We investigated the effects of monocytes on endothelial cell (EC) ectoenzyme activity. Coculture of human aortic ECs with human monocytes (2 x 10(5) monocytes per 2-cm2 well) led to a decrease in EC angiotensin-converting enzyme (ACE) activity (64.5 +/- 3.5% of control) but not aminopeptidase N, aminopeptidase P, and 5'-nucleotidase activities. Similar results were obtained using human umbilical vein EC-human monocyte and porcine aortic EC-porcine monocyte cocultures. The decrease in ACE activity was monocyte concentration and coculture time dependent, reaching a maximum of 65% decrease in activity at 120 hours. Monocyte-mediated reduction in ACE activity did not require cell to cell contact, since exposure of ECs to conditioned medium from cocultures (CCCM) or from monocyte cultures (MCM) produced a decrease in ACE activity similar to that observed in EC-monocyte cocultures. Exogenously added tumor necrosis factor (TNF)-alpha and interleukin (IL)-1 alpha, two known secretory products of monocytes, simulated the effects of monocytes on ACE activity. Western blot analysis revealed a decrease in the amount of ACE protein in TNF-alpha-treated and CCCM-treated ECs compared with control ECs. Both TNF-alpha and IL-1 alpha were present in CCCM and MCM but not EC-conditioned medium. Incubation of the cocultures with a mixture of neutralizing antibodies against TNF-alpha and IL-1 totally abolished the monocyte-induced decrease in ACE activity. In conclusion, monocytes decrease ACE activity in cultured ECs through the release of cytokines such as TNF-alpha and IL-1.