Tissue-resident memory T cells (TRM) persist locally in nonlymphoid tissues where they provide frontline defense against recurring insults. TRM at barrier surfaces express the markers CD103 and/or CD69, which function to retain them in epithelial tissues. In humans, neither the long-term migratory behavior of TRM nor their ability to reenter the circulation and potentially migrate to distant tissue sites has been investigated. Using tissue explant cultures, we found that CD4+CD69+CD103+ TRM in human skin can down-regulate CD69 and exit the tissue. In addition, we identified a skin-tropic CD4+CD69−CD103+ population in human lymph and blood that is transcriptionally, functionally, and clonally related to the CD4+CD69+CD103+ TRM population in the skin. Using a skin xenograft model, we confirmed that a fraction of the human cutaneous CD4+CD103+ TRM population can reenter circulation and migrate to secondary human skin sites where they reassume a TRM phenotype. Thus, our data challenge current concepts regarding the strict tissue compartmentalization of CD4+ T cell memory in humans.

The skin comprises a complex coordinated system of epithelial tissue cells and immune cells that ensure adequate immune reactions against trauma, toxins and pathogens, while maintaining tissue homeostasis. Keratinocytes form the outermost barrier of the skin, and sense changes in barrier integrity, intrusion of microbial components and stress molecules. Thus, they act as sentinels that continuously communicate the status of the organ to the cutaneous immune system. Upon damage the keratinocytes initiate a pro-inflammatory signaling cascade that leads to the activation of resident immune cells. Simultaneously, the tissue mediates and supports immune-suppressive functions to contain inflammation locally. After resolution of inflammation, the skin provides a niche for regulatory and effector memory T cells that can quickly respond to reoccurring antigens. In this review we discuss the central role of keratinocyte-derived signals in controlling cutaneous T cell immunity.

After activation, CD4 + Th cells differentiate into functionally specialized populations that coordinate distinct immune responses and protect against different types of pathogens. In humans, these effector and memory Th cell subsets can be readily identified in peripheral blood based on their differential expression of chemokine receptors that govern their homeostatic and inflammatory trafficking. Foxp3 + regulatory T (Treg) cells can also be divided into subsets that phenotypically mirror each of these effector populations and share expression of key transcription factors and effector cytokines. In this study, we performed comprehensive transcriptional profiling of 11 phenotypically distinct Th and Treg cell subsets sorted from peripheral blood of healthy individuals. Despite their shared phenotypes, we found that mirror Th and Treg subsets were transcriptionally dissimilar and that Treg cell populations showed limited transcriptional diversity compared with Th cells. We identified core transcriptional signatures shared across all Th and Treg cell populations and unique signatures that define each of the Th or Treg populations. Finally, we applied these signatures to bulk Th and Treg RNA-sequencing data and found enrichment of specific Th and Treg cell populations in different human tissues. These results further define the molecular basis for the functional specialization and differentiation of Th and Treg cell populations and provide a new resource for examining Th and Treg specialization in RNA-sequencing data. ImmunoHorizons, 2020, 4: 585-596.

Tissue-resident memory T cells (TRM) persist locally in non-lymphoid tissues where they provide front-line defense against recurring insults. TRM at barrier surfaces express the markers CD103 and/or CD69 which function to retain them in epithelial tissues. In humans, neither the long-term migratory behavior of TRM nor their ability to re-enter the circulation and potentially migrate to distant tissue sites have been investigated. Using tissue explant cultures, we found that CD4 + CD69 + CD103 + TRM in human skin can downregulate CD69 and exit the tissue.Additionally, we identified a skin-tropic CD4 + CD69 -CD103 + population in human lymph and blood that is transcriptionally, functionally and clonally related to the CD4 + CD69 + CD103 + TRM population in the skin. Using a skin xenograft model, we confirmed that a fraction of the human cutaneous CD4 + CD103 + TRM population can re-enter circulation, and migrate to secondary human skin sites where they re-assume a TRM phenotype. Thus, our data challenge current concepts regarding the strict tissue compartmentalization of CD4 + T cell memory in humans.

Background Helicobacter pylori (H. pylori) is a gram-negative bacterium that chronically infects approximately 50% of the world’s human population. While in most cases the infection remains asymptomatic, 10% of infected individuals develop gastric pathologies and 1–3% progress to gastric cancer. AlthoughH. pylori induces severe inflammatory responses, the host’s immune system fails to clear the pathogen and H. pylori can persist in the human stomach for decades. As suppressor of cytokine signaling (SOCS) proteins are important feedback regulators limiting inflammatory responses, we hypothesized thatH. pylori could modulate the host’s immune responses by inducing SOCS expression. Methods The phenotype of human monocyte-derived DCs (moDCs) infected withH. pylori was analyzed by flow cytometry and multiplex technology. SOCS expression levels were monitored by qPCR and signaling studies were conducted by means of Western blot. For functional studies, RNA interference-based silencing of SOCS1–3 and co-cultures with CD4+ T cells were performed. Results We show that H. pylori positive gastritis patients express significantly higher SOCS3, but not SOCS1 andSOCS2, levels compared to H. pylori negative patients. Moreover, infection of human moDCs with H. pylori rapidly inducesSOCS3 expression, which requires the type IV secretion system (T4SS), release of TNFα, and signaling via the MAP kinase p38, but appears to be independent of TLR2, TLR4, MEK1/2 and STAT proteins. Silencing of SOCS3 expression in moDCs prior to H. pylori infection resulted in increased release of both pro- and anti-inflammatory cytokines, upregulation of PD-L1, and decreased T-cell proliferation. Conclusions This study shows that H. pylori induces SOCS3 via an autocrine loop involving the T4SS and TNFα and p38 signaling. Moreover, we demonstrate that high levels of SOCS3 in DCs dampen PD-L1 expression on DCs, which in turn drives T-cell proliferation.

Human skin contains a population of memory T cells that supports tissue homeostasis and provides protective immunity. The study of human memory T cells is often restricted to in vitro studies and to human PBMC serving as primary cell source. Because the tissue environment impacts the phenotype and function of memory T cells, it is crucial to study these cells within their tissue. Here we utilized immunodeficient NOD-scid IL2rγnull (NSG) mice that carried in vivo-generated engineered human skin (ES). ES was generated from human keratinocytes and fibroblasts and was initially devoid of skin-resident immune cells. Upon adoptive transfer of human PBMC, this reductionist system allowed us to study human T cell recruitment from a circulating pool of T cells into non-inflamed human skin in vivo. Circulating human memory T cells preferentially infiltrated ES and showed diverse functional profiles of T cells found in fresh human skin. The chemokine and cytokine microenvironment of ES closely resembled that of non-inflamed human skin. Upon entering the ES T cells assumed a resident memory T cell-like phenotype in the absence of infection, and a proportion of these cutaneous T cells can be locally activated upon injection of monocyte derived dendritic cells (moDCs) that presented Candida albicans. Interestingly, we found that CD69+ memory T cells produced higher levels of effector cytokines in response to Candida albicans, compared to CD69- T cells. Overall, this model has broad utility in many areas of human skin immunology research, including the study of immune-mediated skin diseases.

Human skin contains a population of memory T cells that support tissue homeostasis and provide protective immunity. T cell function in human skin is difficult to study due to a lack of adequate in vivo models. Thus, we utilized immunodeficient NOD-scid IL2rγ null (NSG) mice that carried in vivo-generated engineered skin (ES) and received adoptively transferred human peripheral blood mononuclear cells to follow the migration and function of circulating human memory T cells in human ES in vivo. ES were generated from human keratinocytes and fibroblasts only and initially contained no skin-resident immune cells. This reductionist system allowed us to study human T cell recruitment into non-inflamed and non-infected human skin, reflecting physiological skin conditions. Circulating human memory T cells preferentially infiltrated the human ES and these had the diverse functional profiles of T cells found in fresh human skin. Importantly, the chemokine and cytokine microenvironment found in the ES closely resembled that of non-inflamed human skin, which supports T cell maintenance and function. Upon entering the ES T cells assumed a resident memory T celllike phenotype in the absence of infection and these cutaneous T cells were activated locally upon injection of Candida albicans that was presented by autologous monocyte derived dendritic cells within the human ES. In summary, we have generated a novel mouse model with broad utility in studies of the dynamics of human cutaneous memory T cell generation, antigen-specific memory T cell responses, and the role of the skin microenvironment to skin immunity in vivo.

After activation, CD4 + T helper (Th) cells differentiate into functionally specialized populations that coordinate distinct immune responses and protect against different types of pathogens. In humans, these effector and memory Th cell subsets can be readily identified in peripheral blood based on their differential expression of chemokine receptors that govern their homeostatic and inflammatory trafficking. Foxp3 + regulatory T (Treg) cells can also be divided into subsets that phenotypically mirror each of these effector populations, and share expression of key transcription factors and effector cytokines. In this study, we performed comprehensive transcriptional profiling of 11 phenotypically distinct Th and Treg cell subsets sorted from peripheral blood of healthy individuals. Despite their shared phenotypes, we found that mirror Th and Treg subsets were transcriptionally dissimilar, and that Treg cell populations showed limited transcriptional diversity compared to Th cells. We identified core transcriptional signatures shared across all Th and Treg cell populations, and unique signatures that define each of the Th or Treg populations. Finally, we applied these signatures to bulk Th and Treg RNA-seq data and found enrichment of specific Th and Treg cell populations in different human tissues. These results further define the molecular basis for the functional specialization and differentiation of Th and Treg cell populations, and provide a new resource for examining Th and Treg specialization in RNA-seq data.