IFN-γ and IL-21 Double Producing T Cells Are Bcl6-Independent and Survive into the Memory Phase in Plasmodium chabaudi Infection

CD4 T cells are required to fight malaria infection by promoting both phagocytic activity and B cell responses for parasite clearance. In Plasmodium chabaudi infection, one specific CD4 T cell subset generates anti-parasitic IFN-γ and the antibody-promoting cytokine, IL-21. To determine the lineage of these multifunctional T cells, we followed IFN-γ(+) effector T cells (Teff) into the memory phase using Ifng-reporter mice. While Ifng (+) Teff expanded, the level of the Th1 lineage-determining transcription factor T-bet only peaked briefly. Ifng (+) Teff also co-express ICOS, the B cell area homing molecule CXCR5, and other Tfh lineage-associated molecules including Bcl6, the transcription factor required for germinal center (GC) T follicular helper cells (Tfh) differentiation. Because Bcl6 and T-bet co-localize to the nucleus of Ifng (+) Teff, we hypothesized that Bcl6 controls the Tfh-like phenotype of Ifng (+) Teff cells in P. chabaudi infection. We first transferred Bcl6-deficient T cells into wildtype hosts. Bcl6-deficient T cells did not develop into GC Tfh, but they still generated CXCR5(+)IFN-γ(+)IL-21(+)IL-10(+) Teff, suggesting that this predominant population is not of the Tfh-lineage. IL-10 deficient mice, which have increased IFN-γ and T-bet expression, demonstrated expansion of both IFN-γ(+)IL-21(+)CXCR5(+) cells and IFN-γ(+) GC Tfh cells, suggesting a Th1 lineage for the former. In the memory phase, all Ifng (+) T cells produced IL-21, but only a small percentage of highly proliferative Ifng (+) T cells maintained a T-bet(hi) phenotype. In chronic malaria infection, serum IFN-γ correlates with increased protection, and our observation suggests Ifng (+) T cells are maintained by cellular division. In summary, we found that Ifng (+) T cells are not strictly Tfh derived during malaria infection. T cells provide the host with a survival advantage when facing this well-equipped pathogen, therefore, understanding the lineage of pivotal T cell players will aid in the rational design of an effective malaria vaccine.