IL-15 supports the generation of protective lung-resident memory CD4 T cells.

Tissue-resident memory T cells (T _RM ) provide optimal defense at the sites of infection, but signals regulating their development are unclear, especially for CD4 T cells. Here we identify two distinct pathways that lead to the generation of CD4 T _RM in the lungs following influenza infection. The T _RM are transcriptionally distinct from conventional memory CD4 T cells and share a gene signature with CD8 T _RM . The CD4 T _RM are superior cytokine producers compared with conventional memory cells, can protect otherwise naive mice against a lethal influenza challenge, and display functional specialization by inducing enhanced inflammatory responses from dendritic cells compared with conventional memory cells. Finally, we demonstrate than an interleukin (IL)-2-dependent and a novel IL-2-independent but IL-15-dependent pathway support the generation of cohorts of lung T _RM .