LAUSR

CD4+Foxp3+ regulatory T cells (Tregs) restrain inflammation and immunity. However, the mechanisms underlying Treg suppressor function in inflamed nonlymphoid tissues remain largely unexplored. Here, we restricted immune responses to nonlymphoid tissues and used intravital microscopy to visualize Treg suppression of rejection by effector T cells (Teffs) within inflamed allogeneic islet transplants. Despite their elevated motility, Tregs preferentially contacted antigen-presenting cells (APCs) over Teffs. Interestingly, Tregs specifically targeted APCs that were extensively and simultaneously contacted by Teffs. In turn, Tregs decreased MHC-II expression on APCs and hindered Teff function. Last, we demonstrate that Treg suppressive function within inflamed allografts required ectonucleotidase CD73 activity, which generated the antiinflammatory adenosine. Consequently, CD73–/– Tregs exhibited fewer contacts with APCs within inflamed allografts compared with WT Tregs, but not in spleen. Overall, our findings demonstrate that Tregs suppress immunity within inflamed grafts through CD73 activity and suggest that Treg-APC direct contacts are central to this process.