Intrinsic resistance of feline peritoneal macrophages to coronavirus infection correlates with in vivo virulence

Cats infected with virulent feline coronavirus strains develop feline infectious peritonitis, an invariably fatal, immunologically mediated disease; avirulent strains cause either clinically inapparent infection or mild enteritis. Four virulent coronavirus isolates and five avirulent isolates were assessed by immunofluorescence and virus titration for their ability to infect and replicate in feline peritoneal macrophages in vitro. The avirulent coronaviruses infected fewer macrophages, produced lower virus titers, were less able to sustain viral replication, and spread less efficiently to other susceptible macrophages than the virulent coronaviruses. Thus, the intrinsic resistance of feline macrophages may play a pivotal role in the outcome of coronavirus infection in vivo.

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.

Experimental inoculation of cats with human coronavirus 229E and subsequent challenge with feline infectious peritonitis virus

Minimal-disease cats exposed to live human coronavirus 229E developed homologous antibody responses that suggested little or no replication of the virus in inoculated animals. Oronasal and subcutaneous inoculation of coronavirus 229E did not elicit an antibody response by heterologous (transmissible gastroenteritis virus, canine coronavirus) neutralization or by heterologous (transmissible gastroenteritis virus) kinetics-based enzyme-linked immunosorbent assay. No clinical signs attributable to coronavirus 229E were seen in inoculated cats. Although the number of animals in each of the five experimental groups was small (n = 2), antibodies produced in response to the virus did not appear to sensitize cats to subsequent feline infectious peritonitis virus challenge, but neither did they cross-protect cats against the challenge dose.

Experimental inoculation of cats with canine coronavirus and subsequent challenge with feline infectious peritonitis virus

Oronasal inoculation of minimal-disease cats with a virulent field isolate of canine coronavirus elicited a neutralizing antibody response that increased steadily over a two-month period, indicating in vivo amplification of the inoculated dose. A heterologous neutralizing antibody response to transmissible gastroenteritis virus could be detected three weeks after titers to canine coronavirus were first observed and was found to parallel the homologous response at a level approximately one log10 dilution lower. A modest anamnestic response detectable in both assays was observed after a second exposure to canine coronavirus. Cross-reacting antibodies to transmissible gastroenteritis virus were not detected by a kinetics-based enzyme-linked immunosorbent assay (KELA), however, until after cats had received multiple daily doses of canine coronavirus (third exposure). No clinical signs attributable to canine coronavirus were seen in inoculated cats, and virus could not be isolated from fecal or rectal swabs nor identified in fecal specimens by electron microscopy. Both virus-neutralizing and KELA titers were boosted by aerosol challenge with feline infectious peritonitis virus. Antibodies produced in response to canine coronavirus did not sensitize cats to subsequent feline infectious peritonitis virus challenge, but neither did they protect cats against the challenge dose.

Experimental studies of a coronavirus and coronavirus-like agent in a barrier-maintained feline breeding colony

Minimal-disease (MD) cats raised in a barrier-maintained feline breeding colony routinely become coronavirus (CV) antibody-positive at 5-8 weeks of age and remain seropositive indefinitely. In addition, they shed coronavirus-like particles (CVLPs) in their feces which are morphologically distinct from "typical" coronaviruses. Oronasal, intraperitoneal, intraduodenal, and intravenous inoculations of CVLPs were uniformly unsuccessful in the seroconversion of cats to CV, whereas subsequent prolonged close contact with colony cats did result in seroconversion. Such evidence indicates that MD cats in this barrier colony are infected with at least two non-crossreactive agents: (1) feline enteric CVLPs, which are shed in feces and are morphologically and antigenically distinct from established coronaviruses (CVs), and (2) a virus which is antigenically cross-reactive with CVs, but which is apparently shed by some route other than the lower gastrointestinal tract.