Type 1 regulatory T cells (Tr1) homeostasis and function in aging

Abstract

Abstract Type 1 regulatory T cells (Tr1) are a unique population of CD4+ Foxp3− cells that express high levels of IL-10, and have been defined based on their expression of CD49b and LAG-3. Despite the critical roles played by Tr1 cells in controlling T cell responses in autoimmunity and infection, the mechanisms underlying the homeostasis of Tr1 cells remain unclear. Here, we investigated the homeostasis, phenotype and function of Tr1 cells with age. We found that Tr1 cells accumulated dramatically with age. Further, Tr1 cells produced more IL-10 per cell compared to Foxp3+ Treg. While aged Tr1 cells expressed significant levels of c-Maf and Egr-2, two of the Tr1 defining transcription factors, they largely lacked expression of CD49b and were heterogeneous in LAG-3 and ICOS expression. Despite this heterogeneity, LAG-3 identified a population of aged Tr1 cells that potently inhibited T-cell proliferation in a partially IL-10-dependent manner in vitro and controlled colitis severity in vivo. Additional flow cytometric analysis revealed that roughly half of the IL-10 producing cells co-produced IFN-γ. Further, single cell mRNA-seq analysis revealed that IL-10+IFN-γ− cells had a TGF-beta signature, while IL-10+IFN-γ+ cells had a STAT4 signature. Strikingly, IL-6 was critical for overall Tr1 cell accumulation, as aged IL-6 KO mice showed a profound reduction in Tr1 cells and their expression of c-Maf. Combined, these data show that IL-6 promotes Tr1 accumulation with age and provide a new insight into a novel feedback loop whereby an inflammatory cytokine drives accrual of an anti-inflammatory cell population.