Fig. 5.

Proposed model for IRN cells expressing TMPRSS2 for the recovery of serotype I FIPV from clinical samples. Serotype I FIPV entry likely occurs within endosomes via cathepsin [cyan] cleavage of the CoV spike at S2′ or at the cell surface via protease cleavage of S1/S2 following S engagement of the natural receptor (black) (top). The proposed models for tissue culture-adapted FCoV entry and recovery of serotype I FIPV from clinical samples are depicted in the lower panel. Tissue culture-adapted strains may express spikes (violet) that lose S1/S2 cleavage in favor of heparin (magenta) binding (bottom left). Expressing host factors, such as TMPRSS2 (green), may promote natural receptor usage and cell-surface entry, aiding in recovery of serotype I FCoV strains from clinical samples (bottom right).