IgM+ and IgM- memory B cells represent heterogeneous populations capable of producing class-switched antibodies and germinal center B cells upon rechallenge with P. yoelii

Abstract

Memory B cells (MBCs) are essential for maintaining long-term humoral immunity to infectious organisms, including Plasmodium. MBCs are a heterogeneous population whose function can be dictated by isotype or expression of particular surface proteins. Here, aided by antigen-specific B-cell tetramers, MBC populations were evaluated to discern their phenotype and function in response to infection with a nonlethal strain of P. yoelii. Infection of mice with P. yoelii 17X resulted in 2 predominant MBC populations: somatically hypermutated isotype-switched (IgM^- ) and IgM⁺ MBCs that coexpressed CD73 and CD80 that produced antigen-specific antibodies in response to secondary infection. Rechallenge experiments indicated that IgG-producing cells dominated the recall response over the induction of IgM-secreting cells, with both populations expanding with similar timing during the secondary response. Furthermore, using ZsGreen1 expression as a surrogate for activation-induced cytidine deaminase expression alongside CD73 and CD80 coexpression, ZsGreen1⁺ CD73⁺ CD80⁺ IgM⁺ , and IgM^- MBCs gave rise to plasmablasts that secreted Ag-specific Abs after adoptive transfer and infection with P. yoelii. Moreover, ZsGreen1⁺ CD73⁺ CD80⁺ IgM⁺ and IgM^- MBCs could differentiate into B cells with a germinal center phenotype after adoptive transfer. A third population of B cells (ZsGreen1^- CD73^- CD80^- IgM^- ) that is apparent after infection responded poorly to reactivation in vitro and in vivo, indicating that these cells do not represent a canonical population of MBCs. Together these data indicated that MBC function is not defined by immunoglobulin isotype, nor does coexpression of key surface markers limit the potential fate of MBCs after recall.

Keywords: AID; IgM; plasmablasts; somatic hypermutation.