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Using an in vitro coculture system to mimic the interactions between the cells of the vessel wall, we have previously shown that pericytes and smooth muscle cells (SMC) inhibit the growth of capillary endothelial cells (EC). We have undertaken studies to determine the mechanism of this inhibition. Using conditioned media and affinity-purified antibodies to transforming growth factor beta (TGF-beta), we now demonstrate that activated TGF-beta produced in these cocultures mediates EC growth inhibition. No inhibitory activity was detected when media conditioned by individual cultures of EC, SMC, or pericytes were examined for their effect on EC growth. In contrast, media conditioned by cocultures of EC-SMC and EC-pericytes inhibited EC proliferation to the same degree as the coculture itself. Immunoadsorption of coculture-derived conditioned media with antibodies to TGF-beta eliminated the inhibitory activity …