Up-regulation of LFA-1 allows liver-resident memory T cells to patrol and remain in the hepatic sinusoids

Resident-Memory T Cells in Tissue-Restricted Immune Responses: For Better or Worse?

Tissue-resident-memory CD8+ T cells (T_RM) have been described as a non-circulating memory T cell subset that persists at sites of previous infection. While T_RM in all non-lymphoid organs probably share a core signature differentiation pathway, certain aspects of their maintenance and effector functions may vary. It is well-established that T_RM provide long-lived protective immunity through immediate effector function and accelerated recruitment of circulating immune cells. Besides immune defense against pathogens, other immunological roles of T_RM are less well-studied. Likewise, evidence of a putative detrimental role of T_RM for inflammatory diseases is only beginning to emerge. In this review, we discuss the protective and harmful role of T_RM in organ-specific immunity and immunopathology as well as prospective implications for immunomodulatory therapy.

Tissue Resident CD8 Memory T Cell Responses in Cancer and Autoimmunity

Resident memory (T_RM) cells are a distinct tissue-localized T cell lineage that is crucial for protective immunity in peripheral tissues. While a great deal of effort has focused on defining their role in immunity to infections, studies now reveal T_RM cells as a vital component of the host immune response to cancer. Characterized by cell-surface molecules including CD103, CD69, and CD49a, T_RM-like tumor-infiltrating lymphocytes (TILs) can be found in a wide range of human cancers, where they portend improved prognosis. Recent studies in mouse tumor models have shown that T_RM cells are induced by cancer vaccines delivered in peripheral tissue sites, or by the depletion of regulatory T cells. Such tumor-specific T_RM cells are recognized as both necessary and sufficient for long-lived protection against tumors in peripheral tissue locations. T_RM responses against tumor/self-antigens can concurrently result in the development of pathogenic T_RM responses to self, with a growing number of autoimmune diseases and inflammatory pathologies being attributed to T_RM responses. This review will recount the path to discovering the importance of resident memory CD8 T cells as they pertain to cancer immunity. In addition to highlighting key studies that directly implicate T_RM cells in anti-tumor immunity, we will highlight earlier work that implicitly suggested their importance. Informed by studies in infectious disease models, and instructed by a clear role for T_RM cells in autoimmunity, we will discuss strategies for therapeutically promoting T_RM responses in settings where they don't naturally occur.

Malaria prevention: from immunological concepts to effective vaccines and protective antibodies

Development of a malaria vaccine remains a critical priority to decrease clinical disease and mortality and facilitate eradication. Accordingly, RTS,S, a protein-subunit vaccine, has completed phase III clinical trials and confers ~30% protection against clinical infection over 4 years. Whole-attenuated-sporozoite and viral-subunit vaccines induce between 20% and 100% protection against controlled human malaria infection, but there is limited published evidence to date for durable, high-level efficacy (>50%) against natural exposure. Importantly, fundamental scientific advances related to the potency, durability, breadth and location of immune responses will be required for improving vaccine efficacy with these and other vaccine approaches. In this Review, we focus on the current understanding of immunological mechanisms of protection from animal models and human vaccine studies, and on how these data should inform the development of next-generation vaccines. Furthermore, we introduce the concept of using passive immunization with monoclonal antibodies as a new approach to prevent and eliminate malaria.

Prime and target immunization protects against liver-stage malaria in mice

Despite recent advances in treatment and vector control, malaria is still a leading cause of death, emphasizing the need for an effective vaccine. The malaria life cycle can be subdivided into three stages: the invasion and growth within liver hepatocytes (pre-erythrocytic stage), the blood stage (erythrocytic stage), and, finally, the sexual stage (occurring within the mosquito vector). Antigen (Ag)-specific CD8+ T cells are effectively induced by heterologous prime-boost viral vector immunization and known to correlate with liver-stage protection. However, liver-stage malaria vaccines have struggled to generate and maintain the high numbers of Plasmodium-specific circulating T cells necessary to confer sterile protection. We describe an alternative “prime and target” vaccination strategy aimed specifically at inducing high numbers of tissue-resident memory T cells present in the liver at the time of hepatic infection. This approach bypasses the need for very high numbers of circulating T cells and markedly increases the efficacy of subunit immunization against liver-stage malaria with clinically relevant Ags and clinically tested viral vectors in murine challenge models. Translation to clinical use has begun, with encouraging results from a pilot safety and feasibility trial of intravenous chimpanzee adenovirus vaccination in humans. This work highlights the value of a prime-target approach for immunization against malaria and suggests that this strategy may represent a more general approach for prophylaxis or immunotherapy of other liver infections and diseases.

ICAM-1-LFA-1 Dependent CD8+ T-Lymphocyte Aggregation in Tumor Tissue Prevents Recirculation to Draining Lymph Nodes

The quantity of T-lymphocytes reaching the draining lymph nodes from tumors is likely important to mount effective distant responses and for the establishment of long term systemic memory. Looking into mechanisms behind lymphocyte egress, we directed our attention to leukocyte adhesion mechanisms inside tumors. Here we demonstrate that activated T-cells form intra-tumor aggregates in a LFA-1-ICAM-1-dependent fashion in mouse models of melanoma and breast cancer. We also provide evidence of the presence of T-cell clusters in primary human melanoma. Disruption of LFA-1-ICAM-1 interactions, and thereby T-cell clustering, enhances the arrival of activated CD8+ T-cells to tumor draining lymph nodes in both transplanted and spontaneous cancer models. Interestingly, upon ICAM-1 blockade, the expression of the chemotactic receptor CCR7 augments in tumor infiltrating lymphocytes and in in-vitro de-clustered T cells, as well as their ability to transmigrate across lymphatic endothelial cells. We propose that ICAM-1-mediated homotypic T-lymphocyte aggregation may serve as a tumor-mediated immune retention mechanism entrapping activated CD8+ T cells in the tumor microenvironment. Modulation of T-cell adhesion may be of use to improve the transit of activated lymphocytes toward the lymph nodes and their subsequent recirculation.

Armed and Ready: Transcriptional Regulation of Tissue-Resident Memory CD8 T Cells

A fundamental benefit of immunological memory is the ability to respond in an enhanced manner upon secondary encounter with the same pathogen. Tissue-resident memory CD8 T (T_RM) cells contribute to improved protection against reinfection through the generation of immediate effector responses at the site of pathogen entry. Key to the potential of T_RM cells to develop rapid recall responses is their location within the epithelia of the skin, lungs, and intestines at prime entry sites of pathogens. T_RM cells are among the first immune cells to respond to pathogens that have been previously encountered in an antigen-specific manner. Upon recognition of invading pathogens, T_RM cells release IFN-γ and other pro-inflammatory cytokines and chemokines. These effector molecules activate the surrounding epithelial tissue and recruit other immune cells including natural killer (NK) cells, B cells, and circulating memory CD8 T cells to the site of infection. The repertoire of T_RM effector functions also includes the direct lysis of infected cells through the release of cytotoxic molecules such as perforin and granzymes. The mechanisms enabling T_RM cells to respond in such a rapid manner are gradually being uncovered. In this review, we will address the signals that instruct T_RM generation and maintenance as well as the underlying transcriptional network that keeps T_RM cells in a deployment-ready modus. Furthermore, we will discuss how T_RM cells respond to reinfection of the tissue and how transcription factors may control immediate and proliferative T_RM responses.

A Systematic Review: The Role of Resident Memory T Cells in Infectious Diseases and Their Relevance for Vaccine Development

Background

Resident memory T cells have emerged as key players in the immune response generated against a number of pathogens. Their ability to take residence in non-lymphoid peripheral tissues allows for the rapid deployment of secondary effector responses at the site of pathogen entry. This ability to provide enhanced regional immunity has gathered much attention, with the generation of resident memory T cells being the goal of many novel vaccines.

Objectives

This review aimed to systematically analyze published literature investigating the role of resident memory T cells in human infectious diseases. Known effector responses mounted by these cells are summarized and key strategies that are potentially influential in the rational design of resident memory T cell inducing vaccines have also been highlighted.

Methods

A Boolean search was applied to Medline, SCOPUS, and Web of Science. Studies that investigated the effector response generated by resident memory T cells and/or evaluated strategies for inducing these cells were included irrespective of published date. Studies must have utilized an established technique for identifying resident memory T cells such as T cell phenotyping.

Results

While over 600 publications were revealed by the search, 147 articles were eligible for inclusion. The reference lists of included articles were also screened for other eligible publications. This resulted in the inclusion of publications that studied resident memory T cells in the context of over 25 human pathogens. The vast majority of studies were conducted in mouse models and demonstrated that resident memory T cells mount protective immune responses.

Conclusion

Although the role resident memory T cells play in providing immunity varies depending on the pathogen and anatomical location they resided in, the evidence overall suggests that these cells are vital for the timely and optimal protection against a number of infectious diseases. The induction of resident memory T cells should be further investigated and seriously considered when designing new vaccines.

Niches for the Long-Term Maintenance of Tissue-Resident Memory T Cells

Tissue-resident memory T cells (T_RM cells) are a population of immune cells that reside in the lymphoid and non-lymphoid organs without recirculation through the blood. These important cells occupy and utilize unique anatomical and physiological niches that are distinct from those for other memory T cell populations, such as central memory T cells in the secondary lymphoid organs and effector memory T cells that circulate through the tissues. CD8⁺ T_RM cells typically localize in the epithelial layers of barrier tissues where they are optimally positioned to act as sentinels to trigger antigen-specific protection against reinfection. CD4⁺ T_RM cells typically localize below the epithelial layers, such as below the basement membrane, and cluster in lymphoid structures designed to optimize interactions with antigen-presenting cells upon reinfection. A key feature of T_RM populations is their ability to be maintained in barrier tissues for prolonged periods of time. For example, skin CD8⁺ T_RM cells displace epidermal niches originally occupied by γδ T cells, thereby enabling their stable persistence for years. It is also clear that the long-term maintenance of T_RM cells in different microenvironments is dependent on multiple tissue-specific survival cues, although the specific details are poorly understood. However, not all T_RM persist over the long term. Recently, we identified a new spatial niche for the maintenance of CD8⁺ T_RM cells in the lung, which is created at the site of tissue regeneration after injury [termed repair-associated memory depots (RAMD)]. The short-lived nature of RAMD potentially explains the short lifespans of CD8⁺ T_RM cells in this particular tissue. Clearly, a better understanding of the niche-dependent maintenance of T_RM cells will be important for the development of vaccines designed to promote barrier immunity. In this review, we discuss recent advances in our understanding of the properties and nature of tissue-specific niches that maintain T_RM cells in different tissues.

Tissue-resident memory T cells at the center of immunity to solid tumors

Immune responses in tissues are constrained by the physiological properties of the tissue involved. Tissue-resident memory T cells (T_RM cells) are a recently discovered lineage of T cells specialized for life and function within tissues. Emerging evidence has shown that T_RM cells have a special role in the control of solid tumors. A high frequency of T_RM cells in tumors correlates with favorable disease progression in patients with cancer, and studies of mice have shown that T_RM cells are necessary for optimal immunological control of solid tumors. Here we describe what defines T_RM cells as a separate lineage and how these cells are generated. Furthermore, we discuss the properties that allow T_RM cells to operate in normal and transformed tissues, as well as implications for the treatment of patients with cancer.

LFA-1 in T Cell Migration and Differentiation

Maintenance of homeostatic immune surveillance and development of effective adaptive immune responses require precise regulation of spatial and temporal lymphocyte trafficking throughout the body to ensure pathogen clearance and memory generation. Dysregulation of lymphocyte activation and migration can lead to impaired adaptive immunity, recurrent infections, and an array of autoimmune diseases and chronic inflammation. Central to the recruitment of T cells, integrins are cell surface receptors that regulate adhesion, signal transduction, and migration. With 24 integrin pairs having been discovered to date, integrins are defined not only by the composition of the heterodimeric pair but by cell-type specific expression and their ligands. Furthermore, integrins not only facilitate adhesion but also induce intracellular signaling and have recently been uncovered as mechanosensors providing additional complexity to the signaling pathways. Among several leukocyte-specific integrins, lymphocyte function-associated antigen-1 (LFA-1 or α_Lβ₂; CD11a/CD18) is a key T cell integrin, which plays a major role in regulating T cell activation and migration. Adhesion to LFA-1's ligand, intracellular adhesion receptor 1 (ICAM-1) facilitates firm endothelium adhesion, prolonged contact with antigen-presenting cells, and binding to target cells for killing. While the downstream signaling pathways utilized by LFA-1 are vastly conserved they allow for highly disparate responses. Here, we summarize the roles of LFA-1 and ongoing studies to better understand its functions and regulation.

Humanized Mouse Models for the Study of Human Malaria Parasite Biology, Pathogenesis, and Immunity

Malaria parasite infection continues to inflict extensive morbidity and mortality in resource-poor countries. The insufficiently understood parasite biology, continuously evolving drug resistance and the lack of an effective vaccine necessitate intensive research on human malaria parasites that can inform the development of new intervention tools. Humanized mouse models have been greatly improved over the last decade and enable the direct study of human malaria parasites in vivo in the laboratory. Nevertheless, no small animal model developed so far is capable of maintaining the complete life cycle of Plasmodium parasites that infect humans. The ultimate goal is to develop humanized mouse systems in which a Plasmodium infection closely reproduces all stages of a parasite infection in humans, including pre-erythrocytic infection, blood stage infection and its associated pathology, transmission as well as the human immune response to infection. Here, we discuss current humanized mouse models and the future directions that should be taken to develop next-generation models for human malaria parasite research.

Induction and Maintenance of CX3CR1-Intermediate Peripheral Memory CD8+ T Cells by Persistent Viruses and Vaccines

The induction and maintenance of T cell memory is critical to the success of vaccines. A recently described subset of memory CD8+ T cells defined by intermediate expression of the chemokine receptor CX3CR1 was shown to have self-renewal, proliferative, and tissue-surveillance properties relevant to vaccine-induced memory. We tracked these cells when memory is sustained at high levels: memory inflation induced by cytomegalovirus (CMV) and adenovirus-vectored vaccines. In mice, both CMV and vaccine-induced inflationary T cells showed sustained high levels of CX3R1int cells exhibiting an effector-memory phenotype, characteristic of inflationary pools, in early memory. In humans, CX3CR1int CD8+ T cells were strongly induced following adenovirus-vectored vaccination for hepatitis C virus (HCV) (ChAd3-NSmut) and during natural CMV infection and were associated with a memory phenotype similar to that in mice. These data indicate that CX3CR1int cells form an important component of the memory pool in response to persistent viruses and vaccines in both mice and humans.

Liver sampling: a vital window into HBV pathogenesis on the path to functional cure

In order to optimally refine the multiple emerging drug targets for hepatitis B virus (HBV), it is vital to evaluate virological and immunological changes at the site of infection. Traditionally liver biopsy has been the mainstay of HBV disease assessment, but with the emergence of non-invasive markers of liver fibrosis, there has been a move away from tissue sampling. Here we argue that liver biopsy remains an important tool, not only for the clinical assessment of HBV but also for research progress and evaluation of novel agents. The importance of liver sampling has been underscored by recent findings of specialised subsets of tissue-resident immune subsets capable of efficient pathogen surveillance, compartmentalised in the liver and not sampled in the blood. Importantly, the assessment of virological parameters, such as cccDNA quantitation, also requires access to liver tissue. We discuss strategies to maximise information obtained from the site of infection and disease pathology. Fine needle aspirates of the liver may allow longitudinal sampling of the local virus/host landscape. The careful utilisation of liver tissue and aspirates in conjunction with blood will provide critical information in the assessment of new therapeutics for the functional cure of HBV.

Host genetics in malaria: lessons from mouse studies

Malaria remains a deadly parasitic disease caused by Plasmodium, claiming almost half a million lives every year. While parasite genetics and biology are often the major targets in many studies, it is becoming more evident that host genetics plays a crucial role in the outcome of the infection. Similarly, Plasmodium infections in mice also rely heavily on the genetic background of the mice, and often correlate with observations in human studies, due to their high genetic homology with humans. As such, murine models of malaria are a useful tool for understanding host responses during Plasmodium infections, as well as dissecting host-parasite interactions through various genetic manipulation techniques. Reverse genetic approach such as quantitative trait loci studies and random mutagenesis screens have been employed to discover novel host genes that affect malaria susceptibility in mouse models, while other targeted studies utilize mouse models to validate observation from human studies. Herein, we review the findings from the past and present studies on murine models of hepatic and erythrocytic stages of malaria and speculate on how the current mouse models benefit from the recent development in CRISPR/Cas9 gene editing technology.

Incomplete Memories: The Natural Suppression of Tissue-Resident Memory CD8 T Cells in the Lung

The yearly, cyclic impact of viruses like influenza on human health and the economy is due to the high rates of mutation of traditional antibody targets, which negate any preexisting humoral immunity. However, the seasonality of influenza infections can equally be attributed to an absent or defective memory CD8 T cell response since the epitopes recognized by these cells are derived from essential virus proteins that mutate infrequently. Experiments in mouse models show that protection from heterologous influenza infection is temporally limited and conferred by a population of tissue-resident memory (TRM) cells residing in the lung and lung airways. TRM are elicited by a diverse set of pathogens penetrating mucosal barriers and broadly identified by extravascular staining and expression of the activation and adhesion molecules CD69 and CD103. Interestingly, lung TRM fail to express these molecules, which could limit tissue retention, resulting in airway expulsion or death with concomitant loss of heterologous protection. Here, we make the case that respiratory infections uniquely evoke a form of natural immunosuppression whereby specific cytokines and cell-cell interactions negatively impact memory cell programming and differentiation. Respiratory memory is not only short-lived but most of the memory cells in the lung parenchyma may not be bona fide TRM. Given the quantity of microbes humans inhale over a lifetime, limiting cellular residence could be a mechanism employed by the respiratory tract to preserve organismal vitality. Therefore, successful efforts to improve respiratory immunity must carefully and selectively breach these inherent tissue barriers.

Tissue-specific cellular immune responses to malaria pre-erythrocytic stages

Complete and long-lasting protective immunity against malaria can be achieved through vaccination with invasive live attenuated Plasmodium sporozoites, the motile stage inoculated in the host skin during a mosquito bite. Protective immunity relies primarily on effector CD8⁺ T cells targeting the parasite in the liver. Understanding the tissue-specific features of the immune response is emerging as a vital requirement for understanding protective immunity. The small parasite inoculum, the scarcity of infected cells and the tolerogenic properties of the liver represent hurdles for the establishment of protective immunity in endemic areas. In this review, we discuss recent advances on liver-specific features of immunity including innate recognition of malaria pre-erythrocytic stages, CD8⁺ T cell interactions with infected hepatocytes, antigen presentation for effective CD8⁺ T cell responses and generation of liver-resident memory CD8⁺ T cells. A better understanding of the factors involved in the induction and maintenance of effector CD8⁺ T cell immunity against malaria pre-erythrocytic stages is crucial for the development of an effective vaccine targeting the initial phase of malaria infection.

Emerging Targets for Developing T Cell-Mediated Vaccines for Human Immunodeficiency Virus (HIV)-1

Human immunodeficiency virus (HIV)-1 has infected >75 million individuals globally, and, according to the UN, is responsible for ~2.1 million new infections and 1.1 million deaths each year. Currently, there are ~37 million individuals with HIV infection and the epidemic has already resulted in 35 million deaths. Despite the advances of anti-retroviral therapy (ART), a cost-effective vaccine remains the best long-term solution to end the HIV-1 epidemic especially given that the vast majority of infected individuals live in poor socio-economic regions of the world such as Sub-Saharan Africa which limits their accessibility to ART. The modest efficacy of the RV144 Thai trial provides hope that a vaccine for HIV-1 is possible, but as markers for sterilizing immunity are unknown, the design of an effective vaccine is empirical, although broadly cross-reactive neutralizing antibodies (bNAb) that can neutralize various quasispecies of HIV-1 are considered crucial. Since HIV-1 transmission often occurs at the genito-rectal mucosa and is cell-associated, there is a need to develop vaccines that can elicit CD8⁺ T cell immunity with the capacity to kill virus infected cells at the genito-rectal mucosa and the gut. Here we discuss the recent progress made in developing T cell-mediated vaccines for HIV-1 and emphasize the need to elicit mucosal tissue-resident memory CD8⁺ T (CD8⁺ Trm) cells. CD8⁺ Trm cells will likely form a robust front-line defense against HIV-1 and eliminate transmitter/founder virus-infected cells which are responsible for propagating HIV-1 infections following transmission in vast majority of cases.

Not Just an Adhesion Molecule: LFA-1 Contact Tunes the T Lymphocyte Program

The α_Lβ₂ integrin LFA-1 is known to play a key role in T lymphocyte migration, which is necessary to mount a local immune response, and is also the main driver of autoimmune diseases. This migration-triggering signaling process in T cells is tightly regulated to permit an immune response that is appropriate to the local trigger, as well as to prevent deleterious tissue-damaging bystander effects. Emerging evidence shows that, in addition to prompting a diverse range of downstream signaling cascades, LFA-1 stimulation in T lymphocytes modulates gene-transcription programs, including genetic signatures of TGF-β and Notch pathways, with multifactorial biological outcomes. This review highlights recent findings and discusses molecular mechanisms by which LFA-1 signaling influence T lymphocyte differentiation into the effector subsets Th1, Th17, and induced regulatory T cells. We argue that LFA-1 contact with a cognate ligand, such as ICAM-1, independent of the immune synapse activates a late divergence in T cells' effector phenotypes, hence fine-tuning their functioning.

Systems-guided forward genetic screen reveals a critical role of the replication stress response protein ETAA1 in T cell clonal expansion

T-cell immunity requires extremely rapid clonal proliferation of rare, antigen-specific T lymphocytes to form effector cells. Here we identify a critical role for ETAA1 in this process by surveying random germ line mutations in mice using exome sequencing and bioinformatic annotation to prioritize mutations in genes of unknown function with potential effects on the immune system, followed by breeding to homozygosity and testing for immune system phenotypes. Effector CD8⁺ and CD4⁺ T-cell formation following immunization, lymphocytic choriomeningitis virus (LCMV) infection, or herpes simplex virus 1 (HSV1) infection was profoundly decreased despite normal immune cell development in adult mice homozygous for two different Etaa1 mutations: an exon 2 skipping allele that deletes Gly78-Leu119, and a Cys166Stop truncating allele that eliminates most of the 877-aa protein. ETAA1 deficiency decreased clonal expansion cell autonomously within the responding T cells, causing no decrease in their division rate but increasing TP53-induced mRNAs and phosphorylation of H2AX, a marker of DNA replication stress induced by the ATM and ATR kinases. Homozygous ETAA1-deficient adult mice were otherwise normal, healthy, and fertile, although slightly smaller, and homozygotes were born at lower frequency than expected, consistent with partial lethality after embryonic day 12. Taken together with recently reported evidence in human cancer cell lines that ETAA1 activates ATR kinase through an exon 2-encoded domain, these findings reveal a surprisingly specific requirement for this ATR activator in adult mice restricted to rapidly dividing effector T cells. This specific requirement may provide new ways to suppress pathological T-cell responses in transplantation or autoimmunity.

IL-2high tissue-resident T cells in the human liver: Sentinels for hepatotropic infection

The liver provides a tolerogenic immune niche exploited by several highly prevalent pathogens as well as by primary and metastatic tumors. We have sampled healthy and hepatitis B virus (HBV)-infected human livers to probe for a subset of T cells specialized to overcome local constraints and mediate immunity. We characterize a population of T-bet^loEomes^loBlimp-1^hiHobit^lo T cells found within the intrahepatic but not the circulating memory CD8 T cell pool expressing liver-homing/retention markers (CD69⁺CD103⁺ CXCR6⁺CXCR3⁺). These tissue-resident memory T cells (T_RM) are preferentially expanded in patients with partial immune control of HBV infection and can remain in the liver after the resolution of infection, including compartmentalized responses against epitopes within all major HBV proteins. Sequential IL-15 or antigen exposure followed by TGFβ induces liver-adapted T_RM, including their signature high expression of exhaustion markers PD-1 and CD39. We suggest that these inhibitory molecules, together with paradoxically robust, rapid, cell-autonomous IL-2 and IFNγ production, equip liver CD8 T_RM to survive while exerting local noncytolytic hepatic immunosurveillance.