Origin, antigenicity, and function of a secreted form of ORF2 in hepatitis E virus infection

Significance

Hepatitis E virus (HEV) is a main cause of acute hepatitis worldwide. Recent evidence suggests that HEV-infected cells release a secreted form of ORF2 protein (ORF2 ^S) but its origin and function are unknown. Here we demonstrate that ORF2 ^S and ORF2 ^C (the actual capsid protein) are different translation products and that ORF2 ^S is not essential for the HEV life cycle but inhibits antibody-mediated neutralization of HEV. Our results have important implications for understanding the HEV replication cycle and immune evasion mechanisms. The identified internal start codon in this study is highly conserved in most HEV strains, suggesting that the production of ORF2 ^S is an evolutionary conserved function for HEV.

Abstract

The enterically transmitted hepatitis E virus (HEV) adopts a unique strategy to exit cells by cloaking its capsid (encoded by the viral ORF2 gene) and circulating in the blood as “quasi-enveloped” particles. However, recent evidence suggests that the majority of the ORF2 protein present in the patient serum and supernatants of HEV-infected cell culture exists in a free form and is not associated with virus particles. The origin and biological functions of this secreted form of ORF2 (ORF2 ^S) are unknown. Here we show that production of ORF2 ^S results from translation initiated at the previously presumed AUG start codon for the capsid protein, whereas translation of the actual capsid protein (ORF2 ^C) is initiated at a previously unrecognized internal AUG codon (15 codons downstream of the first AUG). The addition of 15 amino acids to the N terminus of the capsid protein creates a signal sequence that drives ORF2 ^S secretion via the secretory pathway. Unlike ORF2 ^C, ORF2 ^S is glycosylated and exists as a dimer. Nonetheless, ORF2 ^S exhibits substantial antigenic overlap with the capsid, but the epitopes predicted to bind the putative cell receptor are lost. Consistent with this, ORF2 ^S does not block HEV cell entry but inhibits antibody-mediated neutralization. These results reveal a previously unrecognized aspect in HEV biology and shed new light on the immune evasion mechanisms and pathogenesis of this virus.