T-bet and RORα control lymph node formation by regulating embryonic innate lymphoid cell differentiation

Romagnani and colleagues discover an unexpected role for T-bet and ROR alpha during embryonic ILC function and highlight that ROR gamma t is crucial in counteracting the suppressive effects of T-bet. The generation of lymphoid tissues during embryogenesis relies on group 3 innate lymphoid cells (ILC3) displaying lymphoid tissue inducer (LTi) activity and expressing the master transcription factor ROR gamma t. Accordingly, ROR gamma t-deficient mice lack ILC3 and lymphoid structures, including lymph nodes (LN). Whereas T-bet affects differentiation and functions of ILC3 postnatally, the role of T-bet in regulating fetal ILC3 and LN formation remains completely unknown. Using multiple mouse models and single-cell analyses of fetal ILCs and ILC progenitors (ILCP), here we identify a key role for T-bet during embryogenesis and show that its deficiency rescues LN formation in ROR gamma t-deficient mice. Mechanistically, T-bet deletion skews the differentiation fate of fetal ILCs and promotes the accumulation of PLZF(hi) ILCP expressing central LTi molecules in a ROR alpha-dependent fashion. Our data unveil an unexpected role for T-bet and ROR alpha during embryonic ILC function and highlight that ROR gamma t is crucial in counteracting the suppressive effects of T-bet.