SerpinB1 regulates homeostatic expansion of IL-17+ γδ and CD4+ Th17 cells.

SerpinB1 is an endogenous inhibitor of serine proteases recognized for its anti-inflammatory and host-protective properties. Although loss of serpinB1 in mice does not result in gross immune deregulation, serpinb1a−/− mice display increased mortality and inflammation-associated morbidity upon challenge with influenza virus. Here, we show that IL-17A+ γδ and CD4+ Th17 cells are already expanded in the lungs of serpinb1a−/− mice at steady-state. Both γδ and αβ+ CD4+ CCR6+ T cells isolated from the lungs of naive serpinb1a−/− mice displayed a skewed transcriptional profile relative to WT cells, including increased Th17 signature transcripts [Il17a, l17f, and Rorc (RORγt)] and decreased Th1 signature transcripts [Ifng, Cxcr3, and Tbx21 (T-bet)] in γδ T cells. In addition to the lung, IL-17A+ γδ and CD4+ Th17 cells were increased in the spleen of naive serpinb1a−/− mice, despite normal αβ and γδ T cell development in the thymus. Within the γδ T cell compartment, loss of serpinb1a prompted selective expansion of Vγ4+ and Vγ6/Vδ1+ cells, which also displayed elevated expression of the proliferating cell nuclear antigen, Ki-67, and IL-17A. Given that serpinb1a is preferentially expressed in WT IL-17A+ γδ and CD4+ Th17 cell subsets vis-a-vis other T cell lineages, our findings reveal a novel function of serpinB1 in limiting untoward expansion of lymphocytes with a Th17 phenotype.