RAG1 and colitis: Foxp3+ Treg cells are well known for their capacity to limit intestinal inflammation and restore immune homeostasis.28 Thus, to define the contribution of Foxp3+ Treg cells to the PSC-mediated attenuation of colitis, we used the T-cell transfer colitis model, in which Foxp3+ Treg cells are largely absent.29 To that end, we induced colitis in lymphopenic Rag1−/− and Rag1−/−Mdr2−/− mice, by transfer of naïve CD4+Foxp3−CD45RBhigh cells (figure 3A).