Continued Bcl6 expression prevents the transdifferentiation of established Tfh cells into Th1 cells during acute viral infection

T follicular helper (Tfh) cells are crucial for the establishment of germinal centers (GCs) and potent antibody responses. Nevertheless, the T cell-intrinsic factors that are required for the maintenance of already established Tfh cells and GCs remain largely unknown. Here, we used temporally guided gene ablation in CD4+ T cells to dissect the contributions of the Tfh-associated chemokine receptor CXCR5 and the transcription factor Bcl6. Induced ablation of Cxcr5 had has minor effects on the function of established Tfh cells and Cxcr5-ablated cells still exhibited most features of CXCR5-positive Tfh cells. In contrast, continued Bcl6 expression was is critical to maintain the GC Tfh cell phenotype and also the GC reaction. Importantly, Bcl6 ablation during acute viral infection results in transdifferentiation of established Tfh into Th1 cells, thus highlighting the plasticity of Tfh cells. These findings have implications for strategies that boost or restrain Tfh cells and GCs in health and disease.