Studies of T cell-deficient or immunosuppressed animals undergoing arterial endothelial denudation have yielded conflicting results as to the contribution of the immune system to neointimal vascular smooth muscle cell accumulation and proliferation. We investigated the cell types and cytokine expression associated with intimal hyperplasia occurring 14 days after balloon angioplasty of the carotid artery in Sprague-Dawley rats. Immunohistological studies using monoclonal antibodies showed that the carotid luminal occlusion observed was associated with smooth muscle cell proliferation and neointimal accumulation of large numbers of CD4+, ED1+ mononuclear cells but no T cells. There was also wide-spread staining for the inflammatory cytokine interleukin-1B (IL-1 beta), tumor necrosis factor-alpha (TNF-alpha), and IL-8, as well as dense expression of the potent smooth muscle mitogens platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-beta), and protein S. The relationship of smooth muscle cell proliferation to monocyte/macrophage accumulation and cytokine expression was tested by daily intraperitoneal administration for 14 days of a rat CD4-specific monoclonal antibody, BWH-4 (500 micrograms/day). Morphometric analysis at day 14 showed that the intimal area of animals treated with CD4 monoclonal antibody comprised 7%+/-4% of the arterial wall compared with 50%+/-6% in control animals (n= 6/group, P< 0.001). In addition, immunohistological studies showed that CD4 monoclonal antibody treatment markedly reduced the intimal accumulation of mononuclear and smooth muscle cells and essentially abrogated expression of the cytokines PDGF-BB, TGF-beta, IL-1 beta, TNF-alpha, and IL-8, plus the anticoagulant molecule, protein S. Our results document the extensive expression in vivo of cytokines that in vitro promote vascular smooth muscle cell proliferation, and suggest that CD4+ mononuclear cells or their secreted products play a key role in the pathogenesis of intimal hyperplasia after endothelial injury. Furthermore, these observations may have clinical relevance in the development of novel strategies to prevent arteriosclerosis.