Strain-Dependent Cross-Neutralization of Closely Related Asian-Lineage Zika Viruses by Dengue-Immune Sera

Background: Zika virus (ZIKV) and dengue virus (DENV) co-circulate in tropical regions and elicit cross-reactive immunity. However, differential neutralization of closely related ZIKV strains remains unclear. This study characterizes the neutralization profiles of dengue-immune sera against DENV and two Asian-lineage ZIKV strains, P6-740 and PRVABC59. Methods: Sera from suspected dengue infections and from healthy donors were screened for anti-DENV IgM and IgG antibodies and anti-ZIKV IgM and IgG antibodies by ELISA and grouped. Neutralization against DENV serotypes and two ZIKV strains was quantified using the foci reduction neutralization test (FRNT). Complete genome sequences of ZIKV P6-740 and PRVABC59 were compared to identify amino acid differences. Results: Sera containing anti-DENV IgG showed higher neutralizing activity against both DENV and ZIKV than IgG-negative sera. Neutralization was also detected in some ELISA-seronegative samples. Across all immunological groups, neutralization titers were higher against ZIKV P6-740 than ZIKV PRVABC59. Among healthy donors with evidence of previous dengue exposure, 58% exhibited a ≥4-fold difference between the two ZIKV strains, of which 80% favored ZIKV P6-740. Among seronegative healthy blood donors, 45% exhibited a 4-fold difference, of which 89% favored P6-740. Four amino acid substitutions were identified in the envelope (E) protein. Conclusions: Dengue-induced immunity differentially neutralizes closely related ZIKV strains, with generally stronger activity against P6-740, suggesting that viral genetic variation may influence cross-neutralization outcomes.