Different patterns of peripheral migration by memory CD4+ and CD8+ T cells

Tissue-resident memory T cells in cytomegalovirus infection

Herpesviruses establish life-long infection in their hosts and maintain latent reservoirs for sporadic reactivation at peripheral sites, such as skin and mucosae. For herpes simplex virus infection, experimental studies in mice revealed that immediate protection against local reactivation or superinfection events in the skin relies on tissue resident memory T cells (TRM) rather than on their circulating counterparts. Recent evidence extends this notion to cytomegalovirus infection, which potently induces TRM cells in both mice and humans particularly in mucosal tissues that constitute important viral sanctuaries and are relevant entry sites for challenge and superinfections. The discovery unravels promising opportunities to exploit cytomegalovirus based vaccine vectors for the specific induction of tissue resident T cell subsets.

Efforts of the human immune system to maintain the peripheral CD8+ T cell compartment after childhood thymectomy

Background

Homeostatic mechanisms to maintain the T cell compartment diversity indicate an ongoing process of thymic activity and peripheral T cell renewal during human life. These processes are expected to be accelerated after childhood thymectomy and by the influence of cytomegalovirus (CMV) inducing a prematurely aged immune system.

The study aimed to investigate proportional changes and replicative history of CD8+ T cells, of recent thymic emigrants (RTEs) and CD103+ T cells (mostly gut-experienced) and the role of Interleukin-(IL)-7 and IL-7 receptor (CD127)-expressing T cells in thymectomized patients compared to young and old healthy controls.

Results

Decreased proportions of naive and CD31 + CD8+ T cells were demonstrated after thymectomy, with higher proliferative activity of CD127-expressing T cells and significantly shorter relative telomere lengths (RTLs) and lower T cell receptor excision circles (TRECs). Increased circulating CD103+ T cells and a skewed T cell receptor (TCR) repertoire were found after thymectomy similar to elderly persons. Naive T cells were influenced by age at thymectomy and further decreased by CMV.

Conclusions

After childhood thymectomy, the immune system demonstrated constant efforts of the peripheral CD8+ T cell compartment to maintain homeostasis. Supposedly it tries to fill the void of RTEs by peripheral T cell proliferation, by at least partly IL-7-mediated mechanisms and by proportional increase of circulating CD103+ T cells, reminiscent of immune aging in elderly. Although other findings were less significant compared to healthy elderly, early thymectomy demonstrated immunological alterations of CD8+ T cells which mimic features of premature immunosenescence in humans.

The Neonatal CD8+ T Cell Repertoire Rapidly Diversifies during Persistent Viral Infection

CMV is the most common congenital infection in the United States. The major target of congenital CMV is the brain, with clinical manifestations including mental retardation, vision impairment, and sensorineural hearing loss. Previous reports have shown that CD8(+) T cells are required to control viral replication and significant numbers of CMV-specific CD8(+) T cells persist in the brain even after the initial infection has been cleared. However, the dynamics of CD8(+) T cells in the brain during latency remain largely undefined. In this report, we used TCR sequencing to track the development and maintenance of neonatal clonotypes in the brain and spleen of mice during chronic infection. Given the discontinuous nature of tissue-resident memory CD8(+) T cells, we hypothesized that neonatal TCR clonotypes would be locked in the brain and persist into adulthood. Surprisingly, we found that the Ag-specific T cell repertoire in neonatal-infected mice diversified during persistent infection in both the brain and spleen, while maintaining substantial similarity between the CD8(+) T cell populations in the brain and spleen in both early and late infection. However, despite the diversification of, and potential interchange between, the spleen and brain Ag-specific T cell repertoires, we observed that germline-encoded TCR clonotypes, characteristic of neonatal infection, persisted in the brain, albeit sometimes in low abundance. These results provide valuable insights into the evolution of CD8(+) T cell repertoires following neonatal CMV infection and thus have important implications for the development of therapeutic strategies to control CMV in early life.

Interleukin-2-Dependent Allergen-Specific Tissue-Resident Memory Cells Drive Asthma

Exposure to inhaled allergens generates T helper 2 (Th2) CD4(+) T cells that contribute to episodes of inflammation associated with asthma. Little is known about allergen-specific Th2 memory cells and their contribution to airway inflammation. We generated reagents to understand how endogenous CD4(+) T cells specific for a house dust mite (HDM) allergen form and function. After allergen exposure, HDM-specific memory cells persisted as central memory cells in the lymphoid organs and tissue-resident memory cells in the lung. Experimental blockade of lymphocyte migration demonstrated that lung-resident cells were sufficient to induce airway hyper-responsiveness, which depended upon CD4(+) T cells. Investigation into the differentiation of pathogenic Trm cells revealed that interleukin-2 (IL-2) signaling was required for residency and directed a program of tissue homing migrational cues. These studies thus identify IL-2-dependent resident Th2 memory cells as drivers of lung allergic responses.

Tissue-resident memory T cells: local specialists in immune defence

T cells have crucial roles in protection against infection and cancer. Although the trafficking of memory T cells around the body is integral to their capacity to provide immune protection, studies have shown that specialization of some memory T cells into unique tissue-resident subsets gives the host enhanced regional immunity. In recent years, there has been considerable progress in our understanding of tissue-resident T cell development and function, revealing mechanisms for enhanced protective immunity that have the potential to influence rational vaccine design. This Review discusses the major advances and the emerging concepts in this field, summarizes what is known about the differentiation and the protective functions of tissue-resident memory T cells in different tissues in the body and highlights key unanswered questions.

Human skin is protected by four functionally and phenotypically discrete populations of resident and recirculating memory T cells

The skin of an adult human contains about 20 billion memory T cells. Epithelial barrier tissues are infiltrated by a combination of resident and recirculating T cells in mice, but the relative proportions and functional activities of resident versus recirculating T cells have not been evaluated in human skin. We discriminated resident from recirculating T cells in human-engrafted mice and lymphoma patients using alemtuzumab, a medication that depletes recirculating T cells from skin, and then analyzed these T cell populations in healthy human skin. All nonrecirculating resident memory T cells (TRM) expressed CD69, but most were CD4(+), CD103(-), and located in the dermis, in contrast to studies in mice. Both CD4(+) and CD8(+) CD103(+) TRM were enriched in the epidermis, had potent effector functions, and had a limited proliferative capacity compared to CD103(-) TRM. TRM of both types had more potent effector functions than recirculating T cells. We observed two distinct populations of recirculating T cells, CCR7(+)/L-selectin(+) central memory T cells (TCM) and CCR7(+)/L-selectin(-) T cells, which we term migratory memory T cells (TMM). Circulating skin-tropic TMM were intermediate in cytokine production between TCM and effector memory T cells. In patients with cutaneous T cell lymphoma, malignant TCM and TMM induced distinct inflammatory skin lesions, and TMM were depleted more slowly from skin after alemtuzumab, suggesting that TMM may recirculate more slowly. In summary, human skin is protected by four functionally distinct populations of T cells, two resident and two recirculating, with differing territories of migration and distinct functional activities.

Antibodies Are Required for Complete Vaccine-Induced Protection against Herpes Simplex Virus 2

Herpes simplex virus 2 (HSV-2) 0ΔNLS is a live HSV-2 ICP0- mutant vaccine strain that is profoundly attenuated in vivo due to its interferon-hypersensitivity. Recipients of the HSV-2 0ΔNLS vaccine are resistant to high-dose HSV-2 challenge as evidenced by profound reductions in challenge virus spread, shedding, disease and mortality. In the current study, we investigated the requirements for HSV-2 0ΔNLS vaccine-induced protection. Studies using (UV)-inactivated HSV-2 0ΔNLS revealed that self-limited replication of the attenuated virus was required for effective protection from vaginal or ocular HSV-2 challenge. Diminished antibody responses in recipients of the UV-killed HSV-2 vaccine suggested that antibodies might be playing a critical role in early protection. This hypothesis was investigated in B-cell-deficient μMT mice. Vaccination with live HSV-2 0ΔNLS induced equivalent CD8+ T cell responses in wild-type and μMT mice. Vaccinated μMT mice shed ~40-fold more infectious HSV-2 at 24 hours post-challenge relative to vaccinated wild-type (B-cell+) mice, and most vaccinated μMT mice eventually succumbed to a slowly progressing HSV-2 challenge. Importantly, passive transfer of HSV-2 antiserum restored full protection to HSV-2 0ΔNLS-vaccinated μMT mice. The results demonstrate that B cells are required for complete vaccine-induced protection against HSV-2, and indicate that virus-specific antibodies are the dominant mediators of early vaccine-induced protection against HSV-2.

Limited Internodal Migration of T Follicular Helper Cells after Peripheral Infection with Herpes Simplex Virus-1

The ability of CD4 T cells to give rise to specialized T follicular helper cells (TFH) critical to initiating appropriate Ab responses is regulated by environmental cues in lymphoid tissues draining the site of infection. In this study, we used a skin infection with HSV-1 characterized by the successive involvement of interconnected but distinct lymph nodes (LNs), to investigate the anatomical diversification of virus-specific CD4 T cell responses and the migratory capacity of TFH or their precursors. Whereas Th1 effector CD4 T cells expressing peripheral-targeting migration molecules readily migrated from primary to secondary reactive LNs, Bcl6(+) CXCR5(+) PD1(hi) TFH were largely retained at the site of initial activation with little spillover into the downstream LNs involved at later stages of infection. Consistent with this, TFH maintained high-level surface expression of CD69, indicative of impaired migratory capacity. Notably, the biased generation and retention of TFH in primary LNs correlated with a preferential generation of germinal centers at this site. Our results highlight a limited anatomical diversification of TFH responses and germinal center reactions that were imprinted within the first few cell divisions during TFH differentiation in LNs draining the site of initial infection.

T Cell Motility as Modulator of Interactions with Dendritic Cells

It is well established that the balance of costimulatory and inhibitory signals during interactions with dendritic cells (DCs) determines T cell transition from a naïve to an activated or tolerant/anergic status. Although many of these molecular interactions are well reproduced in reductionist in vitro assays, the highly dynamic motility of naïve T cells in lymphoid tissue acts as an additional lever to fine-tune their activation threshold. T cell detachment from DCs providing suboptimal stimulation allows them to search for DCs with higher levels of stimulatory signals, while storing a transient memory of short encounters. In turn, adhesion of weakly reactive T cells to DCs presenting peptides presented on major histocompatibility complex with low affinity is prevented by lipid mediators. Finally, controlled recruitment of CD8(+) T cells to cognate DC-CD4(+) T cell clusters shapes memory T cell formation and the quality of the immune response. Dynamic physiological lymphocyte motility therefore constitutes a mechanism to mitigate low avidity T cell activation and to improve the search for "optimal" DCs, while contributing to peripheral tolerance induction in the absence of inflammation.

Strategies that target leukocyte traffic in inflammatory bowel diseases: recent developments

PURPOSE OF REVIEW

We review the most recent developments regarding the targeting of molecules involved in the traffic of leukocytes for the treatment of inflammatory bowel diseases (IBD).

RECENT FINDINGS

We discuss the most important findings of one published phase II trial that targeted the β7 integrin (etrolizumab), two phase II trials that targeted the α4β7 integrin ligand: mucosal addressin cell adhesion molecule 1 (MAdCAM-1, PF-00547659), a phase II trial targeting the chemokine IP-10 (CXCL10) in Crohn's, and a phase II trial that targeted the sphingosine-1-phosphate receptor-1: ozanimod in patients with ulcerative colitis.

SUMMARY

Targeting molecules involved in leukocyte traffic has recently become an effective and well tolerated strategy for the treatment of IBD. Novel approaches now not only target the integrins on the lymphocyte surface, but also its endothelial ligand: MAdCAM-1. As with vedolizumab, antibodies against MAdCAM-1 appear most effective in ulcerative colitis rather than in Crohn's. Targeting chemokines or their receptors does not appear to have the same efficacy as those that target the most stable integrin: immunoglobulin superfamily interactions between the lymphocyte and endothelium. Preliminary results also suggest that the sphingosine-1-phosphate pathway might also be targeted therapeutically in IBD, no longer with parenterally administered antibodies but with orally administered small molecules.

Bacterial Vaginosis and Subclinical Markers of Genital Tract Inflammation and Mucosal Immunity

Bacterial vaginosis (BV) has been linked to an increased risk of human immunodeficiency virus (HIV) acquisition and transmission in observational studies, but the underlying biological mechanisms are unknown. We measured biomarkers of subclinical vaginal inflammation, endogenous antimicrobial activity, and vaginal flora in women with BV and repeated sampling 1 week and 1 month after completion of metronidazole therapy. We also compared this cohort of women with BV to a healthy control cohort without BV. A longitudinal, open label study of 33 women with a Nugent score of 4 or higher was conducted. All women had genital swabs, cervicovaginal lavage (CVL) fluid, and cervicovaginal biopsies obtained at enrollment and received 7 days of metronidazole treatment. Repeat sampling was performed approximately 1 week and 1 month after completion of therapy. Participant's baseline samples were compared to a healthy, racially matched control group (n=13) without BV. The CVL from women with resolved BV (Nugent 0-3) had significantly higher anti-HIV activity, secretory leukocyte protease inhibitor (SLPI), and growth-related oncogene alpha (GRO-α) levels and their ectocervical tissues had significantly more CD8 cells in the epithelium. Women with persistent BV after treatment had significantly higher levels of interleukin-1β, tumor necrosis factor alpha (TNF-α), and intercellular adhesion molecule 1 (ICAM-1) in the CVL. At study entry, participants had significantly greater numbers of CCR5(+) immune cells and a higher CD4/CD8 ratio in ectocervical tissues prior to metronidazole treatment, compared to a racially matched cohort of women with a Nugent score of 0-3. These data indicate that BV is associated with changes in select soluble immune mediators, an increase in HIV target cells, and a reduction in endogenous antimicrobial activity, which may contribute to the increased risk of HIV acquisition.

The Salivary Gland Acts as a Sink for Tissue-Resident Memory CD8(+) T Cells, Facilitating Protection from Local Cytomegalovirus Infection

Tissue-resident memory T cells (TRM) reside in barrier tissues and provide local immediate protective immunity. Here, we show that the salivary gland (SG) most-effectively induces CD8(+) and CD4(+) TRM cells against murine cytomegalovirus (MCMV), which persists in and spreads from this organ. TRM generation depended on local antigen for CD4(+), but not CD8(+), TRM cells, highlighting major differences in T cell subset-specific demands for TRM development. CMV-specific CD8(+) T cells fail to control virus replication upon primary infection in the SG due to CMV-induced MHC I downregulation in glandular epithelial cells. Using intraglandular infection, we challenge this notion and demonstrate that memory CD8(+) T cells confer immediate protection against locally introduced MCMV despite active viral immune evasion, owing to early viral tropism to cells that largely withstand MHC I downregulation. Thus, we unravel a yet-unappreciated role for memory CD8(+) T cells in protecting mucosal tissues against CMV infection.

Crawling and Gliding: A Computational Model for Shape-Driven Cell Migration

Cell migration is a complex process involving many intracellular and extracellular factors, with different cell types adopting sometimes strikingly different morphologies. Modeling realistically behaving cells in tissues is computationally challenging because it implies dealing with multiple levels of complexity. We extend the Cellular Potts Model with an actin-inspired feedback mechanism that allows small stochastic cell rufflings to expand to cell protrusions. This simple phenomenological model produces realistically crawling and deforming amoeboid cells, and gliding half-moon shaped keratocyte-like cells. Both cell types can migrate randomly or follow directional cues. They can squeeze in between other cells in densely populated environments or migrate collectively. The model is computationally light, which allows the study of large, dense and heterogeneous tissues containing cells with realistic shapes and migratory properties.

Cell migration is involved in vital processes like morphogenesis, regeneration and immune system responses, but can also play a central role in pathological processes like metastasization. Computational models have been successfully employed to explain how single cells migrate, and to study how diverse cell-cell interactions contribute to tissue level behavior. However, there are few models that implement realistic cell shapes in multicellular simulations. The method we present here is able to reproduce two different types of motile cells—amoeboid and keratocyte-like cells. Amoeboid cells are highly motile and deform frequently; many cells can act amoeboid in certain circumstances e.g., immune system cells, epithelial cells, individually migrating cancer cells. Keratocytes are (fish) epithelial cells which are famous for their ability to preserve their shape and direction when migrating individually; during wound healing, keratocytes migrate collectively, in sheets, to the site needing reepithelialization. Our method is computationally simple, improves the realism of multicellular simulations and can help assess the tissue level impact of specific cell shapes. For example, it can be employed to study the tissue scanning strategies of leukocytes, the circumstances in which cancer cells adopt amoeboid migration strategies, or the collective migration of keratocytes.

Hair follicle-derived IL-7 and IL-15 mediate skin-resident memory T cell homeostasis and lymphoma

The skin harbors a variety of resident leukocyte subsets that must be tightly regulated to maintain immune homeostasis. Hair follicles are unique structures in the skin that contribute to skin dendritic cell homeostasis via chemokine production. We demonstrate that CD4⁺ and CD8⁺ skin resident memory T cells (T_RM), responsible for long-term skin immunity, resided predominantly within the hair follicle epithelium of unperturbed epidermis. T_RM tropism for the epidermis and follicles was herein termed epidermotropism. Hair follicle-derived IL-15 was required for CD8⁺ T_RM, and IL-7 for CD8⁺ and CD4⁺ T_RM, to exert epidermotropism. The lack of either cytokine impaired hapten-induced contact hypersensitivity responses. In a model of cutaneous T cell lymphoma, epidermotropic CD4⁺ T_RM lymphoma cell localization depended on hair follicle-derived IL-7. These findings implicate hair follicle-derived cytokines as regulators of malignant and non-malignant T_RM cell tissue residence and suggest they may be targeted therapeutically in inflammatory skin disease and lymphoma.

Memory T Cell Migration

Immunological memory is a key feature of adaptive immunity. It provides the organism with long-lived and robust protection against infection. In organ transplantation, memory T cells pose a significant threat by causing allograft rejection that is generally resistant to immunosuppressive therapy. Therefore, a more thorough understanding of memory T cell biology is needed to improve the survival of transplanted organs without compromising the host’s ability to fight infections. This review will focus on the mechanisms by which memory T cells migrate to the site where their target antigen is present, with particular emphasis on their migration to transplanted organs. First, we will define the known subsets of memory T cells (central, effector, and tissue resident) and their circulation patterns. Second, we will review the cellular and molecular mechanisms by which memory T cells migrate to inflamed and non-inflamed tissues and highlight the emerging paradigm of antigen-driven, trans-endothelial migration. Third, we will discuss the relevance of this knowledge to organ transplantation and the prevention or treatment of allograft rejection.

Functional classification of memory CD8(+) T cells by CX3CR1 expression

Localization of memory CD8(+) T cells to lymphoid or peripheral tissues is believed to correlate with proliferative capacity or effector function. Here we demonstrate that the fractalkine-receptor/CX3CR1 distinguishes memory CD8(+) T cells with cytotoxic effector function from those with proliferative capacity, independent of tissue-homing properties. CX3CR1-based transcriptome and proteome-profiling defines a core signature of memory CD8(+) T cells with effector function. We find CD62L(hi)CX3CR1(+) memory T cells that reside within lymph nodes. This population shows distinct migration patterns and positioning in proximity to pathogen entry sites. Virus-specific CX3CR1(+) memory CD8(+) T cells are scarce during chronic infection in humans and mice but increase when infection is controlled spontaneously or by therapeutic intervention. This CX3CR1-based functional classification will help to resolve the principles of protective CD8(+) T-cell memory.

Cutaneous Infection with Leishmania major Mediates Heterologous Protection against Visceral Infection with Leishmania infantum

Visceral leishmaniasis (VL) is a fatal disease of the internal organs caused by the eukaryotic parasite Leishmania. Control of VL would best be achieved through vaccination. However, this has proven to be difficult partly because the correlates of protective immunity are not fully understood. In contrast, protective immunity against nonfatal cutaneous leishmaniasis (CL) is well defined and mediated by rapidly recruited, IFN-γ-producing Ly6C(+)CD4(+) T cells at the dermal challenge site. Protection against CL is best achieved by prior infection or live vaccination with Leishmania major, termed leishmanization. A long-standing question is whether prior CL or leishmanization can protect against VL. Employing an intradermal challenge model in mice, we report that cutaneous infection with Leishmania major provides heterologous protection against visceral infection with Leishmania infantum. Protection was associated with a robust CD4(+) T cell response at the dermal challenge site and in the viscera. In vivo labeling of circulating cells revealed that increased frequencies of IFN-γ(+)CD4(+) T cells at sites of infection are due to recruitment or retention of cells in the tissue, rather than increased numbers of cells trapped in the vasculature. Shortly after challenge, IFN-γ-producing cells were highly enriched for Ly6C(+)T-bet(+) cells in the viscera. Surprisingly, this heterologous immunity was superior to homologous immunity mediated by prior infection with L. infantum. Our observations demonstrate a common mechanism of protection against different clinical forms of leishmaniasis. The efficacy of leishmanization against VL may warrant the introduction of the practice in VL endemic areas or during outbreaks of disease.

Pre-existing immunity against Ad vectors: humoral, cellular, and innate response, what's important?

Pre-existing immunity against human adenovirus (HAd) serotype 5 derived vector in the human population is widespread, thus hampering its clinical use. Various components of the immune system, including neutralizing antibodies (nAbs), Ad specific T cells and type I IFN activated NK cells, contribute to dampening the efficacy of Ad vectors in individuals with pre-existing Ad immunity. In order to circumvent pre-existing immunity to adenovirus, numerous strategies, such as developing alternative Ad serotypes, varying immunization routes and utilizing prime-boost regimens, are under pre-clinical or clinical phases of development. However, these strategies mainly focus on one arm of pre-existing immunity. Selection of alternative serotypes has been largely driven by the absence in the human population of nAbs against them with little attention paid to cross-reactive Ad specific T cells. Conversely, varying the route of immunization appears to mainly rely on avoiding Ad specific tissue-resident T cells. Finally, prime-boost regimens do not actually circumvent pre-existing immunity but instead generate immune responses of sufficient magnitude to confer protection despite pre-existing immunity. Combining the above strategies and thus taking into account all components regulating pre-existing Ad immunity will help further improve the development of Ad vectors for animal and human use.

Resident memory T cells in human health and disease

Resident memory T cells are non-recirculating memory T cells that persist long-term in epithelial barrier tissues, including the gastrointestinal tract, lung, skin, and reproductive tract. Resident memory T cells persist in the absence of antigens, have impressive effector functions, and provide rapid on-site immune protection against known pathogens in peripheral tissues. A fundamentally distinct gene expression program differentiates resident memory T cells from circulating T cells. Although these cells likely evolved to provide rapid immune protection against pathogens, autoreactive, aberrantly activated, and malignant resident memory cells contribute to numerous human inflammatory diseases including mycosis fungoides and psoriasis. This review will discuss both the science and medicine of resident memory T cells, exploring how these cells contribute to healthy immune function and discussing what is known about how these cells contribute to human inflammatory and autoimmune diseases.

Development and Function of Protective and Pathologic Memory CD4 T Cells

Immunological memory is one of the defining features of the adaptive immune system. As key orchestrators and mediators of immunity, CD4 T cells are central to the vast majority of adaptive immune responses. Generated following an immune response, memory CD4 T cells retain pertinent information about their activation environment enabling them to make rapid effector responses upon reactivation. These responses can either benefit the host by hastening the control of pathogens or cause damaging immunopathology. Here, we will discuss the diversity of the memory CD4 T cell pool, the signals that influence the transition of activated T cells into that pool, and highlight how activation requirements differ between naïve and memory CD4 T cells. A greater understanding of these factors has the potential to aid the design of more effective vaccines and to improve regulation of pathologic CD4 T cells, such as in the context of autoimmunity and allergy.

The role of lymphocytes in the development and treatment of alopecia areata

Alopecia areata (AA) development is associated with both innate and adaptive immune cell activation, migration to peri- and intra-follicular regions, and hair follicle disruption. Both CD4(+) and CD8(+) lymphocytes are abundant in AA lesions; however, CD8(+) cytotoxic T lymphocytes are more likely to enter inside hair follicles, circumstantially suggesting that they have a significant role to play in AA development. Several rodent models recapitulate important features of the human autoimmune disease and demonstrate that CD8(+) cytotoxic T lymphocytes are fundamentally required for AA induction and perpetuation. However, the initiating events, the self-antigens involved, and the molecular signaling pathways, all need further exploration. Studying CD8(+) cytotoxic T lymphocytes and their fate decisions in AA development may reveal new and improved treatment approaches.

Visualization of integrin Mac-1 in vivo

β2 integrins play critical roles in migration of immune cells and in the interaction with other cells, pathogens, and the extracellular matrix. Among the β2 integrins, Mac-1 (Macrophage antigen-1), composed of CD11b and CD18, is mainly expressed in innate immune cells and plays a major role in cell migration and trafficking. In order to image Mac-1-expressing cells both in live cells and mouse, we generated a knock-in (KI) mouse strain expressing CD11b conjugated with monomeric yellow fluorescent protein (mYFP). Expression of CD11b-mYFP protein was confirmed by Western blot and silver staining of CD11b-immunoprecipitates and total cell lysates from the mouse splenocytes. Mac-1-mediated functions of the KI neutrophils were comparable with those in WT cells. The fluorescence intensity of CD11b-mYFP was sufficient to image CD11b expressing cells in live mice using intravital two-photon microscopy. In vitro, dynamic changes in the intracellular localization of CD11b molecules could be measured by epifluorescent microscopy. Finally, CD11b-expressing immune cells from tissue were easily detected by flow cytometry without anti-CD11b antibody staining.

Tissue instruction for migration and retention of TRM cells

During infection, a subset of effector T cells seeds the lymphoid and non-lymphoid tissues and gives rise to tissue-resident memory T cells (TRM). Recent findings have provided insight into the molecular and cellular mechanisms underlying tissue instruction of TRM cell homing, as well as the programs involved in their retention and maintenance. We review these findings here, highlighting both common features and distinctions between CD4 TRM and CD8 TRM cells. In this context we examine the role of memory lymphocyte clusters (MLCs), and propose that the MLCs serve as an immediate response center consisting of TRM cells on standby, capable of detecting incoming pathogens and mounting robust local immune responses to contain and limit the spread of infectious agents.

Tissue-Resident Memory T Cells and Fixed Immune Surveillance in Nonlymphoid Organs

T cell immunity is often defined in terms of memory lymphocytes that use the blood to access a range of organs. T cells are involved in two patterns of recirculation. In one, the cells shuttle back and forth between blood and secondary lymphoid organs, whereas in the second, memory cells recirculate between blood and nonlymphoid tissues. The latter is a means by which blood T cells control peripheral infection. It is now clear that there exists a distinct memory T cell subset that is absent from blood but found within nonlymphoid tissues. These nonrecirculating tissue-resident memory T (TRM) cells develop within peripheral compartments and never spread beyond their point of lodgement. This review examines fixed immune surveillance by TRM cells, highlighting features that make them potent controllers of infection in nonlymphoid tissues. These features provide clues about TRM cell specialization, such as their ability to deal with sequestered, persisting infections confined to peripheral compartments.

Artery Tertiary Lymphoid Organs Control Aorta Immunity and Protect against Atherosclerosis via Vascular Smooth Muscle Cell Lymphotoxin β Receptors

Tertiary lymphoid organs (TLOs) emerge during nonresolving peripheral inflammation, but their impact on disease progression remains unknown. We have found in aged Apoe^−/− mice that artery TLOs (ATLOs) controlled highly territorialized aorta T cell responses. ATLOs promoted T cell recruitment, primed CD4⁺ T cells, generated CD4⁺, CD8⁺, T regulatory (Treg) effector and central memory cells, converted naive CD4⁺ T cells into induced Treg cells, and presented antigen by an unusual set of dendritic cells and B cells. Meanwhile, vascular smooth muscle cell lymphotoxin β receptors (VSMC-LTβRs) protected against atherosclerosis by maintaining structure, cellularity, and size of ATLOs though VSMC-LTβRs did not affect secondary lymphoid organs: Atherosclerosis was markedly exacerbated in Apoe^−/−Ltbr^−/− and to a similar extent in aged Apoe^−/−Ltbr^fl/flTagln-cre mice. These data support the conclusion that the immune system employs ATLOs to organize aorta T cell homeostasis during aging and that VSMC-LTβRs participate in atherosclerosis protection via ATLOs.

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Artery tertiary lymphoid organs control atherosclerosis T cell immunity

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Artery tertiary lymphoid organs generate effector memory T cells

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Artery tertiary lymphoid organs convert naive CD4⁺ T cells into induced Treg cells

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Artery tertiary lymphoid organs protect from atherosclerosis

Spatiotemporally Distinct Interactions with Dendritic Cell Subsets Facilitates CD4+ and CD8+ T Cell Activation to Localized Viral Infection

The dynamics of when and where CD4(+) T cells provide help for CD8(+) T cell priming and which dendritic cells (DCs) activate CD4(+) T cells in vivo after localized infection are poorly understood. By using a cutaneous herpes simplex virus infection model combined with intravital 2-photon imaging of the draining lymph node (LN) to concurrently visualize pathogen-specific CD4(+) and CD8(+) T cells, we found that early priming of CD4(+) T cells involved clustering with migratory skin DCs. CD8(+) T cells did not interact with migratory DCs and their activation was delayed, requiring later clustering interactions with LN-resident XCR1(+) DCs. CD4(+) T cells interacted with these late CD8(+) T cell clusters on resident XCR1(+) DCs. Together, these data reveal asynchronous T cell activation by distinct DC subsets and highlight the key role of XCR1(+) DCs as the central platform for cytotoxic T lymphocyte activation and the delivery of CD4(+) T cell help.

The Characterization of Varicella Zoster Virus-Specific T Cells in Skin and Blood during Aging

Reactivation of the varicella zoster virus (VZV) increases during aging. Although the effects of VZV reactivation are observed in the skin (shingles), the number and functional capacity of cutaneous VZV-specific T cells have not been investigated. The numbers of circulating IFN-γ-secreting VZV-specific CD4(+) T cells are significantly decreased in old subjects. However, other measures of VZV-specific CD4(+) T cells, including proliferative capacity to VZV antigen stimulation and identification of VZV-specific CD4(+) T cells with an major histocompatibility complex class II tetramer (epitope of IE-63 protein), were similar in both age groups. The majority of T cells in the skin of both age groups expressed CD69, a characteristic of skin-resident T cells. VZV-specific CD4(+) T cells were significantly increased in the skin compared with the blood in young and old subjects, and their function was similar in both age groups. In contrast, the number of Foxp3(+) regulatory T cells and expression of the inhibitory receptor programmed cell death -1 PD-1 on CD4(+) T cells were significantly increased in the skin of older humans. Therefore, VZV-specific CD4(+) T cells in the skin of older individuals are functionally competent. However, their activity may be restricted by multiple inhibitory influences in situ.

The emerging role of resident memory T cells in protective immunity and inflammatory disease

Over the past decade, it has become clear that there is an important subset of memory T cells that resides in tissues-tissue-resident memory T (TRM) cells. There is an emerging understanding that TRM cells have a role in human tissue-specific immune and inflammatory diseases. Furthermore, the nature of the molecular signals that maintain TRM cells in tissues is the subject of much investigation. In addition, whereas it is logical for TRM cells to be located in barrier tissues at interfaces with the environment, these cells have also been found in brain, kidney, joint and other non-barrier tissues in humans and mice. Given the biology and behavior of these cells, it is likely that they have a role in chronic relapsing and remitting diseases of both barrier and non-barrier tissues. In this Review we discuss recent insights into the biology of TRM cells with a particular focus on their roles in disease, both proven and putative.

Skin Metabolites Define a New Paradigm in the Localization of Skin Tropic Memory T Cells

The localization of memory T cells to human skin is essential for long-term immune surveillance and the maintenance of barrier integrity. The expression of CCR8 during naive T cell activation is controlled by skin-specific factors derived from epidermal keratinocytes and not by resident dendritic cells. In this study, we show that the CCR8-inducing factors are heat stable and protease resistant and include the vitamin D3 metabolite 1α,25-dihydroxyvitamin D3 and PGE2. The effect of either metabolite alone on CCR8 expression was weak, whereas their combination resulted in robust CCR8 expression. Elevation of intracellular cAMP was essential because PGE2 could be substituted with the adenylyl cyclase agonist forskolin, and CCR8 expression was sensitive to protein kinase A inhibition. For effective induction, exposure of naive T cells to these epidermal factors needed to occur either prior to or during T cell activation even though CCR8 was only detected 4-5 d later in proliferating T cells. The importance of tissue environments in maintaining cellular immune surveillance networks within distinct healthy tissues provides a paradigm shift in adaptive immunity. Epidermal-derived vitamin D3 metabolites and PGs provide an essential cue for the localization of CCR8(+) immune surveillance T cells within healthy human skin.

Correlation between Central Memory T Cell Expression and Proinflammatory Cytokine Production with Clinical Presentation of Multibacillary Leprosy Relapse

BACKGROUND

Despite the efficacy of multidrug therapy, surviving Mycobacterium leprae causes relapse in some leprosy patients, and these patients present signs and symptoms of disease after healing. This study focused on the cellular immune response in relapsed multibacillary patients but also included non-relapsed multibacillary cured individuals, newly diagnosed and untreated multibacillary patients, paucibacillary patients just before the beginning of treatment, and voluntary healthy individuals for comparative analysis.

METHODOLOGY/PRINCIPAL FINDINGS

Inhibition of CD86 expression in the blood-derived monocytes and dendritic cells of relapsed multibacillary patients, either ex vivo or after M. leprae antigen stimulation was observed by flow cytometry. In addition, no significant changes in Interferon-gamma (IFN-g) expression were observed in 5-day culture supernatants of relapsed patients in response to M. leprae, neither before nor after treatment, as measured by ELISA. However, these patients demonstrated a significant increase in central memory CD4+ and CD8+ M. leprae-specific T cells, as assessed by multiparametric flow cytometry. The increase in frequency of central memory T cells in relapsed patients strongly correlated with the bacillary index and the number of skin lesions observed in these subjects. Moreover, cytokine multiplex analysis demonstrated significant antigen-specific production of Interlukin-1beta (IL-1b), IL-6, and Tumour Necrosis Factor (TNF) in the relapsed group with extremely low IL-10 production, which resulted in a high TNF/IL-10 ratio.

CONCLUSIONS/SIGNIFICANCE

Inhibition of CD86 expression may function to reduce effector T cell responses against the M. leprae antigen. Furthermore, the predominance of central memory T cells in association with the high TNF/IL-10 ratio and no observed IFN-g production may be related to the pathogenesis of relapse in multibacillary leprosy. Therefore, our findings may be a direct result of the clinical presentation, including a number of skin lesions and bacterial load, of relapsed patients. To our knowledge, this is the first study correlating immune response parameters with the clinical presentation of relapsed multibacillary patients.

Relay of herpes simplex virus between Langerhans cells and dermal dendritic cells in human skin

The mechanism by which immunity to Herpes Simplex Virus (HSV) is initiated is not completely defined. HSV initially infects mucosal epidermis prior to entering nerve endings. In mice, epidermal Langerhans cells (LCs) are the first dendritic cells (DCs) to encounter HSV, but it is CD103⁺ dermal DCs that carry viral antigen to lymph nodes for antigen presentation, suggesting DC cross-talk in skin. In this study, we compared topically HSV-1 infected human foreskin explants with biopsies of initial human genital herpes lesions to show LCs are initially infected then emigrate into the dermis. Here, LCs bearing markers of maturation and apoptosis formed large cell clusters with BDCA3⁺ dermal DCs (thought to be equivalent to murine CD103⁺ dermal DCs) and DC-SIGN⁺ DCs/macrophages. HSV-expressing LC fragments were observed inside the dermal DCs/macrophages and the BDCA3⁺ dermal DCs had up-regulated a damaged cell uptake receptor CLEC9A. No other infected epidermal cells interacted with dermal DCs. Correspondingly, LCs isolated from human skin and infected with HSV-1 in vitro also underwent apoptosis and were taken up by similarly isolated BDCA3⁺ dermal DCs and DC-SIGN⁺ cells. Thus, we conclude a viral antigen relay takes place where HSV infected LCs undergo apoptosis and are taken up by dermal DCs for subsequent antigen presentation. This provides a rationale for targeting these cells with mucosal or perhaps intradermal HSV immunization.

Herpes Simplex Virus (HSV) is a highly prevalent virus that causes cold sores and genital herpes but also increases the chance of contracting HIV by several folds. In fact, most new cases of HIV in Africa occur in people infected with HSV. Thus, a protective HSV vaccine would have a large impact on public health. Currently, the process by which immunity to HSV is generated is incompletely understood. Paradoxically, the first immune cells to become infected, Langerhans cells in the epidermis, are not the cells that initiate the immune response, while the dermal dendritic cells thought to be responsible for initiating the immune response are not likely to be infected. Here, we have shown, in human skin models and genital herpes lesion biopsies, an interaction between these dendritic cells that could relay HSV to the lymph node. HSV is taken up by the epidermal Langerhans cells that then migrate into the dermis, die and are taken up by another subset of dermal dendritic cells—the homologs of those in mice which stimulate HSV-specific T cells in the lymph node. Thus, a mucosal or intradermal vaccine targeting these two dendritic cells may be required.

Memory CD8 T cell transcriptional plasticity

Memory CD8 T cells generated after acute viral infections or live vaccines can persist for extended periods, in some instances for life, and play an important role in protective immunity. This long-lived immunity is achieved in part through cytokine-mediated homeostatic proliferation of memory T cells while maintaining the acquired capacity for rapid recall of effector cytokines and cytolytic molecules. The ability of memory CD8 T cells to retain their acquired properties, including their ability to remain poised to recall effector functions, is a truly impressive feat given that these acquired properties can be maintained for decades without exposure to cognate antigen. Here, we discuss general mechanisms for acquisition and maintenance of transcriptional programs in memory CD8 T cells and the potential role of epigenetic programming in maintaining the phenotypic and functional heterogeneity of cellular subsets among the pool of memory cells.

Interactions between host factors and the skin microbiome

The skin is colonized by an assemblage of microorganisms which, for the most part, peacefully coexist with their hosts. In some cases, these communities also provide vital functions to cutaneous health through the modulation of host factors. Recent studies have illuminated the role of anatomical skin site, gender, age, and the immune system in shaping the cutaneous ecosystem. Alterations to microbial communities have also been associated with, and likely contribute to, a number of cutaneous disorders. This review focuses on the host factors that shape and maintain skin microbial communities, and the reciprocal role of microbes in modulating skin immunity. A greater understanding of these interactions is critical to elucidating the forces that shape cutaneous populations and their contributions to skin homeostasis. This knowledge can also inform the tendency of perturbations to predispose and/or bring about certain skin disorders.

Peripheral regulation of T cells by dendritic cells during infection

It is well accepted that T cell responses are integral in providing protection during pathogenic infections. In numerous tissues, T cell responses are generated to combat infection. Typically, these T cell responses are primed in draining lymph nodes (LN) by dendritic cells (DC) that have migrated from the infected tissue. Previously, it was thought that after the initial encounter between DC and T cells in the LN, the T cells underwent a programmed response. However, it has become increasingly clear that direct interactions between DCs and T cells in infected, peripheral tissues can modulate the activation, effector function, tissue residence, and memory responses of these T cells. This review will highlight the contribution of local, direct DC: T cell interactions to the regulation of T cell responses in various tissues during inflammation and infection .

Role of the K(Ca)3.1 K+ channel in auricular lymph node CD4+ T-lymphocyte function of the delayed-type hypersensitivity model

BACKGROUND AND PURPOSE

The intermediate-conductance Ca(2+)-activated K(+) channel (K(Ca)3.1) modulates the Ca(2+) response through the control of the membrane potential in the immune system. We investigated the role of K(Ca)3.1 on the pathogenesis of delayed-type hypersensitivity (DTH) in auricular lymph node (ALN) CD4(+) T-lymphocytes of oxazolone (Ox)-induced DTH model mice.

EXPERIMENTAL APPROACH

The expression patterns of K(Ca)3.1 and its possible transcriptional regulators were compared among ALN T-lymphocytes of three groups [non-sensitized (Ox-/-), Ox-sensitized, but non-challenged (Ox+/-) and Ox-sensitized and -challenged (Ox+/+)] using real-time polymerase chain reaction, Western blotting and flow cytometry. KCa 3.1 activity was measured by whole-cell patch clamp and the voltage-sensitive dye imaging. The effects of K(Ca)3.1 blockade were examined by the administration of selective K(Ca)3.1 blockers.

KEY RESULTS

Significant up-regulation of K(Ca)3.1a was observed in CD4(+) T-lymphocytes of Ox+/- and Ox+/+, without any evident changes in the expression of the dominant-negative form, K(Ca)3.1b. Negatively correlated with this, the repressor element-1 silencing transcription factor (REST) was significantly down-regulated. Pharmacological blockade of K(Ca)3.1 resulted in an accumulation of the CD4(+) T-lymphocytes of Ox+/+ at the G0/G1 phase of the cell cycle, and also significantly recovered not only the pathogenesis of DTH, but also the changes in the K(Ca)3.1 expression and activity in the CD4(+) T-lymphocytes of Ox+/- and Ox+/+.

CONCLUSIONS AND IMPLICATIONS

The up-regulation of K(Ca)3.1a in conjunction with the down-regulation of REST may be involved in CD4(+) T-lymphocyte proliferation in the ALNs of DTH model mice; and K(Ca)3.1 may be an important target for therapeutic intervention in allergy diseases such as DTH.

The role of chemokines in cutaneous immunosurveillance

The skin serves as a critical barrier against pathogen entry. This protection is afforded by an array of skin-resident immune cells, which act as first-line responders against barrier breach and infection. The recruitment and positioning of these cells is controlled at multiple levels by endothelial cells, pericytes, perivascular macrophages and mast cells, and by the fibroblasts in the dermis and keratinocytes in the epidermis. Chemokine signalling through chemokine receptors expressed by the various leukocyte subsets is critical for their trafficking into and within the skin. The role of chemokines in the skin is complex, and remains incompletely understood despite three decades of investigation. Here, we review the roles that different chemokine pathways play in the skin, and highlight the recent developments in the field.

CD4 T-cell memory generation and maintenance

Immunologic memory is the adaptive immune system's powerful ability to remember a previous antigen encounter and react with accelerated vigor upon antigen re-exposure. It provides durable protection against reinfection with pathogens and is the foundation for vaccine-induced immunity. Unlike the relatively restricted immunologic purview of memory B cells and CD8 T cells, the field of CD4 T-cell memory must account for multiple distinct lineages with diverse effector functions, the issue of lineage commitment and plasticity, and the variable distribution of memory cells within each lineage. Here, we discuss the evidence for lineage-specific CD4 T-cell memory and summarize the known factors contributing to memory-cell generation, plasticity, and long-term maintenance.

Proinflammatory microenvironments within the intestine regulate the differentiation of tissue-resident CD8⁺ T cells responding to infection

We report that oral infection with Yersinia pseudotuberculosis (Yptb) results in development of two distinct populations of pathogen-specific CD8 tissue-resident memory T (T_RM) cells in the lamina propria (LP). CD103^– T cells did not require transforming-growth factor-β (TGF-β) signaling, but were true resident memory cells. Unlike CD103⁺ CD8 T cells, which were TGF-β-dependent and scattered in the tissue, CD103^– T cells clustered with CD4 T cells and CX3CR1⁺ macrophages and/or dendritic cells around areas of bacterial infection. CXCR3-dependent recruitment to inflamed areas was critical for development of the CD103^– population and pathogen clearance. These studies have identified the preferential development of CD103^– LP T_RM cells in inflammatory microenvironments within the LP and suggest that this subset plays a critical role in controlling infection.

Phenotypic and functional characterization of herpes simplex virus glycoprotein B epitope-specific effector and memory CD8+ T cells from symptomatic and asymptomatic individuals with ocular herpes

Herpes simplex virus 1 (HSV-1) glycoprotein B (gB)-specific CD8(+) T cells protect mice from herpes infection and disease. However, whether and which HSV-1 gB-specific CD8(+) T cells play a key role in the "natural" protection seen in HSV-1-seropositive healthy asymptomatic (ASYMP) individuals (who have never had clinical herpes disease) remain to be determined. In this study, we have dissected the phenotypes and the functions of HSV-1 gB-specific CD8(+) T cells from HLA-A*02:01 positive, HSV-1 seropositive ASYMP and symptomatic (SYMP) individuals (with a history of numerous episodes of recurrent ocular herpes disease). We found the following. (i) Healthy ASYMP individuals maintained a significantly higher proportion of differentiated HSV-1 gB-specific effector memory CD8(+) T cells (TEM cells) (CD45RA(low) CCR7(low) CD44(high) CD62L(low)). In contrast, SYMP patients had frequent less-differentiated central memory CD8(+) T cells (TCM cells) (CD45RA(low) CCR7(high) CD44(low) CD62L(high)). (ii) ASYMP individuals had significantly higher proportions of multifunctional effector CD8(+) T cells which responded mainly to gB342-350 and gB561-569 "ASYMP" epitopes, and simultaneously produced IFN-γ, CD107(a/b), granzyme B, and perforin. In contrast, effector CD8(+) T cells from SYMP individuals were mostly monofunctional and were directed mainly against nonoverlapping gB17-25 and gB183-191 "SYMP" epitopes. (iii) Immunization of an HLA-A*02:01 transgenic mouse model of ocular herpes with "ASYMP" CD8(+) TEM cell epitopes, but not with "SYMP" CD8(+) TCM cell epitopes, induced a strong CD8(+) T cell-dependent protective immunity against ocular herpes infection and disease. Our findings provide insights into the role of HSV-specific CD8(+) TEM cells in protection against herpes and should be considered in the development of an effective vaccine.

IMPORTANCE

A significantly higher proportion of differentiated and multifunctional HSV-1 gB-specific effector memory CD8(+) T cells (TEM cells) (CD45RA(low) CCR7(low) CD44(high) CD62L(low)) were found in healthy ASYMP individuals who are seropositive for HSV-1 but never had any recurrent herpetic disease, while there were frequent less-differentiated and monofunctional central memory CD8(+) T cells (TCM cells) (CD45RA(low) CCR7(high) CD44(low) CD62L(high)) in SYMP patients. Immunization with "ASYMP" CD8(+) TEM cell epitopes, but not with "SYMP" CD8(+) TCM cell epitopes, induced a strong protective HSV-specific CD8(+) T cell response in HLA-A*02:01 transgenic mice. These findings are important for the development of a safe and effective T cell-based herpes vaccine.

Alterations in antigen-specific naive CD4 T cell precursors after sepsis impairs their responsiveness to pathogen challenge

Patients surviving the acute stages of sepsis develop compromised T cell immunity and increased susceptibility to infection. Little is known about the decreased CD4 T cell function after sepsis. We tracked the loss and recovery of endogenous Ag-specific CD4 T cell populations after cecal ligation and puncture-induced sepsis and analyzed the CD4 T cell response to heterologous infection during or after recovery. We observed that the sepsis-induced early loss of CD4 T cells was followed by thymic-independent numerical recovery in the total CD4 T cell compartment. Despite this numerical recovery, we detected alterations in the composition of naive CD4 T cell precursor pools, with sustained quantitative reductions in some populations. Mice that had experienced sepsis and were then challenged with epitope-bearing, heterologous pathogens demonstrated significantly reduced priming of recovery-impaired Ag-specific CD4 T cell responses, with regard to both magnitude of expansion and functional capacity on a per-cell basis, which also correlated with intrinsic changes in Vβ clonotype heterogeneity. Our results demonstrate that the recovery of CD4 T cells from sepsis-induced lymphopenia is accompanied by alterations to the composition and function of the Ag-specific CD4 T cell repertoire.

Molecular mechanisms of CD8(+) T cell trafficking and localization

Cytotoxic CD8(+) T cells are potent mediators of host protection against disease due to their ability to directly kill cells infected with intracellular pathogens and produce inflammatory cytokines at the site of infection. To fully achieve this objective, naïve CD8(+) T cells must be able to survey the entire body for the presence of foreign or "non-self" antigen that is delivered to draining lymph nodes following infection or tissue injury. Once activated, CD8(+) T cells undergo many rounds of cell division, acquire effector functions, and are no longer restricted to the circulation and lymphoid compartments like their naïve counterparts, but rather are drawn to inflamed tissues to combat infection. As CD8(+) T cells transition from naïve to effector to memory populations, this is accompanied by dynamic changes in the expression of adhesion molecules and chemokine receptors that ultimately dictate their localization in vivo. Thus, an understanding of the molecular mechanisms regulating CD8(+) T cell trafficking and localization is critical for vaccine design, control of infectious diseases, treatment of autoimmune disorders, and cancer immunotherapy.

Imaging CD8+ T cells during diverse viral infections

CD8+ T cells play a critical role in host defense against pathogens and tumors. Much of our current knowledge of the activation and subsequent effector activities of CD8+ T cells has been gained using ex vivo approaches examining the T cell population en masse for surface phenotype, activation status and the production of effector molecules. Thus, the precise behaviors and diversity of individual CD8+ T cells responding to virus infection in vivo have not been extensively explored, leaving many unanswered questions relevant to the rational design of antiviral vaccines and therapeutics. Recently, intravital multiphoton microscopy (MPM) has been used to image CD8+ T cell priming after infection with disparate viral pathogens ranging from small RNA viruses encoding few proteins to DNA viruses producing hundreds of viral proteins (many immunomodulatory). After priming, effector CD8+ T cells have been visualized in virus-infected tissue, both during primary infection and after transitioning to tissue resident memory cells (TRM). Here, I highlight recent advances in our understanding of antiviral CD8+ T cell responses revealed through intravital MPM.

Visualization of bone marrow monocyte mobilization using Cx3cr1gfp/+Flt3L-/- reporter mouse by multiphoton intravital microscopy

Monocytes are innate immune cells that play critical roles in inflammation and immune defense. A better comprehension of how monocytes are mobilized and recruited is fundamental to understand their biologic role in disease and steady state. The BM represents a major "checkpoint" for monocyte homeostasis, as it is the primary site for their production and release. Our study determined that the Cx3cr1(gfp/+) mouse strain is currently the most ideal model for the visualization of monocyte behavior in the BM by multiphoton intravital microscopy. However, we observed that DCs are also labeled with high levels of GFP and thus, interfere with the accuracy of monocyte tracking in vivo. Hence, we generated a Cx3cr1(gfp/+)Flt3L(-/-) reporter mouse and showed that whereas monocyte numbers were not affected, DC numbers were reduced significantly, as DCs but not monocytes depend on Flt3 signaling for their development. We thus verified that mobilization of monocytes from the BM in Cx3cr1(gfp/+)Flt3L(-/-) mice is intact in response to LPS. Collectively, our study demonstrates that the Cx3cr1(gfp/+)Flt3L(-/-) reporter mouse model represents a powerful tool to visualize monocyte activities in BM and illustrates the potential of a Cx3cr1(gfp/+)-based, multifunctionality fluorescence reporter approach to dissect monocyte function in vivo.

Tissue-resident memory T cells

Tissue-resident memory T (Trm) cells constitute a recently identified lymphocyte lineage that occupies tissues without recirculating. They provide a first response against infections reencountered at body surfaces, where they accelerate pathogen clearance. Because Trm cells are not present within peripheral blood, they have not yet been well characterized, but are transcriptionally, phenotypically, and functionally distinct from recirculating central and effector memory T cells. In this review, we will summarize current knowledge of Trm cell ontogeny, regulation, maintenance, and function and will highlight technical considerations for studying this population.

Chronic parasitic infection maintains high frequencies of short-lived Ly6C+CD4+ effector T cells that are required for protection against re-infection

In contrast to the ability of long-lived CD8⁺ memory T cells to mediate protection against systemic viral infections, the relationship between CD4⁺ T cell memory and acquired resistance against infectious pathogens remains poorly defined. This is especially true for T helper 1 (Th1) concomitant immunity, in which protection against reinfection coincides with a persisting primary infection. In these situations, pre-existing effector CD4 T cells generated by ongoing chronic infection, not memory cells, may be essential for protection against reinfection. We present a systematic study of the tissue homing properties, functionality, and life span of subsets of memory and effector CD4 T cells activated in the setting of chronic Leishmania major infection in resistant C57Bl/6 mice. We found that pre-existing, CD44⁺CD62L⁻T-bet⁺Ly6C⁺ effector (T_EFF) cells that are short-lived in the absence of infection and are not derived from memory cells reactivated by secondary challenge, mediate concomitant immunity. Upon adoptive transfer and challenge, non-dividing Ly6C⁺ T_EFF cells preferentially homed to the skin, released IFN-γ, and conferred protection as compared to CD44⁺CD62L⁻Ly6C⁻ effector memory or CD44⁺CD62L⁺Ly6C⁻ central memory cells. During chronic infection, Ly6C⁺ T_EFF cells were maintained at high frequencies via reactivation of T_CM and the T_EFF themselves. The lack of effective vaccines for many chronic diseases may be because protection against infectious challenge requires the maintenance of pre-existing T_EFF cells, and is therefore not amenable to conventional, memory inducing, vaccination strategies.

Naturally acquired resistance to reinfection by numerous infectious pathogens including Leishmania, Plasmodium, Mycobacterium, and parasitic worms, typically coincides with an ongoing primary infection. This natural resistance to reinfection, termed concomitant immunity, is often referred to as a memory response and provides the rationale for the vaccine effort against these infectious pathogens. However, immune memory is mediated by populations of long-lived cells that do not require an ongoing primary infection to mediate protection. The requirement for chronic infection to maintain concomitant immunity suggests that the critical cells that mediate this immunity are not memory cells. In the present study we define short-lived effector T cells that pre-exist secondary challenge, not memory cells, as the critical cells that mediate concomitant immunity. These observations provide direct evidence on a cellular level that conventional vaccination strategies against chronic infectious diseases, whose development is predicated upon the belief that concomitant immunity can be mediated by long-lived memory cells, are unlikely to succeed.

The IL-17A-producing CD8+ T-cell population in psoriatic lesional skin comprises mucosa-associated invariant T cells and conventional T cells

IL-17A is pivotal in the etiology of psoriasis, and CD8(+) T cells with the ability to produce this cytokine (Tc17 cells) are over-represented in psoriatic lesions. Here we demonstrate that the frequency of Tc17 cells in peripheral blood of psoriasis patients correlated with the clinical severity of the disease. Analysis of cutaneous-associated lymphocyte antigen expression showed that the blood Tc17 population contains a significantly higher proportion of cells with skin-homing potential compared with the CD8(+) T-cell population lacking IL-17A/IL-22 expression. IL-17A-producing CD8(+) T cells in blood have previously been reported to belong mainly to the mucosa-associated invariant T-cell (MAIT cell) lineage characterized by TCR Vα7.2 chain, CD161, IL-18Rα, and multidrug transporter ABCB1 expression. We demonstrate the presence of CD8(+) MAIT cells in the dermis and epidermis of psoriatic plaques, as well as healthy skin; however, IL-17A-producing CD8(+) MAIT cells were predominantly found in psoriatic skin. Notably, we observed IL-17A production in a large proportion of psoriatic plaque-derived CD8(+) T cells devoid of MAIT cell characteristics, likely representing conventional CD8(+) T cells. In conclusion, we provide supporting evidence that implicates Tc17 cells in the pathogenesis of psoriasis and describe the presence of innate CD8(+) MAIT cells in psoriatic lesions as an alternative source of IL-17A.