Role of antibodies and host cells in plaque enhancement of Murray Valley encephalitis virus

Chicken antisera to Murray Valley encephalitis (MVE) virus, when incubated with virus and assayed for plaques on chicken embryo (CE) monolayers, neutralized MVE virus at high concentrations of antibody, but caused increases in plaque counts at low concentrations of antibody. Plaque enhancement did not occur when the same virus-antibody mixtures were assayed on a continuous line of rhesus monkey kidney cells (LLC-MK2), nor when the anti-MVE antibody was of mammalian origin and the assay system was CE monolayers. Correspondingly, chicken anti-MVE did not enhance the plaque formation of MVE virus in a stable line of mouse macrophages, P-388D1, whereas rabbit and mouse anti-MVE did enhance plaque formation. This enhancing activity was associated with noncytophilic immunoglobulin G (IgG). The Fc terminus of the IgG molecule was required, as no plaque enhancement occurred with chicken anti-MVE Fab. These data indicate that there is a requirement for taxonomic complementarity between Fc termini and Fc receptors in the above systems. CE cell monolayers were found to contain approximately 2% of Fc receptor-bearing cells among the fibroblast-like cells. Fc receptor-bearing cells in CE monolayers were isolated and found to be of the mononuclear phagocytic lineage. These mononuclear phagocytes, which originate in lymphoid tissues and blood associated with CE tissue fragments, are integrated into primary CE monolayers and form infectious centers in the presence of virus and low dilutions of antibody.