Measuring the diaspora for virus-specific CD8+ T cells

A novel tetravalent influenza vaccine based on one chimpanzee adenoviral vector

Highly effective and broad-spectrum influenza vaccines are urgently required to prevent influenza outbreaks. Hemagglutinin (HA), M2 ectodomain (M2e), and nucleoprotein (NP) are crucial target antigens for the development of universal influenza vaccines. To generate a novel multivalent influenza vaccine, the HA genes of influenza B Yamagata (BY) and Victoria (BV) strains, and the NP gene of H1N1 were cloned into the E1 region of the chimpanzee adenoviral vector, AdC68, and M2e epitopes of H1N1 and H3N2 were fused to the loop region of the AdC68 fiber, resulting in the recombinant adenoviral vector vaccine, AdC-Flu-Tet. The immunoprotective effects of AdC-Flu-Tet were evaluated in the mouse models. The results showed that AdC-Flu-Tet successfully induced robust humoral and cellular immune responses and conferred full protection against H1N1, H3N2, BY, and BV infections. In conclusion, AdC-Flu-Tet is a promising candidate as a novel influenza vaccine with high protective efficacy.

Lung Immune Cell Niches and the Discovery of New Cell Subtypes

Immune cells in the lungs are important for maintaining lung function. The importance of immune cells in defending against lung diseases and infections is increasingly recognized. However, a primary knowledge gaps in current studies of lung immune cells is the understanding of their subtypes and functional heterogeneity. Increasing evidence supports the existence of novel immune cell subtypes that engage in the complex crosstalk between lung-resident immune cells, recruited immune cells, and epithelial cells. Therefore, further studies on how immune cells respond to perturbations in the pulmonary microenvironment are warranted. This review explores the processes behind the formation of the immune cell niche during lung development, and the characteristics and cell interaction modes of several major lung-resident immune cells. It indicates that distinct lung microenvironments or inflammatory niches can mediate the formation of different cell subtypes. These findings summarize and clarify paths to identify new cell subtypes that originate from resident progenitor cells and recruited peripheral cells, which are remodeled by the pulmonary microenvironment. The development of new techniques combining transcriptome analysis and location information is essential for identifying new immune cell subtypes and their relative immune niches, as well as for uncovering the molecular mechanisms of immune cell-mediated lung homeostasis.

Causality between immunocytes and polymyositis: A Mendelian randomization analysis

Polymyositis is a prominent subgroup of idiopathic inflammatory myopathy, considered to have an autoimmune etiology. However, research exploring the condition between immunocytes and polymyositis remains limited, indicating the need for further investigation to unravel these intricate associations. We employed bidirectional Mendelian randomization (MR) analysis to ascertain causality between 731 immunocytes and polymyositis. We also compared the positive immunocytes with dermatomyositis. Our primary analytical method was inverse variance weighted, supplemented by 4 other MR techniques. Additionally, Cochran Q test was performed to assess heterogeneity, MR-Egger to appraise pleiotropy, and MR-PRESSO to identify and eliminate potential outliers. Furthermore, the leave-one-out test evaluated the impact of each instrumental variable (IV) on the causal effect. The inverse variance weighted results revealed that 10 immunocytes exert a protective effect against polymyositis (P < .05, OR < 1), while 16 immunocytes are connected with an elevated risk of the disease (P < .05, OR > 1). In reverse MR, polymyositis was found to decrease the levels of 2 immune cells (P < .05, OR < 1) and elevate the expression of 5 immune cell phenotypes (P < .05, OR > 1). A complex correlation was found between polymyositis and the immunocyte phenotypes CD8, CD33dim, HLA-DR, CD11b, and CD45. Additionally, it was discovered that 15 types of immune cells share a causal relationship between polymyositis and dermatomyositis. All analyses demonstrated no heterogeneity or horizontal pleiotropy (P > .05). Our study provides compelling evidence regarding the intricate causal relationships between immunocytes and polymyositis. Polymyositis and dermatomyositis share common immunocytes' regulatory mechanisms. CD8, CD33dim, HLA-DR, CD11b, and CD45 may represent potential immune cell markers for polymyositis. These findings hold implications for planning prognosis and therapeutic strategies for polymyositis, offering novel insights for drug development.

Tumor-Specific CD8+ T Cells from the Bone Marrow Resist Exhaustion and Exhibit Increased Persistence in Tumor-Bearing Hosts as Compared to Tumor Infiltrating Lymphocytes

Immunotherapy is now an integral aspect of cancer therapy. Strategies employing adoptive cell therapy (ACT) have seen the establishment of chimeric antigen receptor (CAR)-T cells using peripheral blood lymphocytes as well as tumor infiltrating lymphocytes (TILs) with significant clinical results. Despite these successes, the limitations of the current strategies are also emerging and novel approaches are needed. The bone marrow (BM) is an immunological niche that houses T cells with specificity for previously encountered antigens, including tumor-associated antigens from certain solid cancers. This study sought to improve our understanding of tumor-specific BM T cells in the context of solid tumors by comparing them with TILs, and to assess whether there is a rationale for using the BM as a source of T cells for ACT against solid malignancies. Herein, we demonstrate that T cells from the BM appear superior to TILs as a source of cells for cellular therapy. Specifically, they possess a memory-enriched phenotype and exhibit improved effector function, greater persistence within a tumor-bearing host, and the capacity for increased tumor infiltration. Taken together, these data provide a foundation for further exploring the BM as a source of tumor-specific T cells for ACT in solid malignancies.

Understanding the Role of HLA Class I Molecules in the Immune Response to Influenza Infection and Rational Design of a Peptide-Based Vaccine

Influenza A virus is a respiratory pathogen that is responsible for regular epidemics and occasional pandemics that result in substantial damage to life and the economy. The yearly reformulation of trivalent or quadrivalent flu vaccines encompassing surface glycoproteins derived from the current circulating strains of the virus does not provide sufficient cross-protection against mismatched strains. Unlike the current vaccines that elicit a predominant humoral response, vaccines that induce CD8+ T cells have demonstrated a capacity to provide cross-protection against different influenza strains, including novel influenza viruses. Immunopeptidomics, the mass spectrometric identification of human-leukocyte-antigen (HLA)-bound peptides isolated from infected cells, has recently provided key insights into viral peptides that can serve as potential T cell epitopes. The critical elements required for a strong and long-living CD8+ T cell response are related to both HLA restriction and the immunogenicity of the viral peptide. This review examines the importance of HLA and the viral immunopeptidome for the design of a universal influenza T-cell-based vaccine.

Persistent Antigen Harbored by Alveolar Macrophages Enhances the Maintenance of Lung-Resident Memory CD8+ T Cells

Lung tissue-resident memory T cells are crucial mediators of cellular immunity against respiratory viruses; however, their gradual decline hinders the development of T cell-based vaccines against respiratory pathogens. Recently, studies using adenovirus (Ad)-based vaccine vectors have shown that the number of protective lung-resident CD8+ TRMs can be maintained long term. In this article, we show that immunization of mice with a replication-deficient Ad serotype 5 expressing influenza (A/Puerto Rico/8/34) nucleoprotein (AdNP) generates a long-lived lung TRM pool that is transcriptionally indistinct from those generated during a primary influenza infection. In addition, we demonstrate that CD4+ T cells contribute to the long-term maintenance of AdNP-induced CD8+ TRMs. Using a lineage tracing approach, we identify alveolar macrophages as a cell source of persistent NP Ag after immunization with AdNP. Importantly, depletion of alveolar macrophages after AdNP immunization resulted in significantly reduced numbers of NP-specific CD8+ TRMs in the lungs and airways. Combined, our results provide further insight to the mechanisms governing the enhanced longevity of Ag-specific CD8+ lung TRMs observed after immunization with recombinant Ad.

T cell-specific constitutive active SHP2 enhances T cell memory formation and reduces T cell activation

Upon antigen recognition by the T cell receptor (TCR), a complex signaling network orchestrated by protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs) regulates the transmission of the extracellular signal to the nucleus. The role of the PTPs Src-homology 2 (SH2) domain-containing phosphatase 1 (SHP1, Ptpn6) and Src-homology 2 (SH2) domain-containing phosphatase 2 (SHP2, Ptpn11) have been studied in various cell types including T cells. Whereas SHP1 acts as an essential negative regulator of the proximal steps in T cell signalling, the role of SHP2 in T cell activation is still a matter of debate. Here, we analyzed the role of the constitutively active SHP2-D61Y-mutant in T cell activation using knock-in mice expressing the mutant form Ptpn11D61Y in T cells. We observed reduced numbers of CD8+ and increased numbers of CD4+ T cells in the bone marrow and spleen of young and aged SHP2-D61Y-mutant mice as well as in Influenza A Virus (IAV)-infected mice compared to controls. In addition, we found elevated frequencies of effector memory CD8+ T cells and an upregulation of the programmed cell death protein 1 (PD-1)-receptor on both CD4+ and CD8+ T cells. Functional analysis of SHP2-D61Y-mutated T cells revealed an induction of late apoptosis/necrosis, a reduced proliferation and altered signaling upon TCR stimulation. However, the ability of D61Y-mutant mice to clear viral infection was not affected. In conclusion, our data indicate an important regulatory role of SHP2 in T cell function, where the effect is determined by the kinetics of SHP2 phosphatase activity and differs in the presence of the permanently active and the temporally regulated phosphatase. Due to interaction of SHP2 with the PD-1-receptor targeting the protein-tyrosine phosphatase might be a valuable tool to enhance T cell activities in immunotherapy.

Influenza infection fortifies local lymph nodes to promote lung-resident heterosubtypic immunity

Influenza infection generates tissue-resident memory T cells (TRMs) that are maintained in the lung and can mediate protective immunity to heterologous influenza strains, but the precise mechanisms of local T cell-mediated protection are not well understood. In a murine heterosubtypic influenza challenge model, we demonstrate that protective lung T cell responses derive from both in situ activation of TRMs and the enhanced generation of effector T cells from the local lung draining mediastinal lymph nodes (medLNs). Primary infection fortified the medLNs with an increased number of conventional dendritic cells (cDCs) that mediate enhanced priming of T cells, including those specific for newly encountered epitopes; cDC depletion during the recall response diminished medLN T cell generation and heterosubtypic immunity. Our study shows that during a protective recall response, cDCs in a fortified LN environment enhance the breadth, generation, and tissue migration of effector T cells to augment lung TRM responses.

Systems Immunology: Revealing Influenza Immunological Imprint

Understanding protective influenza immunity and identifying immune correlates of protection poses a major challenge and requires an appreciation of the immune system in all of its complexity. While adaptive immune responses such as neutralizing antibodies and influenza-specific T lymphocytes are contributing to the control of influenza virus, key factors of long-term protection are not well defined. Using systems immunology, an approach that combines experimental and computational methods, we can capture the systems-level state of protective immunity and reveal the essential pathways that are involved. New approaches and technological developments in systems immunology offer an opportunity to examine roles and interrelationships of clinical, biological, and genetic factors in the control of influenza infection and have the potential to lead to novel discoveries about influenza immunity that are essential for the development of more effective vaccines to prevent future pandemics. Here, we review recent developments in systems immunology that help to reveal key factors mediating protective immunity.

CD8+ T cell landscape in Indigenous and non-Indigenous people restricted by influenza mortality-associated HLA-A*24:02 allomorph

Indigenous people worldwide are at high risk of developing severe influenza disease. HLA-A*24:02 allele, highly prevalent in Indigenous populations, is associated with influenza-induced mortality, although the basis for this association is unclear. Here, we define CD8⁺ T-cell immune landscapes against influenza A (IAV) and B (IBV) viruses in HLA-A*24:02-expressing Indigenous and non-Indigenous individuals, human tissues, influenza-infected patients and HLA-A*24:02-transgenic mice. We identify immunodominant protective CD8⁺ T-cell epitopes, one towards IAV and six towards IBV, with A24/PB2_550-558-specific CD8⁺ T cells being cross-reactive between IAV and IBV. Memory CD8⁺ T cells towards these specificities are present in blood (CD27⁺CD45RA^- phenotype) and tissues (CD103⁺CD69⁺ phenotype) of healthy individuals, and effector CD27^-CD45RA^-PD-1⁺CD38⁺CD8⁺ T cells in IAV/IBV patients. Our data show influenza-specific CD8⁺ T-cell responses in Indigenous Australians, and advocate for T-cell-mediated vaccines that target and boost the breadth of IAV/IBV-specific CD8⁺ T cells to protect high-risk HLA-A*24:02-expressing Indigenous and non-Indigenous populations from severe influenza disease.

CD8+ T-Cell Memory: The Why, the When, and the How

The generation of effective adaptive T-cell memory is a cardinal feature of the adaptive immune system. The establishment of protective T-cell immunity requires the differentiation of CD8⁺ T cells from a naive state to one where pathogen-specific memory CD8⁺ T cells are capable of responding to a secondary infection more rapidly and robustly without the need for further differentiation. The study of factors that determine the fate of activated CD8⁺ T cells into either effector or memory subsets has a long history. The advent of new technologies is now providing new insights into how epigenetic regulation not only impacts acquisition and maintenance of effector function, but also the maintenance of the quiescent yet primed memory state. There is growing appreciation that rather than distinct subsets, memory T-cell populations may reflect different points on a spectrum between the starting naive T-cell population and a terminally differentiated effector CD8⁺ T-cell population. Interestingly, there is growing evidence that the molecular mechanisms that underpin the rapid effector function of memory T cells are also observed in innate immune cells such as macrophages and natural killer (NK) cells. This raises an interesting hypothesis that the memory/effector T-cell state represents a default innate-like response to antigen recognition, and that it is the naive state that is the defining feature of adaptive immunity. These issues are discussed.

Cytokine Panels and Pediatric Acute Respiratory Distress Syndrome: A Translational Investigation

OBJECTIVES

To identify and compare serum and lower respiratory tract fluid biomarkers of lung injury using well-characterized mouse models of lung injury. To explore the relationship between these preclinical biomarkers and clinical outcomes in a discovery cohort of pediatric patients with acute respiratory failure from pneumonia.

DESIGN

Prospective, observational cohort study.

SETTING

A basic science laboratory and the PICU of a tertiary-care children's hospital.

PATIENTS

PICU patients intubated for respiratory failure from a suspected respiratory infection.

INTERVENTIONS

Prospective enrollment and collection of lower respiratory tract fluid samples.

MEASUREMENTS AND MAIN RESULTS

C57BL6/J mice were intranasally inoculated with escalating doses of influenza A virus or toll-like receptor agonists to simulate varying degrees of lung injury. Serum and bronchoalveolar lavage fluid were measured for the presence of cytokines using commercially available multiplex cytokine assays. Elevated levels of C-C motif chemokine ligand 7 at the peak of inflammation in both bronchoalveolar lavage fluid and serum correlated with lethality, with the bronchoalveolar lavage fluid ratio of C-C motif chemokine ligand 7:C-C motif chemokine ligand 22 providing the best prediction in the mouse models. These preclinical biomarkers were examined in the plasma and lower respiratory tract fluid of a discovery cohort of pediatric patients with acute respiratory failure from pneumonia. The primary clinical outcome measure was ventilator-free days, with secondary outcomes of pediatric acute respiratory distress syndrome severity and mortality. Elevation in peak lower respiratory tract fluid C-C motif chemokine ligand 7:C-C motif chemokine ligand 22 ratios demonstrated a significant negative correlation with ventilator-free days (r = -0.805; p < 0.02).

CONCLUSIONS

This study provides evidence that lung immune profiling via lower respiratory tract fluid cytokine analysis is feasible and may provide insight into clinical outcomes. Further validation of markers, including the C-C motif chemokine ligand 7:C-C motif chemokine ligand 22 ratio in this limited study, in a larger cohort of patients is necessary.

Impact of Pre-Existing Immunity to Influenza on Live-Attenuated Influenza Vaccine (LAIV) Immunogenicity

During the previous influenza seasons, between 2010 and 2016, the live attenuated influenza vaccine (LAIV) provided variable efficacy against influenza in the U.S., causing the recommendation against the use of the LAIV. In striking contrast, pre-clinical studies have repeatedly demonstrated superior efficacy of LAIV against mismatched influenza viruses, compared to inactivated influenza vaccines (IIV). This disparity in reported vaccine efficacies between pre-clinical and clinical studies may in part be explained by limitations of the animal models of influenza. In particular, the absence of pre-existing immunity in animal models has recently emerged as a potential explanation for the discrepancies between preclinical findings and human studies. This commentary focuses on the potential impact of pre-existing immunity on LAIV induced immunogenicity with an emphasis on cross-protective immunity.

Tissue-resident lymphocytes: weaponized sentinels at barrier surfaces

Tissue-resident immune cells stably localize in tissues largely independent of the circulatory system. While initial studies have focused on the recognition of CD8 ⁺ tissue-resident memory T (CD8 T _RM) cells, it is now clear that numerous cell types such as CD4 ⁺ T cells, gd T cells, innate lymphoid cells and mucosal-associated invariant T (MAIT) cells form stable populations in tissues. They are enriched at the barrier surfaces and within non-lymphoid compartments. They provide an extensive immune network capable of sensing local perturbations of the body’s homeostasis. This positioning enables immune cells to positively influence immune protection against infection and cancer but paradoxically also augment autoimmunity, allergy and chronic inflammatory diseases. Here, we highlight the recent studies across multiple lymphoid immune cell types that have emerged on this research topic and extend our understanding of this important cellular network. In addition, we highlight the areas that remain gaps in our knowledge of the regulation of these cells and how a deeper understanding may result in new ways to ‘target’ these cells to influence disease outcome and treatments.

Elucidating Specificity Opens a Window to the Complexity of Both the Innate and Adaptive Immune Systems

Science is a tedious and painstaking business. Many discoveries are considered incremental, individually not necessarily earth shattering, but collectively providing the critical broad framework on which pivotal insights can emerge. Transformational discoveries spring from this knowledge legacy of others and spur a fervent discovery process, often driven by technological developments. The seminal discovery of major histocompatibility class restriction I (MHCI) and its role in antiviral infections by Doherty and Zinkernagel in 1974 was one such discovery-the key that unlocked the treasure chest to the rich tapestry of the diversity of the immune system. An army of researchers have teased apart the different elements of the immune response, which now brings us to a deeper understanding of immune memory and protective immunity. In this process, it has uncovered a multitude of cell types that bridge the innate and adaptive arms of the immune system-blurring the line between these two branches-and ultimately fortifying the development of long-term immune protection.

T cells and the skin: from protective immunity to inflammatory skin disorders

Skin is our primary interface with the environment, and T cells are crucial for orchestrating host immune responses against pathogenic microorganisms at this site. Effective skin immune responses require the generation of antigen-specific effector T cells, which home to cutaneous sites of injury or infection. Long-lasting immunity against future immune challenges is mediated by memory T cells. Among the memory T cells found in skin are both recirculating cells that transit between skin and blood and tissue-resident memory T (TRM) cells, which remain in skin for long periods of time and mediate durable protective immunity. These TRM cells also appear to drive many inflammatory diseases of skin. Here, we consider how a better understanding of cutaneous T cell responses can aid in the development of effective new therapies for immune-mediated cutaneous diseases.

Immunity to Respiratory Infection Is Reinforced Through Early Proliferation of Lymphoid TRM Cells and Prompt Arrival of Effector CD8 T Cells in the Lungs

Cross-protection between serologically distinct strains of influenza A virus (IAV) is mediated by memory CD8 T cells that recognize epitopes from conserved viral proteins. Early viral control begins with activation of tissue-resident memory CD8 T cells (T_RM) cells at the site of viral replication. These CD8 T cells do not act in isolation, as protection against disseminated infection is reinforced by multiple waves of effector cells (T_EFF) that enter the lungs with different kinetics. To define how a protective CTL response evolves, we compared the functional properties of antiviral CD8 T cells in the respiratory tract and local lymphoid tissues. When analyzed 30 dpi, large numbers of antiviral CD8 T cells in the lungs and mediastinal lymph nodes (MLNs) expressed canonical markers of T_RM cells (CD69 and/or CD103). The check point inhibitor PD-1 was also highly expressed on NP-specific CD8 T cells in the lungs, while the ratios of CD8 T cells expressing CD69 and CD103 varied according to antigen specificity. We next used in vitro experiments to identify conditions that induce a canonical T_RM phenotype and found that that naïve and newly activated CD8 T cells maintain CD103 expression during culture with transforming growth factor-beta (TGFβ), while central memory CD8 T cells (T_CM) do not express CD103 under similar conditions. In vivo experiments showed that the distribution of antiviral CTLs in the MLN changed when immune mice were treated with reagents that block interactions with PD-L1. Importantly, the lymphoid T_RM cells were poised for early proliferation upon reinfection with a different strain of IAV and defenses in the lungs were augmented by a transient increase in numbers of T_EFF cells at the site of infection. As the interval between infections increased, lymphoid T_RM cells were replaced with T_CM cells which proliferated with delayed kinetics and contributed to an exaggerated inflammatory response in the lungs.

Parallel worlds of the adaptive and innate immune cell networks

Adaptive and innate immune cells have typically been functionally and temporally segregated even though they share a number of salient features. Over the past decade, significant advances have been made in understanding the composition and diversity of both innate and adaptive cell populations. This has shed light on how cells from two distinct pathways are so highly complementary. Innate lymphoid cells (ILCs) are pivotally positioned in tissues to form a stable population akin to tissue-resident T cells that protects the body. Nevertheless, the pathway by which different lymphocytes enter tissues, terminally differentiate and are replenished to maintain populations remains incompletely understood. Recent evidence challenges our assumptions about the sedentary lifestyles of so called 'tissue-resident cells' and pushes us to consider their roles in orchestrating protection of the immune system beyond the classical models.

Single-Cell Approach to Influenza-Specific CD8+ T Cell Receptor Repertoires Across Different Age Groups, Tissues, and Following Influenza Virus Infection

CD8⁺ T cells recognizing antigenic peptides derived from conserved internal viral proteins confer broad protection against distinct influenza viruses. As memory CD8⁺ T cells change throughout the human lifetime and across tissue compartments, we investigated how T cell receptor (TCR) composition and diversity relate to memory CD8⁺ T cells across anatomical sites and immunological phases of human life. We used ex vivo peptide-HLA tetramer magnetic enrichment, single-cell multiplex RT-PCR for both the TCR-alpha (TCRα) and TCR-beta (TCRβ) chains, and new TCRdist and grouping of lymphocyte interactions by paratope hotspots (GLIPH) algorithms to compare TCRs directed against the most prominent human influenza epitope, HLA-A*02:01-M1_58–66 (A2⁺M1₅₈). We dissected memory TCR repertoires directed toward A2⁺M1₅₈ CD8⁺ T cells within human tissues and compared them to human peripheral blood of young and elderly adults. Furthermore, we compared these memory CD8⁺ T cell repertoires to A2⁺M1₅₈ CD8⁺ TCRs during acute influenza disease in patients hospitalized with avian A/H7N9 virus. Our study provides the first ex vivo comparative analysis of paired antigen-specific TCR-α/β clonotypes across different tissues and peripheral blood across different age groups. We show that human A2⁺M1₅₈ CD8⁺ T cells can be readily detected in human lungs, spleens, and lymph nodes, and that tissue A2⁺M1₅₈ TCRαβ repertoires reflect A2⁺M1₅₈ TCRαβ clonotypes derived from peripheral blood in healthy adults and influenza-infected patients. A2⁺M1₅₈ TCRαβ repertoires displayed distinct features only in elderly adults, with large private TCRαβ clonotypes replacing the prominent and public TRBV19/TRAV27 TCRs. Our study provides novel findings on influenza-specific TCRαβ repertoires within human tissues, raises the question of how we can prevent the loss of optimal TCRαβ signatures with aging, and provides important insights into the rational design of T cell-mediated vaccines and immunotherapies.

Brain-resident memory CD8+ T cells induced by congenital CMV infection prevent brain pathology and virus reactivation

Congenital HCMV infection is a leading infectious cause of long-term neurodevelopmental sequelae. Infection of newborn mice with mouse cytomegalovirus (MCMV) intraperitoneally is a well-established model of congenital human cytomegalovirus infection, which best recapitulates the hematogenous route of virus spread to brain and subsequent pathology. Here, we used this model to investigate the role, dynamics, and phenotype of CD8⁺ T cells in the brain following infection of newborn mice. We show that CD8⁺ T cells infiltrate the brain and form a pool of tissue-resident memory T cells (T_RM cells) that persist for lifetime. Adoptively transferred virus-specific CD8⁺ T cells provide protection against primary MCMV infection in newborn mice, reduce brain pathology, and remain in the brain as T_RM cells. Brain CD8⁺ T_RM cells were long-lived, slowly proliferating cells able to respond to local challenge infection. Importantly, brain CD8⁺ T_RM cells controlled latent MCMV and their depletion resulted in virus reactivation and enhanced inflammation in brain.

Staphylococcus aureus Colonization Induces Strain-Specific Suppression of Interleukin-17

Staphylococcus aureus is a pathogen that causes significant morbidity and mortality. Nasal carriage is a major source of transmission and of endogenous infection. Persistent carriage is detected in ∼30% of healthy individuals. While Th17 cells have been shown to play a role in S. aureus infection and clearance, the immune response to carriage is not well understood. Here, we evaluate the Th17 response and its potential inhibitors during S. aureus carriage. We recruited 25 volunteers, of whom 11 were persistent carriers. Volunteers' peripheral blood mononuclear cells (PBMCs) were stimulated with either their endogenous strain (a strain isolated from that carrier) or exogenous ones (strains not carried by that volunteer). Changes in Th17 cell frequency and numbers, interleukin-17 (IL-17) mRNA expression, and IL-17 protein abundance were measured by fluorescence-activated cell sorting, real-time PCR, and enzyme-linked immunosorbent assay. Similarly, responses of IL-17 suppressors (regulatory T cells [FOXP3], IL-10, IL-27, and IL-19) were measured. Th17 and IL-17 levels in response to stimulation with endogenous strains were significantly lower than those in response to stimulation with exogenous ones. Of the suppressive cytokines tested, only IL-19 exhibited a stronger response to endogenous than to exogenous strains. Addition of recombinant IL-19 to exogenous-strain-stimulated PBMCs caused decreased IL-17 expression, whereas addition of IL-19 antibodies to endogenous-strain-stimulated cells resulted in an increased IL-17 response. Together, our results suggest that S. aureus carriage induced a tolerogenic response to a carried strain that could be reproduced through the addition of recombinant IL-19 or prevented by the addition of IL-19 antibodies. This differential immune response may play a role in the determination of S. aureus carriage patterns.

Heterogeneity and Developmental Connections between Cell Types Inhabiting Teeth

Every tissue is composed of multiple cell types that are developmentally, evolutionary and functionally integrated into the unit we call an organ. Teeth, our organs for biting and mastication, are complex and made of many different cell types connected or disconnected in terms of their ontogeny. In general, epithelial and mesenchymal compartments represent the major framework of tooth formation. Thus, they give rise to the two most important matrix–producing populations: ameloblasts generating enamel and odontoblasts producing dentin. However, the real picture is far from this quite simplified view. Diverse pulp cells, the immune system, the vascular system, the innervation and cells organizing the dental follicle all interact, and jointly participate in transforming lifeless matrix into a functional organ that can sense and protect itself. Here we outline the heterogeneity of cell types that inhabit the tooth, and also provide a life history of the major populations. The mouse model system has been indispensable not only for the studies of cell lineages and heterogeneity, but also for the investigation of dental stem cells and tooth patterning during development. Finally, we briefly discuss the evolutionary aspects of cell type diversity and dental tissue integration.

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.

NKG2C/E Marks the Unique Cytotoxic CD4 T Cell Subset, ThCTL, Generated by Influenza Infection

CD4 T cells can differentiate into multiple effector subsets, including ThCTL that mediate MHC class II-restricted cytotoxicity. Although CD4 T cell-mediated cytotoxicity has been reported in multiple viral infections, their characteristics and the factors regulating their generation are unclear, in part due to a lack of a signature marker. We show in this article that, in mice, NKG2C/E identifies the ThCTL that develop in the lung during influenza A virus infection. ThCTL express the NKG2X/CD94 complex, in particular the NKG2C/E isoforms. NKG2C/E⁺ ThCTL are part of the lung CD4 effector population, and they mediate influenza A virus-specific cytotoxic activity. The phenotype of NKG2C/E⁺ ThCTL indicates they are highly activated effectors expressing high levels of binding to P-selectin, T-bet, and Blimp-1, and that more of them secrete IFN-γ and readily degranulate than non-ThCTL. ThCTL also express more cytotoxicity-associated genes including perforin and granzymes, and fewer genes associated with recirculation and memory. They are found only at the site of infection and not in other peripheral sites. These data suggest ThCTL are marked by the expression of NKG2C/E and represent a unique CD4 effector population specialized for cytotoxicity.

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.

Mathematical Model Reveals the Role of Memory CD8 T Cell Populations in Recall Responses to Influenza

The current influenza vaccine provides narrow protection against the strains included in the vaccine, and needs to be reformulated every few years in response to the constantly evolving new strains. Novel approaches are directed toward developing vaccines that provide broader protection by targeting B and T cell epitopes that are conserved between different strains of the virus. In this paper, we focus on developing mathematical models to explore the CD8 T cell responses to influenza, how they can be boosted, and the conditions under which they contribute to protection. Our models suggest that the interplay between spatial heterogeneity (with the virus infecting the respiratory tract and the immune response being generated in the secondary lymphoid organs) and T cell differentiation (with proliferation occurring in the lymphoid organs giving rise to a subpopulation of resident T cells in the respiratory tract) is the key to understand the dynamics of protection afforded by the CD8 T cell response to influenza. Our results suggest that the time lag for the generation of resident T cells in the respiratory tract and their rate of decay following infection are the key factors that limit the efficacy of CD8 T cell responses. The models predict that an increase in the level of central memory T cells leads to a gradual decrease in the viral load, and, in contrast, there is a sharper protection threshold for the relationship between the size of the population of resident T cells and protection. The models also suggest that repeated natural influenza infections cause the number of central memory CD8 T cells and the peak number of resident memory CD8 T cells to reach their plateaus, and while the former is maintained, the latter decays with time since the most recent infection.

Avian Influenza Viruses, Inflammation, and CD8(+) T Cell Immunity

Avian influenza viruses (AIVs) circulate naturally in wild aquatic birds, infect domestic poultry, and are capable of causing sporadic bird-to-human transmissions. AIVs capable of infecting humans include a highly pathogenic AIV H5N1, first detected in humans in 1997, and a low pathogenic AIV H7N9, reported in humans in 2013. Both H5N1 and H7N9 cause severe influenza disease in humans, manifested by acute respiratory distress syndrome, multi-organ failure, and high mortality rates of 60% and 35%, respectively. Ongoing circulation of H5N1 and H7N9 viruses in wild birds and poultry, and their ability to infect humans emphasizes their epidemic and pandemic potential and poses a public health threat. It is, thus, imperative to understand the host immune responses to the AIVs so we can control severe influenza disease caused by H5N1 or H7N9 and rationally design new immunotherapies and vaccines. This review summarizes our current knowledge on AIV epidemiology, disease symptoms, inflammatory processes underlying the AIV infection in humans, and recent studies on universal pre-existing CD8(+) T cell immunity to AIVs. Immune responses driving the host recovery from AIV infection in patients hospitalized with severe influenza disease are also discussed.

Bone Marrow T Cells and the Integrated Functions of Recirculating and Tissue-Resident Memory T Cells

Changes in T cell trafficking accompany the naive to memory T cell antigen-driven differentiation, which remains an incompletely defined developmental step. Upon priming, each naive T cell encounters essential signals – i.e., antigen, co-stimuli and cytokines – in a secondary lymphoid organ; nevertheless, its daughter effector and memory T cells recirculate and receive further signals during their migration through various lymphoid and non-lymphoid organs. These additional signals from tissue microenvironments have an impact on immune response features, including T cell effector function, expansion and contraction, memory differentiation, long-term maintenance, and recruitment upon antigenic rechallenge into local and/or systemic responses. The critical role of T cell trafficking in providing efficient T cell memory has long been a focus of interest. It is now well recognized that naive and memory T cells have different migratory pathways, and that memory T cells are heterogeneous with respect to their trafficking. We and others have observed that, long time after priming, memory T cells are preferentially found in certain niches such as the bone marrow (BM) or at the skin/mucosal site of pathogen entry, even in the absence of residual antigen. The different underlying mechanisms and peculiarities of resulting immunity are currently under study. In this review, we summarize key findings on BM and tissue-resident memory (TRM) T cells and revisit some issues in memory T cell maintenance within such niches. Moreover, we discuss BM seeding by memory T cells in the context of migration patterns and protective functions of either recirculating or TRM T cells.

Infection with a Mouse-Adapted Strain of the 2009 Pandemic Virus Causes a Highly Severe Disease Associated with an Impaired T Cell Response

Despite a relatively low fatality rate, the 2009 H1N1 pandemic virus differed from other seasonal viruses in that it caused mortality and severe pneumonia in the young and middle-aged population (18–59 years old). The mechanisms underlying this increased disease severity are still poorly understood. In this study, a human isolate of the 2009 H1N1 pandemic virus was adapted to the mouse (MAp2009). The pathogenicity of the MAp2009 virus and the host immune responses were evaluated in the mouse model and compared to the laboratory H1N1 strain A/Puerto Rico/8/1934 (PR8). The MAp2009 virus reached consistently higher titers in the lungs over 14 days compared to the PR8 virus, and caused severe disease associated with high morbidity and 85% mortality rate, contrasting with the 0% death rate in the PR8 group. During the early phase of infection, both viruses induced similar pathology in the lungs. However, MAp2009-induced lung inflammation was sustained until the end of the study (day 14), while there was no sign of inflammation in the PR8-infected group by day 10. Furthermore, at day 3 post-infection, MAp2009 induced up to 10- to 40-fold more cytokine and chemokine gene expression, respectively. More importantly, the numbers of CD4⁺ T cells and virus-specific CD8⁺ T cells were significantly lower in the lungs of MAp2009-infected mice compared to PR8-infected mice. Interestingly, there was no difference in the number of dendritic cells in the lung and in the draining lymph node. Moreover, mice infected with PR8 or MAp2009 had similar numbers of CCR5 and CXCR3-expressing T cells, suggesting that the impaired T cell response was not due to a lack of chemokine responsiveness or priming of T cells. This study demonstrates that a mouse-adapted virus from an isolate of the 2009 pandemic virus interferes with the adaptive immune response leading to a more severe disease.

Tissue-Resident Memory T Cells and Fixed Immune Surveillance in Nonlymphoid Organs

T cell immunity is often defined in terms of memory lymphocytes that use the blood to access a range of organs. T cells are involved in two patterns of recirculation. In one, the cells shuttle back and forth between blood and secondary lymphoid organs, whereas in the second, memory cells recirculate between blood and nonlymphoid tissues. The latter is a means by which blood T cells control peripheral infection. It is now clear that there exists a distinct memory T cell subset that is absent from blood but found within nonlymphoid tissues. These nonrecirculating tissue-resident memory T (TRM) cells develop within peripheral compartments and never spread beyond their point of lodgement. This review examines fixed immune surveillance by TRM cells, highlighting features that make them potent controllers of infection in nonlymphoid tissues. These features provide clues about TRM cell specialization, such as their ability to deal with sequestered, persisting infections confined to peripheral compartments.

Recovery from severe H7N9 disease is associated with diverse response mechanisms dominated by CD8⁺ T cells

The avian origin A/H7N9 influenza virus causes high admission rates (>99%) and mortality (>30%), with ultimately favourable outcomes ranging from rapid recovery to prolonged hospitalization. Using a multicolour assay for monitoring adaptive and innate immunity, here we dissect the kinetic emergence of different effector mechanisms across the spectrum of H7N9 disease and recovery. We find that a diversity of response mechanisms contribute to resolution and survival. Patients discharged within 2-3 weeks have early prominent H7N9-specific CD8(+) T-cell responses, while individuals with prolonged hospital stays have late recruitment of CD8(+)/CD4(+) T cells and antibodies simultaneously (recovery by week 4), augmented even later by prominent NK cell responses (recovery >30 days). In contrast, those who succumbed have minimal influenza-specific immunity and little evidence of T-cell activation. Our study illustrates the importance of robust CD8(+) T-cell memory for protection against severe influenza disease caused by newly emerging influenza A viruses.

Peripheral regulation of T cells by dendritic cells during infection

It is well accepted that T cell responses are integral in providing protection during pathogenic infections. In numerous tissues, T cell responses are generated to combat infection. Typically, these T cell responses are primed in draining lymph nodes (LN) by dendritic cells (DC) that have migrated from the infected tissue. Previously, it was thought that after the initial encounter between DC and T cells in the LN, the T cells underwent a programmed response. However, it has become increasingly clear that direct interactions between DCs and T cells in infected, peripheral tissues can modulate the activation, effector function, tissue residence, and memory responses of these T cells. This review will highlight the contribution of local, direct DC: T cell interactions to the regulation of T cell responses in various tissues during inflammation and infection .

The pulmonary localization of virus-specific T lymphocytes is governed by the tissue tropism of infection

The migration of pathogen-specific T cells into nonlymphoid tissues, such as the lung, is critical to control peripheral infections. Use of in vivo intravascular labeling of leukocytes has allowed for improved discrimination between cells located in the blood from cells present within peripheral tissues, such as the lung. This is particularly important in the lung, which is comprised of an intricate network of blood vessels that harbors a large proportion of the total blood volume at any given time. Recent work has demonstrated that >80% of antigen-specific effector CD8 T cells remain in the pulmonary vasculature following an intratracheal infection with a systemic viral pathogen. However, it remains unclear what proportion of effector CD8 T cells are located within lung tissue following a localized respiratory viral infection. We confirm that most effector and memory CD8 T cells are found in the vasculature after an intranasal infection with the systemic pathogens lymphocytic choriomeningitis virus (LCMV) or vaccinia virus (VACV). In contrast, following pulmonary viral infections with either respiratory syncytial virus (RSV) or influenza A virus (IAV), 80 to 90% of the antigen-specific effector CD8 T cells were located within lung tissue. Similarly, the majority of antigen-specific CD4 T cells were present within lung tissue during a pulmonary viral infection. Furthermore, a greater proportion of gamma interferon-positive (IFN-γ(+)) effector CD8 and CD4 T cells were located within lung tissue following a localized respiratory viral infection. Our results indicate that T cells exhibit significantly altered distribution patterns dependent upon the tissue tropism of the infection.

IMPORTANCE

The migration of T cells to nonlymphoid sites, such as the lung, is critical to mediate clearance of viral infections. The highly vascularized lung holds up to 40% of blood, and thus, the T cell response may be a reflection of lymphocytes localized to the pulmonary vasculature instead of lung tissue. We examined the localization of T cell responses within the lung following either a localized or systemic viral infection. We demonstrate that following intranasal infection with a systemic pathogen, most T cells are localized to the pulmonary vasculature. In contrast, T cells are primarily localized to lung tissue following a respiratory viral infection. Our results demonstrate vast differences in the localization of T cell responses within the lung parenchyma between pathogens that can replicate locally versus systemically and that intravascular antibody labeling can be utilized to assess the localization patterns of T cell responses in nonlymphoid organs.

Lung niches for the generation and maintenance of tissue-resident memory T cells

The extent to which tissue-specific viral infections generate memory T cells specifically adapted to and maintained within the target infection site is unknown. Here, we show that respiratory virus-specific memory T cells in mice and humans are generated and maintained in compartmentalized niches in lungs, distinct from populations in lymphoid tissue or circulation. Using a polyclonal mouse model of influenza infection combined with an in vivo antibody labeling approach and confocal imaging, we identify a spatially distinct niche in the lung where influenza-specific T-cell responses are expanded and maintained long term as tissue-resident memory (T(RM)) CD4 and CD8 T cells. Lung T(RM) are further distinguished from circulating memory subsets in lung and spleen based on CD69 expression and persistence independent of lymphoid stores. In humans, influenza-specific T cells are enriched within the lung T(RM) subset, whereas memory CD8 T cells specific for the systemic virus cytomegalovirus are distributed in both lung and spleen, suggesting that the site of infection affects T(RM) generation. Our findings reveal a precise spatial organization to virus-specific T-cell memory, determined by the site of the initial infection, with important implications for the development of targeted strategies to boost immunity at appropriate tissue sites.

Indoleamine 2,3-dioxygenase (IDO) activity during the primary immune response to influenza infection modifies the memory T cell response to influenza challenge

The generation of a heterosubtypic memory T cell response is important for cross-protective immunity against unrelated strains of influenza virus. One way to facilitate the generation of the memory T cell population is to control the activity of immune modulatory agents. The enzyme, indoleamine 2,3-dioxygenase (IDO), is upregulated during influenza infection by the interferon response where IDO activity depletes tryptophan required in T cell response. In this study, IDO activity was pharmacologically inhibited with 1-methyl-tryptophan (1MT) during the primary response to influenza virus infection and the effect on the memory T cell response was evaluated. 1MT treatment improved the memory T cell response to influenza virus challenge by increasing interferon gamma expression by CD4 and CD8 T cells, and numbers of lung virus-specific CD8+ T cells, and increased the Th1 response as well as modifying the immunodominance hierarchy to increase the number of subdominant epitope specific CD8+ T cells, a feature which may be linked to decreased regulatory T cell function. These changes also accompanied evidence of accelerated lung tissue repair upon virus challenge. These findings suggest that modulation of IDO activity could be exploited in influenza vaccine development to enhance memory T cell responses and reduce disease burden.

Influenza infection results in local expansion of memory CD8(+) T cells with antigen non-specific phenotype and function

Primary viral infections induce activation of CD8(+) T cells responsible for effective resistance. We sought to characterize the nature of the CD8(+) T cell expansion observed after primary viral infection with influenza. Infection of naive mice with different strains of influenza resulted in the rapid expansion of memory CD8(+) T cells exhibiting a unique bystander phenotype with significant up-regulation of natural killer group 2D (NKG2D), but not CD25, on the CD44(high) CD8(+) T cells, suggesting an antigen non-specific phenotype. We further confirmed the non-specificity of this phenotype on ovalbumin-specific (OT-I) CD8(+) T cells, which are not specific to influenza. These non-specific CD8(+) T cells also displayed increased lytic capabilities and were observed primarily in the lung. Thus, influenza infection was shown to induce a rapid, antigen non-specific memory T cell expansion which is restricted to the specific site of inflammation. In contrast, CD8(+) T cells of a similar phenotype could be observed in other organs following administration of systemic agonistic anti-CD40 and interleukin-2 immunotherapy, demonstrating that bystander expansion in multiple sites is possible depending on whether the nature of activation is either acute or systemic. Finally, intranasal blockade of NKG2D resulted in a significant increase in viral replication early during the course of infection, suggesting that NKG2D is a critical mediator of anti-influenza responses prior to the initiation of adaptive immunity. These results characterize further the local bystander expansion of tissue-resident, memory CD8(+) T cells which, due to their early induction, may play an important NKG2D-mediated, antigen non-specific role during the early stages of viral infection.

Cutting edge: Expression of FcγRIIB tempers memory CD8 T cell function in vivo

During reinfection, high-affinity IgG Abs form complexes with both soluble Ag and Ag displayed on the surface of infected cells. These interactions regulate cellular activation of both innate cells and B cells, which express specific combinations of activating FcγRs (FcγRI, FcγRIII, FcγRIV) and/or the inhibitory FcγR (FcγRIIB). Direct proof for functional expression of FcγR by Ag-specific CD8 T cells is lacking. In this article, we show that the majority of memory CD8 T cells generated by bacterial or viral infection express only FcγRIIB, and that FcγRIIB could be detected on previously activated human CD8 T cells. Of note, FcγR stimulation during in vivo Ag challenge not only inhibited the cytotoxicity of memory CD8 T cells against peptide-loaded or virus-infected targets, but FcγRIIB blockade during homologous virus challenge enhanced the secondary CD8 T cell response. Thus, memory CD8 T cells intrinsically express a functional FcγRIIB, permitting Ag-Ab complexes to regulate secondary CD8 T cell responses.

T cell immunity as a tool for studying epigenetic regulation of cellular differentiation

Cellular differentiation is regulated by the strict spatial and temporal control of gene expression. This is achieved, in part, by regulating changes in histone post-translational modifications (PTMs) and DNA methylation that in turn, impact transcriptional activity. Further, histone PTMs and DNA methylation are often propagated faithfully at cell division (termed epigenetic propagation), and thus contribute to maintaining cellular identity in the absence of signals driving differentiation. Cardinal features of adaptive T cell immunity include the ability to differentiate in response to infection, resulting in acquisition of immune functions required for pathogen clearance; and the ability to maintain this functional capacity in the long-term, allowing more rapid and effective pathogen elimination following re-infection. These characteristics underpin vaccination strategies by effectively establishing a long-lived T cell population that contributes to an immunologically protective state (termed immunological memory). As we discuss in this review, epigenetic mechanisms provide attractive and powerful explanations for key aspects of T cell-mediated immunity – most obviously and notably, immunological memory, because of the capacity of epigenetic circuits to perpetuate cellular identities in the absence of the initial signals that drive differentiation. Indeed, T cell responses to infection are an ideal model system for studying how epigenetic factors shape cellular differentiation and development generally. This review will examine how epigenetic mechanisms regulate T cell function and differentiation, and how these model systems are providing general insights into the epigenetic regulation of gene transcription during cellular differentiation.

Lung-resident memory CD8 T cells (TRM) are indispensable for optimal cross-protection against pulmonary virus infection

Previous studies have shown that some respiratory virus infections leave local populations of tissue TRM cells in the lungs which disappear as heterosubtypic immunity declines. The location of these TRM cells and their contribution to the protective CTL response have not been clearly defined. Here, fluorescence microscopy is used to show that some CD103(+) TRM cells remain embedded in the walls of the large airways long after pulmonary immunization but are absent from systemically primed mice. Viral clearance from the lungs of the locally immunized mice precedes the development of a robust Teff response in the lungs. Whereas large numbers of virus-specific CTLs collect around the bronchial tree during viral clearance, there is little involvement of the remaining lung tissue. Much larger numbers of TEM cells enter the lungs of the systemically immunized animals but do not prevent extensive viral replication or damage to the alveoli. Together, these experiments show that virus-specific antibodies and TRM cells are both required for optimal heterosubtypic immunity, whereas circulating memory CD8 T cells do not substantially alter the course of disease.

Protective CD8 T cell-mediated immunity against influenza A virus infection following influenza virus-like particle vaccination

The development of influenza A virus (IAV) vaccines capable of inducing cytotoxic CD8 T cell responses could potentially provide superior, long-term protection against multiple, heterologous strains of IAV. Although prior studies demonstrated the effectiveness of baculovirus-derived virus-like particle (VLP) vaccination in generating Ab-mediated protection, the role that CD8 T cell immunity plays in overall VLP-mediated protection is less-well understood. In this article, we demonstrate that intranasal vaccination of mice with a VLP containing the hemagglutinin and matrix 1 proteins of IAV/PR/8/34 leads to a significant increase in hemagglutinin 533-specific CD8 T cells in the lungs and protection following subsequent homologous challenge with IAV. VLP-mediated protection was significantly reduced by CD8 T cell depletion, indicating a critical role for CD8 T cells in protective immunity. Importantly, our results show that VLP vaccine-induced CD8 T cell-mediated protection is not limited to homologous IAV strains. VLP vaccination leads to an increase in protection following heterosubtypic challenge with a strain of IAV that avoids vaccine-induced neutralizing Abs but contains conserved, immunodominant CD8 T cell epitopes. Overall, our results demonstrate the ability of influenza protein-containing VLPs to prime IAV-specific CD8 T cell responses that contribute to protection from homo- and heterosubtypic IAV infections. These results further suggest that vaccination strategies focused on the development of cross-protective CD8 T cell responses may contribute to the development of "universal" IAV vaccines.

Distinct resident and recirculating memory T cell subsets in non-lymphoid tissues

Antigen experienced or memory T cells make a critical contribution to immunity against infection. Many pathogens colonise non-lymphoid tissues and memory T cells in these compartments can deal with such localised infections. Emerging data show that there are at least two phenotypically distinct peripheral T cell subsets, one permanently resident and one recirculating between tissues and blood. A full appreciation of the T cells in the non-lymphoid memory pool and their relationship to those in the circulation is an important step in understanding how to generate and exploit effective peripheral immunity for the purpose of infection control.

The integration of T cell migration, differentiation and function

T cells function locally. Accordingly, T cells' recognition of antigen, their subsequent activation and differentiation, and their role in the processes of infection control, tumour eradication, autoimmunity, allergy and alloreactivity are intrinsically coupled with migration. Recent discoveries revise our understanding of the regulation and patterns of T cell trafficking and reveal limitations in current paradigms. Here, we review classic and emerging concepts, highlight the challenge of integrating new observations with existing T cell classification schemes and summarize the heuristic framework provided by viewing T cell differentiation and function first through the prism of migration.

Intrinsic role of FoxO3a in the development of CD8+ T cell memory

CD8(+) T cells undergo rapid expansion during infection with intracellular pathogens, which is followed by swift and massive culling of primed CD8(+) T cells. The mechanisms that govern the massive contraction and maintenance of primed CD8(+) T cells are not clear. We show in this study that the transcription factor, FoxO3a, does not influence Ag presentation and the consequent expansion of CD8(+) T cell response during Listeria monocytogenes infection, but plays a key role in the maintenance of memory CD8(+) T cells. The effector function of primed CD8(+) T cells as revealed by cytokine secretion and CD107a degranulation was not influenced by inactivation of FoxO3a. Interestingly, FoxO3a-deficient CD8(+) T cells displayed reduced expression of proapoptotic molecules BIM and PUMA during the various phases of response, and underwent reduced apoptosis in comparison with wild-type cells. A higher number of memory precursor effector cells and memory subsets was detectable in FoxO3a-deficient mice compared with wild-type mice. Furthermore, FoxO3a-deficient memory CD8(+) T cells upon transfer into normal or RAG1-deficient mice displayed enhanced survival. These results suggest that FoxO3a acts in a cell-intrinsic manner to regulate the survival of primed CD8(+) T cells.

Guarding the perimeter: protection of the mucosa by tissue-resident memory T cells

Mucosal tissues are continually bombarded with infectious agents seeking to gain entry into the body. The absence of a tough physical exterior layer surrounding these tissues creates a unique challenge for the immune system, which manages to provide broad protection against a plethora of different organisms with the aid of special adaptations that augment immunity at these vulnerable sites. For example, specialized populations of memory T lymphocytes reside at initial sites of pathogen entry into the body, where they provide an important protective barrier. Similar anatomically-confined populations of pathogen-specific CD8 T cells can be found near the outer margins of the body following recovery from a variety of local infections, where they share very similar phenotypic characteristics. How these tissue-resident T cells are retained in a single anatomic location where they can promote immunity is beginning to be defined. Here, we will review current knowledge of the mechanisms that help establish and maintain these regional lymphocytes in the mucosal tissues and discuss relevant data that enhance our understanding of the contribution of these lymphocyte populations to protective immunity against infectious diseases.

Defining the molecular blueprint that drives CD8(+) T cell differentiation in response to infection

A cardinal feature of adaptive, cytotoxic T lymphocyte (CTL)-mediated immunity is the ability of naïve CTLs to undergo a program of differentiation and proliferation upon activation resulting in the acquisition of lineage-specific T cell functions and eventual establishment of immunological memory. In this review, we examine the molecular factors that shape both the acquisition and maintenance of lineage-specific effector function in virus-specific CTL during both the effector and memory phases of immunity.

Early dysregulation of the memory CD8+ T cell repertoire leads to compromised immune responses to secondary viral infection in the aged

BACKGROUND

Virus-specific memory CD8+ T cells persist long after infection is resolved and are important for mediating recall responses to secondary infection. Although the number of memory T cells remains relatively constant over time, little is known about the overall stability of the memory T cell pool, particularly with respect to T cell clonal diversity. In this study we developed a novel assay to measure the composition of the memory T cell pool in large cohorts of mice over time following respiratory virus infection.

RESULTS

We find that the clonal composition of the virus-specific memory CD8+ T cell pool begins to change within months of the initial infection. These early clonal perturbations eventually result in large clonal expansions that have been associated with ageing.

CONCLUSIONS

Maintenance of clonal diversity is important for effective long-term memory responses and dysregulation of the memory response begins early after infection.

Use it or lose it: establishment and persistence of T cell memory

Pre-existing T cell memory provides substantial protection against viral, bacterial, and parasitic infections. The generation of protective T cell memory constitutes a primary goal for cell-mediated vaccines, thus understanding the mechanistic basis of memory development and maintenance are of major importance. The widely accepted idea that T cell memory pools are directly descended from the effector populations has been challenged by recent reports that provide evidence for the early establishment of T cell memory and suggest that the putative memory precursor T cells do not undergo full expansion to effector status. Moreover, it appears that once the memory T cells are established early in life, they can persist for the lifetime of an individual. This is in contrast to the reported waning of naïve T cell immunity with age. Thus, in the elderly, immune memory that was induced at an early age may be more robust than recently induced memory, despite the necessity for long persistence. The present review discusses the mechanisms underlying the early establishment of immunological memory and the subsequent persistence of memory T cell pools in animal models and humans.

Local immunity by tissue-resident CD8(+) memory T cells

Microbial infection primes a CD8⁺ cytotoxic T cell response that gives rise to a long-lived population of circulating memory cells able to provide protection against systemic reinfection. Despite this, effective CD8⁺ T cell surveillance of barrier tissues such as skin and mucosa typically wanes with time, resulting in limited T cell-mediated protection in these peripheral tissues. However, recent evidence suggests that a specialized subset of CD103⁺ memory T cells can permanently lodge and persist in peripheral tissues, and that these cells can compensate for the loss of peripheral immune surveillance by circulating memory T cells. Here, we review evolving concepts regarding the generation and long-term persistence of these tissue-resident memory T cells (T_RM) in epithelial and neuronal tissues. We further discuss the role of T_RM cells in local infection control and their contribution to localized immune phenomena, in both mice and humans.