Absence of antibody-dependent, complement-mediated lysis of feline infectious peritonitis virus-infected cells

Intestinal injury and vasculitis biomarkers in cats with feline enteric coronavirus and effusive feline infectious peritonitis

OBJECTIVE

To investigate intestinal injury, repair and vasculitis biomarkers that may illuminate the progression and/or pathogenesis of feline infectious peritonitis (FIP) or feline enteric coronavirus (FECV) infection.

MATERIALS AND METHODS

A total of 40 cats with effusive FIP (30 with abdominal effusion, AE group; 10 with thoracic effusion, TE group) and 10 asymptomatic but FECV positive cats (FECV group), all were confirmed by reverse transcription polymerase chain reaction either in faeces or effusion samples. Physical examinations and effusion tests were performed. Trefoil factor-3 (TFF-3), intestinal alkaline phosphatase (IAP), intestinal fatty acid binding protein (I-FABP), myeloperoxidase-anti-neutrophilic cytoplasmic antibody (MPO-ANCA) and proteinase 3-ANCA (PR3-ANCA) concentrations were measured both in serum and effusion samples.

RESULTS

Rectal temperature and respiratory rate were highest in the TE group (p < 0.000). Effusion white blood cell count was higher in the AE group than TE group (p < 0.042). Serum TFF-3, IAP and I-FABP concentrations were higher in cats with effusive FIP than the cats with FECV (p < 0.05). Compared with the AE group, TE group had lower effusion MPO-ANCA (p < 0.036), higher IAP (p < 0.050) and higher TFF-3 (p < 0.016) concentrations.

CLINICAL SIGNIFICANCE

Markers of intestinal and epithelial surface injury were higher in cats with effusive FIP than those with FECV. Compared to cats with abdominal effusions, markers of apoptosis inhibition and immunostimulation to the injured epithelium were more potent in cats with thoracic effusion, suggesting the possibility of a poorer prognosis or more advanced disease in these patients.

Complement and COVID-19: Three years on, what we know, what we don't know, and what we ought to know

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus was identified in China in 2019 as the causative agent of COVID-19, and quickly spread throughout the world, causing over 7 million deaths, of which 2 million occurred prior to the introduction of the first vaccine. In the following discussion, while recognising that complement is just one of many players in COVID-19, we focus on the relationship between complement and COVID-19 disease, with limited digression into directly-related areas such as the relationship between complement, kinin release, and coagulation. Prior to the 2019 COVID-19 outbreak, an important role for complement in coronavirus diseases had been established. Subsequently, multiple investigations of patients with COVID-19 confirmed that complement dysregulation is likely to be a major driver of disease pathology, in some, if not all, patients. These data fuelled evaluation of many complement-directed therapeutic agents in small patient cohorts, with claims of significant beneficial effect. As yet, these early results have not been reflected in larger clinical trials, posing questions such as who to treat, appropriate time to treat, duration of treatment, and optimal target for treatment. While significant control of the pandemic has been achieved through a global scientific and medical effort to comprehend the etiology of the disease, through extensive SARS-CoV-2 testing and quarantine measures, through vaccine development, and through improved therapy, possibly aided by attenuation of the dominant strains, it is not yet over. In this review, we summarise complement-relevant literature, emphasise its main conclusions, and formulate a hypothesis for complement involvement in COVID-19. Based on this we make suggestions as to how any future outbreak might be better managed in order to minimise impact on patients.

Characterization of peritoneal cells from cats with experimentally-induced feline infectious peritonitis (FIP) using RNA-seq

Laboratory cats were infected with a serotype I cat-passaged field strain of FIP virus (FIPV) and peritoneal cells harvested 2-3 weeks later at onset of lymphopenia, fever and serositis. Comparison peritoneal cells were collected from four healthy laboratory cats by peritoneal lavage and macrophages predominated in both populations. Differential mRNA expression analysis identified 5621 genes as deregulated in peritoneal cells from FIPV infected versus normal cats; 956 genes showed > 2.0 Log2 Fold Change (Log2FC) and 1589 genes showed < -2.0 Log2FC. Eighteen significantly upregulated pathways were identified by InnateDB enrichment analysis. These pathways involved apoptosis, cytokine-cytokine receptor interaction, pathogen recognition, Jak-STAT signaling, NK cell mediated cytotoxicity, several chronic infectious diseases, graft versus host disease, allograft rejection and certain autoimmune disorders. Infected peritoneal macrophages were activated M1 type based on pattern of RNA expression. Apoptosis was found to involve large virus-laden peritoneal macrophages more than less mature macrophages, suggesting that macrophage death played a role in virus dissemination. Gene transcripts for MHC I but not II receptors were upregulated, while mRNA for receptors commonly associated with virus attachment and identified in other coronaviruses were either not detected (APN, L-SIGN), not deregulated (DDP-4) or down-regulated (DC-SIGN). However, the mRNA for FcγRIIIA (CD16A/ADCC receptor) was significantly upregulated, supporting entry of virus as an immune complex. Analysis of KEGG associated gene transcripts indicated that Th1 polarization overshadowed Th2 polarization, but the addition of relevant B cell associated genes previously linked to FIP macrophages tended to alter this perception.

Feline infectious peritonitis: still an enigma?

Feline infectious peritonitis (FIP) is one of the most important fatal infectious diseases of cats, the pathogenesis of which has not yet been fully revealed. The present review focuses on the biology of feline coronavirus (FCoV) infection and the pathogenesis and pathological features of FIP. Recent studies have revealed functions of many viral proteins, differing receptor specificity for type I and type II FCoV, and genomic differences between feline enteric coronaviruses (FECVs) and FIP viruses (FIPVs). FECV and FIP also exhibit functional differences, since FECVs replicate mainly in intestinal epithelium and are shed in feces, and FIPVs replicate efficiently in monocytes and induce systemic disease. Thus, key events in the pathogenesis of FIP are systemic infection with FIPV, effective and sustainable viral replication in monocytes, and activation of infected monocytes. The host's genetics and immune system also play important roles. It is the activation of monocytes and macrophages that directly leads to the pathologic features of FIP, including vasculitis, body cavity effusions, and fibrinous and granulomatous inflammatory lesions. Advances have been made in the clinical diagnosis of FIP, based on the clinical pathologic findings, serologic testing, and detection of virus using molecular (polymerase chain reaction) or antibody-based methods. Nevertheless, the clinical diagnosis remains challenging in particular in the dry form of FIP, which is partly due to the incomplete understanding of infection biology and pathogenesis in FIP. So, while much progress has been made, many aspects of FIP pathogenesis still remain an enigma.

An update on feline infectious peritonitis: virology and immunopathogenesis

Feline infectious peritonitis (FIP) continues to be one of the most researched infectious diseases of cats. The relatively high mortality of FIP, especially for younger cats from catteries and shelters, should be reason enough to stimulate such intense interest. However, it is the complexity of the disease and the grudging manner in which it yields its secrets that most fascinate researchers. Feline leukemia virus infection was conquered in less than two decades and the mysteries of feline immunodeficiency virus were largely unraveled in several years. After a half century, FIP remains one of the last important infections of cats for which we have no single diagnostic test, no vaccine and no definitive explanations for how virus and host interact to cause disease. How can a ubiquitous and largely non-pathogenic enteric coronavirus transform into a highly lethal pathogen? What are the interactions between host and virus that determine both disease form (wet or dry) and outcome (death or resistance)? Why is it so difficult, and perhaps impossible, to develop a vaccine for FIP? What role do genetics play in disease susceptibility? This review will explore research conducted over the last 5 years that attempts to answer these and other questions. Although much has been learned about FIP in the last 5 years, the ultimate answers remain for yet more studies.

ORF7-encoded accessory protein 7a of feline infectious peritonitis virus as a counteragent against IFN-α-induced antiviral response

The type I IFN-mediated immune response is the first line of antiviral defence. Coronaviruses, like many other viruses, have evolved mechanisms to evade this innate response, ensuring their survival. Several coronavirus accessory genes play a central role in these pathways, but for feline coronaviruses this has never to our knowledge been studied. As it has been demonstrated previously that ORF7 is essential for efficient replication in vitro and virulence in vivo of feline infectious peritonitis virus (FIPV), the role of this ORF in the evasion of the IFN-α antiviral response was investigated. Deletion of ORF7 from FIPV strain 79-1146 (FIPV-Δ7) rendered the virus more susceptible to IFN-α treatment. Given that ORF7 encodes two proteins, 7a and 7b, it was further explored which of these proteins is active in this mechanism. Providing 7a protein in trans rescued the mutant FIPV-Δ7 from IFN sensitivity, which was not achieved by addition of 7b protein. Nevertheless, addition of protein 7a to FIPV-Δ3Δ7, a FIPV mutant deleted in both ORF3 and ORF7, could no longer increase the replication capacity of this mutant in the presence of IFN. These results indicate that FIPV 7a protein is a type I IFN antagonist and protects the virus from the antiviral state induced by IFN, but it needs the presence of ORF3-encoded proteins to exert its antagonistic function.

Quantitative analysis of an anti-viral immune escape compound ML-7 in feline plasma using ultra performance liquid chromatography/electrospray ionization mass spectrometry

An analytical method for the quantitative measurement of ML-7, a product with possible anti-immune escape activity for feline infectious peritonitis virus (FIPV), in feline plasma was developed and validated. The sample preparation consists of a solid-phase extraction step on an MCX cartridge. ML-7 and ML-9, used as the internal standard for the analysis, were separated on an ACQUITY UPLC™ BEH C(18) reversed-phase column (1.7 μm, 50 mm × 2.1 mm I.D.), using isocratic elution with acetonitrile and 0.1% formic acid in water as the mobile phase. Both compounds were subsequently quantified in MRM mode on a Micromass(®) Quattro Premier™ XE triple quadrupole mass spectrometer. The use of a Thermo Scientific(®) Exactive™ orbitrap mass spectrometer made it possible to confirm the proposed fragmentation pattern of both ML-7 and ML-9. A validation study according to EC requirements was carried out, in which the method showed good performance. Linear behaviour was observed in the 1-2500 ng ml(-1) range, which is relevant for real sample analysis. Accuracy and precision were within the criteria requested by the EC requirements throughout this concentration range. Extraction recovery of ML-7 was 72%. Matrix effect for ML-7 was not higher than 8%. The method was successfully used for the monitoring of ML-7 in feline plasma after intravenous, subcutaneous or oral administration of an ML-7 formulation, for the determination of pharmacokinetic parameters, with a limit of quantification of 1 ng ml(-1) and a limit of detection of 0.4 ng ml(-1). The proposed method also shows good characteristics for the analysis of ML-7 in plasma of other animal species and human plasma.