Interleukin-2 Immunotherapy Reveals Human Regulatory T Cell Subsets with Distinct Functional and Tissue-Homing Characteristics

Due to its stimulatory potential for immunomodulatory CD4+ regulatory T (Treg) cells, low-dose interleukin-2 (IL-2) immunotherapy has recently gained considerable attention for treatment of various autoimmune diseases. Although early-stage clinical trials have correlated expansion of circulating Treg cells with clinical response to IL-2 treatment, detailed mechanistic data on responding Treg cell subsets are lacking. Within an investigator-initiated phase-2 clinical trial of low-dose IL-2 immunotherapy in systemic lupus erythematosus (SLE) patients, we performed an in-depth study of circulating and cutaneous Treg cell subsets by imaging mass cytometry, high-parameter spectral flow cytometry, bulk and single-cell RNA sequencing with cellular indexing, ATAC sequencing, and targeted serum proteomics. Low-dose IL-2 stimulated circulating Treg cells with skin-homing properties that appeared in the skin of SLE patients in close interaction with endothelial cells, suggestive of a gatekeeper function. Analysis of surface proteins and transcriptomes detected different IL-2-driven Treg cell programs, including highly proliferative CD38+ HLA-DR+, activated gut-homing CD38+, and skin-homing HLA-DR+ Treg cells. These data identify functionally distinct Treg cell subsets in human blood and skin, including those most responsive to IL-2 immunotherapy, thus providing unprecedented insight into human Treg cell biology.