Stepwise differentiation from precursor intermediates and distinct Th1 checkpoints promote CD4 Th1 cell differentiation during chronic viral infection

CD4⁺ helper T cells are essential for controlling viral infections. During chronic LCMV infection, CD4⁺ T cells differentiate into heterogeneous populations, including a TCF-1^hi progenitor subset that serves as a reservoir to continuously replenish type 1 helper (Th1) and follicular helper (Tfh) T cells. The gradual loss of CD4⁺ Th1 cell responses impairs the immune system's ability to control viral replication and contributes to the development of CD8⁺ T cell exhaustion. However, the mechanisms directing Th1 differentiation and the factors underlying their progressive decline during chronic infection remain poorly understood. In this study, we delineate the stepwise differentiation trajectory of Th1 cells, tracing their progression from TCF-1^hi progenitors through an intermediate state to fully differentiated Th1 cells. We identify an intermediate CD4⁺ T cell subset that serves as a precursor to Th1 cells, demonstrate that PD-1/PD-L1 signaling suppresses the transition from the progenitor to intermediate state, whereas the chromatin-remodeling complex PBAF restricts the terminal differentiation of CD4⁺ T cells into the Th1 subset. Notably, the combined blockade of PD-1/PD-L1 and genetic ablation of PBAF component (ARID2) additively enhanced Th1 differentiation and maintenance, leading to effective viral control. Thus, targeting these mechanisms driving CD4 Th1 cell differentiation and maintenance could enhance therapeutic strategies to restore Th1 function and control chronic infection.