UBC Theses and Dissertations
Mechanisms of hepatic injury in murine hepatitis virus type 3 infection MacPhee, Peggy J.
Murine hepatitis virus type 3 (MHV-3), a member of the coronavirus family, induces a response that varies with the age and genetic background of the host mouse strain. A/J mice are fully resistant to the virus, while Balbc/J are fully susceptible and C3HebFe/J are semi-susceptible, making it possible to predictably reproduce the major human responses to hepatitis viruses. Although there has been considerable discussion of viral pathology in the literature, there has been much less emphasis on pathogenesis. In the experiments described here, histological, biophysical, and immunological techniques have been used to define the processes and cells involved. Transmission electron microscopic observations have confirmed that Kupffer and endothelial cells of hepatic sinusoids show clear changes by 12 hrs post-infection (p.i.), which are more advanced than hepatocellular changes. No replicating virus was seen in altered hepatocytes up to 3 days p.i. Scanning electron microscopy demonstrated that areas of necrosis are focal in nature and at 2-3 days p.i. consist of small spherical areas without flow. In vivo microcirculatory studies confirm the localized nature of the lesion and have shown that red cell velocity can be recorded in individual sinusoids . Velocities were found to vary from zero within a lesion to a normal velocity of 69±31 um/sec over a distance of not more than 3 sinusoids. In-vivo microcirculatory studies also revealed the ability of macrophages to move upstream (against flow) in the hepatic sinusoids. Using fluorescein labelled antibodies to cell surface markers (Thy-1, Lyt-2, and L3T4) it was shown that no T-cells of any subset were present in the areas of hepatocellular necrosis. Furthermore, treatment with cyclosporine A, which would be expected to decrease necrosis due to cell mediated cytotoxicity, did not significantly alter the course of the disease. The only cells which increased in number in the liver post infection were cells of the monocyte/macrophage lineage (Mac 1+), which had increased twofold at 12 hrs (p<.025) p.i. and to greater than twenty fold (p<.005) by 3 days p.i. Resistance in the A/J strain did not reflect an inability of the immunocompetent cells to present and respond to viral antigen. It was demonstrated that MHV-3 infected macrophages from resistant A/J mice are better able to stimulate proliferation of allogeneic and syngeneic lymphocytes than those from the sensitive Balb/cJ strain. In contrast, MHV-3 infection caused a significant enhancement of chemiluminescence from Balb/cJ macrophages, which did not occur in A/J animals. In vivo studies demonstrated a significant increase in free radical reaction products, including conjugated dienes (of long chain free fatty acids and aldehydes), thiobarbituric acid reactive substances, and lipid soluble fluorescent products between 12-72 hours p.i. with MHV-3 in the livers of susceptible Balb/cJ strain mice. All of these are products of oxidative cleavage of cellular and membrane polyunsaturated fatty acids, and result from the action of oxygen free radicals. Free radical inhibitors, or quenchers of free radical reaction products, were able to significantly reduce the liver necrosis in the susceptible mouse strain following infection. Radioimmune assays for antibody to MHV-3 have confirmed the presence of preformed antibodies to (or cross-reactive with) MHV-3 in the sera of both susceptible and resistant mice, pre and post-infection. Immunofluorescent labelled antibodies have also been used to demonstrate the presence of IgG deposits in the sinusoids of the liver both pre and post infection. This suggests the possibility that these mice have been infected with a non-virulent MHV strain prior to these experiments. From these studies, we conclude that the hepatic injury caused by MHV-3 infction in Balb/cJ mice is mediated predominantly by fixed and migratory cells of the mononuclear phagocytic series. Susceptibility and resistance are related to strain dependant differences in the response of macrophages (and Kupffer cells) to infection, and include the release of procoagulant activity (previously shown) and reactive oxygen radicals (and possibly other macrophage activation products such as PAF) that act together to induce hepatocellular necrosis. Preformed non-neutralizing antibody and an intact complement cascade may enhance viral uptake and activation of macrophages in the Balbc/J mice. Resistance to necrosis may be enhanced by a genetic deficiency of C5 in the A/J mice, preventing the formation of the membrane attack complex and hence complement dependant cell lysis, or macrophage activation.
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