Replication of parainfluenza type 3 virus in alveolar macrophages: evidence of in vivo infection and of in vitro temperature sensitivity in virus maturation

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Abstract
Approximately 2% of cultured alveolar macrophages (AM), originally lavaged from the lungs of parainfluenza type 3 virus (PI-3V)-infected calves, were observed to contain viral antigen (by fluorescent antibody method) or viral nucleocapsids (by electron microscopy). Plaque assays, however, indicated that virus titers were generally low when cultures were incubated at 37 degrees C for 10 days. AM, obtained from "in vivo infected" and "noninfected" calves, were found to be equally susceptible to further in vitro PI-3V infection when cultures were incubated at 37 degrees C. AM that were obtained from the lungs of normal calves, cultured at 37 degrees C, and inoculated with PI-3V were observed to produce relatively high virus titers when the incubation temperature was shifted down to 32 degrees C. Results from hemagglutinin assays showed that considerable amounts of hemagglutinin were detected when AM cultures were incubated at 32 degrees C, but only limited amounts were detected at 37 degrees C. Results from electron microscopic examinations at both temperatures substantiated the results of plaque and hemagglutinin assays. The PI-3V, isolated from AM cultures incubated at 32 degrees C, grew well in Madin-Darby bovine kidney cells at 32 degrees C, but little virus was produced at 37 degrees C. In contrast, parent PI-3V grew equally well at both temperatures. The results are discussed in terms of host susceptibility, temperature-sensitivity and virus maturation, and surface viral antigens and persistent viral infection.