The Pathogenesis of Cache Valley Virus in the Ovine Fetus

Cache Valley virus (CVV) induced malformations have been previously reproduced in ovine fetuses; however, no studies have established the CVV infection sequence of the cells targeted by the virus or the development of the antiviral response of the early, infected fetus that results in viral clearance before development of immunocompetency. To address these questions, ovine fetuses at 35 dg were inoculated in utero with CVV and euthanized at 7, 10, 14, 21 and 28 dpi. On postmortem examination arthrogryposis and oligohydramnios were observed in some infected fetuses. Morphologic studies showed necrosis in the central nervous system (CNS) and skeletal muscle of earlier infected fetuses and hydrocephalus, micromyelia and muscular loss in later infected fetuses. Using immunohistochemistry and in situ hybridization, intense CVV viral antigenic signal was detected in the brain, spinal cord, skeletal muscles and fetal membranes of infected fetuses. Viral signal decreased in targeted and infected tissues with the progression of the infection.

To determine specific cell types targeted by CVV in the CNS, indirect immunofluorescence was applied to sections of the CNS using a double labeling technique with antibodies against CVV together with antibodies against neurons, astrocytes and microglia. CVV viral antigen was shown within the cytoplasm of neurons in the brain and spinal cord. No viral signal was observed in microglial cells; however, infected animals had marked microgliosis.

The antiviral immune response in immature fetuses infected with CVV was evaluated. Gene expression associated with an innate, immune response was quantified by real-time, quantitative PCR. Upregulated genes in infected fetuses included ISG15, Mx1, Mx2, IL-1, IL-6, TNF-?, TLR-7 and TLR-8. The amount of Mx protein, an interferon stimulated GTPase capable of restricting growth of bunyaviruses, was elevated in the allantoic and amniotic fluid in infected fetuses. ISG15 protein expression was significantly increased in target tissues of infected animals. B lymphocytes and immunoglobulin-positive cells were detected in lymphoid tissues and in the meninges of infected animals. This demonstrated that the infected ovine fetus is able to stimulate an innate and adaptive immune response before immunocompetency that presumably contributes to viral clearance in infected animals.