The lichenoid tissue reaction. A new concept of pathogenesis

Cytotoxic dermatitis: Review of the interface dermatitis pattern in veterinary skin diseases

Interface dermatitis or lichenoid interface dermatitis refers to a cutaneous inflammatory pattern in which keratinocyte cell death is the essential feature. These terms have evolved from the originally described lichenoid tissue reaction. These lesions are the basis for an important group of skin diseases in animals and people where cytotoxic T-cell-mediated epidermal damage is a major pathomechanism. Yet, for largely historical reasons these commonly used morphological diagnostic terms do not reflect the essential nature of the lesion. An emphasis on subsidiary lesions, such as the presence of a lichenoid band, and definitions based on anatomical features, such as location at the dermo-epidermal location, may cause confusion and even misdiagnosis. This review covers historical aspects of the terminology, including the origin of terms such as "lichenoid." The types of cell death involved and the histopathologic lesions are described. Etiopathogenesis is discussed in terms of aberrations of immune/inflammatory mechanisms focusing on cutaneous lupus erythematosus, erythema multiforme, and Stevens-Johnson syndrome/toxic epidermal necrolysis. Mechanisms have most extensively been studied in humans and laboratory animals and the discussion is centered on these species. As interface dermatitis is firmly entrenched in dermatological parlance, rather than using "cytotoxic" as its substitute, the terminologies "interface cytotoxic dermatitis" and "panepidermal cytotoxic dermatitis" are recommended, based on location and extent of epithelium affected.

Lichen Planus Annularis: An Immunohistochemical Study

Lichen planus annularis is a relatively rare skin manifestation of lichen planus. The mechanisms in the formation of annular lesions are not fully understood. We reported here a 57-year-old female with this disease. The eruption initially occurred as lichen-papules, then enlarged (bean-sized, umbilicated small plaques), and finally developed annular manifestations. We performed immunohistochemical examinations of specimens taken from different types of eruptions. In all specimens, HLA-DR was expressed in the focal keratinocytes adjacent to the dermal HLA-DR positive cell infiltration. Both in the initial papule and in the final annular lesion, expression of ICAM-1 was present only in the keratinocytes above the dermal cell infiltration, similar to HLA-DR. It is of interest that, in the umbilicated small plaques, the peripheral epidermis other than the central site extensively reacted to ICAM-1. LFA-1 expression was most prominent in the mononuclear cells impinging on the dermo-epidermal junction in all specimens. In addition, in the periphery of the umbilicated small plaques, which showed no bandlike dense cell infiltration nor degeneration of basement membrane, TNF-alpha, but not LFA-1, was positive in the infiltrated cells of the upper dermis. These results suggest that expressions of ICAM-1 and TNF-alpha in the peripheral keratinocytes and dermal infiltrated cells are important molecular events in the mechanisms of formation of the annular lesions.

Vitiligo, reactive oxygen species and T-cells

The acquired depigmenting disorder of vitiligo affects an estimated 1% of the world population and constitutes one of the commonest dermatoses. Although essentially asymptomatic, the psychosocial impact of vitiligo can be severe. The cause of vitiligo remains enigmatic, hampering efforts at successful therapy. The underlying pathogenesis of the pigment loss has, however, been clarified to some extent in recent years, offering the prospect of effective treatment, accurate prognosis and rational preventative strategies. Vitiligo occurs when functioning melanocytes disappear from the epidermis. A single dominant pathway is unlikely to account for all cases of melanocyte loss in vitiligo; rather, it is the result of complex interactions of biochemical, environmental and immunological events, in a permissive genetic milieu. ROS (reactive oxygen species) and H2O2 in excess can damage biological processes, and this situation has been documented in active vitiligo skin. Tyrosinase activity is impaired by excess H2O2 through oxidation of methionine residues in this key melanogenic enzyme. Mechanisms for repairing this oxidant damage are also damaged by H2O2, compounding the effect. Numerous proteins and peptides, in addition to tyrosinase, are similarly affected. It is possible that oxidant stress is the principal cause of vitiligo. However, there is also ample evidence of immunological phenomena in vitiligo, particularly in established chronic and progressive disease. Both innate and adaptive arms of the immune system are involved, with a dominant role for T-cells. Sensitized CD8+ T-cells are targeted to melanocyte differentiation antigens and destroy melanocytes either as the primary event in vitiligo or as a secondary promotive consequence. There is speculation on the interplay, if any, between ROS and the immune system in the pathogenesis of vitiligo. The present review focuses on the scientific evidence linking alterations in ROS and/or T-cells to vitiligo.

Involvement of granzyme B and granulysin in the cytotoxic response in lichen planus

BACKGROUND

Lichen planus is an inflammatory dermatosis involving either skin and/or mucosal epithelial surfaces. A cell-mediated cytotoxicity response is the main suspected mechanism of this dermatosis. Granzyme B and granulysin are components of the cytoplasmic granules of cytotoxic T lymphocytes and natural killers. They are involved in cell-mediated apoptosis. This work studies the possible implication of granzyme B and granulysin in the cell-mediated cytotoxicity response in lichen planus.

METHODS

In situ expression of granzyme B and granulysin was studied by real-time reverse transcriptase polymerase chain reaction in 15 biopsies of lichen planus. The distribution and the phenotype of the inflammatory infiltrate and the expression of granzyme B were studied by immunohistochemistry in seven other biopsies of lichen planus.

RESULTS

Granzyme B and granulysin mRNA expression was one to two hundred times greater than in biopsies of normal skin. Immunohistochemical study revealed that the lymphohistiocytic infiltrate consisted mainly of CD4+ and CD8+ lymphocytes. Granzyme B+ cells were observed close to apoptotic keratinocytes.

CONCLUSION

Our results suggest a central role for cell-mediated cytotoxicity by the granule exocytosis pathway probably because of auto-cytotoxic T-cell clones in the pathogenesis of lichen planus.

Possible role of Fas/Fas ligand-mediated apoptosis in the pathogenesis of fixed drug eruption

BACKGROUND

Although epidermal and dermal T cells play roles in the pathogenesis of fixed drug eruption (FDE), not much is known about keratinocyte death and its precise mechanism in FDE.

OBJECTIVES

Our aim is to elucidate the mechanism that underlies keratinocyte death in FDE, that is, the role of apoptosis and its signalling pathway.

METHODS

We first examined the involvement of apoptosis in the active FDE lesions by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labelling (TUNEL) assay and immunohistochemical analysis of caspase-3. We then examined the expressions of Fas and Fas ligand (FasL) to deduce the possible upstream signalling pathway of apoptosis, if apoptosis were involved. We finally characterized the infiltrated T-cell subpopulations in the active FDE lesions.

RESULTS

In the active FDE lesions, TUNEL positivity was strongly observed in the basal keratinocytes, and also weakly observed in the upper dermal infiltrates as well as in a few keratinocytes in the granular layer. The distribution of TUNEL-positive cells was similar to that of the strong staining of active capase-3. Fas was found mainly in the keratinocytes and some infiltrated dermal cells, whereas FasL was identified predominantly in the intraepidermal and dermal infiltrated cells and in some basal keratinocytes. Overlapping expression of Fas and FasL was accompanied by apoptosis in the FDE lesions. Many of the infiltrated mononuclear cells were CD8+. Perforin was rarely observed in the FDE lesions.

CONCLUSIONS

These findings suggest that apoptosis of the keratinocyte is highly likely to be involved in the pathogenesis of FDE, and this cytotoxicity might be predominantly mediated by the FasL of the infiltrating CD8+ T cells, possibly also playing an inflammatory role.

Management of erosive lichen planus with topical tacrolimus and recurrence secondary to metoprolol

Metoprolol, a widely prescribed beta-adrenergic receptor blocker, has occasionally been associated with a diversity of cutaneous reactions. We present a 79-year-old male patient with erosive lichen planus (LP) on the feet and hands who was successfully treated with topical tacrolimus. Six months after the lesions had been cured the patient received the beta-receptor blocker metoprolol for the treatment of hypertonus. Within only 2 weeks of metoprolol intake the erosive lesions on the palms and feet recurred. After discontinuation of the drug and repetitive topical treatment with tacrolimus a complete remission of the lesions could be achieved. The recurrence of erosive LP probably secondary to metoprolol and the therapeutic success of topical tacrolimus in the treatment of LP are discussed.

The role of perforin-mediated apoptosis in lichen planus lesions

Lichen planus is recognized as a T-cell-mediated disease. Histologically, it is characterized by the formation of colloid bodies representing apoptotic keratinocytes. The apoptotic process mediated by CD8(+) cytotoxic T lymphocytes (CTLs) and NK cells mainly involves two distinct pathways: the perforin/granzyme pathway and the Fas/FasL pathway. So far, little is known regarding the role of perforin-mediated apoptosis in lichen planus. In the present study, the expression and distribution of perforin, T and NK cell subsets in the epidermis and dermis of lesional and nonlesional lichen planus skin were studied. Skin biopsy specimens from lesional and nonlesional skin of ten patients with lichen planus and eight healthy persons were analysed by immunohistochemistry. Significant accumulation of T cells, particularly of CD4(+) and CD8(+) subsets, was found in both epidermis and dermis of lichen planus lesions compared with nonlesional and healthy skin. There were no significant differences in the incidence of NK cells (CD16(+) and CD56(+)) between lesional, nonlesional and healthy skin. Perforin expression was significantly upregulated in the epidermis of lichen planus lesions. In conclusion, accumulation of perforin(+) cells in the epidermis of lichen planus lesions suggest a potential role of perforin in the apoptosis of basal keratinocytes.

Blockade of lymphotoxin signaling inhibits the clinical expression of murine graft-versus-host skin disease

Adhesion molecules are essential for the recruitment of T cells into the skin during the development of graft-vs-host skin disease (GVHSD). However, the mechanisms responsible for the regulation of expression of cutaneous adhesion molecules in this setting are still poorly understood. In this study we blocked lymphotoxin (LT) signaling in a murine model of minor histocompatibility Ag system mismatch GVHSD by using an LTbeta receptor-Ig fusion protein (LTbetaR-Ig). The recipient mice treated with control human Ig developed clinically apparent, severe skin lesions. However, none of the mice treated with LTbetaR-Ig developed clinical skin disease. The expression of ICAM-1 in cutaneous tissue was also much lower in mice treated with LTbetaR-Ig than in mice treated with human Ig. Thus, the inhibition of LT signaling via LTbetaR-Ig treatment appears to be capable of markedly ameliorating the development of GVHSD, possibly by inhibiting the expression of adhesion molecules.

Coexistence of Riehl's melanosis and lichen planus

A 61-year-old man presented with purple-red papules and plaques. These lesions on the nape and dorsal aspects of both hands displayed the histological findings characteristic of lichen planus. Riehl's melanosis, which is synonymous with pigmented contact dermatitis, was present on the forehead and bald scalp. Riehl's melanosis typically appears on the face and is induced by cosmetics. Causative factors in this case, however, remained undefined. The coexistence of lichen planus and Riehl's melanosis at separate sites in a single patient suggests that the two diseases represent manifestations of specific immune responses characteristic of lichenoid tissue reaction and situate in the same spectrum. The differences in the skin manifestations of lichen planus and Riehl's melanosis might depend on the severity of immune responses against intrinsic or extrinsic antigens. In addition, the skin manifestations of lichenoid tissue reaction might depend on the disposition of the lesion sites.

'Activation-induced cell death': a special program able to preserve the homeostasis of the skin?

The 'activation-induced cell death' (AICD) is a molecular system leading to death of antigen-activated T lymphocytes, in order to avoid accumulation of harmful cytokine-releasing cells. This article reviews both the molecular mechanisms working in AICD and the role played by such mechanisms in preventing a number of skin diseases. Specifically, because AICD removes activated and autoreactive T cells through a CD95-/CD95-L-mediated suicide, skin diseases were scrutinized in which such valuable machinery could be lacking. Indeed, at least some inflammatory skin diseases, including psoriasis and atopic dermatitis, can be sustained by an increased survival of activated T lymphocytes associated with deficient CD95-/CD95-L-mediated AICD of such strong pro-inflammatory cells. In addition, autoreactive skin diseases, including, e.g. alopecia areata, lichen planus and other lichenoid tissue reactions, can be related to autoreactive T lymphocytes which could be unable to undergo CD95-/CD95-L-mediated AICD. Finally, a lack of AICD may be executive even in favoring cutaneous T cell lymphoma. Thus, because inflammatory, autoreactive and neoplastic skin diseases can be associated with a deficient CD95-/CD95-L-mediated suicide of activated T cells, AICD is likely to represent a fundamental program to preserve the homeostasis of the skin. Therapeutic approaches able to restore the AICD machinery promise to successfully treat such relevant skin diseases.

Oral lichenoid drug eruptions

OBJECTIVES

To identify, from amongst drugs reported as causing lichenoid drug eruptions, those affecting the oral mucous membranes and to review the clinical, histological and immunological features of such oral lichenoid drug eruptions in comparison to oral lichen planus, amalgam contact lesions and lichen planus-like eruption in graft-versus-host disease (GVHD).

DATA SOURCES

Ovid Medline data searches on CD-Rom were carried out for the years 1966-1996 to identify reports of oral lichenoid drug eruptions and their clinical, histological and immunological features. Articles retrieved were examined for further appropriate references in the period 1940-1996.

DATA EXTRACTION AND SYNTHESIS

Each paper was critically examined for evidence of a clinically verifiable lichenoid drug-eruption affecting the oral mucous membranes and the effects of subsequent drug withdrawal. Available clinical, histological and immunological features were recorded. The papers examined were too diverse in nature to permit a structured criticism. The extracted data have been tabulated where appropriate.

CONCLUSIONS

The reports of oral lichenoid drug eruptions are considerably fewer than those of cutaneous eruptions and fewer drugs have been reported as causing oral rather than cutaneous lichenoid eruptions. Histology and immunology cannot be used reliably to differentiate lichenoid drug eruptions from idiopathic lichen planus, amalgam contact lesions and lichen planus-like eruption in GVHD. Lichenoid drug eruptions may also show some histological characteristics of oral discoid lupus erythematosus. An accepted protocol agreed by a number of international centres would permit the gathering of substantial information on LDE and could lead to a greater understanding of the mechanisms involved.

Lichenoid changes in mycosis fungoides

BACKGROUND

The lichenoid tissue reaction (LTR) is present in a variety of dermatoses. Numerous histologic variants of mycosis fungoides (MF) have also been described.

OBJECTIVE

Our purpose was to analyze a subset of patients with mycosis fungoides (MF) who had a prominent LTR at the time of presentation.

METHODS

Clinical and pathologic features were evaluated in 12 patients with MF and an LTR.

RESULTS

The primary complaint was intense pruritus (in 10 of 12 patients). An accelerated course was noted in one half of the patients including four with Sézary syndrome. Four patients died: two with Sézary syndrome and two with extensive patch/plaque MF. Lymphocyte atypia, prominent basal cell layer epidermotropism, and the presence of plasma cells and eosinophils may help to distinguish lichenoid MF from lichen planus.

CONCLUSION

A subset of patients with MF present with lichenoid changes. The pathologic features may have a striking resemblance to lichen planus. Our findings suggest that lichenoid changes in MF tend to be associated with intense pruritus and may connote a poor prognosis.

Three cases of linear lichen planus caused by dental metal compounds

Three cases of linear lichen planus on the lower extremities unaccompanied by mucous lesions are described. Dental metal compounds were thought to be the precipitating factor in all cases. Skin lesions did not respond to topical steroid ointment or antihistamines. Two cases showed a positive patch test reaction to gold (HAuCl4) and a positive lymphocyte stimulation test to gold compound (Gold sodium thiomalate). One case showed a positive patch test reaction to mercury (HgCl2), but a negative lymphocyte stimulation test. Suspected metal compounds were demonstrated in their dental materials. Removal of gold materials in one case gradually improved the lesions within 6 months with a transient erythematous swelling of the face shortly after removal of the metal. Both of these cases responded to oral disodium chromoglycate therapy. These results suggest that metal compound specific T cells might be responsible for the development of linear lichen planus.

The histopathology of oral mucosal lesions associated with amalgam or porcelain-fused-to-metal restorations

OBJECTIVES

To analyse the interface stomatitis patterns of oral lichenoid lesions in contact with amalgam and to compare these with the histologic changes in oral lesions clinically associated with porcelain-fused-to-metal (PFM) restorations. To relate these features to the presence of tissue-bound mercury (Hg).

DESIGN

A retrospective analysis of tissue biopsies, with clinical data collected via a complementary questionnaire.

SUBJECTS AND METHODS

479 biopsies diagnosed in 1987 as 'lichenoid reactions'. From these, we retrieved all with amalgam contact and without candida or medication. From 1990-91, all mucosal lesions stated to be associated with PFM restorations were then retrieved for comparative analysis. The biopsies were examined with routine histologic and autometallographic methods.

RESULTS

77 amalgam-associated lesions were found and could be subdivided into five pre-defined interface stomatitis types. We found 22 lesions associated with PFM and 20 showed histopathologic features similar to those associated with amalgam. Hg accumulations were detected in the majority of amalgam-associated but only in part of the PFM-associated lesions.

CONCLUSIONS

Amalgam-associated lichenoid lesions present a wide spectrum of histopathologic patterns, corresponding to similar patterns in dermatopathology but with no evidence of association with specific disease. PFM-associated lesions tend to display similar lichenoid features, suggestive of common pathogenetic mechanisms. Hg accumulations may play a role to maintain the chronicity of such lichenoid lesions.

Generalized lichen nitidus appearing subsequent to lichen planus

A 27-year-old man was seen with multiple, small, shiny papules on his shoulders, upper arms, and trunk, and hyperpigmented violaceous plaques on his feet. The former was diagnosed as generalized lichen nitidus and the latter, as lichen planus. It is not likely that the coexistence of the two diseases in this patient is a fortuitous one, since generalized lichen nitidus is a very rare condition. The association of lichen nitidus and lichen planus suggests that lichen nitidus is closely related to lichen planus and that the two diseases may be different manifestations of essentially the same pathogenetic process.

Contact stomatitis to mercury associated with spontaneous mononuclear cell infiltrates in brown Norway (BN) rats with HgCl2-induced autoimmunity

Light microscopy and immunocytochemistry have been used to study the tissue reaction to non-irritant concentrations of mercury painted onto the oral mucosa of genetically mercury-sensitive BN rats. Low-dose skin injections of HgCl2 in BN rats result in an autoimmune syndrome, including also a spontaneous migration of T lymphocytes into the oral mucosa. Our results show that such infiltrates confer an increased degree of reactivity (contact stomatitis) to HgCl2 painted onto the BN (Hg) rat oral mucosa. In contrast, results were negative in the LEW rat strain, which is also resistant to development of autoimmunity to skin-injected mercury. The possible involvement of mucosal mercury-loaded macrophages is discussed. The results are also discussed with respect to possible etiologic and pathogenetic mechanisms involved in the development of dental material (amalgam)-associated lichenoid lesions of human oral mucosa.

Hypertrophic lichen planus in three HIV-positive patients: a histologic and immunological study

It is well known that several dermatoses, such as psoriasis vulgaris and seborrheic dermatitis, present with more extensive and severe disease in patients infected with the human immunodeficiency virus (HIV-1). Except for one report, however, lichen planus (LP) has not been described in patients with HIV infection. In this report we describe the clinical and morphological features of 3 HIV-positive patients who presented with extensive hypertrophic LP. To determine if alteration in the immune status in HIV-positive hosts is reflected in the nature of the infiltrate in LP, we determined the proportion of T-helper and T-suppressor cells in the infiltrate in 1 case. The majority of the infiltrating lymphocytes in the dermis were of the T-helper phenotype. Epidermal lymphocytes, however, were predominantly of the T-suppressor phenotype. We conclude that LP in HIV-positive hosts may present with more extensive disease than in immunocompetent hosts. Based on our immunohistochemical studies, we conclude that, similar to immunocompetent hosts, T-helper cells are the predominant cells in the dermal infiltrate of LP in HIV-positive patients. However, in contrast with reports in the literature on LP in immunocompetent hosts, we found that, in the case studied, the epidermal lymphocytes were predominantly of the T-suppressor phenotype.

Unilateral linear lichenoid eruption after bone marrow transplantation: an unmasking of tolerance to an abnormal keratinocyte clone?

Chronic cutaneous graft-versus-host disease may appear clinically as a lichenoid eruption. We describe a 26-year-old man who developed a unilateral linear lichenoid eruption 7 months after allogeneic bone marrow transplantation. We believe this represents an unusual form of localized, chronic graft-versus-host disease. The possible relationship to viral infection or cellular mosaicism and the clinical, histologic, and immunologic similarities to idiopathic lichen planus are discussed.

Immunohistochemical examination of lichen nitidus suggests that it is not a localized papular variant of lichen planus

BACKGROUND

Lichen nitidus is believed, by some, to be a variant of lichen planus, and by others to be a distinct entity.

OBJECTIVE

We examined five cases of lichen nitidus with immunohistochemical reagents designed to characterize the dermal inflammatory infiltrate in an attempt to resolve the uncertainty.

METHODS

We stained formalin-fixed, paraffin-embedded tissue sections with the following antibodies: L26, A6, KP1, BerH2, OPD4, and HECA-452.

RESULTS

The inflammatory infiltrate was 90% A6+, with few L26+ cells. In contrast to lichen planus, KP1+ macrophages were seen and fewer of the lymphocytes demonstrated HECA-452. Fifty percent to 80% of lymphocytes were OPD4 positive, similar to that usually seen in lichen planus. Rare Ki-1+ cells were seen in one case.

CONCLUSION

We believe that the pattern of a mixed cellular infiltrate characterized by macrophages and a helper T cell response with few HECA-452+ cells is somewhat different from the pattern seen in lichen planus, wherein almost all of the cells are CD4+/HECA-452+ lymphocytes. This suggests a different immunologic pathogenesis.

Common histopathological processes of phenytoin drug eruption

Fifteen patients (6 males and 9 females) with phenytoin drug eruptions which ultimately resulted in various skin manifestations were analyzed histopathologically. The following types of skin manifestations were noted; 2 cases of toxic epidermal necrolysis, 2 cases of mucocutaneous ocular syndrome, 6 cases of erythema exudative multiform, 3 cases of lichenoid, and 2 cases of the maculopapular type. All of the biopsied specimens from these different skin manifestations exhibited some of the more common histopathological findings: 1) adhesion of the infiltrated cells to the basal layer of the epidermis, 2) cell infiltration into the epidermis, 3) vacuolation of the basal cells, 4) dyskeratotic cells in the epidermis, and epidermal necrosis. Immunohistopathological examinations in 5 typical cases with different skin manifestations revealed that epidermal cells and infiltrating cells were HLA-DR antigen positive. The infiltrating cells in the dermis consisted of almost equal numbers of CD4+ and CD8+ cells; CD8+ cells were predominant in the cells infiltrating into the epidermis. These findings suggest a possibility that the various clinical features in phenytoin drug eruptions may share some common mechanisms.

Prurigo pigmentosa: role of ICAM-1 in the localization of the eruption

Immunohistochemical studies were carried out on the skin lesions of two cases of prurigo pigmentosa. There was a predominance of CD4+ cells in the dermal infiltrate, whereas those lymphocytes in the epidermis were mainly CD8+ cells. The majority of dermal and epidermotropic lymphocytes showed an intense expression of lymphocyte function-associated antigen 1 (LFA-1). The number of CD1+ cells was increased in the epidermis. There was intense expression of ICAM-1 by keratinocytes in the erythematous papules. Focal expression of ICAM-1 was still observed in the residual pigmented areas and could explain the recurrence of the lesions at these sites.

Lichen planus

Lichen planus, a papulosquamous disease, in its classical presentation is characterized by pruritic violaceous papules most commonly on the extremities of middle-aged adults. It may or may not be accompanied by oral and genital mucous membrane involvement. Its course is generally self-limited for a period of several months to years, but it may last indefinitely. There are many clinical variants described, ranging from lichenoid drug eruptions to association with other diseases such as diabetes mellitus, autoimmune disease, and the graft-versus-host reaction. The relationship of these, if any, to classical lichen planus is questionable. Multiple therapeutic options exist including corticosteroids, retinoids, griseofulvin, PUVA, and cyclosporine.

Acral pseudolymphomatous angiokeratoma of children (APACHE): a case report and immunohistological study

Acral pseudolymphomatous angiokeratoma of children (APACHE) is a new clinical entity that is characterized by angiomatous papules on the extremities. We report a case APACHE in a 14-year-old Japanese girl with asymptomatic red and violaceous papules and nodules on the ring finger of the left hand. Histological and immunohistochemical studies of our patient indicated that APACHE is a pseudolymphoma rather than an angiokeratoma.

Lymphocyte recognition of psoriatic endothelium: evidence for a tissue-specific receptor/ligand interaction

Epidermotropic lymphocytes are an essential cellular component of the skin-associated lymphoid tissues (SALT). Dermal lymphocytic infiltrates are also characteristics of inflammatory dermatoses, such as psoriasis, and may be involved in the pathogenesis of the disease, although the mechanisms by which lymphocytes are recruited into these sites are not known. We have used an in vitro lymphocyte adherence assay to test the hypothesis that specialized endothelial cells are present in inflamed skin, and are capable of supporting lymphocyte adherence and promoting lymphocyte emigration. In this assay, we assessed the binding of lymphocytes overlaid onto frozen sections of normal and psoriatic skin. Peripheral blood mononuclear cells (PBMC) from patients, from healthy volunteers, and from rat thoracic duct bound specifically to dermal endothelia in psoriatic plaques, in steroid-resistant areas of plaques, but not in uninvolved skin or skin from healthy individuals. Analysis of the binding properties of lymphocyte subsets revealed preferential adherence by CD4+ T cells as compared with CD8+ T cells and to B cells. Interestingly, PBMC from patients undergoing ultraviolet light therapy failed to adhere to autologous skin or to lesion-containing skin sections from untreated patients. Additional studies indicate that the lymphocyte-endothelial interaction is an energy- and calcium-dependent process and involves surface glycoprotein and carbohydrate moieties, requirements similar to those found in specific lymphocyte interactions with high endothelial venules in lymph nodes during the homing process. Pretreatment of lymphocytes with antibodies directed against homing receptors mediating migration into lymph nodes and into gut-associated lymphoid tissues, however, did not interfere with lymphocyte adherence to psoriatic endothelium. In contrast, anti-lymphocyte function associated antigen (LFA)-1 antibody partially inhibited lymphocyte binding to chronic plaques. We conclude that a tissue-specific receptor/ligand interaction independent of LFA-1 directs lymphocyte emigration from the dermal microvasculature into the psoriatic dermis and that LFA-1 plays only an accessory role in the adhesion process.