MATERNAL VIRAL INFECTION DURING PREGNANCY PROGRAMS FETAL DEVELOPMENT AND INDUCES A DIFFERENTIAL TYPE 1 INTERFERON RESPONSE

Abstract

The mechanisms of persistent infection are incompletely understood, and bovine viral diarrhea virus (BVDV) provides a useful disease model for studying the complex host-virus interactions involved during the prenatal infection period. The noncytopathic BVDV biotype (ncpBVDV) has the ability to establish infection early in fetal development (<150 d.), creating a persistently infected (PI) calf that is immunotolerant to the virus and unable to resolve the infection. A viremia persists in the animal after birth, and continuous viral shedding in body effluents ensures the continuation of the infection cycle. In contrast, infection after the development of a competent immune system (fetal >150 d. or postnatal) results in a transient infection (TI) that is associated with clearance of the virus and seroconversion. It has been suggested that the establishment of persistent infection with BVDV may involve viral evasion of innate and subsequent adaptive immune responses. Screening of maternal blood cell mRNA on day 190 of pregnancy by microarray analysis revealed an intense upregulation of interferon stimulated gene 15 K (ISG15) after BVDV infection. ISG15 is considered a prominent marker in the cascade of events that occur during the innate immune response. BVDV naï ve heifers (n=6 per treatment group) were infected with ncpBVDV type 2 virus at d. 75 or d. 175 of pregnancy creating PI and TI fetuses, respectively. A control group was maintained free of BVDV. Maternal blood was collected at multiple time points during the experiment, and cesarean sections were performed on d. 190. Infection was confirmed by ELISA and qtRT-PCR. A ponderal index of fetal growth was calculated (body weight/crown rump length3), indicating intrauterine growth restriction of PI fetuses (p<0.05). Decreased megakaryocyte numbers were noted in PI spleen tissues, in addition to a prominent occurrence of BVDV antigen in the spleen, liver, and brain of PI fetuses. No viral antigen was detected in TI or control fetuses. qtRT-PCR analysis of fetal spleen mRNA revealed a dramatic upregulation of ISG15 expression in TI fetuses (p<0.01) in contrast to the low expression of ISG15 in PI and control fetuses. These findings were further confirmed by Western Blot analysis of spleen samples using anti-ISG15 antibody. The strong induction of ISG15 in TI fetuses is characteristic of acute viral infection and the consequent innate immune response, however the mild ISG15 response of PI fetuses, despite their viremic state, indicates a deficient IFN pathway activation in the host tissues. This lack of a robust response by the PI host during prenatal development may contribute to the establishment of the persistent infection, and the associated inhibition of normal fetal growth. NcpBVDV infection during gestation clearly has numerous adverse effects on the developing fetus, in addition to a complex interaction with the innate immune response. Further understanding of the relationship between BVDV and its infected host is crucial to identifying the impact of viral infection on fetal growth and immune development, and the subsequent control of this and other viral pathogens. National Research Initiative Competitive Grant 2006-03907 from the USDA Cooperative State Research, Education, and Extension Service (T.R.H & A.L.V.). (platform)