Production of Trimeric SARS-CoV-2 Spike Protein by CHO Cells for
Serological COVID-19 Testing
Abstract
We describe scalable and cost-efficient production of full length,
His-tagged SARS-CoV-2 spike glycoprotein trimer by CHO cells that can be
used to detect SARS-CoV-2 antibodies in patient sera at high specificity
and sensitivity. Transient production of spike in both HEK and CHO cells
mediated by PEI was increased significantly (up to 10.9-fold) by a
reduction in culture temperature to 32ÂșC to permit extended duration
cultures. Based on these data GS-CHO pools stably producing spike trimer
under the control of a strong synthetic promoter were cultured in
hypothermic conditions with combinations of bioactive small molecules to
increase yield of purified spike product 4.9-fold to 53 mg/L.
Purification of recombinant spike by Ni-chelate affinity chromatography
initially yielded a variety of co-eluting protein impurities identified
as host cell derived by mass spectrometry, which were separated from
spike trimer using a modified imidazole gradient elution. Purified CHO
spike trimer antigen was used in ELISA format to detect IgG antibodies
against SARS-CoV-2 in sera from patient cohorts previously tested for
viral infection by PCR, including those who had displayed COVID-19
symptoms. The antibody assay, validated to ISO 15189 Medical
Laboratories standards, exhibited a specificity of 100% and sensitivity
of 92.3%. Our data show that CHO cells are a suitable host for the
production of larger quantities of recombinant SARS-CoV-2 trimer which
can be used as antigen for mass serological testing.