Heterogeneity and plasticity of tissue-resident memory T cells in skin diseases and homeostasis: a review

Skin tissue-resident memory T (Trm) cells are produced by antigenic stimulation and remain in the skin for a long time without entering the peripheral circulation. In the healthy state Trm cells can play a patrolling and surveillance role, but in the disease state Trm cells differentiate into various phenotypes associated with different diseases, exhibit different localizations, and consequently have local protective or pathogenic roles, such as disease recurrence in vitiligo and maintenance of immune homeostasis in melanoma. The most common surface marker of Trm cells is CD69/CD103. However, the plasticity of tissue-resident memory T cells after colonization remains somewhat uncertain. This ambiguity is largely due to the variation in the functionality and ultimate destination of Trm cells produced from memory cells differentiated from diverse precursors. Notably, the presence of Trm cells is not stationary across numerous non-lymphoid tissues, most notably in the skin. These cells may reenter the blood and distant tissue sites during the recall response, revealing the recycling and migration potential of the Trm cell progeny. This review focuses on the origin and function of skin Trm cells, and provides new insights into the role of skin Trm cells in the treatment of autoimmune skin diseases, infectious skin diseases, and tumors.

Research progress on intestinal tissue-resident memory T cells in inflammatory bowel disease

Tissue-resident memory T (TRM) cells are a recently discovered subpopulation of memory T cells that reside in non-lymphoid tissues such as the intestine and skin and do not enter the bloodstream. The intestine encounters numerous pathogens daily. Intestinal mucosal immunity requires a balance between immune responses to pathogens and tolerance to food antigens and symbiotic microbiota. Therefore, intestinal TRM cells exhibit unique characteristics. In healthy intestines, TRM cells induce necessary inflammation to strengthen the intestinal barrier and inhibit bacterial translocation. During intestinal infections, TRM cells rapidly eliminate pathogens by proliferating, releasing cytokines, and recruiting other immune cells. Moreover, certain TRM cell subsets may have regulatory functions. The involvement of TRM cells in inflammatory bowel disease (IBD) is increasingly recognized as a critical factor. In IBD, the number of pro-inflammatory TRM cells increases, whereas the number of regulatory subgroups decreases. Additionally, the classic markers, CD69 and CD103, are not ideal for intestinal TRM cells. Here, we review the phenotype, development, maintenance, and function of intestinal TRM cells, as well as the latest findings in the context of IBD. Further understanding of the function of intestinal TRM cells and distinguishing their subgroups is crucial for developing therapeutic strategies to target these cells.

Gut tissue-resident memory T cells in coeliac disease

This mini-review describes observations of the 1990ies with culturing of gluten-specific and astrovirus-specific CD4⁺ T cells from duodenal biopsies from subjects who presumably had a long time between the exposure to gluten or astrovirus antigens and the sampling of the biopsy. In these studies, it was also observed that antigen-specific CD4⁺ T cells migrated out of the gut biopsies during overnight culture. The findings are suggestive of memory T cells in tissue which are resident, but which also can be mobilised on antigen stimulation. Of note, these findings were made years before the term tissue-resident memory T cells was invoked. Since that time, many observations have accumulated on these gut T cells, particularly the gluten-specific T cells, and we have insight into the turnover of CD4⁺ T cells in the gut lamina propria. These data make it evident that human antigen-specific CD4⁺ T cells that can be cultured from gut biopsies indeed are bone fide tissue-resident memory T cells.

Tissue-resident regulatory T cells accumulate at human barrier lymphoid organs

Regulatory T cells (Tregs) play a critical role in immune regulation and peripheral tolerance. While different types of Tregs have been identified in both mice and humans, much of our understanding about how these cells maintain immune homeostasis is derived from animal models. In this study, we examined two distinct human lymphoid organs to understand how repeated exposure to infections at the mucosal surface influences the phenotype and tissue localization of Tregs. We show that while Tregs in both tonsils and spleen express a tissue-resident phenotype, they accumulate in greater numbers in tonsils. Tonsillar-resident Tregs exhibit a highly suppressive phenotype with significantly increased expression of CD39, ICOS and CTLA-4 compared with their counterparts in circulation or in the spleen. Functionally, resident Tregs are able effectively to suppress T cell proliferation. We further demonstrate that tonsillar-resident Tregs share key features of T follicular helper cells. Spatial analysis reveals that the vast majority of resident Tregs are localized at the border of the T-zone and B cell follicle, as well as within the lymphocyte pockets enriched with resident memory T cells. Together our findings suggest that resident Tregs are strategically co-localized to maintain immune homeostasis at sites of recurrent inflammation.

Expansible residence decentralizes immune homeostasis

Metazoans relegate specific tasks to dedicated organs that are established early in development, occupy discrete anatomic locations, and typically remain fixed in size. The adult immune system arises from a centralized hematopoietic niche that maintains self-renewing potential^1,2, and upon maturation, becomes distributed throughout the body to monitor environmental perturbations, regulate tissue homeostasis, and mediate organism-wide defense. This study examines how immunity is integrated within adult mouse tissues while addressing issues of durability, expansibility, and contribution to organ cellularity. Focusing on antiviral T cell immunity, we observe durable maintenance of resident memory T cells (T_RM) up to 450 days after infection. Once established, resident T cells did not require the T cell receptor for survival or retention of a poised effector-like state. While resident memory indefinitely dominated most mucosal organs, surgical separation of parabiotic mice unexpectedly revealed a tissue-resident provenance for bloodborne effector memory T cells, and circulating memory slowly made substantial contributions to tissue immunity in some organs. Following additional microbial experiences via serial immunizations or pet shop mice co-housing, for most tissues we find tissue pliancy allows for the accretion of tissue-resident memory, without axiomatic erosion of preexisting antiviral T cell immunity. Extending these findings, we demonstrate tissue residence and organ pliancy are generalizable aspects underlying the homeostasis of innate and adaptive immunity. The immune system-at-large grows commensurate to microbial experience reaching up to 25% of visceral organ cellularity. Regardless of location, many white blood cell populations adopted a tissue residency program within nonlymphoid organs. Thus, residence, rather than renewal or recirculation, typifies nonlymphoid immune surveillance, and organs serve as a pliant storage reservoir that can accommodate the continuous expansion of the cellular immune system throughout life. While hematopoiesis (‘to make blood’) restores certain elements of the immune system, in parallel, nonlymphoid organs sustain an accrual of durable tissue-autonomous cellular immunity, resulting in the progressive decentralization of organismal immune homeostasis.

CD4+ T cells persist for years in the human small intestine and display a TH1 cytokine profile

Studies in mice and humans have shown that CD8⁺ T cell immunosurveillance in non-lymphoid tissues is dominated by resident populations. Whether CD4⁺ T cells use the same strategies to survey peripheral tissues is less clear. Here, examining the turnover of CD4⁺ T cells in transplanted duodenum in humans, we demonstrate that the majority of CD4⁺ T cells were still donor-derived one year after transplantation. In contrast to memory CD4⁺ T cells in peripheral blood, intestinal CD4⁺ T_RM cells expressed CD69 and CD161, but only a minor fraction expressed CD103. Functionally, intestinal CD4⁺ T_RM cells were very potent cytokine producers; the vast majority being polyfunctional T_H1 cells, whereas a minor fraction produced IL-17. Interestingly, a fraction of intestinal CD4⁺ T cells produced granzyme-B and perforin after activation. Together, we show that the intestinal CD4⁺ T-cell compartment is dominated by resident populations that survive for more than 1 year. This finding is of high relevance for the development of oral vaccines and therapies for diseases in the gut.

Defined Intestinal Regions Are Drained by Specific Lymph Nodes That Mount Distinct Th1 and Th2 Responses Against Schistosoma mansoni Eggs

The balance of type 1 and type 2 immune responses plays a crucial role in anti-helminth immunity and can either support chronic infection or drive type 2 mediated expulsion of the parasite. Helminth antigens and secreted molecules directly influence this balance and induce a favorable immunological environment for the parasite’s survival. However, less is known if the site of infection also influences the balance of type 1 and type 2 immunity. Here, we report that tissue-specific immune responses are mounted against helminth antigens, which elicited strong IL-4 responses when injected into the skin, while the same antigen, delivered into the intestinal subserosa, induced increased IFN-γ and reduced Th2 responses. Immune responses in individual mesenteric lymph nodes that drain defined regions of the intestine furthermore displayed a site-specific pattern of type 1 and type 2 immunity after Schistosoma mansoni or Heligmosomoides polygyrus infection. S. mansoni egg-specific Th2 responses were detectable in all mesenteric lymph nodes but Th1 responses were only present in those draining the colon, while H. polygyrus infection elicited mixed Th1 and Th2 responses in the lymph nodes associated with the site of infection. Similar site-specific type 1 and type 2 immune responses were observed in the draining lymph nodes after the controlled delivery of S. mansoni eggs into different segments of the small and large intestine using microsurgical techniques. Different subsets of intestinal dendritic cells were hereby responsible for the uptake and priming of Th1 and Th2 responses against helminth antigens. Migratory CD11b⁺CD103⁻ and especially CD11b⁺CD103⁺ DC2s transported S. mansoni egg antigens to the draining lymph nodes to induce Th1 and Th2 responses, while CD103⁺ DC1s induced only IFN-γ responses. In contrast, H. polygyrus antigens were predominantly transported by CD11b⁺CD103⁻ DC2s and CD103⁺ DC1s and all DC subsets induced similar Th1 but weaker Th2 responses, compared to S. mansoni egg antigens. The development of adaptive anti-helminth immune responses is therefore influenced by the antigen itself, the uptake and priming characteristics of antigen-positive dendritic cell subsets and the site of infection, which shape the level of Th1 and Th2 responses in order to create a favorable immunological environment for the parasite.

Tissue-resident memory-like T cells in tumor immunity: Clinical implications

Tissue-resident memory (T_RM) T cells are distinct population of non-circulating lymphocytes that play an important role in mediating regional immunity. T_RM- like cells have now been identified as a component of tumor-infiltrating lymphocytes in several human tumors and correlate with outcome and response to immunotherapy. T_RM cells have also been shown to mediate anti-tumor immunity in murine models. Biology of T_RM cells has several implications for clinical cancer immunotherapy. Here we discuss newer insights into the biology of T_RM T cells and discuss their implications for understanding the heterogeneity of immune microenvironment in tumors as well as improving the efficacy of cancer vaccines, immune-checkpoint blockade and adoptive cellular therapies in the clinic.

Tissue-Resident Memory T Cells in Mice and Humans: Towards a Quantitative Ecology

In recent years, tissue-resident memory T cells (T_RM) have emerged as essential components of immunological memory. Following antigenic challenge, T_RM remain in nonlymphoid tissues and defend against re-exposure. Although accumulating evidence suggests important roles for T_RM in mediating protective immunity, fundamental aspects of the population biology of T_RM remain poorly understood. In this article, we discuss how results from different systems shed light on the ecological dynamics of T_RM in mice and humans. We highlight the importance of dissecting processes contributing to T_RM maintenance, and how these might vary across phenotypically and spatially heterogeneous subsets. We also discuss how the diversity of T_RM communities within specific tissues may evolve under competition and in response to antigenic perturbation. Throughout, we illustrate how mathematical models can clarify inferences obtained from experimental data and help elucidate the homeostatic mechanisms underpinning the ecology of T_RM populations.

Distribution and storage of inflammatory memory in barrier tissues

Memories of previous immune events enable barrier tissues to rapidly recall distinct environmental exposures. To effectively inform future responses, these past experiences can be stored in cell types that are long-term residents or essential constituents of tissues. There is an emerging understanding that, in addition to antigen-specific immune cells, diverse haematopoietic, stromal, parenchymal and neuronal cell types can store inflammatory memory. Here, we explore the impact of previous immune activity on various cell lineages with the goal of presenting a unified view of inflammatory memory to environmental exposures (such as allergens, antigens, noxious agents and microorganisms) at barrier tissues. We propose that inflammatory memory is distributed across diverse cell types and stored through shifts in cell states, and we provide a framework to guide future experiments. This distribution and storage may promote adaptation or maladaptation in homeostatic, maintenance and disease settings - especially if the distribution of memory favours cellular cooperation during storage or recall.

Memory CD4+ T Cells in Immunity and Autoimmune Diseases

CD4⁺ T helper (Th) cells play central roles in immunity in health and disease. While much is known about the effector function of Th cells in combating pathogens and promoting autoimmune diseases, the roles and biology of memory CD4⁺ Th cells are complex and less well understood. In human autoimmune diseases such as multiple sclerosis (MS), there is a critical need to better understand the function and biology of memory T cells. In this review article we summarize current concepts in the field of CD4⁺ T cell memory, including natural history, developmental pathways, subsets, and functions. Furthermore, we discuss advancements in the field of the newly-described CD4⁺ tissue-resident memory T cells and of CD4⁺ memory T cells in autoimmune diseases, two major areas of important unresolved questions in need of answering to advance new vaccine design and development of novel treatments for CD4⁺ T cell-mediated autoimmune diseases.

Isolation and functional characterisation of lamina propria leukocytes from helminth-infected, murine small intestine

The use of helminth infections as tools to understand the type 2 immune response is a well-established technique and important to many areas of immunological research. The phenotype and function of immune cell populations at the site of infection is a key determinant of pathogen clearance. However, infections with helminths such as the murine nematode Heligomosmoides polygryrus cause increased mucus production and thickening of the intestinal wall, which can result in extensive cell death when isolating and analysing cells from the lamina propria (LP). Populations of larger immune cells such as macrophages and dendritic cells are often trapped within mucus or dying tissues. Here we describe an optimised protocol for isolating LP leukocytes from the small intestine of H.polygyrus -infected mice, and we demonstrate phenotypic and functional identification of myeloid and CD4+ T cell subsets using cytokine staining and flow cytometry. Our protocol may provide a useful experimental method for the immunological analysis of the affected tissue site during helminth infections.