Polymorphic Forms of Human Cytomegalovirus Glycoprotein O Protect against Neutralization of Fibroblast Entry by Antibodies Targeting Epitopes Defined by Glycoproteins H and L

Human cytomegalovirus (CMV) utilizes different glycoproteins to enter into fibroblast and epithelial cells. A trimer of glycoproteins H, L, and O (gH/gL/gO) is required for entry into all cells, whereas a pentamer of gH/gL/UL128/UL130/UL131A is selectively required for infection of epithelial, endothelial, and some myeloid-lineage cells, but not of fibroblasts. Both complexes are of considerable interest for vaccine and immunotherapeutic development but present a conundrum: gH/gL-specific antibodies have moderate potency yet neutralize CMV entry into all cell types, whereas pentamer-specific antibodies are more potent but do not block fibroblast infection. Which cell types and neutralizing activities are important for protective efficacy in vivo remain unclear. Here, we present evidence that certain CMV strains have evolved polymorphisms in gO to evade trimer-specific neutralizing antibodies. Using luciferase-tagged variants of strain TB40/E in which the native gO is replaced by gOs from other strains, we tested the effects of gO polymorphisms on neutralization by monoclonal antibodies (mAbs) targeting four independent epitopes in gH/gL that are common to both trimer and pentamer. Neutralization of fibroblast entry by three mAbs displayed a range of potencies that depended on the gO type, a fourth mAb failed to neutralize fibroblast entry regardless of the gO type, while neutralization of epithelial cell entry by all four mAbs was potent and independent of the gO type. Thus, specific polymorphisms in gO protect the virus from mAb neutralization in the context of fibroblast but not epithelial cell entry. No influence of gO type was observed for protection against CMV hyperimmune globulin or CMV-seropositive human sera, suggesting that antibodies targeting protected gH/gL epitopes represent a minority of the polyclonal neutralizing repertoire induced by natural infection.

Characterization of the novel alleles: HLA-A*01:305, HLA-B*58:120, HLA-C*04:428Q and HLA-C*05:01:56

Four new HLA class I alleles were characterized by next-generation sequencing and haplotypes determined.

Influence of Human Cytomegalovirus Glycoprotein O Polymorphism on the Inhibitory Effect of Soluble Forms of Trimer- and Pentamer-Specific Entry Receptors

Human cytomegalovirus (HCMV) envelope glycoprotein complexes, gH/gL/gO trimer and gH/gL/UL128-131 pentamer, are important for cell-free HCMV entry. While soluble NRP2-Fc (sNRP2-Fc) interferes with epithelial/endothelial cell entry through UL128, soluble platelet-derived growth factor receptor α-Fc (sPDGFRα-Fc) interacts with gO, thereby inhibiting infection of all cell types. Since gO is the most variable subunit, we investigated the influence of gO polymorphism on the inhibitory capacities of sPDGFRα-Fc and sNRP2-Fc. Accordingly, gO genotype 1c (GT1c) sequence was fully or partially replaced by gO GT2b, GT3, and GT5 sequences in the bacterial artificial chromosome (BAC) TB40-BAC4-luc background (where luc is luciferase). All mutants were tested for fibroblast and epithelial cell infectivity, for virion content of gB, gH, and gO, and for infection inhibition by sPDGFRα-Fc and sNRP2-Fc. Full-length and partial gO GT swapping may increase epithelial-to-fibroblast ratios due to subtle alterations in fibroblast and/or epithelial infectivity but without substantial changes in gB and gH levels in mutant virions. All gO GT mutants except recombinant gO GT1c/3 displayed a nearly complete inhibition at 1.25 μg/ml sPDGFRα-Fc on epithelial cells (98% versus 91%), and all experienced complete inhibition on fibroblasts (≥99%). While gO GT replacement did not influence sNRP2-Fc inhibition at 1.25 μg/ml on epithelial cells (97% to 99%), it rendered recombinant mutant GT1c/3 moderately accessible to fibroblast inhibition (40%). In contrast to the steep sPDGFRα-Fc inhibition curves (slope of >1.0), sNRP2-Fc dose-response curves on epithelial cells displayed slopes of ∼1.0, suggesting functional differences between these entry inhibitors. Our findings demonstrate that artificially generated gO recombinants rather than the major gO genotypic forms may affect the inhibitory capacities of sPDGFRα and sNRP2 in a cell type-dependent manner.IMPORTANCE Human cytomegalovirus (HCMV) is known for its broad cell tropism, as reflected by the different organs and tissues affected by HCMV infection. Hence, inhibition of HCMV entry into distinct cell types could be considered a promising therapeutic option to limit cell-free HCMV infection. Soluble forms of cellular entry receptor PDGFRα rather than those of entry receptor neuropilin-2 inhibit infection of multiple cell types. sPDGFRα specifically interacts with gO of the trimeric gH/gL/gO envelope glycoprotein complex. HCMV strains may differ with respect to the amounts of trimer in virions and the highly polymorphic gO sequence. In this study, we show that the major gO genotypes of HCMV that are also found in vivo are similarly well inhibited by sPDGFRα. Novel gO genotypic forms potentially emerging through recombination, however, may evade sPDGFRα inhibition on epithelial cells. These findings provide useful additional information for the future development of anti-HCMV therapeutic compounds based on sPDGFRα.

Differences in Growth Properties among Two Human Cytomegalovirus Glycoprotein O Genotypes

Glycoprotein O (gO) of the human cytomegalovirus (HCMV) is the critical subunit of the envelope trimer gH/gL/gO as it interacts with platelet-derived growth factor alpha receptor upon fibroblast entry, and triggers gB-mediated fusion for fibroblast and epithelial cell infection. Eight genotypes (GT) of the highly polymorphic gO gene are described, yet it is unclear whether the distinct GTs differ in their function. Thus, we aimed to elucidate potential functional differences between two highly diverse gO GTs in an otherwise genomically identical HCMV strain. Therefore, resident gO GT1c sequence of strain TB40-BAC4-luc was entirely replaced by gO GT4 of strain Towne and both, GT1c and GT4 viruses, were investigated for their growth properties in fibroblasts and epithelial cells. In addition, two conserved gO cysteines involved in gH/gL/gO stabilization were mutated to serine either in GT1c (C218S and C343S) or GT4 (C216S and C336S) and their effects on cell-free infectivity were assessed. GT4 viruses displayed a significantly enhanced epithelial cell tropism and this resulted in higher virus release upon replication in epithelial cells when compared to GT1c viruses. Further, when the two cysteines were individually mutated in gO GT1c no impairment in cell-free infectivity was observed. This, however, was in sharp contrast to gO GT4, in which both of the corresponding cysteine mutations led to a substantial reduction in cell-free infectivity which was even more pronounced upon mutation of GT4-C336 than of GT4-C216. In conclusion, these findings provide evidence that the two highly diverse gO genotypes, GT1c and GT4, differ in their functional properties as revealed by their different infection capacities for epithelial cells and by their different responsiveness to mutation of strictly conserved cysteine residues. Thus, it is likely that the gO heterogeneity influences cell-free infectivity of HCMV also in vivo which may have important implications for virus host transmission.

Fusion proteins of flagellin and the major birch pollen allergen Bet v 1 show enhanced immunogenicity, reduced allergenicity, and intrinsic adjuvanticity

BACKGROUND

Recombinant fusion proteins of flagellin and antigens have been demonstrated to induce strong innate and adaptive immune responses. Such fusion proteins can enhance the efficacy of allergen-specific immunotherapy.

OBJECTIVE

We sought to characterize different fusion proteins of flagellin and the major birch pollen allergen Bet v 1 for suitability as allergy vaccines.

METHODS

A truncated version of flagellin (NtCFlg) was genetically fused to the N- or C-terminus of Bet v 1. Toll-like receptor (TLR) 5 binding was assessed with HEK293 cells expressing TLR5. Upregulation of CD40, CD80, CD83, and CD86 on monocyte-derived dendritic cells from allergic patients was analyzed by using flow cytometry. The T cell-stimulatory capacity of the fusion proteins was assessed with naive and Bet v 1-specific T cells. IgE binding was tested in inhibition ELISAs and basophil activation tests. Mice were immunized with the fusion proteins in the absence and presence of aluminum hydroxide. Cellular and antibody responses were monitored. Murine antibodies were tested for blocking capacity in basophil activation tests.

RESULTS

Both fusion proteins matured monocyte-derived dendritic cells through TLR5. Compared with Bet v 1, the fusion proteins showed stronger T cell-stimulatory and reduced IgE-binding capacity and induced murine Bet v 1-specific antibodies in the absence of aluminum hydroxide. However, only antibodies induced by means of immunization with NtCFlg fused to the C-terminus of Bet v 1 inhibited binding of patients' IgE antibodies to Bet v 1.

CONCLUSION

Bet v 1-flagellin fusion proteins show enhanced immunogenicity, reduced allergenicity, and intrinsic adjuvanticity and thus represent promising vaccines for birch pollen allergen-specific immunotherapy. However, the sequential order of allergen and adjuvant within a fusion protein determines its immunologic characteristics.