Graft coronary arteriosclerosis (GCA) frequently limits the long-term success of cardiac transplantation. The pathogenic mechanisms of and stimuli that provoke GCA remain uncertain. Whatever the initiating factors, deranged control of smooth muscle cells (SMC) proliferation likely contributes to the intimal hyperplasia that produces obstructive lesions. To identify mediators that may contribute to ongoing modulation of SMC functions during acute rejection and to explore the mechanisms of the pathogenesis of graft coronary arteriosclerosis, we studied the kinetics of proliferation and the expression of tumor necrosis factor-alpha (TNF-alpha), a proinflammatory and SMC growth-promoting cytokine, in coronary arterial SMCs in rabbit hearts transplanted heterotopically without immunosuppression. Hearts were harvested at 2 (n = 5), 5 (n = 5), and 8.2 +/- 0.4 (mean +/- SD, n = 5) days after transplantation, just before graft failure as judged clinically. SMC proliferation was assessed by continuous bromodeoxyuridine labeling (BrdU 10 mg/kg/d. s.q.). Whole heart cross sections were stained immunohistochemically with monoclonal antibodies that recognize TNF-alpha, BrdU, and SMCs (muscle alpha-actin). Major epicardial coronary arteries (five to nine profiles in each animal) were evaluated. Histological rejection grades by the International Society for Heart and Lung Transplantation scale at 2, 5, and 10 days were 1.6 +/- 0.9, 2.8 +/- 1.1, and 4.0 +/- 0.0, respectively. Medial SMCs in normal hearts and 2 days after transplant expressed little or no TNF-alpha and displayed negligible BrdU incorporation. At 5 days after transplantation, some medial SMCs stained for TNF-alpha and had a low BrdU labeling index (0.5 +/- 0.8%). At 8.2 days after transplant, almost all medial SMCs expressed TNF-alpha intensely and had a high labeling index (29.8 +/- 8.0%). These results demonstrate that acute rejection activates medial SMCs in coronary arteries to express TNF-alpha and that SMC-derived TNF-alpha may contribute to medial SMC proliferation in coronary arteries during acute rejection. This finding of early medial SMC replication suggests a novel and heretofore unsuspected mechanism of intimal expansion consequent to the allogeneic state. These results furnish additional insight into the possible mechanisms that link acute rejection with graft coronary arteriosclerosis. Furthermore, the close association of TNF-alpha expression with SMC replication provides not only a novel marker of SMC activation but also a potential new therapeutic target for the prevention of graft coronary disease.