BLIMP-1 and CEACAM1 cooperate to regulate human regulatory T cell proliferation and function

Abstract

Abstract BLIMP-1, encoded by the PRDM1 gene, is a major transcriptional hub distinct from Foxp3 in human Tregs, but its mechanisms of action have not been fully determined. Here, using CRISPR/Cas9-mediated disruption of PRDM1, we show that BLIMP-1 limits Treg proliferation while supports Treg functional activity. PRDM1KO human Tregs are less suppressive and exhibit increased expansion in a humanized mouse model of xenogeneic GVHD. BLIMP-1 restrains Treg proliferation by lowering IL-2R signaling through downregulation of CD25 and by inhibiting responsiveness to TCR/CD28 signaling and mTOR activation through upregulation of CEACAM1, an immune checkpoint. Expression of CEACAM1 is induced by IL-2 in human, but not mouse, Tregs. Prolonged IL-2R signaling leads to optimal BLIMP-1 expression and chromatin opening of human CEACAM1, leading to high cell surface CEACAM1 through a 3’ regulatory region containing motifs for binding PRDM1 and STATs. Correspondingly, CEACAM1 is highly induced on Tregs from autoimmune patients undergoing low-dose IL-2 therapy, and these Tregs showed reduced proliferation. In vivo studies using xenogeneic GVHD mouse model show that CEACAM1KO human Tregs have higher suppressive activities, indicating a potential role of CEACAM1 in Treg function. Collectively, our findings uncover a novel IL-2-dependent regulatory loop whereby BLIMP-1 and CEACAM1 act to limit human Treg proliferation and contribute to Treg suppressive function.