Background & Aims: T helper (Th) 17 cells that express the retinoid-related orphan receptor (ROR) γt contribute to the development of colitis in mice, yet are found in normal and inflamed intestine. We investigated their development and functions in intestines of mice. Methods: We analyzed intestinal Th17 cells in healthy and inflamed intestinal tissues of mice. We analyzed expression of lymphotoxin (LT)α by Th17 cells and lymphoid tissue inducer-like cells. Results: LTα-/- and RORγt-/- mice had significantly lower percentages of naturally occurring Th17 cells in the small intestine than wild-type mice. Numbers of CD3-CD4+/-interleukin-7Rα+c-kit +CCR6+NKp46- lymphoid tissue inducer-like cells that produce interleukin-17A were increased in LTα-/- and LTα-/-×recombination activating gene (RAG)-2 -/- mice, compared with wild-type mice, but were absent from RORγt-/- mice. Parabiosis of wild-type and LTα -/- mice and bone marrow transplant experiments revealed that LTα-dependent gut-associated lymphoid tissue structures are required for generation of naturally occurring Th17 cells. However, when wild-type or LTα-/- CD4+CD45RBhigh T cells were transferred to RAG-2-/- or LTα-/-×RAG-2 -/- mice, all groups, irrespective of the presence or absence of LTα on the donor or recipient cells, developed colitis and generated Th1, Th17, and Th17/Th1 cells. RAG-2-/- mice that received a second round of transplantation, with colitogenic but not naturally occurring Th17 cells, developed intestinal inflammation. The presence of naturally occurring Th17 cells in the colons of mice inhibited development of colitis after transfer of CD4+CD45RBhigh T cells and increased the numbers of Foxp3+ cells derived from CD4+CD45RBhigh T cells. Conclusions: Gut-associated lymphoid tissue structures are required to generate naturally occurring Th17 cells that have regulatory activities in normal intestines of mice, but not for colitogenic Th17 and Th17/Th1 cells during inflammation.
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