Circulating T cells and resident non-T cells restrict type 2 innate lymphoid cell expansion in the small intestine

Interaction among various adaptive, circulating cells and tissue-resident cells including innate lymphocytes during the establishment and maintenance of the barrier-tissue immune system has only recently started to be explored. Here, we show that the cellular crosstalk with circulating T cells and other resident cells regulated the population size of type 2 innate lymphoid cells (ILC2s) in the small intestine lamina propria. Rag1^-/- mice had excessive numbers of both ILC2s and ILC3s, and such an over-expansion was corrected by establishing parabiosis with wild type mice or by adoptively transferring wild type CD4⁺ T cells. In contrast, anti-CD3 antibody-mediated T cell depletion in wild type mice increased ILC2 but not ILC3 numbers. Unconventional CD4^-CD8^- αβ T and γδ T cells could also restrict ILC2 expansion as the numbers of ILC2s were not altered even in mice treated with anti-CD4/anti-CD8 antibodies. ILC3 restriction seemed to be through the control of proliferation, but that for ILC2s did not. In addition, elevation in ILC2 numbers seen in mice lacking the transcription factors RORγt and STAT6 was found to be T cell-independent. Our current findings altogether uncovered the homeostatic 'quota' restriction imposed on intestinal ILC2s in the steady state by resident non-T cells via RORγt- and STAT6-dependent mechanisms as well as by conventional and nonconventional T cells.