Abstract
The relationship between susceptibility of mouse peritoneal macrophages to lactic dehydrogenase-elevating virus (LDV) infection and expression of I region-coded antigens (Ia) on these cells was investigated. The proportion of Ia-positive cells in resident peritoneal macrophages from adult and suckling mice were 4 to 10% and 50 to 70% respectively. Approximately the same percentage of the cells were susceptible to LDV, as detected by fluorescent antibody staining. In adult mice, double- labelling experiments showed that most of the Ia-positive cells were LDV-infected. When the cells were cultured for more than 24 h in vitro, Ia-positive cells rapidly disappeared and the culture became resistant to LDV. Removal of Ia-positive cells by treatment with anti-Ia plus complement or enrichment using an anti-Ia-coated Petri dish simultaneously removed or enriched for LDV-susceptible cells. Treatment of cells with trypsin (1 mg/ml) removed their I-A and I-E antigens and simultaneously abolished susceptibility for LDV. When LDV was preincubated with subneutralizing amounts of antibody, infectivity for macrophages was enhanced and the proportion of LDV-infected cells was higher than that of Ia-positive cells. This suggests that Fc receptors on macrophages can act as receptors for LDV coated with antiviral IgG.