Immunohistologic identification of antigen-presenting cells in cutaneous sarcoidosis

Adaptive immune responses in primary cutaneous sarcoidosis

Sarcoidosis is a multisystemic inflammatory disorder with cutaneous lesions present in about one-quarter of the patients. Cutaneous lesions have been classified as specific and nonspecific, depending on the presence of nonnecrotizing epithelial cell granulomas on histologic studies. The development and progression of specific cutaneous sarcoidosis involves a complex interaction between cells of the adaptive immune systems, notably T-lymphocytes and dendritic cells. In this paper, we will discuss the role of T-cells and skin dendritic cells in the development of primary cutaneous sarcoidosis and comment on the potential antigenic stimuli that may account for the development of the immunological response. We will further explore the contributions of selected cytokines to the immunopathological process. The knowledge of the adaptive immunological mechanisms operative in cutaneous sarcoidosis may subsequently be useful for identifying prevention and treatment strategies of systemic sarcoidosis.

Cutaneous sarcoidosis: updates in the pathogenesis

Sarcoidosis is a multiorgan granulomatous disease in which the skin is one of the frequently involved target organs. Cutaneous involvement occurs in a third of patients with sarcoidosis and has protean manifestations. More than a century has passed since the initial description of sarcoidosis, but its cause continues to be an enigma. Recent studies have introduced several new insights into the pathogenesis of this disease. The aim of this literature review was to provide a comprehensive overview on the current updates in the pathogenesis of sarcoidosis. This review has revealed that several genetic polymorphisms are associated with an increased risk of developing sarcoidosis, suggesting that genetic susceptibility to sarcoidosis is probably polygenic. Environmental factors may also modify the susceptibility to sarcoidosis. Evidence favouring an infectious aetiology has been accumulating, but the results of studies are conflicting. The current concept is that the pathogenesis of sarcoidosis involves a T-helper-1-mediated immune response to environmental antigens in a genetically susceptible host. The studies carried out on sarcoidosis have largely focused on the pulmonary aspects and have been mainly conducted by respiratory physicians. In contrast, research conducted on the cutaneous aspects of sarcoidosis is comparatively limited. Although tremendous advances have been made, there is a significant gap between the vast knowledge accumulated on sarcoidosis in recent years and the understanding of this disease.

Dendritic cells in the pathogenesis of sarcoidosis

Sarcoidosis is a noncaseating granulomatous disease, likely of autoimmune etiology, that causes inflammation and tissue damage in multiple organs, most commonly the lung, but also skin, and lymph nodes. Reduced dendritic cell (DC) function in sarcoidosis peripheral blood compared with peripheral blood from control subjects suggests that blunted end organ cellular immunity may contribute to sarcoidosis pathogenesis. Successful treatment of sarcoidosis with tumor necrosis factor (TNF) inhibitors, which modulate DC maturation and migration, has also been reported. Together, these observations suggest that DCs may be important mediators of sarcoidosis immunology. This review focuses on the phenotype and function of DCs in the lung, skin, blood, and lymph node of patients with sarcoidosis. We conclude that DCs in end organs are phenotypically and functionally immature (anergic), while DCs in the lymph node are mature and polarize pathogenic Th1 T cells. The success of TNF inhibitors is thus likely secondary to inhibition of DC-mediated Th1 polarization in the lymph node.

Epidermal changes in cutaneous lesions of sarcoidosis

The essential histologic finding of cutaneous sarcoidosis is a noncaseating epithelioid cell granuloma in the dermis or, infrequently, in the subcutaneous tissues. However, there have been cases of cutaneous sarcoidosis with clinical appearances altered by epidermal changes such as ulcers, the formation of psoriasiform or verrucous plaques, and by hypopigmentation. In this study, histologic examinations of the epidermis were performed in cutaneous lesions of 62 cases of sarcoidosis. Seventy-nine percent (49/62) showed epidermal changes including hyperkeratosis (8/49), parakeratosis (10/49), acanthosis (6/49), and epidermal atrophy (35/49). Lymphoid cells extended into the epidermis in 50 of 62 cases. The infiltration patterns were in the forms of a spongiotic reaction (21/50), lichenoid tissue reaction (9/50), and simple exocytosis without epidermal vesiculation (20/50). Immunohistochemical studies showed that the lymphoid cells in the epidermis expressed CD3, 8, 45RO, and 11a. The epidermal changes overlying the granulomatous lesions contribute to the variety of clinical manifestations and are in part associated with the pathogenesis of cutaneous sarcoidosis.

Plasmacytoid monocytes (so-called plasmacytoid T cells) in granulomatous lymphadenitis

Immunohistochemical evidence that the plasmacytoid T cell is closely related to the blood monocyte has been reported, and the term plasmacytoid monocyte has been proposed to describe this cell. The present study was undertaken to analyze the presence and distribution of plasmacytoid monocytes in human reactive lymph nodes showing epithelioid cell reactions. Numerous plasmacytoid monocytes (detected by a panel of monoclonal antibodies) were found in the majority of the lymph nodes studies, usually in close topographical association with epithelioid cells and multinucleated giant cells. The present findings suggest that plasmacytoid monocytes may give rise to epithelioid cells. This is further supported by the ultrastructural similarities between plasmacytoid monocytes and plasmacytoid epithelioid cells, a cell type that has been identified previously in granulomas and considered a direct precursor of the classical epithelioid cell.

Connective tissue responses in blacks in relation to disease: further observations

Additional information is presented in support of the hypothesis (Polednak: Am. J. Phys. Anthropol. 41:49-58, 1974) that in some black populations certain connective-tissue responses, which are involved in protection against infection and repair after injury, also may predispose to specific chronic diseases. These diseases include some autoimmune disorders (i.e., systemic lupus erythematosus, sarcoidosis, and scleroderma) and various benign and malignant tumors involving connective-tissue cells. Complex interactions between genetic factors (HLA and non-HLA loci) and environmental agents may be involved both in the etiology of these autoimmune diseases and in population differences in the incidence of these diseases. A framework is reviewed whereby cellular responses to infectious agents, involving chiefly immunoglobulin-producing cells and macrophages, may have consequences in terms of pathogenesis of specific chronic diseases more common in some black populations. The possible role of natural selection in maintaining some of these diseases is also considered, along with the need for involvement of biomedical anthropologists in their investigation.

Dendritic cells in cutaneous, lymph node and pulmonary lesions of sarcoidosis

To determine whether dendritic cells (DC) are a consistent feature of lesions of sarcoidosis, we have used monoclonal antibodies to identify the HLA-DR-expressing populations of cells in cryostat sections of 15 lymph node, pulmonary and cutaneous lesions. The commonest HLA-DR positive cells in granulomas were epithelioid and giant cells, although lymphocytes within granulomas and tissue macrophages around them were also positive. Dendritic cells with Langerhans cells (NA1/34+ = OKT6+) and interdigitating cell (RFD1+) phenotype were consistently associated with granulomas only in skin lesions. In lymph nodes, interdigitating cells (NA1/34-/RFD1+/HLA-DR++) were confined to paracortical zones as in normal nodes, although a small area of NA1/34+/RFD1+ cells was found in one of three nodes. In lung lesions NA1/34+/RFD1+ dendritic cells were uncommon or absent, except in one chronic case. We conclude that while sometimes present in extracutaneous sites, DC are not an essential feature of sarcoid lesions, and that cells of the classical macrophage group are the most significant HLA-DR-expressing population. We suggest that the presence of DC in lesions of sarcoidosis may indicate an immunological response distinct from that causing granulomas to form. The variability of their involvement may have immunoregulatory significance.