Molecular Variations in Glycoprotein B of Asian Human Cytomegalovirus: Potential Impact on Virus Entry and Immune Evasion in Ocular Diseases

Human cytomegalovirus (HCMV)-associated ocular diseases have gained increasing attention due to a recent rise in cases diagnosed in Asia. A glycoprotein encoded by the virus UL55 gene, glycoprotein B (gB), is essential for viral entry and a primary target for naturally-produced antibodies and vaccine development. gB is classified into five genotypes (gB1-gB5) based on polymorphisms surrounding the furin cleavage site. This study analyzed the UL55 gene in 62 blood and ocular specimens of Japanese patients with CMV viremia and CMV-associated ocular diseases. Distinct gB genotype distributions were found between sample types (p = 0.008): gB2 was the most prevalent genotype in blood samples (41%, 11/27), while gB3 (43%, 15/35) and gB1 (37%, 13/35) predominated in ocular fluids. Viral loads were significantly higher in gB1 and gB3-positive samples compared with gB2 (p = 0.016). A shared gB1/gB3-specific peptide (aa 190-204; SRVIAGTVFVAYHRD), distinct from that of gB2, exhibited reduced HLA class II binding. In addition, a K518R substitution was identified in 80% of gB1 and gB3 variants in our cohort and other Asian-derived GenBank entries, but only 3% of European origin strains. This substitution was significantly enriched in ocular fluids from patients with CMV ocular infection (71%, 17/24), compared with blood from patients with CMV viremia (32%, 8/25) (p = 0.01). The predicted structural modeling infers that this substitution is located in the core of gB Domain III, and potentially increase the local molecular stability in this region. Evolutionary analyses indicated positive selective pressure at this site, implying the biological significance. These findings infer that genetic variations enriched in ocular fluids and Asian-derived HCMV strains, may contribute to ocular pathogenesis through influencing on viral entry and reduced immune recognition.