Virulent and attenuated mutant Venezuelan equine encephalitis virus show marked differences in replication in infection in murine macrophages.

One of the first target cells at the site of inoculation with an alphavirus may be monocytes or macrophages. The replication kinetics of virulent and attenuated molecularly cloned Venezuelan equine encephalitis virus (VEE) in murine macrophages were therefore compared. Infection of both quiescent and activated mouse primary peritoneal macrophages with a molecularly cloned, virulent VEE termed V3000 resulted in peak virus titres of 10(4) plaque forming units (PFU)/ml supernatant by 24 h post-infection (pi), followed by rapidly decreasing virus titres. In contrast, a molecularly cloned attenuated VEE mutant, V3032, that differs from V3000 by a single amino acid at E2 glycoprotein position 209 (glu-->lys) replicated more slowly and to higher titres (10(8) PFU/ml supernatant) that peaked at 72 h pi. Replication of V3032, but not V3000, was sharply restricted by prior activation of macrophages with lipopolysaccharide or interferon-gamma. These results indicate that virulent V3000 and attenuated V3032 differ in their growth kinetics in both quiescent and activated macrophages. Thus, macrophages, and their specific activation state, may play a major role in virulent and attenuated VEE replication and pathogenesis.