Immunoreactivity patterns of CD31 and CD68 in 28 cases of Kaposi's sarcoma - Evidence supporting endothelial differentiation in the spindle cell component

The histogenesis of Kaposi's sarcoma (KS), especially the spindle cell component, has been a subject of debate for many years. Although most researchers have favored vascular endothelium as the cellular origin of KS, lymphatic endothelium, interstitial stem cells, and macrophages have also been proposed. Twenty-eight cases of KS were studied with antibodies to CD31, an antigen purported to be both sensitive and specific for endothelial differentiation, and the monocyte/macrophage marker CD68. Cases chosen for study were of tissues from a variety of anatomic sites, including skin (both patch/plaque and nodular forms), gastrointestinal (GI) tract, lung, and lymph nodes. Nineteen of 28 (68%) patients were infected with the human immunodeficiency virus (HIV); one patient had undergone previous renal transplantation. CD31 stained the endothelial cell lining of small vessels in all cases of KS. In addition, CD31 also stained the spindle cell component in 25 of 28 (89%) cases, typically in a strong and diffuse pattern. Staining was found in 14 of 17 cases of cutaneous KS (eight of 10 cases of patch/plaque stage; six of seven cases of nodular stage), five of five cases of GI KS, three of three cases of nodal KS, and three of three cases of pulmonary KS. Eighteen of 19 (95%) of the HIV-positive patients had CD31-positive tumors in the spindle cell component. Focal, weak CD68 staining was seen in only eight of 28 (29%) cases of KS, including three cases of patch/plaque stage, four cases of nodular cutaneous KS, and one case of nodal KS. Four of the eight patients with CD68-positive tumors were HIV-positive. In conclusion, CD31 is a sensitive marker of KS from a variety of anatomic sites, is independent of tumor stage and HIV status, and provides supporting evidence of endothelial differentiation in the spindle cell component of KS. CD68 demonstrates only weak immunoreactivity in occasional spindle cells in fewer than one third of cases of KS and is not diagnostically helpful.