Memory T cells as an occupying force

Vaccine Strategy That Enhances the Protective Efficacy of Systemic Immunization by Establishing Lung-Resident Memory CD8 T Cells Against Influenza Infection

Most influenza vaccines currently in use target the highly variable hemagglutinin protein to induce neutralizing antibodies and therefore require yearly reformulation. T cell-based universal influenza vaccines focus on eliciting broadly cross-reactive T-cell responses, especially the tissue-resident memory T cell (TRM) population in the respiratory tract, providing superior protection to circulating memory T cells. This study demonstrated that intramuscular (i.m.) administration of the adenovirus-based vaccine expressing influenza virus nucleoprotein (rAd/NP) elicited weak CD8 TRM responses in the lungs and airways, and yielded poor protection against lethal influenza virus challenge. However, a novel "prime-and-deploy" strategy that combines i.m. vaccination of rAd/NP with subsequent intranasal administration of an empty adenovector induced strong NP-specific CD8+ TRM cells and provided complete protection against influenza virus challenge. Overall, our results demonstrate that this "prime-and-deploy" vaccination strategy is potentially applicable to the development of universal influenza vaccines.

Neural-Cadherin Influences the Homing of Terminally Differentiated Memory CD8 T Cells to the Lymph Nodes and Bone Marrow

Memory T (TM) cells play an important role in the long-term defense against pathogen reinvasion. However, it is still unclear how these cells receive the crucial signals necessary for their longevity and homeostatic turnover. To understand how TM cells receive these signals, we infected mice with lymphocytic choriomeningitis virus (LCMV) and examined the expression sites of neural cadherin (N-cadherin) by immunofluorescence microscopy. We found that N-cadherin was expressed in the surroundings of the white pulps of the spleen and medulla of lymph nodes (LNs). Moreover, TM cells expressing high levels of killer cell lectin-like receptor G1 (KLRG1), a ligand of N-cadherin, were co-localized with N-cadherin+ cells in the spleen but not in LNs. We then blocked N-cadherin in vivo to investigate whether it regulates the formation or function of TM cells. The numbers of CD127hiCD62Lhi TM cells in the spleen of memory P14 chimeric mice declined when N-cadherin was blocked during the contraction phase, without functional impairment of these cells. In addition, when CD127loKLRG1hi TM cells were adoptively transferred into anti-N-cadherin-treated mice compared with control mice, the number of these cells was reduced in the bone marrow and LNs, without functional loss. Taken together, our results suggest that N-cadherin participates in the development of CD127hiCD62Lhi TM cells and homing of CD127loKLRG1hi TM cells to lymphoid organs.

Brain tissue-resident immune memory cells are required for long-term protection against CNS infection with rabies virus

Immune memory cells residing in previously infected, nonlymphoid tissues play a role in immune surveillance. In the event that circulating antibodies fail to prevent virus spread to the tissues in a secondary infection, these memory cells provide an essential defense against tissue reinfection. CNS tissues are isolated from circulating immune cells and antibodies by the blood-brain barrier, making the presence of tissue-resident immune memory cells particularly needed to combat recurrent infection by neurotropic viruses. Wild-type and laboratory-engineered rabies viruses are neurotropic, differ in pathogenicity, and have varying effects on BBB functions. These viruses have proven invaluable tools in demonstrating the importance of tissue-resident immune memory cells in the reinfection of CNS tissues. Only Type 1 immune memory is effective at therapeutically clearing a secondary infection with wild-type rabies viruses from the CNS and does so despite the maintenance of blood-brain barrier integrity.

Targeting resident memory T cell immunity culminates in pulmonary and systemic protection against Brucella infection

Brucellosis remains the most common zoonotic disease globally. Currently no vaccines for humans exist, and conventional brucellosis vaccines for livestock fail to confer complete protection; hence, an improved vaccine is needed. Although Brucella infections primarily occur following a mucosal exposure, vaccines are administered parenterally. Few studies have considered mucosal vaccinations, or even targeting of tissue-resident memory T (T_RM) cells. T_RM cells protect against viral infections, but less is known of their role in bacterial infections, and even less for brucellosis. Oral prime, nasal boost with a newly developed Brucella abortus double mutant (znBAZ) confers nearly complete protection against pulmonary challenge with wild-type (wt) B. abortus 2308, and its protective efficacy is >2800-fold better than the RB51 vaccine. Vaccination with znBAZ potently stimulated CD8⁺ T cells, whereas mucosal vaccination with RB51 induced mostly CD4⁺ T cells. Subsequent analysis revealed these pulmonary CD44⁺ CD69⁺ CD8⁺ T cells to be either CD103⁺ or CD103^- T_RM cells, and these sequestered to the lung parenchyma as CXCR3^lo and to the airways as CXCR3^hi. Both CD8⁺ T_RM subsets contained single-positive IFN-γ and TNF-α, as well as, polyfunctional cells. IL-17-producing CD8⁺ T_RM cells were also induced by znBAZ vaccination, but in vivo IL-17 neutralization had no impact upon protection. In vivo depletion of CD4⁺ T cells had no impact upon protection in znBAZ-vaccinated mice. In contrast, CD4⁺ T cell depletion reduced RB51’s protective efficacy in spleens and lungs by two- and three-logs, respectively. Although anti-CD8 mAb-treated znBAZ-vaccinated mice showed a significantly reduced pulmonary efficacy, this treatment failed to completely deplete the lung CD8⁺ T cells, leaving the CD103⁺ and CD103^- CD8⁺ T_RM cell ratios intact. Only znBAZ-vaccinated CD8^-/- mice were fully sensitive to pulmonary challenge with virulent wt B. abortus 2308 since CD8⁺ T_RM cells could not be induced. Collectively, these data demonstrate the key role of mucosal vaccination for the generation of CD8⁺ T_RM cells in protecting against pulmonary challenge with virulent B. abortus.

Brucellosis is the most common zoonotic disease worldwide and is transmitted via the consumption of unpasteurized dairy products or exposure to Brucella-laden aerosols. In fact, mucosal exposure is the most common route of infection for humans and animals, yet parenteral vaccination of livestock remains the preferred route of immunization. To determine whether development of a mucosal vaccination regimen could effectively generate immunity against pulmonary challenge with virulent B. abortus, a double-mutant B. abortus vaccine was administered mucosally, and found to induce CD8⁺ T_RM cells. These conferred complete protection against pulmonary infection and prevented systemic brucellae spread even in the absence of immune recirculating CD8⁺ T cells. These data show that mucosal vaccination can stimulate the induction of T_RM cells, which should be considered as a more effective means to protect against brucellosis. Furthermore, brucellosis needs to be considered a mucosal pathogen to warrant development of approaches different from conventional methods to protect humans and livestock against this disease.

Vaccine-induced gastric CD4+ tissue-resident memory T cells proliferate in situ to amplify immune response against Helicobacter pylori insult

BACKGROUND

Tissue-resident memory T cells accelerate the clearance of pathogens during recall response. However, whether CD4⁺ TRM cells themselves can provide gastric immunity is unclear.

MATERIALS AND METHODS

We established a parabiosis model between the enhanced green fluorescent protein and wild-type mice that the circulation system was shared, and the wild-type partner was vaccinated with H pylori vaccine composed of CCF and silk fibroin in gastric subserous layer to induce gastric EGFP⁺ CD4⁺ TRM cells. Antigen-specific EGFP⁺ CD4⁺ T cells and proliferous TRM cells were analyzed by flow cytometry. The colonization of H pylori was detected by quantitative real-time PCR. EGFP⁺ CD4⁺ TRM cells and the inflammation of the stomach were observed by histology.

RESULTS

A parabiosis animal model was employed to identify the cells that introduced by vaccination in GSL. Antigen-specific EGFP⁺ CD4⁺ T cells could be detected at day 7 post-vaccination. Thirty days later, EGFP⁺ CD4⁺ TRM cells were established with a phenotype of CD69⁺ CD103^- . Of note, we found that when circulating lymphocytes were depleted by FTY720 administration, these TRM cells could proliferate in situ and differentiate into effector Th1 cells after H pylori challenge. A decrease in H pylori colonization was observed in the vaccinated mice but not unvaccinated mice. Further, we found that although FTY720 was administrated, mounted pro-inflammatory myeloid cells still emerged in the stomach of the vaccinated mice, which might contribute to the reduction of H pylori colonization.

CONCLUSIONS

Our study reveals that H pylori vaccine-induced CD4⁺ TRM cells can proliferate and differentiate in situ to enhance gastric local immunity during recall response.

Memory CD8 T cell inflation vs tissue-resident memory T cells: Same patrollers, same controllers?

The induction of long-lived populations of memory T cells residing in peripheral tissues is of considerable interest for T cell-based vaccines, as they can execute immediate effector functions and thus provide protection in case of pathogen encounter at mucosal and barrier sites. Cytomegalovirus (CMV)-based vaccines support the induction and accumulation of a large population of effector memory CD8 T cells in peripheral tissues, in a process called memory inflation. Tissue-resident memory (T_RM ) T cells, induced by various infections and vaccination regimens, constitute another subset of memory cells that take long-term residence in peripheral tissues. Both memory T cell subsets have evoked substantial interest in exploitation for vaccine purposes. However, a direct comparison between these two peripheral tissue-localizing memory T cell subsets with respect to their short- and long-term ability to provide protection against heterologous challenge is pending. Here, we discuss communalities and differences between T_RM and inflationary CD8 T cells with respect to their development, maintenance, function, and protective capacity. In addition, we discuss differences and similarities between the transcriptional profiles of T_RM and inflationary T cells, supporting the notion that they are distinct memory T cell populations.

Tissue-resident lymphocytes: from adaptive to innate immunity

Efficient immune responses against invading pathogens often involve coordination between cells from both the innate and adaptive immune systems. For multiple decades, it has been believed that CD8⁺ memory T cells and natural killer (NK) cells constantly and uniformly recirculate. Only recently was the existence of noncirculating memory T and NK cells that remain resident in the peripheral tissues, termed tissue-resident memory T (T_RM) cells and tissue-resident NK (trNK) cells, observed in various organs owing to improved techniques. T_RM cells populate a wide range of peripheral organs, including the skin, sensory ganglia, gut, lungs, brain, salivary glands, female reproductive tract, and others. Recent findings have demonstrated the existence of T_RM in the secondary lymphoid organs (SLOs) as well, leading to revision of the classic theory that they exist only in peripheral organs. trNK cells have been identified in the uterus, skin, kidney, adipose tissue, and salivary glands. These tissue-resident lymphocytes do not recirculate in the blood or lymphatic system and often adopt a unique phenotype that is distinct from those of circulating immune cells. In this review, we will discuss the recent findings on the tissue residency of both innate and adaptive lymphocytes, with a particular focus on CD8⁺ memory T cells, and describe some advances regarding unconventional T cells (invariant NKT cells, mucosal-associated invariant T cells (MAIT), and γδ T cells) and the emerging family of trNK cells. Specifically, we will focus on the phenotypes and functions of these subsets and discuss their implications in anti-viral and anti-tumor immunity.

IL-15 supports the generation of protective lung-resident memory CD4 T cells

Tissue-resident memory T cells (T_RM) provide optimal defense at the sites of infection, but signals regulating their development are unclear, especially for CD4 T cells. Here we identify two distinct pathways that lead to the generation of CD4 T_RM in the lungs following influenza infection. The T_RM are transcriptionally distinct from conventional memory CD4 T cells and share a gene signature with CD8 T_RM. The CD4 T_RM are superior cytokine producers compared with conventional memory cells, can protect otherwise naive mice against a lethal influenza challenge, and display functional specialization by inducing enhanced inflammatory responses from dendritic cells compared with conventional memory cells. Finally, we demonstrate than an interleukin (IL)-2-dependent and a novel IL-2-independent but IL-15-dependent pathway support the generation of cohorts of lung T_RM.

Trigger-happy resident memory CD4+ T cells inhabit the human lungs

Resident memory T cells (T_RM) reside in the lung epithelium and mediate protective immunity against respiratory pathogens. Although lung CD8⁺ T_RM have been extensively characterized, the properties of CD4⁺ T_RM remain unclear. Here we determined the transcriptional signature of CD4⁺ T_RM, identified by the expression of CD103, retrieved from human lung resection material. Various tissue homing molecules were specifically upregulated on CD4⁺ T_RM, whereas expression of tissue egress and lymph node homing molecules were low. CD103⁺ T_RM expressed low levels of T-bet, only a small portion expressed Eomesodermin (Eomes), and although the mRNA levels for Hobit were increased, protein expression was absent. On the other hand, the CD103⁺ T_RM showed a Notch signature. CD4⁺CD103⁺ T_RM constitutively expressed high transcript levels of numerous cytotoxic mediators that was functionally reflected by a fast recall response, magnitude of cytokine production, and a high degree of polyfunctionality. Interestingly, the superior cytokine production appears to be because of an accessible interferon-γ (IFNγ) locus and was partially because of rapid translation of preformed mRNA. Our studies provide a molecular understanding of the maintenance and potential function of CD4⁺ T_RM in the human lung. Understanding the specific properties of CD4⁺ T_RM is required to rationally improve vaccine design.

Trying to See the Forest through the Trees: Deciphering the Nature of Memory Immunity to Mycobacterium tuberculosis

The purpose of vaccination against tuberculosis and other diseases is to establish a heightened state of acquired specific resistance in which the memory immune response is capable of mediating an accelerated and magnified expression of protection to the pathogen when this is encountered at a later time. In the earliest studies in mice infected with Mycobacterium tuberculosis, memory immunity and the cells that express this were definable both in terms of kinetics of emergence, and soon thereafter by the levels of expression of markers including CD44, CD62L, and the chemokine receptor CCR7, allowing the identification of effector memory and central memory T cell subsets. Despite these initial advances in knowledge, more recent information has not revealed more clarity, but instead, has created a morass of complications—complications that, if not resolved, could harm correct vaccine design. Here, we discuss two central issues. The first is that we have always assumed that memory is induced in the same way, and consists of the same T cells, regardless of whether that immunity is generated by BCG vaccination, or by exposure to M. tuberculosis followed by effective chemotherapy. This assumption is almost certainly incorrect. Second, a myriad of additional memory subsets have now been described, such as resident, stem cell-like, tissue specific, among others, but as yet we know nothing about the relative importance of each, or whether if a new vaccine needs to induce all of these, or just some, to be fully effective.

IRF4 Modulates CD8+ T Cell Sensitivity to IL-2 Family Cytokines

IFN regulatory factor 4 (IRF4) is a key transcription factor that promotes effector CD8⁺ T cell differentiation and expansion. The roles of IRF4 in regulating the CD8⁺ T cell response to cytokines have not been explored. In this article, we show that IL-2 and IL-15 signaling and STAT5 activation regulate IRF4 expression in CD8⁺ T cells. Gene-expression profile analysis has also revealed that IRF4 is required for expression of the receptors of IL-2 family cytokines CD122 and CD127. We found that IRF4 binds directly to CD122 and CD127 gene loci, indicating that it may directly promote CD122 and CD127 gene transcription. As a consequence, IRF4-deficient CD8⁺ T cells show diminished sensitivity to IL-2, IL-15, and IL-7 treatment in vitro. Furthermore, we found that IRF4-deficient CD8⁺ T cells had lower expression of CD122 and CD127 in vivo during influenza virus infection. These data suggest that IRF4 regulates the sensitivity of CD8⁺ T cells to IL-2 family cytokines, which correlates with the diminished effector and memory CD8⁺ T cell responses in IRF4-deficient CD8⁺ T cells.

High pathogen burden in childhood promotes the development of unconventional innate-like CD8+ T cells

Cellular and humoral constituents of the immune system differ significantly between children and adults, yet very little is known about the impact of early-life pathogen exposure on this immunologic transition. We examined CD4+ and CD8+ T cell subsets defined by CCR7 and CD45RA expression in two longitudinal pediatric cohorts experiencing divergent levels of pathogen burden. Using multiparameter flow cytometry, along with serological, cytokine, and transcriptomic data, we show that cumulative pathogen burden promotes the development of atypical CD8dim T cells with an innate-like profile (Granzyme Bhi, IFNγlow, TNFαlow, PLFZhi, ID2hi, IKZF2hi) in contrast to age-matched children residing in a low pathogen-exposure area who display a more conventional CD8bright profile (IFNγ+, TNFα+, CCL4+). Furthermore, these unconventional T cells had stunted proliferation, distinct transcriptional programs, and impaired T cell receptor signaling and were enriched in hallmark TNFα, NF-κB, and IL-6 gene signaling pathways, reminiscent of NK cells and type-1 innate lymphoid cells. Our findings suggest that these unconventional CD8dim T cells arise in a very particular immunological context and may provide a deeper understanding of the heterogeneity in human immune responses.

Tissue resident memory T cells and viral immunity

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

The Effects of Acute Neutrophil Depletion on Resolution of Acute Influenza Infection, Establishment of Tissue Resident Memory (TRM), and Heterosubtypic Immunity

After disease resolution, a small subset of influenza specific CD8⁺ T cells can remain in the airways of the lung as a tissue resident memory population (T_RM). These cells are critical for protection from subsequent infections with heterosubtypic influenza viruses. Although it is well established that expression of the collagen IV binding integrin alpha 1 is necessary for the retention and maintenance of T_RM cells, other requirements allowing them to localize to the airways and persist are less well understood. We recently demonstrated that inhibition of neutrophils or neutrophil derived chemokine CXCL12 during acute influenza virus infection reduces the effector T cell response and affects the ability of these cells to localize to the airways. We therefore sought to determine whether the defects that occur in the absence of neutrophils would persist throughout resolution of the disease and impact the development of the T_RM population. Interestingly, the early alterations in the CD8⁺ T cell response recover by two weeks post-infection, and mice form a protective population of T_RM cells. Overall, these observations show that acute neutrophil depletion results in a delay in the effector CD8⁺ T cell response, but does not adversely impact the development of T_RM.

Compartmentalization of Total and Virus-Specific Tissue-Resident Memory CD8+ T Cells in Human Lymphoid Organs

Disruption of T cell memory during severe immune suppression results in reactivation of chronic viral infections, such as Epstein Barr virus (EBV) and Cytomegalovirus (CMV). How different subsets of memory T cells contribute to the protective immunity against these viruses remains poorly defined. In this study we examined the compartmentalization of virus-specific, tissue resident memory CD8+ T cells in human lymphoid organs. This revealed two distinct populations of memory CD8+ T cells, that were CD69+CD103+ and CD69+CD103-, and were retained within the spleen and tonsils in the absence of recent T cell stimulation. These two types of memory cells were distinct not only in their phenotype and transcriptional profile, but also in their anatomical localization within tonsils and spleen. The EBV-specific, but not CMV-specific, CD8+ memory T cells preferentially accumulated in the tonsils and acquired a phenotype that ensured their retention at the epithelial sites where EBV replicates. In vitro studies revealed that the cytokine IL-15 can potentiate the retention of circulating effector memory CD8+ T cells by down-regulating the expression of sphingosine-1-phosphate receptor, required for T cell exit from tissues, and its transcriptional activator, Kruppel-like factor 2 (KLF2). Within the tonsils the expression of IL-15 was detected in regions where CD8+ T cells localized, further supporting a role for this cytokine in T cell retention. Together this study provides evidence for the compartmentalization of distinct types of resident memory T cells that could contribute to the long-term protection against persisting viral infections.

Evidence for Resident Memory T Cells in Rasmussen Encephalitis

Rasmussen encephalitis (RE) is a rare pediatric neuroinflammatory disease of unknown etiology characterized by intractable seizures, and progressive atrophy usually confined to one cerebral hemisphere. Surgical removal or disconnection of the affected cerebral hemisphere is currently the only intervention that effectively stops the seizures. Histopathological evaluation of resected brain tissue has shown that activated brain resident macrophages (microglia) and infiltrating T cells are involved in the inflammatory reaction. Here, we report that T cells isolated from seven RE brain surgery specimens express the resident memory T cell (T_RM) marker CD103. CD103 was expressed by >50% of CD8⁺ αβ T cells and γδ T cells irrespective of the length of time from seizure onset to surgery, which ranged from 0.3 to 8.4 years. Only ~10% of CD4⁺ αβ were CD103⁺, which was consistent with the observation that few CD4⁺ T cells are found in RE brain parenchyma. Clusters of T cells in brain parenchyma, which are a characteristic of RE histopathology, stained for CD103. Less than 10% of T cells isolated from brain specimens from eight surgical cases of focal cortical dysplasia (FCD), a condition that is also characterized by intractable seizures, were CD103⁺. In contrast to the RE cases, the percent of CD103⁺ T cells increased with the length of time from seizure onset to surgery. In sections of brain tissue from the FCD cases, T cells were predominantly found around blood vessels, and did not stain for CD103. The presence of significant numbers of T_RM cells in RE brain irrespective of the length of time between clinical presentation and surgical intervention supports the conclusion that a cellular immune response to an as yet unidentified antigen(s) occurs at an early stage of the disease. Reactivated T_RM cells may contribute to disease progression.

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.

Antibody-targeted vaccination to lung dendritic cells generates tissue-resident memory CD8 T cells that are highly protective against influenza virus infection

Influenza virus gains entry into the body by inhalation and initiates its replication cycle within the lung. The early stage of infection, while the virus is confined to the lung mucosa, provides the ideal window of opportunity for an effective immune response to control the infection. Tissue-resident memory (Trm) CD8 T cells, located in a variety of tissues including the lung, are ideally situated to act during this window and stall the infection. The factors involved in the differentiation of lung Trm cells remain poorly defined. We demonstrate that recognition of antigen presented locally by dendritic cells (DCs) and transforming growth factor-β (TGFβ) signaling are both required. We exploited this knowledge to develop an antibody-targeted vaccination approach to generate lung Trm cells. Delivering antigen exclusively to respiratory DCs results in the development of lung CD8 Trm cells that are highly protective against lethal influenza challenge. Our results describe an effective vaccination strategy that protects against influenza virus infection.

Delivery strategies to enhance oral vaccination against enteric infections

While the majority of human pathogens infect the body through mucosal sites, most licensed vaccines are injectable. In fact the only mucosal vaccine that has been widely used globally for infant and childhood vaccination programs is the oral polio vaccine (OPV) developed by Albert Sabin in the 1950s. While oral vaccines against Cholera, rotavirus and Salmonella typhi have also been licensed, the development of additional non-living oral vaccines against these and other enteric pathogens has been slow and challenging. Mucosal vaccines can elicit protective immunity at the gut mucosa, in part via antigen-specific secretory immunoglobulin A (SIgA). However, despite their advantages over the injectable route, oral vaccines face many hurdles. A key challenge lies in design of delivery strategies that can protect antigens from degradation in the stomach and intestine, incorporate appropriate immune-stimulatory adjuvants and control release at the appropriate gastrointestinal site. A number of systems including micro and nanoparticles, lipid-based strategies and enteric capsules have significant potential either alone or in advanced combined formulations to enhance intestinal immune responses. In this review we will outline the opportunities, challenges and potential delivery solutions to facilitate the development of improved oral vaccines for infectious enteric diseases.

Role of Ets Proteins in Development, Differentiation, and Function of T-Cell Subsets

Through positive selection, double-positive cells in the thymus differentiate into CD4(+) or CD8(+) T single-positive cells that subsequently develop into different types of effective T cells, such as T-helper and cytotoxic T lymphocyte cells, that play distinctive roles in the immune system. Development, differentiation, and function of thymocytes and CD4(+) and CD8(+) T cells are controlled by a multitude of secreted and intracellular factors, ranging from cytokine signaling modules to transcription factors and epigenetic modifiers. Members of the E26 transformation specific (Ets) family of transcription factors, in particular, are potent regulators of these CD4(+) or CD8(+) T-cell processes. In this review, we summarize and discuss the functions and underlying mechanisms of the Ets family members that have been characterized as involved in these processes. Ongoing research of these factors is expected to identify practical applications for the Ets family members as novel therapeutic targets for inflammation-related diseases.

The emerging role of resident memory T cells in protective immunity and inflammatory disease

Over the past decade, it has become clear that there is an important subset of memory T cells that resides in tissues-tissue-resident memory T (TRM) cells. There is an emerging understanding that TRM cells have a role in human tissue-specific immune and inflammatory diseases. Furthermore, the nature of the molecular signals that maintain TRM cells in tissues is the subject of much investigation. In addition, whereas it is logical for TRM cells to be located in barrier tissues at interfaces with the environment, these cells have also been found in brain, kidney, joint and other non-barrier tissues in humans and mice. Given the biology and behavior of these cells, it is likely that they have a role in chronic relapsing and remitting diseases of both barrier and non-barrier tissues. In this Review we discuss recent insights into the biology of TRM cells with a particular focus on their roles in disease, both proven and putative.

Common clonal origin of central and resident memory T cells following skin immunization

Central memory T (TCM) cells in lymph nodes (LNs) and resident memory T (TRM) cells in peripheral tissues have distinct roles in protective immunity. Both are generated after primary infections, but their clonal origins have been unclear. To address this question, we immunized mice through the skin with a protein antigen, a chemical hapten, or a non-replicating poxvirus. We then analyzed antigen-activated T cells from different tissues using high-throughput sequencing (HTS) of the gene encoding the T cell receptor (TCR) β-chain (Trb, also known as Tcrb) using CDR3 sequences to simultaneously track thousands of unique T cells. For every abundant TRM cell clone generated in the skin, an abundant TCM cell clone bearing the identical TCR was present in the LNs. Thus, antigen-reactive skin TRM and LN TCM cell clones were derived from a common naive T cell precursor after skin immunization, generating overlapping TCR repertoires. Although they bore the same TCR, TRM cells mediated rapid contact hypersensitivity responses, whereas TCM cells mediated delayed and attenuated responses. Studies in human subjects confirmed the generation of skin TRM cells in allergic contact dermatitis. Thus, immunization through skin simultaneously generates skin TRM and LN TCM cells in similar numbers from the same naive T cells.

Notch controls the survival of memory CD4+ T cells by regulating glucose uptake

CD4+ T cells differentiate into memory T cells that protect the host from subsequent infection. In contrast, autoreactive memory CD4+ T cells harm the body by persisting in the tissues. The underlying pathways controlling the maintenance of memory CD4+ T cells remain undefined. We show here that memory CD4+ T cell survival is impaired in the absence of the Notch signaling protein known as recombination signal binding protein for immunoglobulin κ J region (Rbpj). Treatment of mice with a Notch inhibitor reduced memory CD4+ T cell numbers and prevented the recurrent induction of experimental autoimmune encephalomyelitis. Rbpj-deficient CD4+ memory T cells exhibit reduced glucose uptake due to impaired AKT phosphorylation, resulting in low Glut1 expression. Treating mice with pyruvic acid, which bypasses glucose uptake and supplies the metabolite downstream of glucose uptake, inhibited the decrease of autoimmune memory CD4+ T cells in the absence of Notch signaling, suggesting memory CD4+ T cell survival relies on glucose metabolism. Together, these data define a central role for Notch signaling in maintaining memory CD4+ T cells through the regulation of glucose uptake.

Understanding and overcoming the barriers to T cell-mediated immunity against tuberculosis

Despite the overwhelming success of immunization in reducing, and even eliminating, the global threats posed by a wide spectrum of infectious diseases, attempts to do the same for tuberculosis (TB) have failed to date. While most effective vaccines act by eliciting neutralizing antibodies, T cells are the primary mediators of adaptive immunity against TB. Unfortunately, the onset of the T cell response after aerosol infection with Mycobacterium tuberculosis (Mtb), the bacterium that causes TB, is exceedingly slow, and systemically administered vaccines only modestly accelerate the recruitment of effector T cells to the lungs. This delay seems to be orchestrated by Mtb itself to prolong the period of unrestricted bacterial replication in the lung that characterizes the innate phase of the response. When T cells finally arrive at the site of infection, multiple layers of regulation have been established that limit the ability of T cells to control or eradicate Mtb. From this understanding, emerges a strategy for achieving immunity. Lung resident memory T cells may recognize Mtb-infected cells shortly after infection and confer protection before regulatory networks are allowed to develop. Early studies using vaccines that elicit lung resident T cells by targeting the lung mucosa have been promising, but many questions remain. Due to the fundamental nature of these questions, and the need to understand and manipulate the early events in the lung after aerosol infection, only coordinated approaches that utilize tractable animal models to inform human TB vaccine trials will move the field toward its goal.

Adaptation in the innate immune system and heterologous innate immunity

The innate immune system recognizes deviation from homeostasis caused by infectious or non-infectious assaults. The threshold for its activation seems to be established by a calibration process that includes sensing of microbial molecular patterns from commensal bacteria and of endogenous signals. It is becoming increasingly clear that adaptive features, a hallmark of the adaptive immune system, can also be identified in the innate immune system. Such adaptations can result in the manifestation of a primed state of immune and tissue cells with a decreased activation threshold. This keeps the system poised to react quickly. Moreover, the fact that the innate immune system recognizes a wide variety of danger signals via pattern recognition receptors that often activate the same signaling pathways allows for heterologous innate immune stimulation. This implies that, for example, the innate immune response to an infection can be modified by co-infections or other innate stimuli. This "design feature" of the innate immune system has many implications for our understanding of individual susceptibility to diseases or responsiveness to therapies and vaccinations. In this article, adaptive features of the innate immune system as well as heterologous innate immunity and their implications are discussed.

Characterization and functional properties of gastric tissue-resident memory T cells from children, adults, and the elderly

T cells are the main orchestrators of protective immunity in the stomach; however, limited information on the presence and function of the gastric T subsets is available mainly due to the difficulty in recovering high numbers of viable cells from human gastric biopsies. To overcome this shortcoming we optimized a cell isolation method that yielded high numbers of viable lamina propria mononuclear cells (LPMC) from gastric biopsies. Classic memory T subsets were identified in gastric LPMC and compared to peripheral blood mononuclear cells (PBMC) obtained from children, adults, and the elderly using an optimized 14 color flow cytometry panel. A dominant effector memory T (T_EM) phenotype was observed in gastric LPMC CD4⁺ and CD8⁺ T cells in all age groups. We then evaluated whether these cells represented a population of gastric tissue-resident memory T (T_RM) cells by assessing expression of CD103 and CD69. The vast majority of gastric LPMC CD8⁺ T cells either co-expressed CD103/CD69 (>70%) or expressed CD103 alone (~20%). Gastric LPMC CD4⁺ T cells also either co-expressed CD103/CD69 (>35%) or expressed at least one of these markers. Thus, gastric LPMC CD8⁺ and CD4⁺ T cells had the characteristics of T_RM cells. Gastric CD8⁺ and CD4⁺ T_RM cells produced multiple cytokines (IFN-γ, IL-2, TNF-α, IL-17A, MIP-1β) and up-regulated CD107a upon stimulation. However, marked differences were observed in their cytokine and multi-cytokine profiles when compared to their PBMC T_EM counterparts. Furthermore, gastric CD8⁺ T_RM and CD4⁺ T_RM cells demonstrated differences in the frequency, susceptibility to activation, and cytokine/multi-cytokine production profiles among the age groups. Most notably, children’s gastric T_RM cells responded differently to stimuli than gastric T_RM cells from adults or the elderly. In conclusion, we demonstrate the presence of gastric T_RM, which exhibit diverse functional characteristics in children, adults, and the elderly.

Human memory T cells from the bone marrow are resting and maintain long-lasting systemic memory

In the bone marrow, a population of memory T cells has been described that promotes efficient secondary immune responses and has been considered to be preactivated, owing to its expression of CD69 and CD25. Here we show that human bone marrow professional memory T cells are not activated but are resting in terms of proliferation, transcription, and mobility. They are in the G0 phase of the cell cycle, and their transcriptome is that of resting T cells. The repertoire of CD4(+) bone marrow memory T cells compared with CD4(+) memory T cells from the blood is significantly enriched for T cells specific for cytomegalovirus-pp65 (immunodominant protein), tetanus toxoid, measles, mumps, and rubella. It is not enriched for vaccinia virus and Candida albicans-MP65 (immunodominant protein), typical pathogens of skin and/or mucosa. CD4(+) memory T cells specific for measles are maintained nearly exclusively in the bone marrow. Thus, CD4(+) memory T cells from the bone marrow provide long-term memory for systemic pathogens.

Stimulation through very late antigen-4 and -5 improves the multifunctionality and memory formation of CD8⁺ T cells

T cells express multiple integrin molecules. The significance of signaling through these molecules on acquisition of T-cell effector functions and memory formation capacity remains largely unknown. Moreover, the impact of stimulation through these signals on the generation of T cells for adoptive immunotherapy has not been elucidated. In this study, using a recombinant fragment of fibronectin, CH-296, we demonstrated that stimulation via very late Ag (VLA)-4 and VLA-5 in human and BALB/c mouse CD8(+) T cells, in combination with TCR stimulation, enhances effector multifunctionality and in vivo memory formation. Using TCR-transgenic mouse-derived CD8(+) T cells expressing TCR specific for the syngeneic CMS5 fibrosarcoma-derived tumor Ag, we showed that stimulation by CH-296 improved the ability of tumor-specific CD8(+) T cells to inhibit CMS5 tumor growth when adoptively transferred into hosts with progressing tumors. Improved antitumor effects were associated with decreased infiltration of Foxp3(+) CD4(+) Treg cells in tumors. These results suggest that stimulation via VLA-4 and VLA-5 modulates the qualities of effector T cells and could potentially increase the efficacy of adoptive therapy against cancer.

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.

Mucosal immunization with recombinant fusion protein DnaJ-ΔA146Ply enhances cross-protective immunity against Streptococcus pneumoniae infection in mice via interleukin 17A

Pneumolysin (Ply) and its variants are protective against pneumococcal infections in animal models, and as a Toll-like receptor 4 agonist, pneumolysin has been reported to be a mucosal adjuvant. DnaJ has been approved as a useful candidate vaccine protein; we therefore designed novel fusion proteins of DnaJ with a form of Ply that has a deletion of A146 (ΔA146Ply-DnaJ [the C terminus of ΔA146Ply connected with the N terminus of DnaJ] and DnaJ-ΔA146Ply [the C terminus of DnaJ connected with the N terminus of ΔA146Ply]) to test whether they are protective against focal and lethal pneumococcal infections and their potential protective mechanisms. The purified proteins were used to intranasally immunize the animals without additional adjuvant. Immunization with DnaJ-ΔA146Ply or DnaJ plus ΔA146Ply (Ply with a single deletion of A146) could significantly reduce S. pneumoniae colonization in the nasopharynx and lung relative with DnaJ alone. Additionally, we observed the best protection for DnaJ-ΔA146Ply-immunized mice after challenge with lethal doses of S. pneumoniae strains, which was comparable to that achieved by PPV23. Mice immunized with DnaJ-ΔA146Ply produced significantly higher levels of anti-DnaJ IgG in serum and secretory IgA (sIgA) in saliva than those immunized with DnaJ alone. The production of IL-17A was also striking in DnaJ-ΔA146Ply-immunized mice. IL-17A knockout (KO) mice did not benefit from DnaJ-ΔA146Ply immunization in colonization experiments, and sIgA production was impaired in IL-17A KO mice. Collectively, our results indicate a mucosal adjuvant potential for ΔA146Ply and that, without additional adjuvant, DnaJ-ΔA146Ply fusion protein exhibits extensive immune stimulation and is effective against pneumococcal challenges, properties which are partially attributed to the IL-17A-mediated immune responses.

The role of CARMA1 in T cells

Caspase recruitment domain-containing membrane-associated guanylate kinase protein-1 (CARMA1), a member of the membrane associated guanylate kinase (MAGUK) family of kinases, is essential for T lymphocyte activation and proliferation via T-cell receptor (TCR) mediated NF-κB activation. Recent studies suggest a broader role for CARMA1 regulating other T-cell functions as well as a role in non-TCR-mediated signaling pathways important for lymphocyte development and functions. In addition, CARMA1 has been shown to be an important component in the pathogenesis of several human diseases. Thus, comprehensively defining its mechanisms of action and regulation could reveal novel therapeutic targets for T-cell-mediated diseases and lymphoproliferative disorders.

Cytokine release assays: current practices and future directions

As a result of the CD28 superagonist biotherapeutic monoclonal antibody (TGN 1412) "cytokine storm" incident, cytokine release assays (CRA) have become hazard identification and prospective risk assessment tools for screening novel biotherapeutics directed against targets having a potential risk for eliciting adverse pro-inflammatory clinical infusion reactions. Different laboratories may have different strategies, assay formats, and approaches to the reporting, interpretation, and use of data for either decision making or risk assessment. Additionally, many independent contract research organizations (CROs), academic and government laboratories are involved in some aspect of CRA work. As a result, while some pharmaceutical companies are providing CRA data as part of the regulatory submissions when necessary, technical and regulatory practices are still evolving to provide data predictive of cytokine release in humans and that are relevant to safety. This manuscript provides an overview of different approaches employed by the pharmaceutical industry and CROs, for the use and application of CRA based upon a survey and post survey follow up conducted by ILSI-Health and Environmental Sciences Institute (HESI) Immunotoxicology Committee CRA Working Group. Also discussed is ongoing research in the academic sector, the regulatory environment, current limitations of the assays, and future directions and recommendations for cytokine release assays.

Tissue-resident memory T cells: local guards of the thymus

T-cell surveillance of nonlymphoid tissues has traditionally been ascribed to recirculating memory T cells that continuously patrol the body. Extending this concept, recent evidence suggests that T cells also exist as nonmigratory memory cells that provide local immune protection in a broad range of peripheral tissues, including barrier locations such as skin and mucosa. In this issue of the European Journal of Immunology, Pircher and colleagues [Eur. J. Immunol. 2013. 43: 2295-2304] demonstrate, for the first time, the existence of such permanently tissue-resident CD8(+) memory T (TRM) cells in a primary lymphoid organ, the thymus. TRM cells in this location provide potent local immunity, which may help to preserve thymic integrity and normal T-cell development in the face of infection with thymus-invading pathogens.

Tumor-infiltrating lymphocytes expressing the tissue resident memory marker CD103 are associated with increased survival in high-grade serous ovarian cancer

BACKGROUND

The presence of CD8(+) tumor-infiltrating lymphocytes (TIL) is associated with prolonged survival in high-grade serous ovarian cancer (HGSC) and other epithelial cancers. Survival is most strongly associated with intraepithelial versus intrastromal CD8(+) TILs; however, the mechanisms that promote the intraepithelial localization of TILs remain poorly understood. We hypothesized that intraepithelial CD8(+) TILs, like normal mucosal intraepithelial lymphocytes, might express CD103, a subunit of αE/β7 integrin, which binds E-cadherin on epithelial cells.

METHODS

A large collection of primary ovarian tumors (HGSC, endometrioid, mucinous, and clear cell) was analyzed by immunohistochemistry for the presence of TIL-expressing CD103. The activation and differentiation status of CD103(+) TILs were assessed by flow cytometry. The prognostic significance of TIL subsets was evaluated by Kaplan-Meier analysis.

RESULTS

CD103(+) TILs were present in all major ovarian cancer subtypes and were most abundant in HGSC. CD103(+) TILs were preferentially localized to epithelial regions of tumors and were comprised predominantly of CD8(+) T cells expressing activation (HLA-DR, Ki-67, PD-1) and cytolytic (TIA-1) markers, as well as CD56(+) NK cells. Tumor infiltration by CD103(+) TILs was strongly associated with patient survival in HGSC. Tumors containing CD8(+) TILs that were CD103(-) showed poor prognosis equivalent to tumors lacking CD8(+) TILs altogether.

CONCLUSIONS

CD103(+) TILs comprise intraepithelial, activated CD8(+) T cells, and NK cells and are strongly associated with patient survival in HGSC. CD103 may serve as a useful marker for enriching the most beneficial subsets of TILs for immunotherapy.

Transcriptional downregulation of S1pr1 is required for the establishment of resident memory CD8+ T cells

Cell-mediated immunity critically depends on lymphocyte localization at sites of infection. While some memory T cells recirculate, a distinct lineage (resident memory T cells; T_RM) are embedded in non-lymphoid tissues (NLTs) and mediate potent protective immunity. However, the defining transcriptional basis for T_RM establishment is unknown. We report that CD8⁺ T_RM cells lacked expression of the transcription factor KLF2 and its target gene S1pr1 (encoding sphingosine 1-phosphate receptor 1). Forced S1PR1 expression prevented establishment of T_RM. Cytokines inducing T_RM phenotype (including TGF-β, IL-33 and TNF) provoked KLF2 downregulation in a phosphatidylinositol-3-OH kinase (PI(3)K)–Akt-dependent pathway, suggesting environmental regulation. Hence KLF2 and S1PR1 regulation provides a switch, dictating whether CD8⁺ T cells commit to the recirculating or tissue resident memory populations.

The developmental pathway for CD103(+)CD8+ tissue-resident memory T cells of skin

Tissue-resident memory T cells (T(RM) cells) provide superior protection against infection in extralymphoid tissues. Here we found that CD103(+)CD8(+) T(RM) cells developed in the skin from epithelium-infiltrating precursor cells that lacked expression of the effector-cell marker KLRG1. A combination of entry into the epithelium plus local signaling by interleukin 15 (IL-15) and transforming growth factor-β (TGF-β) was required for the formation of these long-lived memory cells. Notably, differentiation into T(RM) cells resulted in the progressive acquisition of a unique transcriptional profile that differed from that of circulating memory cells and other types of T cells that permanently reside in skin epithelium. We provide a comprehensive molecular framework for the local differentiation of a distinct peripheral population of memory cells that forms a first-line immunological defense system in barrier tissues.

T-cell memory differentiation: insights from transcriptional signatures and epigenetics

A critical component of vaccine design is to generate and maintain antigen-specific memory lymphocytes of sufficient quantity and quality to give the host life-long protection against re-infection. Therefore, it is important to understand how memory T cells acquire the ability for self-renewal while retaining a potential for heightened recall of effector functions. During acute viral infection or following vaccination, antigen-specific T cells undergo extensive phenotypic and functional changes during differentiation to the effector and memory phases of the immune response. The changes in cell phenotype that accompany memory T-cell differentiation are predominantly mediated through acquired transcriptional regulatory mechanisms, in part achieved through epigenetic modifications of DNA and histones. Here we review our current understanding of epigenetic mechanisms regulating the off-on-off expression of CD8 and CD4 T-cell effector molecules at naive, effector and memory stages of differentiation, respectively, and how covalent modifications to the genome may serve as a mechanism to preserve 'poised' transcriptional states in homeostatically dividing memory cells. We discuss the potential of such mechanisms to control genes that undergo on-off-on patterns of expression including homing and pro-survival genes, and the implications on the development of effector-memory and central-memory T-cell differentiation. Lastly, we review recent studies demonstrating epigenetic modifications as a mechanism for the progressive loss of transcriptional adaptation in antigen-specific T cells that undergo sustained high levels of T-cell receptor signalling.

Peripheral tissue homing receptor control of naïve, effector, and memory CD8 T cell localization in lymphoid and non-lymphoid tissues

T cell activation induces homing receptors that bind ligands on peripheral tissue vasculature, programing movement to sites of infection and injury. There are three major types of CD8 effector T cells based on homing receptor expression, which arise in distinct lymphoid organs. Recent publications indicate that naïve, effector, and memory T cell migration is more complex than once thought; while many effectors enter peripheral tissues, some re-enter lymph nodes (LN), and contain central memory precursors. LN re-entry can depend on CD62L or peripheral tissue homing receptors. Memory T cells in LN tend to express the same homing receptors as their forebears, but often are CD62Lneg. Homing receptors also control CD8 T cell tumor entry. Tumor vasculature has low levels of many peripheral tissue homing receptor ligands, but portions of it resemble high endothelial venules (HEV), enabling naïve T cell entry, activation, and subsequent effector activity. This vasculature is associated with positive prognoses in humans, suggesting it may sustain ongoing anti-tumor responses. These findings reveal new roles for homing receptors expressed by naïve, effector, and memory CD8 T cells in controlling entry into lymphoid and non-lymphoid tissues.

Peripheral tissue homing receptors enable T cell entry into lymph nodes and affect the anatomical distribution of memory cells

Peripheral tissue homing receptors enable T cells to access inflamed nonlymphoid tissues. In this study, we show that two such molecules, E-selectin ligand and α4β1 integrin, enable activated and memory T cells to enter lymph nodes (LN) as well. This affects the quantitative and qualitative distribution of these cells among regional LN beds. CD8 memory T cells in LN that express these molecules were mostly CD62L(lo) and would normally be classified as effector memory cells. However, similar to central memory cells, they expanded upon Ag re-encounter. This led to differences in the magnitude of the recall response that depended on the route of immunization. These novel cells share properties of both central and effector memory cells and reside in LN based on previously undescribed mechanisms of entry.

Thymus-resident memory CD8+ T cells mediate local immunity

The thymus is a primary lymphoid organ responsible for production and selection of T cells. Nonetheless, mature T cells and in particular activated T cells can reenter the thymus. Here, we identified memory CD8(+) T cells specific for lymphocytic choriomeningitis virus or vaccinia virus in the thymus of mice long-time after the infection. CD8(+) T cells were mainly located in the thymic medulla, but also in the cortical areas. Interestingly, virus-specific memory CD8(+) T cells in the thymus expressed the cell surface markers CD69 and CD103 that are characteristic of tissue-resident memory T cells in a time-dependent manner. Kinetic analyses and selective depletion of peripheral CD8(+) T cells by antibodies further revealed that thymic virus-specific memory CD8(+) T cells did not belong to the circulating pool of lymphocytes. Finally, we demonstrate that these thymus-resident virus-specific memory CD8(+) T cells efficiently mounted a secondary proliferative response, exhibited immediate effector functions and were able to protect the thymus from lymphocytic choriomeningitis virus reinfection. In conclusion, the present study not only describes for the first time virus-specific memory CD8(+) T cells with characteristics of tissue-resident memory T (T(RM)) cells in a primary lymphoid organ but also extends our knowledge about local T-cell immunity in the thymus.

Using epigenetics to define vaccine-induced memory T cells

Memory T cells generated from acute infection or vaccination have the potential to provide the host with life-long immunity against re-infection. Protection by memory T cells is achieved through their acquired ability to persist at anatomical sites of the primary infection as well as maintaining a heightened ability to recall effector functions. The maintenance of CD8 and CD4 T cell function in a state of readiness is key to life-long immunity and manifest through changes in transcriptional regulation. Yet, the ability to identify poised transcriptional programs at the maintenance stage of the response is lacking from most transcriptional profiling studies of memory T cells. Epigenetic profiling allows for the assessment of transcriptionally poised (promoters that are readily accessible for transcription) states of antigen-specific T cells without manipulation of the activation state of the cell. Here we review recent studies that have examined epigenetic programs of effector and memory T cell subsets. These reports demonstrate that acquisition of epigenetic programs during memory T cell differentiation to acute and chronic infections is coupled to, and potentially regulate, the cell's recall response. We discuss the usefulness of epigenetic profiling in characterizing T cell differentiation state and function for preclinical evaluation of vaccines and the current methodologies for single locus versus genome-wide epigenetic profiling.

Memory CD4 T cell-mediated immunity against influenza A virus: more than a little helpful

Recent observations have uncovered multiple pathways whereby CD4 T cells can contribute to protective immune responses against microbial threats. Incorporating the generation of memory CD4 T cells into vaccine strategies thus presents an attractive approach toward improving immunity against several important human pathogens, especially those against which antibody responses alone are inadequate to confer long-term immunity. Here, we review how memory CD4 T cells provide protection against influenza viruses. We discuss the complexities of protective memory CD4 T cell responses observed in animal models and the potential challenges of translating these observations into the clinic. Specifically, we concentrate on how better understanding of organ-specific heterogeneity of responding cells and defining multiple correlates of protection might improve vaccine-generated memory CD4 T cells to better protect against seasonal, and more importantly, pandemic influenza.

Sensing and alarm function of resident memory CD8⁺ T cells

CD8(+) T cells eliminate intracellular infections through two contact-dependent effector functions: cytolysis and secretion of antiviral cytokines. Here we identify the following additional function for memory CD8(+) T cells that persist at front-line sites of microbial exposure: to serve as local sensors of previously encountered antigens that precipitate innate-like alarm signals and draw circulating memory CD8(+) T cells into the tissue. When memory CD8(+) T cells residing in the female mouse reproductive tract encountered cognate antigen, they expressed interferon-γ (IFN-γ), potentiated robust local expression of inflammatory chemokines and induced rapid recruitment of circulating memory CD8(+) T cells. Anamnestic responses in front-line tissues are thus an integrated collaboration between front-line and circulating populations of memory CD8(+) T cells, and vaccines should establish both populations to maximize rapid responses.

Distribution and compartmentalization of human circulating and tissue-resident memory T cell subsets

Knowledge of human T cells derives chiefly from studies of peripheral blood, whereas their distribution and function in tissues remains largely unknown. Here, we present a unique analysis of human T cells in lymphoid and mucosal tissues obtained from individual organ donors, revealing tissue-intrinsic compartmentalization of naive, effector, and memory subsets conserved between diverse individuals. Effector memory CD4(+) T cells producing IL-2 predominated in mucosal tissues and accumulated as central memory subsets in lymphoid tissue, whereas CD8(+) T cells were maintained as naive subsets in lymphoid tissues and IFN-γ-producing effector memory CD8(+) T cells in mucosal sites. The T cell activation marker CD69 was constitutively expressed by memory T cells in all tissues, distinguishing them from circulating subsets, with mucosal memory T cells exhibiting additional distinct phenotypic and functional properties. Our results provide an assessment of human T cell compartmentalization as a new baseline for understanding human adaptive immunity.

CD8 T cell memory: it takes all kinds

Understanding the mechanisms that regulate the differentiation and maintenance of CD8⁺ memory T cells is fundamental to the development of effective T cell-based vaccines. Memory cell differentiation is influenced by the cytokines that accompany T cell priming, the history of previous antigen encounters, and the tissue sites into which memory cells migrate. These cues combine to influence the developing CD8⁺ memory pool, and recent work has revealed the importance of multiple transcription factors, metabolic molecules, and surface receptors in revealing the type of memory cell that is generated. Paired with increasingly meticulous subsetting and sorting of memory populations, we now know the CD8⁺ memory pool to be phenotypically and functionally heterogeneous in nature. This includes both recirculating and tissue-resident memory populations, and cells with varying degrees of inherent longevity and protective function. These data point to the importance of tailored vaccine design. Here we discuss how the diversity of the memory CD8⁺ T cell pool challenges the notion that “one size fits all” for pathogen control, and how distinct memory subsets may be suited for distinct aspects of protective immunity.

Tissue-resident memory CD8+ T cells continuously patrol skin epithelia to quickly recognize local antigen

Recent work has demonstrated that following the clearance of infection a stable population of memory T cells remains present in peripheral organs and contributes to the control of secondary infections. However, little is known about how tissue-resident memory T cells behave in situ and how they encounter newly infected target cells. Here we demonstrate that antigen-specific CD8(+) T cells that remain in skin following herpes simplex virus infection show a steady-state crawling behavior in between keratinocytes. Spatially explicit simulations of the migration of these tissue-resident memory T cells indicate that the migratory dendritic behavior of these cells allows the detection of antigen-expressing target cells in physiologically relevant time frames of minutes to hours. Furthermore, we provide direct evidence for the identification of rare antigen-expressing epithelial cells by skin-patrolling memory T cells in vivo. These data demonstrate the existence of skin patrol by memory T cells and reveal the value of this patrol in the rapid detection of renewed infections at a previously infected site.

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.

Peripheral tissue surveillance and residency by memory T cells

T cell immunity has long been described in terms of two circulating memory populations. Central memory T (T(CM)) cells migrate between the secondary lymphoid organs and are capable of mounting a recall proliferative response on pathogen re-encounter, whereas effector memory T (T(EM)) cells traffic between blood and extralymphoid compartments for effective peripheral immune surveillance. It is now clear that there exists a third category of memory cells that never returns to the circulation. These tissue-resident memory T (T(RM)) cells are phenotypically distinct from T(EM) cells, persist in elevated numbers in areas involved in prior infection and have been implicated in various immune phenomena, such as the control of persisting infections and immune disorders in fixed regions of the body.