Characterization of cellular dermal infiltrates in human cutaneous mastocytosis

Biopsies of lesional and nonlesional skin from 14 patients with localized cutaneous or associated systemic mastocytosis were examined by ultrastructural and immunohistochemical techniques. Mast cells within lesions of the dermis were highly variable between patients with regard to cell number and extent of degranulation, although lesional sites consistently contained more mast cells than did nonlesional sites. Two mast cell patterns were identified based upon granule morphology. In biopsies from 8 patients, the majority of granules contained electron-dense amorphous zones; crystalline lattices; and indistinct, incomplete solid scrolls forming parallel lamellae. In biopsies from 6 patients, in addition to these granules, there were also granules composed of electron-dense amorphous zones, reticulated matrices, and/or distinct scrolls with lucent cores interrupted by dense spheres. The granule morphology for the first group (N = 8) was identical with that seen in the preponderant type of skin mast cell of 6 normal control subjects, whereas the granule morphology of the second group (N = 6) displayed an abnormal ultrastructural phenotype for skin that included granule types normally found not only in skin but also in intestinal lamina propria and lung. For individual patients, the patterns of granule ultrastructure were consistent between clinically nonlesional and lesional skin. A minority of cells in both patient groups appeared primitive ultrastructurally, exhibiting rudimentary, Golgi-associated progranules; monocyte-like morphologic characteristics; and mitotic activity. Moreover, when mast cells in lesional skin were screened for a limited panel of surface antigens, they displayed common patterns of reactivity (M718+, HLA-DR/DQ+, CD4+), and in a selected case, immunoelectron microscopy confirmed the presence of these antigens on mast cell plasma membranes. Dermal mast cells from normal donors (N = 6) lack these epitopes. These observations suggest that infiltrates in cutaneous mastocytosis may exhibit phenotypic characteristics not only of cutaneous mast cells, but in some patients also of mucosal mast cells. In either circumstance, the mast cells may display antigenic determinants common to monocyte/macrophages. Concordance of granule phenotype between lesional and clinically uninvolved skin of individual patients furthers the notion that even localized mastocytosis reflects covertly defective systemic mast cell homeostasis.