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

Defining Parameters That Modulate Susceptibility and Protection to Respiratory Murine Coronavirus MHV1 Infection

Patients infected with SARS-CoV-2 experience variable disease susceptibility, and patients with comorbidities such as sepsis are often hospitalized for COVID-19 complications. However, the extent to which initial infectious inoculum dose determines disease outcomes and whether this can be used for immunological priming in a genetically susceptible host has not been completely defined. We used an established SARS-like murine model in which responses to primary and/or secondary challenges with murine hepatitis virus type 1 (MHV-1) were analyzed. We compared the response to infection in genetically susceptible C3H/HeJ mice, genetically resistant C57BL/6J mice, and genetically diverse, variably susceptible outbred Swiss Webster mice. Although defined as genetically susceptible to MHV-1, C3H/HeJ mice displayed decreasing dose-dependent pathological changes in disease severity and lung infiltrate/edema, as well as lymphopenia. Importantly, an asymptomatic dose (500 PFU) was identified that yielded no measurable morbidity/mortality postinfection in C3H/HeJ mice. Polymicrobial sepsis induced via cecal ligation and puncture converted asymptomatic infections in C3H/HeJ and C57BL/6J mice to more pronounced disease, modeling the impact of sepsis as a comorbidity to β-coronavirus infection. We then used low-dose infection as an immunological priming event in C3H/HeJ mice, which provided neutralizing Ab-dependent, but not circulating CD4/CD8 T cell-dependent, protection against a high-dose MHV-1 early rechallenge. Together, these data define how infection dose, immunological status, and comorbidities modulate outcomes of primary and secondary β-coronavirus infections in hosts with variable susceptibility.

Tissue-Resident Memory T Cells in Allergy

Tissue-resident memory T (TRM) cells constitute a distinct subset within the memory T cell population, serving as the vanguard against invading pathogens and antigens in peripheral non-lymphoid tissues, including the respiratory tract, intestines, and skin. Notably, TRM cells adapt to the specific microenvironment of each tissue, predominantly maintaining a sessile state with distinctive phenotypic and functional attributes. Their role is to ensure continuous immunological surveillance and protection. Recent findings have highlighted the pivotal contribution of TRM cells to the modulation of adaptive immune responses in allergic disorders such as allergic rhinitis, asthma, and dermatitis. A comprehensive understanding of the involvement of TRM cells in allergic diseases bears profound implications for allergy prevention and treatment. This review comprehensively explores the phenotypic characteristics, developmental mechanisms, and functional roles of TRM cells, focusing on their intricate relationship with allergic diseases.

Antigen perception in T cells by long-term Erk and NFAT signaling dynamics

Immune system threat detection hinges on T cells' ability to perceive varying peptide-major histocompatibility complex (pMHC) antigens. As the Erk and NFAT pathways link T cell receptor engagement to gene regulation, their signaling dynamics may convey information about pMHC inputs. To test this idea, we developed a dual reporter mouse strain and a quantitative imaging assay that, together, enable simultaneous monitoring of Erk and NFAT dynamics in live T cells over day-long timescales as they respond to varying pMHC inputs. Both pathways initially activate uniformly across various pMHC inputs but diverge only over longer (9+ h) timescales, enabling independent encoding of pMHC affinity and dose. These late signaling dynamics are decoded via multiple temporal and combinatorial mechanisms to generate pMHC-specific transcriptional responses. Our findings underscore the importance of long timescale signaling dynamics in antigen perception and establish a framework for understanding T cell responses under diverse contexts.

IFN-γ-dependent interactions between tissue-intrinsic γδ T cells and tissue-infiltrating CD8 T cells limit allergic contact dermatitis

BACKGROUND

Elicitation of allergic contact dermatitis (ACD), an inflammatory type 4 hypersensitivity disease, induces skin infiltration by polyclonal effector CD8 αβ T cells and precursors of tissue-resident memory T (TRM) cells. Because TRM have long-term potential to contribute to body-surface immunoprotection and immunopathology, their local regulation needs a fuller understanding.

OBJECTIVE

We sought to investigate how TRM-cell maturation might be influenced by innate-like T cells pre-existing within many epithelia.

METHODS

This study examined CD8+ TRM-cell maturation following hapten-induced ACD in wild-type mice and in strains harboring altered compartments of dendritic intraepidermal γδ T cells (DETCs), a prototypic tissue-intrinsic, innate-like T-cell compartment that reportedly regulates ACD, but by no elucidated mechanism.

RESULTS

In addition to eliciting CD8 TRM, ACD induced DETC activation and an intimate coregulatory association of the 2 cell types. This depended on DETC sensing IFN-γ produced by CD8 cells and involved programmed death-ligand 1 (PD-L1). Thus, in mice lacking DETC or lacking IFN-γ receptor solely on γδ cells, ACD-elicited CD8 T cells showed enhanced proliferative and effector potentials and reduced motility, collectively associated with exaggerated ACD pathology. Comparable dysregulation was elicited by PD-L1 blockade in vitro, and IFN-γ-regulated PD-L1 expression was a trait of human skin-homing and intraepithelial γδ T cells.

CONCLUSIONS

The size and quality of the tissue-infiltrating CD8 T-cell response during ACD can be profoundly regulated by local innate-like T cells responding to IFN-γ and involving PD-L1. Thus, interindividual and tissue-specific variations in tissue-intrinsic lymphocytes may influence responses to allergens and other challenges and may underpin inflammatory pathologies such as those repeatedly observed in γδ T-cell-deficient settings.

Antigen perception in T cells by long-term Erk and NFAT signaling dynamics

Immune system threat detection hinges on T cells' ability to perceive varying peptide major-histocompatibility complex (pMHC) antigens. As the Erk and NFAT pathways link T cell receptor engagement to gene regulation, their signaling dynamics may convey information about pMHC inputs. To test this idea, we developed a dual reporter mouse strain and a quantitative imaging assay that, together, enable simultaneous monitoring of Erk and NFAT dynamics in live T cells over day-long timescales as they respond to varying pMHC inputs. Both pathways initially activate uniformly across various pMHC inputs, but diverge only over longer (9+ hrs) timescales, enabling independent encoding of pMHC affinity and dose. These late signaling dynamics are decoded via multiple temporal and combinatorial mechanisms to generate pMHC-specific transcriptional responses. Our findings underscore the importance of long timescale signaling dynamics in antigen perception, and establish a framework for understanding T cell responses under diverse contexts.

SIGNIFICANCE STATEMENT

To counter diverse pathogens, T cells mount distinct responses to varying peptide-major histocompatibility complex ligands (pMHCs). They perceive the affinity of pMHCs for the T cell receptor (TCR), which reflects its foreignness, as well as pMHC abundance. By tracking signaling responses in single living cells to different pMHCs, we find that T cells can independently perceive pMHC affinity vs dose, and encode this information through the dynamics of Erk and NFAT signaling pathways downstream of the TCR. These dynamics are jointly decoded by gene regulatory mechanisms to produce pMHC-specific activation responses. Our work reveals how T cells can elicit tailored functional responses to diverse threats and how dysregulation of these responses may lead to immune pathologies.

Tumor resident memory CD8 T cells and concomitant tumor immunity develop independently of CD4 help

Tissue resident memory (Trm) CD8 T cells infiltrating tumors represent an enriched population of tumor antigen-specific T cells, and their presence is associated with improved outcomes in patients. Using genetically engineered mouse pancreatic tumor models we demonstrate that tumor implantation generates a Trm niche that is dependent on direct antigen presentation by cancer cells. However, we observe that initial CCR7-mediated localization of CD8 T cells to tumor draining lymph nodes is required to subsequently generate CD103+ CD8 T cells in tumors. We observe that the formation of CD103+ CD8 T cells in tumors is dependent on CD40L but independent of CD4 T cells, and using mixed chimeras we show that CD8 T cells can provide their own CD40L to permit CD103+ CD8 T cell differentiation. Finally, we show that CD40L is required to provide systemic protection against secondary tumors. These data suggest that CD103+ CD8 T cell formation in tumors can occur independent of the two-factor authentication provided by CD4 T cells and highlight CD103+ CD8 T cells as a distinct differentiation decision from CD4-dependent central memory.

Human T lymphocytes at tumor sites

CD4⁺ and CD8⁺ T lymphocytes mediate most of the adaptive immune response against tumors. Naïve T lymphocytes specific for tumor antigens are primed in lymph nodes by dendritic cells. Upon activation, antigen-specific T cells proliferate and differentiate into effector cells that migrate out of peripheral blood into tumor sites in an attempt to eliminate cancer cells. After accomplishing their function, most effector T cells die in the tissue, while a small fraction of antigen-specific T cells persist as long-lived memory cells, circulating between peripheral blood and lymphoid tissues, to generate enhanced immune responses when re-encountering the same antigen. A subset of memory T cells, called resident memory T (T_RM) cells, stably resides in non-lymphoid peripheral tissues and may provide rapid immunity independently of T cells recruited from blood. Being adapted to the tissue microenvironment, T_RM cells are potentially endowed with the best features to protect against the reemergence of cancer cells. However, when tumors give clinical manifestation, it means that tumor cells have evaded immune surveillance, including that of T_RM cells. Here, we review the current knowledge as to how T_RM cells are generated during an immune response and then maintained in non-lymphoid tissues. We then focus on what is known about the role of CD4⁺ and CD8⁺ T_RM cells in antitumor immunity and their possible contribution to the efficacy of immunotherapy. Finally, we highlight some open questions in the field and discuss how new technologies may help in addressing them.

Influenza-Induced CD103+ T Resident Memory Cells Exhibit Enhanced Functional Avidity over CD103- Memory T Cells in the Mediastinal Lymph Node

Influenza virus–specific tissue-resident memory CD8 T cells (Trms) targeting conserved viral proteins provide strain-transcending heterosubtypic immunity to infection. Trms in the lung combat reinfection through rapid cytolytic function and production of inflammatory cytokines to recruit other immune cells. Influenza-specific Trms are also generated in the lung draining mediastinal lymph node (mLN) and can provide immunity to heterologous virus infection in this tissue, although their role in combating influenza infection is less well defined. Functional avidity, a measure of T cell sensitivity to Ag stimulation, correlates with control of viral infection and may be important for immune detection of recently infected cells, when low numbers of surface peptide–MHC complexes are displayed. However, the functional avidity of influenza-specific Trms has not been previously compared with that of other memory CD8 T cell subsets. In this article, a methodology is presented to compare the functional avidity of CD8 T cell subsets across murine tissues, with a focus on influenza-specific mLNs compared with splenic CD8 T cells, by stimulating both populations in the same well to account for CD8 T cell–extrinsic variables. The functional avidity of influenza-specific mLN effector CD8 T cells is slightly increased relative to splenic effector CD8 T cells. However, CD103⁺ mLN Trms display increased functional avidity compared with splenic memory CD8 T cells and CD103⁻ memory CD8 T cells within the mLN. In contrast, lung-derived CD103⁺ Trms did not exhibit enhanced functional avidity. mLN CD103⁺ Trms also exhibit increased TCR expression, providing a potential mechanism for their enhanced functional avidity.

The evolving role of tissue-resident memory T cells in infections and cancer

Resident memory T cells (TRM) form a distinct type of T memory cells that stably resides in tissues. TRM form an integral part of the immune sensing network and have the ability to control local immune homeostasis and participate in immune responses mediated by pathogens, cancer, and possibly autoantigens during autoimmunity. TRM express residence gene signatures, functional properties of both memory and effector cells, and remarkable plasticity. TRM have a well-established role in pathogen immunity, whereas their role in antitumor immune responses and immunotherapy is currently evolving. As TRM form the most abundant T memory cell population in nonlymphoid tissues, they are attractive targets for therapeutic exploitation. Here, we provide a concise review of the development and physiological role of CD8+ TRM, their involvement in diseases, and their potential therapeutic exploitation.

Divide and Conquer: Phenotypic and Temporal Heterogeneity Within CD8+ T Cell Responses

The advent of technologies that can characterize the phenotypes, functions and fates of individual cells has revealed extensive and often unexpected levels of diversity between cells that are nominally of the same subset. CD8+ T cells, also known as cytotoxic T lymphocytes (CTLs), are no exception. Investigations of individual CD8+ T cells both in vitro and in vivo have highlighted the heterogeneity of cellular responses at the levels of activation, differentiation and function. This review takes a broad perspective on the topic of heterogeneity, outlining different forms of variation that arise during a CD8+ T cell response. Specific attention is paid to the impact of T cell receptor (TCR) stimulation strength on heterogeneity. In particular, this review endeavors to highlight connections between variation at different cellular stages, presenting known mechanisms and key open questions about how variation between cells can arise and propagate.

The perception and response of T cells to a changing environment are based on the law of initial value

αβ T cells are critical components of the adaptive immune system and are capable of inducing sterilizing immunity after pathogen infection and eliminating transformed tumor cells. The development and function of T cells are controlled through the T cell antigen receptor, which recognizes peptides displayed on major histocompatibility complex (MHC) molecules. Here, we review how T cells generate the ability to recognize self-peptide-bound MHC molecules and use signals derived from these interactions to instruct cellular development, activation thresholds, and functional specialization in the steady state and during immune responses. We argue that the basic tenants of T cell development and function follow Weber-Fetcher's law of just noticeable differences and Wilder's law of initial value. Together, these laws argue that the ability of a system to respond and the quality of that response are scalable to the basal state of that system. Manifestation of these laws in T cells generates clone-specific activation thresholds that are based on perceivable differences between homeostasis and pathogen encounter (self versus nonself discrimination), as well as poised states for subsequent differentiation into specific effector cell lineages.

A dormant T cell population with autoimmune potential exhibits low self-reactivity and infiltrates islets in type 1 diabetes

The contribution of low affinity T cells to autoimmunity in the context of polyclonal T cell responses is understudied due to the limitations in their capture by tetrameric reagents and low level of activation in response to antigenic stimulation. As a result, low affinity T cells are often disregarded as non-antigen specific cells irrelevant to the immune response. Our study aimed to assess how the level of self-antigen reactivity shapes T cell lineage and effector responses in the context of spontaneous tissue specific autoimmunity observed in NOD mice. Using multi-color flow cytometry in combination with Nur77^GFP reporter of TCR signaling we identified a dormant population of T cells that infiltrated the pancreatic islets of pre-diabetic NOD mice, which exhibited reduced level of self-tissue reactivity based on expression of CD5 and Nur77^GFP . We showed that these CD5^low T cells had a unique TCR repertoire, exhibited low activation and minimal effector function; however, induced rapid diabetes upon transfer. The CD4⁺ CD5^low T cell population displayed transcriptional signature of central memory T cells, consistent with the ability to acquire effector function post-transfer. Transcriptional profile of CD5^low T cells was similar to T cells expressing a low affinity TCR, indicating TCR affinity to be the important factor in shaping CD5^low T cell phenotype and function at the tissue site. Overall, our study suggests that autoimmune tissue can maintain a reservoir of undifferentiated central memory-like autoreactive T cells with pathogenic effector potential that might be an important source for effector T cells during long-term chronic autoimmunity. This article is protected by copyright. All rights reserved.

Tissue-resident memory T cells in the urogenital tract

Our understanding of T cell memory responses changed drastically with the discovery that specialized T cell memory populations reside within peripheral tissues at key pathogen entry sites. These tissue-resident memory T (T_RM) cells can respond promptly to an infection without the need for migration, proliferation or differentiation. This rapid and local deployment of effector functions maximizes the ability of T_RM cells to eliminate pathogens. T_RM cells do not circulate through peripheral tissues but instead form isolated populations in the skin, gut, liver, kidneys, the reproductive tract and other organs. This long-term retention in the periphery might allow T_RM cells to fully adapt to the local conditions of their environment and mount customized responses to counter infection and tumour growth in a tissue-specific manner. In the urogenital tract, T_RM cells must adapt to a unique microenvironment to confer protection against potential threats, including cancer and infection, while preventing the onset of auto-inflammatory disease. In this Review, we discuss insights into the diversification of T_RM cells from other memory T cell lineages, the adaptations of T_RM cells to their local environment, and their enhanced capacity to counter infection and tumour growth compared with other memory T cell populations, especially in the urogenital tract.

Tissue resident memory T cells in the respiratory tract

Owing to their capacity to rapidly spread across the population, airborne pathogens represent a significant risk to global health. Indeed, several of the past major global pandemics have been instigated by respiratory pathogens. A greater understanding of the immune cells tasked with protecting the airways from infection will allow for the development of strategies that curb the spread and impact of these airborne diseases. A specific subset of memory T-cell resident in both the upper and lower respiratory tract, termed tissue-resident memory (Trm), have been shown to play an instrumental role in local immune responses against a wide breadth of both viral and bacterial infections. In this review, we discuss factors that influence respiratory tract Trm development, longevity, and immune surveillance and explore vaccination regimes that harness these cells, such approaches represent exciting new strategies that may be utilized to tackle the next global pandemic.

Staggered starts in the race to T cell activation

How T lymphocytes tune their responses to different strengths of stimulation is a fundamental question in immunology. Recent work using new optogenetic, single-cell genomic, and live-imaging approaches has revealed that stimulation strength controls the rate of individual cell responses within a population. Moreover, these responses have been found to use shared molecular programs, regardless of stimulation strength. However, additional data indicate that stimulation duration or cytokine feedback can impact later gene expression phenotypes of activated cells. In-depth molecular studies have suggested mechanisms by which stimulation strength might modulate the probability of T cell activation. This emerging model allows activating T cells to achieve a wide range of population responses through probabilistic control within individual cells.

Decoding Tissue-Residency: Programming and Potential of Frontline Memory T Cells

Memory T-cell responses are partitioned between the blood, secondary lymphoid organs, and nonlymphoid tissues. Tissue-resident memory T (Trm) cells are a population of immune cells that remain permanently in tissues without recirculating in blood. These nonrecirculating cells serve as a principal node in the anamnestic response to invading pathogens and developing malignancies. Here, we contemplate how T-cell tissue residency is defined and shapes protective immunity in the steady state and in the context of disease. We review the properties and heterogeneity of Trm cells, highlight the critical roles these cells play in maintaining tissue homeostasis and eliciting immune pathology, and explore how they might be exploited to treat disease.

Formation of Tissue-Resident CD8+ T-Cell Memory

Resident memory CD8⁺ T (Trm) cells permanently reside in nonlymphoid tissues where they act as a first line of defense against recurrent pathogens. How and when antigen-inexperienced CD8⁺ T cells differentiate into Trm has been a topic of major interest, as knowledge on how to steer this process may be exploited in the development of vaccines and anticancer therapies. Here, we first review the current understanding of the early signals that CD8⁺ T cells receive before they have entered the tissue and that govern their capacity to develop into tissue-resident memory T cells. Subsequently, we discuss the tissue-derived factors that promote Trm maturation in situ. Combined, these data sketch a model in which a subset of responding T cells develops a heightened capacity to respond to local cues present in the tissue microenvironment, which thereby imprints their ability to contribute to the tissue-resident memory CD8⁺ T-cell pool that provide local control against pathogens.

Tissue-Resident T Cells in Chronic Relapsing-Remitting Intestinal Disorders

Tissue-resident memory T (TRM) cells critically contribute to the rapid immunoprotection and efficient immunosurveillance against pathogens, particularly in barrier tissues, but also during anti-tumor responses. However, the involvement of TRM cells also in the induction and exacerbation of immunopathologies, notably in chronically relapsing auto-inflammatory disorders, is becoming increasingly recognized as a critical factor. Thus, TRM cells may also represent an attractive target in the management of chronic (auto-) inflammatory disorders, including multiple sclerosis, rheumatoid arthritis, celiac disease and inflammatory bowel diseases. In this review, we focus on current concepts of TRM cell biology, particularly in the intestine, and discuss recent findings on their involvement in chronic relapsing-remitting inflammatory disorders. Potential therapeutic strategies to interfere with these TRM cell-mediated immunopathologies are discussed.

Interplay of Inflammatory, Antigen and Tissue-Derived Signals in the Development of Resident CD8 Memory T Cells

CD8 positive, tissue resident memory T cells (TRM) are a specialized subset of CD8 memory T cells that surveil tissues and provide critical first-line protection against tumors and pathogen re-infection. Recently, much effort has been dedicated to understanding the function, phenotype and development of TRM. A myriad of signals is involved in the development and maintenance of resident memory T cells in tissue. Much of the initial research focused on the roles tissue-derived signals play in the development of TRM, including TGFß and IL-33 which are critical for the upregulation of CD69 and CD103. However, more recent data suggest further roles for antigenic and pro-inflammatory cytokines. This review will focus on the interplay of pro-inflammatory, tissue and antigenic signals in the establishment of resident memory T cells.

Age-Related Dynamics of Lung-Resident Memory CD8+ T Cells in the Age of COVID-19

Following respiratory viral infections or local immunizations, lung resident-memory T cells (T_RM) of the CD8 lineage provide protection against the same pathogen or related pathogens with cross-reactive T cell epitopes. Yet, it is now clear that, if homeostatic controls are lost following viral pneumonia, CD8 T_RM cells can mediate pulmonary pathology. We recently showed that the aging process can result in loss of homeostatic controls on CD8 T_RM cells in the respiratory tract. This may be germane to treatment modalities in both influenza and coronavirus disease 2019 (COVID-19) patients, particularly, the portion that present with symptoms linked to long-lasting lung dysfunction. Here, we review the developmental cues and functionalities of CD8 T_RM cells in viral pneumonia models with a particular focus on their capacity to mediate heterogeneous responses of immunity and pathology depending on immune status.

Discipline in Stages: Regulating CD8+ Resident Memory T Cells

Resident memory CD8⁺ T (T_RM) cells are a lymphocyte lineage distinct from circulating memory CD8⁺ T cells. T_RM lodge within peripheral tissues and secondary lymphoid organs where they provide rapid, local protection from pathogens and control tumor growth. However, dysregulation of CD8⁺ T_RM formation and/or activation may contribute to the pathogenesis of autoimmune diseases. Intrinsic mechanisms, including transcriptional networks and inhibitory checkpoint receptors control T_RM differentiation and response. Additionally, extrinsic stimuli such as cytokines, cognate antigen, fatty acids, and damage signals regulate T_RM formation, maintenance, and expansion. In this review, we will summarize knowledge of CD8⁺ T_RM generation and highlight mechanisms that regulate the persistence and responses of heterogeneous T_RM populations in different tissues and distinct microenvironments.

A committed tissue-resident memory T cell precursor within the circulating CD8+ effector T cell pool

An increasing body of evidence emphasizes the role of tissue-resident memory T cells (T_RM) in the defense against recurring pathogens and malignant neoplasms. However, little is known with regard to the origin of these cells and their kinship to other CD8⁺ T cell compartments. To address this issue, we followed the antigen-specific progeny of individual naive CD8⁺ T cells to the T effector (T_EFF), T circulating memory (T_CIRCM), and T_RM pools by lineage-tracing and single-cell transcriptome analysis. We demonstrate that a subset of T cell clones possesses a heightened capacity to form T_RM, and that enriched expression of T_RM–fate-associated genes is already apparent in the circulating T_EFF offspring of such clones. In addition, we demonstrate that the capacity to generate T_RM is permanently imprinted at the clonal level, before skin entry. Collectively, these data provide compelling evidence for early stage T_RM fate decisions and the existence of committed T_RM precursor cells in the circulatory T_EFF compartment.

Influenza-Specific Lung-Resident Memory CD8+ T Cells

Despite the availability of seasonal vaccines, influenza A (IAV) prevails as a leading cause of respiratory infection worldwide. Current vaccination efforts aim at increasing protection against heterologous and potentially pandemic IAV strains. Lung-resident CD8⁺ T cells (Trm) generated upon IAV infection are vital for heterosubtypic immunity to IAV reexposure and provide quick and robust responses upon reactivation. Yet, protection wanes with time as lung Trm cell numbers decline, a contrasting feature with Trm cells at other mucosal sites such as the skin. In this review, we discuss current data on lung Trm compared to Trm cells in other tissues. Furthermore, major knowledge gaps in the generation and maintenance of IAV-specific lung Trm are addressed and mechanisms that may contribute to their decline are discussed. Further understanding in the mechanisms that govern effector function versus immunopathology is paramount for future IAV vaccine design in enhancing durability of lung Trm cells.

Balancing Inflammation and Central Nervous System Homeostasis: T Cell Receptor Signaling in Antiviral Brain TRM Formation and Function

Tissue-resident memory (T_RM) CD8 T cells provide early frontline defense against regional pathogen reencounter. CD8 T_RM are predominantly parked in nonlymphoid tissues and do not circulate. In addition to this anatomic difference, T_RM are transcriptionally and phenotypically distinct from central-memory T cells (T_CM) and effector-memory T cells (T_EM). Moreover, T_RM differ phenotypically, functionally, and transcriptionally across barrier tissues (e.g., gastrointestinal tract, respiratory tract, urogenital tract, and skin) and in non-barrier organs (e.g., brain, liver, kidney). In the brain, T_RM are governed by a contextual milieu that balances T_RM activation and preservation of essential post-mitotic neurons. Factors contributing to the development and maintenance of brain T_RM, of which T cell receptor (TCR) signal strength and duration is a central determinant, vary depending on the infectious agent and modulation of TCR signaling by inhibitory markers that quell potentially pathogenic inflammation. This review will explore our current understanding of the context-dependent factors that drive the acquisition of brain (b)T_RM phenotype and function, and discuss the contribution of T_RM to promoting protective immune responses in situ while maintaining tissue homeostasis.

Area under Immunosurveillance: Dedicated Roles of Memory CD8 T-Cell Subsets

Immunological memory, defined as the ability to respond in an enhanced manner upon secondary encounter with the same pathogen, can provide substantial protection against infectious disease. The improved protection is mediated in part by different populations of memory CD8 T cells that are retained after primary infection. Memory cells persist in the absence of pathogen-derived antigens and enable secondary CD8 T-cell responses with accelerated kinetics and of larger magnitude after reencounter with the same pathogen. At least three subsets of memory T cells have been defined that are referred to as central memory CD8 T cells (Tcm), effector memory CD8 T cells (Tem), and tissue-resident memory CD8 T cells (Trm). Tcm and Tem are circulating memory T cells that mediate bodywide immune surveillance in search of invading pathogens. In contrast, Trm permanently reside in peripheral barrier tissues, where they form a stationary defensive line of sentinels that alert the immune system upon pathogen reencounter. The characterization of these different subsets has been instrumental in our understanding of the strategies that memory T cells employ to counter invading pathogens. It is clear that memory T cells not only have a numerical advantage over naive T cells resulting in improved protection in secondary responses, but also acquire distinct sets of competencies that assist in pathogen clearance. Nevertheless, inherent challenges are associated with the allocation of memory T cells to a limited number of subsets. The classification of memory T cells into Tcm, Tem, and Trm may not take into account the full extent of the heterogeneity that is observed in the memory population. Therefore, in this review, we will revisit the current classification of memory subsets, elaborate on functional and migratory properties attributed to Tcm, Tem, and Trm, and discuss how potential heterogeneity within these populations arises and persists.

Circulating memory CD8+ T cells are limited in forming CD103+ tissue-resident memory T cells at mucosal sites after reinfection

Tissue-resident memory CD8⁺ T cells (T_RM ) localize to barrier tissues and mediate local protection against reinvading pathogens. Circulating central memory (T_CM ) and effector memory CD8⁺ T cells (T_EM ) also contribute to tissue recall responses, but their potential to form mucosal T_RM remains unclear. Here, we employed adoptive transfer and lymphocytic choriomeningitis virus reinfection models to specifically assess secondary responses of T_CM and T_EM at mucosal sites. Donor T_CM and T_EM exhibited robust systemic recall responses, but only limited accumulation in the small intestine, consistent with reduced expression of tissue-homing and -retention molecules. Murine and human circulating memory T cells also exhibited limited CD103 upregulation following TGF-β stimulation. Upon pathogen clearance, T_CM and T_EM readily gave rise to secondary T_EM . T_CM also formed secondary central memory in lymphoid tissues and T_RM in internal tissues, for example, the liver. Both T_CM and T_EM failed to substantially contribute to resident mucosal memory in the small intestine, while activated intestinal T_RM , but not liver T_RM , efficiently reformed CD103⁺ T_RM . Our findings demonstrate that circulating T_CM and T_EM are limited in generating mucosal T_RM upon reinfection. This may pose important implications on cell therapy and vaccination strategies employing memory CD8⁺ T cells for protection at mucosal sites.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

Long-term surviving influenza infected cells evade CD8+ T cell mediated clearance

Influenza A virus (IAV) is a seasonal pathogen with the potential to cause devastating pandemics. IAV infects multiple epithelial cell subsets in the respiratory tract, eliciting damage to the lungs. Clearance of IAV is primarily dependent on CD8+ T cells, which must balance control of the infection with immunopathology. Using a virus expressing Cre recombinase to permanently label infected cells in a Cre-inducible reporter mouse, we previously discovered infected club cells that survive both lytic virus replication and CD8+ T cell-mediated clearance. In this study, we demonstrate that ciliated epithelial cells, type I and type II alveolar cells can also become survivor cells. Survivor cells are stable in the lung long-term and demonstrate enhanced proliferation compared to uninfected cells. When we investigated how survivor cells evade CD8+ T cell killing we observed that survivor cells upregulated the inhibitory ligand PD-L1, but survivor cells did not use PD-L1 to evade CD8+ T cell killing. Instead our data suggest that survivor cells are not inherently resistant to CD8+ T cell killing, but instead no longer present IAV antigen and cannot be detected by CD8+ T cells. Finally, we evaluate the failure of CD8+ T cells to kill these previously infected cells. This work demonstrates that additional cell types can survive IAV infection and that these cells robustly proliferate and are stable long term. By sparing previously infected cells, the adaptive immune system may be minimizing pathology associated with IAV infection.