Association of virulence of murine cytomegalovirus with macrophage susceptibility and with virion-bound non-neutralizing antibody.

A comparison was made of the virulence in vivo and the infectivity in vitro for macrophages of (i) tissue culture-passed mouse cytomegalovirus (MCMV), (ii) salivary gland virus taken 3 weeks after infection (SGV 3w), and (iii) salivary gland virus taken 1 week after infection (SGV 1w). Salivary gland virus (3w) is known to be coated with non-neutralizing antibody, and is more virulent for newborn suckling mice and less infectious for macrophages than tissue culture-passed virus (TCV). Properties of SGV 1w were similar to those of TCV. Infectivity of SGV 3w for macrophages was significantly enhanced by treatment with trypsin (10 micrograms/ml) and at the same time virulence was lost. When SGV 1w or TCV were treated with trypsin the infectivity for macrophages was unaltered as long as the inoculum was adjusted to contain the same number of p.f.u. as assayed in MEF. Trypsin-treated SGV 3w was neutralized not by rabbit anti-mouse IgG Fc, but by anti-Fab, whereas untreated virus was neutralized by both of these anti-mouse immunoglobulins. These results are discussed in terms of the association of virulence with virion-bound antibody and Fc receptors on macrophages.