UCSF

Regulatory T cells (Tregs) are critical to the maintenance of immune homeostasis and the prevention of autoimmunity. In the NOD mouse model of autoimmune diabetes, treatment with therapeutic Tregs can prevent and even reverse disease onset. The means by which Tregs are able to exert their protective effects in vivo have remained incompletely understood. This work focused on elucidating the critical cellular targets of Tregs at the site of inflammation and the effects of Tregs on these target cells. This was accomplished by examining the target tissue, the pancreatic islets of Langerhans, before and after Treg treatment with a variety of approaches. Islets and their immune infiltrates were analyzed at the transcriptional and post-transcriptional levels. Islet immune infiltrates were characterized at the phenotypic and functional levels by flow cytometry and by multiple modes of microscopic imaging. Therapeutic Tregs were found to quickly home to inflamed islets, where they engaged in dynamic interactions with resident dendritic cells (DCs). Treg arrival was followed by a rapid decrease in islet CD8 T cells that corresponded with a reduction in the expression of cytotoxic effectors and cytokines. T cells that persisted in the islets following Treg treatment showed no decrease in proliferative capacity but were suppressed at the level of effector function, where they lacked production of IFNg protein despite continued expression of IFNg mRNA. In CD8 T cells, this inhibition of effector function was related to a decrease in mTOR signaling that was required for sustained IFNg production. In addition to these acute effects on CD8 T cells following Treg treatment, we also observed a more gradual impact on islet DCs. The recruitment of DCs was reduced in the weeks following Treg treatment. Islet DCs were found to arise from a blood monocyte precursor. Overall, our findings that therapeutic Tregs target CD8 T cells during acute disease control and DCs during later stages implicate the importance of these targets in disease pathogenesis and elucidates the mechanisms by which therapeutic Tregs exert their protective effects.