Human Cytomegalovirus gH/gL/gO Promotes the Fusion Step of Entry into All Cell Types, whereas gH/gL/UL128-131 Broadens Virus Tropism through a Distinct Mechanism

Abstract

Interaction between gH/gL and the fusion protein gB is likely a conserved feature of the entry mechanism for all herpesviruses. Human cytomegalovirus (HCMV) gH/gL can be bound by gO or by the set of proteins UL128, UL130, and UL131, forming gH/ gL/gO and gH/gL/UL128-131. The mechanisms by which these complexes facilitate entry are poorly understood. Mutants lacking UL128-131 replicate well on fibroblasts but fail to enter epithelial/endothelial cells, and this has led to the general assumption that gH/gL/UL128-131 promotes gB-mediated fusion on epithelial/endothelial cells whereas gH/gL/gO provides this function on fibroblasts. This was challenged by observations that gO-null mutants were defective on all of these cell types, suggesting that entry into epithelial/endothelial cells requires both of the gH/gL complexes, but the severe replication defect of the gO mutants precluded detailed analysis. We previously reported that the ratio of gH/gL/gO and gH/gL/UL128-131 in the virion envelope varied dramatically among HCMV strains. Here, we show that strains not only differ in the ratio, but also vary in the total amount of gH/gL in the virion. Cell-type-specific particle-to-PFU ratios of HCMV strains that contained different amounts of gH/gL/gO and gH/gL/UL128-131 were determined. Infection of both fibroblasts and epithelial cells was generally correlated with the abundance of gH/gL/gO, but not with that of gH/gL/UL128-131. The low infectivity of virions rich in gH/gL/UL128-131 but low in gH/gL/gO could be overcome by treatment with the chemical fusogen polyethylene glycol (PEG), strongly arguing that gH/gL/gO provides the conserved herpesvirus gH/gL entry function of promoting gB-mediated fusion for entry into all cell types, whereas gH/gL/UL128-131 acts through a distinct mechanism to allow infection of select cell types. IMPORTANCEThe functions of HCMV gH/gL complexes in entry are unclear. Unlike the well-studied Epstein-Barr virus (EBV), where gH/gL and gH/gL/gp42 complexes both seem capable of promoting gB fusion during entry into different cell types, our studies here suggest that for HCMV, gH/gL/gO promotes gB fusion on all cell types, whereas gH/gL/UL128-131 broadens virus tropism through a distinct, as yet unknown mechanism. To our knowledge, this is the first suggestion of a herpesvirus gH/gL that does not act by promoting gB fusion, which might make HCMV a useful model to study the fundamental mechanisms by which herpesvirus gH/gL regulates gB fusion. Moreover, gH/gL/UL128-131 is a candidate vaccine target. Our findings help to explain the cell-type-dependent virus neutralization exhibited by anti-gH/gL/UL128-131 antibodies and underscore the importance of gH/ gL/gO as another important part of vaccine or therapeutic strategies. P rimary infection of healthy adults by human cytomegalovirus (HCMV) is usually subclinical or mildly symptomatic but leads to lifelong persistent or latent infection. Primary infection or reactivation of HCMV in immunocompromised hosts, such those infected with HIV and t...

Global Mapping of O-Glycosylation of Varicella Zoster Virus, Human Cytomegalovirus, and Epstein-Barr Virus

“…Five HCMV envelope glycoproteins are indispensable for replication: gB, gH, gL, gM, and gN (57). Glycoprotein B and gH-gL comprise the conserved fusion machinery; however, gH-gL can additionally be complexed with gO or UL128 -131A to promote infectivity (58). In agreement with alphaherpesviruses, we found two O-glycosites at the N terminus of gB (Fig.…”

The pentameric complex of human Cytomegalovirus: cell tropism, virus dissemination, immune response and vaccine development

“…However, gO and the UL128-131 gene products were reported to compete for binding of gH/gL, thus influencing the ratio of gH/gL/ gO to gH/gL/UL128-131 within virions as well as the HCMV cell tropism [61,62]. As a result, gH/gL/gO was also found to be required for entry into epithelial cells [57,63,64].…”

“…As for the fusion event relevant to HCMV entry into all cell types, it has been reported to be mediated by gH/gL/gO and gB, whereas in epithelial cells efficient fusion would be preceded by a PC-mediated process [64]. Recently, the HCMV gH/gL complex has been shown to form a stable complex with the fusion protein gB in virions including up to 50 % of total gH/gL, suggesting that gH/gL interacts with gB to mediate membrane fusion [65].…”

The Cell Biology of Cytomegalovirus: Implications for Transplantation

“…This suggests that gH/gL/gO provides a conserved herpesvirus "gH/gL entry function" that promotes glycoprotein B (gB)-mediated fusion for entry, whereas gH/gL/UL128-131 acts through a distinct mechanism to allow infection of selected cell types. Unlike EBV, in which gH/gL and gH/gL/gp42 complexes are both capable of promoting gB fusion during cellular entry into various cell types, gH/gL/gO in HCMV promotes gB fusion on cells, whereas gH/gL/UL128-131 broadens viral tropism through another, unknown mechanism (47). Monoclonal antibodies targeting CMV complexes gH/gL and gH/gL/UL128/UL130/UL131 (RG7667) bind neutralizing viral epitopes and are under study for treatment of CMV infection in kidney recipients (48).…”