Epithelioid cell histiocytoma--histogenetic and kinetics analysis of dermal microvascular unit dendritic cell subpopulations.

Abstract

Epithelioid cell histiocytoma (ECH), also known as epithelioid fibrous histiocytoma, is a peculiar dermal tumor, which can mimic melanocytic, vascular, epithelial, or other histiocytic lesions. Thought to arise from dermal dendrocytes, most ECH contain approximately 50% FXIIIa+ histiocytic dendrocytes, but not all lesional cells express FXIIIa. A putative fibroblastic component has not been characterized. We analyzed the differentiation and cell kinetics of dermal microvascular unit cells in 12 previously reported ECH using antibodies to FXIIIa, CD68 (KP1), CD34, CD117, CD31, smooth muscle actin, collagen type 1 aminopropeptide, and MIB-1, using single and double immunostains. In ECH, many variably sized CD34/CD31+ tumor vessels with actin+ myopericytes were surrounded by epithelioid-to-dendritic cells of three types. About 5-80% were dendritic histiocytes that expressed FXIIIa but not CD31 or KP1. Fibroblasts, in some cases showing mild nuclear pleomorphism, were usually collagen type 1+, but CD34 and actin- in 11/12 cases. One 'early' ECH had 40% CD34+ epithelioid cells, admixed with 50% FXIIIa+ histiocytes. Most ECH had about 2-20% KP1+, CD117+ mast cells. Mast cell numbers increased with FXIIIa+ histiocyte numbers and the intensity of FXIIIa expression. MIB-1/FXIIIa double-labeling showed only rare cycling histiocytes, with numerous cycling fibroblasts and endothelial cells. Our findings support the impression that ECH is a vascular fibrous histiocytoma. The constituent cells appear to arise from the activation of resident microvascular CD34+ dermal fibroblasts and the accumulation of FXIIIa+ dendritic stromal assembly histiocytes. The CD34+ cells appear to differentiate toward collagenous fibrocytes in association with histiocytes and mast cells in forming collagenous stroma and vessels. ECH is a tumor composed of all requisite cell types consistent with the origin from the dermal microvascular unit.