(A) Diagram of parabiosis between Rag1^KOTslpr^KO and Rag1^KO mice. Three weeks after successful surgery, the Rag1^KOTslpr^KO mouse (if present in each pair) was given a total dose of 4 million purified naive WT CD4⁺CD25^– T cells i.v. (2 million/mouse). The animals were harvested at the moribund state or on day 15 after T cell transfer. (B) Representative colon H&E staining of Rag1^KOTslpr^KO + Rag1^KO, Rag1^KO + Rag1^KO, and Rag1^KOTslpr^KO + Rag1^KOTslpr^KO parabiotic pairs. Note the abundance of goblet cells in Rag1^KOTslpr^KO + Rag1^KO and Rag1^KO + Rag1^KO pairs, while Rag1^KOTslpr^KO + Rag1^KOTslpr^KO pairs have severe mucosal inflammation, crypt abscess formation, goblet cell loss, crypt distortion, and epithelial hyperplasia. Scale bars: 50 μm. (C) Colitis scoring of Rag1^KOTslpr^KO + Rag1^KO (n = 3 pairs), Rag1^KO + Rag1^KO (n = 2 pairs), and Rag1^KOTslpr^KO + Rag1^KOTslpr^KO (n = 2 pairs) parabiotic pairs. Fisher’s exact test. (D) Representative CD3/CD4 staining of colons from Rag1^KOTslpr^KO + Rag1^KO, Rag1^KO + Rag1^KO, and Rag1^KOTslpr^KO + Rag1^KOTslpr^KO parabiotic pairs. Scale bars: 50 μm. (E) CD4⁺ T cell counts in colon sections from Rag1^KOTslpr^KO + Rag1^KO (n = 3 pairs), Rag1^KO + Rag1^KO (n = 2 pairs), and Rag1^KOTslpr^KO + Rag1^KOTslpr^KO (n = 2 pairs) parabiotic pairs. CD3⁺CD4⁺ mucosal T cells were counted in 10 randomly selected HPFs per mouse at the end of the study. Each dot represents 1 HPF; bar graph shows mean + SD. **P < 0.01; ****P < 0.0001 by ordinary 1-way ANOVA. NS, not significant.