Neuroinvasiveness of the MR766 strain of Zika virus in IFNAR-/- mice maps to prM residues conserved amongst African genotype viruses.

Zika virus (ZIKV) strains are classified into the African and Asian genotypes. The higher virulence of the African MR766 strain, which has been used extensively in ZIKV research, in adult IFNα/β receptor knockout (IFNAR-/-) mice is widely viewed as an artifact associated with mouse adaptation due to at least 146 passages in wild-type suckling mouse brains. To gain insights into the molecular determinants of MR766's virulence, a series of genes from MR766 were swapped with those from the Asian genotype PRVABC59 isolate, which is less virulent in IFNAR-/- mice. MR766 causes 100% lethal infection in IFNAR-/- mice, but when the prM gene of MR766 was replaced with that of PRVABC59, the chimera MR/PR(prM) showed 0% lethal infection. The reduced virulence was associated with reduced neuroinvasiveness, with MR766 brain titers ≈3 logs higher than those of MR/PR(prM) after subcutaneous infection, but was not significantly different in brain titers of MR766 and MR/PR(prM) after intracranial inoculation. MR/PR(prM) also showed reduced transcytosis when compared with MR766 in vitro. The high neuroinvasiveness of MR766 in IFNAR-/- mice could be linked to the 10 amino acids that differ between the prM proteins of MR766 and PRVABC59, with 5 of these changes affecting positive charge and hydrophobicity on the exposed surface of the prM protein. These 10 amino acids are highly conserved amongst African ZIKV isolates, irrespective of suckling mouse passage, arguing that the high virulence of MR766 in adult IFNAR-/- mice is not the result of mouse adaptation. Author summary: Contemporary Asian genotype Zika virus (ZIKV) caused a large epidemic in the Americas and was associated with congenital abnormalities and neurological disease in adults, whereas African ZIKV has not been reported to cause neurological malformations in humans. Whether the lack of reported cases caused by African ZIKV simply reflects under-reporting remains unclear. Numerous studies have attempted to explain how and why these severe manifestations are restricted to ZIKV of Asian origin. Although several studies have reported that African ZIKV strains are more virulent in adult IFNα/β receptor knockout (IFNAR-/-) mice, the representative African ZIKV strain MR766 is believed to have artefactually high virulence in adult IFNAR-/- mice due to its extensive passage history in suckling wild-type mouse brains. However, we provide evidence herein that the lethality of MR766 in the adult IFNAR-/- mice is associated with 10 amino acid residues in the prM protein that are associated with an increased ability to cross the blood-brain barrier. These amino acids are highly conserved in African genotype viruses, irrespective of multiple passage history in suckling mouse brains, suggesting that high virulence is a legitimate characteristic of African ZIKVs. [ABSTRACT FROM AUTHOR]