Objective Interleukin (IL)-7 is mainly produced in bone marrow (BM) that forms the niche for B cells. We previously demonstrated that BM also retains pathogenic memory CD4 T cells in murine models of inflammatory bowel disease (IBD). However, it remains unknown whether BM-derived IL-7 is sufficient for the development of IBD and which cells form the niche for colitogenic memory CD4 T cells in BM.

Design To address these questions, we developed mice in which IL-7 expression was specific for BM, and identified colitis-associated IL-7-expressing mesenchymal stem cells (MSC) in the BM.

Results IL-7^–/–×RAG-1^–/– mice injected with BM cells from IL-7^+/+×RAG-1^–/– mice, but not from IL-7^–/–×RAG-1^–/– mice, expressed IL-7 in BM, but not in their colon, and developed colitis when injected with CD4⁺CD45RB^high T cells. Cultured BM MSC stably expressed a higher level of IL-7 than that of primary BM cells. IL-7-sufficient, but not IL-7-deficient, BM MSC supported upregulation of Bcl-2 in, and homeostatic proliferation of, colitogenic memory CD4 T cells in vitro. Notably, IL-7^–/–×RAG-1^–/– mice transplanted with IL-7-sufficient, but not IL-7-deficient, BM MSC expressed IL-7 in BM, but not in their colon, and developed colitis when transplanted with CD4⁺CD45RB^high T cells.

Conclusions We demonstrate for the first time that BM MSC are a major source of IL-7 and play a pathological role in IBD by forming the niche for colitogenic CD4 memory T cells in BM.