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

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.

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

“…The B cell help quality of antigen-specific memory CD4 + T cells in vaccination and infection was assessed in the autologous T:B co-culture assay, as we described recently 47 . Briefly, cryopreserved PBMCs were revived as mentioned previously with ≥90% cellular viability and surface stained with following monoclonal antibodies for 15 minutes at 4 ºC: fixable viability dye efluor 506, anti-CD4 AF700 (RPA-T4), anti-CD45RO FITC (UCHL1), anti-CD14 PE (MΦP9) and anti-CD20 PE-Cy7 (2H7).…”

“…The B-cell help potential of CD4 + T cells is not yet demonstrated in COVID-19 infection or vaccination. We recently described that supplementation of conventional T-B co-cultures with autologous monocytes substantially enhance the sensitivity and efficiency in defining the B-cell help potential of SARS-CoV-2 specific memory CD4 + T cells 47 . We thus utilized the newly described method to examine the help potential of BBV152-induced CD4 + T cells by enumeration of IL-21 secreting cells and measuring the extent of plasma cells differentiation in the autologous T-B co-cultures (Fig.…”