Haptens can serve as surrogate transplantation antigens in a manner that demonstrates H-2 restriction of graft rejection.

Hapten-immune mice are capable of rejecting syngeneic skin grafts that are derivatized with the relevant hapten, but only if the hapten is applied while the graft is "healing in." This model system was used to demonstrate that the hapten-specific immune effectors responsible for rejection are restricted by H-2 determinants of the recipient. Thus, haptens can be used in vivo as surrogate transplantation antigens for the study of immunopathogenic mechanisms in transplantation immunity.

Depletion of epidermal langerhans cells and Ia immunogenicity from tape-stripped mouse skin.

To explore the relationships among Ia antigen expression, epidermal Langerhans cells, and the immunogenicity of skin allografts, cellophane tape-stripping was used in H-2 congenic and recombinant mice of defined immunogenetic disparity. Tape-stripping of murine abdominal wall skin achieved almost complete depletion of epidermal Langerhans cells within a few hours of application, as measured by cell surface ATPase and expression of Ia antigens. Tape-stripping also reduced, to a considerable degree (but not absolutely), the Ia immunogenicity of skin allografts prepared from stripped surfaces. No comparable reduction in immunogenicity of class I major histocompatibility determinants was observed, suggesting that Langerhans cells are relatively unimportant in the presentation of H-2K antigens in skin grafts. Langerhans cells reappear within 24 h of tape-stripping to anatomically intact skin, but are detectable in orthotopically grafted only after the graft has been in residence for 4 d, i.e., shortly after it has acquired a blood supply. Repopulating Langerhans cells at that time and thereafter are exclusively of host origin. These results indicate that the traffic of Langerhans cells to the skin can be extremely dynamic, especially when the epidermal surface has been markedly disturbed, and the data imply that, under normal circumstances, large numbers of Langerhans cells can be mobilized readily from an available pool of precursors.

Analysis of the mechanism of unresponsiveness produced by haptens painted on skin exposed to low dose ultraviolet radiation.

Acute, low dose ultraviolet B radiation of murine body wall skin followed by local application of DNFB produces a state of antigen-specific unresponsiveness. This state is maintained at least in part by an Lyt-1+ T cell that effects unresponsiveness by impairing the induction phase of contact hypersensitivity. The absence of suppressor cells capable of acting at the effector stage of immunity suggests that tolerogenic signals derived from the skin establish suppressor networks that are incomplete and perhaps different from networks that are induced by systemic administration of tolerogens. It is proposed that ultraviolet radiation produces its effects by impairing the antigen-presenting potential of resident Langerhans cells in whose absence hapten-derivatized keratinocytes (or their products) are able to deliver a tolerogenic signal.

Relationship of psoriasis severity to obesity using same-gender siblings as controls for obesity.

BACKGROUND: Psoriasis is a multifactorial disease affected by both genetic and environmental factors. Several comorbid conditions, such as smoking, depression and obesity have been found to be associated with psoriasis. This study addressed the association of psoriasis and obesity using same-gender full siblings as controls, correlating between body mass index (BMI) and severity of psoriasis as determined by body surface area (BSA) and the Physician's Global Assessment (PGA). METHODS: In total, 88 patients undergoing outpatient treatment for psoriasis were surveyed for demographic information, psoriasis history, social history, personal and family medical history, whether they had a same-gender full sibling and if so, the age, weight and height of the sibling. Height, weight, PGA scores and percentage of BSA affected by psoriasis, were recorded for each patient. BMI was calculated for each patient and their same-gender full sibling. RESULTS: A positive association between psoriasis severity and BMI was found. PGA score increased with BMI (Spearman's correlation, r(s) = 0.29, P = 0.007). There was also a positive correlation between BMI and BSA%, r(s) = 0.24, P = 0.02. A significant difference in BMI between patients with psoriasis and the same-gender full sibling control was seen for women (mean +/- SD 30.2 +/- 10.2 vs. 27.6 +/- 7.3 kg/m(2), respectively, P = 0.02), but not for men. CONCLUSION: In this study, psoriasis severity was found to be related to the level of obesity. Using same-gender siblings as genetic controls for predisposition to both obesity and psoriasis, patients with psoriasis were more likely to have a higher BMI, particularly for women. This study reinforces the need to treat the whole patient and to encourage healthy living, such as maintaining an appropriate weight, proper eating habits and exercise. Limitations of this study include the relatively small number of patients enrolled, potential inaccuracies in sibling BMIs calculated from information provided by patients, and a lack of information about dietary habits, exercise and lifestyle.

A novel mechanism of glucocorticoid-induced immune suppression: the inhibiton of T cell-mediated terminal maturation of a murine dendritic cell line.

Working with the murine epidermal-derived dendritic cell (DC) line XS52, we have observed previously that antigen-specific interaction with T cells stimulates their "terminal maturation" into fully professional DC. In this study we examined the impact of dexamethasone (DEX) on this T cell-induced event. When added to cocultures of XS52 DC and the KLH-specific Th1 clone HDK-1 in the presence of antigen, DEX at relatively low concentrations (10(-9)-10(-7) M) prevented substantially or completely each of the changes that typify terminal maturation, including (a) secretion of relatively large amounts of IL-1beta, IL-6, and TNFalpha; (b) loss of CD115 (colony-stimulating factor-1 receptor) expression and proliferative responsiveness to colony-stimulating factor-1; and (c) elevated expression of CD86 (B7-2). XS52 cells also underwent terminal maturation upon exposure to lipopolysaccharide alone, and DEX also inhibited effectively each of the same changes, indicating that DC can serve as the direct target of DEX. By contrast, DEX inhibited XS52 DC-stimulated IL-2 secretion by HDK-1 T cells, but not other changes that accompany T cell activation, including the secretion of IFNgamma and TNFalpha and the elevated expression of CD25, CD28, and CD44. These results reveal a new immunosuppressive mechanism of glucocorticoid action, that is, direct inhibition of T cell-mediated terminal maturation by DC.

A role for NF-kappaB-dependent gene transactivation in sunburn.

Exposure of skin to ultraviolet (UV) radiation is known to induce NF-kappaB activation, but the functional role for this pathway in UV-induced cutaneous inflammation remains uncertain. In this study, we examined whether experimentally induced sunburn reactions in mice could be prevented by blocking UV-induced, NF-kappaB-dependent gene transactivation with oligodeoxynucleotides (ODNs) containing the NF-kappaB cis element (NF-kappaB decoy ODNs). UV-induced secretion of IL-1, IL-6, TNF-alpha, and VEGF by skin-derived cell lines was inhibited by the decoy ODNs, but not by the scrambled control ODNs. Systemic or local injection of NF-kappaB decoy ODNs also inhibited cutaneous swelling responses to UV irradiation. Moreover, local UV-induced inflammatory changes (swelling, leukocyte infiltration, epidermal hyperplasia, and accumulation of proinflammatory cytokines) were all inhibited specifically by topically applied decoy ODNs. Importantly, these ODNs had no effect on alternative types of cutaneous inflammation caused by irritant or allergic chemicals. These results indicate that sunburn reactions culminate from inflammatory events that are triggered by UV-activated transcription of NF-kappaB target genes, rather than from nonspecific changes associated with tissue damage.

Killing of skin-derived tumor cells by mouse dendritic epidermal T-cells.

Dendritic epidermal T-cells (DETC) are a unique population of T-cells that reside normally in mouse epidermis and express a gamma delta T-cell receptor. We have reported previously that DETC acquire in culture the capacity to lyse the YAC-1 lymphoma, a conventional target for natural killer cells. The aim of the present study was to characterize this cytotoxic potential, using a spectrum of skin-derived mouse tumors. Cytotoxicity was measured by a 51Cr release assay and by the visual assessment of target cell lysis. Long-term DETC lines, established from CBA, AKR, and BALB/c mice by mitogenic stimulation and repeated feeding with interleukin 2 (5 units/ml), were used as effectors. Skin-derived tumor targets included 5 melanoma lines and the transformed keratinocyte line Pam 212. Each DETC line lysed skin-derived tumors as well as YAC-1 targets effectively in the 18-h 51Cr release assay, and target lysis occurred in a non-major histocompatibility complex-restricted manner. By contrast, freshly isolated spleen cells lysed YAC-1 but not skin tumor targets. Moreover, confluent monolayers of melanoma or Pam 212 targets were disrupted completely by added DETC lines but not by spleen cells. The cytolytic activity of DETC appeared to be specific for tumor cells, since normal mouse keratinocyte monolayers remained intact under the same conditions. Finally, DETC freshly isolated from skin failed to exhibit significant cytotoxicity but acquired this capacity 10-14 days after mitogenic stimulation and feeding with interleukin 2 (5 units/ml). We conclude that DETC possess the potential to recognize, bind, and lyse tumor cells that originate in skin.

Epidermal Langerhans cell involvement in cutaneous lupus erythematosus.

Epidermal Langerhans cells (LCs) possess surface markers and functional attributes which identify them as being of macrophage/monocyte lineage, and recent evidence documents their participation in certain immune process which occur in skin. To assess the role of LCs in lupus erythematosus (LE), a disease in which immune system dysfunction predominates, human epidermis from patients with cutaneous LE was studied with 3 LC surface markers: ATPase activity, HLA-DR and OKT-6 antigens. Suction blister top epidermal skin biopsies from patients with 3 clinical types of cutaneous LE exhibited similar features: LCs were less dendritic, they were more irregularly distributed, and they were present in fewer numbers when compared with those in adjacent normal skin. These changes contrasted with those observed in diseases with similar lichenoid histopathological features. LCs appeared increased in number in lichen planus. LCs in skin lesions from one patient with dermatomyositis exhibited similar morphologic alterations, but surface densities and distributions were preserved. Disaggregated epidermal cells from skin lesions of patients with cutaneous LE induced allogeneic lymphocyte proliferation as efficiently as did cells from nonlesional skin, indicating that the morphologic alterations observed were not associated with a decreased alloantigen presenting capacity. These studies have demonstrated that epidermal LC populations in 3 clinical types of cutaneous LE are perturbed in a manner not seen in 2 other lichenoid skin diseases, although these changes were not associated with an altered capacity of such cells to stimulate proliferation by allogeneic lymphocytes.

Langerhans cell function dictates induction of contact hypersensitivity or unresponsiveness to DNFB in Syrian hamsters.

The relationship between distribution and function of Langerhans cells within the epidermis and the capacity of cutaneous surfaces to promote the induction of contact hypersensitivity to DNFB have been examined in inbred Syrian hamsters. In a manner very similar to previous findings in mice, the results indicate that hamster cutaneous surfaces deficient in normally functioning Langerhans cells, naturally (cheek pouch epithelium) or artificially (after perturbation with ultraviolet light), are inefficient at promoting DNFB sensitization. Instead, DNFB applied to these regions of skin results in the induction of a state of specific unresponsiveness. Viable lymphoid cells from unresponsive hamsters can transfer the unresponsiveness to naive hamsters suggesting that active suppression is at least partly responsible, probably mediated by T lymphocytes.

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.

Prolactin stimulates proliferation of cultured human keratinocytes.

The effect of the pituitary hormone prolactin on in vitro proliferation of human keratinocytes has been studied. Cell proliferation was determined by [3H] thymidine incorporation and cell enumeration in culture. Physiologic concentrations of prolactin markedly stimulated proliferation of newborn foreskin keratinocytes in serum-free medium. In addition, it was able to replace almost completely the growth-promoting effects of bovine pituitary extract, a commonly added supplement for keratinocyte culture. This activity was also evident in the absence of epidermal growth factor, but required the presence of insulin. Radioligand-binding studies confirmed the expression of specific prolactin binding sites (Kd 8.9 nM; 1350 sites per cell) on freshly procured keratinocyte membranes. These results extend its hormonal influences to include regulation of in vitro proliferation of human keratinocytes, and suggest the possibility of a completely defined growth medium for keratinocytes.

Mouse dendritic epidermal T cells exhibit chemotactic migration toward PAM 212 keratinocyte culture supernatants.

Dendritic epidermal T cells (DETCs) are Thy-1+, CD45+, CD3+, CD4-, CD8-, and T-cell receptor-V gamma 3/V delta 1+ leukocytes that reside normally in adult mouse skin. We have demonstrated previously that keratinocytes serve as adhesion substrates for DETCs, and that interleukin 7 (IL-7), which is produced by keratinocytes, serves as a growth factor for DETCs. The present study was conducted to address the mechanisms by which DETCs migrate into the epidermis, reasoning that keratinocytes may also be a source of chemotactic activity. Short-term DETC lines were 35S-labeled and tested for migration toward Pam 212 keratinocyte culture supernatants using a modified Boyden chamber method; cell movement from upper chambers toward test samples in lower chambers was traced by counting radioactivity. DETC displayed rapid (within 60 min) and marked (> 50%) migration toward keratinocyte supernatants. The majority of cells that had migrated into keratinocyte supernatants expressed the V gamma 3 T-cell receptor, thus verifying that the migrating cells were DETCs. Addition of keratinocyte supernatants to the upper chambers completely blocked migration, suggesting its chemotactic nature. By contrast, no DETC migration was observed toward 3T3 fibroblast supernatants. Chemotactic activities were 1) produced by Pam 212 cells even in the absence of serum; 2) greater than 12 kD in size; 3) heat and pH labile; 4) trypsin sensitive; and 5) precipitated by 60-100% ammonium sulfate. Several cytokines (e.g., IL-1 alpha and IL-8) failed to mediate DETC migration when added to the lower chambers. Likewise, the same cytokines, when added to the upper chambers, failed to inhibit DETC migration toward Pam 212 supernatants. These results support our hypothesis that keratinocytes facilitate the residence of DETC in epidermis by secreting unique chemotactic factors, by providing adhesion substrates, and by elaborating specific growth factors.

Dendritic epidermal T cells: lessons from mice for humans.

Dendritic epidermal T cells (DETC) in mice form part of a primitive system of epithelial-resident T cells characterized by the expression of gamma delta T-cell receptors (TCR). Critical attributes that characterize DETC include their highly restricted T-cell receptor gene utilization, proliferation and maturation within epidermis, a capacity to kill relevant skin-derived tumor targets, and the ability to modulate immune responses that are initiated and expressed in skin. Contemporary knowledge suggests that DETC and the related skin-directed gamma delta T cells found in humans play important roles in maintaining the immunologic integrity of skin.

Internalization and acidification of surface HLA-DR molecules by epidermal Langerhans cells: a paradigm for antigen processing.

CD4+ T lymphocytes recognize multi-molecular complexes, formed by major histocompatibility complex class II molecules and exogenous antigens, on the surface of antigen-presenting cells (APC). For most protein antigens, processing is required to produce immunogenic peptide fragments that can then form stable associations with class II molecules. These two processes, the modification of antigen and its coupling to class II molecules, are thought to occur in acidic endosomal compartments. Furthermore, membrane class II molecules are endocytosed in APC and may provide ligands for the immunogenic peptides. To gain insight into these processes, we examined the internalization and acidification of membrane HLA-DR molecules by three APC populations: 1) freshly isolated Langerhans cells (LC), 2) LC after 48-72 h of bulk epidermal cell culture, and 3) peripheral blood monocytes (PBM). Using FITC-conjugated anti-HLA-DR monoclonal antibodies (MoAb), endocytosis was studied by fluorescence microscopy and by flow cytometry (pulse width analysis), while acidification was assessed by exploiting the pH sensitivity of fluorescein fluorescence. We observed both freshly isolated LC and PBM to internalize surface HLA-DR molecules into acidic compartments with great efficiency. Endocytosis was inhibited by the addition of azide and 2-deoxy-D-glucose, whereas acidification was partially blocked by treatment with ammonium chloride or chloroquine. The degree of internalization and acidification of HLA-DR molecules was greatly influenced by the degree of Ab cross-linking. On the other hand, cultured LC were capable of internalizing HLA-DR molecules, but were not able to acidify the environments to which these molecules were delivered; this loss of acidification capacity was partially restored by treatment with phorbol 12-myristate 13-acetate.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of dose response of trinitrochlorobenzene contact hypersensitivity induction in mice: pretreatment with cyclophosphamide reveals an optimal sensitizing dose.

A detailed dose-response curve has been established for induction of contact hypersensitivity (CH) in mice with trinitrochlorobenzene (TNCB). It was determined that in BALB/c, CBA/J, and C57BL/6 mice, the dose required to sensitize via epicutaneous application was between 1 and 10 micrograms TNCB. When doses of hapten of 200 micrograms or greater were painted on abdominal skin, CH responses were induced which were only marginally greater than responses induced by sensitizing doses of hapten in the 10-50-micrograms range, implying that no further dose-response relationship exists beyond 50 micrograms of hapten. However, in companion experiments, in which panels of mice were pretreated with cyclophosphamide, it was determined that sensitizing doses of hapten in excess of 50 micrograms induced both CH and concomitant induction of down-regulation of CH. Thus, at 200 micrograms or higher doses of TNCB, CH responses of cyclophosphamide pretreated mice were invariably more intense than in their untreated, hapten-painted cohorts. In the animals pretreated with cyclophosphamide, it was possible to see that a dose-response relationship continued to exist between the amount of epicutaneously applied hapten over a 200 micrograms to 14 mg range and the intensity of the CH induced. We conclude that the optimal dose for immunizing mice epicutaneously with TNCB is between 10 and 50 micrograms. This is considered optimal since animals sensitized in this manner display no evidence of concomitant down-regulation of their CH responses.

Phenotypic and functional heterogeneity among murine epidermal-derived dendritic cell clones.

We have established recently long-term dendritic cell lines from the epidermis of newborn BALB/c mice. These lines, termed XS series, resembled epidermal resident Langerhans cells or their progenitors in terms of surface phenotype, antigen-presenting capacity, and growth factor requirement. We examined in this study the degree of clonal heterogeneity among XS cells with respect to each of these features. Twelve stable clones were established by limiting dilution microculture from 8-10-week-old cultures of the XS52 or XS20 line. Despite the uniform expression of CD45, these clones varied substantially in their expression of Ia, B7-1, and B7-2 molecules. They also varied significantly in their relative efficiency in activating T cells. Finally, remarkable clone-to-clone heterogeneity was also observed in their growth factor responsiveness; some clones responded equally well to granulocyte macrophage-colony-stimulating factor-1, whereas others responded preferentially to one or the other of these factors. We propose that the observed clonal heterogeneity in XS cells reflects possible heterogeneity in the state of maturation and mitotic potential among the starting populations, i.e., skin-associated dendritic cells in newborn mice.

Intravenously injected, TNP-derivatized, Langerhans cell-enriched epidermal cells induce contact hypersensitivity in Syrian hamsters.

The ability of haptenated subpopulations of epidermal cells (EC) to induce contact hypersensitivity (CH) in Syrian hamsters was investigated. Crude haptenated EC and haptenated EC enriched for Langerhans cells (LC) inoculated i.v. into hamsters induced CH that was equivalent in intensity to that induced by epicutaneous application of hapten. By contrast, haptenated EC, relatively depleted of LC, failed to induce CH hypersensitivity responses. Upon subsequent reimmunization of all animals with epicutaneously applied hapten, hamsters that had first received haptenated EC depleted of LC failed to respond in CH assays, indicating that these animals had been rendered unresponsive. These data suggest that haptenated LC are capable of inducing CH regardless of the route of inoculation, whereas haptenated EC, when depleted of LC, deliver a down-regulating signal via the i.v. route.

Profiles of cytokine mRNA expressed by dendritic epidermal T cells in mice.

The epidermis of mice contains, in addition to Langerhans cells, a second dendritic population that is Thy-1+/CD3+/CD4-/CD8-/T-cell receptor-V gamma 3/V delta 1+. These dendritic epidermal T cells (DETC) are now thought to comprise one element in the family of epithelial tissue-resident gamma delta T cells. In the present study, DETCs were examined for their expression of mRNA for cytokines, using a reverse transcription-polymerase chain reaction. Freshly isolated Thy-1+ epidermal cells constitutively expressed mRNA for gamma-interferon, but not IL-2. Within 24 h after stimulation with Con A, these cells then expressed mRNA for gamma-interferon and IL-2, but not IL-4. The rapid onset of expression of mRNA for IL-2 occurred exclusively within the Thy-1+ population, and in a Con A-dependent fashion. When freshly isolated epidermal cells were first stimulated with Con A and then expanded in bulk with rIL-2 for 20-24 d, cells expressing IL-4 mRNA then emerged, upon secondary stimulation with Con A. These "short-term" DETC lines also expressed mRNA for IL-2, interferon-gamma, IL-1 alpha, IL-3, IL-6, IL-7, tumor necrosis factor alpha and beta, and granulocyte macrophage-colony stimulating factor. Interestingly, mRNA for IL-4 and IL-6 was no longer detected in long-term (> 1 year) DETC lines 7-17 and 12-12. In addition, one line (7-17) maintained IL-3 mRNA expression, whereas another (12-12) had lost this capacity. These results emphasize the concept that, as resident cells in epidermis, DETCs exhibit several different immunorelevant activities, and the heterogeneity in cytokine mRNA profiles suggests that DETCs may divide into functional subsets.

Increased number and microtubule-associated dispersal of acidic intracellular compartments accompany differentiation of cultured human keratinocytes.

Intracellular acidic compartments serve several functions, including uptake of nutrients, processing and sorting of secreted and membrane-bound proteins, and even entry of viruses into cells. In this study, we examined the distribution of acidic compartments in normal human keratinocytes cultured in serum-free medium. Acridine orange was used to stain acidic organelles (red fluorescence), and adherent cells were evaluated by fluorescence microscopy and by interactive laser cytometry (ILC). Keratinocytes cultured in low [Ca++] (0.15 mM) exhibited morphologic characteristics associated with basal cells; red acidic vesicles in these cells were aggregated around the nucleus, sparing the peripheral cytoplasm. After 24 h of culture in high [Ca++] (1.5 mM) keratinocytes showed morphologic changes associated with differentiated cells, including increased number and dispersal of red vesicles to the periphery of the cytoplasm. Keratinocytes cultured in 0.15 mM [Ca++], but treated with phorbol 12-myristate 13-acetate (PMA, 5-100 ng/ml) to induce terminal differentiation, developed similar features. Incubation in media with either high [Ca++] or PMA also induced radial extension of the microtubule network, suggesting that the distribution of acidic organelles occurs along this network. Finally, crude keratinocyte membranes were evaluated by radioactive assay for the presence of three ion-translocating ATPase activities, plasma membrane Na/K ATPase, mitochondrial ATPase, and vacuolar H+ pump ATPase, the latter being the activity responsible for acidification of intracellular compartments. Both basaloid and differentiated keratinocytes exhibited similar vacuolar H+ pump ATPase activity, as measured by its sensitivity to bafilomycin.

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.