Tissue-resident memory T (Trm) cells permanently localize to portals of pathogen entry, where they provide immediate protection against reinfection. To enforce tissue retention, Trm cells up-regulate CD69 and down-regulate molecules associated with tissue egress; however, a Trm-specific transcriptional regulator has not been identified. Here, we show that the transcription factor Hobit is specifically up-regulated in Trm cells and, together with related Blimp1, mediates the development of Trm cells in skin, gut, liver, and kidney in mice. The Hobit-Blimp1 transcriptional module is also required for other populations of tissue-resident lymphocytes, including natural killer T (NKT) cells and liver-resident NK cells, all of which share a common transcriptional program. Our results identify Hobit and Blimp1 as central regulators of this universal program that instructs tissue retention in diverse tissue-resident lymphocyte populations.

Although circulating memory T cells provide enhanced protection against pathogen challenge, they often fail to do so if infection is localized to peripheral or extralymphoid compartments. In those cases, it is T cells already resident at the site of virus challenge that offer superior immune protection. These tissue-resident memory T (T RM ) cells are identified by their expression of the α-chain from the integrin α E (CD103)β 7 , and can exist in disequilibrium with the blood, remaining in the local environment long after peripheral infections subside. In this study, we demonstrate that long-lived intraepithelial CD103 + CD8 + T RM cells can be generated in the absence of in situ antigen recognition. Local inflammation in skin and mucosa alone resulted in enhanced recruitment of effector populations and their conversion to the T RM phenotype. The CD8 + T RM cells lodged in these barrier tissues provided long-lived protection against local challenge with herpes simplex virus in skin and vagina challenge models, and were clearly superior to the circulating memory T-cell cohort. The results demonstrate that peripheral T RM cells can be generated and survive in the absence of local antigen presentation and provide a powerful means of achieving immune protection against peripheral infection.

Infections localized to peripheral tissues such as the skin result in the priming of T-cell responses that act to control pathogens. Activated T cells undergo migrational imprinting within the draining lymph nodes, resulting in memory T cells that provide local and systemic protection. Combinations of migrating and resident memory T cells have been implicated in long-term peripheral immunity, especially at the surfaces that form pathogen entry points into the body. However, T-cell immunity consists of separate CD4(+) helper T cells and CD8(+) killer T cells, with distinct effector and memory programming requirements. Whether these subsets also differ in their ability to form a migrating pool involved in peripheral immunosurveillance or a separate resident population responsible for local infection control has not been explored. Here, using mice, we show key differences in the migration and tissue localization of memory CD4(+) and CD8(+) T cells following infection of the skin by herpes simplex virus. On resolution of infection, the skin contained two distinct virus-specific memory subsets; a slow-moving population of sequestered CD8(+) T cells that were resident in the epidermis and confined largely to the original site of infection, and a dynamic population of CD4(+) T cells that trafficked rapidly through the dermis as part of a wider recirculation pattern. Unique homing-molecule expression by recirculating CD4(+) T effector-memory cells mirrored their preferential skin-migratory capacity. Overall, these results identify a complexity in memory T-cell migration, illuminating previously unappreciated differences between the CD4(+) and CD8(+) subsets.

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.

In recent years, various intervention strategies have reduced malaria morbidity and mortality, but further improvements probably depend upon development of a broadly protective vaccine. To better understand immune requirement for protection, we examined liver-stage immunity after vaccination with irradiated sporozoites, an effective though logistically difficult vaccine. We identified a population of memory CD8 T cells that expressed the gene signature of tissue-resident memory T (Trm) cells and remained permanently within the liver, where they patrolled the sinusoids. Exploring the requirements for liver Trm cell induction, we showed that by combining dendritic cell-targeted priming with liver inflammation and antigen recognition on hepatocytes, high frequencies of Trm cells could be induced and these cells were essential for protection against malaria sporozoite challenge. Our study highlights the immune potential of liver Trm cells and provides approaches for their selective transfer, expansion, or depletion, which may be harnessed to control liver infections or autoimmunity.

Tissue-resident memory T cells provide local immune protection in barrier tissues, such as skin and mucosa. However, the molecular mechanisms controlling effector T cell retention and subsequent memory formation in those locations are not fully understood. In this study, we analyzed the role of CD69, an early leukocyte activation marker, in regulating effector T cell egress from peripheral tissues. We provide evidence that CD69 surface expression by skin-infiltrating CD8 T cells can be regulated at multiple levels, including local Ag stimulation and signaling through type I IFNRs, and it coincides with the transcriptional downregulation of the sphingosine-1-phosphate receptor S1P1. Importantly, we demonstrate that expression of CD69, by interfering with sphingosine-1-phosphate receptor function, is a critical determinant of prolonged T cell retention and local memory formation. Our results define an important step in the generation of long-lived adaptive immune memory at body surfaces.

Secondary lymphoid organs are dominant sites of T cell activation, although many T cells are subsequently retained within peripheral tissues. Currently, these nonlymphoid compartments are viewed as sites only of effector T cell function, without the involvement of renewed induction of immunity via the interactions with professional antigen-presenting cells. We describe a method of reactivation of herpes simplex virus to examine the stimulation of tissue-resident T cells during secondary challenge. The results revealed that memory CD8+ T cell responses can be initiated within peripheral tissues through a tripartite interaction that includes CD4+ T cells and recruited dendritic cells. These findings lend evidence for the existence of a sophisticated T cell response mechanism in extra-lymphoid tissues that can act to control localized infection.

Barrier tissues such as the skin contain various populations of immune cells that contribute to protection from infections. These include recently identified tissue-resident memory T cells (T RM ). In the skin, these memory CD8 + T cells reside in the epidermis after being recruited to this site by infection or inflammation. In this study, we demonstrate prolonged persistence of epidermal T RM preferentially at the site of prior infection despite sustained migration. Computational simulation of T RM migration within the skin over long periods revealed that the slow rate of random migration effectively constrains these memory cells within the region of skin in which they form. Notably, formation of T RM involved a concomitant local reduction in dendritic epidermal γδ T-cell numbers in the epidermis, indicating that these populations persist in mutual exclusion and may compete for local survival signals. Accordingly, we show that expression of the aryl hydrocarbon receptor, a transcription factor important for dendritic epidermal γδ T-cell maintenance in skin, also contributes to the persistence of skin T RM . Together, these data suggest that skin tissue-resident memory T cells persist within a tightly regulated epidermal T-cell niche. T he skin is a complex organ that acts as a primary barrier between the body and the environment. Multiple leukocyte subsets reside within the main compartments of the skin, the dermis and the epidermis, as well as the hair follicles that are contiguous with the epidermis. Populations of macrophages, dendritic cells, mast cells, γδ T cells and αβ T cells are present in the dermis, and Langerhans cells (LCs) and dendritic epidermal γδ T cells (DETCs) lie in a strategic network in the epidermis (1). We recently described a population of memory CD8 + T cells that enter the epidermis and hair follicles during infection or inflammation and become long-lived populations of tissue-resident memory T cells (T RM ) (2-5). These memory T cells are sequestered in this site and are distinct from circulating effector memory (T EM ) and central memory (T CM ) populations (3, 6). Memory CD8 + T cells in the epidermis display a dendritic morphology and move at a slower velocity than T cells within the dermis. These memory T cells are sequestered in the skin epithelial layer and do not recirculate to other tissues. In contrast, memory CD4 + T cells are found within the dermis and at least a proportion of these cells are capable of recirculating around the body (3, 7).In this study, we sought to further examine the mechanisms of T RM persistence within the skin. We reveal that skin T RM persist at the site of their formation and despite displaying a sustained mode of random migration, the slow rate of this movement locally constrains these memory cells. Examination of other cells in this environment showed that although epidermal T RM regularly interact with LC, these interactions were not required for persistence. In contrast, skin tissue-resident memory T cells replaced DETCs in the epidermis, seem...