Memory T cell subsets, migration patterns, and tissue residence.

Tissues such as the skin and mucosae are frequently exposed to microbial pathogens. Infectious agents must be quickly and efficiently controlled by our immune system, but the low frequency of naive T cells specific for any one pathogen means dependence on primary responses initiated in draining lymph nodes, often allowing time for serious infection to develop. These responses imprint effectors with the capacity to home to infected tissues; this process, combined with inflammatory signals, ensures the effective targeting of primary immunity. Upon vaccination or previous pathogen exposure, increased pathogen-specific T cell numbers together with altered migratory patterns of memory T cells can greatly improve immune efficacy, ensuring infections are prevented or at least remain subclinical. Until recently, memory T cell populations were considered to comprise central memory T cells (TCM), which are restricted to the secondary lymphoid tissues and blood, and effector memory T cells (TEM), which broadly migrate between peripheral tissues, the blood, and the spleen. Here we review evidence for these two memory populations, highlight a relatively new player, the tissue-resident memory T cell (TRM), and emphasize the potential differences between the migratory patterns of CD4(+) and CD8(+) T cells. This new understanding raises important considerations for vaccine design and for the measurement of immune parameters critical to the control of infectious disease, autoimmunity, and cancer.

Runx3 drives a CD8+ T cell tissue residency program that is absent in CD4+ T cells.

Tissue-resident memory T cells (TRM cells) provide rapid and superior control of localized infections. While the transcription factor Runx3 is a critical regulator of CD8+ T cell tissue residency, its expression is repressed in CD4+ T cells. Here, we show that, as a direct consequence of this Runx3-deficiency, CD4+ TRM cells lacked the transforming growth factor (TGF)-ß-responsive transcriptional network that underpins the tissue residency of epithelial CD8+ TRM cells. While CD4+ TRM cell formation required Runx1, this, along with the modest expression of Runx3 in CD4+ TRM cells, was insufficient to engage the TGF-ß-driven residency program. Ectopic expression of Runx3 in CD4+ T cells incited this TGF-ß-transcriptional network to promote prolonged survival, decreased tissue egress, a microanatomical redistribution towards epithelial layers and enhanced effector functionality. Thus, our results reveal distinct programming of tissue residency in CD8+ and CD4+ TRM cell subsets that is attributable to divergent Runx3 activity.

Discrete tissue microenvironments instruct diversity in resident memory T cell function and plasticity.

Tissue-resident memory T (TRM) cells are non-recirculating cells that exist throughout the body. Although TRM cells in various organs rely on common transcriptional networks to establish tissue residency, location-specific factors adapt these cells to their tissue of lodgment. Here we analyze TRM cell heterogeneity between organs and find that the different environments in which these cells differentiate dictate TRM cell function, durability and malleability. We find that unequal responsiveness to TGFß is a major driver of this diversity. Notably, dampened TGFß signaling results in CD103- TRM cells with increased proliferative potential, enhanced function and reduced longevity compared with their TGFß-responsive CD103+ TRM counterparts. Furthermore, whereas CD103- TRM cells readily modified their phenotype upon relocation, CD103+ TRM cells were comparatively resistant to transdifferentiation. Thus, despite common requirements for TRM cell development, tissue adaptation of these cells confers discrete functional properties such that TRM cells exist along a spectrum of differentiation potential that is governed by their local tissue microenvironment.

Local proliferation maintains a stable pool of tissue-resident memory T cells after antiviral recall responses.

Although tissue-resident memory T cells (TRM cells) are critical in fighting infection, their fate after local pathogen re-encounter is unknown. Here we found that skin TRM cells engaged virus-infected cells, proliferated in situ in response to local antigen encounter and did not migrate out of the epidermis, where they exclusively reside. As a consequence, secondary TRM cells formed from pre-existing TRM cells, as well as from precursors recruited from the circulation. Newly recruited antigen-specific or bystander TRM cells were generated in the skin without displacement of the pre-existing TRM cell pool. Thus, pre-existing skin TRM cell populations are not displaced after subsequent infections, which enables multiple TRM cell specificities to be stably maintained within the tissue.

The skin-resident and migratory immune system in steady state and memory: innate lymphocytes, dendritic cells and T cells.

The skin is a highly complex organ interspersed with a variety of smaller organ-like structures and a plethora of cell types that together perform essential functions such as physical sensing, temperature control, barrier maintenance and immunity. In this Review, we outline many of the innate and adaptive immune cell types associated with the skin, focusing on the steady state in mice and men, and include a broad update of dendritic cell function and T cell surveillance.

The developmental pathway for CD103(+)CD8+ tissue-resident memory T cells of skin.

Tissue-resident memory T cells (T(RM) cells) provide superior protection against infection in extralymphoid tissues. Here we found that CD103(+)CD8(+) T(RM) cells developed in the skin from epithelium-infiltrating precursor cells that lacked expression of the effector-cell marker KLRG1. A combination of entry into the epithelium plus local signaling by interleukin 15 (IL-15) and transforming growth factor-ß (TGF-ß) was required for the formation of these long-lived memory cells. Notably, differentiation into T(RM) cells resulted in the progressive acquisition of a unique transcriptional profile that differed from that of circulating memory cells and other types of T cells that permanently reside in skin epithelium. We provide a comprehensive molecular framework for the local differentiation of a distinct peripheral population of memory cells that forms a first-line immunological defense system in barrier tissues.

Liver-Resident Memory CD8+ T Cells Form a Front-Line Defense against Malaria Liver-Stage Infection.

In recent years, various intervention strategies have reduced malaria morbidity and mortality, but further improvements probably depend upon development of a broadly protective vaccine. To better understand immune requirement for protection, we examined liver-stage immunity after vaccination with irradiated sporozoites, an effective though logistically difficult vaccine. We identified a population of memory CD8+ T cells that expressed the gene signature of tissue-resident memory T (Trm) cells and remained permanently within the liver, where they patrolled the sinusoids. Exploring the requirements for liver Trm cell induction, we showed that by combining dendritic cell-targeted priming with liver inflammation and antigen recognition on hepatocytes, high frequencies of Trm cells could be induced and these cells were essential for protection against malaria sporozoite challenge. Our study highlights the immune potential of liver Trm cells and provides approaches for their selective transfer, expansion, or depletion, which may be harnessed to control liver infections or autoimmunity.

T-box Transcription Factors Combine with the Cytokines TGF-ß and IL-15 to Control Tissue-Resident Memory T Cell Fate.

Tissue-resident memory T (Trm) cells contribute to local immune protection in non-lymphoid tissues such as skin and mucosa, but little is known about their transcriptional regulation. Here we showed that CD8(+)CD103(+) Trm cells, independent of circulating memory T cells, were sufficient for protection against infection and described molecular elements that were crucial for their development in skin and lung. We demonstrated that the T-box transcription factors (TFs) Eomes and T-bet combined to control CD8(+)CD103(+) Trm cell formation, such that their coordinate downregulation was crucial for TGF-ß cytokine signaling. TGF-ß signaling, in turn, resulted in reciprocal T-box TF downregulation. However, whereas extinguishment of Eomes was necessary for CD8(+)CD103(+) Trm cell development, residual T-bet expression maintained cell surface interleukin-15 (IL-15) receptor ß-chain (CD122) expression and thus IL-15 responsiveness. These findings indicate that the T-box TFs control the two cytokines, TGF-ß and IL-15, which are pivotal for CD8(+)CD103(+) Trm cell development and survival.

CD4 helpers put tissue-resident memory cells in their place.

The regulation of tissue-resident memory cell development is poorly understood. In this issue of Immunity, Laidlaw et al. (2014) demonstrate that CD4(+) T cells promote development of lung-resident memory cells by limiting T-bet expression and directing CD8(+) T cells to the airway epithelium.

Dendritic cell subsets in primary and secondary T cell responses at body surfaces.

We examine the role of dendritic cells subsets in immunity to peripheral infections, with emphasis on the differences in the regulation of primary and secondary T cell responses to viruses. Our major focus is on new developments in the understanding of immunity to infections of the skin and lung, which are crucial entry points for a variety of infectious pathogens. Initially we describe a diverse network of dendritic cell subsets, but then we argue for a more generalized model of reduced complexity.

Memory T cells in nonlymphoid tissue that provide enhanced local immunity during infection with herpes simplex virus.

Effective immunity is dependent on long-surviving memory T cells. Various memory subsets make distinct contributions to immune protection, especially in peripheral infection. It has been suggested that T cells in nonlymphoid tissues are important during local infection, although their relationship with populations in the circulation remains poorly defined. Here we describe a unique memory T cell subset present after acute infection with herpes simplex virus that remained resident in the skin and in latently infected sensory ganglia. These T cells were in disequilibrium with the circulating lymphocyte pool and controlled new infection with this virus. Thus, these cells represent an example of tissue-resident memory T cells that can provide protective immunity at points of pathogen entry.

Cross-presentation of viral and self antigens by skin-derived CD103+ dendritic cells.

Skin-derived dendritic cells (DCs) include Langerhans cells, classical dermal DCs and a langerin-positive CD103(+) dermal subset. We examined their involvement in the presentation of skin-associated viral and self antigens. Only the CD103(+) subset efficiently presented antigens of herpes simplex virus type 1 to naive CD8(+) T cells, although all subsets presented these antigens to CD4(+) T cells. This showed that CD103(+) DCs were the migratory subset most efficient at processing viral antigens into the major histocompatibility complex class I pathway, potentially through cross-presentation. This was supported by data showing only CD103(+) DCs efficiently cross-presented skin-derived self antigens. This indicates CD103(+) DCs are the main migratory subtype able to cross-present viral and self antigens, which identifies another level of specialization for skin DCs.

Indiscriminate memories during infection control.

In this issue of Immunity, Soudja et al. (2012) demonstrate that non-antigen-specific stimulation evoked by a variety of pathogens plays an important role in the innate acquisition of effector function by memory CD8(+) T cells.

Cutting Edge: Tissue-Resident Memory T Cells Generated by Multiple Immunizations or Localized Deposition Provide Enhanced Immunity.

Tissue-resident memory T cells (TRM) have been shown to afford superior protection against infection, particularly against pathogens that enter via the epithelial surfaces of the body. Although TRM are often concentrated at sites of prior infection, it has been shown that TRM can disseminate throughout the body. We examined the relative effectiveness of global versus targeted CD8+ TRM lodgment in skin. The site of initial T cell priming made little difference to skin lodgement, whereas local inflammation and Ag recognition enhanced TRM accumulation and retention. Disseminated TRM lodgment was seen with the skin, but required multiple exposures to Ag and was inferior to targeted strategies. As a consequence, active recruitment by inflammation or infection resulted in superior TRM numbers and maximal protection against infection. Overall, these results highlight the potency of localized TRM deposition as a means of pathogen control as well as demonstrating the limitations of global TRM lodgment.

Development of a Novel CD4+ TCR Transgenic Line That Reveals a Dominant Role for CD8+ Dendritic Cells and CD40 Signaling in the Generation of Helper and CTL Responses to Blood-Stage Malaria.

We describe an MHC class II (I-Ab)-restricted TCR transgenic mouse line that produces CD4+ T cells specific for Plasmodium species. This line, termed PbT-II, was derived from a CD4+ T cell hybridoma generated to blood-stage Plasmodium berghei ANKA (PbA). PbT-II cells responded to all Plasmodium species and stages tested so far, including rodent (PbA, P. berghei NK65, Plasmodium chabaudi AS, and Plasmodium yoelii 17XNL) and human (Plasmodium falciparum) blood-stage parasites as well as irradiated PbA sporozoites. PbT-II cells can provide help for generation of Ab to P. chabaudi infection and can control this otherwise lethal infection in CD40L-deficient mice. PbT-II cells can also provide help for development of CD8+ T cell-mediated experimental cerebral malaria (ECM) during PbA infection. Using PbT-II CD4+ T cells and the previously described PbT-I CD8+ T cells, we determined the dendritic cell (DC) subsets responsible for immunity to PbA blood-stage infection. CD8+ DC (a subset of XCR1+ DC) were the major APC responsible for activation of both T cell subsets, although other DC also contributed to CD4+ T cell responses. Depletion of CD8+ DC at the beginning of infection prevented ECM development and impaired both Th1 and follicular Th cell responses; in contrast, late depletion did not affect ECM. This study describes a novel and versatile tool for examining CD4+ T cell immunity during malaria and provides evidence that CD4+ T cell help, acting via CD40L signaling, can promote immunity or pathology to blood-stage malaria largely through Ag presentation by CD8+ DC.

Staged development of long-lived T-cell receptor αß TH17 resident memory T-cell population to Candida albicans after skin infection.

BACKGROUND: Candida albicans is a dimorphic fungus to which human subjects are exposed early in life, and by adulthood, it is part of the mycobiome of skin and other tissues. Neonatal skin lacks resident memory T (TRM) cells, but in adults the C albicans skin test is a surrogate for immunocompetence. Young adult mice raised under specific pathogen-free conditions are naive to C albicans and have been shown recently to have an immune system resembling that of neonatal human subjects. OBJECTIVE: We studied the evolution of the adaptive cutaneous immune response to Candida species. METHODS: We examined both human skin T cells and the de novo and memory immune responses in a mouse model of C albicans skin infection. RESULTS: In mice the initial IL-17-producing cells after C albicans infection were dermal γδ T cells, but by day 7, αß TH17 effector T cells were predominant. By day 30, the majority of C albicans-reactive IL-17-producing T cells were CD4 TRM cells. Intravital microscopy showed that CD4 effector T cells were recruited to the site of primary infection and were highly motile 10 days after infection. Between 30 and 90 days after infection, these CD4 T cells became increasingly sessile, acquired expression of CD69 and CD103, and localized to the papillary dermis. These established TRM cells produced IL-17 on challenge, whereas motile migratory memory T cells did not. TRM cells rapidly clear an infectious challenge with C albicans more effectively than recirculating T cells, although both populations participate. We found that in normal human skin IL-17-producing CD4+ TRM cells that responded to C albicans in an MHC class II-restricted fashion could be identified readily. CONCLUSIONS: These studies demonstrate that C albicans infection of skin preferentially generates CD4+ IL-17-producing TRM cells, which mediate durable protective immunity.

Sustained accumulation of antigen-presenting cells after infection promotes local T-cell immunity.

Antigen-presenting cells (APC), such as dendritic cells (DC) and macrophages, are critical for T-cell-mediated immunity. Although it is established that memory T cells accumulate and persist in peripheral tissues after the resolution of infection, whether this is also the case for APC remains unclear. Here, we report that CCR2-dependent cells infiltrate skin during acute infection with herpes simplex virus (HSV)-1 and subsequently give rise to localized populations of DCs and macrophages. These APC are found at elevated numbers at sites of resolved infection or inflammation compared with unaffected regions of skin. Importantly, this local accumulation of APC is sustained for prolonged periods of time and has important functional consequences, as it promotes interferon-γ responses by virus-specific CD4+ T cells upon localized challenge infection with HSV-1. Thus, our results highlight how infection history determines long-term changes in immune cell composition in skin and how different types of immune cells accumulate, persist and co-operate to provide optimal immunity at this critical barrier site.