Immunological and histochemical analysis of regional variations of epidermal Langerhans cells in normal human skin

Skin microbiome before development of atopic dermatitis: Early colonization with commensal staphylococci at 2 months is associated with a lower risk of atopic dermatitis at 1 year

BACKGROUND

Disease flares of established atopic dermatitis (AD) are generally associated with a low-diversity skin microbiota and Staphylococcus aureus dominance. The temporal transition of the skin microbiome between early infancy and the dysbiosis of established AD is unknown.

METHODS

We randomly selected 50 children from the Cork Babies After SCOPE: Evaluating the Longitudinal Impact Using Neurological and Nutritional Endpoints (BASELINE) longitudinal birth cohort for microbiome sampling at 3 points in the first 6 months of life at 4 skin sites relevant to AD: the antecubital and popliteal fossae, nasal tip, and cheek. We identified 10 infants with AD and compared them with 10 randomly selected control infants with no AD. We performed bacterial 16S ribosomal RNA sequencing and analysis directly from clinical samples.

RESULTS

Bacterial community structures and diversity shifted over time, suggesting that age strongly affects the skin microbiome in infants. Unlike established AD, these patients with infantile AD did not have noticeably dysbiotic communities before or with disease and were not colonized by S aureus. In comparing patients and control subjects, infants who had affected skin at month 12 had statistically significant differences in bacterial communities on the antecubital fossa at month 2 compared with infants who were unaffected at month 12. In particular, commensal staphylococci were significantly less abundant in infants affected at month 12, suggesting that this genus might protect against the later development of AD.

CONCLUSIONS

This study suggests that 12-month-old infants with AD were not colonized with S aureus before having AD. Additional studies are needed to confirm whether colonization with commensal staphylococci modulates skin immunity and attenuates development of AD.

Nerve-Langerhans cell interactions in diabetes and aging

Cutaneous infections are a leading cause of hospitalization of diabetic patients. Langerhans cells (LCs) are antigen-presenting cutaneous dendritic cells that protect against infections, and effects of diabetes and aging on these cells are unclear. We examined LCs in footpads of rats with streptozotocin-induced diabetes at 3 months of age following 4 weeks of diabetes, and at 6 months following 16 weeks of diabetes. Immunostaining of LCs using the selective marker protein langerin showed cutaneous LC composition increased between 3 and 6 months of age owing to increased LC numbers and size in control rats. In diabetic rats, LC numbers increased with age but, unlike 6 month old controls, cell size did not, suggesting that diabetes impairs the increase in cell size that is a hallmark of LC maturation. Diabetes reduced LC numbers after 4 weeks and numbers and sizes following 16 weeks. We examined the relation between LC and innervation and found that, while axon density decreased with aging, it was not affected by 16 weeks of diabetes. However, LCs expressing the neuronal marker PGP9.5 represented a source of error in axonal counts. These findings support the hypothesis that diabetes substantially impacts LC proliferation and maturation independent of effects on cutaneous innervation. Accordingly, the interactions of diabetes and aging on LCs may be important factors in predisposing diabetic patients to cutaneous ulcers and infections.

Dermal dendritic cells in psoriasis, nummular dermatitis, and normal-appearing skin

BACKGROUND

The reason psoriasis (PSO) favors extensor skin is unknown. We hypothesized that PSO may involve extensor skin preferentially because of differences in the number or type of dermal dendritic cells (dDCs) between flexural and extensor skin.

OBJECTIVE

We sought to compare dDC type and distribution in normal-appearing flexural and extensor skin, PSO, and nummular dermatitis (ND).

METHODS

Using immunohistochemical markers, the number, distribution, and type of Langerhans cells, myeloid dendritic cells (DCs), and plasmacytoid DCs was compared in normal-appearing skin, PSO, and ND.

RESULTS

Significant differences in dDC density were not identified between flexural and extensor skin, although extensor skin contained fewer CD11a(+) and CD11c(+) cells. Compared with normal-appearing skin, cells expressing CD11a, CD11c, CD123, CD303, and CD207 were increased in PSO. ND lesions showed similar increases. No significant difference between PSO and ND was evident with the exception of decreased S100A6(+) cells in PSO.

LIMITATIONS

We did not study seasonal variation in DC density or assess nonlesional skin from patients with PSO.

CONCLUSIONS

The data did not support the hypothesis that PSO favors extensor skin because of differences in DC localization. However, dDCs were significantly increased in PSO by comparison with normal-appearing skin, supporting existing evidence that they are involved in the overall pathogenesis of PSO.

Role of epidermal dendritic cells in drug-induced cutaneous adverse reactions

Drug-induced adverse reactions (ADR) include any undesirable pharmacological effect that occurs following drug administration at therapeutic doses. The appearance of ADR significantly limits the use of drugs in as much as their clinical symptoms may range from very mild discomfort such as cutaneous rash, up to very severe, or even fatal tissue necrolysis such as the Stevens Johnson syndrome.One of the most frequently involved organ during ADR is the skin. Drug-induced cutaneous reactions (CDR) incidence is variable but they may appear in 2-3% of ambulatory patients, and it may increase to 10-15% when patients are hospitalized, or even be as high as 60% when co morbidity includes the presence of virus, bacteria, or parasites.Due to the fact that skin is one of the organs most frequently involved in ADR, in this work we analyze and propose a mechanism by which epidermal dendritic cells operating as the sentinels of the skin neuro-immune-endocrine system may contribute to CDR via either immunogenic or tolerogenic immune responses towards drugs, whenever they are administered topic or systemically.

Resident and "inflammatory" dendritic cells in human skin

Dendritic cells (DCs) are a heterogeneous group of antigen-presenting leukocytes that are important in activation of both the innate and adaptive arms of the immune system. Although there are several different DC populations in the body, DCs are globally defined by their capacity for potent antigen presentation and naive T-cell activation. In noninflamed human skin during steady state, there are three main cutaneous DC populations: epidermal Langerhans cells, dermal myeloid DCs, and dermal plasmacytoid DCs. In psoriasis, a model for cutaneous inflammation, there is an additional population of myeloid dermal DCs--"inflammatory DCs"--which appears to be critical for disease pathogenesis.

Mystery of the disappearing allergen: published allergens rarely seen again

Patch testing is an important tool in the diagnosis of allergic contact dermatitis. Although this technique can be accurate, occasionally the results may be inconclusive. A previously positive result to an allergen may become negative upon repeat testing, and this may complicate the process of achieving a definitive diagnosis. There are some potential explanations for such inconsistencies, including the Excited Skin Syndrome, irritant reactions, a need to repeat the diagnostic algorithm, "rogue" reactions, and "contact allergy." These explanations should be taken into account when interpreting these results. However, further knowledge is needed to solve the mystery of an allergen that subsequently disappears.

Deterioration of the Langerhans cell network of the human gingival epithelium with aging

Dendritic cells (DCs) are the professional antigen-presenting cells responsible for initiating of the immune response. Langerhans cells (LCs) are a type of DC that is a permanent resident of the oral epithelium. LCs are organized conforming a network in such a way as to maximize their surface area for efficient apprehension of antigens. To detect age-related changes in the LCs network, fragments of gingival epithelium spontaneously accompanying dental removals were processed by immunohistochemistry. Monoclonal antibody CD1a followed by biotinized immunoglobulin-streptoavidin peroxidase were used to identify the LCs with the light microscope. LC density and LC types were analyzed according to their morphology and intraepithelial distribution. In the older age group (61-74 years) the density was significantly lower than in the younger age groups. Morphologically, LCs showed fewer dendritic-branching processes and had a rounded shape in the older age group. Present observations indicate that the LC network changes markedly with aging. These results suggest that immunological defense of the oral tissue might be compromised in old age.

40nm, but not 750 or 1,500nm, Nanoparticles Enter Epidermal CD1a+ Cells after Transcutaneous Application on Human Skin

Although conventional vaccines have generated major successes in the control of infectious diseases, several obstacles remain in their development against chronic diseases (HIV, tuberculosis), against which no current candidate vaccines yet ensure protection. The transcutaneous route of vaccine administration appears to be a promising approach of targeting vaccines toward antigen-presenting cells (APCs) and thus improving immune responses. We investigated the suitability of nanoparticles in this approach. We found a high density of Langerhans cells (LCs) around hair follicles that, when sorted, readily internalized all size particles. However, flow cytometry after transcutaneous application of 40, 750, or 1,500 nm nanoparticles on human skin samples revealed that only 40 nm particles entered epidermal LC. Fluorescence and laser scan microscopies, which were carried out to identify the penetration pathway of transcutaneously applied nanoparticles, revealed that only 40 nm particles deeply penetrate into vellus hair openings and through the follicular epithelium. We conclude that 40 nm nanoparticles, but not 750 or 1,500 nm nanoparticles, may be efficiently used to transcutaneously deliver vaccine compounds via the hair follicle into cutaneous APCs.

Ultrastructural determination of gingival Langerhans cells in alloxan-induced diabetic rats

The ultrastructure of Langerhans cells has not been fully investigated in diabetes-associated gingival tissues. The present study was carried out to investigate the ultrastructure of gingival Langerhans cells in alloxan-induced diabetic rats. Gingival biopsies were obtained from 22 diabetic and 18 control rats. Langerhans cells were observed by transmission electron microscopy (TEM) in the basal layers of healthy oral epithelium. On rare occasions, Langerhans cells were found in the suprabasal layers of the oral epithelium. Langerhans cells in the oral epithelium of diabetic rats were seen in the basal and suprabasal layers. Usually, Langerhans cells had clear cytoplasm and convoluted or indented nuclei and few or no specific granules. The clear cytoplasm contained mitochondria, lysosomes and a small number of rough-surfaced endoplasmic reticulum regions, but it lacked tonofilament. Occasionally, centrioles were also observed in the cytoplasm. The membrane of Langerhans cells had no junctional complexes such as desmosomes. In diabetic rats, Langerhans cell precursors were developed into specific granule-bearing cells. Both Langerhans cells and their granules were more frequent in the gingiva of diabetic rats than in the control group. These data suggest that Langerhans cells play an important role in explaining the pathogenesis and development of diabetic gingivitis.

Spatial relationship between Merkel cells and Langerhans cells in human hair follicles

The distributions of Merkel cells and Langerhans cells within human hair follicles have been reported. However, there has been no description of the relationship between Merkel cells and Langerhans cells, which were discovered by 19th century German pathologists. Merkel cells and Langerhans cells share some similar characteristics such as the localization of human hair follicles, a close association with peripheral nerves and the expression of several neuropeptides. Merkel cells were stained with CK20 or CAM5.2, while Langerhans cells were stained with CD1a or S-100 protein. We thus immunohistochemically confirmed the preferential localization of Merkel cells and Langerhans cells in normal human hair follicles. Using a double staining technique, two- and three-dimensional observations demonstrated that a small proportion of Merkel cells were closely contacted with Langerhans cells below the sebaceous gland level, presumably indicating the bulge area. Merkel cells and Langerhans cells connected directly or approached each dendrite within the basal layer of the outer root sheath. For the first time, we demonstrated a close anatomical relationship between Merkel cells and Langerhans cells within the bulge area of human hair follicles where follicular stem cells may be present. These morphological observations suggest a functional interaction between follicular Merkel cells and Langerhans cells. We herein hypothesize that Merkel cells communicate with Langerhans cells by characteristic dendrites in which some neuropeptides or cytokines may be stored.

Huriez syndrome: case report with a detailed analysis of skin dendritic cells

We report a 60-year-old man with familial scleroatrophic syndrome of Huriez who developed squamous cell carcinomas on the affected skin of the right palm. Immunohistochemical analysis showed a marked reduction in the number of CD1a+, Lag+ and S100+ epidermal Langerhans cells, but not of CD1b+ and factor XIIIa+ dermal dendritic cells, limited to palmoplantar skin. The Langerhans cell depletion was not associated with an abnormal skin content of mRNA for factors involved in Langerhans cell development or recruitment in the epidermis, including granulocyte/macrophage colony-stimulating factor, transforming growth factor-beta1 and macrophage inflammatory protein-3alpha. The results indicate that other as yet unknown mechanisms may account for the reduced number of Langerhans cells in the affected skin of such patients.

Myocutaneous flaps in patients with head and neck cancer retain their immunological capacities in an activated functional state

Free/pedicled myocutaneous flaps used as functional replacement after radical dissection of advanced stage squamous cell carcinomas of the oral cavity/oropharynx were examined by immunohistochemistry (APAAP-technique). Biopsies from eight patients were taken at the time of surgery and at 3 and 5 months post-operatively. Fifteen monoclonal antibodies were used to detect surface antigens as markers of phenotypic changes of immune competent cells. In post-operative biopsies all antigens investigated increased significantly. Significantly higher numbers of CD45RO+ (P < 0.01) and CD45RA+ (P < 0.001) leukocytes were detectable. The majority of these leukocytes were TcR alpha/beta +/CD3+ T-cells, which increased in the CD4 (P < 0.05) and the CD8 (P < 0.001) subset. In addition, B-cells (P < 0.05), granulocytes (P < 0.05), NK cells (CD16+ lymphocytic cells; P < 0.05) and mature macrophages (25F9+cells; P < 0.01) were increased. Intra- and subepidermally a significantly (P < 0.01) higher number of dendritic-/Langerhans cells (CD1a+) was detectable. In post-operative biopsies, the activation-associated antigens ICAM-1, VCAM and HLA-DR were expressed on significantly more mononuclear-/endothelial cells and on keratinocytes. Our findings indicate that the myocutaneous flaps still contained cells with immunological capacities.

Comparison of the effect of 8-methoxypsoralen (8-MOP) plus UVA (PUVA) on human melanocytes in vitiligo vulgaris and in vitro

In this study, we examined the various effects of PUVA treatment on cultured human melanocytes, and it revealed that 1) the higher the dose of PUVA treatment, the more significant the inhibition of cell DNA and protein synthesis; 2) the higher the dose of PUVA treatment, the more significant the depletion of epidermal growth factor receptor (EGFR) expression; 3) PUVA treatment at 124 mJoule/cm2 depleted the vitiligo-associated melanocyte antigens (VAMA) immediately after irradiation, and both the VAMA and EGFR expression progressively recovered at 24 or 72 h after PUVA; 4) PUVA treatment stimulated tyrosinase activity, but not in a dose-dependent fashion. In vitiligo vulgaris, PUVA treatment may stimulate the regrowth of melanocytes from hair follicles, but deplete the epidermal Langerhans cells in depigmented lesion of patients with stable vitiligo. Comparing the above results obtained from in vivo and in vitro studies, it reveals significantly different biologic responses. Although the precise therapeutic mechanism of PUVA treatment in vitiligo is still not well known, it is proposed that 1) PUVA treatment may stimulate the other components of skin, such as keratinocytes, to release inflammatory mediators and some of them may act as melanocyte growth-stimulatory factors (MGSF), which further enhance the proliferation of remaining melanocytes in hair follicle; and 2) PUVA treatment may deplete the VAMA expression on cell membrane of melanocytes and also deplete epidermal Langerhans cells, which may result in blocking the progressing of antibody-dependent cell-mediated cytotoxicity to melanocytes in vitiligo.

Depletion and repopulation of Langerhans cells in nonsegmental type vitiligo

The role of Langerhans cells in the pathogenesis of nonsegmental type vitiligo is still unknown. In this study, biopsies were taken from 26 patients at various stages of nonsegmental type vitiligo and morphometrically observed to investigate the kinetics of Langerhans cells in patients at various stages of the disease. A marked depletion of OKT6-positive and ATPase-positive epidermal dendritic cells was noted in patients with active nonsegmental type vitiligo. A repopulation of both OKT6-positive and ATPase-positive epidermal dendritic cells was noted in patients with stable nonsegmental type vitiligo. Profound depletion of epidermal OKT6-positive and ATPase-positive dendritic cells was noted in patients with repigmenting nonsegmental type vitiligo receiving treatments involving topical use of 0.05% Fluocinonide cream or PUVA photochemotherapy. Transmission electron microscopy confirmed the absence of epidermal dendritic cells (Langerhans cells and intermediate cells) in patients with active and repigmenting nonsegmental type vitiligo. In active nonsegmental type vitiligo, two possible explanations are proposed for the depletion of OKT6-positive and ATPase-positive epidermal dendritic cells (presumptive Langerhans cells): 1) the cells are destroyed by cytotoxic factors released during the course of destruction of melanocytes in active vitiligo, and/or 2) they leave the epidermis and migrate to regional lymph nodes to present certain antigens which are released from certain destroyed epidermal cells (keratinocytes or melanocytes) during the course of active vitiligo. The repopulated epidermal Langerhans cells may result from phenotypically transformed dermal dendritic cells in the depigmented lesions of patients with stable vitiligo. Since various therapies which result in repigmentation deplete the density of epidermal Langerhans cells markedly, it is suggested that depletion of epidermal Langerhans cells in stable vitiligo may aid in repigmentation. It is also proposed that the repopulated epidermal Langerhans cells may play a role in inhibiting the proliferation of epidermal melanocytes in depigmented lesions of stable vitiligo, thus various methods of treatment which deplete the Langerhans cells may eventually aid in the repigmentation of nonsegmental type vitiligo.

Human epidermal T cells predominantly belong to the lineage expressing alpha/beta T cell receptor

The epidermis of clinically normal-appearing human skin harbors a phenotypically heterogeneous population of T lymphocytes (TCs), the majority of which are CD2+/CD3+/CD5+ "memory" cells, but in an unactivated state, and express the TCR-alpha/beta. In contrast to murine skin, only a very minor subpopulation of CD3+ cells in the human epidermis bears the TCR-gamma/delta. Epidermal TCs primarily are distributed along the rete ridges in the basal keratinocyte layer and are often in close apposition to Langerhans cells (LCs). These TCs were propagated from epidermal cell suspensions after stimulation with TC activating agents (Con A, rIL-1, rIL-2), then evaluated for phenotypic features and TCR diversity. Similar to the in situ situation, most were CD4-/CD8+/TCR-alpha/beta+. In addition, two cultures contained TCR-gamma/delta+ cells; one of these determined to be an adherent CD4-/CD8+ population. Epidermal TCs were significantly (p less than 0.0001) more abundant in the sole than in the other body regions examined (i.e., 40 vs. 7 CD3+ cells/linear centimeter of epidermis) and seemed to have a particular affinity for the acrosyringial epithelium of eccrine sweat ducts. Moreover, the sole usually contained a greater number of CD8+ relative to CD4+ TCs, whereas the epidermal CD4/CD8 ratio in the trunk and extremities was quite variable, although the trend also was towards a slightly larger percentage of CD8+ cells. Collectively, our data suggest that the volar epidermis has a unique microenvironment which is responsible for both the higher density of TCs, preferentially CD8+, and lower number of LCs. This study has not only provided evidence for significant regional variability in the human epidermal TC population of normal skin, but also strengthens the concept for skin-associated lymphoid tissues (SALT), whereby memory TCs recirculate back to the epidermis and interact with resident antigen-presenting cells (i.e., LC).

Cutaneous dermal Ia+ cells are capable of initiating delayed type hypersensitivity responses

The presence of Langerhans cells (LC) within the epidermis has been shown to be critical for inducing T-cell-mediated immune responses in the skin. The purpose of this study was to assess whether cells in the dermis can initiate T-cell-mediated delayed-type hypersensitivity responses in vivo. Initially, back skins from C3H mice were trypsinized to remove the epidermis. The dermis was enzymatically dispersed and filtered to obtain a cell suspension. However, dermal cells exposed to trypsin were contaminated with numerous disaggregated hair follicles. These hair follicles contained Ia+ cells (presumably LC), and upon haptenation in vitro with trinitrophenyl, initiated contact hypersensitivity reactions in vivo. We therefore used dispase in place of trypsin to prevent follicular disaggregation and to allow preparation of dermal cell suspensions free of hair follicles. These hair follicle-free dermal cells were haptenated with trinitrophenyl and injected intradermally. Elicitation of contact hypersensitivity by epicutaneous painting 6 d later revealed the mean +/- SEM incremental ear-swelling response to be 53 +/- 8 mm X 10(-3). In contrast, mice sensitized by injection with dermal cells depleted of Ia+ cells demonstrated only 10 +/- 1 mm X 10(-3) of ear swelling. Thus, like dendritic LC of the epidermis, perivascular dendritic Ia+ cells of the dermis are capable of initiating T-cell-mediated contact hypersensitivity in vivo and may be highly relevant for presentation of antigen to T cells trafficking through the dermis.

Dermatitis due to intravesical mitomycin C: a delayed-type hypersensitivity reaction?

Mitomycin C is an alkylating agent, used by intravesical instillation to treat carcinoma of the bladder. Repeated instillations can induce cystitis and an eczematous eruption affecting the palms, soles and face. If these effects are due to delayed hypersensitivity with sensitization to mitomycin C occurring in the bladder wall, it should be possible to demonstrate antigen-presenting cells in the bladder wall and positive patch tests to the drug. Using an immuno-alkaline phosphatase method we have identified CDI+ cells in bladder epithelium and submucosa and have demonstrated Birbeck granules in a few cells. In further support of our hypothesis it was also possible to demonstrate delayed type hypersensitivity in 13 out of 26 patients who had received mitomycin instillations by applying the allergen as a patch test. These results indicate that the eczematous eruption in this group of patients is most likely a hypersensitivity reaction and that it may be mediated transvesically.

Cutaneous leucocyte composition after human allogeneic bone marrow transplantation: relationship to marrow purging, histology and clinical rash

Immunohistological and morphometric techniques were used to study the skin after marrow transplantation with particular reference to the relationship of marrow purging, the presence of a clinical rash and histological changes to leucocyte numbers and phenotype. Recipients of T-cell-depleted marrow showed significant reductions in CD2+, CD4+ and CD8+ T-lymphocytes in the first 22 d after transplantation but not after this time. T-cell numbers in recipients of unpurged marrow were similar to those of normal donors, indicating a rapid repopulation by cells from the graft. Langerhans cells (CD1+ dendritic cells) and macrophages, on the other hand, were present in similar numbers in both groups of patients within the first 22 d; the former in low and the latter in normal numbers. Biopsies exhibiting graft versus host disease showed increases in CD2+, CD4+ and CD8+ T-lymphocytes with significant lowering of the CD4:CD8 ratio. A proportion expressed markers of activation and HNK1+ cells and macrophages were also increased. Biopsies exhibiting epidermal basal abnormalities only (changes identical to graft versus host disease but without detectable leucocyte infiltration on conventional microscopy) showed a minor increase in macrophages and HNK1+ cells but no other leucocyte alterations to suggest a pathogenetic link with graft versus host disease. Langerhans cells were reduced in these biopsies, however, when taken more than 22 d post-transplant, suggesting that the epidermal changes are associated with Langerhans cell damage or repopulation. We were unable to identify any significant alteration in leucocytes in patients with strong clinical evidence of graft versus host disease but with histologically unremarkable biopsies. Although it is possible that perivascular increases in T-cells and expression of activation markers precede the characteristic histological picture of graft versus host disease the time scale is probably too short to allow diagnostic value.

Long-term effects of local ionizing radiation treatment on Langerhans cells in mouse footpad epidermis

Langerhans cells (LC) were studied with ADPase histochemistry in sheets of hind footpad epidermis from groups of CBA/H mice. Single, local 20-Gy doses of 250-kV x-rays were administered to the right hind feet of the mice when they were 3-4 months old, and LC were counted at intervals ranging from 2 to 24 months later. In unirradiated mice, aged 5-19 months, the mean density of LC in footpad was 1521-1617 cells/mm2. It dropped to 1137 +/- 86 cells/mm2 (mean +/- SE) in untreated 28-month-old mice. At times from 2-15 months after irradiation, normal mean densities of LC were present in footpad epidermis. On average, LC numbers were subsequently reduced to 1078 +/- 65/mm2 by 19 months after irradiation (71% of the cells in age-matched controls) and to 789 +/- 53/mm2 by 24 months (59% of the cells in age-matched controls). Loss of cells was focal. Chronic radiation-induced fibrosis and damage to circulatory function in skin may have contributed to impaired replacement of LC from bone marrow precursors. The possibility that late radiation-related depletion of the LC population permits development of skin tumors as a delayed consequence of exposure to ionizing radiation is discussed.

Morphological and immunohistological changes in the skin in allogeneic bone marrow recipients

Skin biopsies from leukaemic patients undergoing allogeneic bone marrow transplantations and treated prophylactically with cyclosporin A were analysed using histological, morphometric, and immunohistological techniques. Samples from donors were used to establish normal values. Biopsies taken from recipients two days before grafting were all histologically normal, but on immunohistological staining half of them showed a reduction in the number of epidermal Langerhans' cells and 29% a reduction in T inducer lymphocytes. Thirty two biopsies were taken from patients with rashes at various times after transplantation: 14 showed lichenoid changes consistent with graft versus host disease, three eczematous tissue reactions, two vesicular lesions, and 12 no histological abnormality. One sample showed changes intermediate between the lichenoid and eczematous forms. The numbers of epidermal Langerhans' cells were low during the first few weeks after transplantation and were normal or raised later regardless of histological appearances. Unlike epidermal Langerhans' cells, significant reductions in the numbers of lymphocytes were not seen. Lesions of all histological types contained mixtures of T inducer and T suppressor/cytotoxic cells, although the eczematous and vesicular lesions contained higher proportions of T inducer cells. Epidermal infiltrates invariably contained T suppressor/cytotoxic cells but infiltration of epidermis by T inducer cells occurred only in the presence of normal numbers of epidermal Langerhans' cells. Natural killer cells were not identified. The immunological appearances of the various histological subgroups thus change with time after transplantation.