BACKGROUND: Histoid leprosy is a rare form of multibacillary leprosy as the result of secondary or even primary resistance to dapsone. The etiopathogenesis has not been clarified up to now.
METHODS: An immunohistochemical study was carried out for the expression of various markers on epidermal and dermal cell populations using sections of frozen skin specimens from 5 patients with histoid leprosy as compared to specimens from 7 tuberculoid and 7 lepromatous patients.
RESULTS: Dendritic epidermal cells, identified by monoclonal antibodies against CD1, HLA-DR, CD45, and CD36, were found reduced in histoid leprosy as compared to both tuberculoid and lepromatous groups. A gradual reduction of keratinocytic HLA-DR expression from tuberculoid to lepromatous to histoid leprosy was observed. The pattern of CD36, CD4, and CD8 expression of lymphomonocytic cells in the dermis of histoid lesions was similar to that of tuberculoid leprosy, but without the formation of an organized granuloma. CD45+ cells as well as activated lymphocytic cells, expressed by the activation immunophenotype (CD1, HLA-DR, CD25, CD71, EGF-R) were found frequently in all groups.
CONCLUSIONS: The in situ immunohistochemical findings support a modified hypersensitivity reaction of the cellular type that results in an inhibition of the lesional expansion, but not in the destruction of the bacilli within the histoid lesion.
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