Naturally Occurring Polymorphisms in Human Cytomegalovirus gO Exert Epistatic Influences on Cell-Free and Cell-To-Cell Spread, and Antibody Neutralization on gH Epitopes.

Abstract

The human cytomegalovirus (HCMV) glycoproteins H and L (gH/gL) can be bound by either gO, or the UL128-131 proteins to form complexes that facilitate entry and spread and the complexes formed are important targets of neutralizing antibodies. Strains of HCMV vary considerably in the levels of gH/gL/gO and gH/gL/UL128-131 and this can impact infectivity and cell tropism. In this report, we investigated how natural interstrain variation in the amino acid sequence of gO influences the biology of HCMV. Heterologous gO recombinants were constructed in which 6 of the 8 alleles or genotypes (GT) of gO were analyzed in the backgrounds of strain TR and Merlin (ME). The levels of gH/gL complexes were not affected, but there were impacts on entry, spread and neutralization by anti-gH antibodies. AD169 (AD) gO (GT1a) drastically reduced cell-free infectivity of both strains on fibroblasts and epithelial cells. PHgO(GT2a) increased cell-free infectivity of TR in both cell types, but spread in fibroblasts was impaired. In contrast, spread of ME in both cell types was enhanced by Towne (TN) gO (GT4), despite similar cell-free infectivity. TR expressing TNgO(GT4) was resistant to neutralization by anti-gH antibodies AP86 and 14-4b, whereas ADgO(GT1a) conferred resistance to 14-4b, but enhanced neutralization by AP86. Conversely, ME expressing ADgO(GT1a) was more resistant to 14-4b. These results suggest; 1) mechanistically distinct roles for gH/gL/gO in cell-free and cell-to-cell spread, 2) gO isoforms can differentially shield the virus from neutralizing antibodies, and 3) effects of gO polymorphisms are epistatically dependent on other variable loci. IMPORTANCEAdvances in HCMV population genetics have greatly outpaced understanding of the links between genetic diversity and phenotypic variation. Moreover, recombination between genotypes may shuffle variable loci into various combinations with unknown outcomes. UL74(gO) is an important determinant of HCMV infectivity, and one of the most diverse loci in the viral genome. By analyzing interstrain heterologous UL74(gO) recombinants, we show that gO diversity can have dramatic impacts on cell-free and cell-to-cell spread as well as on antibody neutralization and that the manifestation of these impacts can be subject to epistatic influences of the global genetic background. These results highlight the potential limitations of laboratory studies of HCMV biology that use single, isolated genotypes or strains.