Host Immune Response to ZIKV in an Immunocompetent Embryonic Mouse Model of Intravaginal Infection

Abstract

Zika virus (ZIKV) only induces mild symptoms in adults; however, it can cause congenital Zika syndrome (CZS), including microcephaly. Most of the knowledge on ZIKV pathogenesis was gained using immunocompromised mouse models, which do not fully recapitulate human pathology. Moreover, the study of the host immune response to ZIKV becomes challenging in these animals. Thus, the main goal of this study was to develop an immunocompetent mouse model to study the ZIKV spread and teratogeny. FVB/NJ immune competent dams were infected intravaginally with ZIKV during the early stage of pregnancy. We found that the placentae of most fetuses were positive for ZIKV, while the virus was detected in the brain of only about 42% of the embryos. To investigate the host immune response, we measured the expression of several inflammatory factors. Embryos from ZIKV-infected dams had an increased level of inflammatory factors, as compared to Mock. Next, we compared the gene expression levels in embryos from ZIKV-infected dams that were either negative or positive for ZIKV in the brain. The mRNA levels of viral response genes and cytokines were increased in both ZIKV-positive and negative brains. Interestingly, the levels of chemokines associated with microcephaly in humans, including CCL2 and CXCL10, specifically increased in embryos harboring ZIKV in the embryo brains.

Keywords: CCL2; CXCL1 and CXCL10; FVB/NJ mice; ZIKV; intravaginal infection.

Figure 1

The ZIKV effect on FVB/NJ mice dams and fetuses. The FVB/NJ mice were infected intravaginally with 10 µl of ZIKV (1.0 × 10⁵ PFU) or PBS (MOCK infection) on the gestational day (GD) 4.5. The dam weight was determined on the GDs 4.5, 11.5 and 17.5. (B) Representative images of embryos derived from either the MOCK- or ZIKV-infected dams are shown. The graphs show the weight of the (A) dams, (C) fetuses and (D) placentae. The data represent the means ± SEM. * indicates a significant difference, as compared to MOCK (p < 0.05).

Figure 2

The ZIKV presence in the brain tissue modifies the expression of inflammatory factors in the brains collected from embryos derived from the ZIKV-infected FVB/NJ dams. Fetus brains were collected on GD 17.5 and used to extract total RNA. cDNA was amplified and used for a qPCR analysis. The real-time PCR values were normalized to those of GAPDH of the corresponding samples. The relative levels for each gene were calculated in reference to the mock-infected cells using the ∆∆Ct method. The graphs show (A) TLR7, (B) Mx1, (C) CCL2, CXCL1 and CXCL10, and (D) IL-1β, IL-18, TNFα and IL-6 gene transcription levels (fold change to MOCK) in brains that were either positive or negative for ZIKV and that were derived from the ZIKV-infected FVB/NJ dams. The data represent the means ± SEM obtained from 6 ZIKV-positive brains and 8 ZIKV-negative brains. * indicates a significant difference, as compared to the ZIKV-negative brains (p < 0.05).