Comparative pathogenesis of passaged pichinde virus variants in outsred guinea pigs

BACKGROUND: The established animal model for human Lassa fever utilizes a guinea pig passage-adapted derivative of the arenavirus Pichinde. The low passage virus derived from isolate 4763 (4763-P2) is attenuated, whereas the high passage (4763-P16) variant is extremely virulent in guinea pigs. OBJECTIVES: Compare spread through the host and cell and tissue tropism of the attenuated (P2) and virulent (P16) passage-related variants. EXPERIMENTAL DESIGN: Outbred Hartley guinea pigs were intraperitoneally infected with 1000 plaque forming units of the P2 or P16 variant. Peripheral blood and organs were taken at various post-inoculation time points for viral titer and histopathology. Flow cytometry and immunohistochemlstry were performed to determine infected cell types. Peripheral blood and spleen cell populations were immunophenotyped using commercially available antibodies for T cells, B cells, macrophages and CD45 positive leukocytes. RESULTS: In the P16 infected animals, there was a 2 to 3 log increase in splenic viral titers, associated with an earlier onset of viremia, fever, greater weight loss and a longer duration of fever compared to the P2 animals. Flow cytometry suggested that T cells and macrophages are major targets of infection, correlating with histology results. Spleen and liver snowed larger and more numerous foci of viral antigen in P16 vs. P2 animals after day 6. By day 12, the P16 infected guinea pigs showed massive red pulp necrosis of the spleen and immunohistochemical evidence of viral spread to non-lymphoid cells in many organs. In contrast, the P2 infected animals revealed macrophage hyperplasia in the marginal zones of spleen and lymph nodes associated with viral clearance. CONCLUSIONS: Both P2 and P16 viruses display macrophage and lymphocyte tropism. Guinea pigs infected with P2 restrict and clear the virus, white the P16 virus replicates uncontrollably. Mechanisms of pathogenesis and immunity will be examined in future studies.