Cutting edge: gut microenvironment promotes differentiation of a unique memory CD8 T cell population

Regulation of Effector and Memory CD8 T Cell Differentiation by IL-2-A Balancing Act

Interleukin-2 (IL-2) regulates key aspects of CD8 T cell biology–signaling through distinct pathways IL-2 triggers critical metabolic and transcriptional changes that lead to a spectrum of physiological outcomes such as cell survival, proliferation, and effector differentiation. In addition to driving effector differentiation, IL-2 signals are also critical for formation of long-lived CD8 T cell memory. This review discusses a model of rheostatic control of CD8 T cell effector and memory differentiation by IL-2, wherein the timing, duration, dose, and source of IL-2 signals are considered in fine-tuning the balance of key transcriptional regulators of cell fate.

Transcriptional and Epigenetic Regulation of Effector and Memory CD8 T Cell Differentiation

Immune protection and lasting memory are accomplished through the generation of phenotypically and functionally distinct CD8 T cell subsets. Understanding how these effector and memory T cells are formed is the first step in eventually manipulating the immune system for therapeutic benefit. In this review, we will summarize the current understanding of CD8 T cell differentiation upon acute infection, with a focus on the transcriptional and epigenetic regulation of cell fate decision and memory formation. Moreover, we will highlight the importance of high throughput sequencing approaches and single cell technologies in providing insight into genome-wide investigations and the heterogeneity of individual CD8 T cells.

Resident-Memory T Cells in Tissue-Restricted Immune Responses: For Better or Worse?

Tissue-resident-memory CD8+ T cells (T_RM) have been described as a non-circulating memory T cell subset that persists at sites of previous infection. While T_RM in all non-lymphoid organs probably share a core signature differentiation pathway, certain aspects of their maintenance and effector functions may vary. It is well-established that T_RM provide long-lived protective immunity through immediate effector function and accelerated recruitment of circulating immune cells. Besides immune defense against pathogens, other immunological roles of T_RM are less well-studied. Likewise, evidence of a putative detrimental role of T_RM for inflammatory diseases is only beginning to emerge. In this review, we discuss the protective and harmful role of T_RM in organ-specific immunity and immunopathology as well as prospective implications for immunomodulatory therapy.

An Insight Into the Intestinal Web of Mucosal Immunity, Microbiota, and Diet in Inflammation

The rising global incidence of autoimmune and inflammatory conditions can be attributed to changes in the large portion of the immune system that belongs to our gastrointestinal tract (GI). The intestinal immune system serves as a gatekeeper to prevent pathogenic invasions and to preserve a healthier gut microbiota. The gut microbiota has been increasingly studied as a fundamental contributor to the state of health and disease. From food fermentation, the gut microbiota releases metabolites or short chain fatty acids (SCFAs), which have anti-inflammatory properties and preserve gut homeostasis. Immune responses against food and microbial antigens can cause inflammatory disorders such as inflammatory bowel disease (IBD) and celiac disease. As such, many autoimmune and inflammatory diseases also have a “gut origin”. A large body of evidence in recent years by ourselves and others has uncovered the link between the immune system and the SCFAs in specific diseases such as autoimmune type 1 diabetes (T1D), obesity and type 2 diabetes (T2D), cardiovascular disease, infections, allergies, asthma, and IBD. Thus, the power of these three gut dynamic components—the mucosal immunity, the microbiota, and diet—can be harnessed in tandem for the prevention and treatment of many inflammatory and infectious diseases.

Tissue-resident MAIT cell populations in human oral mucosa exhibit an activated profile and produce IL-17

Mucosa‐associated invariant T (MAIT) cells are unconventional T lymphocytes defined by their innate‐like characteristics and broad antimicrobial responsiveness. Whether MAIT cells are part of the tissue‐resident defense in the oral mucosal barrier is unknown. Here, we found MAIT cells present in the buccal mucosa, with a tendency to cluster near the basement membrane, and located in both epithelium and the underlying connective tissue. Overall MAIT cell levels were similar in the mucosa compared to peripheral blood, in contrast to conventional T cells that showed an altered representation of CD4⁺ and CD8⁺ subsets. The major mucosal MAIT cell subset displayed a tissue‐resident and activated profile with high expression of CD69, CD103, HLA‐DR, and PD‐1, as well as a skewed subset distribution with higher representation of CD4^–/CD8^– double‐negative cells and CD8αα⁺ cells. Interestingly, tissue‐resident MAIT cells had a specialized polyfunctional response profile with higher IL‐17 levels, as assessed by polyclonal stimulus and compared to tissue nonresident and circulating populations. Furthermore, resident buccal MAIT cells were low in perforin. Together, these data indicate that MAIT cells form a part of the oral mucosal T cell compartment, where they exhibit a tissue‐resident‐activated profile biased toward IL‐17 production.

Prime and target immunization protects against liver-stage malaria in mice

Despite recent advances in treatment and vector control, malaria is still a leading cause of death, emphasizing the need for an effective vaccine. The malaria life cycle can be subdivided into three stages: the invasion and growth within liver hepatocytes (pre-erythrocytic stage), the blood stage (erythrocytic stage), and, finally, the sexual stage (occurring within the mosquito vector). Antigen (Ag)-specific CD8+ T cells are effectively induced by heterologous prime-boost viral vector immunization and known to correlate with liver-stage protection. However, liver-stage malaria vaccines have struggled to generate and maintain the high numbers of Plasmodium-specific circulating T cells necessary to confer sterile protection. We describe an alternative “prime and target” vaccination strategy aimed specifically at inducing high numbers of tissue-resident memory T cells present in the liver at the time of hepatic infection. This approach bypasses the need for very high numbers of circulating T cells and markedly increases the efficacy of subunit immunization against liver-stage malaria with clinically relevant Ags and clinically tested viral vectors in murine challenge models. Translation to clinical use has begun, with encouraging results from a pilot safety and feasibility trial of intravenous chimpanzee adenovirus vaccination in humans. This work highlights the value of a prime-target approach for immunization against malaria and suggests that this strategy may represent a more general approach for prophylaxis or immunotherapy of other liver infections and diseases.

CD8+ Resident Memory T Cells and Viral Infection

Tissue-resident memory T (Trm) cells are a subset of recently identified memory T cells that mainly reside and serve as sentinels in non-lymphoid peripheral tissues. Unlike the well-characterized circulating central memory T (Tcm) cells and effector memory T (Tem) cells, Trm cells persist in the tissues, do not recirculate into blood, and offer immediate protection against pathogens upon reinfection. In this review, we focus on CD8⁺ Trm cells and briefly introduce their characteristics, development, maintenance, and function during viral infection. We also discuss some unresolved problems, such as how CD8⁺ Trm cells adapt to the local tissue microenvironment, how Trm cells interact with other immune cells during their development and maintenance, and the mechanisms by which CD8⁺ Trm cells confer immune protection. We believe that a better understanding of these problems is of great clinical and therapeutic value and may contribute to more effective vaccination and treatments against viral infection.

CD160 Stimulates CD8+ T Cell Responses and Is Required for Optimal Protective Immunity to Listeria monocytogenes

CD160 promotes NK cell cytotoxicity and IFN-γ production, but the function of CD160 on CD8⁺ T cells remains unclear with some studies supporting a coinhibitory role and others a costimulatory role. In this study, we demonstrate that CD160 has a costimulatory role in promoting CD8⁺ T cell effector functions needed for optimal clearance of oral Listeria monocytogenes infection. CD160^-/- mice did not clear oral L. monocytogenes as efficiently as wild type (WT) littermates. WT RAG^-/- and CD160^-/- RAG^-/- mice similarly cleared L. monocytogenes, indicating that CD160 on NK cells does not contribute to impaired L. monocytogenes clearance. Defective L. monocytogenes clearance is due to compromised intraepithelial lymphocytes and CD8⁺ T cell functions. There was a reduction in the frequencies of granzyme B-expressing intraepithelial lymphocytes in L. monocytogenes-infected CD160^-/- mice as compared with WT littermate controls. Similarly, the frequencies of granzyme B-expressing splenic CD8⁺ T cells and IFN-γ and TNF-α double-producer CD8⁺ T cells were significantly reduced in L. monocytogenes-infected CD160^-/- mice compared with WT littermates. Adoptive transfer studies showed that RAG^-/- recipients receiving CD160^-/- CD8⁺ T cells had a higher mortality, exhibited more weight loss, and had a higher bacterial burden compared with RAG^-/- recipients receiving WT CD8⁺ T cells. These findings demonstrate that CD160 provides costimulatory signals to CD8⁺ T cells needed for optimal CD8⁺ T cell responses and protective immunity during an acute mucosal bacterial infection.

Induction and maintenance of a phenotypically heterogeneous lung tissue-resident CD4+ T cell population following BCG immunisation

Tuberculosis (TB) is the biggest cause of human mortality from an infectious disease. The only vaccine currently available, bacille Calmette-Guérin (BCG), demonstrates some protection against disseminated disease in childhood but very variable efficacy against pulmonary disease in adults. A greater understanding of protective host immune responses is required in order to aid the development of improved vaccines. Tissue-resident memory T cells (T_RM) are a recently-identified subset of T cells which may represent an important component of protective immunity to TB. Here, we demonstrate that intradermal BCG vaccination induces a population of antigen-specific CD4⁺ T cells within the lung parenchyma which persist for >12 months post-vaccination. Comprehensive flow cytometric analysis reveals this population is phenotypically and functionally heterogeneous, and shares characteristics with lung vascular and splenic CD4⁺ T cells. This underlines the importance of utilising the intravascular staining technique for definitive identification of tissue-resident T cells, and also suggests that these anatomically distinct cellular subsets are not necessarily permanently resident within a particular tissue compartment but can migrate between compartments. This lung parenchymal population merits further investigation as a critical component of a protective immune response against Mycobacterium tuberculosis (M. tb).

Armed and Ready: Transcriptional Regulation of Tissue-Resident Memory 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 (T_RM) 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 T_RM cells to develop rapid recall responses is their location within the epithelia of the skin, lungs, and intestines at prime entry sites of pathogens. T_RM 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, T_RM 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 T_RM effector functions also includes the direct lysis of infected cells through the release of cytotoxic molecules such as perforin and granzymes. The mechanisms enabling T_RM cells to respond in such a rapid manner are gradually being uncovered. In this review, we will address the signals that instruct T_RM generation and maintenance as well as the underlying transcriptional network that keeps T_RM cells in a deployment-ready modus. Furthermore, we will discuss how T_RM cells respond to reinfection of the tissue and how transcription factors may control immediate and proliferative T_RM responses.

Developing effective vaccines: Cues from natural infection

The ultimate goal of any vaccine is to generate a heterogeneous and stable pool of memory lymphocytes. Vaccine are designed with the hope to generate antigen specific long-lived T cell responses, as it may be the case in natural infection; however, inducing such response by sub-unit vaccine has been a challenge. Although significant progress has been made, there is lot of scope for designing novel vaccine strategies by taking cues from the natural infection. This review focuses upon the roadblocks and the possible ways to overcome them leading to developing effective vaccines. Here we propose that mimicking the natural course of infection as well as the inclusion of non-target antigens in vaccine formulations might generate heterogeneous pool of memory T cells to ensure long-lived protection.

Listeria Monocytogenes: A Model Pathogen Continues to Refine Our Knowledge of the CD8 T Cell Response

Listeria monocytogenes (Lm) infection induces robust CD8 T cell responses, which play a critical role in resolving Lm during primary infection and provide protective immunity to re-infections. Comprehensive studies have been conducted to delineate the CD8 T cell response after Lm infection. In this review, the generation of the CD8 T cell response to Lm infection will be discussed. The role of dendritic cell subsets in acquiring and presenting Lm antigens to CD8 T cells and the events that occur during T cell priming and activation will be addressed. CD8 T cell expansion, differentiation and contraction as well as the signals that regulate these processes during Lm infection will be explored. Finally, the formation of memory CD8 T cell subsets in the circulation and in the intestine will be analyzed. Recently, the study of CD8 T cell responses to Lm infection has begun to shift focus from the intravenous infection model to a natural oral infection model as the humanized mouse and murinized Lm have become readily available. Recent findings in the generation of CD8 T cell responses to oral infection using murinized Lm will be explored throughout the review. Finally, CD8 T cell-mediated protective immunity against Lm infection and the use of Lm as a vaccine vector for cancer immunotherapy will be highlighted. Overall, this review will provide detailed knowledge on the biology of CD8 T cell responses after Lm infection that may shed light on improving rational vaccine design.

Human Tissue-Resident Memory T Cells Are Defined by Core Transcriptional and Functional Signatures in Lymphoid and Mucosal Sites

Tissue-resident memory T cells (TRMs) in mice mediate optimal protective immunity to infection and vaccination, while in humans, the existence and properties of TRMs remain unclear. Here, we use a unique human tissue resource to determine whether human tissue memory T cells constitute a distinct subset in diverse mucosal and lymphoid tissues. We identify a core transcriptional profile within the CD69⁺ subset of memory CD4⁺ and CD8⁺ T cells in lung and spleen that is distinct from that of CD69^- TEM cells in tissues and circulation and defines human TRMs based on homology to the transcriptional profile of mouse CD8⁺ TRMs. Human TRMs in diverse sites exhibit increased expression of adhesion and inhibitory molecules, produce both pro-inflammatory and regulatory cytokines, and have reduced turnover compared with circulating TEM, suggesting unique adaptations for in situ immunity. Together, our results provide a unifying signature for human TRM and a blueprint for designing tissue-targeted immunotherapies.

Tissue-resident memory T cells at the center of immunity to solid tumors

Immune responses in tissues are constrained by the physiological properties of the tissue involved. Tissue-resident memory T cells (T_RM cells) are a recently discovered lineage of T cells specialized for life and function within tissues. Emerging evidence has shown that T_RM cells have a special role in the control of solid tumors. A high frequency of T_RM cells in tumors correlates with favorable disease progression in patients with cancer, and studies of mice have shown that T_RM cells are necessary for optimal immunological control of solid tumors. Here we describe what defines T_RM cells as a separate lineage and how these cells are generated. Furthermore, we discuss the properties that allow T_RM cells to operate in normal and transformed tissues, as well as implications for the treatment of patients with cancer.

Tumor cure by radiation therapy and checkpoint inhibitors depends on pre-existing immunity

Radiation therapy is a source of tumor antigen release that has the potential to serve as an endogenous tumor vaccination event. In preclinical models radiation therapy synergizes with checkpoint inhibitors to cure tumors via CD8 T cell responses. To evaluate the immune response initiated by radiation therapy, we used a range of approaches to block the pre-existing immune response artifact initiated by tumor implantation. We demonstrate that blocking immune responses at tumor implantation blocks development of a tumor-resident antigen specific T cell population and prevents tumor cure by radiation therapy combined with checkpoint immunotherapy. These data demonstrate that this treatment combination relies on a pre-existing immune response to cure tumors, and may not be a solution for patients without pre-existing immunity.

Detection of HIV-1-specific gastrointestinal tissue resident CD8+ T-cells in chronic infection

Tissue-resident memory (TRM) CD8+ T-cells are non-recirculating, long-lived cells housed in tissues that can confer protection against mucosal pathogens. Human immunodeficiency virus-1 (HIV-1) is a mucosal pathogen and the gastrointestinal tract is an important site of viral pathogenesis and transmission. Thus, CD8+ TRM cells may be an important effector subset for controlling HIV-1 in mucosal tissues. This study sought to determine the abundance, phenotype, and functionality of CD8+ TRM cells in the context of chronic HIV-1 infection. We found that the majority of rectosigmoid CD8+ T-cells were CD69+CD103+S1PR1- and T-betLowEomesoderminNeg, indicative of a tissue-residency phenotype similar to that described in murine models. HIV-1-specific CD8+ TRM responses appeared strongest in individuals naturally controlling HIV-1 infection. Two CD8+ TRM subsets, distinguished by CD103 expression intensity, were identified. CD103Low CD8+ TRM primarily displayed a transitional memory phenotype and contained HIV-1-specific cells and cells expressing high levels of Eomesodermin, whereas CD103High CD8+ TRM primarily displayed an effector memory phenotype and were EomesoderminNeg. These findings suggest a large fraction of CD8+ T-cells housed in the human rectosigmoid mucosa are tissue-resident and that TRM contribute to the anti-HIV-1 immune response. Further exploration of CD8+ TRM will inform development of anti-HIV-1 immune-based therapies and vaccines targeted to the mucosa.

The function and dysfunction of memory CD8+ T cells in tumor immunity

The generation and maintenance of CD8+ T cell memory is crucial to long-term host survival, yet the basic tenets of CD8+ T cell immunity are still being established. Recent work has led to the discovery of tissue-resident memory cells and refined our understanding of the transcriptional and epigenetic basis of CD8+ T cell differentiation and dysregulation. In parallel, the unprecedented clinical success of immunotherapy has galvanized an intense, global research effort to decipher and de-repress the anti-tumor response. However, the progress of immunotherapy is at a critical juncture, since the efficacy of immuno-oncology agents remains confined to a fraction of patients and often fails to provide durable benefit. Unlocking the potential of immunotherapy requires the design of strategies that both induce a potent effector response and reliably forge stable, functional memory T cell pools capable of protecting from recurrence or relapse. It is therefore essential that basic and emerging concepts of memory T cell biology are rapidly and faithfully transposed to advance therapeutic development in cancer immunotherapy. This review highlights seminal and recent reports in CD8+ T cell memory and tumor immunology, and evaluates recent data from solid cancer specimens in the context of the key paradigms from preclinical models. We elucidate the potential significance of circulating effector cells poised downstream of neoantigen recognition and upstream of T cell dysfunction and propose that cells in this immunological 'sweet spot' may be key anti-tumor effectors.

Transcriptional programming of tissue-resident memory CD8+ T cells

Tissue-resident memory CD8⁺ T cells (T_RM) are localized in non-lymphoid tissues throughout the body where they mediate long-lived protective immunity at common sites of pathogen exposure. As the signals controlling T_RM differentiation are uncovered, it is becoming apparent that the dynamic activities of numerous transcription factors are intricately involved in T_RM formation. Here, we highlight known transcriptional regulators of T_RM differentiation and discuss how understanding the transcriptional programming of CD8⁺ T cell residency in non-lymphoid tissues can be leveraged to prevent or treat disease.

Intestinal Intraepithelial Lymphocytes: Sentinels of the Mucosal Barrier

Intestinal intraepithelial lymphocytes (IELs) are a large and diverse population of lymphoid cells that reside between the intestinal epithelial cells (IECs) that form the intestinal mucosal barrier. Although IEL biology has traditionally focused on T cells, recent studies have identified several subsets of T cell receptor (TCR)-negative IELs with intriguing properties. New insight into the development, homeostasis, and functions of distinct IEL subsets has recently been provided. Additional studies have revealed intricate interactions between different IEL subsets, reciprocal interactions between IELs and IECs, and communication of IELs with immune cells that reside outside the intestinal epithelium. We review here sentinel functions of IELs in the maintenance of the mucosal barrier integrity, as well as how dysregulated IEL responses can contribute to pathology.

T Cells in Nonlymphoid Tissues Give Rise to Lymph-Node-Resident Memory T Cells

Immunosurveillance of secondary lymphoid organs (SLO) is performed by central memory T cells that recirculate through blood. Resident memory T (Trm) cells remain parked in nonlymphoid tissues and often stably express CD69. We recently identified Trm cells within SLO, but the origin and phenotype of these cells remains unclear. Using parabiosis of "dirty" mice, we found that CD69 expression is insufficient to infer stable residence of SLO Trm cells. Restimulation of nonlymphoid memory CD8+ T cells within the skin or mucosa resulted in a substantial increase in bona fide Trm cells specifically within draining lymph nodes. SLO Trm cells derived from emigrants from nonlymphoid tissues and shared some transcriptional and phenotypic signatures associated with nonlymphoid Trm cells. These data indicate that nonlymphoid cells can give rise to SLO Trm cells and suggest vaccination strategies by which memory CD8+ T cell immunosurveillance can be regionalized to specific lymph nodes.

Trials and Tribble-ations of tissue TRM cells

CD8+ tissue-resident memory T cells (TRM cells) in two mucosal tissues, the skin and the female reproductive tract, proliferate in situ to generate a secondary pool of TRM cells that does not exit into the circulation.

Intravital mucosal imaging of CD8+ resident memory T cells shows tissue-autonomous recall responses that amplify secondary memory

CD8+ T cell immunosurveillance dynamics influence the outcome of intracellular infections and cancer. Here we used two-photon intravital microscopy to visualize the responses of CD8+ resident memory T cells (TRM cells) within the reproductive tracts of live female mice. We found that mucosal TRM cells were highly motile, but paused and underwent in situ division after local antigen challenge. TRM cell reactivation triggered the recruitment of recirculating memory T cells that underwent antigen-independent TRM cell differentiation in situ. However, the proliferation of pre-existing TRM cells dominated the local mucosal recall response and contributed most substantially to the boosted secondary TRM cell population. We observed similar results in skin. Thus, TRM cells can autonomously regulate the expansion of local immunosurveillance independently of central memory or proliferation in lymphoid tissue.

Incomplete Memories: The Natural Suppression of Tissue-Resident Memory CD8 T Cells in the Lung

The yearly, cyclic impact of viruses like influenza on human health and the economy is due to the high rates of mutation of traditional antibody targets, which negate any preexisting humoral immunity. However, the seasonality of influenza infections can equally be attributed to an absent or defective memory CD8 T cell response since the epitopes recognized by these cells are derived from essential virus proteins that mutate infrequently. Experiments in mouse models show that protection from heterologous influenza infection is temporally limited and conferred by a population of tissue-resident memory (TRM) cells residing in the lung and lung airways. TRM are elicited by a diverse set of pathogens penetrating mucosal barriers and broadly identified by extravascular staining and expression of the activation and adhesion molecules CD69 and CD103. Interestingly, lung TRM fail to express these molecules, which could limit tissue retention, resulting in airway expulsion or death with concomitant loss of heterologous protection. Here, we make the case that respiratory infections uniquely evoke a form of natural immunosuppression whereby specific cytokines and cell-cell interactions negatively impact memory cell programming and differentiation. Respiratory memory is not only short-lived but most of the memory cells in the lung parenchyma may not be bona fide TRM. Given the quantity of microbes humans inhale over a lifetime, limiting cellular residence could be a mechanism employed by the respiratory tract to preserve organismal vitality. Therefore, successful efforts to improve respiratory immunity must carefully and selectively breach these inherent tissue barriers.

Long-lived tissue resident HIV-1 specific memory CD8+ T cells are generated by skin immunization with live virus vectored microneedle arrays

The generation of tissue resident memory (TRM) cells at the body surfaces to provide a front line defence against invading pathogens represents an important goal in vaccine development for a wide variety of pathogens. It has been widely assumed that local vaccine delivery to the mucosae is necessary to achieve that aim. Here we characterise a novel micro-needle array (MA) delivery system fabricated to deliver a live recombinant human adenovirus type 5 vaccine vector (AdHu5) encoding HIV-1 gag. We demonstrate rapid dissolution kinetics of the microneedles in skin. Moreover, a consequence of MA vaccine cargo release was the generation of long-lived antigen-specific CD8+ T cells that accumulate in mucosal tissues, including the female genital and respiratory tract. The memory CD8+ T cell population maintained in the peripheral mucosal tissues was attributable to a MA delivered AdHu5 vaccine instructing CD8+ T cell expression of CXCR3+, CD103+, CD49a+, CD69+, CD127+ homing, retention and survival markers. Furthermore, memory CD8+ T cells generated by MA immunization significantly expanded upon locally administered antigenic challenge and showed a predominant poly-functional profile producing high levels of IFNγ and Granzyme B. These data demonstrate that skin vaccine delivery using microneedle technology induces mobilization of long lived, poly-functional CD8+ T cells to peripheral tissues, phenotypically displaying hallmarks of residency and yields new insights into how to design and deliver effective vaccine candidates with properties to exert local immunosurveillance at the mucosal surfaces.

Inhibition of JAK-STAT Signaling Suppresses Pathogenic Immune Responses in Medium and Large Vessel Vasculitis

BACKGROUND

Giant cell arteritis, a chronic autoimmune disease of the aorta and its large branches, is complicated by aneurysm formation, dissection, and arterial occlusions. Arterial wall dendritic cells attract CD4⁺ T cells and macrophages to form prototypic granulomatous infiltrates. Vasculitic lesions contain a diverse array of effector T cells that persist despite corticosteroid therapy and sustain chronic, smoldering vasculitis. Transmural inflammation induces microvascular neoangiogenesis and results in lumen-occlusive intimal hyperplasia. We have examined whether persistent vessel wall inflammation is maintained by lesional T cells, including the newly identified tissue-resident memory T cells, and whether such T cells are sensitive to the cytokine-signaling inhibitor tofacitinib, a Janus kinase (JAK) inhibitor targeting JAK3 and JAK1.

METHODS

Vascular inflammation was induced in human arteries engrafted into immunodeficient mice that were reconstituted with T cells and monocytes from patients with giant cell arteritis. Mice carrying inflamed human arteries were treated with tofacitinib or vehicle. Vasculitic arteries were examined for gene expression (reverse transcription polymerase chain reaction), protein expression (immunohistochemistry), and infiltrating cell populations (flow cytometry).

RESULTS

Tofacitinib effectively suppressed innate and adaptive immunity in the vessel wall. Lesional T cells responded to tofacitinib with reduced proliferation rates (<10%) and minimal production of the effector molecules interferon-γ, interleukin-17, and interleukin-21. Tofacitinib disrupted adventitial microvascular angiogenesis, reduced outgrowth of hyperplastic intima, and minimized CD4⁺CD103⁺ tissue-resident memory T cells.

CONCLUSIONS

Cytokine signaling dependent on JAK3 and JAK1 is critically important in chronic inflammation of medium and large arteries. The JAK inhibitor tofacitinib effectively suppresses tissue-resident memory T cells and inhibits core vasculitogenic effector pathways.

Runx3 programs CD8+ T cell residency in non-lymphoid tissues and tumours

Tissue-resident memory CD8+ T (TRM) cells are found at common sites of pathogen exposure, where they elicit rapid and robust protective immune responses. However, the molecular signals that control TRM cell differentiation and homeostasis are not fully understood. Here we show that mouse TRM precursor cells represent a unique CD8+ T cell subset that is distinct from the precursors of circulating memory cell populations at the levels of gene expression and chromatin accessibility. Using computational and pooled in vivo RNA interference screens, we identify the transcription factor Runx3 as a key regulator of TRM cell differentiation and homeostasis. Runx3 was required to establish TRM cell populations in diverse tissue environments, and supported the expression of crucial tissue-residency genes while suppressing genes associated with tissue egress and recirculation. Furthermore, we show that human and mouse tumour-infiltrating lymphocytes share a core tissue-residency gene-expression signature with TRM cells that is associated with Runx3 activity. In a mouse model of adoptive T cell therapy for melanoma, Runx3-deficient CD8+ tumour-infiltrating lymphocytes failed to accumulate in tumours, resulting in greater rates of tumour growth and mortality. Conversely, overexpression of Runx3 enhanced tumour-specific CD8+ T cell abundance, delayed tumour growth, and prolonged survival. In addition to establishing Runx3 as a central regulator of TRM cell differentiation, these results provide insight into the signals that promote T cell residency in non-lymphoid sites, which could be used to enhance vaccine efficacy or adoptive cell therapy treatments that target cancer.

Blockade of αEβ7 integrin suppresses accumulation of CD8+ and Th9 lymphocytes from patients with IBD in the inflamed gut in vivo

OBJECTIVE

Therapeutically targeting lymphocyte adhesion is of increasing relevance in IBD. Yet, central aspects of the action of antiadhesion compounds are incompletely understood. We investigated the role of αEβ7 and α4β7 integrins and their blockade by vedolizumab and etrolizumab for trafficking of IBD T lymphocytes in an in vivo model of homing to and retention in the inflamed gut.

DESIGN

We explored integrin expression in patients with IBD by flow cytometry and immunohistochemistry, while regulation of integrins was studied in T cell cultures. The functional relevance of integrins was assessed by adhesion assays and a recently established humanised mouse model in dextran sodium sulfate-treated immunodeficient mice.

RESULTS

High expression of αEβ7 was noted on CD8⁺ and CD4⁺ Th9 cells, while α4β7 was expressed on CD8⁺, Th2 and Th17 cells. T cell receptor stimulation and transforming growth factor β were key inducers of αEβ7 on human T cells, while butyric acid suppressed αEβ7. In comparison to α4β7 blockade via vedolizumab, blockade of β7 via etrolizumab surrogate antibody superiorly reduced colonic numbers of CD8⁺ and Th9 cells in vivo after 3 hours, while no difference was noted after 0.5 hours. AEβ7 expression was higher on CD8⁺ T cells from patients with IBD under vedolizumab therapy.

CONCLUSIONS

AEβ7 is of key relevance for gut trafficking of IBD CD8⁺ T cells and CD4⁺ Th9 cells in vivo and mainly retention might account for this effect. These findings indicate that blockade of αEβ7 in addition to α4β7 may be particularly effective in intestinal disorders with expansion of CD8⁺ and Th9 cells such as IBD.

A Yin and Yang in Epithelial Immunology: The Roles of the αE(CD103)β7 Integrin in T Cells

The proper function(s) of cell-surface receptors is crucial for the regulation of adaptive immune responses. One such receptor is the α_E(CD103)β₇ integrin, whose history in science is closely linked with the evolution of our knowledge of immune regulation. Initially described as a marker of intraepithelial T-lymphocytes, this leukocyte integrin is now seen as a dynamically regulated receptor involved in the functional differentiation of some cytotoxic T cells as well as regulatory T cells, thus presumably contributing to the fine-tuning of immune reactions in epithelial compartments. In this brief overview, we delineate our current view on α_E(CD103)β₇ in T-cell-mediated immune responses.

IL-15 Complexes Induce Migration of Resting Memory CD8 T Cells into Mucosal Tissues

IL-15 is an essential cytokine known to promote T cell survival and activate the effector function of memory phenotype CD8 T cells. Blocking IL-15 signals also significantly impacts tissue-specific effector and memory CD8 T cell formation. In this study, we demonstrate that IL-15 influences the generation of memory CD8 T cells by first promoting their accumulation into mucosal tissues and second by sustaining expression of Bcl-6 and T-bet. We show that the mechanism for this recruitment is largely dependent on mammalian target of rapamycin and its subsequent inactivation of FoxO1. Last, we show that IL-15 complexes delivered locally to mucosal tissues without reinfection is an effective strategy to enhance establishment of tissue resident memory CD8 T cells within mucosal tissues. This study provides mechanistic insight into how IL-15 controls the generation of memory CD8 T cells and influences their trafficking and ability to take up residence within peripheral tissues.

Predominance of weakly cytotoxic, T-betLowEomesNeg CD8+ T-cells in human gastrointestinal mucosa: implications for HIV infection

The gastrointestinal mucosa is an important site of HIV acquisition, viral replication, and pathogenesis. Immune cells in mucosal tissues frequently differ in phenotype and function from their non-mucosal counterparts. Although perforin-mediated cytotoxicity as measured in blood is a recognized correlate of HIV immune control, its role in gastrointestinal tissues is unknown. We sought to elucidate the cytotoxic features of rectal mucosal CD8⁺ T-cells in HIV infected and uninfected subjects. Perforin expression and lytic capacity were significantly reduced in rectal CD8⁺ T-cells compared with their blood counterparts, regardless of HIV clinical status; granzyme B (GrzB) was reduced to a lesser extent. Mucosal perforin and GrzB expression were higher in participants not on antiretroviral therapy compared with those on therapy and controls. Reduction in perforin and GrzB was not explained by differences in memory/effector subsets. Expression of T-bet and Eomesodermin was significantly lower in gut CD8⁺ T-cells compared with blood, and in vitro neutralization of TGF-β partially restored perforin expression in gut CD8⁺ T-cells. These findings suggest that rectal CD8⁺ T-cells are primarily non-cytotoxic, and phenotypically shaped by the tissue microenvironment. Further elucidation of rectal immune responses to HIV will inform the development of vaccines and immunotherapies targeted to mucosal tissues.

Generating long-lived CD8(+) T-cell memory: Insights from epigenetic programs

T-cell-based immunological memory has the potential to provide the host with life-long protection against pathogen reexposure and thus offers tremendous promise for the design of vaccines targeting chronic infections or cancer. In order to exploit this potential in the design of new vaccines, it is necessary to understand how and when memory T cells acquire their poised effector potential, and moreover, how they maintain these properties during homeostatic proliferation. To gain insight into the persistent nature of memory T-cell functions, investigators have turned their attention to epigenetic mechanisms. Recent efforts have revealed that many of the properties acquired among memory T cells are coupled to stable changes in DNA methylation and histone modifications. Furthermore, it has recently been reported that the delineating features among memory T cells subsets are also linked to distinct epigenetic events, such as permissive and repressive histone modifications and DNA methylation programs, providing exciting new hypotheses regarding their cellular ancestry. Here, we review recent studies focused on epigenetic programs acquired during effector and memory T-cell differentiation and discuss how these data may shed new light on the developmental path for generating long-lived CD8(+) T-cell memory.

Tissue-resident and memory properties of human T-cell and NK-cell subsets

Efficient immune responses to invading pathogens are the result of the complex but coordinated synergy between a variety of cell types from both the innate and adaptive arms of the immune system. While adaptive and innate immune responses are highly complementary, some cells types within these two systems perform similar functions, underscoring the need for redundancy and increased flexibility. In this review, we will discuss the striking shared features of immunological memory and tissue residency recently discovered between T cells, a component of the adaptive immune system, and natural killer (NK) cells, members generally assigned to the innate compartment. Specifically, we will focus on the T-cell and NK-cell diversity at the single-cell level, on the discrete function of specific subsets, and on their anatomical location. Finally, we will discuss the implication of such diversity in the generation of long-term memory.

Understanding memory CD8+ T cells

Memory CD8+ T cells were originally thought to exist as two populations (effector and central memory). In recent years, a third population called resident memory T cells has been discovered and further to this these populations are being divided into different subtypes. Understanding the function and developmental pathways of memory CD8+ T cells is key to developing effective therapies against cancer and infectious diseases. Here we have reviewed what is currently known about all three subsets of memory CD8+ T populations and as to how each population was originally discovered and the developmental pathways of each subpopulation. Each memory population appears to play a distinct role in adaptive immune responses but we are still a long way from understanding how the populations are generated and what roles they play in protection against invading pathogens and if they contribute to the pathogenesis of inflammatory diseases.

Tissue adaptation: Implications for gut immunity and tolerance

Faria et al. discuss the concept that immune cells undergo specialized adaptation to tissue-specific conditions and its potential implications for tolerance and immunity.

Tissue adaptation is an intrinsic component of immune cell development, influencing both resistance to pathogens and tolerance. Chronically stimulated surfaces of the body, in particular the gut mucosa, are the major sites where immune cells traffic and reside. Their adaptation to these environments requires constant discrimination between natural stimulation coming from harmless microbiota and food, and pathogens that need to be cleared. This review will focus on the adaptation of lymphocytes to the gut mucosa, a highly specialized environment that can help us understand the plasticity of leukocytes arriving at various tissue sites and how tissue-related factors operate to shape immune cell fate and function.

The PD-1: PD-L1 pathway promotes development of brain-resident memory T cells following acute viral encephalitis

Background

Previous work from our laboratory has demonstrated that during acute viral brain infection, glial cells modulate antiviral T cell effector responses through the PD-1: PD-L1 pathway, thereby limiting the deleterious consequences of unrestrained neuroinflammation. Here, we evaluated the PD-1: PD-L1 pathway in development of brain-resident memory T cells (bT_RM) following murine cytomegalovirus (MCMV) infection.

Methods

Flow cytometric analysis of immune cells was performed at 7, 14, and 30 days post-infection (dpi) to assess the shift of brain-infiltrating CD8⁺ T cell populations from short-lived effector cells (SLEC) to memory precursor effector cells (MPEC), as well as generation of bT_RMs.

Results

In wild-type (WT) animals, we observed a switch in the phenotype of brain-infiltrating CD8⁺ T cell populations from KLRG1⁺ CD127⁻ (SLEC) to KLRG1⁻ CD127⁺ (MPEC) during transition from acute through chronic phases of infection. At 14 and 30 dpi, the majority of CD8⁺ T cells expressed CD127, a marker of memory cells. In contrast, fewer CD8⁺ T cells expressed CD127 within brains of infected, PD-L1 knockout (KO) animals. Notably, in WT mice, a large population of CD8⁺ T cells was phenotyped as CD103⁺ CD69⁺, markers of bT_RM, and differences were observed in the numbers of these cells when compared to PD-L1 KOs. Immunohistochemical studies revealed that brain-resident CD103⁺ bT_RM cells were localized to the parenchyma. Higher frequencies of CXCR3 were also observed among WT animals in contrast to PD-L1 KOs.

Conclusions

Taken together, our results indicate that bT_RMs are present within the CNS following viral infection and the PD-1: PD-L1 pathway plays a role in the generation of this brain-resident population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0860-3) contains supplementary material, which is available to authorized users.

Skin-resident CD4+ T cells protect against Leishmania major by recruiting and activating inflammatory monocytes

Tissue-resident memory T cells are required for establishing protective immunity against a variety of different pathogens, although the mechanisms mediating protection by CD4+ resident memory T cells are still being defined. In this study we addressed this issue with a population of protective skin-resident, IFNγ-producing CD4+ memory T cells generated following Leishmania major infection. We previously found that resident memory T cells recruit circulating effector T cells to enhance immunity. Here we show that resident memory CD4+ T cells mediate the delayed-hypersensitivity response observed in immune mice and provide protection without circulating T cells. This protection occurs rapidly after challenge, and requires the recruitment and activation of inflammatory monocytes, which limit parasites by production of both reactive oxygen species and nitric oxide. Overall, these data highlight a novel role for tissue-resident memory cells in recruiting and activating inflammatory monocytes, and underscore the central role that skin-resident T cells play in immunity to cutaneous leishmaniasis.

Up-regulation of LFA-1 allows liver-resident memory T cells to patrol and remain in the hepatic sinusoids

Liver-resident CD8+ T cells are highly motile cells that patrol the vasculature and provide protection against liver pathogens. A key question is: how can these liver CD8+ T cells be simultaneously present in the circulation and tissue-resident? Because liver-resident T cells do not express CD103 - a key integrin for T cell residence in epithelial tissues - we investigated other candidate adhesion molecules. Using intra-vital imaging we found that CD8+ T cell patrolling in the hepatic sinusoids is dependent upon LFA-1-ICAM-1 interactions. Interestingly, liver-resident CD8+ T cells up-regulate LFA-1 compared to effector-memory cells, presumably to facilitate this behavior. Finally, we found that LFA-1 deficient CD8+ T cells failed to form substantial liver-resident memory populations following Plasmodium or LCMV immunization. Collectively, our results demonstrate that it is adhesion through LFA-1 that allows liver-resident memory CD8+ T cells to patrol and remain in the hepatic sinusoids.

Tissue reservoirs of antiviral T cell immunity in persistent human CMV infection

T cell responses to viruses are initiated and maintained in tissue sites; however, knowledge of human antiviral T cells is largely derived from blood. Cytomegalovirus (CMV) persists in most humans, requires T cell immunity to control, yet tissue immune responses remain undefined. Here, we investigated human CMV-specific T cells, virus persistence and CMV-associated T cell homeostasis in blood, lymphoid, mucosal and secretory tissues of 44 CMV seropositive and 28 seronegative donors. CMV-specific T cells were maintained in distinct distribution patterns, highest in blood, bone marrow (BM), or lymph nodes (LN), with the frequency and function in blood distinct from tissues. CMV genomes were detected predominantly in lung and also in spleen, BM, blood and LN. High frequencies of activated CMV-specific T cells were found in blood and BM samples with low virus detection, whereas in lung, CMV-specific T cells were present along with detectable virus. In LNs, CMV-specific T cells exhibited quiescent phenotypes independent of virus. Overall, T cell differentiation was enhanced in sites of viral persistence with age. Together, our results suggest tissue T cell reservoirs for CMV control shaped by both viral and tissue-intrinsic factors, with global effects on homeostasis of tissue T cells over the lifespan.

Characterization of resident lymphocytes in human pancreatic islets

The current view of type 1 diabetes (T1D) is that it is an immune-mediated disease where lymphocytes infiltrate the pancreatic islets, promote killing of beta cells and cause overt diabetes. Although tissue resident immune cells have been demonstrated in several organs, the composition of lymphocytes in human healthy pancreatic islets have been scarcely studied. Here we aimed to investigate the phenotype of immune cells associated with human islets of non-diabetic organ donors. A flow cytometry analysis of isolated islets from perfused pancreases (n = 38) was employed to identify alpha, beta, T, natural killer (NK) and B cells. Moreover, the expression of insulin and glucagon transcripts was evaluated by RNA sequencing. Up to 80% of the lymphocytes were CD3⁺ T cells with a remarkable bias towards CD8⁺ cells. Central memory and effector memory phenotypes dominated within the CD8⁺ and CD4⁺ T cells and most CD8⁺ T cells were positive for CD69 and up to 50-70% for CD103, both markers of resident memory cells. The frequency of B and NK cells was low in most islet preparations (12 and 3% of CD45⁺ cells, respectively), and the frequency of alpha and beta cells varied between donors and correlated clearly with insulin and glucagon mRNA expression. In conclusion, we demonstrated the predominance of canonical tissue resident memory CD8⁺ T cells associated with human islets. We believe that these results are important to understand more clearly the immunobiology of human islets and the disease-related phenotypes observed in diabetes.

Tissue Microenvironments in the Nasal Epithelium of Rainbow Trout (Oncorhynchus mykiss) Define Two Distinct CD8α+ Cell Populations and Establish Regional Immunity

Mucosal surfaces require balancing different physiological roles and immune functions. To effectively achieve multifunctionality, mucosal epithelia have evolved unique microenvironments that create unique regional immune responses without impairing other normal physiological functions. Whereas examples of regional immunity are known in other mucosal epithelia, to date, no immune microenvironments have been described in the nasal mucosa, a site where the complex functions of olfaction and immunity need to be orchestrated. In this study we identified the presence of CD8α⁺ cells in the rainbow trout (Oncorhynchus mykiss) nasal epithelium. Nasal CD8α⁺ cells display a distinct phenotype suggestive of CD8⁺ T cells with high integrin β₂ expression. Importantly, nasal CD8α⁺ cells are located in clusters at the mucosal tip of each olfactory lamella but scattered in the neuroepithelial region. The grouping of CD8α⁺ cells may be explained by the greater expression of CCL19, ICAM-1, and VCAM-1 in the mucosal tip compared with the neuroepithelium. Whereas viral Ag uptake occurred via both tip and lateral routes, tip-resident MHC class II⁺ cells are located significantly closer to the lumen of the nasal cavity than are their neuroepithelial counterparts, therefore having quicker access to invading pathogens. Our studies reveal compartmentalized mucosal immune responses within the nasal mucosa of a vertebrate species, a strategy that likely optimizes local immune responses while protecting olfactory sensory functions.

Optimal Generation of Tissue-Resident but Not Circulating Memory T Cells during Viral Infection Requires Crosspriming by DNGR-1+ Dendritic Cells

Despite the crucial role of tissue-resident memory T (Trm) cells in protective immunity, their priming remains poorly understood. Here, we have shown differential priming requirements for Trm versus circulating memory CD8⁺ T cells. In vaccinia cutaneous-infected mice, DNGR-1-mediated crosspresentation was required for optimal Trm cell priming but not for their skin differentiation or for circulating memory T cell generation. DNGR-1⁺ dendritic cells (DCs) promoted T-bet transcription-factor induction and retention of CD8⁺ T cells in the lymph nodes (LNs). Inhibition of LN egress enhanced Trm cell generation, whereas genetic or antibody blockade of DNGR-1 or specific signals provided during priming by DNGR-1⁺ DCs, such as interleukin-12 (IL-12), IL-15, or CD24, impaired Trm cell priming. DNGR-1 also regulated Trm cell generation during influenza infection. Moreover, protective immunity depended on optimal Trm cell induction by DNGR-1⁺ DCs. Our results reveal specific priming requirements for CD8⁺ Trm cells during viral infection and vaccination.

CMV-Specific CD8 T Cell Differentiation and Localization: Implications for Adoptive Therapies

Human cytomegalovirus (HCMV) is a ubiquitous virus that causes chronic infection and, thus, is one of the most common infectious complications of immune suppression. Adoptive transfer of HCMV-specific T cells has emerged as an effective method to reduce the risk for HCMV infection and/or reactivation by restoring immunity in transplant recipients. However, the CMV-specific CD8⁺ T cell response is comprised of a heterogenous mixture of subsets with distinct functions and localization, and it is not clear if current adoptive immunotherapy protocols can reconstitute the full spectrum of CD8⁺ T cell immunity. The aim of this review is to briefly summarize the role of these T cell subsets in CMV immunity and to describe how current adoptive immunotherapy practices might affect their reconstitution in patients. The bulk of the CMV-specific CD8⁺ T cell population is made up of terminally differentiated effector T cells with immediate effector function and a short life span. Self-renewing memory T cells within the CMV-specific population retain the capacity to expand and differentiate upon challenge and are important for the long-term persistence of the CD8⁺ T cell response. Finally, mucosal organs, which are frequent sites of CMV reactivation, are primarily inhabited by tissue-resident memory T cells, which do not recirculate. Future work on adoptive transfer strategies may need to focus on striking a balance between the formation of these subsets to ensure the development of long lasting and protective immune responses that can access the organs affected by CMV disease.

Memory T cells in cutaneous leishmaniasis

Leishmania causes a spectrum of diseases that range from self-healing to fatal infections. Control of leishmania is dependent upon generating CD4+ Th1 cells that produce IFNγ, leading to macrophage activation and killing of the intracellular parasites. Following resolution of the disease, short-lived effector T cells, as well as long-lived central memory T cells and skin resident memory T cells, are retained and able to mediate immunity to a secondary infection. However, there is no vaccine for leishmaniasis, and the drugs used to treat the disease can be toxic and ineffective. While a live infection generates immunity, a successful vaccine will depend upon generating memory T cells that can be maintained without the continued presence of parasites. Since both central memory and skin resident memory T cells are long-lived, they may be the appropriate targets for a leishmaniasis vaccine.

CCR7 expression alters memory CD8 T-cell homeostasis by regulating occupancy in IL-7- and IL-15-dependent niches

C-C receptor 7 (CCR7) is important to allow T cells and dendritic cells to migrate toward CCL19- and CCL21-producing cells in the T-cell zone of the spleen and lymph nodes. The role of this chemokine receptor in regulating the homeostasis of effector and memory T cells during acute viral infection is poorly defined, however. In this study, we show that CCR7 expression alters memory CD8 T-cell homeostasis following lymphocytic choriomeningitis virus infection. Greater numbers of CCR7-deficient memory T cells were formed and maintained compared with CCR7-sufficient memory T cells, especially in the lung and bone marrow. The CCR7-deficient memory T cells also displayed enhanced rates of homeostatic turnover, which may stem from increased exposure to IL-15 as a consequence of reduced exposure to IL-7, because removal of IL-15, but not of IL-7, normalized the numbers of CCR7-sufficient and CCR7-deficient memory CD8 T cells. This result suggests that IL-15 is the predominant cytokine supporting augmentation of the CCR7(-/-) memory CD8 T-cell pool. Taken together, these data suggest that CCR7 biases memory CD8 T cells toward IL-7-dependent niches over IL-15-dependent niches, which provides insight into the homeostatic regulation of different memory T-cell subsets.