Langerhans cells: functional aspects revealed by in vivo grafting studies

Allogeneic versus xenogeneic immune reaction to bioengineered skin grafts

There are conflicting reports on the survival and immune reaction to allografts and xenografts of cultured skin substitutes (CSS). In this study, we investigated the allogeneic and xenogeneic responses to CSS of human keratinocytes and genetically engineered CSS expressing keratinocyte growth factor (KGF) that forms a hyperproliferative epidermis. CSS (control and KGF modified) and neonatal human foreskins were evaluated by immunohistochemistry for the expression of MHC class I and II. To study allograft rejection, grafts were transplanted to human peripheral blood mononuclear cell (huPBMC)-reconstituted SCID mice. To study xenograft rejection, grafts were transplanted to immunocompetent mice. Graft survival and immune reaction were assessed visually and microscopically. After transplantation, control CSS formed a normal differentiated epidermis, whereas KGF CSS formed a hyperproliferative epidermis. Control and KGF CSS expressed class I similar to neonatal foreskin, but did not express class II. In the allograft model, rejection of neonatal foreskins was between 5 and 9 days. In contrast, neither control nor KGF CSS was rejected by huPBMC-SCID mice. Histology showed dense mononuclear cell infiltration in human foreskins, with few, if any, mononuclear cells in control or KGF CSS. In contrast to the allogeneic reaction, CSS (control and KGF) were rejected in the xenograft model, but rejection was delayed (9-21 days) compared with neonatal skin (5-8 days). Humanized SCID mice rejected allografts of human neonatal foreskins, but did not reject control CSS or KGF CSS, even though the KGF CSS formed a hyperproliferative epidermis. Rejection of control and KGF CSS by immunocompetent mice in a xenograft model was comparable and their survival was significantly prolonged compared with neonatal skin. These results demonstrate that control CSS and hyperproliferative KGF CSS are less immunogenic than normal human skin and that sustained hyperproliferation of the epidermis does not accelerate rejection.

Skin anatomy and flap physiology

This article focuses on three main areas, which includes an overview of skin anatomy, different types of skin flaps and their vascular supply, and several aspects of flap biomechanics to allow the surgeon to perform the most adequate reconstruction with regard to location and size of defects.

Validity of the two-signal model for activation of CD28-deficient T lymphocytes: quantitative characterization of an alternative costimulatory function of dendritic cells

The observation that primary T-dependent immune responses are generated in mice lacking CD28, the only receptor definitively shown to costimulate naive T cells, has led to ambiguity as to whether costimulation is absolutely required for initiation of T-cell responses. In this report, in vitro analysis of the relationship between cell density and proliferation demonstrates that activation of CD28-/- T cells to immobilized T-cell receptor (TCR)-alpha monoclonal antibodies (MoAb) depends on costimulatory signals provided by other cells in culture and occurs only at cell densities sufficient to permit these intercellular interactions. These signals are necessary even under TCR triggering conditions that obviate the CD28 requirement. Dendritic cells (DCs) provide the necessary costimulation in vitro and prime T cells in vivo in CD28-/- mice. Single-cell and limiting dilution analyses indicate that individual T cells from normal and CD28-/- mice produce equivalent interleukin (IL)-2 in response to DCs. However, half as many T cells produce IL-2 when only the CD28-independent pathway is used. Nonetheless, CD28-/- T cells produce sufficient IL-2 to support clonal expansion comparable to that of CD28+/+ T cells, which may account for the equally robust in vivo responses initiated by DCs in normal and CD28-deficient animals.

Role of graft interleukin-10 expression in the tolerogenicity of neonatal skin allografts

BACKGROUND

Allografts of skin from neonatal donors survive longer than those from adult donors and can induce tolerance in mice that are treated with short-term immunosuppression. Neonatal (< or =24 hr old) epidermal cells (EPC) secrete high levels of interleukin-(IL) 10 and include abundant class II- immature Langerhans cells (LC). In this study, the role of IL-10 in the tolerogenicity of neonatal skin grafts was examined.

METHODS

After a preliminary experiment established that tolerogenesis by neonatal grafts could be supported by monoclonal antilymphocyte antibodies, B10.A(5R) recipients were immunosuppressed with anti-CD4 plus anti-CD8 (days 0, +2) and adult C57B1/6 bone marrow cells (day +7). Recipients were grafted with adult or neonatal C57B1/6 skin from wild-type or IL-10 deficient ("knockout" donors). EPC from wild-type and knockout neonatal skin were compared by flow cytometry, before and after 48 hr culture, to adult cells in terms of class II and costimulatory molecule expression.

RESULTS

Grafts from knockout neonates survived longer than those from adult donors (median survival, MST=81 vs. 61 days), but not as long as those from wild-type neonates (MST=100 days; P<0.05). As with normal neonatal EPC, neonatal knockout EPC expressed little class II antigen. Both types of neonatal EPC acquired class II in culture, and up-regulated CD80 and CD86 in an adult pattern, but failed to up-regulate class II antigen to the high level seen among cultured adult cells.

CONCLUSIONS

The tolerogenicity of neonatal skin grafts derives in part from natural expression of IL-10 by the graft. Another possible contribution to tolerogenicity may be the inability of neonatal antigen presenting cells to up-regulate class II fully. Low expression of class II by neonatal cells is not attributable to epidermal IL-10 secretion.

Inhibition of functional T cell priming and contact hypersensitivity responses by treatment with anti-secondary lymphoid chemokine antibody during hapten sensitization

Recent studies have suggested a pivotal role for secondary lymphoid chemokine (SLC) in directing dendritic cell trafficking from peripheral to lymphoid tissues. As an extension of these studies, we examined the consequences of anti-SLC Ab treatment during Ag priming on T cell function in an inflammatory response. We used a model of T cell-mediated inflammation, contact hypersensitivity (CHS), where priming of the effector T cells is dependent upon epidermal dendritic cell, Langerhans cells, and migration from the hapten sensitization site in the skin to draining lymph nodes. A single injection of anti-SLC Ab given at the time of sensitization with FITC inhibited Langerhans cell migration into draining lymph nodes for at least 3 days. The CHS response to hapten challenge was inhibited by anti-SLC Ab treatment in a dose-dependent manner. Despite the inhibition of CHS, T cells producing IFN-gamma following in vitro stimulation with anti-CD3 mAb or with hapten-labeled cells were present in the skin-draining lymph nodes of mice treated with anti-SLC Ab during hapten sensitization. These T cells were unable, however, to passively transfer CHS to naive recipients. Animals treated with anti-SLC Ab during hapten sensitization were not tolerant to subsequent sensitization and challenge with the hapten. In addition, anti-SLC Ab did not inhibit CHS responses when given at the time of hapten challenge. These results indicate an important role for SLC during sensitization for CHS and suggest a strategy to circumvent functional T cell priming for inflammatory responses through administration of an Ab inhibiting dendritic cell trafficking.

Dendritic cells and cutaneous immune responses to chemical allergens

This article reviews the role of epidermal Langerhans cells (LC) in the development of cutaneous immune responses to chemical allergens. Following topical exposure to sensitizing chemicals, LC, many of which bear allergen, are induced to migrate from the skin, via the afferent lymphatics, to the draining lymph nodes. The phenotypic and functional changes to which LC are subject during this process and their development into active immunostimulatory cells closely resembling lymphoid dendritic cells is discussed. The migration and maturation of LC following skin sensitization is of critical importance to the effective presentation of chemical allergens to T lymphocytes and the induction of allergic responses. Evidence is reviewed which suggests that these events are initiated and regulated by epidermal cytokines. The conclusion drawn is that an early event during the induction of skin sensitization is the production by keratinocytes of cytokines which stimulate the migration of LC from the skin and which also result in the functional maturation of LC into potent antigen-presenting cells.

Acceptance of class II major histocompatibility complex disparate skin grafts associated with suppressor cells and elevated Langerhans cell numbers

Class II major histocompatibility complex (MHC) molecules are only present on Langerhans cells (LC) in normal murine epidermis. Depletion of this antigen with the chemical carcinogen dimethylbenzanthracene (DMBA) causes I-E disparate B10.A(2R) congenic tail skin to be accepted permanently when grafted onto B10.A(4R) recipients. Adoptive transfer of spleen cells from these recipients into naive syngeneic hosts inhibited the ability of the host mice to reject untreated B10.A(2R) tail skin grafts. Hence DMBA-treated LC depleted I-E disparate skin grafts activate suppressor cells which did not inhibit BALB/c mice from rejecting a B10.A(2R) tail skin graft. In contrast, the tobacco derived carcinogen benzo(a)pyrene (BP) increased the number of epidermal LC but had no effect on either class I or class II MHC disparate skin graft survival time. This confirms that the number of class II MHC-positive LC is critical for the initiation of skin graft rejection; when the threshold level is attained graft rejection proceeds at a maximal rate that cannot be enhanced by raising the number of LC. The tolerant skin grafts had increased numbers of LC; this was not observed in syngeneic grafts and therefore may be related to the active suppression of immunity.

Cultured epidermal Langerhans cells activate effector T cells for contact sensitivity

To investigate whether Langerhans cells are capable of inducing contact sensitivity effector T cells, we incubated purified T cells from naive mice with syngeneic cultured trinitrophenyl-modified Langerhans cells for 4-5 d. The cells were then expanded in interleukin-2 and fresh medium for another 6-9 d and injected intravenously into naive syngeneic recipient mice. After the ears of recipient mice were painted with 1% trinitrochlorobenzene, we observed an ear-swelling response peaking at 24-48 h. The ear-swelling response was hapten specific. CD8- but not CD4-depleted T cells mediated strong contact sensitivity. Systemic adoptive transfer into nude mice also lead to a hapten-specific delayed ear-swelling response. However, this response was less protracted than in euthymic animals, suggesting the participation of the recipient (non-immunized) T cells in the ear-swelling response of the euthymic mice. Lymphokine analysis of in vitro primed and restimulated T cells revealed predominant production of interleukin-2 but little or no interleukin-4. These in vitro primed cells therefore resemble type 1 or inflammatory T helper cell clones.

Functional dichotomy between Langerhans cells that present antigen to naive and to memory/effector T lymphocytes

The general thrust of this volume is to review the roles of accessory cells in regulating T and B lymphocytes. To that end, we have summarized the evidence that indicates the crucial role that Langerhans cells play in the induction and expression of immunity to antigens that gain access to, or arise within, skin. Langerhans cells accomplish this important goal by their abilities to (a) activate naive T cells to antigens not previously encountered by the host, and (b) activate memory/effector T cells specific for previously encountered antigens. Arguments have been advanced to support the view that the functional properties of Langerhans cells used to present antigens to naive T cells differ substantially from the properties that equip Langerhans cells to activate effector T cells. The arguments are based in part on the fact that Langerhans cells carry out these functions in two very different environments: in the epidermis, and in the draining lymph node. The arguments are also based on results of in vitro experiments that reveal distinct differences in antigen processing and presenting properties of Langerhans cells freshly obtained from mouse and human skin as compared to Langerhans cells that have been cultured in vitro for 2-3 days. We propose that freshly explanted Langerhans cells faithfully reflect the functional program of intraepidermal Langerhans cells, and are able to present antigen to memory/effector T cells that enter the epidermal compartment. To accomplish this task, epidermal LC pick up environmental antigens, process them with great efficiency, and then present them in situ, without further upregulation of "accessory" signals (cell-adhesion molecules, secretion of additional cytokines). They can carry out this function, even in the presence of TGFB--a a cytokine which is constitutively made by keratinocytes, and which we have found to profoundly inhibit antigen presentation by most other types of "professional" antigen-presenting cells. Intraepidermal Langerhans cells are also capable of carrying cutaneous antigens through the dermal epidermal junction and migrating to the draining lymph node. We further propose that cultured Langerhans cells are fated to present antigens to unprimed/naive T cells, and thereby to initiate immune responses to new cutaneous antigens. Cultured LC process antigens less efficiently than fresh cells, but their unique capacity to present antigen effectively to unprimed T cells rests chiefly on the fact that they have significantly upregulated cell surface adhesion molecules, expression of MHC molecules, and secretion of activating cytokines--the "accessory" signals that are required for arousing naive T cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Aspects of the immune response to contact allergens: opportunities for the development and modification of predictive test methods

A variety of guinea-pig tests are currently employed to assess the skin-sensitizing potential of chemicals. Although some such tests, in particular the guinea-pig maximization test and the occluded patch test of Buehler, have become well established, widely applied and are of proven value in the safety evaluation of chemicals, they have certain limitations. It is the purpose of this review to examine various aspects of the immune response to contact allergens and the way in which an understanding of the molecular and cellular events that characterize the induction and elicitation of contact sensitivity may be applied to the development and modification of predictive test methods. Attention is focused on the role of dendritic cell migration and T lymphocyte activation during the induction phase of skin allergy and the association of acute-phase proteins and vasoactive amines with the elicitation reaction.

Antigen-presenting cells in the induction of contact hypersensitivity in mice: evidence that Langerhans cells are sufficient but not required

One explanation for the fact that certain genetically defined strains of mice prove to be resistant to effects of low dose ultraviolet B radiation on the induction of contact hypersensitivity is that ultraviolet B resistant mice possess a second pathway for antigen presentation through the skin--a pathway that is independent of epidermal Langerhans cells and beyond the reach of the damaging effects of ultraviolet B light. As a corollary, ultraviolet-B susceptible mice would be expected to be deficient in this pathway. Several experimental strategies were employed to determine whether Langerhans cells are required for the induction of contact hypersensitivity by epicutaneously applied hapten. The results reveal that tape-stripped skin supports the induction of contact hypersensitivity, whereas surgical excision of hapten-painted skin within 1 h of application fails to permit the development of contact hypersensitivity. Because the former selectively eliminates epidermal Langerhans cells while the latter deletes both Langerhans cells and dermal antigen-presenting cells, we conclude that either Langerhans cells or dermal cells are sufficient to provide antigen presentation in the induction of contact hypersensitivity. When large amounts of hapten are painted epicutaneously, or when hapten is injected subcutaneously or painted on sub-dermal tissues, contact hypersensitivity also results, indicating that induction of contact hypersensitivity does not require that antigen processing and presentation be provided by cutaneous cells. Reasons are presented for concluding that under physiologic circumstances induction of contact hypersensitivity by epicutaneous hapten application relies primarily upon the antigen-presenting capabilities of epidermal (Langerhans cells) and dermal cells.

Ontogeny of Langerhans cells in human embryonic and fetal skin: cell densities and phenotypic expression relative to epidermal growth

Langerhans cells (LCs) positive for HLA-DR antigens were present in developing human epidermis by at least 7 weeks estimated gestational age (EGA). Most were negative for CD1 (T6) until 12-13 weeks EGA when they underwent a dramatic increase in CD1 reactivity. To gain insight into the density of LCs during ontogeny and to assess whether their distribution was coordinated with epidermal growth, the number of cells positive for both HLA-DR and CD1 antigens was determined relative to surface area and to volume of developing, interfollicular epidermis. LCs differed in their phenotype, distribution (follicular vs. interfollicular), size, and shape between 7 and 21 weeks EGA; however, during this period they maintained a statistically equivalent (P greater than .25) density (65 cells/mm2 and 1,750/mm3) even though the epidermis increased in thickness and the fetus rapidly expanded its surface area. While LCs were evenly distributed within the epidermal sheets at all gestational ages, those in embryonic skin were much smaller and less dendritic than the older cells. The density, size, and shape of LCs in developing skin seemed to be independent of epidermal status (e.g., thickness of keratinization, and number of cell layers) but rather were correlated with gestational age. The number of fetal LCs, through at least 23 weeks EGA, was only 10-20% of the adult LC density. Thus, we can conclude that the increase in LC density to adult levels must occur either during the third trimester or after birth.

Genetic basis of ultraviolet-B effects on contact hypersensitivity

The genetic basis of the effects of ultraviolet B (UVB) radiation on the induction of contact hypersensitivity (CH) to dinitrofluorobenzene (DNFB) has been explored in genetically defined mice. It was found that acute, low-dose UVB radiation produced profound depletion of epidermal Langerhans cells (LC) at UVB-treated sites in all strains of mice tested. However, when DNFB was applied to UVB radiation sites, unresponsiveness developed in some strains of mice, but vigorous contact hypersensitivity was induced in others. The UVB-susceptible phenotype proved dominant or codominant in F1 hybrids derived from parental strains of the susceptible and UVB-resistant phenotypes. Experiments conducted in one set of F1 hybrids derived from two UVB-susceptible parental strains displayed UVB resistance, suggesting gene complementation, and showed that more than one genetic locus was involved. Segregant backcross populations, analyzed for the capacity to develop CH after UVB treatment and skin painting with DNFB, revealed that at least two, and probably three, independent genetic loci participate in determining UVB resistance. Results of experiments with H-2 congenic and recombinant mice derived from the B10 background implicated class I genes of the major histocompatibility complex as relevant genetic factors. These results indicate that there is a dissociation between the effects of UVB radiation on epidermal Langerhans cells and the capacity of a cutaneous surface to support the induction of contact hypersensitivity. The data indicate that the induction of CH to haptens is dependent on normal numbers of functional LC at the skin painting site only in some strains of mice. The data imply that in the so-called UVB-resistant strains of mice, alternative (non-Langerhans cell-dependent) mechanisms allow for the induction of CH. Several independent genetic loci, one of which appears to be H-2, govern this UVB-related effect.

Formation of lymphoepithelial tissue in the sheep's palatine tonsil

Formation of lymphoepithelial tissue was studied in 1-, 10- and 21-day-old sheep. From each of the animals one of the tonsils was fixed in 4% glutaraldehyde for light- and electron microscopy while the other was frozen in liquid nitrogen for immunohistochemistry. These examinations revealed sequential histological events during the formation of reticular epithelium. (i) Appearance of a distinct epithelial cell indicates the initiation of the reticulation. The electron density of these epithelial cells is much lower than that of the common keratinocytes but the presence of cytoplasmic tonofibrils and desmosomes provides evidence that they are of epithelial origin. They may represent the precursors of M cells. Their appearance may be followed by expression of Ia+ molecules on the surrounding keratinocytes showed by isolated Ia+ areas in the epithelium. (ii) In the mesenchyme underneath the Ia+ epithelial areas, Ia+ dendritic-like cells emerge which immigrate through the basement membrane into the epithelium establishing a provisional dendro-epithelial tissue. In this stage of the reticular epithelium's formation large Ia+ areas are shown by immunostaining, which include the epithelium and mesenchyme. The origin of the Ia+ dendritic-like cells is uncertain but their distribution and dense accumulation underneath the epithelium suggest that they are transformed tonsillar mesenchymal cells. Similar cell transformation of mesenchymal cells takes place in the bursa of Fabricius prior to development of lymphoepithelial tissue. (iii) The M cell precursors together with the Ia+ dendritic-like cells adapt the epithelium to be suitable for receiving a large number of lymphoid cells. Immigration of the lymphoid cells into the epithelium transforms the dendro-epithelial tissue to a real lymphoepithelial one.

Effects of low or high doses of short wavelength ultraviolet light (UVB) on Langerhans cells and skin allograft survival

Since Langerhans cells (LC) are normally the only cells within the epidermis to express the class II major histocompatibility complex (MHC) transplantation antigens, depletion of LC could be expected to prolong skin allograft survival by reducing the antigenic disparity between host and recipient. To assess this hypothesis, donor C57BL mouse shaved dorsal trunk or tail skin was exposed to high (200 mJ/cm2) or low (40 mJ/cm2) doses of short wavelength ultraviolet light (UVB) before grafting on to the thorax of BALB/c mouse recipients of the same sex. These strains have different major and minor transplantation antigens. The effects of UVB treatments on LC were determined by electronmicroscopy. Skin grafted 1-14 days following a single high dose of UVB irradiation was ultrastructurally depleted of LC and survived significantly longer than unirradiated skin before being rejected. After a 21-day interval between exposure and grafting when LC were again present in the epidermis there was no significant difference between treated and control graft survival. Exposure to low dose UVB irradiation only significantly increased graft survival for skin transplanted 1-3 days after irradiation; skin grafted 4 days following irradiation survived for a similar period to unirradiated control skin grafts. Electronmicroscopy showed that the low UVB dose did not deplete LC from the epidermis. We conclude that after low dose UVB treatment the class II MHC antigens on the LC plasma membrane were lost temporarily, thus prolonging graft survival, but when the plasma membrane antigens were re-expressed graft survival returned to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative studies on Langerhans cells in mouse corneal epithelium following infection with herpes simplex virus

Dendritic Langerhans cells (LCs) were identified in flat-mount preparations of mouse corneal epithelium after staining for ATPase activity. They were found predominantly in the limbus, but after inoculating the cornea with HSV1 strain SC16 LC, numbers increased both in the limbus and the central cornea. Numbers of LCs reached a maximum on day 8 and if severe keratitis was present remained high at least until day 22. A small but significant increase in LCs was also found in the opposite, uninoculated eye in mice with severe damage in the inoculated eye. After HSV inoculation on the snout, 60% of mice had corneal disease in the eye on the inoculated side; in such mice corneal LCs were at a maximum 18 days after inoculation. The increase in LC numbers was similar whether inoculation was into the cornea or in the snout. After corneal inoculation the cells were distributed fairly evenly over the corneal surface, with accumulations limited to epithelial ulcers. However, after inoculation on the snout, numerous clusters were seen over the epithelial surface, often surround by epithelium devoid of LCs.

PUVA therapy prevents sensitization to mechlorethamine in patients with psoriasis

Two groups of 20 patients with psoriasis were treated with mechlorethamine applied topically (group A) or with PUVA combined with mechlorethamine (group B). In group B mechlorethamine was started after six PUVA treatments. Results showed a significant decrease of the incidence of contact dermatitis in group B (30%) compared with group A (75%). Allergic dermatitis, demonstrated by a positive patch test to mechlorethamine with an histology of eczema, was observed in 55% of patients in group A and 20% in group B. The incidence of irritant dermatitis was not significantly different in the two groups. Allergic dermatitis was observed later in group B: after an average of 32.2 applications of mechlorethamine compared with 25 applications in group A. Possible mechanisms responsible for these results are reduction of epidermal Langerhans cells by PUVA therapy and induction of antigen-specific suppressor T cells. Patients living far from a specialized centre might be treated initially with PUVA therapy then with mechlorethamine alone, at home. This schedule may reduce the incidence of contact dermatitis to mechlorethamine.

Activation of herpes simplex following dermabrasion. Report of a patient successfully treated with intravenous acyclovir and brief review of the literature

Herpes simplex labialis developed in a patient immediately following dermabrasion. The patient was hospitalized because the infection had spread rapidly over the dermabraded face and was complicated by secondary impetiginization. Herpesvirus hominis type I and Enterobacter aerogenes were isolated from cultures. Intravenous acyclovir and oral antibiotics were administered. On this regimen, new vesicle formation ceased in 36 hours. Complete resolution of the infection occurred within 6 days, with an excellent cosmetic result. Observation at 1 month confirmed no sequelae (in particular, scarring). The "at risk" patient with a history of recurrent herpes labialis should be identified prospectively in an attempt to prevent possible reactivation; should this complication ensue, appropriate treatment should be immediately administered because a state of local immunocompromise exists. We believe our patient benefited greatly from vigorous treatment, with significant shortening of time to healing. Prophylaxis with oral acyclovir of "at risk" patients prior to dermabrasion is proposed.

Influence of ultraviolet radiation treatment on the survival of heterotopic skin grafts in the mouse

Isolated mouse tail skin was UV-irradiated in vitro at a dose of 40 mJ/cm2 from both sides to remove the Ia immunogenicity. Immediately after irradiation the skin was transplanted onto the flank of allogeneic mice. When there was a total H-2 difference between donor and recipient, the UV-irradiated skin did not show a prolonged survival compared to control grafts. In the case of an I-region difference only, i.e., B10.AQR grafts onto B10.T (6R) recipients, a significant prolongation of the survival time was observed, whereas 50% of the UV-treated grafts were not rejected at all.

Langerhans cells: antigen presenting cells of the epidermis

While epidermis in the skin provides an excellent barrier to the environment, it is an incomplete one. Some antigenic material can penetrate through the stratum corneum (or be introduced pathologically) where strategically placed epidermal Langerhans cells reside. In this review, we have assembled relevant data concerning the antigen presenting potential of epidermal Langerhans cells. Strong circumstantial evidence derived from in vitro studies of epidermal cell suspensions enriched for Langerhans cells indicates that Langerhans cells possess this capability. In vivo studies with intact skin indicate that critical numbers of functioning Langerhans cells are essential for successful induction of contact hypersensitivity by epicutaneously applied haptens. And within the past several months, experiments with purified preparations of epidermal Langerhans cells have proven that these cells, and perhaps they alone among epidermal cells, possess the capacity of processing and presenting haptenic determinants to the immune system. The challenge for the future is to determine the extent to which this unique property of Langerhans cells affords physiologic protection to the skin and under what pathologic circumstances altered Langerhans cell function leads to disease.

Prolongation of skin graft survival in mice by in vitro PUVA treatment and failure of induction of specific immunological memory by PUVA-treated grafts

Treatment of murine skin grafts in vitro with 8-methoxypsoralen and longwave ultraviolet radiation prolonged their subsequent survival on allogeneic recipients, but not in cases where the recipients had been presensitized by a former skin graft of the same donor strain. In contrast to normal skin, grafts pretreated with 8-methoxypsoralen and longwave ultraviolet radiation were not able to induce an immunological memory as revealed by a second transplantation of normal skin. The results show that primary and secondary skin graft rejection can be affected by the combined action of psoralen and ultraviolet radiation.

Anatomical mapping of epidermal Langerhans cell densities in adults

The densities of T6 antigen-bearing Langerhans cells in 112 biopsies of human skin from sixteen surgical out-patients and four cadavers were determined for eight anatomical regions. The regional mean densities (+/- s.e.m.) of epidermal Langerhans cells per mm2 were: head and neck, 489 +/- 27; chest, 466 +/- 22; back, 466 +/- 11; upper extremities, 458 +/- 25; lower extremities, 431 +/- 30; buttocks, 411 +/- 11; genitalia, 298 +/- 45; soles, 58 +/- 12. No statistically significant differences were found between any of these Langerhans cell densities except for that of the soles which was lower than those of all other regions (P less than 0.002). No significant differences were detected between the mean densities of patients and cadavers, Caucasians and Hispanics or males and females.

Strain variation in the induction of tolerance by epicutaneous application of trinitrochlorobenzene

It has been postulated that a relationship exists between the density of epidermal Langerhans cells and the capacity of the epidermis to promote the induction of contact sensitization. This postulate was developed, in part, because (1) mouse tail epidermis contains fewer ATPase-positive (presumably Langerhans) cells than does abdominal epidermis, and (2) when tails of C57Bl/6 mice were painted with dinitrofluorobenzene (DNFB), the mice were less sensitive than those painted on the abdomen. In addition, tail-painted mice were shown to be tolerant to subsequent attempts at sensitization with DNFB. In this study we found that by painting the tails of mice with the hapten trinitrochlorobenzene (TNCB), sensitization was induced in certain mouse strains (BALB/c, A/J, and CBA--haplotypes H-2d, H-2a, H-2k, respectively), but tolerance resulted from painting the tails of other strains (C57Bl/6, C57Bl/10, and AB.Y--haplotype H-2b). The ability to become sensitive or tolerant is not related to Langerhans cell density as detected by ATPase staining. While the mechanism for this strain difference in the induction of tolerance is unknown, tolerance induced in C57Bl/6 mice is mediated in part by the generation of suppressor cells.

Biology of Langerhans cells: selective migration of Langerhans cells into allogeneic and xenogeneic grafts on nude mice

A major question challenging immunobiologists relates to those mechanisms that control the selective movement of cells involved in immune and inflammatory processes at various tissue sites such as the skin. Little is known about those influences that control the selective migration of macrophage-like Langerhans cells (LC) to normal epidermis, where it is uniformly distributed. Mechanistically, this includes the interaction of blood-borne LC precursors with the vascular endothelium of the skin and those factors that control the migration of the LC into the avascular epidermal component of the skin. By using (i) monoclonal antibodies specific for I-region associated Ia antigens found on LC from various inbred strains of animals and (ii) the congenitally athymic (nude) mouse as an immunologically compromised recipient of allografts and selected xenografts, we developed a model system to study the factors that restrict LC migration into the epidermis. Using this model, which excludes the need to lethally x-irradiate graft recipients, we established that: (i) the ingress of LC does not show major histocompatibility complex restriction [LC of the nude host are capable of migrating into the epidermis of allogeneic and certain xenogeneic (rat) skin grafts]; (ii) host LC are incapable of migrating into the epidermis of guinea pig or human skin grafts; (iii) the ingress of host LC into the epidermis of the graft is not accompanied by an overgrowth of the graft by host epidermis; and (iv) LC or LC precursors are capable of dividing in the skin or, alternatively, represent an extremely long-lived cell population. The specificity of this model system provides a powerful tool to help understand many aspects of LC biology. Grafting human skin to the nude mouse not only provides a biologic support system for the graft but also is, by design, a system that is devoid of contaminating circulating precursor cell types. Manipulation of the experimental conditions is quite easy and provides a highly specific means to investigate many parameters of LC function.

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.

Failure of periodic ultraviolet radiation treatments to prevent sensitization to nitrogen mustard: a case report

We have reported that prior ultraviolet radiation (UVR) exposure significantly delayed development of contact sensitivity to nitrogen mustard (Halprin et al., 1981). We felt that this effect was due to disruption of functional Langerhans cells in skin by UVR and suggested that periodic UVR treatments might prevent sensitization to the mustard. We now report on a patient with mycosis fungoides whose epidermal Langerhans cell count was monitored with the ATP-ase stain in order to determine when such 'booster" UVR therapy was to be given. Our attempts to interfere with Langerhans cell function in this manner failed to prevent delayed contact sensitivity to nitrogen mustard and may have been partly responsible for the development of contact urticaria to nitrogen mustard after 28 days of use. Whether the reaction was a delayed, cell-mediated reaction, or an antibody mediated reaction is not clear, but the use of UVR did fail to prevent contact sensitivity to the nitrogen mustard in our patient.

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.

Disappearance of epidermal Langerhans cells during PUVA therapy

The numbers and morphological appearance of epidermal Langerhans cells (LCs) were studied in twenty-five patients with psoriasis receiving treatment with 8-methoxypsoralen (8-MOP) and long wavelength UV irradiation (UV-A) (PUVA). After a single exposure, LCs showed loss of fine dendritic processes. Repeated treatments resulted in a reduction of the number of LCs from the mean pretreatment value of 713/mm2 to less than 60/mm2 after seven treatments. The number of LCs remained low while treatment continued for up to 4 weeks. This finding may explain the impaired contact hypersensitivity observed in patients with psoriasis receiving PUVA therapy.

Ultraviolet light treatment delays contact sensitization to nitrogen mustard

Contact sensitivity to nitrogen mustard was delayed after first treating patients with u.v. light. While the number of patients becoming sensitive to nitrogen mustard after ultraviolet light exposure was not significantly different from the control, the number of patients who became sensitive in the first 30 days was significantly less than the control group. It is postulated than this delay in sensitization is due to an alteration of the patients' epidermal Langerhans cells, produced by the u.v. light exposure.

Langerhans cells as macrophages in skin and lymphoid organs

Properties of epidermal Langerhans cell were compared with those of a number of other dendritic cells in lymphoid organs and of mononuclear phagocytes. Among the dendritic "reticulum" cells included were indeterminate cells from the epidermis, interdigitating "reticulum" cells from T-dependent areas of lymphoid tissue and thymus, follicular dendritic cells of Nossal, and the dendritic cells described by Steinman and Cohn. Interdigitating cells with typical Birbeck granules, in the thymus and in the paracortices of lymph nodes, which are morphologically indistinguishable from Langerhans cells and indeterminate dendritic cells in the epidermis, appear to belong to the same system and possibly represent a subpopulation of "macrophages." On the basis of their similarity to these other dendritic cells, we believe Langerhans cells may function in antigen presentation, lymphokine production, provision of a microenvironment for T lymphocytes, and prostaglandin secretion.