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

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.