Fig. 2. ZIKV sfRNA is required for placental infection and viral dissemination into the fetal brain.

(A, F, and G) Serum viremia in ZIKV-infected dams. Pregnant IFNAR^−/− mice were inoculated with 10⁴ FFU (A) and 10⁶ FFU (F) of WT or sfRNA-deficient ZIKV Natal via subcutaneous injection. (G) Side-by-side comparison of viremia curves in mice infected with WT ZIKV at 10⁴ FFU and xrRNA2′ ZIKV at 10⁶ FFU. Values in (G) are means ± SD. (B to E) Infection rate (B and D) and viral titers (C and E) in fetal tissues collected from mice infected at 10⁴ FFU. (H to K) Infection rate (I and J) and viral titers (H and K) in fetal tissues collected from mice infected at 10⁶ FFU. Fetal material was collected at 5 dpi and weighed. Placentas and fetal heads were separated, homogenized, and used for virus titration. Deformed fetuses (fetal tissue masses) were not used for virus titration, as placentas and heads could not be separated. Viral titers were normalized to the tissue weight. The only virus-positive placenta sample for xrRNA2′ mutant in (E) contains reverse mutation to the WT sequence. Statistical analyses for differences in viremias in (A) and (F) are shown in tables S2 and S3. Statistical analyses for differences in viral loads in (G) are shown in tables S4 and S5. Statistical analyses for (B), (D), (J), and (I) were undertaken using Fisher’s exact tests and by Mann-Whitney U tests for (C), (E), (H), and (K); P values are in comparison to the WT-infected group. All titers were determined by a foci-forming immunoassay on C6/36 cells.