Transcriptional programming of early-forming memory B cells arises independently of cognate CD4+ T-cell interactions

Memory B cells (MBCs) are an integral part of the humoral immune response with the capacity to both reseed germinal center reactions and rapidly form antibody-secreting plasma cells (ASCs) upon secondary antigen encounter. MBCs arise via both T cell-dependent and -independent routes and while CD4+ T cells are not required for their formation, it is still not clear if or how initial T:B-cell interactions influence the molecular programming and diversity of T cell-independent MBCs. To address this, we characterized MBCs that form in response to influenza infection in major histocompatibility complex class II knockout (MHCII-KO) mice, which lack CD4+ T cells. Consistent with T cell-independent responses, the MBCs that formed in MHCII-KO mice were reduced in number and did not acquire surface expression of CCR6 and class-switched BCR. Transcriptional profiling identified cytokine- and activation-induced genes that were reduced in expression in MHCII-KO MBCs compared to wild type (WT). Adoptive transfer of MHCII-KO B cells into WT hosts, which cannot receive peptide/MHCII-TCR cognate interactions, revealed an ability of MHCII-KO cells to form MBCs and ASCs. Single-cell RNA sequencing revealed minimal transcriptional differences between MHCII-KO and WT MBCs, indicating that cognate CD4+ T-cell interactions provide limited early programming instruction to MBCs. MHCII-KO MBCs were able to clonally expand, class-switch, and form the same diverse transcriptional clusters as WT. These data indicate that early differentiating MBCs can seed a diverse pool of MBC populations in response to influenza infection independent of cognate T-cell help.