Heterogeneity and plasticity of tissue-resident memory T cells in skin diseases and homeostasis: a review

Skin tissue-resident memory T (Trm) cells are produced by antigenic stimulation and remain in the skin for a long time without entering the peripheral circulation. In the healthy state Trm cells can play a patrolling and surveillance role, but in the disease state Trm cells differentiate into various phenotypes associated with different diseases, exhibit different localizations, and consequently have local protective or pathogenic roles, such as disease recurrence in vitiligo and maintenance of immune homeostasis in melanoma. The most common surface marker of Trm cells is CD69/CD103. However, the plasticity of tissue-resident memory T cells after colonization remains somewhat uncertain. This ambiguity is largely due to the variation in the functionality and ultimate destination of Trm cells produced from memory cells differentiated from diverse precursors. Notably, the presence of Trm cells is not stationary across numerous non-lymphoid tissues, most notably in the skin. These cells may reenter the blood and distant tissue sites during the recall response, revealing the recycling and migration potential of the Trm cell progeny. This review focuses on the origin and function of skin Trm cells, and provides new insights into the role of skin Trm cells in the treatment of autoimmune skin diseases, infectious skin diseases, and tumors.

Innate phase production of IFN-γ by memory and effector T cells expressing early activation marker CD69 during infection with Cryptococcus deneoformans in the lungs

Cryptococcus deneoformans is a yeast-type fungus that causes fatal meningoencephalitis in immunocompromised patients and evades phagocytic cell elimination through an escape mechanism. Memory T (Tm) cells play a central role in preventing the reactivation of this fungal pathogen. Among these cells, tissue-resident memory T (TRM) cells quickly respond to locally invaded pathogens. This study analyzes the kinetics of effector T (Teff) cells and Tm cells in the lungs after cryptococcal infection. Emphasis is placed on the kinetics and cytokine expression of TRM cells in the early phase of infection. CD4+ Tm cells exhibited a rapid increase by day 3, peaked at day 7, and then either maintained their levels or exhibited a slight decrease until day 56. In contrast, CD8+ Tm cells reached their peak on day 3 and thereafter decreased up to day 56 post-infection. These Tm cells were predominantly composed of CD69+ TRM cells and CD69+ CD103+ TRM cells. Disruption of the CARD9 gene resulted in reduced accumulation of these TRM cells and diminished interferon (IFN) -γ expression in TRM cells. TRM cells were derived from T cells with T cell receptors non-specific to ovalbumin in OT-II mice during cryptococcal infection. In addition, TRM cells exhibited varied behavior in different tissues. These results underscore the importance of T cells, which produce IFN-γ in the lungs during the early stage of infection, in providing early protection against cryptococcal infection through CARD9 signaling.

Tissue-resident memory T cells in human kidney transplants have alloreactive potential

The extent to which tissue-resident memory T (TRM) cells in transplanted organs possess alloreactivity is uncertain. This study investigates the alloreactive potential of TRM cells in kidney explants from 4 patients who experienced severe acute rejection leading to graft loss. Alloreactive T cell receptor (TCR) clones were identified in pretransplant blood samples through mixed lymphocyte reactions, followed by single-cell RNA and TCR sequencing of the proliferated recipient T cells. Subsequently, these TCR clones were traced in the TRM cells of kidney explants, which were also subjected to single-cell RNA and TCR sequencing. The proportion of recipient-derived TRM cells expressing an alloreactive TCR in the 4 kidney explants varied from 0% to 9%. Notably, these alloreactive TCRs were predominantly found among CD4+ and CD8+ TRM cells with an effector phenotype. Intriguingly, these clones were present not only in recipient-derived TRM cells but also in donor-derived TRM cells, constituting up to 4% of the donor population, suggesting the presence of self-reactive TRM cells. Overall, our study demonstrates that T cells with alloreactive potential present in the peripheral blood prior to transplantation can infiltrate the kidney transplant and adopt a TRM phenotype.

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.

Human adipose tissue as a major reservoir of cytomegalovirus-reactive T cells

Introduction

Cytomegalovirus (CMV) is a common herpesvirus with a high prevalence worldwide. After the acute infection phase, CMV can remain latent in several tissues. CD8 T cells in the lungs and salivary glands mainly control its reactivation control. White adipose tissue (WAT) contains a significant population of memory T cells reactive to viral antigens, but CMV specificity has mainly been studied in mouse WAT. Therefore, we obtained blood, omental WAT (oWAT), subcutaneous WAT (sWAT), and liver samples from 11 obese donors to characterize the human WAT adaptive immune landscape from a phenotypic and immune receptor specificity perspective.

Methods

We performed high-throughput sequencing of the T cell receptor (TCR) locus to analyze tissue and blood TCR repertoires of the 11 donors. The presence of TCRs specific to CMV epitopes was tested through ELISpot assays. Moreover, phenotypic characterization of T cells was carried out through flow cytometry.

Results

High-throughput sequencing analyses revealed that tissue TCR repertoires in oWAT, sWAT, and liver samples were less diverse and dominated by hyperexpanded clones when compared to blood samples. Additionally, we predicted the presence of TCRs specific to viral epitopes, particularly from CMV, which was confirmed by ELISpot assays. Remarkably, we found that oWAT has a higher proportion of CMV-reactive T cells than blood or sWAT. Finally, flow cytometry analyses indicated that most WAT-infiltrated lymphocytes were tissue-resident effector memory CD8 T cells.

Discussion

Overall, these findings postulate human oWAT as a major reservoir of CMV-specific T cells, presumably for latent viral reactivation control. This study enhances our understanding of the adaptive immune response in human WAT and highlights its potential role in antiviral defense.

Tissue-resident memory T (TRM ) cells: Front-line workers of the immune system

Tissue-resident memory T (TRM ) cells play a vital role in local immune protection against infection and cancer. The location of TRM cells within peripheral tissues at sites of pathogen invasion allows for the rapid detection and elimination of microbes, making their generation an attractive goal for the development of next-generation vaccines. Here, we discuss differential requirements for CD8+ TRM cell development across tissues with implications for establishing local prophylactic immunity, emphasizing the role of tissue-derived factors, local antigen, and adjuvants on TRM cell generation in the context of vaccination.

Usefulness of Biopsy Specimens for Evaluating CD103+ Tumor-resident Memory T Cells in Esophageal Cancer

BACKGROUND/AIM

CD103+ tissue-resident memory T cells (TRM) in tumor sites are associated with a favorable prognosis and predict the effectiveness of immune checkpoint inhibitors. The detection of CD103+ TRM infiltration in biopsy samples could be beneficial for patients without surgical indications. However, the usefulness of TRM detection in biopsy tissue and the difference in TRM status between biopsy and surgical specimens' post-neoadjuvant chemotherapy have not been elucidated. In the present study, we aimed to elucidate whether we can detect TRM in biopsy specimens and the impact of chemotherapy on TRM infiltration.

MATERIALS AND METHODS

Tissue sections were obtained from 46 patients with esophageal cancer who received neoadjuvant chemotherapy and underwent radical esophagectomy in 2017. Immunohistochemistry was performed using an anti-CD103 antibody for biopsy and surgical specimens. We examined the relationship between CD103 expression, clinicopathological features, and prognosis for each patient.

RESULTS

TRM infiltration was detected in the biopsy specimens. CD103 expression in biopsy specimens correlated with that in surgical specimens. Although there was no statistical significance in clinicopathological findings between CD103high and CD103low, patients with CD103high biopsy specimens exhibited favorable prognosis. The number of CD103+ cells was increased by chemotherapy: though with no survival benefit.

CONCLUSION

Regardless of surgical indication, we were able to determine the TRM status even in biopsy specimens. CD103 evaluation at biopsy may be more useful and practical than evaluation in surgical specimens, enabling prediction of prognosis and response to immune therapy.

OMIP-096: A 24-color flow cytometry panel to identify and characterize CD4+ and CD8+ tissue-resident T cells in human skin, intestinal, and type II mucosal tissue

There is a great need to understand human immune cells within tissue, where disease manifests and infection occurs. Tissue-resident memory T cells (TRMs) were discovered over a decade ago, there is a great need to understand their role in human disease. We developed a 24-color flow cytometry panel to comprehensively interrogate CD4+ and CD8+ TRMs isolated from human tissues. When interrogating cells within human tissue, enzymatic methods used to liberate cells from within the tissue can cause cleavage of cell surface markers needed to phenotype these cells. Here we carefully select antibody clones and evaluate the effect of enzymatic digestion on the expression of markers relevant to the identification of T cell residency, as well as markers relevant to the activation and immunoregulation status of these cells. We have designed this panel to be applicable across a range of human tissues including skin, intestine, and type II mucosae such as the vagina.

Small Molecule Targeting Immune Cells: A Novel Approach for Cancer Treatment

Conventional and cancer immunotherapies encompass diverse strategies to address various cancer types and stages. However, combining these approaches often encounters limitations such as non-specific targeting, resistance development, and high toxicity, leading to suboptimal outcomes in many cancers. The tumor microenvironment (TME) is orchestrated by intricate interactions between immune and non-immune cells dictating tumor progression. An innovative avenue in cancer therapy involves leveraging small molecules to influence a spectrum of resistant cell populations within the TME. Recent discoveries have unveiled a phenotypically diverse cohort of innate-like T (ILT) cells and tumor hybrid cells (HCs) exhibiting novel characteristics, including augmented proliferation, migration, resistance to exhaustion, evasion of immunosurveillance, reduced apoptosis, drug resistance, and heightened metastasis frequency. Leveraging small-molecule immunomodulators to target these immune players presents an exciting frontier in developing novel tumor immunotherapies. Moreover, combining small molecule modulators with immunotherapy can synergistically enhance the inhibitory impact on tumor progression by empowering the immune system to meticulously fine-tune responses within the TME, bolstering its capacity to recognize and eliminate cancer cells. This review outlines strategies involving small molecules that modify immune cells within the TME, potentially revolutionizing therapeutic interventions and enhancing the anti-tumor response.

Site-specific development and progressive maturation of human tissue-resident memory T cells over infancy and childhood

Infancy and childhood are critical life stages for generating immune memory to protect against pathogens; however, the timing, location, and pathways for memory development in humans remain elusive. Here, we investigated T cells in mucosal sites, lymphoid tissues, and blood from 96 pediatric donors aged 0-10 years using phenotypic, functional, and transcriptomic profiling. Our results revealed that memory T cells preferentially localized in the intestines and lungs during infancy and accumulated more rapidly in mucosal sites compared with blood and lymphoid organs, consistent with site-specific antigen exposure. Early life mucosal memory T cells exhibit distinct functional capacities and stem-like transcriptional profiles. In later childhood, they progressively adopt proinflammatory functions and tissue-resident signatures, coincident with increased T cell receptor (TCR) clonal expansion in mucosal and lymphoid sites. Together, our findings identify staged development of memory T cells targeted to tissues during the formative years, informing how we might promote and monitor immunity in children.

Anti-CD3 inhibits circulatory and tissue-resident memory CD4 T cells that drive asthma exacerbations in mice

BACKGROUND

Exacerbations of asthma are thought to be strongly dependent on reactivation of allergen-induced lung tissue-resident and circulatory memory CD4 T cells. Strategies that broadly inhibit multiple T cell populations might then be useful to limit asthma. Accordingly, we tested whether targeting CD3 during exposure to inhaled allergen could prevent the accumulation of lung-localized effector memory CD4 T cells and block exacerbations of asthmatic inflammation.

METHODS

House dust mite-sensitized and repetitively challenged BL/6 mice were transiently treated therapeutically with F(ab')2 anti-CD3ε and memory T cell responses and lung inflammation were assessed. PBMCs from HDM-allergic donors were examined for the effect of anti-CD3 on expansion of allergen-reactive T cells.

RESULTS

Allergen-sensitized mice undergoing exacerbations of asthma were protected from lung inflammation by transient therapeutic treatment with F(ab')2 anti-CD3. Regardless of whether sensitized mice underwent a secondary or tertiary recall response to inhaled allergen, anti-CD3 inhibited all phenotypes of effector memory CD4 T cells in the lung tissue and lung vasculature by 80%-90%, including those derived from tissue-resident and circulatory memory T cells. This did not depend on Treg cells suggesting it was primarily a blocking effect on memory T cell signaling. Correspondingly, anti-CD3 also strongly inhibited proliferation of human allergen-reactive memory CD4 T cells from allergic individuals. In contrast, the number of surviving tissue-resident memory CD4 T cells that were maintained in the lungs at later times was not robustly reduced by anti-CD3.

CONCLUSION

Anti-CD3 F(ab')2 administration at the time of allergen exposure represents a viable strategy for limiting the immediate activity of allergen-responding memory T cells and asthma exacerbations.

Lung-resident CD69+ST2+ TH2 cells mediate long-term type 2 memory to inhaled antigen in mice

BACKGROUND

Chronic airway diseases such as asthma are characterized by persistent type 2 immunity in the airways. We know little about the mechanisms that explain why type 2 inflammation continues in these diseases.

OBJECTIVE

We used mouse models to investigate the mechanisms involved in long-lasting immune memory.

METHODS

Naive mice were exposed intranasally to ovalbumin (OVA) antigen with Alternaria extract as an adjuvant. Type 2 memory was analyzed by parabiosis model, flow cytometry with in vivo antibody labeling, and intranasal OVA recall challenge. Gene-deficient mice were used to analyze the mechanisms.

RESULTS

In the parabiosis model, mice previously exposed intranasally to OVA with Alternaria showed more robust antigen-specific immune responses and airway inflammation than mice with circulating OVA-specific T cells. After a single airway exposure to OVA with Alternaria, CD69+ST2+ TH2-type T cells, which highly express type 2 cytokine messenger RNA and lack CD62L expression, appeared in lung tissue within 5 days and persisted for at least 84 days. When exposed again to OVA in vivo, these cells produced type 2 cytokines quickly without involving circulating T cells. Development of tissue-resident CD69+ST2+ TH2 cells and long-term memory to an inhaled antigen were abrogated in mice deficient in ST2 or IL-33, but not TSLP receptor.

CONCLUSION

CD69+ST2+ TH2 memory cells develop quickly in lung tissue after initial allergen exposure and persist for a prolonged period. The ST2/IL-33 pathway may play a role in the development of immune memory in lung to certain allergens.

Functional heterogeneity of human skin-resident memory T cells in health and disease

The human skin is populated by a diverse pool of memory T cells, which can act rapidly in response to pathogens and cancer antigens. Tissue-resident memory T cells (TRM ) have been implicated in range of allergic, autoimmune and inflammatory skin diseases. Clonal expansion of cells with TRM properties is also known to contribute to cutaneous T-cell lymphoma. Here, we review the heterogeneous phenotypes, transcriptional programs, and effector functions of skin TRM . We summarize recent studies on TRM formation, longevity, plasticity, and retrograde migration and contextualize the findings to skin TRM and their role in maintaining skin homeostasis and altered functions in skin disease.

Systemic priming and intranasal booster with a BcfA-adjuvanted acellular pertussis vaccine generates CD4+ IL-17+ nasal tissue resident T cells and reduces B. pertussis nasal colonization

Introduction

Resurgence of pertussis, caused by Bordetella pertussis, necessitates novel vaccines and vaccination strategies to combat this disease. Alum-adjuvanted acellular pertussis vaccines (aPV) delivered intramuscularly reduce bacterial numbers in the lungs of immunized animals and humans, but do not reduce nasal colonization. Thus, aPV-immunized individuals are sources of community transmission. We showed previously that modification of a commercial aPV (Boostrix) by addition of the Th1/17 polarizing adjuvant Bordetella Colonization Factor A (BcfA) attenuated Th2 responses elicited by alum and accelerated clearance of B. pertussis from mouse lungs. Here we tested whether a heterologous immunization strategy with systemic priming and mucosal booster (prime-pull) would reduce nasal colonization.

Methods

Adult male and female mice were immunized intramuscularly (i.m.) with aPV or aPV/BcfA and boosted either i.m. or intranasally (i.n.) with the same formulation. Tissue-resident memory (TRM) responses in the respiratory tract were quantified by flow cytometry, and mucosal and systemic antibodies were quantified by ELISA. Immunized and naïve mice were challenged i.n. with Bordetella pertussis and bacterial load in the nose and lungs enumerated at days 1-14 post-challenge.

Results

We show that prime-pull immunization with Boostrix plus BcfA (aPV/BcfA) generated IFNγ+ and IL-17+ CD4+ lung resident memory T cells (TRM), and CD4+IL-17+ TRM in the nose. In contrast, aPV alone delivered by the same route generated IL-5+ CD4+ resident memory T cells in the lungs and nose. Importantly, nasal colonization was only reduced in mice immunized with aPV/BcfA by the prime-pull regimen.

Conclusions

These results suggest that TH17 polarized TRM generated by aPV/BcfA may reduce nasal colonization thereby preventing pertussis transmission and subsequent resurgence.

Mycobacterium bovis BCG Given at Birth Followed by Inactivated Respiratory Syncytial Virus Vaccine Prevents Vaccine-Enhanced Disease by Promoting Trained Macrophages and Resident Memory T Cells

Respiratory syncytial virus (RSV) infects more than 60% of infants in their first year of life. Since an experimental formalin-inactivated (FI) RSV vaccine tested in the 1960s caused enhanced respiratory disease (ERD), few attempts have been made to vaccinate infants. ERD is characterized by Th2-biased responses, lung inflammation, and poor protective immune memory. Innate immune memory displays an increased nonspecific effector function upon restimulation, a process called trained immunity, or a repressed effector function upon restimulation, a process called tolerance, which participates in host defense and inflammatory disease. Mycobacterium bovis bacillus Calmette-Guérin (BCG) given at birth can induce trained immunity as well as heterologous Th1 responses. We speculate that BCG given at birth followed by FI-RSV may alleviate ERD and enhance protection through promoting trained immunity and balanced Th immune memory. Neonatal mice were given BCG at birth and then vaccinated with FI-RSV+Al(OH)3. BCG/FI-RSV+Al(OH)3 induced trained macrophages, tissue-resident memory T cells (TRM), and specific cytotoxic T lymphocytes (CTL) in lungs and inhibited Th2 and Th17 cell immune memory, all of which contributed to inhibition of ERD and increased protection. Notably, FI-RSV+Al(OH)3 induced tolerant macrophages, while BCG/FI-RSV+Al(OH)3 prevented the innate tolerance through promoting trained macrophages. Moreover, inhibition of ERD was attributed to trained macrophages or TRM in lungs but not memory T cells in spleens. Therefore, BCG given at birth to regulate trained immunity and TRM may be a new strategy for developing safe and effective RSV killed vaccines for young infants. IMPORTANCE RSV is the leading cause of severe lower respiratory tract infection of infants. ERD, characterized by Th2-biased responses, inflammation, and poor immune memory, has been an obstacle to the development of safe and effective killed RSV vaccines. Innate immune memory participates in host defense and inflammatory disease. BCG given at birth can induce trained immunity as well as heterologous Th1 responses. Our results showed that BCG/FI-RSV+Al(OH)3 induced trained macrophages, TRM, specific CTL, and balanced Th cell immune memory, which contributed to inhibition of ERD and increased protection. Notably, FI-RSV+Al(OH)3 induced tolerant macrophages, while BCG/FI-RSV+Al(OH)3 prevented tolerance through promoting trained macrophages. Moreover, inhibition of ERD was attributed to trained macrophages or TRM in lungs but not memory T cells in spleens. BCG at birth as an adjuvant to regulate trained immunity and TRM may be a new strategy for developing safe and effective RSV killed vaccines for young infants.

Bystander activation of Bordetella pertussis-induced nasal tissue-resident memory CD4 T cells confers heterologous immunity to Klebsiella pneumoniae

Tissue-resident memory CD4 T (T_RM ) cells induced by infection with Bordetella pertussis persist in respiratory tissues and confer long-term protective immunity against reinfection. However, it is not clear how they are maintained in respiratory tissues. Here, we demonstrate that B. pertussis-specific CD4 T_RM cells produce IL-17A in response to in vitro stimulation with LPS or heat-killed Klebsiella pneumoniae (HKKP) in the presence of dendritic cells. Furthermore, IL-17A-secreting CD4 T_RM cells expand in the lung and nasal tissue of B. pertussis convalescent mice following in vivo administration of LPS or HKKP. Bystander activation of CD4 T_RM cells was suppressed by anti-IL-12p40 but not by anti-MHCII antibodies. Furthermore, purified respiratory tissue-resident, but not circulating, CD4 T cells from convalescent mice produced IL-17A following direct stimulation with IL-23 and IL-1β or IL-18. Intranasal immunization of mice with a whole-cell pertussis vaccine induced respiratory CD4 T_RM cells that were reactivated following stimulation with K. pneumoniae. Furthermore, the nasal pertussis vaccine conferred protective immunity against B. pertussis but also attenuated infection with K. pneumoniae. Our findings demonstrate that CD4 T_RM cells induced by respiratory infection or vaccination can undergo bystander activation and confer heterologous immunity to an unrelated respiratory pathogen.

Identification of a tissue resident memory CD8 T cell-related risk score signature for colorectal cancer, the association with TME landscapes and therapeutic responses

Backgrounds: The tissue resident memory CD8 T cell (Trm) constitutes an important component of the local immunity. In the context of malignant tumors, mounting evidence also supports the potential anti-tumor property of this cell subset. Therefore, identification of Trm marker genes and exploration of the causative effect of Trm in shaping tumor microenvironment (TME) heterogeneity might provide novel insights for the comprehensive management of cancer patients. Methods: By dissecting a single T cell transcriptome dataset, we acquired marker genes for Trm, which were latter applied to bulk RNA sequencing profiles of two large colorectal cancer (CRC) patient cohorts downloaded from TCGA and GEO databases. First, colorectal cancer patients were divided into different Trm clusters using consensus clustering algorithm. Then, we established a Trm-related gene (TRMRG) risk score signature and tested its efficacy in predicting prognosis for colorectal cancer patients. Moreover, a sequence of rigorous and robust analyses were also carried out to investigate the potential role of Trm-related gene risk score in tumor microenvironment remodeling and therapeutic utility of it in colorectal cancer treatment. Results: A total of 49 Trm marker genes were identified by analyzing single cell RNA sequencing profiles. First, colorectal cancer patients were successfully classified into two Trm clusters with significant heterogeneity in functional enrichment patterns and tumor microenvironment landscapes. Then, we developed a Trm-related gene risk score signature and divided patients into different risk levels. High risk patients were characterized by attenuated immunogenicity, weakened sensitivity to immunotherapy, as well as adverse clinical outcomes. While low risk patients with advantages in survival exhibited increased immunogenicity, stronger metabolic activity and improved immunotherapeutic responses. Conclusion: Through combinatorial analysis of single cell and bulk RNA sequencing data, the present study identified Trm to play a non-negligible role in regulating the complexity and heterogeneity of tumor microenvironment for colorectal cancer. Moreover, the Trm-related gene risk score signature developed currently was corroborated to be tightly correlated with prognosis and therapeutic responses of colorectal cancer patients, thus exhibiting potential application value for clinical practice.

Tissue-resident memory T cells in immune-related adverse events: friend or foe?

Many cancer patients experience toxicity during checkpoint blockade immunotherapy, which often leads to treatment discontinuation. To this end, understanding the mechanisms mediating immune-related adverse events (irAE) should ultimately enable improvement in clinical outcomes. Recent work has revealed that tissue-resident memory T (T_RM) cells are locally expanded in irAE-dermatitis and -colitis.

Intestinal tissue-resident T cell activation depends on metabolite availability

The role of tissue-resident memory T cells has come to the forefront, and intestinal intraepithelial lymphocytes (IELs) are abundant. The cells protect the host against invasion and regulate tissue homeostasis and tolerance. Microbial invasion and aberrant IEL activation can contribute to disorders such as inflammatory bowel disease. IELs are kept in a state with limited metabolic activity. We describe the functional analysis of IELs compared with their CD8 circulating counterparts. Although proliferative bursts are similar, IEL metabolism is rapid. Metabolic pathway analysis highlights a tight connection of OXPHOS and glycolysis in IELs and a reliance on pyruvate oxidation. Glucose availability in the local environment can regulate IEL activity, resulting in rapid clearance of an intestinal parasite infection.

The metabolic capacity of many cells is tightly regulated and can adapt to changes in metabolic resources according to environmental changes. Tissue-resident memory (T_RM) CD8⁺ T cells are one of the most abundant T cell populations and offer rapid protection against invading pathogens, especially at the epithelia. T_RM cells metabolically adapt to their tissue niche, such as the intestinal epithelial barrier. In the small intestine, the types of T_RM cells are intraepithelial lymphocytes (IELs), which contain high levels of cytotoxic molecules and express activation markers, suggesting a heightened state of activation. We hypothesize that the tissue environment may determine IEL activity. We show that IEL activation, in line with its semiactive status, is metabolically faster than circulating CD8⁺ T cells. IEL glycolysis and oxidative phosphorylation (OXPHOS) are interdependently regulated and are dependent on rapid access to metabolites from the environment. IELs are restrained by local availability of metabolites, but, especially, glucose levels determine their activity. Importantly, this enables functional control of intestinal T_RM cells by metabolic means within the fragile environment of the intestinal epithelial barrier.

T-cell exhaustion and residency dynamics inform clinical outcomes in hepatocellular carcinoma

BACKGROUND & AIMS

Despite recent translation of immunotherapies into clinical practice, the immunobiology of hepatocellular carcinoma (HCC), in particular the role and clinical relevance of exhausted and liver-resident T cells remain unclear. We therefore dissected the landscape of exhausted and resident T cell responses in the peripheral blood and tumor microenvironment of patients with HCC.

METHODS

Lymphocytes were isolated from the blood, tumor and tumor-surrounding liver tissue of patients with HCC (n = 40, n = 10 treated with anti-PD-1 therapy). Phenotype, function and response to anti-PD-1 were analyzed by mass and flow cytometry ex vivo and in vitro, tissue residence was further assessed by immunohistochemistry and imaging mass cytometry. Gene signatures were analyzed in silico.

RESULTS

We identified significant enrichment of heterogeneous populations of exhausted CD8+ T cells (TEX) in the tumor microenvironment. Strong enrichment of severely exhausted CD8 T cells expressing multiple immune checkpoints in addition to PD-1 was linked to poor progression-free and overall survival. In contrast, PD-1 was also expressed on a subset of more functional and metabolically active CD103+ tissue-resident memory T cells (TRM) that expressed few additional immune checkpoints and were associated with better survival. TEX enrichment was independent of BCLC stage, alpha-fetoprotein levels or age as a variable for progression-free survival in our cohort. These findings were in line with in silico gene signature analysis of HCC tumor transcriptomes from The Cancer Genome Atlas. A higher baseline TRM/TEX ratio was associated with disease control in anti-PD-1-treated patients.

CONCLUSION

Our data provide information on the role of peripheral and intratumoral TEX-TRM dynamics in determining outcomes in patients with HCC. The dynamics between exhausted and liver-resident T cells have implications for immune-based diagnostics, rational patient selection and monitoring during HCC immunotherapies.

LAY SUMMARY

The role of the immune response in hepatocellular carcinoma (HCC) remains unclear. T cells can mediate protection against tumor cells but are frequently dysfunctional and exhausted in cancer. We found that patients with a predominance of exhausted CD8+ T cells (TEX) had poor survival compared to patients with a predominance of tissue-resident memory T cells (TRM). This correlated with the molecular profile, metabolic and functional status of these cell populations. The enrichment of TEX was independently associated with prognosis in addition to disease stage, age and tumor markers. A high TRM proportion was also associated with better outcomes following checkpoint therapy. Thus, these T-cell populations are novel biomarkers with relevance in HCC.

Mucosal immunization with an adenoviral vector vaccine confers superior protection against RSV compared to natural immunity

Respiratory syncytial virus (RSV) infections are the leading cause of severe respiratory illness in early infancy. Although the majority of children and adults mount immune responses against RSV, recurrent infections are frequent throughout life. Humoral and cellular responses contribute to an effective immunity but also their localization at respiratory mucosae is increasingly recognized as an important factor. In the present study, we evaluate a mucosal vaccine based on an adenoviral vector encoding for the RSV fusion protein (Ad-F), and we investigate two genetic adjuvant candidates that encode for Interleukin (IL)-1β and IFN-β promoter stimulator I (IPS-1), respectively. While vaccination with Ad-F alone was immunogenic, the inclusion of Ad-IL-1β increased F-specific mucosal immunoglobulin A (IgA) and tissue-resident memory T cells (T_RM). Consequently, immunization with Ad-F led to some control of virus replication upon RSV infection, but Ad-F+Ad-IL-1β was the most effective vaccine strategy in limiting viral load and weight loss. Subsequently, we compared the Ad-F+Ad-IL-1β-induced immunity with that provoked by a primary RSV infection. Systemic F-specific antibody responses were higher in immunized than in previously infected mice. However, the primary infection provoked glycoprotein G-specific antibodies as well eventually leading to similar neutralization titers in both groups. In contrast, mucosal antibody levels were low after infection, whereas mucosal immunization raised robust F-specific responses including IgA. Similarly, vaccination generated F-specific T_RM more efficiently compared to a primary RSV infection. Although the primary infection resulted in matrix protein 2 (M2)-specific T cells as well, they did not reach levels of F-specific immunity in the vaccinated group. Moreover, the infection-induced T cell response was less biased towards T_RM compared to vaccine-induced immunity. Finally, our vaccine candidate provided superior protection against RSV infection compared to a primary infection as indicated by reduced weight loss, virus replication, and tissue damage. In conclusion, our mucosal vaccine candidate Ad-F+Ad-IL-1β elicits stronger mucosal immune responses and a more effective protection against RSV infection than natural immunity generated by a previous infection. Harnessing mucosal immune responses by next-generation vaccines is therefore a promising option to establish effective RSV immunity and thereby tackle a major cause of infant hospitalization.

Using Full-Spectrum Flow Cytometry to Phenotype Memory T and NKT Cell Subsets with Optimized Tissue-Specific Preparation Protocols

Full-spectrum flow cytometry is now routinely used in many laboratories internationally, and the demand for this technology is rapidly increasing. With capacity to use larger and more complex staining panels, standardized protocols are required for optimal panel design and analysis. Importantly, for ex vivo analysis, tissue preparation methods also need to be optimized to ensure samples are truly representative of tissues in situ. This is particularly relevant given the recent interest in adaptive immune cells that form residency in specific organs. Here we provide optimized protocols for tissue processing and phenotyping of memory T cells and natural killer T (NKT) cell subsets from liver, lung, spleen, and lymph node using full-spectrum flow cytometry. We provide a 21-color antibody panel for identification of different memory subsets, including tissue-resident memory T (T_RM ) cells, which are increasingly regarded as important effectors in adaptive immunity. We show that processing procedures can affect outcomes, with liver T_RM cells particularly sensitive to heat, such that accurate evaluation requires fast processing at defined temperatures. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Processing mouse liver for flow cytometric analysis of memory T and NKT cell subsets Basic Protocol 2: Processing mouse spleen for flow cytometric analysis of memory T and NKT cell subsets Basic Protocol 3: Processing mouse lungs for flow cytometric analysis of memory T and NKT cell subsets Basic Protocol 4: Processing mouse lymph nodes for flow cytometric analysis of memory T and NKT cell subsets Basic Protocol 5: Staining and flow cytometric analysis of samples for memory T and NKT cell subsets Support Protocol: Obtaining cell counts from flow cytometry data.

A Fraction of CD8+ T Cells from Colorectal Liver Metastases Preferentially Repopulate Autologous Patient-Derived Xenograft Tumors as Tissue-Resident Memory T Cells

The diversity of T cells in the human liver may reflect the composition of TILs in CRLM. Our ex vivo characterization of CRLM vs. adjacent liver tissue detected CD103+CD39+CD8+ T_RM cells predominantly in CRLM, which prompted further assessments. These T_RM cells responded to cognate antigens in vitro. As functional activities of autologous TILs are central to the implementation of personalized cancer treatments, we applied a patient-derived xenograft (PDX) model to monitor TILs' capacity to control CRLM-derived tumors in vivo. We established PDX mice with CRLMs from two patients, and in vitro expansion of their respective TILs resulted in opposing CD4+ vs. CD8+ TIL ratios. These CRLMs also displayed mutated KRAS, which enabled trametinib-mediated inhibition of MEK. Regardless of the TIL subset ratio, persistent or transient control of CRLM-derived tumors of limited size by the transferred TILs was observed only after trametinib treatment. Of note, a portion of transferred TILs was observed as CD103+CD8+ T_RM cells that strictly accumulated within the autologous CRLM-derived tumor rather than in the spleen or blood. Thus, the predominance of CD103+CD39+CD8+ T_RM cells in CRLM relative to the adjacent liver and the propensity of CD103+CD8+ T_RM cells to repopulate the autologous tumor may identify these TILs as strategic targets for therapies against advanced CRC.

Vaccine Based on Dendritic Cells Electroporated with an Exogenous Ovalbumin Protein and Pulsed with Invariant Natural Killer T Cell Ligands Effectively Induces Antigen-Specific Antitumor Immunity

Simple Summary

This study shows the potential of a novel dendritic cell vaccine therapy in antitumor immunity, in which bone marrow-derived dendritic cells are electroporated with an exogenous ovalbumin protein and simultaneously pulsed with α-galactosylceramide. This strategy enhances the induction of cytotoxic CD8⁺ T cells specific for tumor-associated antigens through the activation of invariant natural killer T cells, natural killer cells, and intrinsic dendritic cells. Moreover, this strategy sustains antigen-specific antitumor T cell responses over time.

Abstract

(1) Background: Cancer vaccines are administered to induce cytotoxic CD8⁺ T cells (CTLs) specific for tumor antigens. Invariant natural killer T (iNKT) cells, the specific T cells activated by α-galactosylceramide (α-GalCer), play important roles in this process as they are involved in both innate and adaptive immunity. We developed a new cancer vaccine strategy in which dendritic cells (DCs) were loaded with an exogenous ovalbumin (OVA) protein by electroporation (EP) and pulsed with α-GalCer. (2) Methods: We generated bone marrow-derived DCs from C57BL/6 mice, loaded full-length ovalbumin proteins to the DCs by EP, and pulsed them with α-GalCer (OVA-EP-galDCs). The OVA-EP-galDCs were intravenously administered to C57BL/6 mice as a vaccine. We then investigated subsequent immune responses, such as the induction of iNKT cells, NK cells, intrinsic DCs, and OVA-specific CD8⁺ T cells, including tissue-resident memory T (T_RM) cells. (3) Results: The OVA-EP-galDC vaccine efficiently rejected subcutaneous tumors in a manner primarily dependent on CD8⁺ T cells. In addition to the OVA-specific CD8⁺ T cells both in early and late phases, we observed the induction of antigen-specific T_RM cells in the skin. (4) Conclusions: The OVA-EP-galDC vaccine efficiently induced antigen-specific antitumor immunity, which was sustained over time, as shown by the T_RM cells.

Delayed antiretroviral therapy in HIV-infected individuals leads to irreversible depletion of skin- and mucosa-resident memory T cells

People living with HIV (PLWH) are at increased risk for developing skin and mucosal malignancies despite systemic reconstitution of CD4⁺ T cells upon antiretroviral therapy (ART). The underlying mechanism of chronic tissue-related immunodeficiency in HIV is unclear. We found that skin CD4⁺ tissue-resident memory T (Trm) cells were depleted after HIV infection and replenished only upon early ART initiation. TCR clonal analysis following early ART suggested a systemic origin for reconstituting CD4⁺ Trm cells. Single-cell RNA sequencing in PLWH that received late ART treatment revealed a loss of CXCR3⁺ Trm cells and a tolerogenic skin immune environment. Human papilloma virus-induced precancerous lesion biopsies showed reduced CXCR3⁺ Trm cell frequencies in the mucosa in PLWH versus HIV^- individuals. These results reveal an irreversible loss of CXCR3⁺ Trm cells confined to skin and mucosa in PLWH who received late ART treatment, which may be a precipitating factor in the development of HPV-related cancer.

The Roles of Tissue-Resident Memory T Cells in Lung Diseases

The unique environment of the lungs is protected by complex immune interactions. Human lung tissue-resident memory T cells (T_RM) have been shown to position at the pathogen entry points and play an essential role in fighting against viral and bacterial pathogens at the frontline through direct mechanisms and also by orchestrating the adaptive immune system through crosstalk. Recent evidence suggests that T_RM cells also play a vital part in slowing down carcinogenesis and preventing the spread of solid tumors. Less beneficially, lung T_RM cells can promote pathologic inflammation, causing chronic airway inflammatory changes such as asthma and fibrosis. T_RM cells from infiltrating recipient T cells may also mediate allograft immunopathology, hence lung damage in patients after lung transplantations. Several therapeutic strategies targeting T_RM cells have been developed. This review will summarize recent advances in understanding the establishment and maintenance of T_RM cells in the lung, describe their roles in different lung diseases, and discuss how the T_RM cells may guide future immunotherapies targeting infectious diseases, cancers and pathologic immune responses.

The Inhibitory Receptor GPR56 (Adgrg1) Is Specifically Expressed by Tissue-Resident Memory T Cells in Mice But Dispensable for Their Differentiation and Function In Vivo

Tissue-resident memory T (T_RM) cells with potent antiviral and antibacterial functions protect the epithelial and mucosal surfaces of our bodies against infection with pathogens. The strong proinflammatory activities of T_RM cells suggest requirement for expression of inhibitory molecules to restrain these memory T cells under steady state conditions. We previously identified the adhesion G protein-coupled receptor GPR56 as an inhibitory receptor of human cytotoxic lymphocytes that regulates their cytotoxic effector functions. Here, we explored the expression pattern, expression regulation, and function of GPR56 on pathogen-specific CD8⁺ T cells using two infection models. We observed that GPR56 is expressed on T_RM cells during acute infection and is upregulated by the T_RM cell-inducing cytokine TGF-β and the T_RM cell-associated transcription factor Hobit. However, GPR56 appeared dispensable for CD8⁺ T-cell differentiation and function upon acute infection with LCMV as well as Listeria monocytogenes. Thus, T_RM cells specifically acquire the inhibitory receptor GPR56, but the impact of this receptor on T_RM cells after acute infection does not appear essential to regulate effector functions of T_RM cells.

Tissue-specific differentiation of CD8+ resident memory T cells

CD8⁺ tissue-resident memory T (T_RM) cells play crucial roles in defense against infections and cancer and have been implicated in autoimmune diseases such as psoriasis. In mice and humans, they exist in all nonlymphoid organs and share key characteristics across all tissues, including downregulation of tissue egress and lymph node homing pathways. However, recent studies demonstrate considerable heterogeneity across T_RM cells lodged in different tissues - linked to the activity of tissue-specific molecules, including chemokines, cytokines, and transcription factors. Current work indicates that transforming growth factor (TGF)-β plays a major role in generating T_RM heterogeneity at phenotypic and functional levels. Here, we review common and unique features of T_RM cells in different tissues and discuss putative strategies aimed at harnessing T_RM cells for site-specific protection against infectious and malignant diseases.

The Potential of Tissue-Resident Memory T Cells for Adoptive Immunotherapy against Cancer

Tissue-resident memory T cells (T_RM) comprise an important memory T cell subset that mediates local protection upon pathogen re-encounter. T_RM populations preferentially localize at entry sites of pathogens, including epithelia of the skin, lungs and intestine, but have also been observed in secondary lymphoid tissue, brain, liver and kidney. More recently, memory T cells characterized as T_RM have also been identified in tumors, including but not limited to melanoma, lung carcinoma, cervical carcinoma, gastric carcinoma and ovarian carcinoma. The presence of these memory T cells has been strongly associated with favorable clinical outcomes, which has generated an interest in targeting T_RM cells to improve immunotherapy of cancer patients. Nevertheless, intratumoral T_RM have also been found to express checkpoint inhibitory receptors, such as PD-1 and LAG-3. Triggering of such inhibitory receptors could induce dysfunction, often referred to as exhaustion, which may limit the effectiveness of T_RM in countering tumor growth. A better understanding of the differentiation and function of T_RM in tumor settings is crucial to deploy these memory T cells in future treatment options of cancer patients. The purpose of this review is to provide the current status of an important cancer immunotherapy known as TIL therapy, insight into the role of T_RM in the context of antitumor immunity, and the challenges and opportunities to exploit these cells for TIL therapy to ultimately improve cancer treatment.

Skin Resident Memory T Cells May Play Critical Role in Delayed-Type Drug Hypersensitivity Reactions

Delayed-type drug hypersensitivity reactions (dtDHR) are immune-mediated reactions with skin and visceral manifestations ranging from mild to severe. Clinical care is negatively impacted by a limited understanding of disease pathogenesis. Though T cells are believed to orchestrate disease, the type of T cell and the location and mechanism of T cell activation remain unknown. Resident memory T cells (T_RM) are a unique T cell population potentially well situated to act as key mediators in disease pathogenesis, but significant obstacles to defining, identifying, and testing T_RM in dtDHR preclude definitive conclusions at this time. Deeper mechanistic interrogation to address these unanswered questions is necessary, as involvement of T_RM in disease has significant implications for prediction, diagnosis, and treatment of disease.

Tissue-Resident Memory T Cells in Skin Diseases: A Systematic Review

In health, the non-recirculating nature and long-term persistence of tissue-resident memory T cells (TRMs) in tissues protects against invading pathogens. In disease, pathogenic TRMs contribute to the recurring traits of many skin diseases. We aimed to conduct a systematic literature review on the current understanding of the role of TRMs in skin diseases and identify gaps as well as future research paths. EMBASE, PubMed, SCOPUS, Web of Science, Clinicaltrials.gov and WHO Trials Registry were searched systematically for relevant studies from their inception to October 2020. Included studies were reviewed independently by two authors. This study was conducted in accordance with the PRISMA-S guidelines. This protocol was registered with the PROSPERO database (ref: CRD42020206416). We identified 96 studies meeting the inclusion criteria. TRMs have mostly been investigated in murine skin and in relation to infectious skin diseases. Pathogenic TRMs have been characterized in various skin diseases including psoriasis, vitiligo and cutaneous T-cell lymphoma. Studies are needed to discover biomarkers that may delineate TRMs poised for pathogenic activity in skin diseases and establish to which extent TRMs are contingent on the local skin microenvironment. Additionally, future studies may investigate the effects of current treatments on the persistence of pathogenic TRMs in human skin.

Tissue-resident regulatory T cells accumulate at human barrier lymphoid organs

Regulatory T cells (Tregs) play a critical role in immune regulation and peripheral tolerance. While different types of Tregs have been identified in both mice and humans, much of our understanding about how these cells maintain immune homeostasis is derived from animal models. In this study, we examined two distinct human lymphoid organs to understand how repeated exposure to infections at the mucosal surface influences the phenotype and tissue localization of Tregs. We show that while Tregs in both tonsils and spleen express a tissue-resident phenotype, they accumulate in greater numbers in tonsils. Tonsillar-resident Tregs exhibit a highly suppressive phenotype with significantly increased expression of CD39, ICOS and CTLA-4 compared with their counterparts in circulation or in the spleen. Functionally, resident Tregs are able effectively to suppress T cell proliferation. We further demonstrate that tonsillar-resident Tregs share key features of T follicular helper cells. Spatial analysis reveals that the vast majority of resident Tregs are localized at the border of the T-zone and B cell follicle, as well as within the lymphocyte pockets enriched with resident memory T cells. Together our findings suggest that resident Tregs are strategically co-localized to maintain immune homeostasis at sites of recurrent inflammation.

Engineering Vaccines for Tissue-Resident Memory T Cells

In recent years, tissue-resident memory T cells (TRM) have attracted significant attention in the field of vaccine development. Distinct from central and effector memory T cells, TRM cells take up residence in home tissues such as the lung or urogenital tract and are ideally positioned to respond quickly to pathogen encounter. TRM have been found to play a role in the immune response against many globally important infectious diseases for which new or improved vaccines are needed, including influenza and tuberculosis. It is also increasingly clear that TRM play a pivotal role in cancer immunity. Thus, vaccines that can generate this memory T cell population are highly desirable. The field of immunoengineering-that is, the application of engineering principles to study the immune system and design new and improved therapies that harness or modulate immune responses-is ideally poised to provide solutions to this need for next-generation TRM vaccines. This review covers recent developments in vaccine technologies for generating TRM and protecting against infection and cancer, including viral vectors, virus-like particles, and synthetic and natural biomaterials. In addition, it offers critical insights on the future of engineering vaccines for tissue-resident memory T cells.

Implication of CD69+ CD103+ tissue-resident-like CD8+ T cells as a potential immunotherapeutic target for cholangiocarcinoma

BACKGROUND

The heterogeneous immune landscapes of intrahepatic cholangiocarcinoma (ICC) remain largely unknown. Here we aimed to investigate the implications of tissue-resident memory (TRM)-related features of tumour-infiltrating CD8⁺ T cells (CD8⁺ TILs) from ICC patients.

METHODS

From ICC patients, we obtained blood samples and ICC surgical specimens (n = 33). We performed multicolour flow cytometry, multiplexed immunohistochemistry and RNA sequencing.

RESULTS

When compared to peripheral CD8⁺ T cells, the CD8⁺ TILs included significantly higher proportions of the CD69⁺ CD103^- and CD69⁺ CD103⁺ TRM-like subsets (P < .001 for both). Relative to CD69^- and CD69⁺ CD103^- cells, the CD69⁺ CD103⁺ CD8⁺ TILs harboured higher levels of T-cell markers representing tumour specificity (ie CD39), proliferation (ie Ki-67) and T-cell activation (ie HLA-DR and CD38) (all P < .001). Moreover, compared to the stroma, the tumour margin and core density each had a significantly higher density of CD103⁺ CD8⁺ TILs (P < .001 for both). ICCs with high proportions of CD69⁺ CD103⁺ cells displayed higher levels of parameters associated with response to immune checkpoint inhibitors (ICIs)-including number of CD8⁺ TIL infiltrates (P = .019), PD-L1 expression in the tumour (P = .046) and expression of the T cell-inflamed gene signature (P < .001). ICCs with lower proportions of CD69⁺ CD103⁺ CD8⁺ TILs exhibited significant enrichment of genes related to the Wnt/β-catenin (P < .001) and TGF-β pathways (P = .002).

CONCLUSION

CD69⁺ CD103⁺ TRM-like CD8⁺ TILs represent prominent tumour-specific immune responses and hold promise as a potential therapeutic target in ICC patients. Differential TRM-related features of ICCs may help develop future immunotherapeutic strategies such as maximizing TRM responses or inhibiting pathways contributing to immune evasion.

Rapid Isolation of Functional ex vivo Human Skin Tissue-Resident Memory T Lymphocytes

Studies in animal models have shown that skin tissue-resident memory T (T_RM) cells provide enhanced and immediate effector function at the site of infection. However, analyses of skin T_RM cells in humans have been hindered by the lack of an optimized isolation protocol. Here, we present a combinatorial strategy-the 6-h collagenase IV digestion and gentle tissue dissociation – for rapid and efficient isolation of skin T_RM cells with skin tissue-specific immune features. In comparison with paired blood circulating memory T cells, these ex vivo isolated skin T cells express typical T_RM cell markers and display higher polyfunctional properties. Moreover, these isolated cells can also be assessed for longer periods of time in ex vivo cultures. Thus, the optimized isolation protocol provides a valuable tool for further understanding of human skin T_RM cells, especially for direct comparison with peripheral blood T cells at the same sample collection time.

Staphylococcus aureus-Specific Tissue-Resident Memory CD4+ T Cells Are Abundant in Healthy Human Skin

The skin is an immunocompetent tissue that harbors several kinds of immune cells and a plethora of commensal microbes constituting the skin microbiome. Staphylococcus aureus is a prominent skin pathogen that colonizes a large proportion of the human population. We currently have an incomplete understanding of the correlates of protection against S. aureus infection, however genetic and experimental evidence has shown that CD4⁺ T cells play a key role in orchestrating a protective anti-S. aureus immune response. A high S. aureus-specific memory CD4⁺ T cell response has been reported in the blood of healthy subjects. Since T cells are more abundant in the skin than in blood, we hypothesized that S. aureus-specific CD4⁺ T cells could be present in the skin of healthy individuals. Indeed, we observed proliferation of tissue-resident memory CD4⁺ T cells and production of IL-17A, IL-22, IFN-γ and TNF-β by cells isolated from abdominal skin explants in response to heat-killed S. aureus. Remarkably, these cytokines were produced also during an ex vivo epicutaneous S. aureus infection of human skin explants. These findings highlight the importance of tissue-resident memory CD4⁺ T cells present at barrier sites such as the skin, a primary entry site for S. aureus. Further phenotypical and functional characterization of these cells will ultimately aid in the development of novel vaccine strategies against this elusive pathogen.

Legend of the Sentinels: Development of Lung Resident Memory T Cells and Their Roles in Diseases

SARS-CoV-2 is wreaking havoc around the world. To get the world back on track, hundreds of vaccines are under development. A deeper understanding of how the immune system responds to SARS-CoV-2 re-infection will certainly help. Studies have highlighted various aspects of T cell response in resolving acute infection and preventing re-infections. Lung resident memory T (TRM) cells are sentinels in the secondary immune response. They are mostly differentiated from effector T cells, construct specific niches and stay permanently in lung tissues. If the infection recurs, locally activated lung TRM cells can elicit rapid immune response against invading pathogens. In addition, they can significantly limit tumor growth or lead to pathologic immune responses. Vaccines targeting TRM cells are under development, with the hope to induce stable and highly reactive lung TRM cells through mucosal administration or "prime-and-pull" strategy. In this review, we will summarize recent advances in lung TRM cell generation and maintenance, explore their roles in different diseases and discuss how these cells may guide the development of future vaccines targeting infectious disease, cancer, and pathologic immune response.

T Cell Immunity against Influenza: The Long Way from Animal Models Towards a Real-Life Universal Flu Vaccine

Current flu vaccines rely on the induction of strain-specific neutralizing antibodies, which leaves the population vulnerable to drifted seasonal or newly emerged pandemic strains. Therefore, universal flu vaccine approaches that induce broad immunity against conserved parts of influenza have top priority in research. Cross-reactive T cell responses, especially tissue-resident memory T cells in the respiratory tract, provide efficient heterologous immunity, and must therefore be a key component of universal flu vaccines. Here, we review recent findings about T cell-based flu immunity, with an emphasis on tissue-resident memory T cells in the respiratory tract of humans and different animal models. Furthermore, we provide an update on preclinical and clinical studies evaluating T cell-evoking flu vaccines, and discuss the implementation of T cell immunity in real-life vaccine policies.

Skin-resident natural killer T cells participate in cutaneous allergic inflammation in atopic dermatitis

BACKGROUND

Natural killer T (NKT) cells are unconventional T cells that bridge innate and adaptive immunity. NKT cells have been implicated in the development of atopic dermatitis (AD).

OBJECTIVE

We aimed to investigate the role of NKT cells in AD development, especially in skin.

METHODS

Global proteomic and transcriptomic analyses were performed by using skin and blood from human healthy-controls and patients with AD. Levels of CXCR4 and CXCL12 expression in skin NKT cells were analyzed in human AD and mouse AD models. By using parabiosis and intravital imaging, the role of skin CXCR4⁺ NKT cells was further evaluated in models of mice with AD by using CXCR4-conditionally deficient or CXCL12 transgenic mice.

RESULTS

CXCR4 and its cognate ligand CXCL12 were significantly upregulated in the skin of humans with AD by global transcriptomic and proteomic analyses. CXCR4⁺ NKT cells were enriched in AD skin, and their levels were consistently elevated in our models of mice with AD. Allergen-induced NKT cells participate in cutaneous allergic inflammation. Similar to tissue-resident memory T cells, the predominant skin NKT cells were CXCR4⁺ and CD69⁺. Skin-resident NKT cells uniquely expressed CXCR4, unlike NKT cells in the liver, spleen, and lymph nodes. Skin fibroblasts were the main source of CXCL12. CXCR4⁺ NKT cells preferentially trafficked to CXCL12-rich areas, forming an enriched CXCR4⁺ tissue-resident NKT cells/CXCL12⁺ cell cluster that developed in acute and chronic allergic inflammation in our models of mice with AD.

CONCLUSIONS

CXCR4⁺ tissue-resident NKT cells may form a niche that contributes to AD, in which CXCL12 is highly expressed.

Total Recall: Intestinal TRM Cells in Health and Disease

Tissue-resident memory T cells (T_RM cells) have crucial functions in host defense in mucosal tissues. They provide local adaptive immune surveillance and allow the fast initiation of targeted adaptive immune responses in case of antigen re-exposure. Recently, an aberrant activation in the case of immunologically mediated diseases has been increasingly acknowledged. As the organ with the largest interface to the environment, the gastrointestinal tract faces billions of antigens every day. Tightly balanced processes are necessary to ensure tolerance towards non-hazardous antigens, but to set up a powerful immune response against potentially dangerous ones. In this complex nexus of immune cells and their mediators, T_RM cells play a central role and have been shown to promote both physiological and pathological events. In this review, we will summarize the current knowledge on the homeostatic functions of T_RM cells and delineate their implication in infection control in the gut. Moreover, we will outline their commitment in immune dysregulation in gastrointestinal chronic inflammatory conditions and shed light on T_RM cells as current and potential future therapeutic targets.

Tissue-resident memory T cells invade the brain parenchyma in multiple sclerosis white matter lesions

Multiple sclerosis is a chronic inflammatory, demyelinating disease, although it has been suggested that in the progressive late phase, inflammatory lesion activity declines. We recently showed in the Netherlands Brain Bank multiple sclerosis-autopsy cohort considerable ongoing inflammatory lesion activity also at the end stage of the disease, based on microglia/macrophage activity. We have now studied the role of T cells in this ongoing inflammatory lesion activity in chronic multiple sclerosis autopsy cases. We quantified T cells and perivascular T-cell cuffing at a standardized location in the medulla oblongata in 146 multiple sclerosis, 20 neurodegenerative control and 20 non-neurological control brain donors. In addition, we quantified CD3+, CD4+, and CD8+ T cells in 140 subcortical white matter lesions. The location of CD8+ T cells in either the perivascular space or the brain parenchyma was determined using CD8/laminin staining and confocal imaging. Finally, we analysed CD8+ T cells, isolated from fresh autopsy tissues from subcortical multiple sclerosis white matter lesions (n = 8), multiple sclerosis normal-appearing white matter (n = 7), and control white matter (n = 10), by flow cytometry. In normal-appearing white matter, the number of T cells was increased compared to control white matter. In active and mixed active/inactive lesions, the number of T cells was further augmented compared to normal-appearing white matter. Active and mixed active/inactive lesions were enriched for both CD4+ and CD8+ T cells, the latter being more abundant in all lesion types. Perivascular clustering of T cells in the medulla oblongata was only found in cases with a progressive disease course and correlated with a higher percentage of mixed active/inactive lesions and a higher lesion load compared to cases without perivascular clusters in the medulla oblongata. In all white matter samples, CD8+ T cells were located mostly in the perivascular space, whereas in mixed active/inactive lesions, 16.3% of the CD8+ T cells were encountered in the brain parenchyma. CD8+ T cells from mixed active/inactive lesions showed a tissue-resident memory phenotype with expression of CD69, CD103, CD44, CD49a, and PD-1 and absence of S1P1. They upregulated markers for homing (CXCR6), reactivation (Ki-67), and cytotoxicity (GPR56), yet lacked the cytolytic enzyme granzyme B. These data show that in chronic progressive multiple sclerosis cases, inflammatory lesion activity and demyelinated lesion load is associated with an increased number of T cells clustering in the perivascular space. Inflammatory active multiple sclerosis lesions are populated by CD8+ tissue-resident memory T cells, which show signs of reactivation and infiltration of the brain parenchyma.

Lost Soldiers or Special Forces?

The immune system plays a critical role during pregnancy, but the specific mechanisms and immune cell function needed to support pregnancy remain incompletely understood. Despite decades of research efforts, it is still unclear how the immune system maintains tolerance of fetal-derived tissues, which include most cells of the placenta and of course the fetus itself, without forfeiting the ability to protect against harmful infections. T cells recognize antigen in the context of major histocompatibility complex (MHC) encoded proteins, but classical MHC class I and II expression are diminished in fetal-derived cells. Can T cells present at the maternal–fetal interface (MFI) protect these cells from infection? Here we review what is known in regard to tissue-resident memory T (Trm) cells at the MFI. We mainly focus on how Trm cells can contribute to protection in the context of the unique features of the MFI, such as limited MHC expression as well as the temporary nature of the MFI, that are not found in other tissues.

T Cell Immunity and the Quest for Protective Vaccines against Staphylococcus aureus Infection

Staphylococcus aureus is a wide-spread human pathogen, and one of the top causative agents of nosocomial infections. The prevalence of antibiotic-resistant S. aureus strains, which are associated with higher mortality and morbidity rates than antibiotic-susceptible strains, is increasing around the world. Vaccination would be an effective preventive measure against S. aureus infection, but to date, every vaccine developed has failed in clinical trials, despite inducing robust antibody responses. These results suggest that induction of humoral immunity does not suffice to confer protection against the infection. Evidence from studies in murine models and in patients with immune defects support a role of T cell-mediated immunity in protective responses against S. aureus. Here, we review the current understanding of the mechanisms underlying adaptive immunity to S. aureus infections and discuss these findings in light of the recent S. aureus vaccine trial failures. We make the case for the need to develop anti-S. aureus vaccines that can specifically elicit robust and durable protective memory T cell subsets.

Can immunological manipulation defeat SARS-CoV-2? Why G-CSF induced neutrophil expansion is worth a clinical trial: G-CSF treatment against COVID-19

Immunity against SARS-CoV-2 that is acquired by convalescent COVID-19 patients is examined in reference to (A) the Th17 cell generation system in psoriatic epidermis and (B) a recently discovered phenomenon in which Th17 cells are converted into tissue-resident memory T (T_RM ) cells with Th1 phenotype. Neutrophils that are attracted to the site of infection secrete IL-17A, which stimulates lung epithelial cells to express CCL20. Natural Th17 (nTh17) cells are recruited to the infection site by CCL20 and expand in the presence of IL-23. These nTh17 cells are converted to T_RM cells upon encounter with SARS-CoV-2 and continue to exist as ex-Th17 cells, which exert Th1-like immunity during a memory response. G-CSF can induce nTh17 cell accumulation at the infection site because it promotes neutrophil egress from the bone marrow. Hence, G-CSF may be effective against COVID-19. Administration of G-CSF to patients infected with SARS-CoV-2 is worth a clinical trial.

Induction of Progenitor Exhausted Tissue-Resident Memory CD8+ T Cells Upon Salmonella Typhi Porins Adjuvant Immunization Correlates With Melanoma Control and Anti-PD-1 Immunotherapy Cooperation

Immunotherapy has improved the clinical response in melanoma patients, although a relevant percentage of patients still cannot be salvaged. The search for the immune populations that provide the best tumor control and that can be coaxed by immunotherapy strategies is a hot topic in cancer research nowadays. Tumor-infiltrating TCF-1⁺ progenitor exhausted CD8⁺ T cells seem to grant the best melanoma prognosis and also efficiently respond to anti-PD-1 immunotherapy, giving rise to a TIM-3⁺ terminally exhausted population with heightened effector activity. We tested Porins from Salmonella Typhi as a pathogen associated molecular pattern adjuvant of natural or model antigen in prophylactic and therapeutic immunization approaches against murine melanoma. Porins induced protection against melanomas, even upon re-challenging of tumor-free mice. Porins efficiently expanded IFN-γ-producing CD8⁺ T cells and induced central and effector memory in lymph nodes and tissue-resident (Trm) T cells in the skin and tumors. Porins induced TCF-1⁺ PD-1⁺ CD8⁺ Trm T cells in the tumor stroma and the presence of this population correlated with melanoma growth protection in mice. Porins immunization also cooperated with anti-PD-1 immunotherapy to hamper melanoma growth. Importantly, the potentially protective Trm populations induced by Porins in the murine model were also observed in melanoma patients in which their presence also correlated with disease control. Our data support the use of cancer vaccination to sculpt the tumor stroma with efficient and lasting Trm T cells with effector activities, highlighting the use of Porins as an adjuvant. Furthermore, our data place CD8⁺ Trm T cells with a progenitor exhausted phenotype as an important population for melanoma control, either independently or in cooperation with anti-PD-1 immunotherapy.

White matter lesions in multiple sclerosis are enriched for CD20dim CD8+ tissue-resident memory T cells

Brain CD8⁺ CD69⁺ tissue-resident memory T (T_RM ) cells comprise a CD20^dim subset, which is proportionally larger in CD103-negative T_RM cells. In multiple sclerosis (MS) lesions, CD20^dim T_RM -cell proportions are increased. CD20-expression is associated with higher levels of CXCR6, Ki-67, and granzyme B, supporting CD20^dim T_RM cells as a relevant subset in MS.

Immunological Memory in Imiquimod-Induced Murine Model of Psoriasiform Dermatitis

Psoriasis is a common chronic inflammatory skin condition manifested by T cell responses and characterized by preferential recurrence at previously inflamed sites upon withdrawal of treatment. The site-specific disease memory in psoriasis has been linked to CD8⁺CD103⁺ tissue-resident memory T cells (Trm) in the epidermis which were previously thought to only provide “frontline” protection against pathogens and immunosurveillance during cancer development. In this study, we correlated the presence of a subset of the Trm cells which are also CD49a⁺ with disease severity in human psoriatic lesions with acute and chronic disease. Using an imiquimod (IMQ)-induced murine model of psoriasiform dermatitis, we also investigated the level of CD49a⁺ Trm cells in acute, chronic and resolved psoriatic lesions. Investigation of clinical human samples showed that patient disease severity highly correlated with the numbers of epidermal CD49a⁺ Trm cells. Additionally, this subset of Trm cells was shown to persist in resolved lesions of murine psoriasiform dermatitis once clinical disease features had subsided. Importantly, these CD49a⁺ Trm cells showed significantly higher levels of granzyme B (GzmB) production compared to acute disease, suggesting a potential role of CD49a⁺ Trm cells for psoriatic re-occurrence in resolved patients. Better understanding of epidermal CD49a⁺ Trm cell activity is necessary for development of advanced treatment strategies for psoriasis to permit long-term, continuous disease control.

NK1.1+ innate lymphoid cells in salivary glands inhibit establishment of tissue-resident memory CD8+ T cells in mice

NK1.1⁺ cells found in salivary glands (SG) represent a unique cell population of innate lymphoid cells (ILC) with characteristics of both conventional NK cells and ILC1. Here, we demonstrate that these NK1.1⁺  cells limit the accumulation and differentiation of virus-specific tissue-resident memory CD8⁺ T cells (T_RM  cells) in SG of mice infected with lymphocytic choriomeningitis virus (LCMV). The negative regulation of LCMV-specific CD8⁺ T_RM  cells by NK1.1⁺  cells in SG is independent of NKG2D, NKp46, TRAIL, and perforin. Moreover, analysis of NKp46^iCre+ Eomes^fl/fl mice revealed that Eomes-dependent conventional NK cells are dispensable for negative regulation. Since the SG are prone to autoimmune reactions, regulation of T_RM  cells by tissue-resident ILC may be particularly important to prevent immunopathology in this organ.

Perivascular tissue resident memory T cells as therapeutic target in multiple sclerosis

INTRODUCTION

Multiple sclerosis (MS) is characterized by inflammatory attacks of infiltrating leukocytes at onset but evolves into a smoldering, progressive disease within the central nervous system at its later stages. The authors discuss the contribution of white matter lesions to the pathology of advanced MS, thereby paying particular attention to the role of T cells.

AREAS COVERED

Diagnostic biopsy and autopsy studies of white matter lesions in early MS show different pathological patterns of demyelination and leukocyte infiltration. Brain autopsies from advanced MS display substantial inflammation without distinct patterns and suggest a role for perivascular CD8⁺ tissue-resident memory T (T_RM) cells in active and mixed active/inactive MS white matter lesions. When compared to control and normal-appearing white matter, these lesions are enriched for parenchymal CD8⁺ T cells. In the perivascular space, cuffs containing CD8⁺ T_RM cells are observed also in progressive MS, and could be sites of local reactivation.

EXPERT OPINION

Recent findings point toward the perivascular space as an immunological hotspot, which could be targeted in order to suppress a contribution of T_RM cells to ongoing white matter lesion activity in advanced progressive MS. The authors discuss approaches, which may be explored to suppress T_RM-cell reactivation in the perivascular space.