T-box Transcription Factors Combine with the Cytokines TGF-β and IL-15 to Control Tissue-Resident Memory T Cell Fate

Human intrahepatic CD69 + CD8+ T cells have a tissue resident memory T cell phenotype with reduced cytolytic capacity

Tissue resident memory T cells (T_RM) have been identified in various tissues, however human liver T_RM to date remain unidentified. T_RM can be recognized by CD69 and/or CD103 expression and may play a role in the pathology of chronic hepatitis B (CHB) and hepatitis C virus infection (CHC). Liver and paired blood mononuclear cells from 17 patients (including 4 CHB and 6 CHC patients) were isolated and CD8+ T cells were comprehensively analysed by flowcytometry, immunohistochemistry and qPCR. The majority of intrahepatic CD8+ T cells expressed CD69, a marker used to identify T_RM, of which a subset co-expressed CD103. CD69 + CD8+ T cells expressed low levels of S1PR1 and KLF2 and a large proportion (>90%) was CXCR6+, resembling liver T_RM in mice and liver resident NK cells in human. Cytotoxic proteins were only expressed in a small fraction of liver CD69 + CD8+ T cells in patients without viral hepatitis, however, in livers from CHB patients more CD69 + CD8+ T cells were granzyme B+. In CHC patients, less intrahepatic CD69 + CD8+ T cells were Hobit+ as compared to CHB and control patients. Intrahepatic CD69 + CD8+ T cells likely T_RM which have a reduced cytolytic potential. In patients with chronic viral hepatitis T_RM have a distinct phenotype.

Enhanced anti-tumour immunity requires the interplay between resident and circulating memory CD8+ T cells

The goal of successful anti-tumoural immunity is the development of long-term protective immunity to prevent relapse. Infiltration of tumours with CD8⁺ T cells with a resident memory (Trm) phenotype correlates with improved survival. However, the interplay of circulating CD8⁺ T cells and Trm cells remains poorly explored in tumour immunity. Using different vaccination strategies that fine-tune the generation of Trm cells or circulating memory T cells, here we show that, while both subsets are sufficient for anti-tumour immunity, the presence of Trm cells improves anti-tumour efficacy. Transferred central memory T cells (Tcm) generate Trm cells following viral infection or tumour challenge. Anti-PD-1 treatment promotes infiltration of transferred Tcm cells within tumours, improving anti-tumour immunity. Moreover, Batf3-dependent dendritic cells are essential for reactivation of circulating memory anti-tumour response. Our findings show the plasticity, collaboration and requirements for reactivation of memory CD8⁺ T cells subsets needed for optimal tumour vaccination and immunotherapy.

Tissue-resident memory T cells live off the fat of the land

The consumption of exogenous free fatty acids by tissue-resident memory T (Trm) cells is critical for their long-term survival and antiviral function, and appears to be a conserved feature of Trm cells in both mouse and man, a recent paper published in Nature demonstrates.

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.

Distinct Transcriptional and Alternative Splicing Signatures of Decidual CD4+ T Cells in Early Human Pregnancy

Decidual CD4⁺ T (dCD4 T) cells are crucial for the maternal-fetal immune tolerance required for a healthy pregnancy outcome. However, their molecular and functional characteristics are not well elucidated. In this study, we performed the first analysis of transcriptional and alternative splicing (AS) landscapes for paired decidual and peripheral blood CD4⁺ T (pCD4 T) cells in human early pregnancy using high throughput mRNA sequencing. Our data showed that dCD4 T cells are endowed with a unique transcriptional signature when compared to pCD4 T cells: dCD4 T cells upregulate 1,695 genes enriched in immune system process whereas downregulate 1,011 genes mainly related to mRNA catabolic process and the ribosome. Moreover, dCD4 T cells were observed to be at M phase, and show increased activation, proliferation, and cytokine production, as well as display an effector-memory phenotype and a heterogenous nature containing Th1, Th17, and Treg cell subsets. However, dCD4 T cells undergo a comparable number of upregulated and downregulated AS events, both of which are enriched in the genes related to cellular metabolic process. And the changes at the AS event level do not reflect measurable differences at the gene expression level in dCD4 T cells. Collectively, our findings provide a comprehensive portrait of the unique transcriptional signature and AS profile of CD4⁺ T cells in human decidua and help us gain more understanding of the functional characteristic of these cells during early pregnancy.

Resident memory CD8+ T cells in the upper respiratory tract prevent pulmonary influenza virus infection

Nasal epithelial tissue of the upper respiratory tract is the first site of contact by inhaled pathogens such as influenza virus. We show that this region is key to limiting viral spread to the lower respiratory tract and associated disease pathology. Immunization of the upper respiratory tract leads to the formation of local tissue-resident memory CD8⁺ T cells (Trm cells). Unlike Trm cells in the lung, these cells develop independently of local cognate antigen recognition and transforming growth factor-β signaling and persist with minimal decay, representing a long-term protective population. Repertoire characterization revealed unexpected differences between lung and nasal tissue Trm cells, the composition of which was shaped by the developmental need for lung, but not nasal, Trm cells to recognize antigen within their local tissue. We show that influenza-specific Trm cells in the nasal epithelia can block the transmission of influenza virus from the upper respiratory tract to the lung and, in doing so, prevent the development of severe pulmonary disease. Our findings reveal the protective capacity and longevity of upper respiratory tract Trm cells and highlight the potential of targeting these cells to augment protective responses induced to respiratory viral vaccines.

Regulation of the Immune Response by TGF-β: From Conception to Autoimmunity and Infection

Transforming growth factor β (TGF-β) is a pleiotropic cytokine involved in both suppressive and inflammatory immune responses. After 30 years of intense study, we have only begun to elucidate how TGF-β alters immunity under various conditions. Under steady-state conditions, TGF-β regulates thymic T-cell selection and maintains homeostasis of the naïve T-cell pool. TGF-β inhibits cytotoxic T lymphocyte (CTL), Th1-, and Th2-cell differentiation while promoting peripheral (p)Treg-, Th17-, Th9-, and Tfh-cell generation, and T-cell tissue residence in response to immune challenges. Similarly, TGF-β controls the proliferation, survival, activation, and differentiation of B cells, as well as the development and functions of innate cells, including natural killer (NK) cells, macrophages, dendritic cells, and granulocytes. Collectively, TGF-β plays a pivotal role in maintaining peripheral tolerance against self- and innocuous antigens, such as food, commensal bacteria, and fetal alloantigens, and in controlling immune responses to pathogens.

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.

Persistence in Temporary Lung Niches: A Survival Strategy of Lung-Resident Memory CD8+ T Cells

Respiratory virus infections, such as those mediated by influenza virus, parainfluenza virus, respiratory syncytial virus (RSV), severe acute respiratory syndrome coronavirus (SARS-CoV), rhinovirus, and adenovirus, are responsible for substantial morbidity and mortality, especially in children and older adults. Furthermore, the potential emergence of highly pathogenic strains of influenza virus poses a significant public health threat. Thus, the development of vaccines capable of eliciting long-lasting protective immunity to those pathogens is a major public health priority. CD8⁺ Tissue-resident memory T (T_RM) cells are a newly defined population that resides permanently in the nonlymphoid tissues including the lung. These cells are capable of providing local protection immediately after infection, thereby promoting rapid host recovery. Recent studies have offered new insights into the anatomical niches that harbor lung CD8⁺ T_RM cells, and also identified the requirement and limitations of T_RM maintenance. However, it remains controversial whether lung CD8⁺ T_RM cells are continuously replenished by new cells from the circulation or permanently lodged in this site. A better understanding of how lung CD8⁺ T_RM cells are generated and maintained and the tissue-specific factors that drive local T_RM formation is required for optimal vaccine development. This review focuses on recent advance in our understanding of CD8⁺ T_RM cell establishment and maintenance in the lung, and describes how those processes are uniquely regulated in this tissue.

Influenza-induced lung Trm: not all memories last forever

In the light of new findings that lung tissue resident memory CD8⁺ T cells (T_rm) represent major mediators of heterosubtypic immunity against influenza virus, it is of utmost importance to understand the basic biological mechanisms behind induction, formation and maintenance of this cell population. Addressing these important knowledge gaps will potentially inform development of superior, broadly protective influenza vaccines and new immunization strategies.

Novel Insights into the Mechanisms of Gut Homing and Antiadhesion Therapies in Inflammatory Bowel Diseases

Therapeutic compounds interfering with T cell trafficking are a new column of inflammatory bowel disease (IBD) treatment. Currently, the anti-α4β7 integrin antibody vedolizumab is successfully used in the clinic and further drugs are likely to follow. Despite these clinical advances, the precise mechanistic background of their action is only gradually elucidated and still a matter of intensive research. Only recently, advances made with the help of new in vivo models and human studies have contributed to shape our concept of T cell trafficking in IBD by deciphering some important and so far unanswered questions. At the same time, basic and clinical data have generated new issues to be addressed on the way toward a clear perception of trafficking mechanisms and toward elucidation of the action of compounds interfering with this process. In this review, we will give a comprehensive outline of all components of T cell trafficking in regard to IBD before discussing the current knowledge concerning targeted interference with integrins in this complex network. Moreover, we will summarize remaining ambiguity and give an outlook on potential future targets.

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.

Transcription Factor T-bet Orchestrates Lineage Development and Function in the Immune System

T-bet was originally described as the key transcription factor defining type 1 T helper (Th) cells. However, it is now clear that it drives the orchestrated generation of effector and memory cells in multiple different lymphocyte lineages. In addition to Th1 cells, CD8 T cells, B cells and some innate lymphocyte populations require T-bet for their development or differentiation in response to antigen. Furthermore, other Th cell populations, including T follicular helper and Th17, as well as regulatory T cells can co-opt T-bet expression to promote functional diversification and colocalization. Thus, T-bet broadly regulates transcriptional programs in response to type 1 inflammatory signals and mediates the coordinated differentiation, function, migration and survival of effector and memory lymphocyte subsets in the affected tissue. Therefore, T-bet expression is essential for effective clearance of pathogens and maintenance of immunity.

Early transcriptional and epigenetic regulation of CD8+ T cell differentiation revealed by single-cell RNA sequencing

During microbial infection, responding CD8⁺ T lymphocytes differentiate into heterogeneous subsets that together provide immediate and durable protection. To elucidate the dynamic transcriptional changes that underlie this process, we applied a single-cell RNA-sequencing approach and analyzed individual CD8⁺ T lymphocytes sequentially throughout the course of a viral infection in vivo. Our analyses revealed a striking transcriptional divergence among cells that had undergone their first division and identified previously unknown molecular determinants that controlled the fate specification of CD8⁺ T lymphocytes. Our findings suggest a model for the differentiation of terminal effector cells initiated by an early burst of transcriptional activity and subsequently refined by epigenetic silencing of transcripts associated with memory lymphocytes, which highlights the power and necessity of single-cell approaches.

Survival of tissue-resident memory T cells requires exogenous lipid uptake and metabolism

Tissue-resident memory T (T_RM) cells persist indefinitely in epithelial barrier tissues and protect the host against pathogens. However, the biological pathways that enable the long-term survival of T_RM cells are obscure. Here we show that mouse CD8⁺ T_RM cells generated by viral infection of the skin differentially express high levels of several molecules that mediate lipid uptake and intracellular transport, including fatty-acid-binding proteins 4 and 5 (FABP4 and FABP5). We further show that T-cell-specific deficiency of Fabp4 and Fabp5 (Fabp4/Fabp5) impairs exogenous free fatty acid (FFA) uptake by CD8⁺ T_RM cells and greatly reduces their long-term survival in vivo, while having no effect on the survival of central memory T (T_CM) cells in lymph nodes. In vitro, CD8⁺ T_RM cells, but not CD8⁺ T_CM cells, demonstrated increased mitochondrial oxidative metabolism in the presence of exogenous FFAs; this increase was not seen in Fabp4/Fabp5 double-knockout CD8⁺ T_RM cells. The persistence of CD8⁺ T_RM cells in the skin was strongly diminished by inhibition of mitochondrial FFA β-oxidation in vivo. Moreover, skin CD8⁺ T_RM cells that lacked Fabp4/Fabp5 were less effective at protecting mice from cutaneous viral infection, and lung Fabp4/Fabp5 double-knockout CD8⁺ T_RM cells generated by skin vaccinia virus (VACV) infection were less effective at protecting mice from a lethal pulmonary challenge with VACV. Consistent with the mouse data, increased FABP4 and FABP5 expression and enhanced extracellular FFA uptake were also demonstrated in human CD8⁺ T_RM cells in normal and psoriatic skin. These results suggest that FABP4 and FABP5 have a critical role in the maintenance, longevity and function of CD8⁺ T_RM cells, and suggest that CD8⁺ T_RM cells use exogenous FFAs and their oxidative metabolism to persist in tissue and to mediate protective immunity.

Regulation of Innate and Adaptive Immunity by TGFβ

Immune regulation by cytokines is crucial in maintaining immune homeostasis, promoting responses to infection, resolving inflammation, and promoting immunological memory. Additionally, cytokine responses drive pathology in immune-mediated disease. A crucial cytokine in the regulation of all aspects of an immune response is transforming growth factor beta (TGFβ). Although best known as a crucial regulator of T cell responses, TGFβ plays a vital role in regulating responses mediated by virtually every innate and adaptive immune cell, including dendritic cells, B cells, NK cells, innate lymphoid cells, and granulocytes. Here, we review our current knowledge of how TGFβ regulates the immune system, highlighting the multifunctional nature of TGFβ and how its function can change depending on location and context of action.

Cutting Edge: Tissue-Resident Memory T Cells Generated by Multiple Immunizations or Localized Deposition Provide Enhanced Immunity

Tissue-resident memory T cells (T_RM) have been shown to afford superior protection against infection, particularly against pathogens that enter via the epithelial surfaces of the body. Although T_RM are often concentrated at sites of prior infection, it has been shown that T_RM can disseminate throughout the body. We examined the relative effectiveness of global versus targeted CD8⁺ T_RM lodgment in skin. The site of initial T cell priming made little difference to skin lodgement, whereas local inflammation and Ag recognition enhanced T_RM accumulation and retention. Disseminated T_RM lodgment was seen with the skin, but required multiple exposures to Ag and was inferior to targeted strategies. As a consequence, active recruitment by inflammation or infection resulted in superior T_RM numbers and maximal protection against infection. Overall, these results highlight the potency of localized T_RM deposition as a means of pathogen control as well as demonstrating the limitations of global T_RM lodgment.

T cell responses in the central nervous system

T cells are required for immune surveillance of the central nervous system (CNS); however, they can also induce severe immunopathology in the context of both viral infections and autoimmunity. The mechanisms that are involved in the priming and recruitment of T cells to the CNS are only partially understood, but there has been renewed interest in this topic since the 'rediscovery' of lymphatic drainage from the CNS. Moreover, tissue-resident memory T cells have been detected in the CNS and are increasingly recognized as an autonomous line of host defence. In this Review, we highlight the main mechanisms that are involved in the priming and CNS recruitment of CD4⁺ T cells, CD8⁺ T cells and regulatory T cells. We also consider the plasticity of T cell responses in the CNS, with a focus on viral infection and autoimmunity.

Diminished CD103 (αEβ7) Expression on Resident T Cells from the Female Genital Tract of HIV-Positive Women

Background:

Tissue resident memory T cells (TrM) provide an enhanced response against infection at mucosal surfaces, yet their function has not been extensively studied in humans, including the female genital tract (FGT).

Methods:

Using polychromatic flow cytometry, we studied TrM cells, defined as CD62L-CCR7-CD103⁺CD69⁺ CD4⁺ and CD8⁺ T cells in mucosa-derived T cells from healthy and HIV-positive women.

Results:

We demonstrate that TrM are present in the FGT of healthy and HIV-positive women. The expression of the mucosal retention receptor, CD103, from HIV-positive women was reduced compared to healthy women and was lowest in women with CD4 counts < 500 cells/mm³. Furthermore, CD103 expression on mucosa-derived CD8⁺ T cells correlated with antigen-specific IFN-γ production by mucosal CD4⁺ T cells and was inversely correlated with T-bet from CD8⁺CD103⁺ mucosa-derived T cells.

Conclusions:

These data suggest that CD4⁺ T cells, known to be impaired during HIV-1 infection and necessary for the expression of CD103 in murine models, may play a role in the expression of CD103 on resident T cells from the human FGT.

Dynamics of influenza-induced lung-resident memory T cells underlie waning heterosubtypic immunity

Lung-resident memory CD8 T cells (T_RM) induced by influenza A virus (IAV) that are pivotal for providing subtype-transcending protection against IAV infection (heterosubtypic immunity) are not maintained long term, causing gradual loss of protection. The short-lived nature of lung T_RM contrasts sharply with long-term maintenance of T_RM induced by localized infections in the skin and in other tissues. We show that the decline in lung T_RM is determined by an imbalance between apoptosis and lung recruitment and conversion to T_RM of circulating memory cells. We show that circulating effector memory cells (T_EM) rather than central memory cells (T_CM) are the precursors for conversion to lung T_RM Time-dependent changes in expression of genes critical for lymphocyte trafficking and T_RM differentiation, in concert with enrichment of T_CM, diminish the capacity of circulating memory CD8 T cells to form T_RM with time, explaining why IAV-induced T_RM are not stably maintained. Systemic booster immunization, through increasing the number of circulating T_EM, increases lung T_RM, providing a potential new avenue to enhance IAV vaccines.

Very Late Antigen-1 Marks Functional Tumor-Resident CD8 T Cells and Correlates with Survival of Melanoma Patients

A major limiting factor in the success of immunotherapy is tumor infiltration by CD8⁺ T cells, a process that remains poorly understood. In the present study, we characterized homing receptors expressed by human melanoma-specific CD8⁺ T cells. Our data reveal that P-selectin binding and expression of the retention integrin, very late antigen (VLA)-1, by vaccine-induced T cells correlate with longer patient survival. Furthermore, we demonstrate that CD8⁺VLA-1⁺ tumor-infiltrating lymphocytes (TILs) are highly enriched in melanoma metastases in diverse tissues. VLA-1-expressing TIL frequently co-express CD69 and CD103, indicating tissue-resident memory T cells (T_RM) differentiation. We employed a mouse model of melanoma to further characterize VLA-1-expressing TIL. Our data show that VLA-1⁺ T_RM develop in murine tumors within 2 weeks, where they exhibit increased activation status, as well as superior effector functions. In addition, in vivo blockade of either VLA-1 or CD103 significantly impaired control of subcutaneous tumors. Together, our data indicate that VLA-1⁺ T_RM develop in tumors and play an important role in tumor immunity, presenting novel targets for the optimization of cancer immunotherapy.

Antigen-dependent competition shapes the local repertoire of tissue-resident memory CD8+ T cells

Tissue-resident memory CD8⁺ T cells (T_RM) constitute a major component of the immune-surveillance system in nonlymphoid organs. Local, noncognate factors are both necessary and sufficient to support the programming of T_RM cell fate in tissue-infiltrating T cells. Recent evidence suggests that TCR signals received in infected nonlymphoid tissues additionally contribute to T_RM cell formation. Here, we asked how antigen-dependent pathways influence the generation of skin-resident memory T cells that arise from a polyclonal repertoire of cells induced by infection with an antigenically complex virus and recombinant vaccine vector. We found that CD8⁺ T cells of different specificities underwent antigen-dependent competition in the infected tissue, which shaped the composition of the local pool of T_RM cells. This local cross-competition was active for T cells recognizing antigens that are coexpressed by infected cells. In contrast, T_RM cell development remained largely undisturbed by the presence of potential competitors when antigens expressed in the same tissue were segregated through infection with antigenically distinct viral quasispecies. Functionally, local cross-competition might serve as a gatekeeping mechanism to regulate access to the resident memory niche and to fine-tune the local repertoire of antiviral T_RM cells.

Specific niches for lung-resident memory CD8+ T cells at the site of tissue regeneration enable CD69-independent maintenance

Takamura et al. show that most lung CD8⁺ T_RM cells are not maintained in the inducible bronchus-associated lymphoid tissue (iBALT) but are maintained in specific niches created at the site of tissue regeneration, which are termed as repair-associated memory depots (RAMDs).

CD8⁺ tissue-resident memory T cells (T_RM cells) reside permanently in nonlymphoid tissues and provide a first line of protection against invading pathogens. However, the precise localization of CD8⁺ T_RM cells in the lung, which physiologically consists of a markedly scant interstitium compared with other mucosa, remains unclear. In this study, we show that lung CD8⁺ T_RM cells localize predominantly in specific niches created at the site of regeneration after tissue injury, whereas peripheral tissue-circulating CD8⁺ effector memory T cells (T_EM cells) are widely but sparsely distributed in unaffected areas. Although CD69 inhibited sphingosine 1–phosphate receptor 1–mediated egress of CD8⁺ T cells immediately after their recruitment into lung tissues, such inhibition was not required for the retention of cells in the T_RM niches. Furthermore, despite rigid segregation of T_EM cells from the T_RM niche, prime-pull strategy with cognate antigen enabled the conversion from T_EM cells to T_RM cells by creating de novo T_RM niches. Such damage site–specific localization of CD8⁺ T_RM cells may be important for efficient protection against secondary infections by respiratory pathogens.

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.

Mechanisms of T cell organotropism

Protective immunity relies upon T cell differentiation and subsequent migration to target tissues. Similarly, immune homeostasis requires the localization of regulatory T cells (Tregs) to the sites where immunity takes place. While naïve T lymphocytes recirculate predominantly in secondary lymphoid tissue, primed T cells and activated Tregs must traffic to the antigen rich non-lymphoid tissue to exert effector and regulatory responses, respectively. Following priming in draining lymph nodes, T cells acquire the 'homing receptors' to facilitate their access to specific tissues and organs. An additional level of topographic specificity is provided by T cells receptor recognition of antigen displayed by the endothelium. Furthermore, co-stimulatory signals (such as those induced by CD28) have been shown not only to regulate T cell activation and differentiation, but also to orchestrate the anatomy of the ensuing T cell response. We here review the molecular mechanisms supporting trafficking of both effector and regulatory T cells to specific antigen-rich tissues.

Brain-resident memory T cells represent an autonomous cytotoxic barrier to viral infection

During viral infections, brain tissue–resident memory T cells (bTRM) prevent fatal brain infection after acquiring perforin- and IFN-γ–dependent effector functions through a pathway that involves presentation of cognate antigen on MHC-I.

Tissue-resident memory T cells (T_RM) persist at sites of prior infection and have been shown to enhance pathogen clearance by recruiting circulating immune cells and providing bystander activation. Here, we characterize the functioning of brain-resident memory T cells (bT_RM) in an animal model of viral infection. bT_RM were subject to spontaneous homeostatic proliferation and were largely refractory to systemic immune cell depletion. After viral reinfection in mice, bT_RM rapidly acquired cytotoxic effector function and prevented fatal brain infection, even in the absence of circulating CD8⁺ memory T cells. Presentation of cognate antigen on MHC-I was essential for bT_RM-mediated protective immunity, which involved perforin- and IFN-γ–dependent effector mechanisms. These findings identify bT_RM as an organ-autonomous defense system serving as a paradigm for T_RM functioning as a self-sufficient first line of adaptive immunity.

Environmental cues orchestrate regional immune surveillance and protection by pulmonary CTLs

Tissue-resident memory CD8 T cells (T_RM) provide preemptive immunity against infections that begin in peripheral tissues by guarding the site of initial pathogen exposure. Their role in immunity to respiratory virus infection is particularly important because severe damage to the alveoli can be avoided when local populations of T_RM cells reduce viral burdens and dampen the responses of effector CD8 T cells in the lungs. Although a connection between rapid immune activation and early viral control is well established, the signals that keep T_RM cells poised for action in the local tissues remain poorly defined. Recent studies have shown that environmental cues influence the fate decisions of activated CTLs during memory formation. Manipulation of these signaling pathways could provide new ways to capitalize on protection from T_RM cells in mucosal tissues, while reducing collateral damage and pathology during vaccination.

Local antigen in nonlymphoid tissue promotes resident memory CD8+ T cell formation during viral infection

CD8⁺ T cells activated during viral infection migrate to infected skin in an antigen-independent manner. Local recognition of antigens drives the differentiation into Trm CD8⁺ T cells.

Tissue-resident memory (Trm) CD8⁺ T cells are functionally distinct from their circulating counterparts and are potent mediators of host protection against reinfection. Whether local recognition of antigen in nonlymphoid tissues during infection can impact the formation of Trm populations remains unresolved. Using skin infections with vaccinia virus (VacV)–expressing model antigens, we found that local antigen recognition had a profound impact on Trm formation. Activated CD8⁺ T cells trafficked to VacV-infected skin in an inflammation-dependent, but antigen-independent, manner. However, after viral clearance, there was a subsequent ∼50-fold increase in Trm formation when antigen was present in the tissue microenvironment. Secondary antigen stimulation in nonlymphoid tissue caused CD8⁺ T cells to rapidly express CD69 and be retained at the site of infection. Finally, Trm CD8⁺ T cells that formed during VacV infection in an antigen-dependent manner became potent stimulators of localized antigen-specific inflammatory responses in the skin. Thus, our studies indicate that the presence of antigen in the nonlymphoid tissue microenvironment plays a critical role in the formation of functional Trm CD8⁺ T cell populations, a finding with relevance for both vaccine design and prevention of inflammatory disorders.

Front-Line Memory T Cells Think Outside the T-box… Mostly

Tissue-resident memory T (Trm) cells patrol barrier tissues. In this issue of Immunity, Carbone and colleagues show that downregulation of T-box proteins Eomesodermin and T-bet is critical for their differentiation, but residual levels of T-bet are important for long-term Trm survival and function.