Feline infectious peritonitis virus-infected monocytes internalize viral membrane-bound proteins upon antibody addition.

Feline infectious peritonitis virus (FIPV) may cause a highly lethal infection in cats, in spite of a usually strong humoral immune response. Antibodies seem unable to identify infected cells and mediate antibody-dependent cell lysis. In this study, the effect of antibodies on Feline coronavirus (FCoV)-infected monocytes was investigated. Upon addition of FCoV-specific antibodies, surface-expressed viral proteins were internalized through a highly efficient process, resulting in cells without visually detectable viral proteins on their plasma membrane. The internalization was also induced by mAbs against the Spike and Membrane proteins, suggesting that both proteins play a role in the process. The internalization did not occur spontaneously, as it was not observed in cells incubated with medium or non-specific antibodies. Further, the internalization could not be reproduced in feline cell lines, indicating its cell-type specificity. This study sheds new light on the immune-evasive nature of FIPV infections.