ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
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.
