Relationship of factor XIIIa-positive dermal dendrocytes to Kaposi's sarcoma

The histogenesis of Kaposi's sarcoma (KS) has been the subject of controversy, much of which has centered around whether the spindle cells of KS are derived from vascular endothelium or from lymphatics. Recently, some investigators have speculated that the spindle cells of KS are derived from dermal dendrocytes, a population of mononuclear dendritic cells normally present in the papillary and upper reticular dermis. These cells have been shown to proliferate in response to a variety of stimuli and have been reported to express the plasma proenzyme factor XIIIa. We examined immunohistochemically sections fixed in formaldehyde solution and embedded in paraffin from 20 tumor-stage, 15 patch-stage, and 15 plaque-stage lesions of KS with antibodies directed against factor XIIIa, factor VIII-related antigen, Ulex europaeus lectin, and LN3 (anti-HLA-DR) to investigate the relationship of dermal dendrocytes to KS in general and to try to clarify the histogenesis of this tumor. Our results revealed that the dermis of patch- and plaque-stage KS lesions contains an increased number of factor XIIIa-positive dermal dendrocytes compared with normal dermis and that some of these cells are spindle shaped. Many of the spindle cells in patch- and plaque-stage lesions of KS, however, are negative for factor XIIIa. The cells lining the slitlike spaces and some spindle-shaped cells in close proximity to the vascular spaces stain for factor VIII-related antigen and for Ulex europaeus lectin. LN3 labeled many cells resembling macrophages within the lesions and in papillary dermis. Less than 25% of the dendritic cells within the lesions and in the adjacent dermis expressed both factor XIIIa and LN3. Tumor-stage lesions showed focal but unequivocal staining of the spindle cells for factor VIII-related antigen and Ulex europaeus lectin. Tumor spindle cells were negative for factor XIIIa. Factor XIIIa-positive dendrocytes were plentiful in the uninvolved dermis and were aggregated around the periphery of the tumor nodules. The expression of factor VIII-related antigen and Ulex europaeus lectin by the spindle cells of nodular KS, and their lack of expression of factor XIIIa, suggests that the spindle-shaped tumor cells in all stages of KS are derived from endothelial cells and not from dermal dendrocytes. Dermal dendrocytes appear to undergo hyperplasia in response to KS of all stages. In patch- and early plaque-stage KS lesions, dermal dendrocytes are near factor VIII-related antigen-positive spindle cells and tumor vessels. The mechanism for reactive dermal dendrocyte hyperplasia in KS remains obscure.