Vaccinia virus-induced inhibition of nitric oxide production

Background. The role of nitric oxide (NO) in the host defense against viruses has not been well defined. Several studies have implicated NO as responsible for the destruction of a variety of viruses. However, others have reported that certain viruses can impair the ability of macrophages to produce NO. This study was initiated to determine the ability of macrophages to produce NO in response to vaccinia virus infection. Methods. RAW 264.7 murine macrophages in minimum essential medium were exposed to virus-containing supernatants for 1 h before stimulation with Escherichia coli lipopolysaccharide (LPS, 0.001 and 1.0 mug/ml). After further 24-h incubations, nitrite concentration, cell viability, and inducible nitric oxide synthase (iNOS) were quantitated. Results. The viral preparation alone did not stimulate nitric oxide synthesis (measured as nitrite) by macrophages. However, macrophages exposed to 0.001 and 1.0 mug/ml LPS produced 7.7 +/- 0.6 and 16.6 +/- 0.8 nmole/1.1 x 10(6) cells/24-h nitrite, respectively. Production of nitrite caused cell death. Macrophages incubated with vaccinia virus prior to exposure to LPS resulted in a dose-dependent decrease in nitrite production. An 80% inhibition of nitrite was noted when macrophages were exposed to vaccinia virus (m.o.i. 10(-4)) plus LPS (1.0 mug/ml) (P < 0.05). Further study showed that this inhibition was not associated with changes in cell viability or substrate availability, but was associated with a marked reduction in iNOS protein. When the virus was inactivated with UV-irradiation, the same incubation caused a 46% inhibition of nitrite production (P < 0.05 vs active virus). However, this effect occurred without altering the quantity of iNOS protein. Conclusion. These results indicate that active vaccinia virus inhibits the ability of stimulated macro-phages to produce NO by hindering iNOS protein expression. Because live viral particles were not entirely required for this inhibition, it is possible that by products of viral infection, such as soluble viral proteins, may also be responsible for this effect. (C) 2003 Elsevier Inc. All rights reserved.