SUMMARYThe maximum proportion of skeletal and/or skin defects induced by the Semliki Forest virus (SFV) mutant ts22 in the 17-day-old foetal mouse occurred following infection of the mother at day 10 of pregnancy. The skeletal defects were detected using a combination of Alcian blue staining for cartilage and Alizarin red staining for bone.Using immunogold-silver staining with anti-SFV IgG and in situ hybridization with a cDNA probe to SFV non-structural sequences, we have shown that mesenchymal cells in the dermis and surrounding developing cartilaginous plates were heavily infected in most foetuses at day 17 of pregnancy, following infection of the mother at day 10. Other infected foetal tissues contained less viral antigen and nucleic acid; they included the liver, muscle (including myocardium), lung and kidney. The central nervous system contained only small amounts of viral antigen and nucleic acid. It is proposed that the skeletal and skin defects induced in mouse foetuses by ts22 infection result from the tropism of the virus for mesenchymal cells involved in the development of such tissue.
The A7 strain of Semliki Forest virus induces rapid fetal death in pregnant mice, whereas the ts22 mutant derived from it is teratogenic for a proportion of fetuses. Both A7 and ts22 induce viremia and infect the central nervous systems and fetuses of pregnant mice. Using immunogold-silver staining, a cDNA probe for a Semliki Forest virus nonstructural sequence, and a riboprobe derived from the same sequence, we showed that the skin and musculoskeletal systems of fetuses from mothers infected with ts22 were often heavily infected but the central nervous systems were not labeled before day 17 of pregnancy. Damage to the neural tube, including open-neural-tube defects, was detected in fetuses following infection of the mother at days 8 and 10 of pregnancy with both A7 and ts22. For ts22, neural tube damage induced by fetal infection before day 17 of pregnancy appeared to be indirect and caused by virus infection of mesenchymal cells surrounding the developing neural tube.
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