PAGE of pure spike ectodomain incubated at 37°C for 24 hours. Samples were collected every hour. OBJECTIVES OF THE INVENTION The main objective of the present invention is to establish a simple, and industrially compatible process for the production and purification of recombinant, prefusion stabilized, and untagged, SARS-CoV-2 spike glycoprotein ectodomain and its use as immunogen. Another objective of the present invention is to establish a mammalian cell line-based process for high yield production of recombinant SARS CoV-2 spike ectodomain. Yet, another objective of the present invention is to provide a simple and straight-forward, highly efficient process of purification, wherein the recombinant spike protein is purified with more than 95% purity using tangential flow filtration and liquid chromatography. Yet another objective of the present invention is to assess the immunogenicity of the recombinant, SARS-CoV-2 spike protein ectodomain. Yet another objective is to evaluate the efficiency and breadth of neutralization of SARS-CoV- 2 variants with anti-sera generated from the spike protein immunization. Yet another objective is to assess the thermostability of the recombinant, trimeric SARS-CoV- 2 spike protein ectodomain. SUMMARY OF THE INVENTION Accordingly, the present invention provides a process for the production and purification of recombinant SARS-CoV-2 spike protein. One aspect of the present invention describes a process of stable cell line/clone generation, bioreactor production, purification of SARS-CoV- 2 prefusion stabilized spike protein ectodomain (PSSPE), and/or full-length protein. Another aspect of the present invention describes the production process for spike protein at shake flask as well as at fermenter level. The scalability of the process is also described. In another aspect of the present invention, the purification process of the spike protein from the cell culture broth is described. The purification steps involving the separation of cells from the liquid broth using filtration/centrifugation followed by the chromatography purification of the protein were described. The processes describe different strategies where a two-step as well as one-step chromatographic purification yielded in 95% pure protein. The process yield is also described. Still another aspect of the present invention describes the immunogenicity of the spike protein in the BALB/c mouse model. The sera collected from the mice were analyzed using ELISA. In yet another aspect of the present invention, the thermostability of the purified, trimeric spike protein ectodomain is described. The protein, purified and stored at -80°C almost a year ago, was incubated at room temperature, and 37°C for 24 hours. Samples were collected every 4 hours and every hour, respectively. The protein was also subjected to multiple freeze-thaw cycles and samples were collected after every thaw. The collected samples were analyzed by SDS-PAGE. In an embodiment of the present invention, it provides a process for producing and purifying recombinant SARS-CoV-2 spike protein ectodomain-based immunogen, the process comprising the steps of: a) providing a linearized plasmid having MTCC 25619 containing the gene encoding the recombinant spike protein having SEQ ID NO: 1, b) transfecting the CHO-S cells with the linearized plasmid obtained in step (a), c) subjecting the transfected cells obtained in step (b) to selection pressure, involving repeated subculturing media having Methotrexate (MTX) and Puromycin along with anti- clumping agent to obtain the stable cells, d) subjecting the stable cells pools obtained in step (c) to 7-14 days production phase, e) subjecting the culture obtained in step (d) to nutrient supplement feeding on alternate days in the ratio of 10:1, making up 5% of the volume of the entire culture; f) subjecting the culture obtained in step (e) to temperature at 37°C and lowering the temperature to 32°C on day 5 of production, g) harvesting the culture obtained in step (f) by physical separation techniques comprising of centrifugation and filtration, thereby to obtain the clarified broth for purification, h) subjecting the clarified broth obtained in step (g) to tangential flow filtration (TFF), to recover the concentrated retentate, containing the recombinant SARS-CoV-2 spike protein ectodomain, i) subjecting the retentate obtained in step (h), recovered from TFF, to liquid chromatography techniques In another embodiment of the present invention provides a process wherein the harvested culture of step (f) is obtained on day 7-14. In yet another embodiment of the present invention provides a process wherein the said process is performed in fed-batch bioreactor. In an embodiment of the present invention provides a process wherein the liquid chromatography technique is selected from the group comprising of cation exchange chromatography, size-exclusion chromatography. In another embodiment of the present invention provides a process, wherein the yield of recombinant spike protein ectodomain is in the range of 150-500mg/L. In yet another embodiment of the present invention provides a process wherein the purity of recombinant SARS-CoV-2 spike protein ectodomain is 95%.In an embodiment of the present invention provides a process wherein obtained recombinant SARS-CoV-2 spike protein ectodomain having induces neutralizing antibodies against Wild-Type, Alpha, Beta, and Delta pseudoviruses with 99.9% neutralizing efficiency at a 1:100 dilution of sera. In another embodiment of the present invention provides a process wherein the obtained SARS- CoV-2 spike protein is trimeric, untagged, pre-fusion stabilized and thermostable up to - 80°C.In yet another embodiment of the present invention provides a process wherein the obtained SARS-CoV-2 spike protein use for vaccines, biotherapeutics and diagnostic activities. DETAILED DESCRIPTION OF THE INVENTION In order to manufacture spike protein, one of the objectives is to provide a method for selecting the gene and designing the construct when expressing the spike glycoprotein in the CHO cell line by which the protein can be expressed in a stable form in the extracellular medium. The second objective is to make a scalable process with which a high titre of the protein can be obtained. Another objective is to model a process of purification for the produced spike glycoprotein. In order to achieve the purpose, the invention adopts the following technical scheme: process for high yield recombinant production of SARS-CoV-2 spike protein immunogen in stable mammalian cells comprises the following steps (S): S1, linearizing the plasmid pCHO 1.0_IMT-C20 (vide Accession No.: MTCC25619) containing the gene (SEQ ID NO: 1) encoding the recombinant spike protein ectodomain, using the enzyme NruI, transfecting the CHO-S cells with the linearized plasmid using lipofection kit. preparing selection media for phase I selection comprises of CD FortiCHO media with 10 µg/ml Puromycin and 100 nM MTX. Transferring cell, at a viable cell density of 5x10 ⁵ cells/ml after 48 hour of transfection, to T150 flasks containing 40 ml selection media, incubating the flask in a static humidified incubator at 37°C with 8% CO2, counting and determining the viability of cells after 7 days, transferring the cells to shake flask once the viability is 30% or above and incubating in 8% CO2 incubator at 37 °C and 150 rpm, maintaining cells by passaging every 3 ^rd day till viability crosses 85%, preparing for selection phase II media with 2 different puromycin (30 µg/ml and 50 µg/ml) and MTX (500 nM and 1000 nM) concentrations. Transferring the cells from selection phase I to selection phase II once the viable cell density reaches more than 85%, making stocks on completion of selection phase II at a cell density of 1x10 ⁷ cells/ml in 10 % DMSO + CD FortiCHO media. S2, Performing fed-batch cultures at shake flask level in 125 mL Erlenmeyer flasks with a starting volume of 30mL/flask and a VCD of 3 x 10 ⁵ cells/mL, using Complete CD FortiCHO™ media (supplemented with 8 mM glutamine) with 1% anti-clumping agent, using different feeding strategies and temperature conditions to observe their effects on the productivity, combining Feed A and B (Cell Boost™ 7a and 7b, HyClone, Cytiva, USA), in the ratio of 10:1 and adding to the culture as per the different feeding strategies, 5% feed on alternate days, 1%, 2%, 2.5% and 3% feed every day, coupled with a shift from 37°C to 32°C on day 5 of production, collecting samples everyday and analyzing them by SDS-PAGE and HPLC, selecting the condition having high expression with cell viability above 85% for scale-up in bioreactor. S3, production of spike protein in fed-batch bioreactor: thawing the stable cell stocks in 30 ml ActiCHO™ media in a 125 ml vented cap Erlenmeyer flask, subculturing the cells with an inoculation density of 3x10 ⁵ cells/ml in 100 ml media, preparing the bioreactor with the following condition: vessel volume 2 L; initial working volume 1 L; pH 7.2 in cascading with CO2; Temperature at 37°C; gassing 3 gas mix air, CO2, O2; DO set at 30% cascading with the total gas flow and O ₂ flow, inoculating the fermenter at a viable cell density of 5x10 ⁵ cells/ml, taking samples at every 24 hour to check the viable cell density, metabolite, spike protein production, adding 5% Feed (Feed A + Feed B, combined in the ratio of 10:1) on alternate days starting from day 3, harvesting the broth on day 14 or before if the viability drops below 80%. S4, purification of spike protein from the harvested broth: separating the solid and liquid part from harvested broth from S3 by centrifugation at 1000 g for 10 min followed by supernatant centrifugation at 10000 g for 10 min in a refrigerated centrifuge, filtering the supernatant obtained after centrifugation through 0.22 µm filter, using tangential flow filtration (TFF) with a 30kDa membrane (30kDa Minimate ^TM TFF Capsule, Pall Corporation, USA) for diafiltering the filtrate with five diafiltration volume (DV) using binding buffer used in the next step, preparing binding buffer (20 mM sodium phosphate) and elution buffer (20 mM sodium phosphate + 2 M NaCl) to use in anion exchange chromatography, equilibrating CaptoQ column with 10 column volumes (CV) of binding buffer, passing the diafiltered solution through the column, eluting the bound protein from the column with different gradients of elution buffer, re-equilibrating the column with the binding buffer till the chromatographic parameter (absorbance, conductivity, pH, etc.) become stable. S5, Sodium Dodecyl Sulphate-Polyacrylamide Gel Electrophoresis (SDS-PAGE): running a 10% SDS-PAGE with the reduced sample, the day-wise sample collected from the production experiment in S3 along with the BSA standards of known concentrations, analyzing the gel using ImageJ software for densitometric quantification of the samples, running samples from S7 in reduced and non-reduced conditions to assess the purity level. S6, Liquid chromatography/mass spectroscopy (LC/MS) was performed for the determination of the mass of purified spike ectodomain monomer. The purified spike was diluted in reduction buffer (25 mM Tris, 25 mM NaCl, pH 7.5) to achieve a final concentration of 1mg/mL and a concentrated TCEP solution was added to the diluted protein to a final concentration of 5 mM. The sample was incubated for 30 mins at 37°C before adding formic acid to a final concentration of 1%. The final concentration of protein samples used for LC/MS analyses was 0.2 mg/mL. C8 (150 × 3.0 mm, 300 Å) column was used with Buffer A (1% Trifluoroacetic acid in water) and Buffer B (1% TFA acetonitrile) as mobile phases. The sample volume loaded onto the column was 2 µl and a linear gradient of Buffer B from 10%-70% in 15 min, 70–90% in 15-18 min, and 90%-100% in 18-19 min was applied at a flow rate of 0.4 mL/min and total ion chromatogram (TIC) was recorded. Agilent MassHunter qualitative analysis software was used to analyze the MS spectra, and the maximum entropy algorithm was used for the deconvolution of spectra. S7, Negative stain-electron microscopy (nsEM) of the purified recombinant spike protein ectodomain was performed. 30-40µl of sample was allowed to bind onto a glow- discharged, carbon-coated grid for about 10 min, followed by blotting, negative staining with phosphotungstic acid (PTA) and air drying. Images were obtained using an electron microscope (JEOL JEM-2100 Electron Microscope). Around 40 individual molecules were selected from different images, their lengths and widths were measured and plotted to deduce the approximate dimensions of the trimeric spike protein ectodomain. S8, Immunogenicity of the purified protein was assessed by injecting (10 μg) into BALB/c mice through intramuscular route. Different adjuvants (e.g., squalene-based oil-in-water nano-emulsion, Alum, Alum+CPG, squalene-based oil-in-water nano-emulsion+CPG) were used for the injection formulation. Two doses at an interval of 28 days were administered. Sera were collected after 10 days of dose 2 and tested for the presence of anti-spike antibodies using ELISA. S9, ELISA was performed to check the immunogenic potential of the spike protein ectodomain. 100 ng of Spike protein was coated in the plates an overnight incubation at 4°C was given. The plate was washed three times with PBST and blocked with 1% BSA in PBS for 3 hours at room temperature. After blocking the plate was washed with PBST 4 times. Sera in different dilutions were added to wells and incubated for 3 hours at room temperature and washed 5 times with PBST post-incubation. HRP labelled anti-mouse IgG secondary antibody was added to wells and incubated for 3 hours at room temperature in dark. Post incubation plate was washed with PBST six times before adding the substrate (TMB) for HRP.15 min incubation was given at room temperature before adding the stop solution (1 N H ₂ SO ₄ ). The plates were read at 450 nm. The results were plotted using GraphPad Prism 8. S10, Pseudovirus neutralization assays were performed to assess the in-vitro neutralization potency of mice anti-sera generated against the recombinant, untagged, trimeric SARS- CoV-2 spike ectodomain. The assays were set up in 96-well plate format. Wild-type, Alpha, Beta, and Delta variants of SARS-CoV-2 pseudoviruses and HEK293T cells expressing SARS-CoV-2 spike were used for the assay. Infection of the indicator cell line with pseudovirus was measured in terms of relative luminescence unit (RLU) and percent neutralization was determined with reference to the control in which no serum was added to the pseudovirus. Neutralization titres (Median infective dose; ID50) were determined as the serum dilution at which infectivity was reduced by 50% by using GraphPad Prism™ software. S11, Thermostability assessment of the spike protein ectodomain was done. The purified protein, which was purified and stored at -80°C was incubated at room temperature and 37°C for 24 hours, and was also subjected to multiple freeze-thaw cycles. The samples were collected every 4 hours, every hour, and after every freeze-thaw cycle, respectively, and analyzed by SDS-PAGE. We describe herein an industrially-compatible and scalable process for production of SARS Cov-2 spike protein at bioreactor level. The process involves novel purification methods which use either one step chromatography or two step chromatography coupled with tangential flow filtration (TFF) to get recombinant protein with high purity. The total protein obtained from our stable cell line-based processes were 150-500 mg/l which are higher as compared to other published studies (Esposito et al., 2020; Stuible et al., 2021). The current process uses cGMP grade industrially-compatible CHO-S cell line (Thermo Fischer Scientific) and expression vectors for production of spike protein. Proteins that are used for biopharmaceutical application like vaccine, require proteins to be tag-free. Purification of tag-free protein is a technologically difficult and cumbersome process and requires multiple chromatography steps. In the present invention we have developed one step chromatography using anion exchange (CaptoQ/ANX) and 2 step chromatography using anion exchange (CaptoQ/ANX) and SEC to purify tag-free protein with high purity. EXAMPLES The following examples are given by way of illustration of the present invention and therefore should not be construed to limit the scope of the present invention Example 1 Process of Stable Cell Line Generation In one example, the plasmid pCHO 1.0_IMT-C20 (vide Accession No.: MTCC25619) containing the gene (SEQ ID NO: 1), having the gene encoding the recombinant spike protein ectodomain, was linearized using the enzyme NruI (NEB, USA) (Patent No.: IN202211030140). CHO cells (Thermo Fischer Scientific) were transfected linearized plasmid using lipofection kit. The transfected cells were incubated at 37°C, 8 % CO ₂ , and 130 rpm for 48 hours. The cells were then passaged in two T150 flasks containing 40 ml complete media (including 1% anti-clumping agent) and were subjected to selection pressure by adding Puromycin and MTX at 10 µg/ml and 100 nM concentration in flask 1 and 20 µg/ml and 200 nM concentration in flask 2. The cells were incubated in CO ₂ static incubator with 8% CO ₂ , 37°C, and >80% relative humidity. A sample was taken on day 7. If the viability was <30%, it was further incubated till day 10. If the viability was still <30% a complete media exchange was performed while maintaining the respective selection pressures and reducing the culture volume such that it maintained a cell density above 3x10 ⁵ cells/ml, but if at any point, the viability becomes >30%, the cells were passaged in shake flasks with 30 ml media and were incubated in CO ₂ shaking incubator (8% CO ₂ , 37°C, >80% relative humidity, 130 rpm). Every 3-4 days, the cells were passaged in SF125 with 30 ml media maintaining a constant cell density of 3x10^5 cells/ml while also maintaining the selection pressure till the viability increases. Selection phase 1 is complete when viability is >85% and cell density is >1x10 ⁶ cells/mL. At this point, a small fraction of cells was advanced to selection phase 2, which involves higher selection pressures, while the other fraction of cells was cryopreserved. The cells were passaged into two SF125 flasks (labeled as A and B) with 30 ml media each at a cell density of 4x10 ⁵ cells/ml and were subjected to puromycin and MTX at 30 µg/ml and 500 nM concentration in flask A and 50 µg/ml and 1000 nM concentration in flask B respectively. Both flasks were incubated under the same conditions as before. The cells were passaged every 3-4 days in 30 ml media at 3x10 ⁵ cells/ml cell density while maintaining the respective selection pressure till the viability increased above 90%, at this point selection phase 2 was said to be complete. At this point, the majority of the cells were cryopreserved and a small fraction of the cells were proceeded to the production phase. The cells were passaged in SF125 with 30 ml media at cell density 3x10 ⁵ cells/ml. No selection pressure was applied during production. The cells were incubated in a shaker incubator under the same conditions as before. Sampling was done daily while feeding was started on day 3 on alternate days till day 14. The production phase was proceeded either till day 14 or till the viability dropped down to 70%, whichever stage came first. The protein production was assessed by SDS-PAGE followed by Western Blotting of the day-wise samples collected during the production phase. Example 2 Optimization of conditions at shake-flask and bioreactor level. In one example, fed-batch cultures at shake flask level were performed in 125 mL Erlenmeyer flasks with a starting volume of 30mL/flask and a VCD of 3 x 105 cells/mL. ActiCHO™ media (Cytiva HyClone™, USA), supplemented with 8 mM glutamine, was used with 1% anti- clumping agent. 5% feed, composed of Feed A and Feed B in a 10:1 ratio, was added every alternate day. The samples were taken at regular intervals for VCD, glucose and lactate analyses, viability %, and SDS-PAGE analyses. In another example, 1% feed, composed of Feed A and Feed B in a 10:1 ratio, was added daily, and samples collected every day were assessed for VCD, glucose and lactate analyses, viability %, and SDS-PAGE analyses. In another example, 2% feed, composed of Feed A and Feed B in a 10:1 ratio, was added daily, and samples collected every day were assessed for VCD, glucose and lactate analyses, viability %, and SDS-PAGE analyses. In another example, 2.5% feed, composed of Feed A and Feed B in a 10:1 ratio, was added daily, and samples collected every day were assessed for VCD, glucose and lactate analyses, viability %, and SDS-PAGE analyses. In another example, 3% feed, composed of Feed A and Feed B in a 10:1 ratio, was added daily, and samples collected every day were assessed for VCD, glucose and lactate analyses, viability %, and SDS-PAGE analyses. In another example, 5% feed, composed of Feed A and Feed B in a 10:1 ratio, was added on alternate days and on day 5, the culture was shifted from 37°C to 32°C. Samples collected every day were assessed for VCD, glucose and lactate analyses, viability %, and SDS-PAGE analyses. In another example, 2.5% feed, composed of Feed A and Feed B in a 10:1 ratio, was added every day and on day 5, the culture was shifted from 37°C to 32°C. Samples collected every day were assessed for VCD, glucose and lactate analyses, viability %, and SDS-PAGE analyses. In another example, 3% feed, composed of Feed A and Feed B in a 10:1 ratio, was added every day, and on day 5, the culture was shifted from 37°C to 32°C. Samples collected every day were assessed for VCD, glucose and lactate analyses, viability %, and SDS-PAGE analyses. Example 3 Production at Bioreactor-Level In one example, scale-up studies were carried out using the BioFlo 320 bioreactor (Eppendorf, Germany) to optimize the fed-batch production of the recombinant spike using a 2L glass water-jacketed vessel. The batch was started with a working volume of 1L ActiCHO™ media. The inoculum was prepared by thawing and reviving 1 vial of C20-2a stable cell pool first in 30 ml media and then subculturing into 100 ml and then 200 mL ActiCHO™ media (10% V/V) for the bioreactor. CO2 and O2 were used to control the pH and DO of the culture, respectively during the bioreactor operation. 100 mL inoculum (3x105 cells/mL VCD) was added to the bioreactor. During the bioreactor operation, samples were collected daily and checked for VCD, viability, glucose and lactic acid concentration profile. Data collection was done using the BioCommand software (Eppendorf, Germany). The culture was harvested on day 14. Example 4 Purification of untagged, trimeric SARS-CoV-2 spike protein ectodomain In one example, the harvest was centrifuged first at 4000 rpm, to separate the cell-mass from the broth, and then at 8000-10000 rpm to remove the cellular debris from the broth. The supernatant was the filtered using a 0.22 µm sterile filter. The culture supernatant thus obtained was concentrated to about half of the original volume using Tangential Flow Filtration (TFF) with a 30 kDa TFF Membrane (Minimate ^TM TFF Capsule, Pall Corporation, USA). The spent media of the concentrated supernatant was exchanged with the binding buffer, which will be further used for column chromatography.5 DV were exchanged with the binding buffer. The protein recovered from the TFF reservoir was then allowed to bind on 1 ml ANX column pre- equilibrated with about 5 column volumes of binding buffer. The protein was allowed to bind at a flow rate of 1 ml/min and the flow-through was collected in a separate flask. After binding, the column was washed with the binding buffer, and elution was initiated. Elution was done in a step-gradient manner at 10%, 20%, 30%, 40%, and 100% of the elution buffer. In another example, the column was changed from ANX (1 ml) to CaptoQ (1 ml). The buffer conditions were kept the same. Elution was done first in a linear gradient manner and then based on the results of the linear gradient, a step gradient was performed at 32%, 60%, and 100%. In another example, the flow-through mode was used. The column was CaptoQ, but the pH of the binding and elution buffers was reduced to pH 6.0. Buffer exchange was performed using this buffer. The binding flow-though was essentially collected and elution was performed in a step-gradient manner directly at 100%. In another example, based on the results of the previous example, the same flow-through mode was followed, but the pH of the binding and elution buffers was slightly raised to pH 6.25. Buffer exchange was performed using this buffer. The binding flow-through was essentially collected and elution was performed directly at 100%. In yet another example, a two-step purification process was used to purify the protein. A CaptoQ column was used in flow through mode followed by a CaptoS chromatography in bind and elute mode. In another example the CaptoS chromatography uses different buffer pH such as 6.5, 6.0, and 4.7. In yet another example the alternate polishing step uses size exclusion chromatography instead of CaptoS chromatography. In another example, a single-step SEC only has been used to purify protein. The harvested broth was dia-filtered and concentrated using a 10 or 30 kDa MWCO membrane to remove media components and loaded onto the column (Superdex 200pg) and eluted peaks were checked for the spike protein. The spike protein was eluted in vary initial peaks as per SDS- PAGE analysis of the peaks. Example 5 Characterization of the expressed recombinant SARS-CoV-2 Spike protein a) Sodium Dodecyl Sulphate-Polyacrylamide Gel Electrophoresis (SDS-PAGE): In one example, 10% polyacrylamide gel was prepared.21 µl of the purified recombinant spike protein was mixed with 7 µl of Laemmli sample buffer, and boiled at 95°C for 10min. The samples were allowed to cool, and then were given a short spin.25 µl of the prepared protein samples were loaded in the gel along with 1.5 µl of BioRad protein ladder b) Liquid Chromatography/Mass Spectroscopy (LC/MS): The liquid chromatography/mass spectroscopy (LC/MS) system (Agilent 6550 iFunnel QTOF, Agilent, USA) was used for the determination of the mass of spike monomer. The purified spike ectodomain was diluted in reduction buffer (25 mM Tris, 25 mM NaCl, pH 7.5) to achieve a final concentration of 1mg/mL and a concentrated TCEP solution was added to the diluted protein to a final concentration of 5 mM. The sample was incubated for 30 mins at 37°C before adding formic acid to a final concentration of 1%. The final concentration of protein samples used for LC/MS analyses was 0.2 mg/mL. C8 (150 × 3.0 mm, 300 Å) column was used with Buffer A (1% Trifluoroacetic acid in water) and Buffer B (1% TFA acetonitrile) as mobile phases. The sample volume loaded onto the column was 2 µl and a linear gradient of Buffer B from 10%-70% in 15 min, 70–90% in 15-18 min, and 90%-100% in 18-19 min was applied at a flow rate of 0.4 mL/min and total ion chromatogram (TIC) was recorded. Agilent MassHunter qualitative analysis software was used to analyze the MS spectra, and the maximum entropy algorithm was used for the deconvolution of spectra. c) Transmission Electron Microscopy: In one of the examples, negative stain-electron microscopy (nsEM) of the purified recombinant spike protein ectodomain was performed.30-40µl of sample was allowed to bind onto a glow- discharged, carbon-coated grid for about 10 min, followed by blotting, negative staining with phosphotungstic acid (PTA) and air drying. Images were obtained using an electron microscope (JEOL JEM-2100 Electron Microscope). Around 40 individual molecules were selected from different images, their lengths and widths were measured and plotted to deduce the approximate dimensions of the trimeric spike protein ectodomain. Example 6 Immunogenicity assessment In one of the examples, the purified protein was injected (10 μg) into BALB/c mice through intramuscular injection. Different adjuvants (e.g., squalene-based oil-in-water nano-emulsion, Alum, Alum+CPG, squalene-based oil-in-water nano-emulsion+CPG) were used for the injection formulation. Two doses were given with an interval time of 28 days. The sera were collected after 10 days of dose 2 and tested for the presence of anti-spike antibodies using ELISA. For ELISA 100 ng of Spike protein was coated in the plates an overnight incubation at 4°C was given. The plate was washed three times with PBST and blocked with 1% BSA in PBS for 3 hour at room temperature. After blocking the plate was washed with PBST 4 times. Sera in different dilutions were added to wells and incubated for 3 hour at room temperature and washed 5 times with PBST post-incubation. HRP labeled anti-mouse IgG secondary antibody was added to wells and incubated for 3 hour at room temperature in dark. Post incubation plate was washed with PBST 6 times before adding the substrate (TMB) for HRP. 15 min incubation was given at room temperature before adding the stop solution (1 N H2SO4). The plates were read at 450 nm. The results were plotted using Graphpad Prism 8. Example 7 Pseudovirus neutralization assay In one example, the in-vitro neutralization potency of mice anti-sera was assessed by SARS- CoV-2 pseudovirus neutralization assay. Pseudoviruses were prepared by transiently transfecting HEK293T cells with SARS-CoV-2 spike protein-expressing plasmid vector and a molecular clone pHIV-1NL4.3Δenv-nanoLuc which expresses all the structural proteins of HIV except Env-glycoprotein and a reporter nanoLuc luciferase enzyme (Promega, USA). Co- transfection of these two plasmid DNA vectors leads to the generation of pseudovirus particles decorated with SARS-CoV-2 spike protein on the surface and capable of a single entry into the cell. The neutralization assays were set up in 96-well plate format in which mice sera were serially diluted in 50µL growth medium, mixed with 50µL pseudovirus and incubated for 1 hour at 37 ^o C. After incubation, 1x10 ⁴ cells (293T-hACE2-TMPRSS2) in 100µL was added and the assay plate was incubated at 37°C for 48 hours. Infection of the indicator cell line with pseudovirus was measured in terms of relative luminescence unit (RLU) and percent neutralization was determined with reference to the control in which no serum was added to the pseudovirus. Neutralization titres (Median infective dose; ID50) were determined as the serum dilution at which infectivity was reduced by 50% by using GraphPad Prism™ software. Example 8 Thermostability Assessments In one example, the purified recombinant SARS-CoV-2 spike protein was incubated at room temperature for 24 hours. Samples were collected every 4 hours, and analyzed by SDS-PAGE. In another example, the purified recombinant SARS-CoV-2 spike protein was incubated at 37°C for 24 hours. Samples were collected every hour, and analyzed by SDS-PAGE. In another example, the purified recombinant SARS-CoV-2 spike protein was subjected to 11 freeze-thaw cycles. Samples were collected after every freeze-thaw cycle, and analyzed by SDS-PAGE. ADVANTAGES OF THE INVENTION 1. Production and purification of recombinant, untagged, trimeric SARS-CoV-2 spike protein ectodomain. 2. Stable cell-based rapid production of spike protein which can be implemented for any other glycosylated and complex protein expression. 3. Very simple and industrially compatible, non-affinity-based purification process, involving one-step and two-step chromatography, yields highly pure recombinant, untagged SARS- CoV-2 spike protein. 4. The trimeric, untagged pre-fusion stabilized spike protein ectodomain is highly thermostable: stable at -80°C for more than a year. 5. The recombinant. untagged, trimeric SARS-CoV-2 spike protein ectodomain induces robust antibody response in preclinical models. 6. Effective neutralization of SARS-CoV-2 pseudoviruses by the anti-sera obtained from immunized animals. 7. This process can be used for an efficient, rapid, and affordable production of large quantities of other complex proteins that find use in various biotherapeutic and diagnostic activities. REFERENCES Literature 1. Alonso, Andres Mariano, and Luis Diambra. “SARS-CoV-2 Codon Usage Bias Downregulates Host Expressed Genes With Similar Codon Usage.” Frontiers in Cell and Developmental Biology 8 (2020): 831. 2. Calcagnile, Matteo, Tiziano Verri, Maurizio Salvatore Tredici, Patricia Forgez, Marco Alifano, and Pietro Alifano. “Codon Usage, Phylogeny and Binding Energy Estimation Predict the Evolution of SARS-CoV-2.” One Health 13 (December 1, 2021): 100352. 3. Dilucca, Maddalena, Sergio Forcelloni, Alexandros G. Georgakilas, Andrea Giansanti, and Athanasia Pavlopoulou. “Codon Usage and Phenotypic Divergences of SARS- CoV-2 Genes.” Viruses 12, no.5 (May 2020): 498. 4. Duan, Liangwei, Qianqian Zheng, Hongxia Zhang, Yuna Niu, Yunwei Lou, and Hui Wang. “The SARS-CoV-2 Spike Glycoprotein Biosynthesis, Structure, Function, and Antigenicity: Implications for the Design of Spike-Based Vaccine Immunogens.” Frontiers in Immunology 11 (2020): 2593. 5. Esposito D, Mehalko J, Drew M, et al. Optimizing high-yield production of SARS- COv-2 soluble spike trimers for serology assays. Protein Expr Purif.2020;174:105686. doi:10.1016/J.PEP.2020.105686 6. Herrera N. G. et al., Characterization of the SARS-CoV-2 S Protein- Biophysical, Biochemical, Structural, and Antigenic Analysis. ACS Omega. 2021, 6, 85-102 doi: 10.1101/2020.06.14.150607 7. Hou, Wei. “Characterization of Codon Usage Pattern in SARS-CoV-2.” Virology Journal 17, no.1 (September 14, 2020): 138. 8. Hussain, Snawar, Sahibzada Tasleem Rasool, and Shinu Pottathil. “The Evolution of Severe Acute Respiratory Syndrome Coronavirus-2 during Pandemic and Adaptation to the Host.” Journal of Molecular Evolution 89, no.6 (July 1, 2021): 341–56. 9. Jitender, B. Vikram Kumar, Sneha Singh, Geetika Verma, Reetesh Kumar, Pranaya M. Mishra, Sahil Kumar, Santhosh K. Nagaraj, Joydeep Nag, Christy M. Joy, Bhushan Nikam, Dharmendra Singh, Pooja, Nidhi Kalidas, Shubham Singh, Mumtaz, Ashwani K. Bhardwaj, Dhananjay S. Mankotia, Rajesh P. Ringe, Nimesh Gupta, Shashank Tripathi, Ravi P.N. Mishra; A broadly protective CHO cell expressed recombinant spike protein subunit vaccine (IMT-CVAX) against SARS-CoV-2; bioRxiv 2023.04.03.534161; doi: https://doi.org/10.1101/2023.04.03.534161 10. Leach, Adam, Ami Miller, Emma Bentley, Giada Mattiuzzo, Jemima Thomas, Craig McAndrew, Rob Van Montfort, and Terence Rabbitts. “Implementing a Method for Engineering Multivalency to Substantially Enhance Binding of Clinical Trial Anti- SARS-CoV-2 Antibodies to Wildtype Spike and Variants of Concern Proteins.” Scientific Reports 11, no.1 (May 18, 2021): 10475. 11. Malik, Yashpal Singh, Mohd Ikram Ansari, Jobin Jose Kattoor, Rahul Kaushik, Shubhankar Sircar, Anbazhagan Subbaiyan, Ruchi Tiwari, et al. “Evolutionary and Codon Usage Preference Insights into Spike Glycoprotein of SARS-CoV-2.” Briefings in Bioinformatics 22, no.2 (March 22, 2021): 1006–22. 12. Ong, Edison, Xiaoqiang Huang, Robin Pearce, Yang Zhang, and Yongqun He. “Computational Design of SARS-CoV-2 Spike Glycoproteins to Increase Immunogenicity by T Cell Epitope Engineering.” Computational and Structural Biotechnology Journal 19 (January 1, 2021): 518–29. 13. Rihn, Suzannah J., Andres Merits, Siddharth Bakshi, Matthew L. Turnbull, Arthur Wickenhagen, Akira J. T. Alexander, Carla Baillie, et al. “A Plasmid DNA- Launched SARS-CoV-2 Reverse Genetics System and Coronavirus Toolkit for COVID-19 Research.” PLOS Biology 19, no.2 (February 25, 2021): e3001091. 14. Sarkar, Bishajit, Md Asad Ullah, Yusha Araf, and Mohammad Shahedur Rahman. “Engineering a Novel Subunit Vaccine against SARS-CoV-2 by Exploring Immunoinformatics Approach.” Informatics in Medicine Unlocked 21 (January 1, 2020): 100478. 15. Stuible M, Gervais C, Lord-Dufour S, et al. Rapid, high-yield production of full-length SARS-CoV-2 spike ectodomain by transient gene expression in CHO cells. J Biotechnol.2021;326:21-27. doi:10.1016/j/jbiotec.2020.12.005 16. Toh, Yee-Huan, Yu-Weng Huang, Yo-Chen Chang, Yi-Ting Chen, Ya-Ting Hsu, and Guang-Huey Lin. “Reactivity of Human Antisera to Codon Optimized SARS-CoV2 Viral Proteins Expressed in Escherichia Coli.” Tzu-Chi Medical Journal 33, no. 2 (March 9, 2021): 146–53. 17. Yuan, Meng, Hejun Liu, Nicholas C. Wu, Chang-Chun D. Lee, Xueyong Zhu, Fangzhu Zhao, Deli Huang, et al. “Structural Basis of a Shared Antibody Response to SARS- CoV-2.” Science 369, no.6507 (August 28, 2020): 1119–23. Patents: 1. Patent No.: WO2021178623A1 Date: 2021-09-10 Lanying et. al. (2021), Immunogenic compositions against severe acute respiratory syndrome coronavirus 2 2. Patent No.: US 11020474 Date: 2021-06-01 Yang Xiang and Jun Li (2021), Producing recombinant SARS-CoV-2 spike protein in a pre-fusion state 3. Patent No.: US 10953089B1 Date: 2021-03-23 Smith et. al. (2021), Coronavirus vaccine formulations 4. Patent No.: WO2021246910A1 Date: 2021-12-09 Дмитрий Виктрович ЩЕБЛЯКОВ et. al. (2021), Sars-cov-2 virus rbd recombinant protein and method for producing same 5. Patent No.: IN 202211030140 Date: 25-05-2022 Mishra et. al. (2022), Recombinant protein for management of SARS-CoV-2 infection