Transient expression of ZBTB32 in anti-viral CD8+ T cells limits the magnitude of the effector response and the generation of memory

Virus infections induce CD8⁺ T cell responses comprised of a large population of terminal effector cells and a smaller subset of long-lived memory cells. The transcription factors regulating the relative expansion versus the long-term survival potential of anti-viral CD8⁺ T cells are not completely understood. We identified ZBTB32 as a transcription factor that is transiently expressed in effector CD8⁺ T cells. After acute virus infection, CD8⁺ T cells deficient in ZBTB32 showed enhanced virus-specific CD8⁺ T cell responses, and generated increased numbers of virus-specific memory cells; in contrast, persistent expression of ZBTB32 suppressed memory cell formation. The dysregulation of CD8⁺ T cell responses in the absence of ZBTB32 was catastrophic, as Zbtb32^-/- mice succumbed to a systemic viral infection and showed evidence of severe lung pathology. We found that ZBTB32 and Blimp-1 were co-expressed following CD8⁺ T cell activation, bound to each other, and cooperatively regulated Blimp-1 target genes Eomes and Cd27. These findings demonstrate that ZBTB32 is a key transcription factor in CD8⁺ effector T cells that is required for the balanced regulation of effector versus memory responses to infection.

CD8⁺ T lymphocytes are essential for immune protection against viruses. In response to an infection, these cells are activated, proliferate, and generate antiviral effector cells that eradicate the infection. Following this, the majority of these effector cells die, leaving a small subset of long-lived virus-specific memory T cells. Our study identifies a transcription factor, ZBTB32, that is required for the regulation of CD8⁺ T cell responses. In its absence, antiviral CD8⁺ T cell numbers increase to abnormally high levels, and generate an overabundance of memory T cells. When this dysregulated response occurs following infection with a virus that cannot be rapidly eliminated by the immune system, the infected animals die from immune-mediated tissue damage, indicating the importance of this pathway.

Diversity of tissue-resident NK cells

Although natural killer (NK) cells were initially named for their spontaneous tumor-killing capacity, their concept has been greatly expanded with more than 40 years of extensive investigation. Currently, NK cells are known as a heterogeneous population of innate lymphoid cell (ILC) family, consisting of different subsets with unique phenotypic and functional features. Recent studies have shown that tissue-resident NK (trNK) cells, which are distinct from conventional NK (cNK) cells, preferentially distribute in non-lymphoid tissues, such as the liver, uterus, salivary gland, and adipose. In this review, we provide a comprehensive overview of the current knowledge about the phenotype, function and development of trNK cells across different tissues and describe the similarities and differences between diverse trNK cells and cNK cells, with a particular focus on the tissue-specific characteristics of different trNK cells.

Tissue-resident memory T cells play a key role in the efficacy of cancer vaccines

Resident memory CD8⁺T cells (T_RM) usually defined by the CD103 marker represent a new subset of long-lived memory T cells that remain in the tissues. We directly demonstrate their specific role in cancer vaccine-induced tumor regression. In human, they also seem to play a major role in tumor immunosurveillance.

Human T cell immune surveillance: Phenotypic, functional and migratory heterogeneity for tailored immune responses

The human immune system constantly provides a balance between pathogen clearance as well as tolerance for autoantigens and the commensal microbiota. This is achieved by immune responses, which are highly specialized and diversified in terms of their phenotype, function, regulation and location. Despite the complexity that is inherent to human immunity, our current knowledge is primarily shaped by very reductionist insights gained from peripheral blood T cells. Since only 2% of human T cells recirculate in the blood, the vast majority remains undetected by common sampling strategies and therefore unexplored. This review highlights and discusses recent developments in human T cell immune surveillance with a particular focus on functional and migratory T cell heterogeneity and provides a critical framework for new conceptual ideas, which could serve as a starting point in the quest for novel targeted therapies for chronic tissue restricted inflammatory diseases.

Tissue-resident memory features are linked to the magnitude of cytotoxic T cell responses in human lung cancer

Therapies that boost the anti-tumor responses of cytotoxic T lymphocytes (CTLs) have shown promise; however, clinical responses to the immunotherapeutic agents currently available vary considerably, and the molecular basis of this is unclear. We performed transcriptomic profiling of tumor-infiltrating CTLs from treatment-naive patients with lung cancer to define the molecular features associated with the robustness of anti-tumor immune responses. We observed considerable heterogeneity in the expression of molecules associated with activation of the T cell antigen receptor (TCR) and of immunological-checkpoint molecules such as 4-1BB, PD-1 and TIM-3. Tumors with a high density of CTLs showed enrichment for transcripts linked to tissue-resident memory cells (TRM cells), such as CD103, and CTLs from CD103hi tumors displayed features of enhanced cytotoxicity. A greater density of TRM cells in tumors was predictive of a better survival outcome in lung cancer, and this effect was independent of that conferred by CTL density. Here we define the 'molecular fingerprint' of tumor-infiltrating CTLs and identify potentially new targets for immunotherapy.

SMAD4 impedes the conversion of NK cells into ILC1-like cells by curtailing non-canonical TGF-β signaling

Among the features that distinguish type 1 innate lymphoid cells (ILC1s) from natural killer (NK) cells is a gene signature indicative of 'imprinting' by cytokines of the TGF-β family. We studied mice in which ILC1s and NK cells lacked SMAD4, a signal transducer that facilitates the canonical signaling pathway common to all cytokines of the TGF-β family. While SMAD4 deficiency did not affect ILC1 differentiation, NK cells unexpectedly acquired an ILC1-like gene signature and were unable to control tumor metastasis or viral infection. Mechanistically, SMAD4 restrained non-canonical TGF-β signaling mediated by the cytokine receptor TGFβR1 in NK cells. NK cells from a SMAD4-deficient person affected by polyposis were also hyper-responsive to TGF-β. These results identify SMAD4 as a previously unknown regulator that restricts non-canonical TGF-β signaling in NK cells.

Hobit expression by a subset of human liver-resident CD56bright Natural Killer cells

Immune responses show a high degree of tissue specificity shaped by factors influencing tissue egress and retention of immune cells. The transcription factor Hobit was recently shown to regulate tissue-residency in mice. Whether Hobit acts in a similar capacity in humans remains unknown. Our aim was to assess the expression and contribution of Hobit to tissue-residency of Natural Killer (NK) cells in the human liver. The human liver was enriched for CD56^bright NK cells showing increased expression levels of the transcription factor Hobit. Hobit^pos CD56^bright NK cells in the liver exhibited high levels of CD49a, CXCR6 and CD69. Hobit^pos CD56^bright NK cells in the liver furthermore expressed a unique set of transcription factors with higher frequencies and levels of T-bet and Blimp-1 when compared to Hobit^neg CD56^bright NK cells. Taken together, we show that the transcription factor Hobit identifies a subset of NK cells in human livers that express a distinct set of adhesion molecules and chemokine receptors consistent with tissue residency. These data suggest that Hobit is involved in regulating tissue-residency of human intrahepatic CD56^bright NK cells in a subset of NK cells in inflamed livers.

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.

Wnt Signaling as Master Regulator of T-Lymphocyte Responses: Implications for Transplant Therapy

T cell-mediated immune responses to the grafted tissues are the major reason for failed organ transplantation. The regulation of T cell responses is complex and involves major histocompatibility complex molecules on transplanted organs, cytokines, regulatory cells, and antigen-presenting cells. The evolutionary conserved Wnt signal transduction pathway has long been known for its importance in development of stem cells and immature T cells in the thymus. Recent evidence indicates the Wnt pathway as a master regulator of T cell immune responses via governing the balance between T helper 17/regulatory T cells and by regulating the formation of effector and memory cytotoxic CD8 T cell responses. In doing so, Wnt signals influence the outcome of immune responses in transplantation settings.

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.

IL-2high tissue-resident T cells in the human liver: Sentinels for hepatotropic infection

The liver provides a tolerogenic immune niche exploited by several highly prevalent pathogens as well as by primary and metastatic tumors. We have sampled healthy and hepatitis B virus (HBV)-infected human livers to probe for a subset of T cells specialized to overcome local constraints and mediate immunity. We characterize a population of T-bet^loEomes^loBlimp-1^hiHobit^lo T cells found within the intrahepatic but not the circulating memory CD8 T cell pool expressing liver-homing/retention markers (CD69⁺CD103⁺ CXCR6⁺CXCR3⁺). These tissue-resident memory T cells (T_RM) are preferentially expanded in patients with partial immune control of HBV infection and can remain in the liver after the resolution of infection, including compartmentalized responses against epitopes within all major HBV proteins. Sequential IL-15 or antigen exposure followed by TGFβ induces liver-adapted T_RM, including their signature high expression of exhaustion markers PD-1 and CD39. We suggest that these inhibitory molecules, together with paradoxically robust, rapid, cell-autonomous IL-2 and IFNγ production, equip liver CD8 T_RM to survive while exerting local noncytolytic hepatic immunosurveillance.

Tissue-resident lymphocytes: sentinel of the transformed tissue

Tumor cells can be detected and cleared by lymphocytes in a process termed cancer immunosurveillance. However, the contributing cell types had not been fully characterized. Using oncogene-induced murine models of epithelial cancer, a recent study showed that cell transformation triggers expansion of tissue-resident lymphocytes derived from innate, T cell receptor (TCR) αβ and TCRγδ lineages. These type-1-like innate lymphoid cells (ILC1ls) and type 1 innate-like T cells (ILTC1s) share a gene expression program distinct from those of conventional lymphocytes, and exhibit cytolytic activities against tumor cells. Further deciphering such a tumor-elicited immunosurveillance mechanism may 1 day be harnessed for novel cancer immunotherapy.

The Transcription Factor ZNF683/HOBIT Regulates Human NK-Cell Development

We identified ZNF683/HOBIT as the most highly upregulated transcription factor gene during ex vivo differentiation of human CD34⁺ cord blood progenitor cells to CD56⁺ natural killer (NK) cells. ZNF683/HOBIT mRNA was preferentially expressed in NK cells compared to other human peripheral blood lymphocytes and monocytes. During ex vivo differentiation, ZNF683/HOBIT mRNA started to increase shortly after addition of IL-15 and further accumulated in parallel to the generation of CD56⁺ NK cells. shRNA-mediated knockdown of ZNF683/HOBIT resulted in a substantial reduction of CD56⁻CD14⁻ NK-cell progenitors and the following generation of CD56⁺ NK cells was largely abrogated. The few CD56⁺ NK cells, which escaped the developmental inhibition in the ZNF683/HOBIT knockdown cultures, displayed normal levels of NKG2A and KIR receptors. Functional analyses of these cells showed no differences in degranulation capacity from control cultures. However, the proportion of IFN-γ-producing cells appeared to be increased upon ZNF683/HOBIT knockdown. These results indicate a key role of ZNF683/HOBIT for the differentiation of the human NK-cell lineage and further suggest a potential negative control on IFN-γ production in more mature human NK cells.

Implications of Resident Memory T Cells for Transplantation

Recent studies have established resident memory T cells (TRM ) as the dominant memory lymphocyte population surveying most nonlymphoid tissues. Unlike other memory T cell lineages, TRM do not recirculate through blood and are permanently confined to their tissue of residence. TRM orchestrate local immune responses and have been shown to accelerate local pathogen control in many experimental infection models. Here we briefly summarize recent advances in TRM differentiation, maintenance, and their protective function. While little is known, we have speculated on the potential implications of TRM for transplantation biology. Areas of emphasis include the role of passenger TRM in controlling latent viral recrudescence in donor organs, donor TRM as a source of graft-versus-host disease, the ability of TRM to potently induce inflammation through sensing and alarm functions, and differentiation of host T cells into TRM in response to local cues inside the allograft. Further investigation of TRM in the context of transplantation might identify therapeutic targets to prolong graft survival.

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.

The emerging role of ECM crosslinking in T cell mobility as a hallmark of immunosenescence in humans

Immunosenescence is thought to result from cellular aging and to reflect exposure to environmental stressors and antigens, including cytomegalovirus (CMV). However, not all of the features of immunosenescence are consistent with this view, and this has led to the emergence of the sister theory of "inflammaging". The recently discovered diffuse tissue distribution of resident memory T cells (T_RM) which don't recirculate, calls these theories into question. These cells account for most T cells residing in barrier epithelia which sit in and travel through the extracellular matrix (ECM). With almost all studies to date carried out on peripheral blood, the age-related changes of the ECM and their consequences for T cell mobility, which is crucial for the function of these cells, have been largely ignored. We propose an update of the theoretical framework of immunosenescence, based on a novel hypothesis: the increasing stiffness and cross-linking of the senescent ECM lead to a progressive immunodeficiency due to an age-related decrease in T cell mobility and eventually the death of these cells. A key element of this mechanism is the mechanical stress to which the cell cytoplasm and nucleus are subjected during passage through the ECM. This hypothesis is based on an "evo-devo" perspective bringing together some major characteristics of aging, to create a single interpretive framework for immunosenescence.

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.

TCR stimulation strength is inversely associated with establishment of functional brain-resident memory CD8 T cells during persistent viral infection

Establishing functional tissue-resident memory (T_RM) cells at sites of infection is a newfound objective of T cell vaccine design. To directly assess the impact of antigen stimulation strength on memory CD8 T cell formation and function during a persistent viral infection, we created a library of mouse polyomavirus (MuPyV) variants with substitutions in a subdominant CD8 T cell epitope that exhibit a broad range of efficiency in stimulating TCR transgenic CD8 T cells. By altering a subdominant epitope in a nonstructural viral protein and monitoring memory differentiation of donor monoclonal CD8 T cells in immunocompetent mice, we circumvented potentially confounding changes in viral infection levels, virus-associated inflammation, size of the immunodominant virus-specific CD8 T cell response, and shifts in TCR affinity that may accompany temporal recruitment of endogenous polyclonal cells. Using this strategy, we found that antigen stimulation strength was inversely associated with the function of memory CD8 T cells during a persistent viral infection. We further show that CD8 T_RM cells recruited to the brain following systemic infection with viruses expressing epitopes with suboptimal stimulation strength respond more efficiently to challenge CNS infection with virus expressing cognate antigen. These data demonstrate that the strength of antigenic stimulation during recruitment of CD8 T cells influences the functional integrity of T_RM cells in a persistent viral infection.

Tissue-resident memory (T_RM) cells are a subset of memory T cells that primarily reside in non-lymphoid tissues and serve as sentinels and effectors against secondary infections. T_RM cells have been extensively characterized in mucosal barriers, but much less is known about this population in non-barrier sites such as the brain. In this study, we designed a novel strategy to evaluate the impact of T cell stimulation strength on the generation and functionality of memory CD8 T cells in both lymphoid and nonlymphoid tissues. Using a mouse polyomavirus (MuPyV) library expressing variants of a subdominant epitope recognized by TCR transgenic CD8 T cells, we found that systemic infection producing weaker responses during T cell priming was sufficient for recruitment of effector cells to the brain. Furthermore, lower stimulation conferred greater functionality to memory T cells in the spleen and to brain T_RM cells. Our findings demonstrate that the strength of antigenic stimulation experienced by a naïve T cell early in infection is a determinant of memory functional integrity during viral persistence in a non-barrier organ.

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.

CD103+ CD8 T Cells in the Toxoplasma-Infected Brain Exhibit a Tissue-Resident Memory Transcriptional Profile

During chronic infection, memory T cells acquire a unique phenotype and become dependent on different survival signals than those needed for memory T cells generated during an acute infection. The distinction between the role of effector and memory T cells in an environment of persistent antigen remains unclear. Here, in the context of chronic Toxoplasma gondii infection, we demonstrate that a population of CD8 T cells exhibiting a tissue-resident memory (T_RM) phenotype accumulates within the brain. We show that this population is distributed throughout the brain in both parenchymal and extraparenchymal spaces. Furthermore, this population is transcriptionally distinct and exhibits a transcriptional signature consistent with the T_RM observed in acute viral infections. Finally, we establish that the CD103⁺ T_RM population has an intrinsic capacity to produce both IFN-γ and TNF-α, cytokines critical for parasite control within the central nervous system (CNS). The contribution of this population to pro-inflammatory cytokine production suggests an important role for T_RM in protective and ongoing immune responses in the infected CNS.

Accession number: GSE95105

Developmental options and functional plasticity of innate lymphoid cells

Innate lymphoid cells (ILCs) are lineage- and antigen receptor-negative lymphocytes including natural killer (NK) cells and at least three distinguishable cell subsets (ILC1, ILC2, ILC3) that rapidly produce cytokines (IFN-γ, IL-5, IL-13, IL-17A, IL-22) upon activation. As such, ILCs can act as first-line defenders in the context of infection, inflammation and cancer. Because of the strong conservation between the expression of key transcription factors that can drive signature cytokine outputs in ILCs and differentiated helper T cells, it has been proposed that ILCs represent innate counterparts of the latter. Several distinct ILC precursors (ILCP) with pan-ILC (giving rise to all ILCs) or subset-restricted potentials have been described in both mouse and man. How and where these different ILCP give rise to more mature tissue-resident ILCs remains unclear. Recently, environmental signals have been shown to epigenetically influence canonical ILC differentiation pathways, generating substantial functional plasticity. These new results suggest that while ILC differentiation may be 'fixed' in principle, it remains 'flexible' in practice. A more comprehensive knowledge in the molecular mechanisms that regulate ILC development and effector functions may allow for therapeutic manipulation of ILCs for diverse disease conditions.

The Transcription Factor Hobit Identifies Human Cytotoxic CD4+ T Cells

The T cell lineage is commonly divided into CD4-expressing helper T cells that polarize immune responses through cytokine secretion and CD8-expressing cytotoxic T cells that eliminate infected target cells by virtue of the release of cytotoxic molecules. Recently, a population of CD4⁺ T cells that conforms to the phenotype of cytotoxic CD8⁺ T cells has received increased recognition. These cytotoxic CD4⁺ T cells display constitutive expression of granzyme B and perforin at the protein level and mediate HLA class II-dependent killing of target cells. In humans, this cytotoxic profile is found within the human cytomegalovirus (hCMV)-specific, but not within the influenza- or Epstein–Barr virus-specific CD4⁺ T cell populations, suggesting that, in particular, hCMV infection induces the formation of cytotoxic CD4⁺ T cells. We have previously described that the transcription factor Homolog of Blimp-1 in T cells (Hobit) is specifically upregulated in CD45RA⁺ effector CD8⁺ T cells that arise after hCMV infection. Here, we describe the expression pattern of Hobit in human CD4⁺ T cells. We found Hobit expression in cytotoxic CD4⁺ T cells and accumulation of Hobit⁺ CD4⁺ T cells after primary hCMV infection. The Hobit⁺ CD4⁺ T cells displayed highly overlapping characteristics with Hobit⁺ CD8⁺ T cells, including the expression of cytotoxic molecules, T-bet, and CX3CR1. Interestingly, γδ⁺ T cells that arise after hCMV infection also upregulate Hobit expression and display a similar effector phenotype as cytotoxic CD4⁺ and CD8⁺ T cells. These findings suggest a shared differentiation pathway in CD4⁺, CD8⁺, and γδ⁺ T cells that may involve Hobit-driven acquisition of long-lived cytotoxic effector function.

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.

The checkpoint for agonist selection precedes conventional selection in human thymus

The thymus plays a central role in self-tolerance, partly by eliminating precursors with a T cell receptor (TCR) that binds strongly to self-antigens. However, the generation of self-agonist-selected lineages also relies on strong TCR signaling. How thymocytes discriminate between these opposite outcomes remains elusive. Here, we identified a human agonist-selected PD-1⁺ CD8αα⁺ subset of mature CD8αβ⁺ T cells that displays an effector phenotype associated with agonist selection. TCR stimulation of immature post-β-selection thymocyte blasts specifically gives rise to this innate subset and fixes early T cell receptor alpha variable (TRAV) and T cell receptor alpha joining (TRAJ) rearrangements in the TCR repertoire. These findings suggest that the checkpoint for agonist selection precedes conventional selection in the human thymus.

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.

Tissue resident memory T cells and viral immunity

Tissue resident memory T cells (TRM) constitute a recently identified T cell lineage that is responsible for frontline defense against viral infections. In contrast to central and effector memory T cells, which constitutively recirculate between tissues and blood, TRM reside permanently within tissues. As the main surveyors of non-lymphoid tissues, TRM are positioned to rapidly respond upon reinfection at barrier sites. During a viral reinfection, TRM trigger the local tissue environment to activate and recruit immune cells and establish an antiviral state. Consistent with this function, there is empirical evidence that TRM accelerate control in the event of reinfection or possible reactivation of latent infections in solid organs and barrier tissues. Here we review recent literature highlighting the protective functions of TRM in multiple viral challenge models and contextualize the implications of these findings for vaccine development.

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.

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.

The three Rs: Recruitment, Retention and Residence of leukocytes in the liver

The composition of leukocytes in the liver is highly distinct from that of the blood and lymphoid organs. In particular, the liver is highly enriched in non-conventional T cells such as natural killer T (NKT) cells, γδ T cells and mucosal-associated invariant T cells. In addition, there are significant populations of tissue-resident NK cells (or innate lymphoid cells (ILC1)) and memory CD8+ T cells. These cells are joined in conditions of inflammation by neutrophils, monocytes and macrophages. In recent years a multitude of studies have generated insights into how these cells arrest, move and remain resident in the liver. This new understanding has largely been due to the use of intra-vital microscopy to track immune cells in the liver, coupled with gene expression profiling and parabiosis techniques. These studies have revealed that leukocyte recruitment in the liver does not correspond to the classical paradigm of the leukocyte adhesion cascade. Rather, both lymphoid and myeloid cells have been found to adhere in the liver sinusoids in a platelet-dependent manner. Leukocytes have also been observed to patrol the hepatic sinusoids using a characteristic crawling motility. Moreover, T cells have been observed surveying hepatocytes for antigen through the unique fenestrated endothelium of the liver sinusoids, potentially negating the need for extravasation. In this review we highlight some of these recent discoveries and examine the different molecular interactions required for the recruitment, retention and-in some cases-residence of diverse leukocyte populations within the liver.

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.

Protective immunity to liver-stage malaria

Despite decades of research and recent clinical trials, an efficacious long-lasting preventative vaccine for malaria remains elusive. This parasite infects mammals via mosquito bites, progressing through several stages including the relatively short asymptomatic liver stage followed by the more persistent cyclic blood stage, the latter of which is responsible for all disease symptoms. As the liver acts as a bottleneck to blood-stage infection, it represents a potential site for parasite and disease control. In this review, we discuss immunity to liver-stage malaria. It is hoped that the knowledge gained from animal models of malaria immunity will translate into a more powerful and effective vaccine to reduce this global health problem.

Type 1 Innate Lymphoid Cell Biology: Lessons Learnt from Natural Killer Cells

Group 1 innate lymphoid cells (ILCs) comprise the natural killer (NK) cells and ILC1s that reside within peripheral tissues. Several different ILC1 subsets have recently been characterized; however, no unique markers have been identified that uniquely define these subsets. Whether ILC1s and NK cells are in fact distinct lineages, or alternately exhibit transitional molecular programs that allow them to adapt to different tissue niches remains an open question. NK cells are the prototypic member of the Group 1 ILCs and have been historically assigned the functions of what now appears to be a multi-subset family that are distributed throughout the body. This raises the question of whether each of these populations mediate distinct functions during infection and tumor immunosurveillance. Here, we review the diversity of the Group 1 ILC subsets in their transcriptional regulation, localization, mobility, and receptor expression, and highlight the challenges in unraveling the individual functions of these different populations of cells.

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.