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.

Inducible T cell co-stimulator (ICOS) plays a key role in the differentiation and maintenance of follicular helper T (Tfh) cells and thus germinal center (GC) formation. Previously, our lab showed in a Plasmodium chabaudi infection model that Icos -/- mice were significantly impaired in their ability to form GCs despite a persistent infection and thus a continued antigen (Ag) load. Here, we show that resolution of a primary infection with P. yoelii , was delayed in Icos -/- mice. This phenotype was associated with a reduction in the accumulation of Tfh-like and GC Tfh cells and an early deficiency in Ag-specific antibody (Ab) production. However, Icos -/- mice could form GCs, though they were less frequent in number than in wild-type (WT) mice. Nonetheless, the Ag-specific Abs from Icos -/- mice lacked signs of affinity maturation, suggesting functional defects associated with these GCs. Eventually, these GC structures dissipated more rapidly in Icos -/- mice than in WT mice. Moreover, the ability of Icos -/- mice to form these GC structures is not reliant on the high Ag load associated with P. yoelii infections, as GC formation was preserved in Icos -/- mice treated with atovaquone. Finally, mice were unable to form secondary GCs in the absence of ICOS after re-challenge. Overall, these data demonstrate the necessity of ICOS in the maintenance of Tfh cells, the formation and maintenance of sufficient numbers of functioning GCs, and the ability to generate new GC structures after re-infection with P. yoelii .

Inducible T cell co-stimulator (ICOS) plays a key role in the differentiation and maintenance of follicular helper T (Tfh) cells and thus germinal center (GC) formation. Previously, our lab showed in a Plasmodium chabaudi infection model that Icos-/- mice did not form GCs despite a persistent infection and thus a continued antigen (Ag) load. Here, we show that resolution of a primary infection with P. yoelii, was delayed in Icos-/- mice. This phenotype was associated with a reduction in the accumulation of Tfh-like and GC Tfh cells and an early deficiency in Ag-specific antibody (Ab) production. However, Icos-/- mice maintained their ability to form GCs, though they were less frequent in number than in wild-type (WT) mice. Furthermore, while Ab production in Icos-/- mice matched that of WT mice after the infection cleared, the Abs lacked signs of affinity maturation, suggesting functional defects associated with these GCs. Eventually, these GC structures dissipated more rapidly in Icos-/-mice than in WT mice. Moreover, the ability of Icos-/- mice to form these GC structures is not reliant on the high Ag load associated with P. yoelii infections, as GC formation was preserved in Icos-/- mice treated with early with atovaquone. Finally, mice were unable to form secondary GCs in the absence of ICOS after re-challenge. Overall, these data demonstrate the necessity of ICOS in the maintenance of Tfh cells, the formation and maintenance of sufficient numbers of functioning GCs, and the ability to generate new GC structures after re-infection 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 the use of 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 three predominant MBC populations: somatically hypermutated isotype-switched (swIg+) and IgM+ MBCs that co-expressed CD73 and CD80, and CD73-CD80- unmutated swIg+ B cells. Re-challenge experiments indicated that IgG-producing cells dominated the recall response with minimal induction of IgM-secreting cells. Furthermore, using fluorescent protein expression as a surrogate for CD73 and CD80 co-expression, ZsGreen1+IgM+ MBCs gave rise to class switched IgG-producing plasmablasts that induced comparable titers of Ag-specific Abs as their swIg+ counterparts after adoptive transfer and P. yoelii infection. Moreover, ZsGreen1+ IgM+ and swIg+ MBCs gave rise to B cells with a germinal center phenotype. 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.