Asymmetry and redundancy of STAT5 paralogs across CD8 T cell differentiation states.

Fostering STAT5 signaling is key to immunotherapies that leverage CD8 T cell biology. Using mouse models, we demonstrate that the two mammalian STAT5 paralogs, STAT5A and STAT5B, are at once redundant and functionally distinct in CD8 T cells. Specifically, we establish that they are , exhibiting both widespread homology at molecular level and functional asymmetry at cellular level, with STAT5B emerging as dominant. In fact, compared to STAT5A, STAT5B deficiency had greater impact on nearly all parameters tested. As a mechanism, we determined STAT5B is twice as abundant, accounting for two-thirds of the total STAT5 pool. We also defined both cytokine- and cell state-restricted STAT5B functions, and a core gene signature that highlights universal effects. Together, these studies affirm the centrality of STAT5 in CD8 T cells, reveal common and circumscribed activities, and present a unifying model for paralog redundancy that foregrounds and explains the dominance of STAT5B.