Reciprocal and dynamic control of CD8 T cell homing by dendritic cells from skin- and gut-associated lymphoid tissues

Tissue-resident memory T cells

Tissues such as the genital tract, skin, and lung act as barriers against invading pathogens. To protect the host, incoming microbes must be quickly and efficiently controlled by the immune system at the portal of entry. Memory is a hallmark of the adaptive immune system, which confers long-term protection and is the basis for efficacious vaccines. While the majority of existing vaccines rely on circulating antibody for protection, struggles to develop antibody-based vaccines against infections such as herpes simplex virus (HSV) and human immunodeficiency virus (HIV) have underscored the need to generate memory T cells for robust antiviral control. The circulating memory T-cell population is generally divided into two subsets: effector memory (TEM ) and central memory (TCM ). These two subsets can be distinguished by their localization, as TCM home to secondary lymphoid organs and TEM circulate through non-lymphoid tissues. More recently, studies have identified a third subset, called tissue-resident memory (TRM ) cells, based on its migratory properties. This subset is found in peripheral tissues that require expression of specific chemoattractants and homing receptors for T-cell recruitment and retention, including barrier sites such as the skin and genital tract. In this review, we categorize different tissues in the body based on patterns of memory T-cell migration and tissue residency. This review also describes the rules for TRM generation and the properties that distinguish them from circulating TEM and TCM cells. Finally, based on the failure of recent T-cell-based vaccines to provide optimal protection, we also discuss the potential role of TRM cells in vaccine design against microbes that invade through the peripheral tissues and highlight new vaccination strategies that take advantage of this newly described memory T-cell subset.

Depletion of host CCR7(+) dendritic cells prevented donor T cell tissue tropism in anti-CD3-conditioned recipients

We reported previously that anti-CD3 mAb treatment before hematopoietic cell transplantation (HCT) prevented graft-versus-host disease (GVHD) and preserved graft-versus-leukemia (GVL) effects in mice. These effects were associated with downregulated donor T cell expression of tissue-specific homing and chemokine receptors, marked reduction of donor T cell migration into GVHD target tissues, and deletion of CD103(+) dendritic cells (DCs) in mesenteric lymph nodes (MLN). MLN CD103(+) DCs and peripheral lymph node (PLN) DCs include CCR7(+) and CCR7(-) subsets, but the role of these DC subsets in regulating donor T cell expression of homing and chemokine receptors remain unclear. Here, we show that recipient CCR7(+), but not CCR7(-), DCs in MLN induced donor T cell expression of gut-specific homing and chemokine receptors in a retinoid acid-dependent manner. CCR7 regulated activated DC migration from tissue to draining lymph node, but it was not required for the ability of DCs to induce donor T cell expression of tissue-specific homing and chemokine receptors. Finally, anti-CD3 treatment depleted CCR7(+) but not CCR7(-) DCs by inducing sequential expansion and apoptosis of CCR7(+) DCs in MLN and PLN. Apoptosis of CCR7(+) DCs was associated with DC upregulation of Fas expression and natural killer cell but not T, B, or dendritic cell upregulation of FasL expression in the lymph nodes. These results suggest that depletion of CCR7(+) host-type DCs, with subsequent inhibition of donor T cell migration into GVHD target tissues, can be an effective approach in prevention of acute GVHD and preservation of GVL effects.

CD11b+ migratory dendritic cells mediate CD8 T cell cross-priming and cutaneous imprinting after topical immunization

Topical antigen application is a focus of current vaccine research. This immunization route mimics natural antigen exposure across a barrier tissue and generates T cells imprinted for skin-selective homing. Soluble antigens introduced through this route require cross-presentation by DC to generate CD8 T cell responses. Here we have explored the relative contribution of various skin-derived DC subsets to cross-priming and skin-selective imprinting. In our model, DC acquire soluble Ag in vivo from immunized murine skin for cross-presentation to naïve CD8 T cells ex vivo. We find CD11b⁺ migratory DC to be the relevant cross-priming DC in this model. Both Langerin⁺ and Langerin^- CD11b⁺ migratory DC can cross-present antigen in our system, but only the Langerin⁺ subset can induce expression of the skin-selective addressin E-selectin ligand. Thus, the CD11b⁺ Langerin⁺ migratory DC population, comprised primarily of Langerhans cells, both cross-primes naïve CD8 T cells and imprints them with skin-homing capabilities.

Memory CD4 T cells in influenza

Influenza A virus is a significant cause of morbidity and mortality worldwide, particularly among young children and the elderly. Current vaccines induce neutralizing antibody responses directed toward highly variable viral surface proteins, resulting in limited heterosubtypic protection to new viral serotypes. By contrast, memory CD4 T cells recognize conserved viral proteins and are cross-reactive to multiple influenza strains. In humans, virus-specific memory CD4 T cells were found to be the protective correlate in human influenza challenge studies, suggesting their key role in protective immunity. In mouse models, memory CD4 T cells can mediate protective responses to secondary influenza infection independent of B cells or CD8 T cells, and can influence innate immune responses. Importantly, a newly defined, tissue-resident CD4 memory population has been demonstrated to be retained in lung tissue and promote optimal protective responses to an influenza infection. Here, we review the current state of results regarding the generation of memory CD4 T cells following primary influenza infection, mechanisms for their enhanced efficacy in protection from secondary challenge including their phenotype, localization, and function in the context of both mouse models and human infection. We also discuss the generation of memory CD4 T cells in response to influenza vaccines and its future implications for vaccinology.

Chemokines and chemokine receptors: positioning cells for host defense and immunity

Chemokines are chemotactic cytokines that control the migratory patterns and positioning of all immune cells. Although chemokines were initially appreciated as important mediators of acute inflammation, we now know that this complex system of approximately 50 endogenous chemokine ligands and 20 G protein-coupled seven-transmembrane signaling receptors is also critical for the generation of primary and secondary adaptive cellular and humoral immune responses. Recent studies demonstrate important roles for the chemokine system in the priming of naive T cells, in cell fate decisions such as effector and memory cell differentiation, and in regulatory T cell function. In this review, we focus on recent advances in understanding how the chemokine system orchestrates immune cell migration and positioning at the organismic level in homeostasis, in acute inflammation, and during the generation and regulation of adoptive primary and secondary immune responses in the lymphoid system and peripheral nonlymphoid tissue.

γδ T cells exhibit multifunctional and protective memory in intestinal tissues

The study of T cell memory and the target of vaccine design have focused on memory subsumed by T cells bearing the αβ T cell receptor. Alternatively, γδ T cells are thought to provide rapid immunity, particularly at mucosal borders. Here, we have shown that a distinct subset of mucosal γδ T cells mounts an immune response to oral Listeria monocytogenes (Lm) infection and leads to the development of multifunctional memory T cells capable of simultaneously producing interferon-γ and interleukin-17A in the murine intestinal mucosa. Challenge infection with oral Lm, but not oral Salmonella or intravenous Lm, induced rapid expansion of memory γδ T cells, suggesting contextual specificity to the priming pathogen. Importantly, memory γδ T cells were able to provide enhanced protection against infection. These findings illustrate that γδ T cells play a role with hallmarks of adaptive immunity in the intestinal mucosa.

Tissue competence imprinting and tissue residency of CD8 T cells

T cell immunity is characterized by striking tissue specialization. Tissue-specificity imprinting starts during priming by tissue-derived migratory dendritic cells in the non-random, specialized micro-anatomical area of the draining lymph node and is influenced by constitutive and induced cues from local environment. Besides tissue-specific effectors, memory cells also exhibit a tissue-specificity. Long-lived tissue-resident memory T cells likely play a considerable role in preventing pathogen invasion. Understanding of the mechanisms of tissue specialization of T cells is of major importance for the design of optimal vaccination strategies and therapeutic interventions in tissue/organ-specific inflammatory diseases. The present review summarizes our current knowledge and hypothesis about tissue-specificity imprinting and tissue residency of T cells.

Tissue specific heterogeneity in effector immune cell response

Post pathogen invasion, migration of effector T-cell subsets to specific tissue locations is of prime importance for generation of robust immune response. Effector T cells are imprinted with distinct “homing codes” (adhesion molecules and chemokine receptors) during activation which regulate their targeted trafficking to specific tissues. Internal cues in the lymph node microenvironment along with external stimuli from food (vitamin A) and sunlight (vitamin D₃) prime dendritic cells, imprinting them to play centre stage in the induction of tissue tropism in effector T cells. B cells as well, in a manner similar to effector T cells, exhibit tissue-tropic migration. In this review, we have focused on the factors regulating the generation and migration of effector T cells to various tissues along with giving an overview of tissue tropism in B cells.

Human CD1c⁺ myeloid dendritic cells acquire a high level of retinoic acid-producing capacity in response to vitamin D₃

All-trans-retinoic acid (RA) plays a critical role in maintaining immune homeostasis. Mouse intestinal CD103⁺ dendritic cells (DCs) produce a high level of RA by highly expressing retinal dehydrogenase (RALDH)2, an enzyme that converts retinal to RA, and induce gut-homing T cells. However, it has not been identified which subset of human DCs produce a high level of RA. In this study, we show that CD1c⁺ blood myeloid DCs (mDCs) but not CD141(high) mDCs or plasmacytoid DCs exhibited a high level of RALDH2 mRNA and aldehyde dehydrogenase (ALDH) activity in an RA- and p38-dependent manner when stimulated with 1α,25-dihydroxyvitamin D₃ (VD₃) in the presence of GM-CSF. The ALDH activity was abrogated by TLR ligands or TNF. CD103⁻ rather than CD103⁺ human mesenteric lymph node mDCs gained ALDH activity in response to VD₃. Furthermore, unlike in humans, mouse conventional DCs in the spleen and mesenteric lymph nodes gained ALDH activity in response to GM-CSF alone. RALDH2(high) CD1c⁺ mDCs stimulated naive CD4⁺ T cells to express gut-homing molecules and to produce Th2 cytokines in an RA-dependent manner. This study suggests that CD1c⁺ mDCs are a major human DC subset that produces RA in response to VD₃ in the steady state. The "vitamin D-CD1c⁺mDC-RA" axis may constitute an important immune component for maintaining tissue homeostasis in humans.

Lung dendritic cells induce migration of protective T cells to the gastrointestinal tract

Developing efficacious vaccines against enteric diseases is a global challenge that requires a better understanding of cellular recruitment dynamics at the mucosal surfaces. The current paradigm of T cell homing to the gastrointestinal (GI) tract involves the induction of α4β7 and CCR9 by Peyer's patch and mesenteric lymph node (MLN) dendritic cells (DCs) in a retinoic acid-dependent manner. This paradigm, however, cannot be reconciled with reports of GI T cell responses after intranasal (i.n.) delivery of antigens that do not directly target the GI lymphoid tissue. To explore alternative pathways of cellular migration, we have investigated the ability of DCs from mucosal and nonmucosal tissues to recruit lymphocytes to the GI tract. Unexpectedly, we found that lung DCs, like CD103(+) MLN DCs, up-regulate the gut-homing integrin α4β7 in vitro and in vivo, and induce T cell migration to the GI tract in vivo. Consistent with a role for this pathway in generating mucosal immune responses, lung DC targeting by i.n. immunization induced protective immunity against enteric challenge with a highly pathogenic strain of Salmonella. The present report demonstrates novel functional evidence of mucosal cross talk mediated by DCs, which has the potential to inform the design of novel vaccines against mucosal pathogens.

Peripheral tissue homing receptor control of naïve, effector, and memory CD8 T cell localization in lymphoid and non-lymphoid tissues

T cell activation induces homing receptors that bind ligands on peripheral tissue vasculature, programing movement to sites of infection and injury. There are three major types of CD8 effector T cells based on homing receptor expression, which arise in distinct lymphoid organs. Recent publications indicate that naïve, effector, and memory T cell migration is more complex than once thought; while many effectors enter peripheral tissues, some re-enter lymph nodes (LN), and contain central memory precursors. LN re-entry can depend on CD62L or peripheral tissue homing receptors. Memory T cells in LN tend to express the same homing receptors as their forebears, but often are CD62Lneg. Homing receptors also control CD8 T cell tumor entry. Tumor vasculature has low levels of many peripheral tissue homing receptor ligands, but portions of it resemble high endothelial venules (HEV), enabling naïve T cell entry, activation, and subsequent effector activity. This vasculature is associated with positive prognoses in humans, suggesting it may sustain ongoing anti-tumor responses. These findings reveal new roles for homing receptors expressed by naïve, effector, and memory CD8 T cells in controlling entry into lymphoid and non-lymphoid tissues.

Lung dendritic cells imprint T cell lung homing and promote lung immunity through the chemokine receptor CCR4

T cell trafficking into the lung is critical for lung immunity, but the mechanisms that mediate T cell lung homing are not well understood. Here, we show that lung dendritic cells (DCs) imprint T cell lung homing, as lung DC-activated T cells traffic more efficiently into the lung in response to inhaled antigen and at homeostasis compared with T cells activated by DCs from other tissues. Consequently, lung DC-imprinted T cells protect against influenza more effectively than do gut and skin DC-imprinted T cells. Lung DCs imprint the expression of CCR4 on T cells, and CCR4 contributes to T cell lung imprinting. Lung DC-activated, CCR4-deficient T cells fail to traffic into the lung as efficiently and to protect against influenza as effectively as lung DC-activated, CCR4-sufficient T cells. Thus, lung DCs imprint T cell lung homing and promote lung immunity in part through CCR4.

Peripheral tissue homing receptors enable T cell entry into lymph nodes and affect the anatomical distribution of memory cells

Peripheral tissue homing receptors enable T cells to access inflamed nonlymphoid tissues. In this study, we show that two such molecules, E-selectin ligand and α4β1 integrin, enable activated and memory T cells to enter lymph nodes (LN) as well. This affects the quantitative and qualitative distribution of these cells among regional LN beds. CD8 memory T cells in LN that express these molecules were mostly CD62L(lo) and would normally be classified as effector memory cells. However, similar to central memory cells, they expanded upon Ag re-encounter. This led to differences in the magnitude of the recall response that depended on the route of immunization. These novel cells share properties of both central and effector memory cells and reside in LN based on previously undescribed mechanisms of entry.

Location, location, location: tissue-specific regulation of immune responses

Discovery of DCs and PRRs has contributed immensely to our understanding of induction of innate and adaptive immune responses. Activation of PRRs leads to secretion of inflammatory cytokines that regulate priming and differentiation of antigen-specific T and B lymphocytes. Pathogens enter the body via different routes, and although the same set of PRRs is likely to be activated, it is becoming clear that the route of immune challenge determines the nature of outcome of adaptive immunity. In addition to the signaling events initiated following innate-immune receptor activation, the cells of the immune system are influenced by the microenvironments in which they reside, and this has a direct impact on the resulting immune response. Specifically, immune responses could be influenced by specialized DCs, specific factors secreted by stromal cells, and also, by commensal microbiota present in certain organs. Following microbial detection, the complex interactions among DCs, stromal cells, and tissue-specific factors influence outcome of immune responses. In this review, we summarize recent findings on the phenotypic heterogeneity of innate and adaptive immune cells and how tissue-specific factors in the systemic and mucosal immune system influence the outcome of adaptive-immune responses.

Regulation and migratory role of P-selectin ligands during intestinal inflammation

Dendritic cells from mesenteric lymph nodes (MLN) can convert retinal to retinoic acid (RA), which promotes induction of the gut-specific homing receptor α4β7. In contrast, priming within peripheral lymph nodes leads to upregulation of E- and P-selectin ligands (E- and P-lig). Apart from its α4β7 promoting effect, RA was shown to suppress E- and P-lig induction in vitro. However, enhanced frequencies of P-lig(+) CD4(+) T cells were reported during intestinal inflammation. To understand this contradiction, we first determined whether location of intestinal inflammation, that is, ileitis or colitis, affects P-lig induction. Both conditions promoted P-lig expression on CD4(+) T cells; however, P-lig expressed on T cells facilitated Th1 cell recruitment only into the inflamed colon but not into inflamed small intestine induced by oral Toxoplasma gondii infection. A majority of P-lig(+)CD4(+) T cells found within MLN during intestinal inflammation co-expressed α4β7 confirming their activation in the presence of RA. Mesenteric P-lig(+)CD4(+) cells co-expressed the 130 kDa isoform of CD43 which requires activity of core 2 (beta)1,6-N-acetyl-glycosaminyltransferase-I (C2GlcNAcT-I) suggesting that C2GlcNAcT-I contributes to P-lig expression under these conditions. To test whether inflammatory mediators can indeed overrule the inhibitory effect of RA on P-lig expression we stimulated CD4(+) T cells either polyclonal in the presence of IL-12 and IFNγ or by LPS-activated MLN-derived dendritic cells. Both conditions promoted P-lig induction even in the presence of RA. While RA impeded the induction of fucosyltransferase-VII it did not affect IL-12-dependent C2GlcNAcT-I induction suggesting that C2GlcNAcT-I can support P-lig expression even if fucosyltransferase-VII mRNA upregulation is dampened.

Intestinal dendritic cells: their role in intestinal inflammation, manipulation by the gut microbiota and differences between mice and men

The intestinal immune system maintains a delicate balance between immunogenicity against invading pathogens and tolerance of the commensal microbiota and food antigens. Dendritic cells (DC) generate primary T-cell responses, and determine whether these responses are immunogenic or tolerogenic. The regulatory role of DC is of particular importance in the gut due to the high antigenic load. Intestinal DC act as sentinels, sampling potentially pathogenic antigens but also harmless antigens including the commensal microbiota. Following antigen acquisition, intestinal DC migrate to secondary lymphoid organs to activate naive T-cells. DC also imprint specific homing properties on T-cells that they stimulate; gut DC specifically induce gut-homing properties on T-cells upon activation, enabling T-cell migration back to intestinal sites. Data regarding properties on gut DC in humans is scarce, although evidence now supports the role of DC as important players in intestinal immunity in humans. Here, we review the role of intestinal DC in shaping mucosal immune responses and directing tissue-specific T-cell responses, with a special focus on the importance of distinguishing DC subsets from macrophages at intestinal sites. We compare and contrast human DC with their murine counterparts, and discuss the ability of the gut microbiota to shape intestinal DC function, and how this may be dysregulated in inflammatory bowel disease (IBD). Lastly, we describe recent advances in the study of probiotics on intestinal DC function, including the use of soluble secreted bacterial products.

Intestinal DC in migrational imprinting of immune cells

Dendritic cells (DCs) have a pivotal role in instructing antigen-specific immune responses, processing and presenting antigens to CD4(+) and CD8(+) T cells and producing factors capable to modulate the quality of T-cell responses. In this review, we will provide an historic overview on the identification of the mechanisms controlling lymphocyte migration into the largest immune organ of the body: the gut, and we will describe how in recent years an unexpected role for DCs has emerged as the architects in programming gut-homing immune cells. Specifically, we will review how intestinal DCs utilize the dietary vitamin A metabolite retinoic acid (RA) to program gut-homing lymphocytes and how intestinal DCs acquire the unique capacity to become RA producers.

Systemic vaccination induces clonally diverse SIV-specific CD8+ T-cell populations in systemic and mucosal compartments

An HIV-1 vaccine must elicit a clonally diverse virus-specific CD8+ T-cell response to contain mutant virus forms, and these responses must be present in mucosal tissues, which are the site of early HIV-1 replication. We show that systemic delivery of prototype vaccine vectors in rhesus monkeys induced SIV (simian immunodeficiency virus)-specific CD8+ T-cell responses in systemic and mucosal compartments with comparable clonal compositions. Although clonal sharing was maintained between the peripheral blood and lungs, the clonal constituents of the vaccine-induced CD8+ T-cell populations in the gastrointestinal mucosal tissues evolved away from the peripheral blood population. A phenotypic characterization indicated that the divergence was a consequence of differential trafficking and retention of the vaccine-induced cells in mucosal compartments. These findings highlight the circulation of vaccine-induced CD8+ T-cell populations between systemic and mucosal compartments and the importance of the expression of specific homing molecules for localization in mucosal tissues.

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.

The dynamic lives of T cells: new approaches and themes

Activated T cells have classically been thought to progress unidirectionally through discrete phenotypic states and differentiate into static lineages. It is increasingly evident, however, that T cells exhibit much more complex and flexible dynamic behaviors than initially appreciated, and that these behaviors influence the efficacy of T cell responses to immunological challenges. In this review, we discuss how new technologies for monitoring the dynamics of T cells are enhancing the resolution of the fine phenotypic and functional heterogeneity within populations of T cells and revealing how individual T cells transition among a continuum of states. Such insights into the dynamic properties of T cells should improve immune monitoring and inform strategies for therapeutic interventions.

Intravaginal immunization with HPV vectors induces tissue-resident CD8+ T cell responses

The induction of persistent intraepithelial CD8+ T cell responses may be key to the development of vaccines against mucosally transmitted pathogens, particularly for sexually transmitted diseases. Here we investigated CD8+ T cell responses in the female mouse cervicovaginal mucosa after intravaginal immunization with human papillomavirus vectors (HPV pseudoviruses) that transiently expressed a model antigen, respiratory syncytial virus (RSV) M/M2, in cervicovaginal keratinocytes. An HPV intravaginal prime/boost with different HPV serotypes induced 10-fold more cervicovaginal antigen-specific CD8+ T cells than priming alone. Antigen-specific T cell numbers decreased only 2-fold after 6 months. Most genital antigen-specific CD8+ T cells were intra- or subepithelial, expressed αE-integrin CD103, produced IFN-γ and TNF-α, and displayed in vivo cytotoxicity. Using a sphingosine-1-phosphate analog (FTY720), we found that the primed CD8+ T cells proliferated in the cervicovaginal mucosa upon HPV intravaginal boost. Intravaginal HPV prime/boost reduced cervicovaginal viral titers 1,000-fold after intravaginal challenge with vaccinia virus expressing the CD8 epitope M2. In contrast, intramuscular prime/boost with an adenovirus type 5 vector induced a higher level of systemic CD8+ T cells but failed to induce intraepithelial CD103+CD8+ T cells or protect against recombinant vaccinia vaginal challenge. Thus, HPV vectors are attractive gene-delivery platforms for inducing durable intraepithelial cervicovaginal CD8+ T cell responses by promoting local proliferation and retention of primed antigen-specific CD8+ T cells.

Local immunity by tissue-resident CD8(+) memory T cells

Microbial infection primes a CD8⁺ cytotoxic T cell response that gives rise to a long-lived population of circulating memory cells able to provide protection against systemic reinfection. Despite this, effective CD8⁺ T cell surveillance of barrier tissues such as skin and mucosa typically wanes with time, resulting in limited T cell-mediated protection in these peripheral tissues. However, recent evidence suggests that a specialized subset of CD103⁺ memory T cells can permanently lodge and persist in peripheral tissues, and that these cells can compensate for the loss of peripheral immune surveillance by circulating memory T cells. Here, we review evolving concepts regarding the generation and long-term persistence of these tissue-resident memory T cells (T_RM) in epithelial and neuronal tissues. We further discuss the role of T_RM cells in local infection control and their contribution to localized immune phenomena, in both mice and humans.

A combination of local inflammation and central memory T cells potentiates immunotherapy in the skin

Adoptive T cell therapy uses the specificity of the adaptive immune system to target cancer and virally infected cells. Yet the mechanism and means by which to enhance T cell function are incompletely described, especially in the skin. In this study, we use a murine model of immunotherapy to optimize cell-mediated immunity in the skin. We show that in vitro-derived central but not effector memory-like T cells bring about rapid regression of skin-expressing cognate Ag as a transgene in keratinocytes. Local inflammation induced by the TLR7 receptor agonist imiquimod subtly yet reproducibly decreases time to skin graft rejection elicited by central but not effector memory T cells in an immunodeficient mouse model. Local CCL4, a chemokine liberated by TLR7 agonism, similarly enhances central memory T cell function. In this model, IL-2 facilitates the development in vivo of effector function from central memory but not effector memory T cells. In a model of T cell tolerogenesis, we further show that adoptively transferred central but not effector memory T cells can give rise to successful cutaneous immunity, which is dependent on a local inflammatory cue in the target tissue at the time of adoptive T cell transfer. Thus, adoptive T cell therapy efficacy can be enhanced if CD8(+) T cells with a central memory T cell phenotype are transferred, and IL-2 is present with contemporaneous local inflammation.

Immunotherapy of cancer in 2012

The immunotherapy of cancer has made significant strides in the past few years due to improved understanding of the underlying principles of tumor biology and immunology. These principles have been critical in the development of immunotherapy in the laboratory and in the implementation of immunotherapy in the clinic. This improved understanding of immunotherapy, enhanced by increased insights into the mechanism of tumor immune response and its evasion by tumors, now permits manipulation of this interaction and elucidates the therapeutic role of immunity in cancer. Also important, this improved understanding of immunotherapy and the mechanisms underlying immunity in cancer has fueled an expanding array of new therapeutic agents for a variety of cancers. Pegylated interferon-α2b as an adjuvant therapy and ipilimumab as therapy for advanced disease, both of which were approved by the United States Food and Drug Administration for melanoma in March 2011, are 2 prime examples of how an increased understanding of the principles of tumor biology and immunology have been translated successfully from the laboratory to the clinical setting. Principles that guide the development and application of immunotherapy include antibodies, cytokines, vaccines, and cellular therapies. The identification and further elucidation of the role of immunotherapy in different tumor types, and the development of strategies for combining immunotherapy with cytotoxic and molecularly targeted agents for future multimodal therapy for cancer will enable even greater progress and ultimately lead to improved outcomes for patients receiving cancer immunotherapy.

CCL25 induces α₄β₇ integrin-dependent migration of IL-17⁺ γδ T lymphocytes during an allergic reaction

Herein, we provide evidence that during allergic inflammation, CCL25 induces the selective migration of IL-17(+) γδ T cells mediated by α(4) β(7) integrin. Intrapleural injection of CCL25 into ovalbumin (OVA)-immunized C57BL/6 mice triggered the accumulation of γδ T lymphocytes expressing CCR9 (CCL25 receptor) and α(4) β(7) integrin in the pleura, but failed to attract αβ T lymphocytes. CCL25 attracted CCR6(+) γδ T cells producing IL-17 (but not IFN-γ or IL-4). OVA challenge triggered increased production of CCL25 followed by the accumulation of CCR9(+) , α(4) β(7) (+) , and CCR6(+) /IL-17(+) γδ T cells into the pleural cavities of OVA-immunized mice, which was inhibited by the in vivo neutralization of CCL25. The in vivo blockade of α(4) β(7) integrin also inhibited the migration of IL-17(+) γδ T lymphocytes (but not of αβ T lymphocytes) into mouse pleura after OVA challenge, suggesting that the CCL25/α(4) β(7) integrin pathway is selective for γδ T cells. In addition, α(4) β(7) integrin blockade impaired the in vitro transmigration of γδ T cells across endothelium (which expresses α(4) β(7) ligands VCAM-1 and MadCAM-1), which was induced by CCL25 and by cell-free pleural washes recovered from OVA-challenged mice. Our results reveal that during an allergic reaction, CCL25 drives IL-17(+) γδ T-cell mobilization to inflamed tissue via α(4) β(7) integrin and modulates IL-17 levels.

Connecting liver and gut: murine liver sinusoidal endothelium induces gut tropism of CD4+ T cells via retinoic acid

Gut-activated T cells migrating into the liver can cause extraintestinal manifestations of inflammatory bowel disease. T cells acquire a gut-homing phenotype dependent on retinoic acid (RA) provided by intestinal dendritic cells (DC). We investigated whether liver antigen-presenting cells can induce gut tropism supporting an enterohepatic lymphocyte circulation. Priming of CD4(+) T cells by liver sinusoidal endothelial cells (LSEC) supported migration into gut and gut-associated lymphoid tissue. As observed for T cells primed by intestinal DCs, this gut tropism depended on α(4) β(7) integrin and CC chemokine receptor 9 (CCR9) expression by LSEC-primed CD4(+) T cells. The induction of gut-homing molecules was mediated by RA, a derivate of vitamin A that is stored in large amounts within the liver. LSECs expressed functional retinal dehydrogenases and could convert vitamin A to RA. Conversely, the lack of signaling via the RA receptor prevented the expression of α(4) β(7) integrin and CCR9 on LSEC-primed CD4(+) T cells, consequently reducing their in vivo migration to the intestine. Other liver antigen-presenting cells failed to support high expression of α(4) β(7) integrin on CD4(+) T cells, thus, the potential to induce gut homing is restricted to LSECs.

CONCLUSION

The capacity to promote gut tropism via vitamin A use is not unique for intestinal DCs but is also a feature of LSECs. Our data support the assumption that CD4(+) T cells can migrate from the liver to the gut as one branch of a postulated enterohepatic lymphocyte circulation.

Tumor microenvironment and lymphocyte infiltration

There is ample evidence that the presence of tumor-infiltrating T lymphocytes is associated with a favorable prognostic in patients. These observations suggest that a limiting step to immune resistance and immunotherapy could be the capacity of tumor-specific T cells to reach tumor bed. In this article, we review some factors that may influence this infiltration, and in particular the nature of the vasculature, the expression of chemokines or tumor antigens and the presence of dendritic cells and CD4+ T lymphocytes.

T cell homing to epithelial barriers in allergic disease

Allergic inflammation develops in tissues that have large epithelial surface areas that are exposed to the environment, such as the lung, skin and gut. In the steady state, antigen-experienced memory T cells patrol these peripheral tissues to facilitate swift immune responses against invading pathogens. In at least two allergy-prone organs, the skin and the gut, memory T cells are programmed during the initial antigen priming to express trafficking receptors that enable them to preferentially home to these organs. In this review we propose that tissue-specific memory and inflammation-specific T cell trafficking facilitates the development of allergic disease in these organs. We thus review recent advances in our understanding of tissue-specific T cell trafficking and how regulation of T cell trafficking by the chemokine system contributes to allergic inflammation in mouse models and in human allergic diseases of the skin, lung and gut. Inflammation- and tissue-specific T lymphocyte trafficking pathways are currently being targeted as new treatments for non-allergic inflammatory diseases and may yield effective new therapeutics for allergic diseases.

Innate immunity in the central nervous system

Immune responses in the CNS are common, despite its perception as a site of immune privilege. These responses can be mediated by resident microglia and astrocytes, which are innate immune cells without direct counterparts in the periphery. Furthermore, CNS immune reactions often take place in virtual isolation from the innate/adaptive immune interplay that characterizes peripheral immunity. However, microglia and astrocytes also engage in significant cross-talk with CNS-infiltrating T cells and other components of the innate immune system. Here we review the cellular and molecular basis of innate immunity in the CNS and discuss what is known about how outcomes of these interactions can lead to resolution of infection, neurodegeneration, or neural repair depending on the context.

Naive, effector and memory CD8 T-cell trafficking: parallels and distinctions

Trafficking of CD8 T cells, in both the steady-state and during episodes of infection or inflammation, is a highly dynamic process and involves a variety of receptor-ligand interactions. A thorough, mechanistic understanding of how this process is regulated could potentially lead to disease prevention strategies, through either enhancing (for infectious diseases or tumors) or limiting (for autoimmunity) recruitment of antigen-specific CD8 T cells to areas of tissue inflammation. As CD8 T cells transition from naive to effector to memory cells, changes in gene expression will ultimately dictate anatomical localization of these cells in vivo. In this article, we discuss recent advances in understanding how antigenic stimulation influences expression of various trafficking receptors and ligands, and how this determines the tissue localization of CD8 T cells.

IL-1 receptor signaling is required at multiple stages of sensitization and elicitation of the contact hypersensitivity response

Contact hypersensitivity (CHS) is a CD8 T cell-mediated response to hapten skin sensitization and challenge. The points at which IL-1R signaling is required during this complex, multistep immune response have not been clearly delineated. The role of IL-1R signaling during 2, 4 dinitro-1-fluorobenezene (DNFB) sensitization to induce hapten-specific CD8 effector T cells and in the trafficking of the effector T cells to the DNFB challenge site to elicit the response were investigated using IL-1R deficient mice. DNFB-sensitized IL-1R(-/-) mice had low CHS responses to hapten challenge that were caused in part by marked decreases in hapten-specific CD8 T cell development to IL-17- and IFN-γ-producing cells during sensitization. Hapten-primed wild type CD8 T cell transfer to naive IL-1R(-/-) mice did not result in T cell activation in response to hapten challenge, indicating a need for IL-1R signaling for the localization or activation, or both, of the CD8 T cells at the challenge site. Decreased CD8 T cell priming in sensitized IL-1R(-/-) mice was associated with marked decreases in hapten-presenting dendritic cell migration from the sensitized skin to draining lymph nodes. Transfer of hapten-presenting dendritic cells from wild type donors to naive IL-1R(-/-) mice resulted in decreased numbers of the dendritic cells in the draining lymph nodes and decreased priming of hapten-specific CD8 T cells compared with dendritic cell transfer to naive wild type recipients. These results indicate that IL-1R signaling is required at multiple steps during the course of sensitization and challenge to elicit CHS.

Shining a light on intestinal traffic

Inflammatory bowel disease (IBD), encompassing Crohn's disease and ulcerative colitis, is associated with enhanced leukocyte infiltration to the gut, which is directly linked to the clinical aspects of these disorders. Thus, leukocyte trafficking is a major target for IBD therapy. Past and emerging techniques to study leukocyte trafficking both in vitro and in vivo have expanded our knowledge of the leukocyte migration process and the role of inhibitors. Various strategies have been employed to target chemokine- and integrin-ligand interactions within the multistep adhesion cascade and the S1P/S1PR1 axis in leukocyte migration. Though there is an abundance of preclinical data demonstrating efficacy of leukocyte trafficking inhibitors, many have yet to be confirmed in clinical studies. Vigilance for toxicity and further research is required into this complex and emerging area of IBD therapy.

Dendritic cells in cancer immunotherapy: vaccines or autologous transplants?

Dendritic cells (DCs) are the most powerful immunostimulatory cells specialized in the induction and regulation of immune responses. Their properties and the feasibility of their large-scale ex vivo generation led to the application of ex vivo-educated DCs to bypass the dysfunction of endogenous DCs in cancer patients and to induce therapeutic anti-cancer immunity. While multiple paradigms of therapeutic application of DCs reflect their consideration as cancer "vaccines", numerous features of DC-based vaccination resemble those of autologous transplants, resulting in challenges and opportunities that distinguish them from classical vaccines. In addition to the functional heterogeneity of DC subsets and plasticity of the individual DC types, the unique features of DCs are the kinetic character of their function, limited functional stability, and the possibility to imprint in maturing DCs distinct functions relevant for the induction of effective cancer immunity, such as the induction of different effector functions or different homing properties of tumor-specific T cells (delivery of "signal 3" and "signal 4"). These considerations highlight the importance of the application of optimized, potentially patient-specific conditions of ex vivo culture of DCs and their delivery, with the logistic and regulatory implications shared with transplantation and other surgical procedures.

Retinoic acid induces homing of protective T and B cells to the gut after subcutaneous immunization in mice

Diarrheal diseases represent a major health burden in developing countries. Parenteral immunization typically does not induce efficient protection against enteropathogens because it does not stimulate migration of immune cells to the gut. Retinoic acid (RA) is critical for gut immunity, inducing upregulation of gut-homing receptors on activated T cells. In this study, we have demonstrated that RA can redirect immune responses elicited by s.c. vaccination of mice from skin-draining inguinal LNs (ingLNs) to the gut. When present during priming, RA induced robust upregulation of gut-homing receptors in ingLNs, imprinting gut-homing capacity on T cells. Concurrently, RA triggered the generation of gut-tropic IgA+ plasma cells in ingLNs and raised the levels of antigen-specific IgA in the intestinal lumen and blood. RA applied s.c. in vivo induced autonomous RA production in ingLN DCs, further driving efficient induction of gut-homing molecules on effector cells. Importantly, RA-supplemented s.c. immunization elicited a potent immune response in the small intestine that protected mice from cholera toxin–induced diarrhea and diminished bacterial loads in Peyer patches after oral infection with Salmonella. Thus, the use of RA as a gut-homing navigator represents a powerful tool to induce protective immunity in the intestine after s.c. immunization, offering what we believe to be a novel approach for vaccination against enteropathogens.

The human cutaneous chemokine system

Irrespective of the immune status, the vast majority of all lymphocytes reside in peripheral tissues whereas those present in blood only amount to a small fraction of the total. It has been estimated that T cells in healthy human skin outnumber those present in blood by at least a factor of two. How lymphocytes within these two compartments relate to each other is not well understood. However, mounting evidence suggest that the study of T cell subsets present in peripheral blood does not reflect the function of their counterparts at peripheral sites. This is especially true under steady-state conditions whereby long-lived memory T cells in healthy tissues, notably those in epithelial tissues at body surfaces, are thought to fulfill a critical immune surveillance function by contributing to the first line of defense against a series of local threats, including microbes, tumors, and toxins, and by participating in wound healing. The relative scarcity of information regarding peripheral T cells and the factors regulating their localization is primarily due to inherent difficulties in obtaining healthy tissue for the extraction and study of immune cells on a routine basis. This is most certainly true for humans. Here, we review our current understanding of T cell homing to human skin and compare it when possible with gut-selective homing. We also discuss candidate chemokines that may account for the tissue selectivity in this process and present a model whereby CCR8, and its ligand CCL1, selectively regulate the homeostatic migration of memory lymphocytes to skin tissue.

Regional and mucosal memory T cells

After infection, most antigen-specific memory T cells reside in nonlymphoid tissues. Tissue-specific programming during priming leads to directed migration of T cells to the appropriate tissue, which promotes the development of tissue-resident memory in organs such as intestinal mucosa and skin. Mechanisms that regulate the retention of tissue-resident memory T cells include transforming growth factor-β (TGF-β)-mediated induction of the E-cadherin receptor CD103 and downregulation of the chemokine receptor CCR7. These pathways enhance protection in internal organs, such as the nervous system, and in the barrier tissues--the mucosa and skin. Memory T cells that reside at these surfaces provide a first line of defense against subsequent infection, and defining the factors that regulate their development is critical to understanding organ-based immunity.

Vitamin A deficiency impairs vaccine-elicited gastrointestinal immunity

Vitamin A deficiency is highly prevalent in much of the developing world, where vaccination programs are of paramount importance to public health. However, the impact of vitamin A deficiency on the immunogenicity and protective efficacy of vaccines has not been defined previously. In this article, we show that the vitamin A metabolite retinoic acid is critical for trafficking of vaccine-elicited T lymphocytes to the gastrointestinal mucosa and for vaccine protective efficacy in mice. Moderate vitamin A deficiency abrogated Ag-specific T lymphocyte trafficking to the gastrointestinal tract, gastrointestinal cellular immune responses, and protection against a mucosal challenge following immunization with a recombinant adenovirus vaccine vector. Oral vitamin A supplementation as well as retinoic acid administration fully restored the mucosal immune responses and vaccine protective efficacy. These data suggest that oral vitamin A supplementation may be important for optimizing the success of vaccines against HIV-1 and other mucosal pathogens in the developing world, highlighting a critical relationship between host nutritional status and vaccine efficacy.

From the bench to the clinic: New aspects on immunoregulation by vitamin D analogs

The skin is a vital organ that plays a crucial role in defending us from pathogens. Multiple players from the innate and adaptive immune system are involved, such as neutrophils, dendritic cells, lymphocytes and antimicrobial peptides. Chronic inflammatory skin diseases can be mediated by inflammatory T cells and their interactions with other cells in the skin. Vitamin D is generated in the skin upon sun exposure and has a variety of effects. Vitamin D and its analogs have been used with success in treating mild to moderate T cell-mediated skin diseases, but how they mediate the beneficial effects is not well understood. In the recent years, emerging evidence is rising that vitamin D analogs and its modulation on the immune system plays a major role. It has been shown that vitamin D analogs can induce the generation of regulatory T cells, which are able to suppress proliferation and alter the function of inflammatory T cells. This may help explain the therapeutic effects that are observed and at the same time give hope that in combination with other therapy or used alone, vitamin D analogs may be helpful when treating more severe forms of the diseases.

Targeting OX40 promotes lung-resident memory CD8 T cell populations that protect against respiratory poxvirus infection

One goal of vaccination is to promote development of mucosal effector cells that can immediately respond to peripheral infection. This is especially important for protection against viruses that enter the host through the respiratory tract. We show that targeting the OX40 costimulatory receptor (CD134) strongly promotes mucosal memory in the CD8 T cell compartment. Systemic injection of an agonist antibody to OX40 strongly enhanced development of polyfunctional effector CD8 T cells that were induced after intraperitoneal infection with a highly virulent strain of vaccinia virus. These cells were located in lymphoid organs and also the lung, and importantly, long-term memory CD8 T cells were maintained in the lung over 1 year. Anti-OX40 also boosted memory development when mice were vaccinated subcutaneously with viral peptide. These CD8 T cells were sufficient to provide protection from lethal respiratory infection with live vaccinia virus independent of CD4 T cells and antibody. Again, the CD8 T cell populations that were induced after secondary infection displayed polyfunctionality and were maintained in the lung for over a year. These data suggest that agonists to the OX40 costimulatory receptor represent potential candidates for incorporation into vaccines for respiratory viruses.

MyD88-dependent TLR1/2 signals educate dendritic cells with gut-specific imprinting properties

Gut-associated dendritic cells (DC) synthesize all-trans retinoic acid, which is required for inducing gut-tropic lymphocytes. Gut-associated DC from MyD88(-/-) mice, which lack most TLR signals, expressed low levels of retinal dehydrogenases (critical enzymes for all-trans retinoic acid biosynthesis) and were significantly impaired in their ability to induce gut-homing T cells. Pretreatment of extraintestinal DC with a TLR1/2 agonist was sufficient to induce retinal dehydrogenases and to confer these DC with the capacity to induce gut-homing lymphocytes via a mechanism dependent on MyD88 and JNK/MAPK. Moreover, gut-associated DC from TLR2(-/-) mice, or from mice in which JNK was pharmacologically blocked, were impaired in their education to imprint gut-homing T cells, which correlated with a decreased induction of gut-tropic T cells in TLR2(-/-) mice upon immunization. Thus, MyD88-dependent TLR2 signals are necessary and sufficient to educate DC with gut-specific imprinting properties and contribute in vivo to the generation of gut-tropic T cells.

Oral exposure to Trypanosoma cruzi elicits a systemic CD8⁺ T cell response and protection against heterotopic challenge

Trypanosoma cruzi infects millions of people in Latin America and often leads to the development of Chagas disease. T. cruzi infection can be acquired at or near the bite site of the triatomine vector, but per os infection is also a well-documented mode of transmission, as evidenced by recent microepidemics of acute Chagas disease attributed to the consumption of parasite-contaminated foods and liquids. It would also be convenient to deliver vaccines for T. cruzi by the oral route, particularly live parasite vaccines intended for the immunization of reservoir hosts. For these reasons, we were interested in better understanding immunity to T. cruzi following oral infection or oral vaccination, knowing that the route of infection and site of antigen encounter can have substantial effects on the ensuing immune response. Here, we show that the route of infection does not alter the ability of T. cruzi to establish infection in muscle tissue nor does it impair the generation of a robust CD8(+) T cell response. Importantly, oral vaccination with attenuated parasites provides protection against wild-type (WT) T. cruzi challenge. These results strongly support the development of whole-organism-based vaccines targeting reservoir species as a means to alleviate the burden of Chagas disease in affected regions.

Epicutaneous challenge of orally immunized mice redirects antigen-specific gut-homing T cells to the skin

Patients with atopic dermatitis (AD) often suffer from food allergy and develop flares upon skin contact with food allergens. However, it is unclear whether T cells sensitized to allergens in the gut promote this skin inflammation. To address this question, we orally immunized WT mice and mice lacking the skin-homing chemokine receptor Ccr4 (Ccr4-/- mice) with OVA and then challenged them epicutaneously with antigen. Allergic skin inflammation developed in the WT mice but not in the mutants and was characterized by epidermal thickening, dermal infiltration by eosinophils and CD4+ T cells, and upregulation of Th2 cytokines. T cells purified from mesenteric lymph nodes (MLNs) of orally immunized WT mice transferred allergic skin inflammation to naive recipients cutaneously challenged with antigen, but this effect was lost in T cells purified from Ccr4-/- mice. In addition, the ability of adoptively transferred OVA-activated T cells to home to the skin following cutaneous OVA challenge was ablated in mice that lacked lymph nodes. These results indicate that cutaneous exposure to food antigens can reprogram gut-homing effector T cells in LNs to express skin-homing receptors, eliciting skin lesions upon food allergen contact in orally sensitized AD patients.

Tissue-based class control: the other side of tolerance

In this Essay, we offer a new perspective on how immune responses are regulated. We do not cover how they are turned on and off, but focus instead on the second major aspect of an immune response: the control of effector class. Although it is generally thought that the class of an immune response is tailored to fit the invading pathogen, we suggest here that it is primarily tailored to fit the tissue in which the response occurs. To this end, we cover such topics as the nature of T helper (T(H)) cell subsets (current and yet to be discovered), the nature of privileged sites, the difference between oral tolerance and oral vaccination, why the route of immunization matters, whether the T(H)1-type response is really the immune system's primary defense, and whether there might be a different role for some regulatory T cells.

Competitive homing assays to study gut-tropic t cell migration

In order to exert their function lymphocytes need to leave the blood and migrate into different tissues in the body. Lymphocyte adhesion to endothelial cells and tissue extravasation is a multistep process controlled by different adhesion molecules (homing receptors) expressed on lymphocytes and their respective ligands (addressions) displayed on endothelial cells (1 2). Even though the function of these adhesion receptors can be partially studied ex vivo, the ultimate test for their physiological relevance is to assess their role during in vivo lymphocyte adhesion and migration. Two complementary strategies have been used for this purpose: intravital microscopy (IVM) and homing experiments. Although IVM has been essential to define the precise contribution of specific adhesion receptors during the adhesion cascade in real time and in different tissues, IVM is time consuming and labor intensive, it often requires the development of sophisticated surgical techniques, it needs prior isolation of homogeneous cell populations and it permits the analysis of only one tissue/organ at any given time. By contrast, competitive homing experiments allow the direct and simultaneous comparison in the migration of two (or even more) cell subsets in the same mouse and they also permit the analysis of many tissues and of a high number of cells in the same experiment. Here we describe the classical competitive homing protocol used to determine the advantage/disadvantage of a given cell type to home to specific tissues as compared to a control cell population. We chose to illustrate the migratory properties of gut-tropic versus non gut-tropic T cells, because the intestinal mucosa is the largest body surface in contact with the external environment and it is also the extra-lymphoid tissue with the best-defined migratory requirements. Moreover, recent work has determined that the vitamin A metabolite all-trans retinoic acid (RA) is the main molecular mechanism responsible for inducing gut-specific adhesion receptors (integrin a4b7and chemokine receptor CCR9) on lymphocytes. Thus, we can readily generate large numbers of gut-tropic and non gut-tropic lymphocytes ex vivoby activating T cells in the presence or absence of RA, respectively, which can be finally used in the competitive homing experiments described here.

Intraepithelial lymphocytes: to serve and protect

The mucosal immune system is constantly exposed to a wide range of commensal and potentially pathogenic microbial species. Chronic exposure to foreign organisms makes generation of an appropriate immune response critical in maintaining a balance between elimination of harmful pathogens, peaceful coexistence with commensals, and prevention of autoimmunity. Intestinal intraepithelial lymphocytes provide a first line of defense at this extensive barrier with the outside world, and as such, understanding their role in immunity is critical.

CD4+ T cell effects on CD8+ T cell location defined using bioluminescence

T lymphocytes of the CD8+ class are critical in delivering cytotoxic function and in controlling viral and intracellular infections. These cells are “helped” by T lymphocytes of the CD4+ class, which facilitate their activation, clonal expansion, full differentiation and the persistence of memory. In this study we investigated the impact of CD4+ T cells on the location of CD8+ T cells, using antibody-mediated CD4+ T cell depletion and imaging the antigen-driven redistribution of bioluminescent CD8+ T cells in living mice. We documented that CD4+ T cells influence the biodistribution of CD8+ T cells, favoring their localization to abdominal lymph nodes. Flow cytometric analysis revealed that this was associated with an increase in the expression of specific integrins. The presence of CD4+ T cells at the time of initial CD8+ T cell activation also influences their biodistribution in the memory phase. Based on these results, we propose the model that one of the functions of CD4+ T cell “help” is to program the homing potential of CD8+ T cells.