UL74 of human cytomegalovirus reduces the inhibitory effect of gH-specific and gB-specific antibodies

The human cytomegalovirus (HCMV) glycoproteins gH (UL75) and gL (UL115) can form complexes with gO (UL74) or with proteins of the UL128-UL131A locus. Deletion of gO abolishes cell-free virus transmission and renders cell-associated virus transmission in fibroblasts more sensitive to inhibition by human anti-HCMV serum. To test whether the latter effect is specific for gO, we compared mutants with deletions in UL74, UL99 and the UL128-131A locus regarding their sensitivity to anti-HCMV antibodies. UL74 deletion mutants were more sensitive to a further restriction by polyspecific or gH-specific antibodies than control mutants, showing that gO specifically protects focal growth against inhibitory antibodies. This effect was not confined to gH-specific antibodies, as UL74 deletion mutants were also inhibited by an anti-gB antibody. In conclusion, gO specifically promotes focal spread in the presence of gH and gB antibodies, thus contributing to the ability of HCMV to resist the host’s immune response.