Title: Transcriptional control of tissue-resident memory T cell generation
Description: Tissue-resident memory T cells (TRM) are a non-circulating subset of memory that are maintained at sites of pathogen entry and mediate optimal protection against reinfection. Lung TRM can be generated in response to respiratory infection or vaccination, however, the molecular pathways involved in CD4+TRM establishment have not been defined. Here, we performed transcriptional profiling of influenza-specific lung CD4+TRM following influenza infection to identify pathways implicated in CD4+TRM generation and homeostasis. Lung CD4+TRM displayed a unique transcriptional profile distinct from spleen memory, including up-regulation of a gene network induced by the transcription factor IRF4, a known regulator of effector T cell differentiation. In addition, the gene expression profile of lung CD4+TRM was enriched in gene sets previously described in tissue-resident regulatory T cells. Up-regulation of immunomodulatory molecules such as CTLA-4, PD-1, and ICOS, suggested a potential regulatory role for CD4+TRM in tissues.

Using loss-of-function genetic experiments in mice, we demonstrate that IRF4 is required for the generation of lung-localized pathogen-specific effector CD4+T cells during acute influenza infection. Influenza-specific IRF4−/− T cells failed to fully express CD44, and maintained high levels of CD62L compared to wild type, suggesting a defect in complete differentiation into lung-tropic effector T cells. This finding identifies IRF4 as an important regulator of CD4+TRM generation in response to respiratory infection. Furthermore, comparing whole transcriptome profiling of mouse and human lung memory T cell subsets, we define a lung CD4+TRM gene signature common to mice and humans. IRF4 protein was specifically up-regulated in lung CD4+TRM but not in circulating memory subsets, in both humans and mice previously infected with influenza. This result suggest that high expression of IRF4 contributes to a cross-species conserved molecular pathway of long term maintenance of CD4+TRM in the lung.

Overall, our findings confirm lung CD4+TRM as a unique memory T cell subset regulated by tissue-specific transcription factors. These results have important implications in focusing future studies of tissue resident memory T cells to factors with translational potential. Importantly, by determining the lung CD4+TRM gene signature common to mice and humans, we motivate future genetic studies that could lead to the complete identification of the mechanisms of TRM maintenance in humans.