Mechanisms of attenuation of the live Junin virus vaccine strain, Candid #1

Millions of individuals are at risk of arenavirus infection worldwide, with untreated infections often resulting in potentially lethal disease. Junin virus (JUNV), the causative agent of Argentine hemorrhagic fever (AHF), is currently the only pathogenic arenavirus for which a vaccine is currently in use. The Candidate #1 (Candid #1) vaccine strain, which was obtained from the 44th mouse brain passage of the parental XJ strain, has significantly reduced the incidence of disease in endemic areas. However, the mechanisms of attenuation and protection remain largely unknown. We have previously identified the glycoprotein (GP) gene as the primary attenuating gene of Candid #1 in our Hartley guinea pig model of AHF. Here, I provide a detailed mechanism through which the Candid #1 GP negatively effects the highly regulated life cycle of the virus within infected cells. Candid #1 loses an N-linked glycosylation motif prior to the 44th mouse brain passage of the XJ strain, which coincides with a significant decrease in fully processed and cleaved glycoprotein complexes (GPCs) and a significant increase in full-length GP. JUNV GPs in which this N-linked glycosylation motif are absent fail to reach the cell surface and aggregate within the endoplasmic reticulum (ER). Evidence of ER-associated autophagy is present with aggregation of the full-length GP, and upregulation of both ER stress-associated markers and LC3-II. Mutations present in the matrix protein (Z) contribute to the attenuation of Candid #1 to a smaller degree. An amino acid substitution within the RING domain of the Candid #1 Z protein results in the protein budding from the cell independently, and there is a significant decrease in interaction between GP and Z in a plasmid expression system. Together, these results demonstrate that the Candid #1 has a significantly altered life cycle compared to that of a wild-type arenavirus, including the overexpression of viral proteins, accumulation of GPC in the ER, and independent budding of the Z protein.