Dendritic Epidermal T Cells: Lessons from Mice for Humans

Developing the right tools for the job: Lin28 regulation of early life T-cell development and function

T cells comprise a functionally heterogeneous cell population that has important roles in the immune system. While T cells are broadly considered to be a component of the antigen-specific adaptive immune response, certain T-cell subsets display innate-like effector characteristics whereas others perform immunosuppressive functions. These functionally diverse T-cell populations preferentially arise at different stages of ontogeny and are tailored to the immunological priorities of the organism over time. Many differences in early life versus adult T-cell phenotypes can be attributed to the cell-intrinsic properties of the distinct progenitors that seed the thymus throughout development. It is becoming clear that Lin28, an evolutionarily conserved, heterochronic RNA-binding protein that is differentially expressed among early life and adult hematopoietic progenitor cells, plays a substantial role in influencing early T-cell development and function. Here, we discuss the mechanisms by which Lin28 shapes the T-cell landscape to protect the developing fetus and newborn. Manipulation of the Lin28 gene regulatory network is being considered as one means of improving hematopoietic stem cell transplant outcomes; as such, understanding the impact of Lin28 on T-cell function is of clinical relevance.

Human αβ and γδ T Cells in Skin Immunity and Disease

γδ T lymphocytes maintain skin homeostasis by balancing keratinocyte differentiation and proliferation with the destruction of infected or malignant cells. An imbalance in skin-resident T cell function can aggravate skin-related autoimmune diseases, impede tumor eradication, or disrupt proper wound healing. Much of the published work on human skin T cells attributes T cell function in the skin to αβ T cells, while γδ T cells are an often overlooked participant. This review details the roles played by both αβ and γδ T cells in healthy human skin and then focuses on their roles in skin diseases, such as psoriasis and alopecia areata. Understanding the contribution of skin-resident and skin-infiltrating T cell populations and cross-talk with other immune cells is leading to the development of novel therapeutics for patients. However, there is still much to be learned in order to effectively modulate T cell function and maintain healthy skin homeostasis.

CD1b-autoreactive T cells contribute to hyperlipidemia-induced skin inflammation in mice

A large proportion of human T cells are autoreactive to group 1 CD1 proteins, which include CD1a, CD1b, and CD1c. However, the physiological role of the CD1 proteins remains poorly defined. Here, we have generated a double-transgenic mouse model that expresses human CD1b and CD1c molecules (hCD1Tg) as well as a CD1b-autoreactive TCR (HJ1Tg) in the ApoE-deficient background (hCD1Tg HJ1Tg Apoe-/- mice) to determine the role of CD1-autoreactive T cells in hyperlipidemia-associated inflammatory diseases. We found that hCD1Tg HJ1Tg Apoe-/- mice spontaneously developed psoriasiform skin inflammation characterized by T cell and neutrophil infiltration and a Th17-biased cytokine response. Anti-IL-17A treatment ameliorated skin inflammation in vivo. Additionally, phospholipids and cholesterol preferentially accumulated in diseased skin and these autoantigens directly activated CD1b-autoreactive HJ1 T cells. Furthermore, hyperlipidemic serum enhanced IL-6 secretion by CD1b+ DCs and increased IL-17A production by HJ1 T cells. In psoriatic patients, the frequency of CD1b-autoreactive T cells was increased compared with that in healthy controls. Thus, this study has demonstrated the pathogenic role of CD1b-autoreactive T cells under hyperlipidemic conditions in a mouse model of spontaneous skin inflammation. As a large proportion of psoriatic patients are dyslipidemic, this finding is of clinical significance and indicates that self-lipid-reactive T cells might serve as a possible link between hyperlipidemia and psoriasis.

Expression of the heat shock protein-27 in the adult human scalp skin and hair follicle: Hair cycle–dependent changes

BACKGROUND

Heat shock protein (HSP) is a molecular chaperone involved in protein folding, assembly, and transport and in the regulation of cell growth and differentiation. HSP27 protein is expressed in murine hair follicle (HF) and in human skin during fetal development. In this investigation we hypothesized that HSP27 protein is expressed in the human scalp skin and its expression in HF changes with the transitions form anagen --> catagen --> telogen stages.

METHODS

To test this hypothesis, the immunoreactivity of HSP27 protein was examined in human scalp skin by immunofluorescent method. A total of 50 normal human scalp skin biopsy specimens were examined (healthy women, age 53-57 years). In each case, 50 HF were analyzed (35, 10, and 5 follicles in anagen, catagen, and telogen, respectively).

RESULTS

HSP27 protein expression was prominent in human scalp anagen, and weak in both catagen and telogen HFs. Within HF, HSP27 protein immunoreactivity was prominent in the outer root sheath, inner root sheath, precorteocytes, and corteocytes of the hair shaft. In addition, HSP27 protein expression was prominent in the epidermis, sebaceous glands, sweat glands, and arrector pili muscles.

LIMITATIONS

Only some types of heat shock proteins are known to date. Also, our knowledge about the exact molecular mechanisms involved in the interactions among these protein and other molecular chaperones is still incomplete.

CONCLUSIONS

Our investigation reports, for the first time, the expression patterns of HSP27 in human scalp skin and HF. The differential expression of HSP27 during HF cycling suggests its possible roles in human HF biology.

Homing and function of human skin gammadelta T cells and NK cells: relevance for tumor surveillance

Normal (noninflamed) human skin contains a network of lymphocytes, but little is known about the homing and function of these cells. The majority of alphabeta T cells in normal skin express CCR8 and produce proinflammatory cytokines. In this study we examined other subsets of cutaneous lymphocytes, focusing on those with potential function in purging healthy tissue of transformed and stressed cells. Human dermal cell suspensions contained significant populations of Vdelta1(+) gammadelta T cells and CD56(+)CD16(-) NK cells, but lacked the subsets of Vdelta2(+) gammadelta T cells and CD56(+)CD16(+) NK cells, which predominate in peripheral blood. The skin-homing receptors CCR8 and CLA were expressed by a large fraction of both cell types, whereas chemokine receptors associated with lymphocyte migration to inflamed skin were absent. Neither cell type expressed CCR7, although gammadelta T cells up-regulated this lymph node-homing receptor upon TCR triggering. Stimulation of cutaneous Vdelta1(+) gammadelta T cell lines induced secretion of large amounts of TNF-alpha, IFN-gamma, and the CCR8 ligand CCL1. In contrast to cutaneous alphabeta T cells, both cell types had the capacity to produce intracellular perforin and displayed strong cytotoxic activity against melanoma cells. We therefore propose that gammadelta T cells and NK cells are regular constituents of normal human skin with potential function in the clearance of tumor and otherwise stressed tissue cells.

Ultraviolet radiation and tumor immunity

Ultraviolet (UV) radiation induces a specific tolerance toward UV-induced skin tumors. This phenomenon has been known and studied for more than 25 years, but the mechanisms by which protective tumor immunity or tumor tolerance is induced are still largely obscure. In parallel with these studies, short-term assays on UV-induced immunosuppression and tolerance toward simple chemicals (e.g., dinitrochlorobenzene) have been analyzed, particularly with respect to the role of cytokines (most notably, interleukin (IL)-10 vs IL-12). However, these short-term assays are not likely to be fully adequate models of the long-term UV-induced tumor tolerance. The important nodal points of action in these immune reactions appear to be the T cells and the antigen-presenting cells (APCs) that prime them. The main focus should probably be on CD8(+) T cells as the ultimate effector of the cytotoxic response against UV-induced skin cancers. APC-mediated activation of these cells depends strongly on cosignaling of CD4(+) T cells. In a tumor tolerant state the activity of the cytotoxic CD8(+) T cells appears to be inhibited through CTLA-4(+) and natural killer T cells. The latter cells are CD1-restricted, which indicates the importance of "unconventional" antigens to UV-induced tumor tolerance.

Migration of Langerhans cells and gammadelta dendritic cells from UV-B-irradiated sheep skin

Depletion of dendritic cells from UV-B-irradiated sheep skin was investigated by monitoring migration of these cells towards regional lymph nodes. By creating and cannulating pseudoafferent lymphatic vessels draining a defined region of skin, migrating cells were collected and enumerated throughout the response to UV-B irradiation. In the present study, the effects of exposing sheep flank skin to UV-B radiation clearly demonstrated a dose-dependent increase in the migration of Langerhans cells (LC) from the UV-B-exposed area to the draining lymph node. The range of UV-B doses assessed in this study included 2.7 kJ/m2, a suberythemal dose; 8 kJ/m2, 1 minimal erythemal dose (MED); 20.1 kJ/m2; 40.2 kJ/m2; and 80.4 kJ/m2, 10 MED. The LC were the cells most sensitive to UV-B treatment, with exposure to 8 kJ/m2 or greater reproducibly causing a significant increase in migration. Migration of gammadelta+ dendritic cells (gammadelta+ DC) from irradiated skin was also triggered by exposure to UV-B radiation, but dose dependency was not evident within the range of UV-B doses examined. This, in conjunction with the lack of any consistent correlation between either the timing or magnitude of migration peaks of these two cell types, suggests that different mechanisms govern the egress of LC and gammadelta+ DC from the skin. It is concluded that the depression of normal immune function in the skin after exposure to erythemal doses of UV-B radiation is associated with changes in the migration patterns of epidermal dendritic cells to local lymph nodes.

Dendritic epidermal gamma/delta T cells (DETC) activated in vivo proliferate in vitro in response to Mycobacterium leprae antigens

BACKGROUND

gamma/delta T-cell receptor (TCR)+ dendritic epidermal T cells (DETC) are part of a primitive defense system in the skin; they are capable of responding only to a limited number of antigens. The aim of the present study was to test whether DETC can proliferate in vitro in response to antigens of Mycobacterium leprae.

METHODS

DETC were obtained from CBA mouse ear skin by trypsinization and Histopaque gradient centrifugation. The resulting epidermal cell suspension contained up to 20% DETC, as analyzed by the fluorescence activated cell sorter (FACS) after staining with anti-Thy-1 or anti-gamma/delta TCR monoclonal antibodies (mAbs). The freshly isolated cells, or DETC cultured up to 4 weeks with interleukin-2 (IL-2), were exposed in vitro for up to 6 days to varying doses of the following M. leprae antigens: (1) integral (live) M. leprae bacilli; (2) Dharmendra antigen; and (3) PGL-1 (phenolic glycolipid of M. leprae). The DETC response was assessed by tritiated thymidine (3H-TdR) incorporation.

RESULTS

The freshly isolated DETC, or DETC cultured up to 4 weeks with IL-2, did not respond significantly to any of the M. leprae antigens, although at the same time they were able to respond vigorously to concanavalin A (Con A), as positive control. If, however, DETC were isolated from skin, painted 7 days before with croton oil (10 microL/cm2 to cause irritant dermatitis, they were able to respond to all M. leprae antigens by a 3-4-fold incrase in the 3H-TdR uptake. The most effective stimulator was a 1 : 1 mixture of Dharmendra and PGL-1 (0. 01 microg/mL), which was as effective as 10-fold higher doses of either antigen alone. Cell counts confirmed that increased DNA synthesis was associated with cell proliferation. Experiments employing alpha/beta-TCR CBA murine spleen cells and epidermal cell suspension treated with anti-gamma/delta or antialpha/beta mAbs + C' proved that only the gamma/delta DETC were the responder cells to M. leprae antigens.

CONCLUSIONS

The results suggest that activation of DETC in vivo may make them responsive to M. leprae antigens. A significant increase in the number of class II major histocompatibility complex (MHC) positive, nondendritic cells was observed in the croton oil-treated epidermis. We hypothesize that croson oil-induced upregulation of class II MHC expression, which endows epidermal cells with antigen-presenting capabilities, might be an important factor in vivo in delivering an immunogenic signal to resident DETC in the skin.

Generation and cyclic remodeling of the hair follicle immune system in mice

In this immunohistomorphometric study, we have defined basic characteristics of the hair follicle (HF) immune system during follicle morphogenesis and cycling in C57BL/6 mice, in relation to the skin immune system. Langerhans cells and gammadelta T cell receptor immunoreactive lymphocytes were the predominant intraepithelial hematopoietic cells in neonatal mouse skin. After their numeric increase in the epidermis, these cells migrated into the HF, although only when follicle morphogenesis was almost completed. In contrast to Langerhans cells, gammadelta T cell receptor immunoreactive lymphocytes entered the HF only via the epidermis. Throughout HF morphogenesis and cycling, both cell types remained strikingly restricted to the distal outer root sheath. On extremely rare occasions, CD4+ or CD8+ alphabetaTC were detected within the HF epithelium or the sebaceous gland. Major histocompatibility complex class II+, MAC-1+ cells of macrophage phenotype and numerous mast cells appeared very early on during HF development in the perifollicular dermis, and the percentage of degranulated mast cells significantly increased during the initiation of synchronized HF cycling (first catagen). During both depilation- and cyclosporine A-induced HF cycling, the numbers of intrafollicular Langerhans cells, gammadelta T cell receptor immunoreactive lymphocytes, and perifollicular dermal macrophages fluctuated significantly. Yet, no numeric increase of perifollicular macrophages was detectable during HF regression, questioning their proposed role in catagen induction. In summary, the HF immune system is generated fairly late during follicle development, shows striking differences to the extrafollicular skin immune system, and undergoes substantial hair cycle-associated remodeling. In addition, synchronized HF cycling is accompanied by profound alterations of the skin immune system.

The anagen hair cycle induces systemic immunosuppression of contact hypersensitivity in mice

Contact hypersensitivity (CHS) to picryl chloride (PCl) is depressed in C57BL/6 mice when CHS is induced via early anagen skin. We have now further dissected this phenomenon in vivo. The elicitation phase for CHS was suppressed when anagen was induced 4 days after PCl sensitization of telogen animals. Sensitization of mice via abdominal skin with all hair follicles in telogen, and back skin follicles in anagen, significantly reduced the magnitude of the ear swelling response. Consecutive applications of two sensitizing doses of hapten, first on induced anagen back skin and then on telogen abdominal skin 7 days later, failed to induce tolerance. Furthermore, spleen cell transfer of sensitized anagen mice into telogen mice did not inhibit CHS response in the recipients. The current study suggests that a temporary state of systemic hyporesponsiveness, mediated, e.g., by hair cycle-dependent production of immunosuppressive cytokines rather than hapten-specific T suppressor cell activities, plays a critical role.

Characterization of the cutaneous inflammatory infiltrate in canine atopic dermatitis

Sections from lesional atopic, clinically normal atopic, and normal canine skin were investigated by light microscopy and an immunoperoxidase method using monoclonal antibodies specific for canine leukocyte antigens. We confirmed that skin-infiltrating cells of canine atopic dermatitis are constituted of mast cells, dendritic antigen-presenting cells, memory helper T-lymphocytes, low numbers of eosinophils and neutrophils, and rare B-lymphocytes. The presence of epidermal eosinophil microaggregates and clustered Langerhans' cells supports the hypothesis of epidermal allergen contact. The hyperplasia of epidermal T-cells expressing the gamma/delta T-cell receptor appears specific to canine atopic dermatitis compared with its human counterpart. This finding could be explained by an interspecies difference in skin immune systems or, alternatively, by an active participation of these epitheliotropic gamma/delta T-cells in the cutaneous allergic immune response in dogs. The paucity of dermal neutrophils in spontaneous lesions of canine atopic dermatitis is notably different from the neutrophil-rich late-phase reactions provoked by intradermal allergen injections in allergic dogs. This difference in the cellular infiltrate probably results from variations in the immune reaction between single and repeated allergen exposure as well as epidermal versus dermal antigen contact.

Spontaneous hair follicle cycling may influence the development of murine contact photosensitivity by modulating keratinocyte cytokine production

The development of murine contact hypersensitivity is influenced by hair follicle cycling. Here, we have examined hair cycle-associated fluctuations of murine contact photosensitivity (CPS) to tetrachlorosalicylanilide (TCSA) and its immunologic mechanism(s). When the CPS outcome was monitored in correlation with their spontaneous, synchronized hair cycling, mice aged 8 and 14 weeks, with most of their hair follicles in telogen, exhibited strong CPS responses, whereas 4-, 11-, and 16-week-old mice with a predominance of anagen follicles in a large area of their integument exhibited lower responses. This suggests that the development of CPS is inhibited in mice with anagen hair follicles. Antigen-specific, T-cell receptor V beta 7+ suppressor T cells, which are recognized to down-regulate the CPS response to TCSA, were not generated in sensitized anagen mice. Culture supernatants of epidermal cells derived from mice with anagen hair follicles contained factor(s) that suppress in vivo the development of CPS. It was found that levels of mRNA for tumor necrosis factor alpha (TNF alpha) were markedly decreased in epidermal cells from early anagen to telogen mice, whereas message for IL-1 receptor antagonist (IL-1ra) was transcribed increasingly during this hair cycling. These findings suggest that altered keratinocyte production of these cytokines is involved in mediating the anagen-associated depression of CPS.

gamma delta cells involved in contact sensitivity preferentially rearrange the Vgamma3 region and require interleukin-7

Ptak and Askenase showed that both alphabeta and gammadelta cells are required for transfer of contact sensitivity (CS). This study confirms that day 4 immune cells depleted of gammadelta cells fail to transfer CS to trinitrochlorobenzene (TNP-Cl) systemically and demonstrates that administration of anti-gammadelta monoclonal antibodies (mAb) in vivo abolishes the CS reaction. Moreover, gammadelta cells accumulate at the antigen challenge site: these cells have the unusual phenotype CD8alpha+, CD8beta-, IL-4 R+ which we suggest is due to their state of activation. Following immunization with contact sensitizer on the skin, the absolute number of gammadelta cells increases in the regional lymph nodes with a peak at 4 days. Of the gammadelta cells, 80 %, both in the lymph nodes of TNP-Cl-immune mice and accumulating at the antigen challenge site are Vgamma3+. The gammadelta cells expressing Vgamma3, which is characteristic of dendritic epithelial T cells (DETC), obtained 4 days after sensitization, proliferate in response to interleukin (IL)-7, but only poorly to IL-2 and IL-4. They also respond to concanavalin A and immobilized anti-gammadelta mAb, but not to haptens or heat-shocked syngeneic spleen cells. Furthermore, injection of mice with mAb to IL-7 inhibits accumulation of Vgamma3+ cells both in the lymph nodes after skin sensitization and at the antigen-challenge site. Altogether, these results strongly support the view that DETC are related to, or the original source of, the gammadelta cells found in the lymph node after skin sensitization and at the site of challenge, and that IL-7 is implicated in these phenomena.

Functional roles for granzymes in murine epidermal gamma(delta) T-cell-mediated killing of tumor targets

Granzymes, a family of serine proteases contained in cytoplasmic granules of cytotoxic T lymphocytes and natural killer cells, play a critical role in killing tumor targets by triggering rapid breakdown of DNA and subsequent apoptosis. We have reported previously that dendritic epidermal T cells, which are skin-specific members of the tissue-type gamma(delta) T-cell family in mice, are capable of killing selected tumor cell lines. Here we report that short-term cultured dendritic epidermal T-cell lines contain significant N-alpha-benzyloxycarbonyl-L-Lys-thiobenzyl esterase activity, produce granzyme A protein, and express constitutively mRNA for granzymes A and B. Messenger RNA expression for granzyme B was also confirmed in freshly procured Thy-1+ epidermal cells (i.e., dendritic epidermal T cells). Finally, preincubation of dendritic epidermal T cell lines with a granzyme inhibitor, dichloroisocoumarin, but not with a cysteine protease inhibitor, E-64, abrogated completely their capacity to trigger DNA breakdown in YAC-1 target cells. These results reinforce the concept that dendritic epidermal T cells represent skin-resident killer cells that share several functional properties with conventional killer leukocytes, thereby playing a local immunosurveillance role against tumor development.

Keratinocytes and cytokine/growth factors

Cytokines are polypeptide growth factors produced by most nucleated cells in the body, including epithelial cells, keratinocytes, and Langerhans cells in the skin. Cytokines can be classified into interleukins, tumor necrosis factors, chemokines, colony-stimulating factor, interferons, and growth factors. Like classic hormones, cytokines bind to specific receptors to transmit their messages to target cells. Cytokine receptors can be divided into three cytokine receptor superfamilies: the immunoglobulin superfamily, the hematopoietin family, and the tumor necrosis factor family. Following cytokine/cytokine-receptor binding (first messenger), a signal transduction pathway is initiated. Factors affecting homeostasis in the skin and oral mucosa include a delicate balance between cytokines/cytokine-receptors and their antagonists. An imbalance in these variables can influence the development of cutaneous and oral diseases-such as lichen planus, autoimmune disorders, and some neoplastic processes- and can affect wound healing. Potential uses of cytokines include cancer and antiviral therapy.

Hair cycle-dependent changes in skin immune functions: anagen-associated depression of sensitization for contact hypersensitivity in mice

To assess whether hair follicle cycling influences skin immunity, we examined the association between highly synchronized hair follicle cycling and experimental contact hypersensitivity in C57BL/6 mice. Hair cycle synchronization was performed by depilation of hair shafts on the back with telogen skin. Mice were sensitized on the lower back skin with picryl chloride between 0 and 25 d, after anagen induction by depilation, and challenged on the earlobes with picryl chloride 5 d later. The magnitude of contact hypersensitivity was significantly decreased in mice sensitized on day 1, was minimal on day 3 (early anagen), and slowly increased thereafter, reaching level comparable to day 0 on day 25 (telogen). The significantly depressed contact hypersensitivity response in anagen skin was confirmed in mice with spontaneously developed follicles. Lymph node cells taken from mice sensitized with picryl chloride on days 0, 1, and 3 after depilation were cultured in vitro in the presence of syngeneic, haptenized, Langerhans cell-enriched epidermal cells. Marked proliferative responses of lymph node cells to haptenized cells were found in mice not only of day 0, but also of days 1 and 3, suggesting that immune T cells exist even lymph node cells of the low-responsive mice. Flow cytometric analyses demonstrated that the number of intraepidermal Langerhans cells and their functions, including the expression of major histocompatibility complex class II, CD54, and CD86, and mixed epidermal cell lymphocyte reactions, were not changed in skin on days 0,1, and 3. These findings demonstrated that contact hypersensitivity is induced most effectively via skin with telogen hair follicles and that the depressed response in early anagen skin is not simply due to failure in Langerhans cell function or sensitization of T cells.

The role of dendritic cells in cutaneous immunity

This article reviews the role of dendritic cells in cutaneous immunity. Langerhans cells (LC) found in the epidermis are the best-characterized dendritic cell population. They have the ability to process antigen in the periphery, transport it to the draining lymph nodes (DLN) where they are able to cluster with, and activate, antigen-specific naive T cells. During migration LC undergo phenotypic and functional changes which enable them to perform this function. There are other less well-characterized dendritic cells including dendritic epidermal T cells, dermal dendrocytes and dermal "LC-like' cells. Although there is no evidence that dendritic epidermal T cells (DETC) can present antigen or migrate to lymph nodes, they do influence the intensity of cutaneous immune responses to chemical haptens. Antigen-presenting cells (APC) in the dermis may provide alternative routes of antigen presentation which could be important in the regulation of skin immune responses. Therefore, dendritic cells are vital for the induction of immune responses to antigens encountered via the skin. LC are particularly important in primary immune responses due to their ability to activate naive T cells. The faster kinetics of secondary responses, and the ability of nonprofessional APC to induce effector function in previously activated cells, suggest that antigen presentation in the DLN may be less important in responses to previously encountered antigens. In these secondary responses, dendritic and nondendritic APC in the skin may directly induce effector functions from antigen-specific recirculating cells.

Expression of the common cytokine receptor gamma chain by murine dendritic cells including epidermal Langerhans cells

The common cytokine receptor gamma chain (gamma c) is an indispensable component of interleukin (IL)-2, IL-4, IL-7, IL-9, and IL-15 receptors, and its expression has been detected in several leukocyte populations, including T cells, B cells, monocytes, natural killer cells, and neutrophils. The purpose of this study was to determine whether gamma c receptors are expressed by dendritic cells (DC). Constitutive gamma c mRNA expression was observed by reverse transcription polymerase chain reaction and/or Northern blotting for: (a) Ia+ epidermal Langerhans cells (LC), (b) 4F7+ splenic DC, (c) granulocyte/macrophage colony-stimulated factor-propagated bone marrow-derived DC, and (d) the epidermal-derived DC line, XS52, which retains important functions of epidermal LC. Exposure of XS52 cells to recombinant IL-4 induced a rapid up-regulation of c-myc mRNA expression, and this IL-4-dependent signaling was blocked almost completely by anti-gamma c monoclonal antibody (mAb) TUGm2 in a soluble form. Moreover, c-myc up-regulation was inducible in XS52 cells by the same mAb in an immobilized form. These results imply that molecules recognized by this antibody (i.e. gamma c receptors) are expressed on XS52 cell surfaces. We thus conclude that DC express functional gamma c receptors, which then mediate cytokine-dependent regulation of DC functions.

Interleukin (IL)-15 promotes the growth of murine epidermal gamma delta T cells by a mechanism involving the beta- and gamma c-chains of the IL-2 receptor

Dendritic epidermal T cells (DETC) are skin-specific members of the epithelial gamma delta T-cell family in mice. We have reported previously that the growth of DETC is promoted by interleukin (IL)-2 in an autocrine fashion, or by IL-7, which is secreted by neighboring keratinocytes. Here we report that DETC growth is promoted by IL-15, a newly discovered T-cell growth factor that is produced in lymphoid as well as nonlymphoid tissues. Recombinant IL-15 promoted the growth of the 7-17 DETC line in a time- and dose-dependent fashion. Using monoclonal antibodies against alpha-, beta-, or gamma c-chains of the IL-2 receptor complex, we observed that the combination of anti-beta chain and anti-gamma c chain antibodies blocked IL-15 responsiveness completely, whereas anti-alpha chain had no effect. These results indicate that this gamma delta T-cell line uses the beta/gamma c heterodimer for proliferative responses to IL-15. Antibodies against IL-2 or IL-7 did not block IL-15-driven proliferation of 7-17 DETC, indicating that IL-15 promotes their growth in an IL-2- and IL-7-independent manner. Both the surface expression of beta/gamma c heterodimers and the IL-15 responsiveness of 7-17 DETC were highest 1 to 8 days after concanavalin A stimulation, and both declined substantially 21 days after stimulation, illustrating regulation by the state of cell activation. Working with epidermal cells that were freshly procured from CBA mice, we noted that IL-15 promoted conavalin-A-triggered growth of Thy-1+ cells (i.e., DETC), but not of the Thy-1- cells. The gamma c-chain was not expressed by freshly procured DETC, becoming detectable within 48 h after concanavalin A stimulation. We propose that IL-15 facilitates the growth of epithelial gamma delta T cells by a beta/gamma c receptor-dependent mechanism.

UVB-dependent modulation of epidermal cytokine network: roles in UVB-induced depletion of Langerhans cells and dendritic epidermal T cells

The epidermis of mice consists of three cellular components, i.e., keratinocytes, Langerhans cells (LC), and dendritic epidermal T cells (DETC). Each epidermal subpopulation produces a different set of cytokines, thereby forming a unique cytokine milieu. These cytokines, in turn, support the survival and growth of LC and DETC and regulate their immunological functions. LC and DETC play important, but distinct, effector roles in protective immunity against antigens that are generated in or penetrate into the epidermis. Acute or chronic exposure of mice to ultraviolet B (UVB) radiation is known to impair this cutaneous immunity, as evidenced by the failure to induce T cell-mediated immune reactions, by the generation of antigen-specific immunological unresponsiveness, and by the development of skin cancers. Importantly, these changes are associated with reduced densities of LC and DETC in UVB-exposed skin, suggesting that the deficiency in these epidermal leukocytes may account for some of the deleterious influences of UVB radiation on skin. Here I will review the recent advance in our understanding of the mechanisms by which UVB radiation may deplete LC and DETC from epidermis. More specifically, I will discuss the following possibilities: a) UVB-mediated suppression of the production of relevant growth factors for LC and DETC, b) UVB-induced abrogation of surface expression of growth factor receptors, and c) UVB-triggered apoptotic cell death in epidermal leukocytes.

2B4, a new member of the immunoglobulin gene superfamily, is expressed on murine dendritic epidermal T cells and plays a functional role in their killing of skin tumors

Dendritic epidermal T cells (DETC), which are skin-specific members of the tissue-resident gamma delta T-cell family, are characterized by their potential to kill selected tumor targets by a non-major histocompatibility complex (MHC)-restricted mechanism. We have recently identified a new receptor molecule, 2B4, that appears to be associated with non-MHC-restricted recognition of tumor targets by natural killer cells. The purpose of this study was to determine whether DETC express 2B4 molecules, and, if so, to assess their functional roles in DETC-mediated killing of tumor targets. Short-term DETC lines as well as DETC freshly procured from skin expressed surface 2B4, as detected with a specific monoclonal antibody. Removal of interleukin (IL)-2 from DETC cultures caused substantial reduction in 2B4 expression levels as well as a reduction in cytotoxic capacity against YAC-1 targets in a standard 51Cr-release assay. Conversely, exposure to IL-2, but not to IL-7, elevated both 2B4 expression and cytotoxicity. To assess the functional roles played by surface 2B4, we pretreated DETC lines with anti-2B4 antibody and then tested for their killing potential. Anti-2B4, but not the control antibody, augmented their capacity to lyse YAC-1 targets (51Cr-release assays) and to disrupt the monolayers of Pam-212-transformed keratinocytes (visual assessment). Thus, we conclude that DETC express, in an IL-2-dependent manner, 2B4 molecules, which may play a unique role in the killing of skin-derived tumors.

Identity and functional properties of novel skin-derived fibroblast lines (NS series) that support the growth of epidermal-derived dendritic cell lines

We have established recently a series of unique cell lines (NS series) from dispase-separated mouse epidermis that promote the growth of epidermal-derived antigen-presenting cell lines (XS series). The purposes of this study were to determine the identity of NS cells and to characterize their functional properties. NS cells were distinguishable from leukocytes by the lack of typical surface markers and by the failure to respond to leukocyte growth factors. Despite their epidermal derivation, NS cells were distinct from keratinocytes by the absence of cytokeratins. On the other hand, NS cells were indistinguishable from lines of dermal fibroblasts by their a) morphology, b) surface phenotype, and c) intracellular deposits of type I collagen. Growth of the XS antigen-presenting lines has been promoted by co-culturing with gamma-irradiated NS cells, and this activity could be replaced with NS cell-conditioned media alone, but not with paraformaldehyde-fixed NS cells. Each clone derived from the NS01 line secreted XS cell-growth-promoting activity, and this activity was blocked by monoclonal antibodies against colony-stimulating factor-1 receptors. Dermal fibroblasts also promoted the growth of XS cells in a colony-stimulating factor-1-dependent manner. By contrast, culture supernatants from other cell lines derived from skin (e.g., Pam 212 keratinocytes, 7-17 dendritic epidermal T cells, or XS lines) failed to promote XS cell growth. These results indicate that NS cells belong to the fibroblast lineage and that they share the intrinsic property to secrete large amounts of colony-stimulating factor-1 with dermal fibroblasts. Dermal cells may support the growth of skin-associated antigen-presenting cells in vivo.

Interleukin-7-dependent interaction of dendritic epidermal T cells with keratinocytes

Dendritic epidermal T cells (DETC), a member of the epithelial tissue-type gamma delta T-cell family, are characterized by their exclusive residence within mouse epidermis, their dendritic morphology, and their monoclonal nature in the T-cell-receptor configuration. Here we review our recent studies on the interleukin (IL)-7-dependent interaction of DETC with neighboring keratinocytes. Keratinocytes express constitutively the mRNAs for IL-7 and secrete biologically relevant amounts of IL-7. This cytokine, in turn, serves as a growth factor for DETC, as evidenced by the proliferative responses to recombinant or keratinocyte-derived IL-7 of the 7-17 DETC line and of DETC freshly purified from mouse skin. The 7-17 DETC line undergoes apoptotic cell death in response to external stimuli known to deplete DETC in situ (e.g., ultraviolet B radiation or corticosteroid treatment), and IL-7 prevents this apoptosis, thereby promoting long-term survival. These results document the crucial role played by IL-7 in maintaining the survival and growth of DETC in epidermis. IL-7 mRNA expression in keratinocytes is abrogated by ultraviolet B radiation, whereas it is up-regulated by interferon-gamma, which is secreted by DETC upon activation. More specifically, interferon-gamma induces the preferential expression of truncated forms (2.6 and 1.5 kb) of IL-7 transcripts, in addition to the 2.9- and 1.7-kb transcripts that are expressed constitutively, and this regulation occurs through the usage of alternative transcription initiation sites. These results suggest unique pathways through which IL-7 production is regulated in keratinocytes by external stimuli (e.g., ultraviolet B) as well as T-cell-derived cytokines (e.g., interferon-gamma). We propose that keratinocyte-derived IL-7 is an essential component of the epidermal cytokine milieu.

Cytokine-dependent regulation of growth and maturation in murine epidermal dendritic cell lines

We have recently established dendritic cell (DC) lines (XS series) from the epidermis of newborn mice by repeated feeding with granulocyte/macrophage-colony-stimulating factor (GM-CSF) and culture supernatants from skin-derived stromal cell lines (NS series). XS lines resemble resident Langerhans cell (LC), which are immature DC that reside in epidermis, by their surface phenotype and antigen-presenting profile. XS lines further resemble resident LC in that they express mRNA for interleukin-1 beta and macrophage inflammatory protein (MIP)-1 alpha, and by the absence of mRNA for IL-6. Their growth is promoted by GM-CSF, colony-stimulating factor-1 (CSF-1), or NS culture supernatant, and inhibited by interferon-gamma or tumor necrosis factor-alpha. The expression by the XS lines of Ia molecules is up-regulated by GM-CSF, and down-regulated by NS supernatant. These results suggest the existence of negative regulatory mechanisms in which the growth and/or maturation of DC is suppressed by selected cytokines.

Activation of murine epidermal gamma delta T cells through surface 2B4

Dendritic epidermal T cells (DETC) are gamma delta T cells that normally reside in murine skin. They express on their surface the 2B4 molecule, a 66-kDa glycoprotein of the immunoglobulin gene superfamily thought to be associated with anti-tumor cytotoxicity by natural killer and lymphokine-activated killer cells. Here, we show that ligation of surface 2B4 transduces cell activation signals in DETC. Treatment with anti-2B4 monoclonal antibodies triggers the secretion of interferon-gamma and interleukin-2 by DETC lines, induces proliferation of resting DETC lines, amplifies anti-CD3-dependent proliferation of DETC freshly isolated from mouse skin; and up-regulates egr-1 and c-fos mRNA expression. These results indicate a unique pathway for DETC activation.

Defective lymphoid development in mice lacking expression of the common cytokine receptor gamma chain

The common gamma chain (gamma c) of the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors is defective in humans with XSCID. Mice lacking gamma c expression had hypoplastic thymuses; the thymocytes responded to gamma c-independent mitogens, but not gamma c-dependent stimuli. Splenic T cells were diminished at 3 weeks of age, but CD4+ T cells markedly increased by 4 weeks. B cells were greatly diminished in contrast with the situation in XSCID. NK cells, gamma delta intestinal intraepithelial lymphocytes, dendritic epidermal T cells, peripheral lymph nodes, and gut-associated lymphoid tissue were absent. These findings underscore the importance of gamma c in lymphoid development. Moreover, differences in humans and mice lacking gamma c expression indicate species-specific differences in the roles of gamma c-dependent cytokines or in the existence of redundant pathways. These mice provide an important model for studying the pathophysiology provide an important model for studying the pathophysiology of and gene therapy for human XSCID.

Heat-shock protein 65 and activated gamma/delta T cells in injured arteries

Because immune mechanisms are implicated in atherogenesis, we investigated the T-lymphocyte subset and factors related to its activation after acute arterial ligation (22 ligated and 13 non-ligated specimens). Ligated arteries produced heat-shock protein 65 (hsp65) and were infiltrated with activated T cells (mostly dendritic, CD3+, CD4-, CD8-, and gamma/delta T-cell-receptor bearing). The protein was found with dendritic T cells, with immunogold-labelled hsp65 beside the dendritic processes. Thus, the immune reaction after acute arterial injury may be associated with binding and recognition of in-situ hsp65 by dendritic gamma/delta T-cells.

Expression of classical and non-classical MHC class I antigens in murine hair follicles

Not all keratinocytes in human and rat hair follicles express MHC class I antigens (MHC I). In the present study, we report the first immunohistological profile of classical and non-classical MHC I expression in the skin of adolescent C57 BL-6 mice during the induced hair cycle. MHC I immunoreactivity (H-2b, H-2Db) is absent in the matrix and inner root sheath of growing (= anagen) hair follicles, and the dermal papillae are H-2b negative during catagen and telogen. This lack of normal MHC I expression may serve to sequester potentially damaging autoantigens from immune recognition. In addition, we present the first evidence of non-classical MHC class I antigen expression in normal mammalian skin: during the entire hair cycle, the distal hair follicle shows strong Qa-2 immunoreactivity, which appears to be restricted to an epithelial follicle compartment densely populated by gamma-delta T cells with which Qa-2 molecules may interact as part of a primitive antibacterial defense system of the follicle. The murine hair cycle is an attractive model for dissecting the functional roles of H-2b and Qa-2 molecules in hair biology and in related tissue-interaction systems.

Reciprocal cytokine-mediated cellular interactions in mouse epidermis: promotion of gamma delta T-cell growth by IL-7 and TNF alpha and inhibition of keratinocyte growth by gamma IFN

A unique subset of gamma delta T cells, termed dendritic epidermal T cells (DETC), resides in symbiosis with keratinocytes in mouse epidermis. We have shown previously that interleukin 7 (IL-7) which is produced by keratinocytes, promotes growth and prevents apoptosis in DETC. To extend this observation, we examined 12 cytokines, each of which is expressed by epidermal cells at mRNA and/or protein levels, for their capacities to modulate the growth of DETC. Cytokines examined included IL-1 alpha, IL-2, IL-3, IL-4, IL-6, IL-7, IL-8, IL-10, tumor necrosis factor-alpha (TNF alpha), interferon-gamma (IFN gamma), granulocyte/macrophage-colony stimulating factor (GM-CSF), and macrophage inflammatory protein-1 alpha (MIP-1 alpha). When tested individually, IL-2 and IL-7 promoted maximal growth of the long-term cultured DETC line 7-17. When tested in combinations, synergistic growth-promoting effects were seen with IL-2 and IL-4 or IL-7, and with IL-7 and IL-4 or TNF alpha. Dose-response experiments demonstrated that TNF alpha, which is produced by keratinocytes, enhances IL-7-induced DETC proliferation, but inhibits IL-2-induced proliferation. The mouse keratinocyte-derived cell line Pam 212 was used to test these cytokines for their capacities to regulate keratinocyte growth. Only gamma IFN, which is produced by DETC, inhibited proliferation in a dose-dependent fashion. These results illustrate three reciprocal pathways by which epidermal cytokines regulate the growth of epidermal cells: 1) a paracrine mechanism by which keratinocyte-derived cytokines (e.g., IL-7 and TNF alpha) promote the growth of DETC, 2) an autocrine mechanism by which DETC-derived cytokines (e.g., IL-2 and IL-4) support their own growth, and 3) a reciprocal pathway in which a cytokine produced by resident epidermal leukocytes (e.g., gamma IFN) modulates the growth of keratinocytes.