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

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 non-lethal strain of P. yoelii. Infection of mice with P. yoelii 17X resulted in the production of two predominant MBC populations: somatically hypermutated isotype-switched (IgM-) and IgM+ MBCs that co-expressed CD73 and CD80 that produced antigen-specific antibodies in response to secondary infection. Re-challenge 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 co-expression, ZsGreen1+CD73+CD80+IgM+ MBCs gave rise to class-switched IgG-producing plasmablasts that induced comparable titers of Ag-specific Abs as their IgM- counterparts after adoptive transfer and infection with P. yoelii. Moreover, ZsGreen1+CD73+CD80+ IgM+ and IgM- MBCs differentiated into B cells with a germinal center phenotype after adoptive transfer. A third population of B cells (ZsGreen1-CD73-CD80-IgM-) that emerges after infection responded poorly to reactivation in vitro and in vivo, indicating that these cells do not represent a population of MBCs. Together these data indicated that MBC function is not defined by immunoglobulin isotype, nor does co-expression of key surface markers limit the potential fate of MBCs after recall. SummaryIgM+ and IgM- MBCs that co-express CD73 and CD80 can differentiate into plasmablasts and GC B cells after re-challenge with P. yoelii.