Tissue-resident memory T (Trm) cells contribute to local immune protection in non-lymphoid tissues such as skin and mucosa, but little is known about their transcriptional regulation. Here we showed that CD8+CD103+ Trm cells, independent of circulating memory T cells, were sufficient for protection against infection and described molecular elements that were crucial for their development in skin and lung. We demonstrated that the T-box transcription factors (TFs) Eomes and T-bet combined to control CD8+CD103+ Trm cell formation, such that their coordinate downregulation was crucial for TGF-β cytokine signaling. TGF-β signaling, in turn, resulted in reciprocal T-box TF downregulation. However, whereas extinguishment of Eomes was necessary for CD8+CD103+ Trm cell development, residual T-bet expression maintained cell surface interleukin-15 (IL-15) receptor β-chain (CD122) expression and thus IL-15 responsiveness. These findings indicate that the T-box TFs control the two cytokines, TGF-β and IL-15, which are pivotal for CD8+CD103+ Trm cell development and survival.
Molecular events that regulate Trm cell formation remain poorly defined. Mackay and colleagues demonstrate that a complex interplay between Eomes and T-bet underpins CD8+CD103+ Trm cell formation. Although both T-box transcription factors must be downregulated, Eomes must be completely extinguished, whereas residual T-bet expression is essential for IL-15-mediated long-term survival.