Human Cytomegalovirus Cell Tropism and Host Cell Receptors

Abstract

In the 1970s-1980s, a striking increase in the number of disseminated human cytomegalovirus (HCMV) infections occurred in immunosuppressed patient populations. Autopsy findings documented the in vivo disseminated infection (besides fibroblasts) of epithelial cells, endothelial cells, and polymorphonuclear leukocytes. As a result, multiple diagnostic assays, such as quantification of HCMV antigenemia (pp65), viremia (infectious virus), and DNAemia (HCMV DNA) in patient blood, were developed. In vitro experiments showed that only low passage or endothelial cell-passaged clinical isolates, and not laboratory-adapted strains, could reproduce both HCMV leuko- and endothelial cell-tropism, which were found through genetic analysis to require the three viral genes UL128, UL130, and UL131 of the HCMV UL128 locus (UL128L). Products of this locus, together with gH/gL, were shown to form the gH/gL/pUL128L pentamer complex (PC) required for infection of epithelial cells/endothelial cells, whereas gH/gL and gO form the gH/gL/gO trimer complex (TC) required for infection of all cell types. In 2016, following previous work, a receptor for the TC that mediates entry into fibroblasts was identified as PDGFRα, while in 2018, a receptor for the PC that mediates entry into endothelial/epithelial cells was identified as neuropilin2 (Nrp2). Furthermore, the olfactory receptor family member OR14I1 was recently identified as a possible additional receptor for the PC in epithelial cells. Thus, current data support two models of viral entry: (i) in fibroblasts, following interaction of PDGFRα with TC, the latter activates gB to fuse the virus envelope with the cell membrane, whereas (ii) in epithelial cells/endothelial cells, interaction of Nrp2 (and OR14I1) with PC promotes endocytosis of virus particles, followed by gB activation by gH/gL/gO (or gH/gL) and final low-pH entry into the cell.

Keywords: HCMV; Nrp2; PDGFRα; cell tropism; cellular receptors; epithelial cells/endothelial cells.

Figure 1

Circulating cytomegalic endothelial cells stained with: (a) the endothelial cell-specific PAL-E monoclonal antibody; (b) a pp65-specific human cytomegalovirus (HCMV) monoclonal antibody (a pp65-positive polymorphonuclear leukocyte is shown in close proximity). (c) Several cytomegalic endothelial cells are present along the vessel wall and in the bloodstream of a prostatic vessel of an AIDS patient with disseminated HCMV infection (from [12]).

Figure 2

Analysis of the gH/gL/gO-PDGFRα complex. Size exclusion chromatography profile of the gH/gL/gO alone (a), or complexed with PDGFRα (b), or PDGFRα and the Fab fragment of the anti-gH mAb 13H11 (c). The shift in the elution time of (b) and (c) complexes compared to (a) is indicated by vertical dotted lines. (d) SDS-PAGE in reducing conditions of individual proteins and complexes, as indicated. (e–g) EM negative staining of gH/gL/gO alone (e), complexed with PDGFRα (indicated by a red dotted line) bound to gO (f), and with PDGFRα (indicated by a red dotted line) and the Fab of mAb anti-gH 13H11 (indicated by a yellow dotted line) bound to the twisted region of gH (g). (h) EM 3D map of gH/gL/gO at 19.3 Å resolution (from [68]).

Figure 3

(a) Molecular architecture of the HCMV pentamer/Nrp2/mAb 3G16 Fab complex. (b) Several cross-links were observed between the Nrp2 a1 domain and residues of pUL130, pUL128, and gL. Additional cross-links were observed between the a2 and b1 domains and residues of pUL131 and pUL128. (c) Interactions between the HCMV pentamer complex (PC) and a2-b1-b2 domains of Nrp2 are observed in Insets 1 and 2 (from [72]).

Figure 4

(a) Summary of the HCMV pentamer epitope mapping. (b) In vitro binding of neutralizing mAbs to pentamer epitopes, which, with the exception of mAbs targeting site 3/7, sterically block Nrp2 pentamer binding (from [72]).

Figure 5

Models of HCMV entry into human cells using the trimer complex (TC) or PC complexes. The TC interacts with PDGFRα via gO, then gH/gL/gO activates the fusogenic activity of gB at the plasma membrane of human fibroblasts. Conversely, the PC first binds Nrp2, then the gH/gL component of the PC (or gH/gL/gO) induces endocytosis of virus particles [71], which are released into cytoplasm from endosomal membranes, following low-pH-induced activation of the gB fusion machinery (from [72]).