Involvement of nitric oxide in IFN-gamma-mediated reduction of microvessel smooth muscle cell proliferation

Previous studies in our laboratory demonstrated that murine cerebral microvessel smooth muscle cells (SMC) activate syngeneic CD4+ T-cells in vitro. These T-cells, or their culture supernatants, in turn, strongly inhibit proliferation of the SMC. The present study focuses on IFN-gamma as a mediator of inhibition of SMC proliferation, and addresses the molecular mechanism of this inhibition. IFN-gamma profoundly reduced the proliferation of murine brain microvessel smooth muscle cells in vitro. Three lines of evidence indicate that nitric oxide contributed to this effect: (1) IFN-gamma-mediated inhibition of proliferation correlated with the quantity of nitrite, a stable breakdown product of nitric oxide, in culture supernatants; (2) the addition of N(g)- monomethyl-l-arginine, and inhibitor of nitric oxide synthesis, restored proliferation to control or near control levels; and (3) the addition of hemoglobin, which has a high affinity for, and thus sequesters nitric oxide, also resulted in significant restoration of the proliferative response. However, the nitric oxide donating chemical sodium nitro-prusside, at concentrations up to 100 microM, had no direct cytostatic effect. These results suggest that nitric oxide is a necessary but insufficient component in IFN-gamma-mediated inhibition of microvessel smooth muscle cell proliferation. TNF-alpha also stimulated nitric oxide production by the smooth muscle cells, but was not as potent as IFN-gamma at inhibiting proliferation. Knowledge of the physiological effects of lymphokines on cells of the brain microvasculature will contribute towards a better understanding of inflammatory processes in diseases such as multiple sclerosis and infectious encephalitis.