Feline infectious peritonitis (FIP) is an enigmatic, intractable fatal illness of cats, where many aspects of its diagnosis, treatment and prevention continue to elude scientists. It is generally accepted that FIP originates from a mutation of the ubiquitous feline enteric coronavirus (FECV). FECV is mucosally transmitted, replicates in the intestinal tract, and most commonly persists in the colonic epithelium. Some cats are able to control FECV replication and eliminate the infection, while others remain persistently infected and/or intermittent shedders. It has been experimentally demonstrated that reinfection with FECV can occur. If FECV mutation occurs allowing immune evasion and systemic spread, fatal FIP can develop.
The FIP-viral mutant originating from FECV acquires expanded host cell range and capability to replicate in monocytes and macrophages, thereby largely evading the host’s immune response. FIP presents a unique quandary: how does a relatively innocuous enteric pathogen (FECV) evolve into a systemic pathogen inducing a destructive immunologic storm driven by lymphocyte apoptosis, antibody complex deposition and granulomatous inflammation?
The objective of the present study was to characterize the mucosal immune response that controls and eliminates FECV. To date, few attempts have been made to control FECV – the source of the FIP-causing virus. The authors posit that characterizing the mucosal immune response could aid in developing strategies for controlling FECV and thereby aiding in the control of FIPV. The current investigation was designed as an experimental study.
Thirty-three domestic shorthair (DSH) cats from a closed, specific pathogen-free breeding colony at the Colorado State University were involved in the present study. Cats ranged from 1-9 years of age, and included 22 intact females, 10 neutered males and 1 intact male. The colony is known to have circulating FEC. The genome of the circulating virus was sequenced and determined to be serotype 1.
Blood, fecal, and colonic biopsy samples were collected to assess mucosal and systemic immunologic and virologic profile. For the purposes of the investigation, the population was divided in to three groups: naive/immunologically quiescent cats (n=18, seronegative and virus negative throughout study), convalescent cats (n=6, seropositive at the time of colonoscopy but never positive for virus in feces or colon mucosa), and actively infected cats (n=9, seropositive and positive for virus in feces). Samples were collected at 7 timepoints, 2 months apart each. Histology and immunohistochemistry (IHC) were performed on all biopsy samples. Flow cytometry was performed for cell immunophenotyping. Specific IgA and IgG measurements were performed, and viral RNA was extracted from both fecal sample and colonic biopsy samples. Extracted RNA underwent real-time quantitative polymerase chain reaction (RT-PCR) analysis, and interferon gamma, interleukin 17 (IL17) and Fox Protein 3 (FoxP3) were measured and quantified.
Cats with active FECV infections showed strong mucosal IgA and sytemic IgG responses that waned after viral clearance. Significant virus-specific mucosal T-cell interferon gamma responses were not detected in any group, indicating minimal cell-mediated response. A shift toward an inflammatory state was detected, as suggested by an increase in IL17 and FoxP3 expression in infected cats. No histologic abnormalities were detected in any cats, and perturbations in the colon (the primary reservoir organ) were minimal. No shifts in lymphocyte subpopulation proliferation were observed. The average time of observed shedding to controlled viral replication was 2.6 months, and the average time to seropositivity after virus replication began was 2.5 months.
The authors speculate that a reasonable approach to FIPV emergence would involve understanding and controlling FECV replication. The results of the present study suggest that FECV replication and control are mediated by humoral mucosal and systemic responses, and these may be important in limiting viral replication. Data suggest that T-cell-mediated control of FECV infection is unlikely to play a role in the colonic mucosa. The authors further theorize that control of FECV at intestinal mucosa might be achievable by a vaccine-induced humoral response, with the goal of disrupting the endemic persistence of FECV in cat populations, thereby reducing the chance of FIPV emergence in individuals.
Few potential limitations of the present study exist. For example, colonic inflammation can be multifocal, and it is possible that the colonic biopsies obtained in the present study were not representative of inflammatory states possibly triggered by viral replication elsewhere in the colon. Also, though unlikely, it is possible that other FECV serotypes may have induced slightly different responses.
Despite few possible limitations, the authors were able to characterize, for the first time, the mucosal humoral and cellular responses that are necessary for FECV control given the lack of demonstrable cell-mediated immune responses. Further work is required to elucidate viral targets and antibody effector functions needed to produce effective vaccines against FECV. (HM)