IgM and IgG Immunoreactivity of SARS-CoV-2 Recombinant M Protein

Abstract

Diagnostic evaluation of specific antibodies against the SARS-CoV-2 virus is mainly based on spike (S) and nucleocapsid (N) proteins. Despite the critical functions in virus infection and contribution to the pattern of immunodominance in COVID-19, exploitation of the most abundant membrane (M) protein in the SARS-CoV-2 serology tests is minimal. This study investigated the recombinant M protein’s immunoreactivity with the sera from COVID-19 convalescents. In silico designed protein was created from the outer N-terminal part (19 aa) and internal C-terminal tail (101–222 aa) of the M protein (YP_009724393.1) and was recombinantly produced and purified. The designed M protein (16,498.74 Da, pI 8.79) revealed both IgM and IgG reactivity with serum samples from COVID-19 convalescents in Western blot. In ELISA, more than 93% (28/30) of COVID-19 sera were positive for IgM detection, and more than 96% (29/30) were positive for specific IgG detection to M protein. Based on the capacity to provoke an immune response and its strong antigenic properties, as shown here, and the fact that it is also involved in the virion entry into host cells, the M protein of the SARS-CoV-2 virus as a good antigen has the potential in diagnostic purposes and vaccine design.

Anti-membrane and anti-spike antibodies are long-lasting and together discriminate between past COVID-19 infection and vaccination

“…In this study, in addition to confirming that anti-RBD antibodies are long-lasting and antinucleocapsid IgG declines rapidly [3][4][5][6][7] , we show for the first time that anti-membrane IgG is present in the vast majority of COVID-19 convalescent patients and persists for at least a year. Our findings are consistent with findings for IgG against a recombinant membrane antigen (polypeptide of aa 1-19 and 101-222) in the early convalescent period 12 . In contrast, a separate study found that only ~20% of non-hospitalized COVID-19 convalescent subjects had IgG against a similar membrane peptide (aa 1-20) to ours (aa 8-23) in the early convalescent period 13 .…”

“…However, serological testing of anti-membrane protein antibodies is in its infancy. Antibodies against membrane antigens are present days to months after SARS-CoV-2 infection [11][12][13] , but it is unknown how long they persist. Thus, there are currently no widely available serologic tests that uniquely and accurately detect past COVID-19 infection, limiting research and public health efforts.…”

Peptide microarrays coupled to machine learning reveal individual epitopes from human antibody responses with neutralizing capabilities against SARS-CoV-2

“…The global statistical analysis does not take protein structures into account that may make certain epitopes inaccessible. When mapping the responses across the M protein topology [ 22 ], we found that its transmembrane regions are hardly targeted by antibodies, whereas the two extraparticular regions were frequently bound (Figure S2). In fact, the extraparticular N-terminus of M constituted one of the most specific epitopes in the study that was addressed in 88% of positive samples.…”

Application of Baculovirus Expression Vector system (BEV) for COVID-19 diagnostics and therapeutics: a review

“…So far, only spike and nucleocapsid proteins have been produced in BEV for the diagnosis of COVID-19. The membrane protein of SARS-CoV-2 has been produced in other expression systems such as the E. coli expression system [ 38 ]. Hence, the potential of the M protein to be produced in BEV for diagnostic purposes remains to be explored further, especially with the emerging of new variants that reduce the sensitivity of the current diagnosis of COVID-19 disease [ 39 ].…”