Cutting edge: IL-7 regulates the peripheral pool of adult RORγ⁺ lymphoid tissue inducer cells

During fetal life, CD4⁺CD3^- lymphoid tissue inducer (LTi) cells are required for lymph node and Peyer's patch development in mice. In adult animals, CD4⁺CD3^- cells are found in low numbers in lymphoid organs. Whether adult CD4⁺CD3^- cells are LTi cells and are generated and maintained through cytokine signals has not been directly addressed. In this study we show that adult CD4⁺CD3 ^- cells adoptively transferred into neonatal CXCR5^-/- mice induced the formation of intestinal lymphoid tissues, demonstrating for the first time their bona fide LTi function. Increasing IL-7 availability in wild-type mice either by IL-7 transgene expression or treatment with IL-7/anti-IL-7 complexes increased adult LTi cell numbers through de novo generation from bone marrow cells and increased the survival and proliferation of LTi cells. Our observations demonstrate that adult CD4 ⁺lineage^- cells are LTi cells and that the availability of IL-7 determines the size of the adult LTi cell pool.