Fig 3. Cross-reactive flavivirus antibodies enhance ZIKV infection in placental explants.

A&B: Term human placental explants were infected with 1.0x10⁵ TCID₅₀/mL ZIKV alone or ZIKV that was preincubated with flavivirus naïve serum (ZIKV+control) or serum containing DENV-2 neutralizing antibodies (ZIKV+DENV nAbs), both in a dilution of 1:250, or with a humanized panflavirus monoclonal antibody (hu4G2, 1μg/mL). ZIKV titers were determined in supernatants (A) and ZIKV RNA levels were determined with RT-PCR in tissue lysates (B). C&D: Term human placental explants were pre-treated with FcyR blocking antibodies or protein G was added to ZIKV–DENV nAbs immune complexes. Subsequently, the explants were infected with either 1.0x10⁵ TCID₅₀/mL ZIKV or ZIKV+DENV nAbs. ZIKV titers were determined in supernatants at two dpi (C) and ZIKV RNA levels were determined in tissue lysates with RT-PCR at two dpi (D). N = 3–4 donors per condition, on average 12 explants per donor. Horizontal lines in the violin plots represent median and the 10^th and 90^th percentile cut-off. Statistical significance was determined using the Kruskal-Wallis test followed by Dunn’s post hoc test. * P<0.05, **P<0.01, ****P<0.0001.