T cell intrinsic SerpinB1 and a cathepsin protease serve as yin and yang to control Th17 differentiation (LYM3P.732)

SerpinB1, an endogenous inhibitor of neutrophil serine proteases, was recognized to restrict infection-associated inflammation. Recently, we found that loss of SerpinB1 leads to expansion of IL-17A+ γδ and Th17 cells even at steady-state. To date, little is known about how proteases and their inhibitors influence the differentiation of T cell subsets. Here, using differentiation of sorted naive CD4+ T cells, we show that, under Th17 conditions (IL-6 and TGF-β), deficiency of SerpinB1 significantly increased the IL-17+ population (~ 50% increase over WT) and reciprocally diminished the FoxP3+ population. In contrast, there were no genotype-differences in Th1 differentiation. Although neutrophil elastase is considered the primary target of SerpinB1, elastase-/- mice have IL-17+ γδ and Th17 cell numbers comparable to WT mice. Systematic screening of pure proteases using peptidase assays revealed that SerpinB1 also inhibits a subset of cysteine cathepsin proteases. Treating naive CD4 cells with the broad cysteine protease inhibitors E64d and FA-fmk decreased differentiation to IL17+ cells and reciprocally increased FoxP3+ cells. Use of specific cathepsin inhibitors including CLIK195 and LHVS further suggests a role for cathepsin-L or a cathepsin-L like protease. Thus, these findings show that, in addition to the known transcriptional regulation paradigm, the balance between Th17 cells and Tregs is tightly regulated by SerpinB1 acting through a cysteine cathepsin.