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Tissue-resident memory CD8+ T (TRM) cells that develop in the epithelia at portals of pathogen entry are important for improved protection against re-infection. CD8+ TRM cells within the skin and the small intestine are long-lived and maintained independently of circulating memory CD8+ T cells. In contrast to CD8+ TRM cells at these sites, CD8+ TRM cells that arise after influenza virus infection within the lungs display high turnover and require constant recruitment from the circulating memory pool for long-term persistence. The distinct characteristics of CD8+ TRM cell maintenance within the lungs may suggest a unique program of transcriptional regulation of influenza-specific CD8+ TRM cells. We have previously demonstrated that the transcription factors Hobit and Blimp-1 are essential for the formation of CD8+ TRM cells across several tissues, including skin, liver, kidneys, and the small intestine. Here, we addressed the roles of Hobit and Blimp-1 in CD8+ TRM cell differentiation in the lungs after influenza infection using mice deficient for these transcription factors. Hobit was not required for the formation of influenza-specific CD8+ TRM cells in the lungs. In contrast, Blimp-1 was essential for the differentiation of lung CD8+ TRM cells and inhibited the differentiation of central memory CD8+ T (TCM) cells. We conclude that Blimp-1 rather than Hobit mediates the formation of CD8+ TRM cells in the lungs, potentially through control of the lineage choice between TCM and TRM cells during the differentiation of influenza-specific CD8+ T cells.

A fundamental benefit of immunological memory is the ability to respond in an enhanced manner upon secondary encounter with the same pathogen. Tissue-resident memory CD8 T (TRM) cells contribute to improved protection against reinfection through the generation of immediate effector responses at the site of pathogen entry. Key to the potential of TRM cells to develop rapid recall responses is their location within the epithelia of the skin, lungs, and intestines at prime entry sites of pathogens. TRM cells are among the first immune cells to respond to pathogens that have been previously encountered in an antigen-specific manner. Upon recognition of invading pathogens, TRM cells release IFN-γ and other pro-inflammatory cytokines and chemokines. These effector molecules activate the surrounding epithelial tissue and recruit other immune cells including natural killer (NK) cells, B cells, and circulating memory CD8 T cells to the site of infection. The repertoire of TRM effector functions also includes the direct lysis of infected cells through the release of cytotoxic molecules such as perforin and granzymes. The mechanisms enabling TRM cells to respond in such a rapid manner are gradually being uncovered. In this review, we will address the signals that instruct TRM generation and maintenance as well as the underlying transcriptional network that keeps TRM cells in a deployment-ready modus. Furthermore, we will discuss how TRM cells respond to reinfection of the tissue and how transcription factors may control immediate and proliferative TRM responses.