Inactivated virus vaccine BBV152/Covaxin elicits robust cellular immune memory to SARS-CoV-2 and variants of concern

The characteristics of immune memory established in response to inactivated SARS-CoV-2 vaccines remains unclear. We determined the magnitude, quality and persistence of cellular and humoral memory responses up to 6 months after vaccination with BBV152/Covaxin. Here, we show that the quantity of vaccine-induced spike- and nucleoprotein-antibodies is comparable to that following natural infection and the antibodies are detectable up to 6 months. The RBD-specific antibodies decline in the range of 3 to 10-fold against the SARS-CoV-2 variants in the order of alpha (B.1.1.7) > delta (B.1.617.2) > beta (B.1.351), with no observed impact of gamma (P.1) and kappa (B.1.617.1) variant. We found that the vaccine induces memory B cells, similar to natural infection, which are impacted by virus variants in the same order as antibodies. The vaccine further induced antigen-specific functionally potent multi-cytokine expressing CD4+ T cells in [~]85% of the subjects, targeting spike and nucleoprotein of SARS-CoV-2. Marginal [~]1.3 fold-reduction was observed in vaccine-induced CD4+ T cells against the beta variant, with no significant impact of the alpha and the delta variants. The antigen-specific CD4+ T cells were populated in the central memory compartment and persisted up to 6 months of vaccination. Importantly the vaccine generated Tfh cells that are endowed with B cell help potential, similar to the Tfh cells induced after natural infection. Altogether, these findings establish that the inactivated virus vaccine BBV152 induces robust immune memory to SARS-CoV-2 and variants of concern, which persist for at least 6 months after vaccination. This study provides insight into the attributes of BBV152-elicited immune memory, and has implication for future vaccine development, guidance for use of inactivated virus vaccine, and booster immunization.

A highly efficient T-cell immunoassay provides assessment of B cell help function of SARS-CoV-2 specific memory CD4+ T cells

The B cell help function of CD4+ T cells may serve as an immunologic correlate of protective adaptive immunity. The quantitative assessment of the B cell help potential of CD4+ T cells is limited by the lack of suitable antigen-specific functional assays. Here, we describe a highly efficient antigen-specific T-B co-cultures for quantitative measurement of T-dependent B cell responses. Using Mycobacterium tuberculosis specific setup, we show that early priming and activation of CD4+ T cells is important for the mutualistic collaboration between antigen-specific T and B cells, which could be achieved by supplementing the co-cultures with autologous monocytes. We further show that monocyte-derived growth factors provide the impetus for productive T-B collaboration by conferring optimal survivability in the cultured cells. This study provides first evidence of C-type lectin domain family 11 member A (CLEC11A/SCGF) as an essential growth factor for B cell survival. Importantly, we demonstrate the successful translation of monocyte supplemented T-B co-cultures in qualitative assessment of SARS-CoV-2 specific memory CD4+ T cells by quantifying several correlates of productive T-B cross-talk like plasma cell output, secreted antibody, antibody secreting cells and IL21 secreting T cells. Thus, the method described here can provides qualitative assessment of SARS-CoV-2 spike CD4+ T cells after natural infection and can be applied to assess the B cell help function of memory CD4+ T cells generated in response to COVID-19 vaccine. One sentence summaryQualitative assessment of antigen-specific CD4+ T cells for T-dependent B cell responses.

Immune memory in mild COVID-19 patients and unexposed donors from India reveals persistent T cell responses after SARS-CoV-2 infection

Understanding the causes of the diverse outcome of COVID-19 pandemic in different geographical locations is important for the worldwide vaccine implementation and pandemic control responses. We analyzed 42 unexposed healthy donors and 28 mild COVID-19 subjects up to 5 months from the recovery for SARS-CoV-2 specific immunological memory. Using HLA class II predicted peptide megapools, we identified SARS-CoV-2 cross-reactive CD4+ T cells in around 66% of the unexposed individuals. Moreover, we found detectable immune memory in mild COVID-19 patients several months after recovery in the crucial arms of protective adaptive immunity; CD4+ T cells and B cells, with a minimal contribution from CD8+ T cells. Interestingly, the persistent immune memory in COVID-19 patients is predominantly targeted towards the Spike glycoprotein of the SARS-CoV-2. This study provides the evidence of both high magnitude pre-existing and persistent immune memory in Indian population. By providing the knowledge on cellular immune responses to SARS-CoV-2, our work has implication for the development and implementation of vaccines against COVID-19.

A haemagglutination test for rapid detection of antibodies to SARS-CoV-2

Serological detection of antibodies to SARS-CoV-2 is essential for establishing rates of seroconversion in populations, detection of seroconversion after vaccination, and for seeking evidence for a level of antibody that may be protective against COVID-19 disease. Several high-performance commercial tests have been described, but these require centralised laboratory facilities that are comparatively expensive, and therefore not available universally. Red cell agglutination tests have a long history in blood typing, and general serology through linkage of reporter molecules to the red cell surface. They do not require special equipment, are read by eye, have short development times, low cost and can be applied as a Point of Care Test (POCT). We describe a red cell agglutination test for the detection of antibodies to the SARS-CoV-2 receptor binding domain (RBD). We show that the Haemagglutination Test ("HAT") has a sensitivity of 90% and specificity of 99% for detection of antibodies after a PCR diagnosed infection. The HAT can be titrated, detects rising titres in the first five days of hospital admission, correlates well with a commercial test that detects antibodies to the RBD, and can be applied as a point of care test. The developing reagent is composed of a previously described nanobody to a conserved glycophorin A epitope on red cells, linked to the RBD from SARS-CoV-2. It can be lyophilised for ease of shipping. We have scaled up production of this reagent to one gram, which is sufficient for ten million tests, at a cost of [~]0.27 UK pence per test well. Aliquots of this reagent are ready to be supplied to qualified groups anywhere in the world that need to detect antibodies to SARS-CoV-2, but do not have the facilities for high throughput commercial tests.