Langerhans cells activate naive self-antigen-specific CD8 T cells in the steady state

TRPV4 Activation Increases the Expression of CD207 (Langerin) of Monocyte-Derived Langerhans Cells without Affecting their Maturation

Langerhans cells (LCs) are the sole professional antigen-presenting cell normally found in the human epidermal compartment. Research into their physiological role is hindered by the fact that they are invariably activated during isolation from the skin. To overcome this challenge, we turned to a monocyte-derived LC (moLC) model, which we characterized with RNA sequencing, and compared the transcriptome of moLCs with that of donor-matched immature dendritic cells. We found that moLCs express markers characteristic of LC2 cells as well as TRPV4. TRPV4 is especially important in the skin because it has been linked to the conservation of the skin barrier, immunological responses, as well as acute and chronic itch, but we know little about its function on LCs. Our results show that TRPV4 activation increased the expression of Langerin and led to increased intracellular calcium concentration in moLCs. Regarding the functionality of moLCs, we found that TRPV4 agonism had a mitigating effect on their inflammatory responses because it decreased their cytokine production and T-cell activating capability. Because TRPV4 has emerged as a potential therapeutic target in dermatological conditions, it is important to highlight LCs as, to our knowledge, a previously unreported target of these therapies.

B cell tolerance to epidermal ribonuclear-associated neo-autoantigen in vivo

Defining how self-antigens are perceived by the immune system is pivotal to understand how tolerance is maintained under homeostatic conditions. Clinically relevant, natural autoantigens targeted by autoantibodies, in e.g. systemic lupus erythematosus (SLE), commonly have an intrinsic ability to engage not only the B cell receptor (BCR), but also a co-stimulatory pathway in B cells, such as the Toll-like receptor (TLR)-7 pathway. Here we developed a novel mouse model displaying inducible expression of a fluorescent epidermal neo-autoantigen carrying an OT-II T cell epitope, B cell antigen and associated ribonucleic acids capable of stimulating TLR-7. The neo-autoantigen was expressed in skin, but did not drain in intact form into draining lymph nodes, even after ultraviolet B (UVB)-stimulated induction of apoptosis in the basal layer. Adoptively transferred autoreactive B cells were excluded follicularly and perished at the T-B border in the spleen, preventing their recirculation and encounter with antigen peripherally. This transitional check-point was bypassed by crossing the reporter to a BCR knock-in line on a C4-deficient background. Adoptively transferred OT-II T cells homed rapidly into cutaneous lymph nodes and up-regulated CD69. Surprisingly, however, tolerance was not broken, as the T cells subsequently down-regulated activation markers and contracted. Our results highlight how sequestration of intracellular and peripheral antigen, the transitional B cell tolerance check-point and T cell regulation co-operate to maintain immunological tolerance in vivo.

Langerhans Cells Maintain Local Tissue Tolerance in a Model of Systemic Autoimmune Disease

Systemic autoimmune diseases such as lupus affect multiple organs, usually in a diverse fashion where only certain organs are affected in individual patients. It is unclear whether the "local" immune cells play a role in regulating tissue specificity in relation to disease heterogeneity in systemic autoimmune diseases. In this study, we used skin as a model to determine the role of tissue-resident dendritic cells (DCs) in local and systemic involvement within a systemic lupus disease model. Skin-resident DCs, namely, Langerhans cells (LCs), have been implicated in regulating tolerance or autoimmunity using elegant transgenic models, however, their role in local versus systemic immune regulation is unknown. We demonstrate that although lymphocytes from skin-draining lymph nodes of autoimmune-prone MRL/MpJ-Fas(lpr/lp) (r) (MRL-lpr) mice react spontaneously to a physiological skin self-Ag desmoglein-3, epicutaneous applications of desmoglein-3 induced tolerance that is dependent on LCs. Inducible ablation of LCs in adult preclinical MRL-lpr and MRL/MpJ-Fas(+/+) mice resulted in increased autoantibodies against skin Ags and markedly accelerated lupus dermatitis with increased local macrophage infiltration, but had no effect on systemic autoantibodies such as anti-dsDNA Abs or disease in other organs such as kidneys, lung, and liver. Furthermore, skin-draining lymph nodes of LC-ablated MRL-lpr mice had significantly fewer CD4(+) T cells producing anti-inflammatory cytokine IL-10 than LC-intact controls. These results indicate that a skin-resident DC population regulates local tolerance in systemic lupus and emphasize the importance of the local immune milieu in preventing tissue-specific autoimmunity, yet have no effect on systemic autoimmunity.

Self-Antigen Presentation by Keratinocytes in the Inflamed Adult Skin Modulates T-Cell Auto-Reactivity

Keratinocytes have a pivotal role in the regulation of immune responses, but the impact of antigen presentation by these cells is still poorly understood, particularly in a situation where the antigen will be presented only in adult life. Here, we generated a transgenic mouse model in which keratinocytes exclusively present a myelin basic protein (MBP) peptide covalently linked to the major histocompatibility complex class II β-chain, solely under inflammatory conditions. In these mice, inflammation caused by epicutaneous contact sensitizer treatment resulted in keratinocyte-mediated expansion of MBP-specific CD4(+) T cells in the skin. Moreover, repeated contact sensitizer application preceding a systemic MBP immunization reduced the reactivity of the respective CD4(+) T cells and lowered the symptoms of the resulting experimental autoimmune encephalomyelitis. This downregulation was CD4(+) T-cell-mediated and dependent on the presence of the immune modulator Dickkopf-3. Thus, presentation of a neo self-antigen by keratinocytes in the inflamed, adult skin can modulate CD4(+) T-cell auto-aggression at a distal organ.

Programmed cell death 1 and Helios distinguish TCR-αβ+ double-negative (CD4-CD8-) T cells that derive from self-reactive CD8 T cells

TCR-αβ(+) double-negative (DN; CD4(-)CD8(-)) T cells represent a poorly understood cellular subset suggested to contribute to the pathogenesis of the autoimmune disease systemic lupus erythematosus. DN T cells have been proposed to derive from CD8(+) cells. However, the conditions that govern the loss of CD8 expression after Ag encounter are unknown. In this study, we tracked the fate of CD8 T cells from transgenic TCR mice exposed to their cognate Ags as self or in the context of infection. We demonstrate that CD8 T cells lose CD8 expression and become DN only when cognate Ag is sensed as self. This process is restricted to tissues where the Ag is present. We also show that DN T cells derived from self-reactive CD8 cells express the inhibitory molecules PD-1 and Helios. These molecules identify a subset of DN T cells in normal mice. A similar population expands when CD8 T cells from repertoires enriched in self-reactive cells (Aire-deficient) are transferred into cognate hosts. Collectively, our data suggest that a subset of DN T cells, identified by the expression of PD-1 and Helios, represent self-reactive cells. Our results provide an explanation for the origin of DN T cells and introduce CD8 loss as a process associated with self-Ag encounter.

Development and function of dendritic cell subsets

Classical dendritic cells (cDCs) form a critical interface between innate and adaptive immunity. As myeloid immune cell sentinels, cDCs are specialized in the sensing of pathogen challenges and cancer. They translate the latter for T cells into peptide form. Moreover, cDCs provide additional critical information on the original antigen context to trigger a diverse spectrum of appropriate protective responses. Here we review recent progress in our understanding of cDC subsets in mice. We will discuss cDC subset ontogeny and transcription factor dependencies, as well as emerging functional specializations within the cDC compartment in lymphoid and nonlymphoid tissues.

Enhanced responses to tumor immunization following total body irradiation are time-dependent

The development of successful cancer vaccines is contingent on the ability to induce effective and persistent anti-tumor immunity against self-antigens that do not typically elicit immune responses. In this study, we examine the effects of a non-myeloablative dose of total body irradiation on the ability of tumor-naïve mice to respond to DNA vaccines against melanoma. We demonstrate that irradiation followed by lymphocyte infusion results in a dramatic increase in responsiveness to tumor vaccination, with augmentation of T cell responses to tumor antigens and tumor eradication. In irradiated mice, infused CD8⁺ T cells expand in an environment that is relatively depleted in regulatory T cells, and this correlates with improved CD8⁺ T cell functionality. We also observe an increase in the frequency of dendritic cells displaying an activated phenotype within lymphoid organs in the first 24 hours after irradiation. Intriguingly, both the relative decrease in regulatory T cells and increase in activated dendritic cells correspond with a brief window of augmented responsiveness to immunization. After this 24 hour window, the numbers of dendritic cells decline, as does the ability of mice to respond to immunizations. When immunizations are initiated within the period of augmented dendritic cell activation, mice develop anti-tumor responses that show increased durability as well as magnitude, and this approach leads to improved survival in experiments with mice bearing established tumors as well as in a spontaneous melanoma model. We conclude that irradiation can produce potent immune adjuvant effects independent of its ability to induce tumor ablation, and that the timing of immunization and lymphocyte infusion in the irradiated host are crucial for generating optimal anti-tumor immunity. Clinical strategies using these approaches must therefore optimize such parameters, as the correct timing of infusion and vaccination may mean the difference between an ineffective treatment and successful tumor eradication.

Initial afferent lymphatic vessels controlling outbound leukocyte traffic from skin to lymph nodes

Tissue drains fluid and macromolecules through lymphatic vessels (LVs), which are lined by a specialized endothelium that expresses peculiar differentiation proteins, not found in blood vessels (i.e., LYVE-1, Podoplanin, PROX-1, and VEGFR-3). Lymphatic capillaries are characteristically devoid of a continuous basal membrane and are anchored to the ECM by elastic fibers that act as pulling ropes which open the vessel to avoid edema if tissue volume increases, as it occurs upon inflammation. LVs are also crucial for the transit of T lymphocytes and antigen presenting cells from tissue to draining lymph nodes (LN). Importantly, cell traffic control across lymphatic endothelium is differently regulated under resting and inflammatory conditions. Under steady-state non-inflammatory conditions, leukocytes enter into the lymphatic capillaries through basal membrane gaps (portals). This entrance is integrin-independent and seems to be mainly guided by CCL21 chemokine gradients acting on leukocytes expressing CCR7. In contrast, inflammatory processes in lymphatic capillaries involve a plethora of cytokines, chemokines, leukocyte integrins, and other adhesion molecules. Importantly, under inflammation a role for integrins and their ligands becomes apparent and, as a consequence, the number of leukocytes entering the lymphatic capillaries multiplies several-fold. Enhancing transmigration of dendritic cells en route to LN is conceivably useful for vaccination and cancer immunotherapy, whereas interference with such key mechanisms may ameliorate autoimmunity or excessive inflammation. Recent findings illustrate how, transient cell-to-cell interactions between lymphatic endothelial cells and leukocytes contribute to shape the subsequent behavior of leukocytes and condition the LV for subsequent trans-migratory events.

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.

Immunobiology of dendritic cells and the influence of HIV infection

Recent progress in phenotyping of human dendritic cells (DCs) has allowed a closer alignment of the classification and functions of murine and human dendritic cell subsets. Marked differences in the functions of these human DC subsets and their response to HIV infection have become apparent, relevant to HIV pathogenesis and vaccine and microbicide development. Systems biology approaches to studying HIV uptake and infection of dendritic cells has revealed how markedly HIV subverts their functions, especially in relation to the trafficking pathways and viral transfer to T cells. Furthermore the interactions between DCs and other innate immune cells, NK cells, NKT cells and gamma delta T cells are now known to influence DC and T cell function and are also disturbed by HIV infection in vitro and in vivo. Such cellular interactions are potential targets for vaccine adjuvants and immunotherapy.

Layers of dendritic cell-mediated T cell tolerance, their regulation and the prevention of autoimmunity

The last decades of Nobel prize-honored research have unequivocally proven a key role of dendritic cells (DCs) at controlling both T cell immunity and tolerance. A tight balance between these opposing DC functions ensures immune homeostasis and host integrity. Its perturbation could explain pathological conditions such as the attack of self tissues, chronic infections, and tumor immune evasion. While recent insights into the complex DC network help to understand the contribution of individual DC subsets to immunity, the tolerogenic functions of DCs only begin to emerge. As these consist of many different layers, the definition of a “tolerogenic DC” is subjected to variation. Moreover, the implication of DCs and DC subsets in the suppression of autoimmunity are incompletely resolved. In this review, we point out conceptual controversies and dissect the various layers of DC-mediated T cell tolerance. These layers include central tolerance, Foxp3⁺ regulatory T cells (Tregs), anergy/deletion and negative feedback regulation. The mode and kinetics of antigen presentation is highlighted as an additional factor shaping tolerance. Special emphasis is given to the interaction between layers of tolerance as well as their differential regulation during inflammation. Furthermore, potential technical caveats of DC depletion models are considered. Finally, we summarize our current understanding of DC-mediated tolerance and its role for the suppression of autoimmunity. Understanding the mechanisms of DC-mediated tolerance and their complex interplay is fundamental for the development of selective therapeutic strategies, e.g., for the modulation of autoimmune responses or for the immunotherapy of cancer.

Langerhans cells protect from allergic contact dermatitis in mice by tolerizing CD8(+) T cells and activating Foxp3(+) regulatory T cells

Allergic contact dermatitis is the most frequent occupational disease in industrialized countries. It is caused by CD8(+) T cell-mediated contact hypersensitivity (CHS) reactions triggered at the site of contact by a variety of chemicals, also known as weak haptens, present in fragrances, dyes, metals, preservatives, and drugs. Despite the myriad of potentially allergenic substances that can penetrate the skin, sensitization is relatively rare and immune tolerance to the substance is often induced by as yet poorly understood mechanisms. Here we show, using the innocuous chemical 2,4-dinitrothiocyanobenzene (DNTB), that cutaneous immune tolerance in mice critically depends on epidermal Langerhans cells (LCs), which capture DNTB and migrate to lymph nodes for direct presentation to CD8(+) T cells. Depletion and adoptive transfer experiments revealed that LCs conferred protection from development of CHS by a mechanism involving both anergy and deletion of allergen-specific CD8(+) T cells and activation of a population of T cells identified as ICOS(+)CD4(+)Foxp3(+) Tregs. Our findings highlight the critical role of LCs in tolerance induction in mice to the prototype innocuous hapten DNTB and suggest that strategies targeting LCs might be valuable for prevention of cutaneous allergy.

Innate immune factors associated with HIV-1 transmission

PURPOSE OF REVIEW

Relatively little is known with regards to the mechanisms of HIV-1 transmission across a mucosal surface and more specifically what effects host factors have on influencing infection and early viral dissemination. The purpose of this review is to summarize which factors of the innate immune response can influence mucosal transmission of HIV-1.

RECENT FINDINGS

A large array of cell types reside at the mucosal surface ranging from Langerhans cells, dendritic cells, macrophages as well as CD4⁺ lymphocytes, all of which interact with the virus in a unique and different way and which can contribute to risk of HIV-1 transmission. Numerous factors present in bodily secretions as well as the carrier fluids of HIV-1 (breast milk, vaginal secretions, semen and intestinal mucus) can influence transmission and early virus replication. These range from cytokines, chemokines, small peptides, glycoproteins as well as an array of host intracellular molecules which can influence viral uncoating, reverse transcription as well as egress from the infected cell.

SUMMARY

Better understanding the cellular mechanisms of HIV-1 transmission and how different host factor can influence infection will aide in the future development of vaccines, microbicides, and therapies.

TLR7 enables cross-presentation by multiple dendritic cell subsets through a type I IFN-dependent pathway

Conjugation of TLR agonists to protein or peptide antigens has been demonstrated in many studies to be an effective vaccine formula in inducing cellular immunity. However, the molecular and cellular mediators involved in TLR-induced immune responses have not been carefully examined. In this study, we identify Type I IFN and IL-12 as critical mediators of cross-priming induced by a TLR7 agonist-antigen conjugate. We demonstrate that TLR7-driven cross-priming requires both Type I IFN and IL-12. Signaling through the IFN-αβR was required for the timely recruitment and accumulation of activated dendritic cells in the draining lymph nodes. Although IL-12 was indispensable during cross-priming, it did not regulate DC function. Therefore, the codependency for these 2 cytokines during TLR7-induced cross-priming is the result of their divergent effects on different cell-types. Furthermore, although dermal and CD8α(+) DCs were able to cross-prime CD8(+) T cells, Langerhans cells were unexpectedly found to potently cross-present antigen and support CD8(+) T-cell expansion, both in vitro and in vivo. Collectively, the data show that a TLR7 agonist-antigen conjugate elicits CD8(+) T-cell responses by the coordinated recruitment and activation of both tissue-derived and lymphoid organ-resident DC subsets through a Type I IFN and IL-12 codependent mechanism.

Specific immunotherapy and turning off the T cell: how does it work?

OBJECTIVE

To examine T-regulatory (Treg) cell functions in allergic immune responses and their roles during allergen specific immunotherapy based on recent developments and current understanding of immune regulation.

DATA SOURCES

PubMed search of English-language articles regarding Treg cells and allergen specific immunotherapy.

STUDY SELECTION

Articles on the subject matter were selected and reviewed.

RESULTS

Allergen specific immunotherapy is the ultimate treatment modality targeting the immunopathogenic mechanisms of allergic disorders. A diminished allergen-specific T-cell proliferation and suppressed secretion of T(H)1- and T(H)2-type cytokines are the characteristic hallmarks. In addition, Treg cells inhibit the development of allergen-specific T(H)2 and T(H)1 cell responses and therefore exert key roles in healthy immune response to allergens. Treg cells potently suppress IgE production and directly or indirectly control the activity of effector cells of allergic inflammation, such as eosinophils, basophils, and mast cells.

CONCLUSION

As advancements in the field of allergen specific immunotherapy ensue, they may provide novel progression of more rational and safer approaches for the prevention and treatment of allergic disorders. Currently, the Treg cell field is an open research area to increase our understanding in mechanisms of peripheral tolerance to allergens.

Langerhans cells regulate cutaneous injury by licensing CD8 effector cells recruited to the skin

Langerhans cells (LCs) are a distinct population of dendritic cells that form a contiguous network in the epidermis of the skin. Although LCs possess many of the properties of highly proficient dendritic cells, recent studies have indicated that they are not necessary to initiate cutaneous immunity. In this study, we used a tractable model of cutaneous GVHD, induced by topical application of a Toll-like receptor agonist, to explore the role of LCs in the development of tissue injury. By adapting this model to permit inducible and selective depletion of host LCs, we found that GVHD was significantly reduced when LCs were absent. However, LCs were not required either for CD8 T-cell activation within the draining lymph node or subsequent homing of effector cells to the epidermis. Instead, we found that LCs were necessary for inducing transcription of IFN-γ and other key effector molecules by donor CD8 cells in the epidermis, indicating that they license CD8 cells to induce epithelial injury. These data demonstrate a novel regulatory role for epidermal LCs during the effector phase of an inflammatory immune response in the skin.

Glucocorticosteroids modify Langerhans cells to produce TGF-β and expand regulatory T cells

Although glucocorticosteroids (GCSs) have been used for many decades in transplantation and (auto)inflammatory diseases, the exact mechanisms responsible for their immunosuppressive properties are not fully understood. The purpose of this study was to characterize the effects of oral GCSs on the cutaneous immune response. We analyzed, by immunofluorescence staining and quantitative RT-PCR, residual skin biopsy material from a clinical study in which we had used oral GCS as positive control for determining the effects of candidate anti-inflammatory compounds on epicutaneous patch tests of Ni-allergic patients. Expectedly, oral GCS treatment led to a reduction of clinical symptoms and infiltrating leukocytes. Notably, we observed increased numbers of dermal FOXP3(+)CD25(+) T cells and epidermal Langerhans cells (LCs) that were associated with upregulated mRNA expression of TGF-β in lesions of GCS-treated Ni-allergic patients. To investigate this phenomenon further, we exposed purified LCs to GCS. They exhibited, in contrast to GCS-nonexposed LCs, 1) a more immature phenotype, 2) higher intracellular amounts of TGF-β, and 3) increased receptor activator for NF-κB expression, conditions that reportedly favor the expansion of regulatory T cells (Tregs). Indeed, we observed an enhancement of functionally suppressive FOXP3(+) T cells when CD3(+) cells were incubated with GCS-pretreated LCs. The expansion of Tregs was inhibited by TGF-β blockage alone, and their suppressive activity was neutralized by a combination of anti-TGF-β and anti-IL-10 Abs. Our data show that systemically applied GCSs endow LCs with Treg-promoting properties and thus shed new light on the mechanisms of GCS-mediated immunosuppression.

Viruses and Langerhans cells

Langerhans cells (LCs) are the resident dendritic cells (DCs) of epidermis in human mucosal stratified squamous epithelium and the skin. A phenotypically similar DC has recently been discovered as a minor population in the murine dermis. In epidermis, LCs function as sentinel antigen-presenting cells that can capture invading viruses such as herpes simplex virus (HSV), varicella-zoster virus (VZV) and human immunodeficiency virus (HIV). This interaction between LCs and viruses results in highly variable responses, depending on the virus as discussed in this review. For example, HSV induces apoptosis in LCs but HIV does not. LCs seem to be the first in a complex chain of antigen presentation to T cells in lymph nodes for HSV and possibly VZV, or they transport virus to T cells, as described for HIV and maybe VZV. Together with epidermal keratinocytes they may also have a role in the initial innate immune response at the site of infection in the epidermis, although this is not fully known. The full spectrum of biological responses of LCs even to these viruses has yet to be understood and will require complementary studies in human LCs in vitro and in murine models in vivo.

The use of mouse models to better understand mechanisms of autoimmunity and tolerance

A major emphasis of our studies has been on developing a better understanding of how and why the skin serves as a target for immune reactions as well as how the skin evades becoming a target for destruction. For these studies we developed transgenic mice that express a membrane-tethered form of a model self antigen, chicken ovalbumin (mOVA), under the control of a keratin 14 (K14) promoter. K14-mOVA transgenic mice that express OVA mRNA and protein in the epithelia have been assessed for their immune responsiveness to OVA and are being used as targets for T cells obtained from OT-1 transgenic mice whose CD8+ T cells carry a Vα2/Vβ5-transgenic T cell receptor with specificity for the OVA(257-264)-peptides (OVAp) in association with class I MHC antigens. Some of the K14-mOVA transgenic mice develop a graft-versus-host-like disease (GvHD) when the OT-1 cells are injected while others appear to be tolerant to the OT-1 cells. We found that γc cytokines, especially IL-15, determine whether autoimmunity or tolerance ensues in K14-mOVA Tg mice. We also developed transgenic mice that express soluble OVA under the control of a K14 promoter (K14-sOVA) that die within 5-8 days after adoptive transfer of OT-1 cells and identified these mice as a model for more acute GvHD-like reactions. Spontaneous autoimmunity occurs when these K14-sOVA mice are crossed with the OT-I mice. In contrast, we found that preventive or therapeutic OVAp injections induced a dose-dependent increase in survival. In this review the characterization of 5 strains of K14-OVATg mice and underlying mechanisms involved in autoimmune reactions in these Tg mice are discussed. We also describe a strategy to break tolerance and describe how the autoimmunity can be obviated using OVAp. Finally, a historical overview of using transgenic mice to assess the mechanisms of tolerance is also provided.

Langerhans cells and more: langerin-expressing dendritic cell subsets in the skin

Langerhans cells (LCs) are antigen-presenting dendritic cells (DCs) that reside in epithelia. The best studied example is the LC of the epidermis. By electron microscopy, their identifying feature is the unique rod- or tennis racket-shaped Birbeck granule. The phenotypic hallmark is their expression of the C-type lectin receptor langerin/CD207. Langerin, however, is also expressed on a recently discovered population of DC in the dermis and other tissues of the body. These 'dermal langerin(+) dendritic cells' are unrelated to LCs. The complex field of langerin-negative dermal DCs is not dealt with here. In this article, we briefly review the history, ontogeny, and homeostasis of LCs. More emphasis is laid on the discussion of functional properties in vivo. Novel models using genetically engineered mice are contributing tremendously to our understanding of the role of LCs in eliciting adaptive immune responses against pathogens or tumors and in inducing and maintaining tolerance against self antigens and innocuous substances in vivo. Also, innate effector functions are increasingly being recognized. Current activities in this area are reviewed, and possibilities for future exploitation of LC in medicine, e.g. for the improvement of vaccines, are contemplated.

The evolving function of Langerhans cells in adaptive skin immunity

Langerhans cells (LC) are dendritic cell that resides in the epidermis of skin. Paul Langerhans originally observed and named this epinonymous cell more than 140 years ago. Their network-like distribution and dendritic processes that extended up into the stratum corneum convinced him that they represented peripheral nerve cells. It was not determined until almost 100 years later that LC are, in fact, bone marrow-derived and function as skin-resident antigen presenting cells. Many studies have shown that LC are highly immunostimulatory. Recently, data have begun to accumulate suggesting LC have immunoregulatory properties. This review will focus on the participation of LC in the development and regulation of adaptive immune responses.

Revisiting the tolerogenicity of epidermal Langerhans cells

Langerhans cells (LC) are unique members of the dendritic cell (DC) family residing in the epidermis of skin and mucosa. Specific autocrine and environmental factors shape the biology of LC, such as TGF-beta1, IL-10, vitamin D(3), UV light or neuropeptides, which are required for LC development but also influence their capacity to induce immunity or tolerance. Both, immunogenic and tolerogenic functions require antigen transport from the skin to the draining lymph nodes, but the LC maturation grade directs the differential outcome. In this review, we recapitulate early indications for LC tolerogenicity and oppose them to more recent findings with gene-targeted mice, which dramatically challenged some of the early results. The newly discovered Langerin(+) dermal DC subset (DDC) seems to be responsible also for many tolerogenic effects that were initially attributed to steady state migratory LC. Transfer of antigens from LC to other DC subsets as well as transport of HIV are discussed as part of the complex interactions between LC and other cells or as mechanisms of immune evasion. Finally, the first clinical trials on allergy therapies targeting skin DC in the steady state are mentioned as they may open the door to curative tolerogenic therapies.

Epidermal gammadelta T cells sense precancerous cellular dysregulation and initiate immune responses

Hyperplasia associated with a loss of tissue homeostasis can induce DNA replication stress, leading to precancerous dysregulation. Epidermal gammadelta T cells reside in the primary barrier that protects against diverse environmental insults; however, the functions of these T cells in tissue surveillance are not completely understood. In mice with inducible Notch1 inactivation in keratinocytes that causes epidermal hyperplasia, epidermal gammadelta T cells sensed stressed keratinocytes and migrated into the cutaneous draining lymph nodes. Simultaneous induction of beta-galactosidase (beta-Gal) as a putative antigen expressed in the process of precancerous dysregulation and Notch1 ablation in the epidermis resulted in elevated beta-Gal-specific IgG2a production. Epidermal gammadelta T cells were found to have the capacity to express chemokine (C-C motif) receptor 7 and migrate into the lymph nodes. Cutaneous draining lymph node cells in Notch1-inactivated mice expressed high levels of IFN-gamma upon anti-CD3 plus anti-CD28 stimulation. Furthermore, induced expression of beta-Gal in mice that lacked epidermal gammadelta T cells failed to induce anti-beta-Gal IgG. These results suggest that epidermal gammadelta T cells play an essential role in the initiation process of epidermal antigen-specific humoral immune responses and demonstrate the importance of epidermal gammadelta T cells in sensing precancerous dysregulation and activating adaptive immunity.

Cyclophosphamide enhances immunity by modulating the balance of dendritic cell subsets in lymphoid organs

Cyclophosphamide (CTX), a commonly used chemotherapeutic agent can enhance immune responses. The ability of CTX to promote the proliferation of effector T cells and abrogate the function of regulatory T cells (Tregs) has been described. In this study, we examined the effects of CTX treatment on dendritic cell (DC) subsets and the subsequent outcome on the effector and suppressive arms of adaptive immunity. In secondary lymphoid tissues, tissue-derived migratory DCs (migratory DCs), lymphoid tissue-resident DCs (resident DCs), and plasmacytoid DCs (pDCs) are well described. CTX has profound and selective cytotoxic effects on CD8(+) resident DCs, but not skin-derived migratory DCs or pDCs in lymph nodes (LNs) and spleen, causing an imbalance among these DC subsets. CTX treatment increases the potency of DCs in antigen presentation and cytokine secretion, and partially inhibits the suppressor activity of Tregs. Adoptive transfer of CD8(+) DCs can reconstitute this population in regional draining LNs and abrogate the immune-enhancing effects of CTX in vivo. These findings demonstrate that CTX may improve immune responses by preferentially depleting CD8(+) lymphoid-resident DCs, which leads to diminished Treg suppression and enhanced effector T-cell function in vivo.

Human langerhans cells are more efficient than CD14(-)CD1c(+) dermal dendritic cells at priming naive CD4(+) T cells

Few data are available regarding the role of human skin dendritic cells (DCs) in driving T-cell responses. In this study we analyzed the relative capacity of Langerhans cells (LCs) and dermal CD14(-)CD1c(+) DCs (DDCs) to trigger naive CD4(+) T-cell proliferation and differentiation. DC subsets were purified after a 2-day migration from epidermis and dermis of the same skin sample. Migratory LCs showed far more activated phenotype than CD1c(+)DDCs and distinct expression of new molecules of the B7 family; when compared with LCs, CD1c(+)DDCs showed higher PD-L1 and lower inducible co-stimulator ligand (ICOS-L) expression. As expected, CD1c(+)DDCs showed lower allostimulatory property than LCs, a process that was partly reversed by anti-PD-L1 mAb. LCs were significantly more efficient than CD1c(+)DDCs at inducing allogeneic naive CD4(+) T cells to secrete both T helper cell 1 (Th1; IFN-gamma and tumor necrosis factor-alpha ) and Th2 (IL-4 and IL-5) cytokines. Moreover, anti-PD-L1 mAb increased the production of IFN-gamma by both LC- and CD1c(+)DDC-stimulated T cells. Globally, these results argue for a preponderant role of human LCs in inducing naive CD4(+) T-cell priming. Low expression of co-stimulatory molecules together with high expression of PD-L1 might limit the efficiency of CD1c(+)DDCs at inducing naive CD4(+) T-cell proliferation and secretion of cytokines.

Dendritic cell migration limits the duration of CD8+ T-cell priming to peripheral viral antigen

CD8(+) T cells (T(CD8(+))) play a crucial role in immunity to viruses. Antiviral T(CD8(+)) are initially activated by recognition of major histocompatibility complex (MHC) class I-peptide complexes on the surface of professional antigen-presenting cells (pAPC). Migration of pAPC from the site of infection to secondary lymphoid organs is likely required during a natural infection. Migrating pAPC can be directly infected with virus or may internalize antigen derived from virus-infected cells. The use of experimental virus infections to assess the requirement for pAPC migration in initiation of T(CD8(+)) responses has proven difficult to interpret because injected virus can readily drain to secondary lymphoid organs without the need for cell-mediated transport. To overcome this ambiguity, we examined the generation of antigen-specific T(CD8(+)) after immunization with recombinant adenoviruses that express antigen driven by skin-specific or ubiquitous promoters. We show that the induction of T(CD8(+)) in response to tissue-targeted antigen is less efficient than the response to ubiquitously expressed antigen and that the resulting T(CD8(+)) fail to clear all target cells pulsed with the antigenic peptide. This failure to prime a fully functional T(CD8(+)) response results from a reduced period of priming to peripherally expressed antigen versus ubiquitously expressed antigen and correlated with a brief burst of pAPC migration from the skin, a requirement for induction of the response to peripheral antigen. These results indicate that a reduced duration of pAPC migration after virus infection likely reduces the amplitude of the T(CD8(+)) response, allowing persistence of the peripheral virus.

Induction of peripheral CD4+ T-cell tolerance and CD8+ T-cell cross-tolerance by dendritic cells

DC can present and cross-present self-antigens to autoreactive CD4(+) and CD8(+) T cells, respectively, and incapacitate them by inducing anergy, deletion or converting them into Treg. In this review, we summarize the recent progress in immune tolerance research, which has been achieved by employing antigen- and TCR-transgenic mice. We cover the numerous discoveries that have furthered our knowledge of the DC subsets and maturation pathways involved in tolerance; the signals, such as CD70, TGF-beta, B7-H1/PD-L1, which dictate the decision between immunity and tolerance; and the in vivo role of DC in the maintenance of CD4(+) T-cell tolerance and CD8(+) T-cell cross-tolerance.

Keratinocytes function as accessory cells for presentation of endogenous antigen expressed in the epidermis

The precise contribution(s) of skin dendritic cells (DCs) to immune responses in the skin has not been well delineated. We developed an intradermal (i.d.) injection model in which CD8+ T (OT-I) cells that express ovalbumin (OVA) peptide-specific TCRs (Valpha2/Vbeta5) are delivered directly to the dermis of transgenic (Tg) mice expressing OVA in the epidermis. After i.d. injection, these mice reliably develop skin graft-versus-host disease (GVHD) by day 7. To determine the relative contribution of Langerhans cells (LCs) to the ensuing GVHD-like reaction, we generated K14-OVA x Langerin-diphtheria-toxin-receptor (Langerin-DTR) Tg mice to allow conditional ablation of LCs in the epidermis. To delineate the role of dermal DCs (dDCs) in the reaction, we also generated K14-OVA Tg chimeras using beta(2)-microglobulin-deficient (beta(2)m) congenic donor bone marrow cells. Dermal DCs in these mice cannot present OVA to autoreactive T cells (OT-I cells), whereas the LCs are antigen presentation-competent. Unexpectedly, OT-I cell injection into diphtheria toxin (DT)-treated beta(2)m --> K14-OVA x Langerin-DTR Tg mice resulted in skin GVHD. Thus, in vivo, both LC and dDC appear to be dispensable for the induction of keratinocyte-directed, CD8-mediated effector immune responses. Furthermore and surprisingly, OVA-expressing epidermal cells depleted of LCs that could not initiate allogeneic epidermal lymphocyte reactions activated naive OT-I cells in vitro. These results indicate that keratinocytes may function as accessory cells competent to prime naive skin-reactive T cells.JID JOURNAL CLUB ARTICLE: For questions, answers, and open discussion about this article, please go to http://network.nature.com/group/jidclub.

T regulatory cells and their counterparts: masters of immune regulation

The interaction of environmental and genetic factors with the immune system can lead to the development of allergic diseases. The essential step in this progress is the generation of allergen-specific CD4(+) T-helper (Th) type 2 cells that mediate several effector functions. The influence of Th2 cytokines leads to the production of allergen-specific IgE antibodies by B cells, development and recruitment of eosinophils, mucus production and bronchial hyperreactivity, as well as tissue homing of other Th2 cells and eosinophils. Meanwhile, Th1 cells may contribute to chronicity and the effector phases. T cells termed T regulatory (Treg) cells, which have immunosuppressive functions and cytokine profiles distinct from that of either Th1 or Th2 cells, have been intensely investigated during the last 13 years. Treg cell response is characterized by an abolished allergen-specific T cell proliferation and the suppressed secretion of Th1 and Th2-type cytokines. Treg cells are able to inhibit the development of allergen-specific Th2 and Th1 cell responses and therefore play an important role in a healthy immune response to allergens. In addition, Treg cells potently suppress IgE production and directly or indirectly suppress the activity of effector cells of allergic inflammation, such as eosinophils, basophils and mast cells. Currently, Treg cells represent an exciting area of research, where understanding the mechanisms of peripheral tolerance to allergens may soon lead to more rational and safer approaches for the prevention and cure of allergic diseases.

Maintenance of peripheral tolerance through controlled tissue homing of antigen-specific T cells in K14-mOVA mice

Immunological tolerance is crucial to avoid autoimmune and inflammatory diseases; however, the mechanisms involved are incompletely understood. To study peripheral tolerance to skin-associated Ags, we generated new transgenic mice expressing a membrane-bound form of OVA in skin under the human keratin 14 (K14) promoter (K14-mOVA mice). In contrast to other transgenic mice expressing similar self-Ags in skin, adoptive transfer of Ag-specific T cells does not induce inflammatory skin disease in our K14-mOVA mice. OVA-specific T cells transferred into K14-mOVA mice are activated in lymphoid tissues, undergo clonal expansion, and eventually acquire effector function. Importantly, these Ag-specific T cells selectively up-regulate expression of E-selectin ligand in cutaneous lymph nodes but not in mesenteric lymph nodes and spleen, demonstrating that expression of endogenous self-Ags in skin dictates imprinting of skin tissue homing in vivo. However, an additional inflammatory signal, here induced by tape stripping, is required in K14-mOVA mice to induce T cell migration to skin and development of inflammatory skin disease. Depletion of regulatory CD4(+)CD25(+) T cells did not provoke homing of transferred T cells to skin under steady-state conditions, indicating that these cells are not the key regulators for inhibiting T cell homing in K14-mOVA mice. Both skin-derived and lymph node-resident CD8alpha(+) dendritic cells are responsible for Ag presentation in vivo and induce tolerance to skin Ags, as we show by selective depletion of langerin(+) and CD11c(+) dendritic cells. Taken together, controlled skin homing of T cells is critical for the maintenance of peripheral immune tolerance to epidermal self-Ags.

Langerhans cells are not required for the CD8 T cell response to epidermal self-antigens

Langerhans cells (LC) are APC that reside at the barrier surfaces. Mice expressing an OVA peptide in the epidermis (K14-OVAp) were used to study CD8(+) T cell responses to an epidermal self-Ag. Earlier results suggested that LC were the predominant APC, inducing a robust T cell response and autoimmunity. In this study, we used a whole protein model system, the K14-mOVA mouse, in which a transmembrane form of OVA was expressed in keratinocytes. In contrast to K14-OVAp mice, T cells in K14-mOVA mice were activated, but did not expand and instead died by apoptosis. Furthermore, in double-transgenic mice expressing both mOVA and OVAp, robust OT-I expansion occurred, indicating that tolerance to this Ag is not dominant and was due to lack of activating signals. We sought to identify the relevant APC in K14 mice using bone marrow chimeras and found that radioresistant cells (presumably LC) were able to cross-present the OVA Ag from keratinocytes to naive T cells in the lymph node. However, use of LC-deficient mice indicated that LC were not required for the expansion of OT-I in K14-OVAp or the deletion of OT-I in K14-mOVA mice. These data suggest that radioresistant non-LC present self-Ag in K14-OVAp mice and drive a robust CD8 T cell response.

Strategies and challenges in eliciting immunity to melanoma

The ability of CD8+ T cells to recognize melanoma tumors has led to the development of immunotherapeutic approaches that use the antigens CD8+ T cells recognize. However, clinical response rates have been disappointing. Here we summarize our work to understand the mechanisms of self-tolerance that limit responses to currently utilized antigens and our approach to identify new antigens directly tied to malignancy. We also explore several aspects of the anti-tumor immune response induced by peptide-pulsed dendritic cells (DCs). DCs differentially augment the avidity of recall T cells specific for self-antigens and overcome a process of aberrant CD8+ T-cell differentiation that occurs in tumor-draining lymph nodes. DC migration is constrained by injection route, resulting in immune responses in localized lymphoid tissue, and differential control of tumors depending on their location in the body. We demonstrate that CD8+ T-cell differentiation in different lymphoid compartments alters the expression of homing receptor molecules and leads to the presence of systemic central memory cells. Our studies highlight several issues that must be addressed to improve the efficacy of tumor immunotherapy.

Regulation of cutaneous immunity by the environment: an important role for UV irradiation and vitamin D

Cutaneous immunity can be controlled by environmental factors such as ultraviolet (UV) irradiation. UV irradiation affects keratinocytes, antigen presenting cells, such as epidermal Langerhans cells (LC), and T lymphocytes. LC are specialized in antigen presentation. Upon encountering exogenous antigens they migrate to skin draining lymph nodes where they present skin-acquired antigens to naive T cells resulting in effector T cell differentiation. T cell effector functions depend on the activation state of LC, which can be influenced by UV irradiation. After completion T cell mediated cutaneous immune responses need to be downregulated. In this context, CD4(+)CD25(+) regulatory T cells have been shown to play an important role in the suppression of cellular immune responses via inhibition of T cell proliferation. Naturally occurring regulatory T cells develop in the thymus and on the molecular level members of the B7- and TNF-superfamilies are critically involved in the peripheral maintenance of CD4(+)CD25(+) T cells. Substantial evidence exists that peripheral regulatory T cells are responsive to environmental stimuli including UV irradiation. UV-induced regulatory T cells are expanded by UV-exposed cutaneous LC and recently, epidermal expression of vitamin D3 or RANKL (CD254) has been shown to connect the environment to the immune system via expansion of CD4(+)CD25(+) regulatory T cells.

Differential roles of migratory and resident DCs in T cell priming after mucosal or skin HSV-1 infection

Although mucosal surfaces represent the main portal of entry for pathogens, the mechanism of antigen presentation by dendritic cells (DCs) that patrol various mucosal tissues remains unclear. Instead, much effort has focused on the understanding of initiation of immune responses generated against antigens delivered by injection. We examined the contributions of migratory versus lymph node–resident DC populations in antigen presentation to CD4 and CD8 T cells after needle injection, epicutaneous infection, or vaginal mucosal herpes simplex virus (HSV) 1 infection. We show that upon needle injection, HSV-1 became lymph-borne and was rapidly presented by lymph node–resident DCs to CD4 and CD8 T cells. In contrast, after vaginal HSV-1 infection, antigens were largely presented by tissue-derived migrant DCs with delayed kinetics. In addition, migrant DCs made more frequent contact with HSV-specific T cells after vaginal infection compared with epicutaneous infection. Thus, both migrant and resident DCs play an important role in priming CD8 and CD4 T cell responses, and their relative importance depends on the mode of infection in vivo.

Peripheral tolerance induction by lymph node stroma

In this review we have highlighted the role of LNSCs in the regulation of CD8+ T cell immune responses in peripheral lymph nodes, thereby adding another layer of protection, in addition to the role of resting DCs, against autoimmunity. LNSCs have recently been implicated in the induction of peripheral CD8+ T cell tolerance due to their ability to endogenously express, process, and present PTAs. Furthermore, LNSCs express surface molecules, such as MHC class II and PD-L1, similar to those expressed by mTECs in the thymus and APCs. For future studies it will be important to address some of the new questions that have emerged with respect to the biology and function of LNSCs. Further work will help us to (1) dissect the specific roles that DCs and LNSCs have in the induction and maintenance of tolerance to intestinal antigens, (2) gain a more in-depth understanding of the molecular mechanisms underlying self-tolerance induction by LNSCs and the impact of inflammation on this function, (3) evaluate the relationship of LNSCs to the FRN, and (4) determine if the APC function of LNSCs extends to the acquisition and presentation of exogenous antigens. Finally, it is important to mention that so far the studies done on LNSCs have focused on their role in CD8+ T cell tolerance. At the moment, we do not know if presentation of PTAs by LNSCs can also induce tolerance of CD4+ T cells. Based on the finding that LNSCs express MHC class II (I-A(b)) molecules it is possible that they may present self-antigens to CD4+ T cells and induce tolerance. However, this has yet to be elucidated.

Cutting edge: migration of langerhans dendritic cells is impaired in autoimmune dermatitis

Tissue-resident dendritic cells, such as Langerhans cells (LC), normally carry Ags from tissues to lymph nodes to induce immunity to tissue Ags. In this study, we report that LC are reduced in the skin-draining lymph nodes of MRL-Fas(lpr/lpr) and MRL-Fas(+/+) mice that develop T cell-mediated autoimmune skin inflammation as compared with MHC-matched healthy strains. This deficiency of LC in skin-draining lymph nodes is due to a profound impairment of LC migration, resulting in the accumulation of activated LC in the skin. Such a defect in LC migration develops before the onset of skin lesions and correlates with the onset and severity of dermatitis. The reduced, rather than increased, migration of LC from skin to skin-draining lymph nodes represents a novel functional abnormality of LC in autoimmune dermatitis.

Direct presentation of a melanocyte-associated antigen in peripheral lymph nodes induces cytotoxic CD8+ T cells

Encounter of self-antigens in the periphery by mature T cells induces tolerance in the steady-state. Hence, it is not understood why the same peripheral antigens are also promiscuously expressed in the thymus to mediate central tolerance. Here, we analyzed CD8(+) T-cell tolerance to such an antigen constituted by ovalbumin under the control of the tyrosinase promoter. As expected, endogenous CD8(+) T-cell responses were altered in the periphery of transgenic mice, resulting from promiscuous expression of the self-antigen in mature medullary epithelial cells and deletion of high-affinity T cells in the thymus. In adoptive T-cell transfer experiments, we observed constitutive presentation of the self-antigen in peripheral lymph nodes. Notably, this self-antigen presentation induced persisting cytotoxic cells from high-affinity CD8(+) T-cell precursors. Lymph node resident melanoblasts expressing tyrosinase directly presented the self-antigen to CD8(+) T cells, independently of bone marrow-derived antigen-presenting cells. This peripheral priming was independent of the subcellular localization of the self-antigen, indicating that this mechanism may apply to other melanocyte-associated antigens. Hence, central tolerance by promiscuous expression of peripheral antigens is a mandatory, rather than a superfluous, mechanism to counteract the peripheral priming, at least for self-antigens that can be directly presented in lymph nodes. The peripheral priming by lymph node melanoblasts identified here may constitute an advantage for immunotherapies based on adoptive T-cell transfer.

Insights into Langerhans cell function from Langerhans cell ablation models

Langerhans cells (LC) are the principal dendritic cell (DC) population in the epidermis of the skin. Owing to their prominent position at the environmental barrier, LC have long been considered to be prototypic sentinel DC. More recently, the precise role of LC in the initiation and control of cutaneous immune responses has become debatable. To elucidate their contribution to immune regulation in the skin, our laboratories have generated genetically modified mice in which LC can be followed in situ by expression of enhanced green fluorescent protein and can be either inducibly or constitutively depleted in vivo. This review highlights the similarities and differences between these mouse models, discusses the discovery and functional significance of Langerin(+) dermal DC, and examines some recent data that help to shed light on LC function.

Langerhans cells and viral immunity

Langerhans cells (LC) are a unique dendritic cell subset that are located in mucosal stratified squamous epithelium and skin epidermis. Their location is ideally suited for their function as antigen presenting cells that capture invading viruses and induce anti-viral immunity. However, it is becoming evident that the interaction between LC and viruses can result in different responses, depending on the virus and the receptors involved. Here we will discuss the recent data on the similarities and differences in roles of LC in viral immunity to and infection with HIV, herpes simplex and varicella-zoster virus. Although all three viruses interact with LC during initial infection, the effects can be quite different, reflecting differences in biology and pathogenesis.

Selective delivery of beta cell antigen to dendritic cells in vivo leads to deletion and tolerance of autoreactive CD8+ T cells in NOD mice

Type 1 diabetes (T1D) is an autoimmune disease resulting from defects in central and peripheral tolerance and characterized by T cell-mediated destruction of islet beta cells. Cytotoxic CD8(+) T cells, reactive to beta cell antigens, are required for T1D development in the NOD mouse model of the disease, and CD8(+) T cells specific for beta cell antigens can be detected in the peripheral blood of T1D patients. It has been evident that in nonautoimmune-prone mice, dendritic cells (DCs) present model antigens in a tolerogenic manner in the steady state, e.g., in the absence of infection, and cause T cells to proliferate initially but then to be deleted or rendered unresponsive. However, this fundamental concept has not been evaluated in the setting of a spontaneous autoimmune disease. To do so, we delivered a mimotope peptide, recognized by the diabetogenic CD8(+) T cell clone AI4, to DCs in NOD mice via the endocytic receptor DEC-205. Proliferation of transferred antigen-specific T cells was initially observed, but this was followed by deletion. Tolerance was achieved because rechallenge of mice with the mimotope peptide in adjuvant did not induce an immune response. Thus, targeting of DCs with beta cell antigens leads to deletion of autoreactive CD8(+) T cells even in the context of ongoing autoimmunity in NOD mice with known tolerance defects. Our results provide support for the development of DC targeting of self antigens for treatment of chronic T cell-mediated autoimmune diseases.

Antigen controls IL-7R alpha expression levels on CD8 T cells during full activation or tolerance induction

The high-affinity chain of the IL-7 receptor, IL-7Ralpha (CD127), is expressed by effector CD8 T cells that have the capacity to become memory cells. IL-7Ralpha expression is uniformly high on naive CD8 T cells, and the majority of these cells down-regulate expression upon antigenic challenge. At the peak of expansion, the fraction of effectors expressing high IL-7Ralpha varies depending on the response examined. The signals that a CD8 T cell receives during a response to Ag that lead to altered expression of IL-7Ralpha have not been fully defined. In vitro experiments demonstrated that Ag alone is sufficient to down-regulate IL-7Ralpha on all cells and most of the cells rapidly re-express the receptor upon removal from Ag. Expression was not altered by the B7.1 costimulatory ligand or when IL-12 was present to provide the signal needed for development of effector functions, indicating that TCR engagement is sufficient to regulate IL-7Ralpha expression. Consistent with this, in vivo priming with peptide Ag resulted in IL-7Ralpha expression that inversely correlated with Ag levels, and expression levels were not changed when IL-12 or adjuvant were administered with Ag. A large fraction of the cells present at the peak of expansion had re-expressed IL-7Ralpha, but most of these cells failed to survive; those that did survive expressed high IL-7Ralpha levels. Thus, Ag-dependent signals regulate IL-7Ralpha levels on responding CD8 T cells, and this occurs whether the responding cells become fully activated or are rendered tolerant by administration of peptide Ag alone.