Dermal-resident versus recruited γδ T cell response to cutaneous vaccinia virus infection

T Cell Surveillance during Cutaneous Viral Infections

The skin is a complex tissue that provides a strong physical barrier against invading pathogens. Despite this, many viruses can access the skin and successfully replicate in either the epidermal keratinocytes or dermal immune cells. In this review, we provide an overview of the antiviral T cell biology responding to cutaneous viral infections and how these responses differ depending on the cellular targets of infection. Much of our mechanistic understanding of T cell surveillance of cutaneous infection has been gained from murine models of poxvirus and herpesvirus infection. However, we also discuss other viral infections, including flaviviruses and papillomaviruses, in which the cutaneous T cell response has been less extensively studied. In addition to the mechanisms of successful T cell control of cutaneous viral infection, we highlight knowledge gaps and future directions with possible impact on human health.

γδ T Cells Mediate a Requisite Portion of a Wound Healing Response Triggered by Cutaneous Poxvirus Infection

Infection at barrier sites, e.g., skin, activates local immune defenses that limit pathogen spread, while preserving tissue integrity. Phenotypically distinct γδ T cell populations reside in skin, where they shape immunity to cutaneous infection prior to onset of an adaptive immune response by conventional αβ CD4+ (TCD4+) and CD8+ (TCD8+) T cells. To examine the mechanisms used by γδ T cells to control cutaneous virus replication and tissue pathology, we examined γδ T cells after infection with vaccinia virus (VACV). Resident γδ T cells expanded and combined with recruited γδ T cells to control pathology after VACV infection. However, γδ T cells did not play a role in control of local virus replication or blockade of systemic virus spread. We identified a unique wound healing signature that has features common to, but also features that antagonize, the sterile cutaneous wound healing response. Tissue repair generally occurs after clearance of a pathogen, but viral wound healing started prior to the peak of virus replication in the skin. γδ T cells contributed to wound healing through induction of multiple cytokines/growth factors required for efficient wound closure. Therefore, γδ T cells modulate the wound healing response following cutaneous virus infection, maintaining skin barrier function to prevent secondary bacterial infection.

Widespread and dynamic expression of granzyme C by skin-resident antiviral T cells

After recognition of cognate antigen (Ag), effector CD8+ T cells secrete serine proteases called granzymes in conjunction with perforin, allowing granzymes to enter and kill target cells. While the roles for some granzymes during antiviral immune responses are well characterized, the function of others, such as granzyme C and its human ortholog granzyme H, is still unclear. Granzyme C is constitutively expressed by mature, cytolytic innate lymphoid 1 cells (ILC1s). Whether other antiviral effector cells also produce granzyme C and whether it is continually expressed or responsive to the environment is unknown. To explore this, we analyzed granzyme C expression in different murine skin-resident antiviral lymphocytes. At steady-state, dendritic epidermal T cells (DETCs) expressed granzyme C while dermal γδ T cells did not. CD8+ tissue-resident memory T cells (TRM) generated in response to cutaneous viral infection with the poxvirus vaccinia virus (VACV) also expressed granzyme C. Both DETCs and virus-specific CD8+ TRM upregulated granzyme C upon local VACV infection. Continual Ag exposure was not required for maintained TRM expression of granzyme C, although re-encounter with cognate Ag boosted expression. Additionally, IL-15 treatment increased granzyme C expression in both DETCs and TRM. Together, our data demonstrate that granzyme C is widely expressed by antiviral T cells in the skin and that expression is responsive to both environmental stimuli and TCR engagement. These data suggest that granzyme C may have functions other than killing in tissue-resident lymphocytes.

Murine CXCR3+CXCR6+γδT Cells Reside in the Liver and Provide Protection Against HBV Infection

Gamma delta (γδ) T cells play a key role in the innate immune response and serve as the first line of defense against infection and tumors. These cells are defined as tissue-resident lymphocytes in skin, lung, and intestinal mucosa. They are also relatively abundant in the liver; however, little is known about the residency of hepatic γδT cells. By comparing the phenotype of murine γδT cells in liver, spleen, thymus, and small intestine, a CXCR3⁺CXCR6⁺ γδT-cell subset with tissue-resident characteristics was found in liver tissue from embryos through adults. Liver sinusoidal endothelial cells mediated retention of CXCR3⁺CXCR6⁺ γδT cells through the interactions between CXCR3 and CXCR6 and their chemokines. During acute HBV infection, CXCR3⁺CXCR6⁺ γδT cells produced high levels of IFN-γ and adoptive transfer of CXCR3⁺CXCR6⁺ γδT cells into acute HBV-infected TCRδ^−/− mice leading to lower HBsAg and HBeAg expression. It is suggested that liver resident CXCR3⁺CXCR6⁺ γδT cells play a protective role during acute HBV infection. Strategies aimed at expanding and activating liver resident CXCR3⁺CXCR6⁺ γδT cells both in vivo or in vitro have great prospects for use in immunotherapy that specifically targets acute HBV infection.

Evolved to protect, designed to destroy: IL-17-producing γδ T cells in infection, inflammation, and cancer

T cells of the gamma delta (γδ) lineage are evolutionary conserved from jawless to cartilaginous and bony fish to mammals and represent the "swiss army knife" of the immune system capable of antigen-dependent or independent responses, memory, antigen presentation, regulation of other lymphocytes, tissue homeostasis, and mucosal barrier maintenance, to list a few. Over the last 10 years, γδ T cells that produce the cytokine IL-17 (γδT17) have taken a leading position in our understanding of how our immune system battles infection, inflicts tissue damage during inflammation, and gets rewired by the tumor microenvironment. A lot of what we know about γδT17 cells stems from mouse models, however, increasing evidence implicates these cells in numerous human diseases. Herein, we aim to give an overview of the most common mouse models that have been used to study the role of γδT17 cells in infection, inflammation, and cancer, while at the same time we will evaluate evidence for their importance in humans. We hope and believe that in the next 10 years, means to take advantage of the protective and destructive properties of γδ T and in particular γδT17 cells will be part of our standard immunotherapy toolkit.

Tissues: the unexplored frontier of antibody mediated immunity

Pathogen-specific immunity evolves in the context of the infected tissue. However, current immune correlates analyses and vaccine efficacy metrics are based on immune functions from peripheral cells. Less is known about tissue-resident mechanisms of immunity. While antibodies represent the primary correlate of immunity following most clinically approved vaccines, how antibodies interact with localized, compartment-specific immune functions to fight infections, remains unclear. Emerging data demonstrate a unique community of immune cells that reside within different tissues. These tissue-specific immunological communities enable antibodies to direct both expected and unexpected local attack strategies to control, disrupt, and eliminate infection in a tissue-specific manner. Defining the full breadth of antibody effector functions, how they selectively contribute to control at the site of infection may provide clues for the design of next-generation vaccines able to direct the control, elimination, and prevention of compartment specific diseases of both infectious and non-infectious etiologies.

Dissecting the complexity of γδ T-cell subsets in skin homeostasis, inflammation, and malignancy

γδ T cells are much less common than αβ T cells, accounting for 0.5% to 5% of all T lymphocytes in the peripheral blood and lymphoid tissues in mice and humans. However, they are the most abundant T-lymphocyte subset in some epithelial barriers such as mouse skin. γδ T cells are considered innate lymphocytes because of their non-MHC restricted antigen recognition, as well as because of their rapid response to cytokines, invading pathogens, and malignant cells. Exacerbated expansion and activation of γδ T cells in the skin is a common feature of acute and chronic skin inflammation such as psoriasis and contact or atopic dermatitis. Different γδ T-cell subsets showing differential developmental and functional features are found in mouse and human skin. This review discusses the state of the art of research and future perspectives about the role of the different subsets of γδ T-cells detected in the skin in steady-state, psoriasis, dermatitis, infection, and malignant skin diseases. Also, we highlight the differences between human and mouse γδ T cells in skin homeostasis and inflammation, as understanding the differential role of each subtype of skin γδ T cells will improve the discovery of new therapies.

CCR6-Positive γδ T Cells Provide Protection Against Intracorneal HSV-1 Infection

Purpose

γδ T cells offer an important early immune defense against many different pathogens, both bacterial and viral. Herein, we examined the capacity of γδ T cell subsets to provide protection in the cornea against herpes simplex virus-1 (HSV-1).

Methods

C57Bl/6 (wild-type [WT]), γδ T-cell deficient (TCRδ-/-) and CCR6-deficient (CCR6-/-) mice were infected intracorneally with HSV-1. At multiple time points following infection, corneas were excised, and cells were immunostained for surface markers, intracellular cytokines, and analyzed using flow cytometry. WT and CCR6-/- γδ T cells were adoptively transferred into TCRδ-/- mice and corneal scores and survival were measured.

Results

Intracorneal infection of mice lacking γδ T cells exhibited increased corneal opacity scores, elevated viral titers, and higher mortality compared with WT mice. Both CCR6+ and CCR6neg γδ T cell subsets were observed in corneas after virus infection. CCR6+ γδ T cells produced IL-17A and were predominantly CD44+CD62L+, consistent with natural IL-17+ γδ T cells. In contrast IL-17A production by CCR6neg γδ T cells was infrequent, and this subset was largely single positive for CD62L or CD44. The CCR6+ subset appeared to provide protection against HSV-1 as follows: (1) CCR6-/- mice had more severe corneal opacity compared with WT mice; and (2) adoptive transfer of γδ T cells from WT mice restored protection in TCRδ-/- mice whereas transfer of γδ T cells from CCR6-/- mice did not.

Conclusions

γδ T cells in the cornea can be divided into CCR6+ and CCR6neg subsets with the former conferring protection early after intracorneal HSV-1 infection.

The Roles of Liver-Resident Lymphocytes in Liver Diseases

Tissue-resident lymphocytes usually reside in barrier sites and are involved in innate and adaptive immunity. In recent years, many studies have shown that multiple types of lymphocytes are resident in the liver, including memory CD8⁺ T (T_RM) cells; "unconventional" T cells, such as invariant natural killer T (iNKT) cells, mucosal associated invariant T (MAIT) cells, and γδT cells; innate lymphoid cells (ILCs) such as natural killer (NK) cells and other ILCs. Although diverse types of tissue-resident lymphocytes share similar phenotypes, functional properties, and transcriptional regulation, the unique microenvironment of the liver can reshape their phenotypic and functional characteristics. Liver-resident lymphocytes serve as sentinels and perform immunosurveillance in response to infection and non-infectious insults, and are involved in the maintenance of liver homeostasis. Under the pathological conditions, distinct liver-resident lymphocytes exert protective or pathological effects in the process of various liver diseases. In this review, we highlight the unique properties of liver-resident lymphocytes, and discuss their functional characteristics in different liver diseases.

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.

Immunological memories of the bone marrow

Memory for antigens once encountered is a hallmark of the immune system of vertebrates, providing us with an immunity adapted to pathogens of our environment. Despite its fundamental relevance, the cells and genes representing immunological memory are still poorly understood. Here we discuss the concept of a circulating, proliferating, and ubiquitous population of effector lymphocytes vs concepts of resting and dormant populations of dedicated memory lymphocytes, distinct from effector lymphocytes and residing in defined tissues, particularly in barrier tissues and in the bone marrow. The lifestyle of memory plasma cells of the bone marrow may serve as a paradigm, showing that persistence of memory lymphocytes is not defined by intrinsic "half-lives", but rather conditional on distinct survival signals provided by dedicated niches. These niches are organized by individual mesenchymal stromal cells. They define the capacity of immunological memory and regulate its homeostasis.

IL-1β and IL-23 Promote Extrathymic Commitment of CD27+CD122- γδ T Cells to γδT17 Cells

γδT17 cells are a subset of γδ T cells committed to IL-17 production and are characterized by the expression of IL-23R and CCR6 and lack of CD27 expression. γδT17 cells are believed to arise within a narrow time window during prenatal thymic development. In agreement with this concept, we show in this study that adult Rag1^-/- recipient mice of Il23r^gfp/+ (IL-23R reporter) bone marrow selectively lack IL-23R⁺ γδT17 cells. Despite their absence in secondary lymphoid tissues during homeostasis, γδT17 cells emerge in bone marrow chimeric mice upon induction of skin inflammation by topical treatment with imiquimod cream (Aldara). We demonstrate that IL-1β and IL-23 together are able to promote the development of bona fide γδT17 cells from peripheral CD122^-IL-23R^- γδ T cells, whereas CD122⁺ γδ T cells fail to convert into γδT17 cells and remain stable IFN-γ producers (γδT1 cells). IL-23 is instrumental in expanding extrathymically generated γδT17 cells. In particular, TCR-Vγ4⁺ chain-expressing CD122^-IL-23R^- γδ T cells are induced to express IL-23R and IL-17 outside the thymus during skin inflammation. In contrast, TCR-Vγ1⁺ γδ T cells largely resist this process because prior TCR engagement in the thymus has initiated their commitment to the γδT1 lineage. In summary, our data reveal that the peripheral pool of γδ T cells retains a considerable degree of plasticity because it harbors "naive" precursors, which can be induced to produce IL-17 and replenish peripheral niches that are usually occupied by thymus-derived γδT17 cells.

IL-17A-producing resident memory γδ T cells orchestrate the innate immune response to secondary oral Listeria monocytogenes infection

Memory γδ T cells are important for the clearance of Listeria monocytogenes infection in the intestinal mucosa. However, the mechanisms by which memory γδ T cells provide protection against secondary oral infection are poorly understood. Here we used a recombinant strain of L. monocytogenes that efficiently invades the intestinal epithelium to show that Vγ4(+) memory γδ T cells represent a resident memory (Trm) population in the mesenteric lymph nodes (MLNs). The γδ Trm exhibited a remarkably static pattern of migration that radically changed following secondary oral L. monocytogenes infection. The γδ Trms produced IL-17A early after rechallenge and formed organized clusters with myeloid cells surrounding L. monocytogenes replication foci only after a secondary oral infection. Antibody blocking studies showed that in addition to IL-17A, the chemokine receptor C-X-C chemokine receptor 3 (CXCR3) is also important to enable the local redistribution of γδ Trm cells and myeloid cells specifically near the sites of L. monocytogenes replication within the MLN to restrict bacterial growth and spread. Our findings support a role for γδ Trms in orchestrating protective immune responses against intestinal pathogens.

Nociceptive Sensory Fibers Drive Interleukin-23 Production from CD301b+ Dermal Dendritic Cells and Drive Protective Cutaneous Immunity

Innate resistance to Candida albicans in mucosal tissues requires the production of interleukin-17A (IL-17A) by tissue-resident cells early during infection, but the mechanism of cytokine production has not been precisely defined. In the skin, we found that dermal γδ T cells were the dominant source of IL-17A during C. albicans infection and were required for pathogen resistance. Induction of IL-17A from dermal γδ T cells and resistance to C. albicans required IL-23 production from CD301b(+) dermal dendritic cells (dDCs). In addition, we found that sensory neurons were directly activated by C. albicans. Ablation of sensory neurons increased susceptibility to C. albicans infection, which could be rescued by exogenous addition of the neuropeptide CGRP. These data define a model in which nociceptive pathways in the skin drive production of IL-23 by CD301b(+) dDCs resulting in IL-17A production from γδ T cells and resistance to cutaneous candidiasis.

Inflammation induces dermal Vγ4+ γδT17 memory-like cells that travel to distant skin and accelerate secondary IL-17-driven responses

Gamma delta (γδ) T cells represent a major IL-17 committed T-cell population (γδT17 cells) in the mouse dermis. Following exposure to the inflammatory agent imiquimod (IMQ) the Vγ4(+) subset of γδT cells produce IL-17 in the skin and expand rapidly in draining lymph nodes (LNs). Local IMQ treatment in humans is known to exacerbate psoriasis skin lesion activity at distant sites. Whether expanded γδT17 cells sensitize distant sites to inflammation has been unknown. Here we show that expanded Vγ4(+) γδT17 cells egress from LNs in a fingolimod (FTY720)-sensitive manner and use C-C chemokine receptor type 2 to accumulate in inflamed skin where they augment neutrophil recruitment and inflammation. They also travel to noninflamed skin and peripheral LNs and remain in elevated numbers at these distant sites for at least 3 mo. Sensitized mice show more rapid skin inflammation and greater proliferation and IL-17 production by Vγ4(+) γδT cells upon imiquimod challenge. Transfer experiments confirm that memory-like Vγ4(+) γδT17 cells respond more rapidly. Memory-like Vγ4(+) γδT17 cells are distinguished by greater IL-1R1 expression and more proliferation in response to IL-1β. These findings establish that local skin inflammation leads to faster and stronger secondary responses to the same stimulus through long-term and systemic changes in the composition and properties of the dermal γδT-cell population.