BLIMP ‐1 and CD39 expression define human effector memory regulatory T cells in blood and recall responses to viruses

BLIMP‐1, a transcriptional repressor initially identified in plasma B‐cell differentiation, also regulates CD8 + T cells and is differentially expressed between naïve and effector/memory CD4 + T cells. However, the functions of BLIMP‐1 in human CD4 + regulatory T cells (Tregs) remain unclear. We found BLIMP‐1 mRNA levels were low in naïve Tregs from thymus and cord blood, but high in effector/memory Tregs from adult peripheral blood. We confirmed the expression of BLIMP‐1 mRNA by microarray of healthy T‐cell populations and single‐cell RNA‐sequencing of SARS‐CoV‐2 spike‐specific CD4 + T cells from convalescent COVID‐19 patients. We found activated memory Tregs expressed the highest levels of BLIMP‐1, FOXP3, and CD39 transcripts and protein. We show that CD39 + Tregs, regardless of in vitro or in vivo activation, had the highest levels of BLIMP‐1 protein expression and significantly more IL‐10 mRNA than CD39 neg cells. The same result was found through reanalysis of public single cell RNA‐sequencing data from lung (tissue and lymph node) SARS‐CoV‐2 spike‐specific Tregs, where cells with high levels of BLIMP‐1 mRNA had significantly more IL‐10 mRNA than cells with low BLIMP‐1 mRNA. Single‐cell RNA‐sequencing identified an extended transcriptomic profile of these CD39 + Tregs that includes the genes encoding HPGD, CTLA‐4, HELIOS, TIGIT, CXCR6, CCR8, and GARP. Together, these data indicate that both circulating and lung resident effector memory Tregs maintain a conserved phenotype following stimulation by cognate antigen. Our data provide further insight into the functions and potential therapeutic targets of the large regulatory T‐cell component of CD4 + T‐cell responses to pathogens.