IL7R⍺, but not Flk2/Flt3, is required for hematopoietic stem cell reconstitution of tissue-resident lymphoid cells

Tissue-resident lymphoid cells (TLCs) span the spectrum of innate-to-adaptive immune function. Unlike traditional, circulating lymphocytes that are continuously generated from hematopoietic stem cells (HSCs), many TLCs are of fetal origin and poorly generated from adult HSCs. Here, we sought to further understand murine TLC development and the roles of Flk2 and IL7R⍺, two cytokine receptors with known function in traditional lymphopoiesis. Using Flk2- and Il7r-Cre lineage tracing, we found that peritoneal B1a cells, splenic marginal zone B (MZB) cells, lung ILC2s and regulatory T cells (Tregs) were highly labeled. Despite high labeling, loss of Flk2 minimally affected the generation of these cells. In contrast, loss of IL7R⍺, or combined deletion of Flk2 and IL7R⍺, dramatically reduced the number of B1a cells, MZBs, ILC2s, and Tregs both in situ and upon transplantation, indicating an intrinsic and essential role for IL7Rα. Surprisingly, reciprocal transplants of WT HSCs showed that an IL7Rα^−/- environment selectively impaired reconstitution of TLCs when compared to TLC numbers in situ. Taken together, our data defined Flk2- and IL7Rα-positive TLC differentiation paths, and revealed functional roles of Flk2 and IL7Rα in TLC establishment.