Isolation and identification of feline peritoneal macrophages for in vitro studies of coronavirus-macrophage interactions.

Feline peritoneal cells were collected by lavage with isotonic saline without the use of irritants or need for euthanasia of the cats. Macrophages were purified by centrifugation on Percoll followed by selective adherence. Although few macrophages could be obtained from an initial lavage, a second lavage performed on the same cat 9-11 days later yielded six times as many macrophages as the first lavage, providing sufficient numbers of cells for characterization and infection experiments. Macrophages from these subsequent lavages were not more functionally activated in phagocytosis assays than the resident macrophages from the initial lavage, and they were equally susceptible to infection with feline infectious peritonitis virus (FIPV). Infected cultures produced peak titers of 10(5.0) TCID50 per ml, and FIPV antigen was detected in a small subset (0.1-1.0%) of cells by indirect immunofluorescence. The FIPV-infected cells were identified as macrophages by their characteristic morphology and ability to phagocytize rhodamine-labeled latex beads. The successful isolation of large numbers of unactivated feline macrophages will permit in vitro studies of feline coronavirus-macrophage interactions that otherwise would not have been possible. Such studies will undoubtedly provide valuable insights into the pathogenesis of feline infectious peritonitis, an invariably fatal disease of domestic and exotic cats.