Bhlhe40 limits early IL-10 production from CD4+ T cells during Plasmodium yoelii 17X infection.

The cytokine IL-10 suppresses T-cell-mediated immunity, which is required to control infection with Plasmodium yoelii. Consequently, IL-10 can delay the time needed to resolve this infection, leading to a higher parasite burden. While the pathways that lead to IL-10 production by CD4+ T cells are well defined, much less is known about the mediators that suppress the expression of this potent anti-inflammatory cytokine. Here, we show that the transcription factor basic helix-loop-helix family member e40 (Bhlhe40) contributes to controlling parasite burden in response to P. yoelii infection in mice. Loss of Bhlhe40 expression in mice results in higher Il10 expression, higher peak parasitemia, and a delay in parasite clearance. The observed phenotype was not due to defects in T-cell activation and proliferation or the humoral response. Nor was it due to changes in regulatory T-cell numbers. However, blocking IL-10 signaling reversed the outcome in Bhlhe40-/ - mice, suggesting that excess IL-10 production limits their ability to control the infection properly. In addition to suppressing Il10 expression in CD4+ T cells, Bhlhe40 can promote Ifng expression. Indeed, IFN-γ production by CD4+ T cells isolated from the liver was significantly affected by the loss of Bhlhe40. Lastly, Bhlhe40 deletion in T cells resulted in a phenotype similar to that observed in the Bhlhe40-/ - mice, indicating that Bhlhe40 expression in T cells contributes to the ability of mice to control infection with P. yoelii.

ICOS Expression Is Required for Maintenance but Not the Formation of Germinal Centers in the Spleen in Response to Plasmodium yoelii Infection.

Inducible T cell costimulator (ICOS) plays a key role in the differentiation and maintenance of follicular helper T (Tfh) cells and, thus, germinal center (GC) formation. Previously, our laboratory showed in a Plasmodium chabaudi infection model that Icos-/- mice were significantly impaired in their ability to form GCs despite persistent infection and, thus, a continued antigen (Ag) load. Here, we show that the resolution of primary infection with Plasmodium 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, although 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 loads 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 rechallenge. 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 reinfection with P. yoelii.