IL17A and colitis: Transfer of naïve CD4+ T cells (CD45RBhiCD25−CD4+) into Rag1−/− mice induced severe colitis, marked by progressive weight loss (Fig. 3B), shortened colons (Fig. 3C), splenomegaly and enlarged lymph nodes (Fig. 3D), histological evidence of colonic tissue damage (Fig. 3E), and increased frequency of inflammatory IFN-γ–producing (but not IL-17A–producing) CD4+ T cells in the colon (Fig. 3F).