Lymphotoxin limits Foxp3 + regulatory T cell development from Foxp3 lo precursors via IL-4 signaling.

Regulatory T cells (T reg ) are critical players of immune tolerance that develop in the thymus via two distinct developmental pathways involving CD25 + Foxp3 - and CD25 - Foxp3 lo precursors. However, the mechanisms regulating the recently identified Foxp3 lo precursor pathway remain unclear. Here, we find that the membrane-bound lymphotoxin α 1 β 2 (LTα 1 β 2 ) heterocomplex is upregulated during T reg development upon TCR/CD28 and IL-2 stimulation. We show that Lta expression limits the maturational development of T reg from Foxp3 lo precursors by regulating their proliferation, survival, and metabolic profile. Transgenic reporter mice and transcriptomic analyses further reveal that medullary thymic epithelial cells (mTEC) constitute an unexpected source of IL-4. We demonstrate that LTα 1 β 2 -lymphotoxin β receptor-mediated interactions with mTEC limit T reg development by down-regulating IL-4 expression in mTEC. Collectively, our findings identify the lymphotoxin axis as the first inhibitory checkpoint of thymic T reg development that fine-tunes the Foxp3 lo T reg precursor pathway by limiting IL-4 availability.