Environmental and antigen receptor-derived signals support sustained surveillance of the lungs by pathogen-specific cytotoxic T lymphocytes

Contribution of Resident Memory CD8+ T Cells to Protective Immunity Against Respiratory Syncytial Virus and Their Impact on Vaccine Design

Worldwide, human respiratory syncytial virus (RSV) is the most common etiological agent for acute lower respiratory tract infections (ALRI). RSV-ALRI is the major cause of hospital admissions in young children, and it can cause in-hospital deaths in children younger than six months old. Therefore, RSV remains one of the pathogens deemed most important for the generation of a vaccine. On the other hand, the effectiveness of a vaccine depends on the development of immunological memory against the pathogenic agent of interest. This memory is achieved by long-lived memory T cells, based on the establishment of an effective immune response to viral infections when subsequent exposures to the pathogen take place. Memory T cells can be classified into three subsets according to their expression of lymphoid homing receptors: central memory cells (T_CM), effector memory cells (T_EM) and resident memory T cells (T_RM). The latter subset consists of cells that are permanently found in non-lymphoid tissues and are capable of recognizing antigens and mounting an effective immune response at those sites. T_RM cells activate both innate and adaptive immune responses, thus establishing a robust and rapid response characterized by the production of large amounts of effector molecules. T_RM cells can also recognize antigenically unrelated pathogens and trigger an innate-like alarm with the recruitment of other immune cells. It is noteworthy that this rapid and effective immune response induced by T_RM cells make these cells an interesting aim in the design of vaccination strategies in order to establish T_RM cell populations to prevent respiratory infectious diseases. Here, we discuss the biogenesis of T_RM cells, their contribution to the resolution of respiratory viral infections and the induction of T_RM cells, which should be considered for the rational design of new vaccines against RSV.

A pro-inflammatory CD8+ T-cell subset patrols the cervicovaginal tract

The immune system of the cervicovaginal tract (CVT) must balance immunosurveillance and active immunity against pathogens with maintenance of tolerance to resident microbiota and to fetal and partner antigens for reproductive purposes. Thus, we predicted that CVT immunity is characterized by distinctive features compared to blood and other tissue compartments. Indeed, we found that CVT CD8+ T-cells had unique transcriptional profiles, particularly in their cytokine signature, compared to that reported for CD8+ T-cells in other tissue sites. Among these CVT CD8+ T-cells, we identified a CD69- CD103- subset that was characterized by reduced migration in response to tissue-exit signals and higher pro-inflammatory potential as compared to their blood counterpart. These inflammatory mucosal CD8+ T-cells (Tim) were increased in frequency in the CVT of individuals with chronic infection, pointing to a potential role in perpetuating inflammation. Our findings highlight the specialized nature of immunity within the CVT and identify Tim cells as potential therapeutic targets to tame tissue inflammation upon chronic infection.

Sphingosine-1-phosphate receptor 2 restrains egress of γδ T cells from the skin

Maintenance of a population of IL-17-committed γδ T cells in the dermis is important in promoting tissue immunity. However, the signals facilitating γδ T cell retention within the dermis remain poorly understood. Here, we find that sphingosine-1-phosphate receptor 2 (S1PR2) acts in a cell-intrinsic manner to oppose γδ T cell migration from the dermis to the skin draining lymph node (dLN). Migration of dermal γδ T cells to the dLN under steady-state conditions occurs in an S1PR1-dependent manner. S1PR1 and CD69 are reciprocally expressed on dermal γδ T cells, with loss of CD69 associated with increased S1PR1 expression and enhanced migration to the dLN. γδ T cells lacking both S1PR2 and CD69 are impaired in their maintenance within the dermis. These findings provide a mechanism for how IL-17+ γδ T cells establish residence within the dermis and identify a role for S1PR2 in restraining the egress of tissue-resident lymphocytes.

The Functional Requirement for CD69 in Establishment of Resident Memory CD8+ T Cells Varies with Tissue Location

Recent studies have characterized populations of memory CD8⁺ T cells that do not recirculate through the blood but are, instead, retained in nonlymphoid tissues. Such CD8⁺ tissue resident memory T cells (T_RM) are critical for pathogen control at barrier sites. Identifying T_RM and defining the basis for their tissue residency is therefore of considerable importance for understanding protective immunity and improved vaccine design. Expression of the molecule CD69 is widely used as a definitive marker for T_RM, yet it is unclear whether CD69 is universally required for producing or retaining T_RM Using multiple mouse models of acute immunization, we found that the functional requirement for CD69 was highly variable, depending on the tissue examined, playing no detectable role in generation of T_RM at some sites (such as the small intestine), whereas CD69 was critical for establishing resident cells in the kidney. Likewise, forced expression of CD69 (but not expression of a CD69 mutant unable to bind the egress factor S1PR1) promoted CD8⁺ T_RM generation in the kidney but not in other tissues. Our findings indicate that the functional relevance of CD69 in generation and maintenance of CD8⁺ T_RM varies considerably, chiefly dependent on the specific nonlymphoid tissue studied. Together with previous reports that suggest uncoupling of CD69 expression and tissue residency, these findings prompt caution in reliance on CD69 expression as a consistent marker of CD8⁺ T_RM.

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.

Establishment and Maintenance of Conventional and Circulation-Driven Lung-Resident Memory CD8+ T Cells Following Respiratory Virus Infections

Antigen-specific CD8⁺ tissue-resident memory T cells (T_RM cells) persist in the lung following resolution of a respiratory virus infection and provide first-line defense against reinfection. In contrast to other memory T cell populations, such as central memory T cells that circulate between lymph and blood, and effector memory T cells (T_EM cells) that circulate between blood and peripheral tissues, T_RM cells are best defined by their permanent residency in the tissues and their independence from circulatory T cell populations. Consistent with this, we recently demonstrated that CD8⁺ T_RM cells primarily reside within specific niches in the lung (Repair-Associated Memory Depots; RAMD) that normally exclude CD8⁺ T_EM cells. However, it has also been reported that circulating CD8⁺ T_EM cells continuously convert into CD8⁺ T_RM cells in the lung interstitium, helping to sustain T_RM numbers. The relative contributions of these two mechanisms of CD8⁺ T_RM cells maintenance in the lung has been the source of vigorous debate. Here we propose a model in which the majority of CD8⁺ T_RM cells are maintained within RAMD (conventional T_RM) whereas a small fraction of T_RM are derived from circulating CD8⁺ T_EM cells and maintained in the interstitium. The numbers of both types of T_RM cells wane over time due to declines in both RAMD availability and the overall number of T_EM in the circulation. This model is consistent with most published reports and has important implications for the development of vaccines designed to elicit protective T cell memory in the lung.

4-1BB Regulates Effector CD8 T Cell Accumulation in the Lung Tissue through a TRAF1-, mTOR-, and Antigen-Dependent Mechanism to Enhance Tissue-Resident Memory T Cell Formation during Respiratory Influenza Infection

The TNFR superfamily member 4-1BB is important in the establishment of tissue-resident memory T cells (Trm) in the lung tissue following influenza infection. Moreover, supraphysiological boosting of 4-1BB in the airways during the boost phase of a prime-boost immunization regimen increases the long-lived Trm population, correlating with increased protection against heterotypic challenge. However, little is known about how 4-1BB contributes to the establishment of the lung Trm population. In this study, we show that effects of 4-1BB on lung Trm accumulation are already apparent at the effector stage, suggesting that the major role of 4-1BB in Trm formation is to allow persistence of CD8 T effector cells in the lung as they transition to Trm. Using supraphysiological stimulation of 4-1BB in the boost phase of a prime-boost immunization, we show that the effect of 4-1BB on Trm generation requires local delivery of both Ag and costimulation, is inhibited by rapamycin treatment during secondary CD8 effector T cell expansion, and is dependent on the signaling adaptor TRAF1. The decrease in lung Trm following early rapamycin treatment is accompanied by increased circulating memory T cells, as well as fewer effectors, suggesting a role for mammalian target of rapamycin (mTOR) in the formation of Trm through effects on the accumulation of effector precursors. Taken together, these data point to an important role for 4-1BB, TRAF1, and mTOR in the persistence of CD8 effector T cells in the lung parenchyma, thereby allowing the transition to Trm.

Persistent viral replication and the development of T-cell responses after intranasal infection by MCMV

Natural transmission of cytomegalovirus (CMV) has been difficult to observe. However, recent work using the mouse model of murine (M)CMV demonstrated that MCMV initially infects the nasal mucosa after transmission from mothers to pups. We found that intranasal (i.n.) inoculation of C57BL/6J mice resulted in reliable recovery of replicating virus from the nasal mucosa as assessed by plaque assay. After i.n. inoculation, CD8⁺ T-cell priming occurred in the mandibular, deep-cervical, and mediastinal lymph nodes within 3 days of infection. Although i.n. infection induced "memory inflation" of T cells specific for the M38_316-323 epitope, there were no detectable CD8⁺ T-cell responses against the late-appearing IE3_416-423 epitope, which contrasts with intraperitoneal (i.p.) infection. MCMV-specific T cells migrated into the nasal mucosa where they developed a tissue-resident memory (T_RM) phenotype and this could occur independently of local virus infection or antigen. Strikingly however, virus replication was poorly controlled in the nasal mucosa and MCMV was detectable by plaque assay for at least 4 months after primary infection, making the nasal mucosa a second site for MCMV persistence. Unlike in the salivary glands, the persistence of MCMV in the nasal mucosa was not modulated by IL-10. Taken together, our data characterize the development of local and systemic T-cell responses after intranasal infection by MCMV and define the nasal mucosa, a natural site of viral entry, as a novel site of viral persistence.

Vitiligo as a skin memory disease: The need for early intervention with immunomodulating agents and a maintenance therapy to target resident memory T cells

The understanding of the immune mechanisms of vitiligo has profoundly improved over the past years. The recent discovery of a new population of antigen-experienced memory T cells called resident memory T cells (T_RM ) has changed the concept of immune surveillance in peripheral tissue as skin, and the presence of melanocyte-specific T_RM is clearly demonstrated in vitiligo, a disease that could be now seen such as a memory skin disease. This review summarizes the recent knowledge on skin T_RM and their role in vitiligo. Future management or therapies for this disease will have the goal to block their migration/differentiation, to dampen their activation and/or their accumulation in the vitiligo skin to prevent flare-up or to promote repigmentation.

Comparative analysis reveals a role for TGF-β in shaping the residency-related transcriptional signature in tissue-resident memory CD8+ T cells

Tissue-resident CD8⁺ memory T (TRM) cells are immune cells that permanently reside at tissue sites where they play an important role in providing rapid protection against reinfection. They are not only phenotypically and functionally distinct from their circulating memory counterparts, but also exhibit a unique transcriptional profile. To date, the local tissue signals required for their development and long-term residency are not well understood. So far, the best-characterised tissue-derived signal is transforming growth factor-β (TGF-β), which has been shown to promote the development of these cells within tissues. In this study, we aimed to determine to what extent the transcriptional signatures of TRM cells from multiple tissues reflects TGF-β imprinting. We activated murine CD8⁺ T cells, stimulated them in vitro by TGF-β, and profiled their transcriptomes using RNA-seq. Upon comparison, we identified a TGF-β-induced signature of differentially expressed genes between TGF-β-stimulated and -unstimulated cells. Next, we linked this in vitro TGF-β-induced signature to a previously identified in vivo TRM-specific gene set and found considerable (>50%) overlap between the two gene sets, thus showing that a substantial part of the TRM signature can be attributed to TGF-β signalling. Finally, gene set enrichment analysis further revealed that the altered gene signature following TGF-β exposure reflected transcriptional signatures found in TRM cells from both epithelial and non-epithelial tissues. In summary, these findings show that TGF-β has a broad footprint in establishing the residency-specific transcriptional profile of TRM cells, which is detectable in TRM cells from diverse tissues. They further suggest that constitutive TGF-β signaling might be involved for their long-term persistence at tissue sites.

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.

IL-33 Augments Virus-Specific Memory T Cell Inflation and Potentiates the Efficacy of an Attenuated Cytomegalovirus-Based Vaccine

Candidate vaccines designed to generate T cell-based immunity are typically vectored by nonpersistent viruses, which largely fail to elicit durable effector memory T cell responses. This limitation can be overcome using recombinant strains of CMV. Proof-of-principle studies have demonstrated the potential benefits of this approach, most notably in the SIV model, but safety concerns require the development of nonreplicating alternatives with comparable immunogenicity. In this study, we show that IL-33 promotes the accumulation and recall kinetics of circulating and tissue-resident memory T cells in mice infected with murine CMV. Using a replication-deficient murine CMV vector, we further show that exogenous IL-33 boosts vaccine-induced memory T cell responses, which protect against subsequent heterologous viral challenge. These data suggest that IL-33 could serve as a useful adjuvant to improve the efficacy of vaccines based on attenuated derivatives of CMV.

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.

Viral MHCI inhibition evades tissue-resident memory T cell formation and responses

Tissue-resident memory CD8+ T cells (TRMs) confer rapid protection and immunity against viral infections. Many viruses have evolved mechanisms to inhibit MHCI presentation in order to evade CD8+ T cells, suggesting that these mechanisms may also apply to TRM-mediated protection. However, the effects of viral MHCI inhibition on the function and generation of TRMs is unclear. Herein, we demonstrate that viral MHCI inhibition reduces the abundance of CD4+ and CD8+ TRMs, but its effects on the local microenvironment compensate to promote antigen-specific CD8+ TRM formation. Unexpectedly, local cognate antigen enhances CD8+ TRM development even in the context of viral MHCI inhibition and CD8+ T cell evasion, strongly suggesting a role for in situ cross-presentation in local antigen-driven TRM differentiation. However, local cognate antigen is not required for CD8+ TRM maintenance. We also show that viral MHCI inhibition efficiently evades CD8+ TRM effector functions. These findings indicate that viral evasion of MHCI antigen presentation has consequences on the development and response of antiviral TRMs.

Advax augments B and T cell responses upon influenza vaccination via the respiratory tract and enables complete protection of mice against lethal influenza virus challenge

Administration of influenza vaccines via the respiratory tract has potential benefits over conventional parenteral administration, inducing immunity directly at the site of influenza exposure as well as being needle free. In this study, we investigated the suitability of Advax™, a stable particulate polymorph of inulin, also referred to as delta inulin, as a mucosal adjuvant for whole inactivated influenza vaccine (WIV) administered either as a liquid or dry powder formulation. Spray freeze-drying produced Advax-adjuvanted WIV powder particles in a size range (1-5 μm) suitable for inhalation. The physical and biological characteristics of both WIV and Advax remained unaltered both by admixing WIV with Advax and by spray freeze drying. Upon intranasal or pulmonary immunization, both liquid and dry powder formulations containing Advax induced significantly higher systemic, mucosal and cellular immune responses than non-adjuvanted WIV formulations. Furthermore, pulmonary immunization with Advax-adjuvanted WIV led to robust memory B cell responses along with an increase of lung localization factors i.e. CXCR3, CD69, and CD103. A less pronounced but still positive effect of Advax was seen on memory T cell responses. In contrast to animals immunized with WIV alone, all animals pulmonary immunized with a single dose of Advax-adjuvanted WIV were fully protected with no visible clinical symptoms against a lethal dose of influenza virus. These data confirm that Advax is a potent mucosal adjuvant that boosts vaccine-induced humoral and cellular immune responses both in the lung and systemically with major positive effects on B-cell memory and complete protection against live virus. Hence, respiratory tract immunization, particularly via the lungs, with Advax-adjuvanted WIV formulation as a liquid or dry powder is a promising alternative to parenteral influenza vaccination.

Tissue-Resident Lymphocytes Across Innate and Adaptive Lineages

Lymphocytes are an integral component of the immune system. Classically, all lymphocytes were thought to perpetually recirculate between secondary lymphoid organs and only traffic to non-lymphoid tissues upon activation. In recent years, a diverse family of non-circulating lymphocytes have been identified. These include innate lymphocytes, innate-like T cells and a subset of conventional T cells. Spanning the innate-adaptive spectrum, these tissue-resident lymphocytes carry out specialized functions and cross-talk with other immune cell types to maintain tissue integrity and homeostasis both at the steady state and during pathological conditions. In this review, we provide an overview of the heterogeneous tissue-resident lymphocyte populations, discuss their development, and highlight their functions both in the context of microbial infection and cancer.

CD8+ Resident Memory T Cells and Viral Infection

Tissue-resident memory T (Trm) cells are a subset of recently identified memory T cells that mainly reside and serve as sentinels in non-lymphoid peripheral tissues. Unlike the well-characterized circulating central memory T (Tcm) cells and effector memory T (Tem) cells, Trm cells persist in the tissues, do not recirculate into blood, and offer immediate protection against pathogens upon reinfection. In this review, we focus on CD8⁺ Trm cells and briefly introduce their characteristics, development, maintenance, and function during viral infection. We also discuss some unresolved problems, such as how CD8⁺ Trm cells adapt to the local tissue microenvironment, how Trm cells interact with other immune cells during their development and maintenance, and the mechanisms by which CD8⁺ Trm cells confer immune protection. We believe that a better understanding of these problems is of great clinical and therapeutic value and may contribute to more effective vaccination and treatments against viral infection.

Spontaneous Proliferation of CD4+ T Cells in RAG-Deficient Hosts Promotes Antigen-Independent but IL-2-Dependent Strong Proliferative Response of Naïve CD8+ T Cells

The fast and intense proliferative responses have been well documented for naïve T cells adoptively transferred into chronic lymphopenic hosts. This response known as spontaneous proliferation (SP), unlike antigen-independent lymphopenia-induced proliferation (LIP), is driven in a manner dependent on antigens derived from commensal microbiota. However, the precise nature of the SP response and its impact on homeostasis and function for T cells rapidly responding under this lymphopenic condition are still unclear. Here we demonstrate that, when naïve T cells were adoptively transferred into specific pathogen-free (SPF) but not germ-free (GF) RAG^-/- hosts, the SP response of these cells substantially affects the intensity and tempo of the responding T cells undergoing LIP. Therefore, the resulting response of these cells in SPF RAG^-/- hosts was faster and stronger than the typical LIP response observed in irradiated B6 hosts. Although the intensity and tempo of such augmented LIP in SPF RAG^-/- hosts were analogous to those of antigen-dependent SP, the former was independent of antigenic stimulation but most importantly, dependent on IL-2. Similar observations were also apparent in other acute lymphopenic settings where antigen-dependent T cell activation can strongly occur and induce sufficient levels of IL-2 production. Consequently, the resulting T cells undergoing IL-2-driven strong proliferative responses showed the ability to differentiate into functional effector and memory cells that can control infectious pathogens. These findings therefore reveal previously unappreciated role of IL-2 in driving the intense form of T cell proliferative responses in chronic lymphopenic hosts.

IL-1β as mucosal vaccine adjuvant: the specific induction of tissue-resident memory T cells improves the heterosubtypic immunity against influenza A viruses

A universal influenza vaccine must provide protection against antigenically divergent influenza viruses either through broadly neutralizing antibodies or cross-reactive T cells. Here, intranasal immunizations with recombinant adenoviral vectors (rAd) encoding hemagglutinin (HA) and nucleoprotein (NP) in combination with rAd-Interleukin-(IL)-1β or rAd-IL-18 were evaluated for their efficacy in BALB/c mice. Mucosal delivery of rAd-IL-1β enhanced HA-specific antibody responses including strain-specific neutralizing antibodies. Nevertheless, the beneficial effects on the local T cell responses were much more impressive reflected by increased numbers of CD103⁺CD69⁺ tissue-resident memory T cells (T_RM). This increased immunogenicity translated into superior protection against infections with homologous and heterologous strains including H1N1, pH1N1, H3N2, and H7N7. Inhibition of the egress of circulating T cells out of the lymph nodes during the heterologous infection had no impact on the degree of protection underscoring the unique potential of T_RM for the local containment of mucosal infections. The local co-expression of IL-1β and antigen lead to the activation of critical checkpoints in the formation of T_RM including activation of epithelial cells, expression of chemokines and adhesion molecules, recruitment of lung-derived CD103⁺ DCs, and finally local T_RM imprinting. Given the importance of T_RM-mediated protection at mucosal barriers, this study has major implications for vaccine development.

Listeria Monocytogenes: A Model Pathogen Continues to Refine Our Knowledge of the CD8 T Cell Response

Listeria monocytogenes (Lm) infection induces robust CD8 T cell responses, which play a critical role in resolving Lm during primary infection and provide protective immunity to re-infections. Comprehensive studies have been conducted to delineate the CD8 T cell response after Lm infection. In this review, the generation of the CD8 T cell response to Lm infection will be discussed. The role of dendritic cell subsets in acquiring and presenting Lm antigens to CD8 T cells and the events that occur during T cell priming and activation will be addressed. CD8 T cell expansion, differentiation and contraction as well as the signals that regulate these processes during Lm infection will be explored. Finally, the formation of memory CD8 T cell subsets in the circulation and in the intestine will be analyzed. Recently, the study of CD8 T cell responses to Lm infection has begun to shift focus from the intravenous infection model to a natural oral infection model as the humanized mouse and murinized Lm have become readily available. Recent findings in the generation of CD8 T cell responses to oral infection using murinized Lm will be explored throughout the review. Finally, CD8 T cell-mediated protective immunity against Lm infection and the use of Lm as a vaccine vector for cancer immunotherapy will be highlighted. Overall, this review will provide detailed knowledge on the biology of CD8 T cell responses after Lm infection that may shed light on improving rational vaccine design.

Aging impacts CD103+ CD8+ T cell presence and induction by dendritic cells in the genital tract

As women age, susceptibility to systemic and genital infections increases. Tissue-resident memory T cells (TRMs) are CD103⁺ CD8⁺ long-lived lymphocytes that provide critical mucosal immune protection. Mucosal dendritic cells (DCs) are known to induce CD103 expression on CD8⁺ T cells. While CD103⁺ CD8⁺ T cells are found throughout the female reproductive tract (FRT), the extent to which aging impacts their presence and induction by DCs remains unknown. Using hysterectomy tissues, we found that endometrial CD103⁺ CD8⁺ T cells were increased in postmenopausal compared to premenopausal women. Endometrial DCs from postmenopausal women were significantly more effective at inducing CD103 expression on allogeneic naïve CD8⁺ T cells than DCs from premenopausal women; CD103 upregulation was mediated through membrane-bound TGFβ signaling. In contrast, cervical CD103⁺ T cells and DC numbers declined in postmenopausal women with age. Decreases in DCs correlated with decreased CD103⁺ T cells in endocervix, but not ectocervix. Our findings demonstrate a previously unrecognized compartmentalization of TRMs in the FRT of postmenopausal women, with loss of TRMs and DCs in the cervix with aging, and increased TRMs and DC induction capacity in the endometrium. These findings are relevant to understanding immune protection in the FRT and to the design of vaccines for women of all ages.

The CD8 T Cell Response to Respiratory Virus Infections

Humans are highly susceptible to infection with respiratory viruses including respiratory syncytial virus (RSV), influenza virus, human metapneumovirus, rhinovirus, coronavirus, and parainfluenza virus. While some viruses simply cause symptoms of the common cold, many respiratory viruses induce severe bronchiolitis, pneumonia, and even death following infection. Despite the immense clinical burden, the majority of the most common pulmonary viruses lack long-lasting efficacious vaccines. Nearly all current vaccination strategies are designed to elicit broadly neutralizing antibodies, which prevent severe disease following a subsequent infection. However, the mucosal antibody response to many respiratory viruses is not long-lasting and declines with age. CD8 T cells are critical for mediating clearance following many acute viral infections in the lung. In addition, memory CD8 T cells are capable of providing protection against secondary infections. Therefore, the combined induction of virus-specific CD8 T cells and antibodies may provide optimal protective immunity. Herein, we review the current literature on CD8 T cell responses induced by respiratory virus infections. Additionally, we explore how this knowledge could be utilized in the development of future vaccines against respiratory viruses, with a special emphasis on RSV vaccination.

Harnessing the Power of T Cells: The Promising Hope for a Universal Influenza Vaccine

Next-generation vaccines that utilize T cells could potentially overcome the limitations of current influenza vaccines that rely on antibodies to provide narrow subtype-specific protection and are prone to antigenic mismatch with circulating strains. Evidence from animal models shows that T cells can provide heterosubtypic protection and are crucial for immune control of influenza virus infections. This has provided hope for the design of a universal vaccine able to prime against diverse influenza virus strains and subtypes. However, multiple hurdles exist for the realisation of a universal T cell vaccine. Overall primary concerns are: extrapolating human clinical studies, seeding durable effective T cell resident memory (Trm), population human leucocyte antigen (HLA) coverage, and the potential for T cell-mediated immune escape. Further comprehensive human clinical data is needed during natural infection to validate the protective role T cells play during infection in the absence of antibodies. Furthermore, fundamental questions still exist regarding the site, longevity and duration, quantity, and phenotype of T cells needed for optimal protection. Standardised experimental methods, and eventually simplified commercial assays, to assess peripheral influenza-specific T cell responses are needed for larger-scale clinical studies of T cells as a correlate of protection against influenza infection. The design and implementation of a T cell-inducing vaccine will require a consensus on the level of protection acceptable in the community, which may not provide sterilizing immunity but could protect the individual from severe disease, reduce the length of infection, and potentially reduce transmission in the community. Therefore, increasing the standard of care potentially offered by T cell vaccines should be considered in the context of pandemic preparedness and zoonotic infections, and in combination with improved antibody vaccine targeting methods. Current pandemic vaccine preparedness measures and ongoing clinical trials under-utilise T cell-inducing vaccines, reflecting the myriad questions that remain about how, when, where, and which T cells are needed to fight influenza virus infection. This review aims to bring together basic fundamentals of T cell biology with human clinical data, which need to be considered for the implementation of a universal vaccine against influenza that harnesses the power of T cells.

Crucial role for CD69 in allergic inflammatory responses: CD69-Myl9 system in the pathogenesis of airway inflammation

CD69 has been known as an early activation marker of lymphocytes; whereas, recent studies demonstrate that CD69 also has critical functions in immune responses. Early studies using human samples revealed the involvement of CD69 in various inflammatory diseases including asthma. Moreover, murine disease models using Cd69^-/- mice and/or anti-CD69 antibody (Ab) treatment have revealed crucial roles for CD69 in inflammatory responses. However, it had not been clear how the CD69 molecule contributes to the pathogenesis of inflammatory diseases. We recently elucidated a novel mechanism, in which the interaction between CD69 and its ligands, myosin light chain 9, 12a and 12b (Myl9/12) play a critical role in the recruitment of activated T cells into the inflammatory lung. In this review, we first summarize CD69 function based on its structure and then introduce the evidence for the involvement of CD69 in human diseases and murine disease models. Then, we will describe how we discovered CD69 ligands, Myl9 and Myl12, and how the CD69-Myl9 system regulates airway inflammation. Finally, we will discuss possible therapeutic usages of the blocking Ab to the CD69-Myl9 system.

Tissue-Resident Memory CD8+ T Cells: From Phenotype to Function

Tissue-resident memory CD8⁺ T cells are an important first line of defense from infection in peripheral non-lymphoid tissues, such as the mucosal tissues of the respiratory, digestive, and urogenital tracts. This memory T cell subset is established late during resolution of primary infection of those tissues, has a distinct genetic signature, and is often defined by the cell surface expression of CD69, CD103, CD49a, and CD44 in both mouse and human studies. The stimuli that program or imprint the unique gene expression and cell surface phenotypes on T_RM are beginning to be defined, but much work remains to be done. It is not clear, for example, when and where the T_RM precursors receive these signals, and there is evidence that supports imprinting in both the lymph node and the peripheral tissue sites. In most studies, expression of CD49a, CD103, and CD69 on T cells in the tissues appears relatively late in the response, suggesting there are precise environmental cues that are not present at the height of the acute response. CD49a and CD103 are not merely biomarkers of T_RM, they confer substrate specificities for cell adhesion to collagen and E-cadherin, respectively. Yet, little attention has been paid to how expression affects the positioning of T_RM in the peripheral tissues. CD103 and CD49a are not mutually exclusive, and not always co-expressed, although whether they can compensate for one another is unknown. In fact, they may define different subsets of T_RM in certain tissues. For instance, while CD49a⁺CD8⁺ memory T cells can be found in almost all peripheral tissues, CD103 appears to be more restricted. In this review, we discuss the evidence for how these hallmarks of T_RM affect positioning of T cells in peripheral sites, how CD49a and CD103 differ in expression and function, and why they are important for immune protection conferred by T_RM in mucosal tissues such as the respiratory tract.

Cellular Immune Correlates Preventing Disease Against Respiratory Syncytial Virus by Vaccination with Virus-Like Nanoparticles Carrying Fusion Proteins

Cellular immune correlates conferring protection against respiratory syncytial virus (RSV) but preventing vaccine-enhanced respiratory disease largely remain unclear. We investigated cellular immune correlates that contribute to preventing disease against human respiratory syncytial virus (RSV) by nanoparticle vaccine delivery. Formalin-inactivated RSV (FI-RSV) vaccines and virus-like nanoparticles carrying RSV fusion proteins (F VLP) were investigated in mice. The FI-RSV vaccination caused severe weight loss and histopathology by inducing interleukin (IL)-4⁺, interferon (IFN)-γ⁺, IL-4⁺IFN-γ⁺ CD4⁺ T cells, eosinophils, and lung plasmacytoid dendritic cells (DCs), CD103⁺ DCs, and CD11b⁺ DCs. In contrast, the F VLP-immune mice induced protection against RSV without disease by inducing natural killer cells, activated IFN-γ⁺, and IFN-γ⁺ tumor necrosis factor (TNF)-α⁺ CD8⁺ T cells in the lung and bronchiolar airways during RSV infection but not disease-inducing DCs and effector T cells. Clodronate-mediated depletion studies provided evidence that alveolar macrophages that were present at high levels in the F VLP-immune mice play a role in modulating protective cellular immune phenotypes. There was an intrinsic difference between the F VLP and FI-RSV treatments in stimulating proinflammatory cytokines. The F VLP nanoparticle vaccination induced distinct innate and adaptive cellular subsets that potentially prevented lung disease after RSV infection.

ZEB1, ZEB2, and the miR-200 family form a counterregulatory network to regulate CD8+ T cell fates

Guan et al. identify genetic cooperativity between the transcription factor ZEB1 and the miR-200 family in memory CD8⁺ T cell development, which contrasts with that observed in the EMT. This study also shows that ZEB1 and its closely related homologue, ZEB2, play functionally distinct roles in CD8⁺ T cell differentiation.

Long-term immunity depends partly on the establishment of memory CD8⁺ T cells. We identified a counterregulatory network between the homologous transcription factors ZEB1 and ZEB2 and the miR-200 microRNA family, which modulates effector CD8⁺ T cell fates. Unexpectedly, Zeb1 and Zeb2 had reciprocal expression patterns and were functionally uncoupled in CD8⁺ T cells. ZEB2 promoted terminal differentiation, whereas ZEB1 was critical for memory T cell survival and function. Interestingly, the transforming growth factor β (TGF-β) and miR-200 family members, which counterregulate the coordinated expression of Zeb1 and Zeb2 during the epithelial-to-mesenchymal transition, inversely regulated Zeb1 and Zeb2 expression in CD8⁺ T cells. TGF-β induced and sustained Zeb1 expression in maturing memory CD8⁺ T cells. Meanwhile, both TGF-β and miR-200 family members selectively inhibited Zeb2. Additionally, the miR-200 family was necessary for optimal memory CD8⁺ T cell formation. These data outline a previously unknown genetic pathway in CD8⁺ T cells that controls effector and memory cell fate decisions.

The Chemokine Receptor CXCR3 Promotes CD8+ T Cell Accumulation in Uninfected Salivary Glands but Is Not Necessary after Murine Cytomegalovirus Infection

Recent work indicates that salivary glands are able to constitutively recruit CD8⁺ T cells and retain them as tissue-resident memory T cells, independently of local infection, inflammation, or Ag. To understand the mechanisms supporting T cell recruitment to the salivary gland, we compared T cell migration to the salivary gland in mice that were infected or not with murine CMV (MCMV), a herpesvirus that infects the salivary gland and promotes the accumulation of salivary gland tissue-resident memory T cells. We found that acute MCMV infection increased rapid T cell recruitment to the salivary gland but that equal numbers of activated CD8⁺ T cells eventually accumulated in infected and uninfected glands. T cell recruitment to uninfected salivary glands depended on chemokines and the integrin α₄ Several chemokines were expressed in the salivary glands of infected and uninfected mice, and many of these could promote the migration of MCMV-specific T cells in vitro. MCMV infection increased the expression of chemokines that interact with the receptors CXCR3 and CCR5, but neither receptor was needed for T cell recruitment to the salivary gland during MCMV infection. Unexpectedly, however, the chemokine receptor CXCR3 was critical for T cell accumulation in uninfected salivary glands. Together, these data suggest that CXCR3 and the integrin α₄ mediate T cell recruitment to uninfected salivary glands but that redundant mechanisms mediate T cell recruitment after MCMV infection.

Formation and function of tissue-resident memory T cells during viral infection

Memory T cells are an important component of the adaptive immune response. Tissue-resident memory T cells (T_RM) are a recently described subset of memory T cells that reside in peripheral tissues and are maintained independently of circulating subsets of memory T cells. Importantly, T_RM are frequently found in barrier tissues that commonly serve as entry portals for pathogens such as viruses. Mounting evidence shows that T_RM are superior to their circulating counterparts in conferring protective immunity against a wide range of viruses. In this review, we will discuss the role of T_RM in controlling viral infection with a focus on CD8+ T_RM, the factors that regulate differentiation and a potential role for T_RM in future vaccine development.

IL-7 plays a critical role for the homeostasis of allergen-specific memory CD4 T cells in the lung and airways

Memory T cells respond rapidly to repeated antigen exposure and can maintain their population for extended periods through self-renewal. These characteristics of memory T cells have mainly been studied during viral infections, whereas their existence and functions in allergic diseases have been studied incompletely. Since allergic patients can suffer repeated relapses caused by intermittent allergen exposure, we hypothesized that allergen- specific memory Th2 cells are present and the factors necessary for the maintenance of these cells are provided by the lung and airways. Using a murine model of airway inflammation, we found that allergen-specific CD4 T cells survived longer than 70 days in the lung and airways in an IL-7 dependent fashion. These T cells showing homeostatic proliferation were largely found in the mediastinal lymph node (mLN), rather than the airways; however, cells residing in the lung and airways developed recall responses successfully. We also found that CD4 T cells exhibited differential phenotypes in the mLN and in the lung. Altogether, we believe that allergen-specific memory T cells reside and function in the lung and airways, while their numbers are replenished through homeostatic turnover in the mLNs. Furthermore, we determined that IL-7 signaling is important for the homeostasis of these cells.

Intrinsic 4-1BB signals are indispensable for the establishment of an influenza-specific tissue-resident memory CD8 T-cell population in the lung

The induction of long-lived heterotypic T-cell protection against influenza virus remains elusive, despite the conservation of T-cell epitopes. T-cell protection against influenza is critically dependent on lung-resident memory T cells (Trm). Here we show that intranasal administration of 4-1BBL along with influenza nucleoprotein in a replication-defective adenovirus vector to influenza pre-immune mice induces a remarkably stable circulating effector memory CD8 T-cell population characterized by higher IL-7Rα expression than control-boosted T cells, as well as a substantial lung parenchymal CD69⁺ CD8 Trm population, including both CD103⁺ and CD103^- cells. These T-cell responses persist to greater than 200 days post-boost and protect against lethal influenza challenge in aged (year old) mice. The expansion of the nucleoprotein-specific CD8 Trm population during boosting involves recruitment of circulating antigen-specific cells and is critically dependent on local rather than systemic administration of 4-1BBL as well as on 4-1BB on the CD8 T cells. Moreover, during primary influenza infection of mixed bone marrow chimeras, 4-1BB-deficient T cells fail to contribute to the lung-resident Trm population. These findings establish both endogenous and supraphysiological 4-1BBL as a critical regulator of lung-resident memory CD8 T cells during influenza infection.

Expression and role of VLA-1 in resident memory CD8 T cell responses to respiratory mucosal viral-vectored immunization against tuberculosis

Lung resident memory T cells (T_RM) characterized by selective expression of mucosal integrins VLA-1 (α1β1) and CD103 (α_Eβ7) are generated following primary respiratory viral infections. Despite recent progress, the generation of lung T_RM and the role of mucosal integrins following viral vector respiratory mucosal immunization still remains poorly understood. Here by using a replication-defective viral vector tuberculosis vaccine, we show that lung Ag-specific CD8 T cells express both VLA-1 and CD103 following respiratory mucosal immunization. However, VLA-1 and CD103 are acquired in differential tissue sites with the former acquired during T cell priming in the draining lymph nodes and the latter acquired after T cells entered the lung. Once in the lung, Ag-specific CD8 T cells continue to express VLA-1 at high levels through the effector/expansion, contraction, and memory phases of T cell responses. Using a functional VLA-1 blocking mAb, we show that VLA-1 is not required for trafficking of these cells to the lung, but it negatively regulates them in the contraction phase. Furthermore, VLA-1 plays a negligible role in the maintenance of these cells in the lung. Our study provides new information on vaccine-inducible lung T_RM and shall help develop effective viral vector respiratory mucosal tuberculosis vaccination strategies.

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

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

Pulmonary immunity to viruses

Mucosal surfaces, such as the respiratory epithelium, are directly exposed to the external environment and therefore, are highly susceptible to viral infection. As a result, the respiratory tract has evolved a variety of innate and adaptive immune defenses in order to prevent viral infection or promote the rapid destruction of infected cells and facilitate the clearance of the infecting virus. Successful adaptive immune responses often lead to a functional state of immune memory, in which memory lymphocytes and circulating antibodies entirely prevent or lessen the severity of subsequent infections with the same virus. This is also the goal of vaccination, although it is difficult to vaccinate in a way that mimics respiratory infection. Consequently, some vaccines lead to robust systemic immune responses, but relatively poor mucosal immune responses that protect the respiratory tract. In addition, adaptive immunity is not without its drawbacks, as overly robust inflammatory responses may lead to lung damage and impair gas exchange or exacerbate other conditions, such as asthma or chronic obstructive pulmonary disease (COPD). Thus, immune responses to respiratory viral infections must be strong enough to eliminate infection, but also have mechanisms to limit damage and promote tissue repair in order to maintain pulmonary homeostasis. Here, we will discuss the components of the adaptive immune system that defend the host against respiratory viral infections.

Implications of Resident Memory T Cells for Transplantation

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

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

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

Differentiation of distinct long-lived memory CD4 T cells in intestinal tissues after oral Listeria monocytogenes infection

Mucosal antigen-specific CD4 T-cell responses to intestinal pathogens remain incompletely understood. Here we examined the CD4 T-cell response after oral infection with an internalin A 'murinized' Listeria monocytogenes (Lm). Oral Lm infection induced a robust endogenous listeriolysin O (LLO)-specific CD4 T-cell response with distinct phenotypic and functional characteristics in the intestine. Circulating LLO-specific CD4 T cells transiently expressed the 'gut-homing' integrin α₄β₇ and accumulated in the intestinal lamina propria and epithelium where they were maintained independent of interleukin (IL)-15. The majority of intestinal LLO-specific CD4 T cells were CD27^- Ly6C^- and CD69⁺ CD103^- while the lymphoid LLO-specific CD4 T cells were heterogeneous based on CD27 and Ly6C expression and predominately CD69^-. LLO-specific effector CD4 T cells transitioned into a long-lived memory population that phenotypically resembled their parent effectors and displayed hallmarks of residency. In addition, intestinal effector and memory CD4 T cells showed a predominant polyfunctional Th1 profile producing IFNγ, TNFα, and IL-2 at high levels with minimal but detectable levels of IL-17A. Depletion of CD4 T cells in immunized mice led to elevated bacterial burden after challenge infection highlighting a critical role for memory CD4 T cells in controlling intestinal intracellular pathogens.

Tissue resident memory T cells and viral immunity

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

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

Background:

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

Methods:

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

Results:

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

Conclusions:

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

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

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

Characterization of resident lymphocytes in human pancreatic islets

The current view of type 1 diabetes (T1D) is that it is an immune-mediated disease where lymphocytes infiltrate the pancreatic islets, promote killing of beta cells and cause overt diabetes. Although tissue resident immune cells have been demonstrated in several organs, the composition of lymphocytes in human healthy pancreatic islets have been scarcely studied. Here we aimed to investigate the phenotype of immune cells associated with human islets of non-diabetic organ donors. A flow cytometry analysis of isolated islets from perfused pancreases (n = 38) was employed to identify alpha, beta, T, natural killer (NK) and B cells. Moreover, the expression of insulin and glucagon transcripts was evaluated by RNA sequencing. Up to 80% of the lymphocytes were CD3⁺ T cells with a remarkable bias towards CD8⁺ cells. Central memory and effector memory phenotypes dominated within the CD8⁺ and CD4⁺ T cells and most CD8⁺ T cells were positive for CD69 and up to 50-70% for CD103, both markers of resident memory cells. The frequency of B and NK cells was low in most islet preparations (12 and 3% of CD45⁺ cells, respectively), and the frequency of alpha and beta cells varied between donors and correlated clearly with insulin and glucagon mRNA expression. In conclusion, we demonstrated the predominance of canonical tissue resident memory CD8⁺ T cells associated with human islets. We believe that these results are important to understand more clearly the immunobiology of human islets and the disease-related phenotypes observed in diabetes.

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

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

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

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

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

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.

Distinct recirculation potential of CD69+CD103- and CD103+ thymic memory CD8+ T cells

Tissue-resident memory T (T_RM) cells occupy peripheral and lymphoid tissues where they confer protection against local infection. While epithelial CD8⁺ T_RM cells coexpress CD69 and CD103, CD103^- memory cells have been described in various organs and are often presumed non-recirculating based on their expression of CD69. We found that both CD69⁺CD103⁺ and CD69⁺CD103^- memory cells populated the thymus upon transfer of CD8⁺ effector T cells into uninfected recipients. Transcriptionally and phenotypically, CD103⁺ thymic cells resembled non-lymphoid T_RM cells, whereas CD69⁺CD103^- cells displayed a profile that was more closely related to recirculating cells. Although CD69 was required for optimal CD103⁺ T_RM formation, its expression alone did not identify permanently resident cells, as CD69⁺CD103^- cells disappeared from the thymus following antibody-mediated depletion of recirculating cells. Our findings highlight a distinct migration potential of phenotypically divergent thymic CD8⁺ memory T cells and emphasise the inadequacy of CD69 as a marker of tissue residency.

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.