Abstract-Heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF), a member of the EGF family, has a potent mitogenic activity for vascular smooth muscle cells (SMCs). We previously reported that HB-EGF is involved in atherogenesis of human aorta and coronary arteries. ProHB-EGF (the membrane-anchored form of HB-EGF) has also been demonstrated to possess a mitogenic activity, which is Ϸ30-fold increased when coexpressed with CD9 in mouse L cells. Thus, in the process of atherogenesis, CD9 may be involved in the proliferation of SMCs. We immunohistochemically investigated the localization of CD9 and proHB-EGF in the human aorta and coronary arteries. In normal aorta and coronary arteries, CD9 immunostaining was virtually negative, whereas proHB-EGF immunostaining was positive, especially in the arteries of babies. In contrast, in atherosclerotic lesions, some intimal SMCs were strongly positive for CD9 and proHB-EGF immunostaining. The juxtacrine growth activities of human aortic SMCs were inhibited in vitro by adding neutralization antibodies for CD9 or adding the specific inhibitor of HB-EGF. Besides, coexpressed CD9 and proHB-EGF cells markedly incorporated T he proliferation of vascular smooth muscle cells (SMCs) in the intima of arteries is one of the most important events in plaque formation 1 and in coronary restenosis after balloon angioplasty. 2 Heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF), a member of the EGF family, has been identified as a potent mitogen for SMCs that is comparable to platelet-derived growth factor. 3 Localization and function of HB-EGF have been demonstrated in human hepatocellular carcinoma, 4 gastric mucosa, 5 wound fluid, 6 and balloon-injured rat carotid arteries. 7 Moreover, we have reported that HB-EGF is produced by macrophages and SMCs in the human aorta 8 and coronary arteries. 9 The HB-EGF precursor (proHB-EGF) is expressed on the cell membrane as a membrane-anchored form and can be cleaved to yield a mature biologically active form (soluble HB-EGF). 10 HB-EGF released by SMCs showed mitogenic function in an autocrine and a paracrine fashion. 11 Furthermore, proHB-EGF has also been demonstrated to have a mitogenic activity under the condition of cell-to-cell contact 12 by a so-called juxtacrine mechanism. 13 The juxtacrine growth factor activity of proHB-EGF in mouse L cells is dramatically upregulated Ϸ30-fold when coexpressed with CD9, 12 which was first identified as a cell surface antigen on lymphohematopoietic cells. 14,15 Thus, in the process of atherogenesis, CD9 may be involved in the proliferation of SMCs. In the present study, we investigated the immunohistochemical localization of CD9 and proHB-EGF in human coronary arteries and aorta and examined in vitro the effects of CD9 on the proliferation of human aortic SMCs.