Tissue-Resident T Cells and Other Resident Leukocytes

Distribution and storage of inflammatory memory in barrier tissues

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

Distinct cellular immune properties in cerebrospinal fluid are associated with cognition in HIV-infected individuals initiating antiretroviral therapy

We examined the relationship between CSF immune cells and neurocognition and neuronal damage in HIV+ individuals before and after initiating antiretroviral therapy. Multivariate analysis at baseline indicated that greater CD4+ T cell abundance was associated with better cognition (p = .017), while higher CSF HIV RNA was associated with increased neuronal damage (p = .014). Following 24 weeks of antiretroviral therapy, CD8+ T cells, HLA-DR expressing CD4+ and CD8+ T cells, B cells, NK cells, and non-classical monocyte percentage decreased in CSF. Female gender was negatively associated with cognitive performance over time, as was higher percentage of HLA-DR expressing CD8+ T cells at baseline.

Immune outposts in the adventitia: One foot in sea and one on shore

Advances in microscopy, genetically modified mice, and single-cell RNA sequencing have begun to deconvolute the composition and function of tissue immune niches. Here we discuss the evidence that the adventitia, the outermost layer of larger blood vessels, is a conserved niche and tissue immune outpost for multiple immune cells, including group 2 innate lymphoid cells (ILC2) and subsets of tissue-resident memory T cells, macrophages, and dendritic cells. We also describe the unique non-immune composition at adventitial regions, including fibroblast-like stromal cell subsets, lymphatic and blood endothelial cells, and neurons, and review how immune-stromal crosstalk impacts regional tissue immunity, organ adaptation, and disease.

Organ-specific isoform selection of fatty acid–binding proteins in tissue-resident lymphocytes

Tissue-resident memory T (TRM) cells exist throughout the body, where they are poised to mediate local immune responses. Although studies have defined a common mechanism of residency independent of location, there is likely to be a level of specialization that adapts TRM cells to their given tissue of lodgment. It has been shown that TRM cells in the skin rely on the uptake of exogenous fatty acids for their survival and up-regulate fatty acid–binding protein 4 (FABP4) and FABP5 as part of their transcriptional program. However, FABPs exist as a larger family of isoforms, with different members selected in a tissue-specific fashion that is optimized for local fatty acid availability. Here, we show that although TRM cells in a range of tissue widely express FABPs, they are not restricted to FABP4 and FABP5. Instead, TRM cells show varying patterns of isoform usage that are determined by tissue-derived factors. These patterns are malleable because TRM cells relocated to different organs modify their FABP expression in line with their new location. As a consequence, these results argue for tissue-specific overlays to the TRM cell residency program, including FABP expression that is tailored to the particular tissue of TRM cell lodgment.

Defining T Cell Tissue Residency in Humans: Implications for HIV Pathogenesis and Vaccine Design

PURPOSE OF REVIEW

This review summarizes recent literature defining tissue-resident memory T cells (TRM) and discusses implications for HIV pathogenesis, vaccines, and eradication efforts.

RECENT FINDINGS

Investigations using animal models and human tissues have identified a TRM transcriptional profile and elucidated signals within the tissue microenvironment leading to TRM development and maintenance. TRM are major contributors to host response in infectious diseases and cancer; in addition, TRM contribute to pathogenic inflammation in a variety of settings. Although TRM are daunting to study in HIV infection, recent work has helped define their molecular signatures and effector functions and tested strategies for their mobilization. Exclusive reliance on blood sampling to gain an understanding of host immunity overlooks the contribution of TRM, which differ in significant ways from their counterparts in circulation. It is hoped that greater understanding of these cells will lead to novel approaches to prevent and/or eradicate HIV infection.

Migratory DCs activate TGF-β to precondition naïve CD8+ T cells for tissue-resident memory fate

Epithelial resident memory T (eT_RM) cells serve as sentinels in barrier tissues to guard against previously encountered pathogens. How eT_RM cells are generated has important implications for efforts to elicit their formation through vaccination or prevent it in autoimmune disease. Here, we show that during immune homeostasis, the cytokine transforming growth factor β (TGF-β) epigenetically conditions resting naïve CD8⁺ T cells and prepares them for the formation of eT_RM cells in a mouse model of skin vaccination. Naïve T cell conditioning occurs in lymph nodes (LNs), but not in the spleen, through major histocompatibility complex class I-dependent interactions with peripheral tissue-derived migratory dendritic cells (DCs) and depends on DC expression of TGF-β-activating α_V integrins. Thus, the preimmune T cell repertoire is actively conditioned for a specialized memory differentiation fate through signals restricted to LNs.

Tissue-resident memory T cells in breast cancer control and immunotherapy responses

The presence of tumour-infiltrating lymphocytes (TILs) is associated with favourable outcomes in patients with breast cancer as well as in those with other solid tumours. T cells make up a considerable proportion of TILs and current evidence suggests that CD8⁺ T cells are a crucial determinant of favourable clinical outcomes. Studies involving tumour material from numerous solid tumour types, including breast cancer, demonstrate that the CD8⁺ TILs include a subpopulation of tissue-resident memory T (T_RM) cells. This subpopulation has features consistent with those of T_RM cells, which have been described as having a role in peripheral immune surveillance and viral immunity in both humans and mice. Patients with early-stage triple-negative breast cancers harbouring greater numbers of T_RM cells have a substantially improved prognosis and longer overall survival. Furthermore, patients with advanced-stage breast cancers with higher levels of T_RM cells have increased response rates to anti-PD-1 antibodies. These findings have motivated efforts to explore whether CD8⁺ T_RM cells include tumour-specific T cells, their functional responses to cognate antigens and their role in responses to immune checkpoint inhibition. In this Review, we focus on the clinical significance of CD8⁺ T_RM cells and the potential ways that these cells can be targeted to improve the success of immunotherapeutic approaches in patients with breast cancer, as well as in those with other solid tumour types.

Vaccination Against Tuberculosis: Revamping BCG by Molecular Genetics Guided by Immunology

Tuberculosis (TB) remains a major health threat. Although a vaccine has been available for almost 100 years termed Bacille Calmette-Guérin (BCG), it is insufficient and better vaccines are urgently needed. This treatise describes first the basic immunology and pathology of TB with an emphasis on the role of T lymphocytes. Better understanding of the immune response to Mycobacterium tuberculosis (Mtb) serves as blueprint for rational design of TB vaccines. Then, disease epidemiology and the benefits and failures of BCG vaccination will be presented. Next, types of novel vaccine candidates are being discussed. These include: (i) antigen/adjuvant subunit vaccines; (ii) viral vectored vaccines; and (III) whole cell mycobacterial vaccines which come as live recombinant vaccines or as dead whole cell or multi-component vaccines. Subsequently, the major endpoints of clinical trials as well as administration schemes are being described. Major endpoints for clinical trials are prevention of infection (PoI), prevention of disease (PoD), and prevention of recurrence (PoR). Vaccines can be administered either pre-exposure or post-exposure with Mtb. A central part of this treatise is the description of the viable BCG-based vaccine, VPM1002, currently undergoing phase III clinical trial assessment. Finally, new approaches which could facilitate design of refined next generation TB vaccines will be discussed.

New Approaches to Targeting B Cells for Myasthenia Gravis Therapy

Current therapies for myasthenia gravis (MG) are limited, and many investigations have recently focused on target-specific therapies. B cell-targeting monoclonal antibody (mAb) therapies for MG are increasingly attractive due to their specificity and efficacy. The targeted B cell biomarkers are mainly the cluster of differentiation (CD) proteins that mediate maturation, differentiation, or survival of pathogenic B cells. Additional B cell-directed therapies include non-specific peptide inhibitors that preferentially target specific B cell subsets. The primary goals of such therapies are to intercept autoantibodies and prevent the generation of an inflammatory response that contributes to the pathogenesis of MG. Treatment of patients with MG using B cell-directed mAbs, antibody fragments, or selective inhibitors have exhibited moderate to high efficacy in early studies, and some of these therapies appear to be highly promising for further drug development. Numerous other biologics targeting various B cell surface molecules have been approved for the treatment of other conditions or are either in clinical trials or preclinical development stages. These approaches remain to be tested in patients with MG or animal models of the disease. This review article provides an overview of B cell-targeted treatments for MG, including those already available and those still in preclinical and clinical development. We also discuss the potential benefits as well as the shortcomings of these approaches to development of new therapies for MG and future directions in the field.

Developmental plasticity allows outside-in immune responses by resident memory T cells

Central memory T (T_CM) cells patrol lymph nodes and perform conventional memory responses on restimulation: proliferation, migration and differentiation into diverse T cell subsets while also self-renewing. Resident memory T (T_RM) cells are parked within single organs, share properties with terminal effectors and contribute to rapid host protection. We observed that reactivated T_RM cells rejoined the circulating pool. Epigenetic analyses revealed that T_RM cells align closely with conventional memory T cell populations, bearing little resemblance to recently activated effectors. Fully differentiated T_RM cells isolated from small intestine epithelium exhibited the potential to differentiate into T_CM cells, effector memory T cells and T_RM cells on recall. Ex-T_RM cells, former intestinal T_RM cells that rejoined the circulating pool, heritably maintained a predilection for homing back to their tissue of origin on subsequent reactivation and a heightened capacity to redifferentiate into T_RM cells. Thus, T_RM cells can rejoin the circulation but are advantaged to re-form local T_RM when called on.

Tissue Determinants of Human NK Cell Development, Function, and Residence

Immune responses in diverse tissue sites are critical for protective immunity and homeostasis. Here, we investigate how tissue localization regulates the development and function of human natural killer (NK) cells, innate lymphocytes important for anti-viral and tumor immunity. Integrating high-dimensional analysis of NK cells from blood, lymphoid organs, and mucosal tissue sites from 60 individuals, we identify tissue-specific patterns of NK cell subset distribution, maturation, and function maintained across age and between individuals. Mature and terminally differentiated NK cells with enhanced effector function predominate in blood, bone marrow, spleen, and lungs and exhibit shared transcriptional programs across sites. By contrast, precursor and immature NK cells with reduced effector capacity populate lymph nodes and intestines and exhibit tissue-resident signatures and site-specific adaptations. Together, our results reveal anatomic control of NK cell development and maintenance as tissue-resident populations, whereas mature, terminally differentiated subsets mediate immunosurveillance through diverse peripheral sites. VIDEO ABSTRACT.

PD-1+ stemlike CD8 T cells are resident in lymphoid tissues during persistent LCMV infection

The migratory patterns of virus-specific CD8 T cells during chronic viral infection are not well understood. To address this issue, we have done parabiosis experiments during chronic lymphocytic choriomeningitis virus (LCMV) infection of mice. We found that despite the high frequency of virus-specific CD8 T cells in both lymphoid and nonlymphoid tissues there was minimal migration of virus-specific CD8 T cells between the chronically infected conjoined parabiont mice. This was in contrast to parabionts between mice that had undergone an acute LCMV infection where virus-specific CD8 T cells established equilibrium demonstrating circulation of memory T cells generated after viral clearance. We have identified a population of PD-1+ TCF1+CXCR5+Tim-3- stemlike virus-specific CD8 T cells that reside in lymphoid tissues and act as resource cells for maintaining the T cell response during chronic infection. These are the cells that proliferate and give rise to the more terminally differentiated PD-1+ CXCR5-Tim-3+ CD8 T cells. Both the stemlike CD8 T cells and their terminally differentiated progeny showed minimal migration during chronic infection and the few LCMV-specific CD8 T cells that were present in circulation were the recently emerging progeny from the stemlike CD8 T cells. The PD-1+ TCF1+CXCR5+ stemlike CD8 T cells were truly resident in lymphoid tissues and did not circulate in the blood. We propose that this residency in specialized niches within lymphoid tissues is a key aspect of their biology and is essential for maintaining their quiescence and stemlike program under conditions of a chronic viral infection.

Polyfunctional, Proinflammatory, Tissue-Resident Memory Phenotype and Function of Synovial Interleukin-17A+CD8+ T Cells in Psoriatic Arthritis

Objective

Genetic associations imply a role for CD8+ T cells and the interleukin‐23 (IL‐23)/IL‐17 axis in psoriatic arthritis (PsA) and other spondyloarthritides (SpA). IL‐17A+CD8+ (Tc17) T cells are enriched in the synovial fluid (SF) of patients with PsA, and IL‐17A blockade is clinically efficacious in PsA/SpA. This study was undertaken to determine the immunophenotype, molecular profile, and function of synovial Tc17 cells in order to elucidate their role in PsA/SpA pathogenesis.

Methods

Peripheral blood (PB) and SF mononuclear cells were isolated from patients with PsA or other types of SpA. Cells were phenotypically, transcriptionally, and functionally analyzed by flow cytometry (n = 6–18), T cell receptor β (TCRβ) sequencing (n = 3), RNA‐Seq (n = 3), quantitative reverse transcriptase–polymerase chain reaction (n = 4), and Luminex or enzyme‐linked immunosorbent assay (n = 4–16).

Results

IL‐17A+CD8+ T cells were predominantly TCRαβ+ and their frequencies were increased in the SF versus the PB of patients with established PsA (P < 0.0001) or other SpA (P = 0.0009). TCRβ sequencing showed that these cells were polyclonal in PsA (median clonality 0.08), while RNA‐Seq and deep immunophenotyping revealed that PsA synovial Tc17 cells had hallmarks of Th17 cells (RORC/IL23R/CCR6/CD161) and Tc1 cells (granzyme A/B). Synovial Tc17 cells showed a strong tissue‐resident memory T (Trm) cell signature and secreted a range of proinflammatory cytokines. We identified CXCR6 as a marker for synovial Tc17 cells, and increased levels of CXCR6 ligand CXCL16 in PsA SF (P = 0.0005), which may contribute to their retention in the joint.

Conclusion

Our results identify synovial Tc17 cells as a polyclonal subset of Trm cells characterized by polyfunctional, proinflammatory mediator production and CXCR6 expression. The molecular signature and functional profiling of these cells may help explain how Tc17 cells can contribute to synovial inflammation and disease persistence in PsA and possibly other types of SpA.

Integrating resident memory into T cell differentiation models

Advances in the field of T cell memory, including the discovery of tissue residency, continue to add to the list of defined T cell subsets. Here, we briefly review the role of resident memory T cells (T_RM) in protective immunity, and propose that they exhibit developmental and migrational plasticity. We discuss T cell classification, the concept of cell type versus 'subset', and the difficulty of inferring developmental relationships between cells occupying malleable differentiation states. We propose that popular subsetting strategies do not perfectly define boundaries of developmental potential. We integrate T_RM into a 'terrace' model that classifies memory T cells along a continuous axis of decreasing developmental potential. This model also segregates cells on the basis of migration properties, although different migration properties are viewed as parallel differentiation states that may be permissive to change.

Using single-cell technologies to map the human immune system - implications for nephrology

Advances in single-cell technologies are transforming our understanding of cellular identity. For instance, the application of single-cell RNA sequencing and mass cytometry technologies to the study of immune cell populations in blood, secondary lymphoid organs and the renal tract is helping researchers to map the complex immune landscape within the kidney, define cell ontogeny and understand the relationship of kidney-resident immune cells with their circulating counterparts. These studies also provide insights into the interactions of immune cell populations with neighbouring epithelial and endothelial cells in health, and across a range of kidney diseases and cancer. These data have translational potential and will aid the identification of drug targets and enable better prediction of off-target effects. The application of single-cell technologies to clinical renal biopsy samples, or even cells within urine, will improve diagnostic accuracy and assist with personalized prognostication for patients with various kidney diseases. A comparison of immune cell types in peripheral blood and secondary lymphoid organs in healthy individuals and in patients with systemic autoimmune diseases that affect the kidney will also help to unravel the mechanisms that underpin the breakdown in self-tolerance and propagation of autoimmune responses. Together, these immune cell atlases have the potential to transform nephrology.

Neutrophils do not impact viral load or the peak of disease severity during RSV infection

Lung and airway neutrophils are a hallmark of severe disease in infants with respiratory syncytial virus (RSV)-induced lower respiratory tract infections. Despite their abundance in the lungs during RSV infection of both mice and man, the role of neutrophils in viral control and in immune pathology is not clear. Here, antibody mediated neutrophil depletion was used to investigate the degree to which neutrophils impact the lung immune environment, the control of viral replication and the peak severity of disease after RSV infection of mice. Neutrophil depletion did not substantially affect the levels of inflammatory mediators such as type I interferons, IL-6, TNF-α or IL-1β in response to RSV. In addition, the lack of neutrophils did not change the viral load during RSV infection. Neither neutrophil depletion nor the enhancement of lung neutrophils by administration of the chemoattractant CXCL1 during RSV infection affected disease severity as measured by weight loss. Therefore, in this model of RSV infection, lung neutrophils do not offer obvious benefits to the host in terms of increasing anti-viral inflammatory responses or restricting viral replication and neutrophils do not contribute to disease severity.

Local heroes or villains: tissue-resident memory T cells in human health and disease

Tissue-resident memory T (T_RM) cells are increasingly associated with the outcomes of health and disease. T_RM cells can mediate local immune protection against infections and cancer, which has led to interest in T_RM cells as targets for vaccination and immunotherapies. However, these cells have also been implicated in mediating detrimental pro-inflammatory responses in autoimmune skin diseases such as psoriasis, alopecia areata, and vitiligo. Here, we summarize the biology of T_RM cells established in animal models and in translational human studies. We review the beneficial effects of T_RM cells in mediating protective responses against infection and cancer and the adverse role of T_RM cells in driving pathology in autoimmunity. A further understanding of the breadth and mechanisms of T_RM cell activity is essential for the safe design of strategies that manipulate T_RM cells, such that protective responses can be enhanced without unwanted tissue damage, and pathogenic T_RM cells can be eliminated without losing local immunity.

CD8+ T Cells Form the Predominant Subset of NKG2A+ Cells in Human Lung Cancer

Background: NKG2A is an inhibitory receptor of both T cells and natural killer (NK) cells. Persistent activation promotes T cells and NK cells to express NKG2A and results in the progression of chronic infection and cancer. However, the characteristics and subsets of NKG2A⁺ lymphocytes in human lung cancer are still unclear.

Methods: Here, we used the Tumor Immune Estimation Resource database and immune profiling of paired biospecimens to uncover the correlation between NKG2A expression and immune infiltration levels in human cancer as well as the characteristics of NKG2A⁺ lymphocytes in human lung cancer.

Results: We found that KLRC1 expression was especially correlated with CD8⁺ T-cell infiltration levels in 34 types of human cancer through the Tumor Immune Estimation Resource database. Moreover, NKG2A⁺ CD8⁺ T cells were the predominant subset of NKG2A⁺ lymphocytes in human lung cancer. In contrast, the NKG2A⁺ NK cells were decreased in tumors compared with the paired normal lung tissue. Tumor-infiltrating NKG2A⁺ CD8⁺ T cells expressed tissue-resident memory T cell (T_RM cell) and exhausted T-cell markers. Cytokines and cytotoxic molecules secreted by tumor-infiltrating NKG2A⁺ CD8⁺ T cells were significantly lower than those secreted by NKG2A⁻ CD8⁺ T cells in vitro. When stimulated with T-cell receptor activator, tumor-infiltrating NKG2A⁺ CD8⁺ T cells could secrete large amounts of granzyme B.

Conclusions: Our findings demonstrate that tumor-infiltrating NKG2A⁺ CD8⁺ T cells form the predominant subset of NKG2A⁺ cells in human lung cancer and suggest that targeting NKG2A⁺ CD8⁺ T cells is a promising approach for future anti-lung cancer immunotherapy.

The Murine Intravaginal HSV-2 Challenge Model for Investigation of DNA Vaccines

DNA vaccines have been licensed in veterinary medicine and have promise for humans. This format is relatively immunogenic in mice and guinea pigs, the two principle HSV-2 animal models, permitting rapid assessment of vectors, antigens, adjuvants, and delivery systems. Limitations include the relatively poor immunogenicity of naked DNA in humans and the profound differences in HSV-2 pathogenesis between host species. Herein, we detail lessons learned investigating candidate DNA vaccines in the progesterone-primed female mouse vaginal model of HSV-2 infection as a guide to investigators in the field.

Prevention of tuberculosis in macaques after intravenous BCG immunization

Mycobacterium tuberculosis (Mtb) is the leading cause of death from infection worldwide¹. The only available vaccine, BCG (Bacillus Calmette-Guérin), is given intradermally and has variable efficacy against pulmonary tuberculosis, the major cause of mortality and disease transmission^1,2. Here we show that intravenous administration of BCG profoundly alters the protective outcome of Mtb challenge in non-human primates (Macaca mulatta). Compared with intradermal or aerosol delivery, intravenous immunization induced substantially more antigen-responsive CD4 and CD8 T cell responses in blood, spleen, bronchoalveolar lavage and lung lymph nodes. Moreover, intravenous immunization induced a high frequency of antigen-responsive T cells across all lung parenchymal tissues. Six months after BCG vaccination, macaques were challenged with virulent Mtb. Notably, nine out of ten macaques that received intravenous BCG vaccination were highly protected, with six macaques showing no detectable levels of infection, as determined by positron emission tomography-computed tomography imaging, mycobacterial growth, pathology and granuloma formation. The finding that intravenous BCG prevents or substantially limits Mtb infection in highly susceptible rhesus macaques has important implications for vaccine delivery and clinical development, and provides a model for defining immune correlates and mechanisms of vaccine-elicited protection against tuberculosis.

Role of the PD-1/PD-L1 Dyad in the Maintenance of Pancreatic Immune Tolerance for Prevention of Type 1 Diabetes

The human pancreas, like almost all organs in the human body, is immunologically tolerated despite the presence of innate and adaptive immune cells that promptly mediate protective immune responses against pathogens in situ. The PD-1/PD-L1 inhibitory pathway seems to play a key role in the maintenance of immune tolerance systemically and within the pancreatic tissue. Tissue resident memory T cells (TRM), T regulatory cells (Treg), macrophages and even β cells exhibit PD-1 or PD-L1 expression that contributes in controlling pancreatic immune homeostasis and tolerance. Dysregulation of the PD-1/PD-L1 axis as shown by animal studies and our recent experience with checkpoint inhibitory blockade in humans can lead to immune dysfunctions leading to chronic inflammatory disease and to type 1 diabetes (T1D) in genetically susceptible individuals. In this review, we discuss the role of the PD-1/PD-L1 axis in pancreatic tissue homeostasis and tolerance, speculate how genetic and environmental factors can regulate the PD-1/PD-L1 pathway, and discuss PD-1/PD-L1-based therapeutic approaches for pancreatic islet transplantation and T1D treatment.

Generation of protective pneumococcal-specific nasal resident memory CD4+ T cells via parenteral immunization

The generation of tissue-resident memory T cells (T_RM) is an essential aspect of immunity at mucosal surfaces, and it has been suggested that preferential generation of T_RM is one of the principal advantages of mucosally administered vaccines. We have previously shown that antigen-specific, IL-17-producing CD4⁺ T cells can provide capsular antibody-independent protection against nasal carriage of Streptococcus pneumoniae; but whether pneumococcus-responsive T_RM are localized within the nasal mucosa and are sufficient for protection from carriage has not been determined. Here, we show that intranasal administration of live or killed pneumococci to mice generates pneumococcus-responsive IL-17A-producing CD4⁺ mucosal T_RM. Furthermore, we show that these cells are sufficient to mediate long-lived, neutrophil-dependent protection against subsequent pneumococcal nasal challenge. Unexpectedly, and in contrast with the prevailing paradigm, we found that parenteral administration of killed pneumococci also generates protective IL-17A⁺CD4⁺ T_RM in the nasal mucosa. These results demonstrate a critical and sufficient role of T_RM in prevention of pneumococcal colonization, and further that these cells can be generated by parenteral immunization. Our findings therefore have important implications regarding the generation of immune protection at mucosal surfaces by vaccination.

You Shall Not Pass: Memory CD8 T Cells in Liver-Stage Malaria

Each year over 200 million malaria infections occur, with over 400 000 associated deaths. Vaccines formed with attenuated whole parasites can induce protective memory CD8 T cell responses against liver-stage malaria; however, widespread administration of such vaccines is logistically challenging. Recent scientific findings are delineating how protective memory CD8 T cell populations are primed and maintained and how such cells mediate immunity to liver-stage malaria. Memory CD8 T cell anatomic localization and expression of transcription factors, homing receptors, and signaling molecules appear to play integral roles in protective immunity to liver-stage malaria. Further investigation of how such factors contribute to optimal protective memory CD8 T cell generation and maintenance in humans will inform efforts for improved vaccines.

Decoding the Body Language of Immunity: Tackling the Immune System at the Organism Level

The immune system is a dynamic mesh of molecules, cells and tissues spanning the entire organism. Despite a wealth of knowledge about the components of the immune system, little is known about the general rules governing the organismal circuitry of immunity. Deciphering the immune system at the scale of the whole organism is crucial to understanding fundamental problems in immunobiology and physiology, and to manipulate immunity for maintaining health and preventing disease. Here I discuss the emerging principles of inter-organ communications during immune responses by focusing on three common themes that are the regulation of the (i) composition, (ii) condition and (iii) coordination of communicating organs by molecular and cellular factors. Based on these common principles, I emphasize fundamental gaps in our knowledge of organismal immune processes and the outlook to tackle immunity at the scale of the whole organism.

Chemokine regulation of inflammation during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) can cause severe lower respiratory tract infections especially in infants, immunocompromised individuals and the elderly and is the most common cause of infant hospitalisation in the developed world. The immune responses against RSV are crucial for viral control and clearance but, if dysregulated, can also result in immunopathology and impaired gas exchange. Lung immunity to RSV and other respiratory viruses begins with the recruitment of immune cells from the bloodstream into the lungs. This inflammatory process is controlled largely by chemokines, which are small proteins that are produced in response to innate immune detection of the virus or the infection process. These chemokines serve as chemoattractants for granulocytes, monocytes, lymphocytes and other leukocytes. In this review, we highlight recent advances in the field of RSV infection and disease, focusing on how chemokines regulate virus-induced inflammation.

Monalizumab: inhibiting the novel immune checkpoint NKG2A

The implementation of immune checkpoint inhibitors to the oncology clinic signified a new era in cancer treatment. After the first indication of melanoma, an increasing list of additional cancer types are now treated with immune system targeting antibodies to PD-1, PD-L1 and CTLA-4, alleviating inhibition signals on T cells. Recently, we published proof-of-concept results on a novel checkpoint inhibitor, NKG2A. This receptor is expressed on cytotoxic lymphocytes, including NK cells and subsets of activated CD8+ T cells. Blocking antibodies to NKG2A unleashed the reactivity of these effector cells resulting in tumor control in multiple mouse models and an early clinical trial. Monalizumab is inhibiting this checkpoint in human beings and future clinical trials will have to reveal its potency in combination with other cancer treatment options.

An innate-like Vδ1+ γδ T cell compartment in the human breast is associated with remission in triple-negative breast cancer

Innate-like tissue-resident γδ T cell compartments capable of protecting against carcinogenesis are well established in mice. Conversely, the degree to which they exist in humans, their potential properties, and their contributions to host benefit are mostly unresolved. Here, we demonstrate that healthy human breast harbors a distinct γδ T cell compartment, primarily expressing T cell receptor (TCR) Vδ1 chains, by comparison to Vδ2 chains that predominate in peripheral blood. Breast-resident Vδ1+ cells were functionally skewed toward cytolysis and IFN-γ production, but not IL-17, which has been linked with inflammatory pathologies. Breast-resident Vδ1+ cells could be activated innately via the NKG2D receptor, whereas neighboring CD8+ αβ T cells required TCR signaling. A comparable population of Vδ1+ cells was found in human breast tumors, and when paired tumor and nonmalignant samples from 11 patients with triple-negative breast cancer were analyzed, progression-free and overall survival correlated with Vδ1+ cell representation, but not with either total γδ T cells or Vδ2+ T cells. As expected, progression-free survival also correlated with αβ TCRs. However, whereas in most cases TCRαβ repertoires focused, typical of antigen-specific responses, this was not observed for Vδ1+ cells, consistent with their innate-like responsiveness. Thus, maximal patient benefit may accrue from the collaboration of innate-like responses mounted by tissue-resident Vδ1+ compartments and adaptive responses mounted by αβ T cells.

CD103hi Treg cells constrain lung fibrosis induced by CD103lo tissue-resident pathogenic CD4 T cells

Tissue-resident memory T cells (T_RM cells) are crucial mediators of adaptive immunity in nonlymphoid tissues. However, the functional heterogeneity and pathogenic roles of CD4⁺ T_RM cells that reside within chronic inflammatory lesions remain unknown. We found that CD69^hiCD103^lo CD4⁺ T_RM cells produced effector cytokines and promoted the inflammation and fibrotic responses induced by chronic exposure to Aspergillus fumigatus. Simultaneously, immunosuppressive CD69^hiCD103^hiFoxp3⁺ CD4⁺ regulatory T cells were induced and constrained the ability of pathogenic CD103^lo T_RM cells to cause fibrosis. Thus, lung tissue-resident CD4⁺ T cells play crucial roles in the pathology of chronic lung inflammation, and CD103 expression defines pathogenic effector and immunosuppressive tissue-resident cell subpopulations in the inflamed lung.

Adventitial Cuffs: Regional Hubs for Tissue Immunity

Inflammation must be effective, while limiting excessive tissue damage. To walk this line, immune functions are grossly compartmentalized by innate cells that act locally and adaptive cells that function systemically. But what about the myriad tissue-resident immune cells that are critical to this balancing act and lie on a spectrum of innate and adaptive immunity? We propose that mammalian perivascular adventitial 'cuffs' are conserved sites in multiple organs, enriched for these tissue-resident lymphocytes and dendritic cells, as well as lymphatics, nerves, and subsets of specialized stromal cells. Here, we argue that these boundary sites integrate diverse tissue signals to regulate the movement of immune cells and interstitial fluid, facilitate immune crosstalk, and ultimately act to coordinate regional tissue immunity.

JMJD3 regulates CD4 T cell trafficking by targeting actin cytoskeleton regulatory gene Pdlim4

Histone H3K27 demethylase, JMJD3 plays a critical role in gene expression and T-cell differentiation. However, the role and mechanisms of JMJD3 in T cell trafficking remain poorly understood. Here we show that JMJD3 deficiency in CD4+ T cells resulted in an accumulation of T cells in the thymus, and reduction of T cell number in the secondary lymphoid organs. We identified PDLIM4 as a significantly down-regulated target gene in JMJD3-deficient CD4+ T cells by gene profiling and ChIP-seq analyses. We further showed that PDLIM4 functioned as an adaptor protein to interact with S1P1 and filamentous actin (F-actin), thus serving as a key regulator of T cell trafficking. Mechanistically, JMJD3 bound to the promoter and gene body regions of Pdlim4 gene and regulated its expression by interacting with zinc finger transcription factor KLF2. Our findings have identified Pdlim4 as a JMJD3 target gene that affects T-cell trafficking by cooperating with S1P1, and provided insights into the molecular mechanisms by which JMJD3 regulates genes involved in T cell trafficking.

Stability and flexibility in chromatin structure and transcription underlies memory CD8 T-cell differentiation

The process by which naïve CD8 T cells become activated, accumulate, and terminally differentiate as well as develop into memory cytotoxic T lymphocytes (CTLs) is central to the development of potent and durable immunity to intracellular infections and tumors. In this review, we discuss recent studies that have elucidated ancestries of short-lived and memory CTLs during infection, others that have shed light on gene expression programs manifest in individual responding cells and chromatin remodeling events, remodeling factors, and conventional DNA-binding transcription factors that stabilize the differentiated states after activation of naïve CD8 T cells. Several models have been proposed to conceptualize how naïve cells become memory CD8 T cells. A parsimonious solution is that initial naïve cell activation induces metastable gene expression in nascent CTLs, which act as progenitor cells that stochastically diverge along pathways that are self-reinforcing and result in shorter- versus longer-lived CTL progeny. Deciphering how regulatory factors establish and reinforce these pathways in CD8 T cells could potentially guide their use in immunotherapeutic contexts.

Human lung tissue resident memory T cells in health and disease

The human lung contains a heterogeneous population of immune cells which mediate protective responses, maintain tissue homeostasis, but can also promote immunopathology in disease. The majority of T cells in the human lung are tissue resident memory T cells (T_RM) which have been shown in mouse models to provide vital roles in the protection against multiple respiratory pathogens, and contribute to heterosubtypic protection in the context of vaccination. In this review, we will discuss recent studies in humans identifying lung TRM, their role in maintaining tissue homeostasis, and emerging evidence implicating T_RM in anti-tumor immunity and immune surveillance as well as their potential for immunopathology in chronic airway inflammation.

The Impact of TCR Signal Strength on Resident Memory T Cell Formation during Influenza Virus Infection

Resident memory T cells (T_RM) in the lung are vital for heterologous protection against influenza A virus (IAV). Environmental factors are necessary to establish lung T_RM; however, the role of T cell-intrinsic factors like TCR signal strength have not been elucidated. In this study, we investigated the impact of TCR signal strength on the generation and maintenance of lung T_RM after IAV infection. We inserted high- and low-affinity OT-I epitopes into IAV and infected mice after transfer of OT-I T cells. We uncovered a bias in T_RM formation in the lung elicited by lower affinity TCR stimulation. TCR affinity did not impact the overall phenotype or long-term maintenance of lung T_RM Overall, these findings demonstrate that T_RM formation is negatively correlated with increased TCR signal strength. Lower affinity cells may have an advantage in forming T_RM to ensure diversity in the Ag-specific repertoire in tissues.

Pertussis vaccines and protective immunity

Despite high vaccine coverage, reported cases of pertussis have increased steadily over the last twenty years. This resurgence has stimulated interest in host responses to pertussis infection and vaccination with the goal of developing more effective next-generation vaccines and vaccination strategies. Optimal protection against Bordetella pertussis appears to be multifactorial requiring both humoral and cellular responses. Natural infection and whole-cell pertussis vaccination induce Th1 and Th17-dominated responses. In contrast, acellular vaccines induce Th2-dominated responses. Available immunological data indicate that while antibodies provide protection against disease, Th1 and Th17-mediated immune responses are required for bacterial clearance and long-lasting protection. The nature of the priming in children appears to be important in modulating bias and durability of immune responses required to provide protection against B. pertussis. This review summarizes the current understanding of differences in immune responses and their role in protection against B. pertussis following infection or vaccination.

Lung Tissue Resident Memory T-Cells in the Immune Response to Mycobacterium tuberculosis

Despite widespread BCG vaccination and effective anti-TB drugs, Tuberculosis (TB) remains the leading cause of death from an infectious agent worldwide. Several recent publications give reasons to be optimistic about the possibility of a more effective vaccine, but the only full-scale clinical trial conducted failed to show protection above BCG. The immunogenicity of vaccines in humans is primarily evaluated by the systemic immune responses they generate, despite the fact that a correlation between these responses and protection from TB disease has not been demonstrated. A novel approach to tackling this problem is to study the local immune responses that occur at the site of TB infection in the human lung, rather than those detectable in blood. There is a growing understanding that pathogen specific T-cell immunity can be highly localized at the site of infection, due to the existence of tissue resident memory T-cells (Trm). Notably, these cells do not recirculate in the blood and thus may not be represented in studies of the systemic immune response. Here, we review the potential role of Trms as a component of the TB immune response and discuss how a better understanding of this response could be harnessed to improve TB vaccine efficacy.