T. muris accelerates T cell–induced colitis. (A) Fresh cecal contents were diluted in phosphate-buffered saline and visualized under a microscope. (B) T. muris from the cecal contents of Rip2^−/−Rag1^−/− mice were stained with Giemsa and visualized. (C–G) T. muris was isolated from the cecum of Rip2^−/−Rag1^−/− mice, and 10⁶ protozoans were orally gavaged into Rag1^−/− mice. 2 wk later, mice were injected i.p. with 0.5 × 10⁶ CD45RB^High CD4 T cells. (C) Body weight was measured, and mice were sacrificed at week 4. (D–F) Colons were fixed, H&E stained, and scored for colitis severity. Data from three experiments were pooled with 12 mice per group. Bars: (A) 100 μm; (B) 10 μm; (F) 200 μm. (G) Proximal colon explants from week 4 after T cell transfer T. muris–infected and control mice were cultured for 18 h, and supernatants were analyzed by ELISA for cytokine production. Data from two experiments were pooled with 9–10 samples per group. (C–E and G) Mean ± SEM is shown with *, P ≤ 0.05; **, P ≤ 0.01 using an unpaired Student’s t test.