Beyond macrophages: FIPV tropism includes T and B lymphocytes.

If untreated, feline infectious peritonitis (FIP) is a fatal disease that is caused by feline infectious peritonitis virus (FIPV), a virulent biotype of feline coronavirus (FCoV) that disseminates broadly and triggers severe systemic inflammation. While the prevailing model holds that FIPV selectively infects monocytes/macrophages, the full range of susceptible cell types and the mechanisms of immune cell invasion remain poorly defined. Here, we applied single-cell RNA sequencing, multiplex immunofluorescence, and in situ hybridization to mesenteric lymph node aspirates and formalin fixed and paraffin embedded lymph node tissues from cats with naturally occurring effusive FIP. We identified FIPV RNA and nucleocapsid protein in T and B lymphocytes and myeloid cells, and subgenomic viral RNA in T cells, demonstrating cell entry and viral genomic replication across multiple immune compartments. Rare FIPV RNA-positive lymphocytes persisted after antiviral treatment cessation and resolution of clinical signs. These findings revise current models of FIPV pathogenesis and reveal new insights into coronavirus-driven immune dysregulation, viral persistence, and relapse. Our study highlights the utility of FIP as a naturally occurring animal model for exploring adaptive immune cell infection in coronavirus diseases, providing a translational platform for understanding virus-host interactions that drive chronic or relapsing immunopathology.