Characterizing replication kinetics and plaque production of type I feline infectious peritonitis virus in three feline cell lines

Nanoparticle ocular immunotherapy for herpesvirus surface eye infections evaluated in cat infection model

Ocular herpes simplex type 1 (HSV-1) infections can trigger conjunctivitis, keratitis, uveitis, and occasionally retinitis, and is a major cause of blindness worldwide. The infections are lifelong and can often recrudesce during periods of stress or immune suppression. Currently HSV-1 infections of the eye are managed primarily with anti-viral eye drops, which require frequent administration, can cause irritation, and may take weeks for full resolution of symptoms. We therefore evaluated the effectiveness of an ocular immune activating nanoparticle eye drop as a novel approach to treating HSV-1 infection, using a cat feline herpesvirus -1 (FHV-1) ocular infection model. In vitro studies demonstrated significant induction of both type I and II interferon responses by the liposome-dual TLR 3/9 agonist nanoparticles, along with suppression of FHV-1 replication. In cats with naturally occurring eye infections either proven or suspected to involve FHV-1, ocular nanoparticle treated animals experienced resolution of signs within several days of treatment, including resolution of keratitis and corneal ulcers. In a cat model of recrudescent FHV-1 infection, cats treated twice daily with immune nanoparticle eye drops experienced significant lessening of ocular signs of infection and significantly fewer episodes of viral shedding compared to control cats. Treatment was well-tolerated by all cats, without signs of drug-induced ocular irritation. We concluded therefore that non-specific ocular immunotherapy offers significant promise as a novel approach to treatment of HSV-1 and FHV-1 ocular infections.

An Optimized Bioassay for Screening Combined Anticoronaviral Compounds for Efficacy against Feline Infectious Peritonitis Virus with Pharmacokinetic Analyses of GS-441524, Remdesivir, and Molnupiravir in Cats

Feline infectious peritonitis (FIP) is a fatal disease of cats that currently lacks licensed and affordable vaccines or antiviral therapeutics. The disease has a spectrum of clinical presentations including an effusive ("wet") form and non-effusive ("dry") form, both of which may be complicated by neurologic or ocular involvement. The feline coronavirus (FCoV) biotype, termed feline infectious peritonitis virus (FIPV), is the etiologic agent of FIP. The objective of this study was to determine and compare the in vitro antiviral efficacies of the viral protease inhibitors GC376 and nirmatrelvir and the nucleoside analogs remdesivir (RDV), GS-441524, molnupiravir (MPV; EIDD-2801), and β-D-N⁴-hydroxycytidine (NHC; EIDD-1931). These antiviral agents were functionally evaluated using an optimized in vitro bioassay system. Antivirals were assessed as monotherapies against FIPV serotypes I and II and as combined anticoronaviral therapies (CACT) against FIPV serotype II, which provided evidence for synergy for selected combinations. We also determined the pharmacokinetic properties of MPV, GS-441524, and RDV after oral administration to cats in vivo as well as after intravenous administration of RDV. We established that orally administered MPV at 10 mg/kg, GS-441524 and RDV at 25 mg/kg, and intravenously administered RDV at 7 mg/kg achieves plasma levels greater than the established corresponding EC₅₀ values, which are sustained over 24 h for GS-441514 and RDV.

Second round of an interlaboratory comparison of SARS-CoV2 molecular detection assays used by 45 veterinary diagnostic laboratories in the United States

The COVID-19 pandemic presents a continued public health challenge. Veterinary diagnostic laboratories in the United States use RT-rtPCR for animal testing, and many laboratories are certified for testing human samples; hence, ensuring that laboratories have sensitive and specific SARS-CoV2 testing methods is a critical component of the pandemic response. In 2020, the FDA Veterinary Laboratory Investigation and Response Network (Vet-LIRN) led an interlaboratory comparison (ILC1) to help laboratories evaluate their existing RT-rtPCR methods for detecting SARS-CoV2. All participating laboratories were able to detect the viral RNA spiked in buffer and PrimeStore molecular transport medium (MTM). With ILC2, Vet-LIRN extended ILC1 by evaluating analytical sensitivity and specificity of the methods used by participating laboratories to detect 3 SARS-CoV2 variants (B.1; B.1.1.7 [Alpha]; B.1.351 [Beta]) at various copy levels. We analyzed 57 sets of results from 45 laboratories qualitatively and quantitatively according to the principles of ISO 16140-2:2016. More than 95% of analysts detected the SARS-CoV2 RNA in MTM at ≥500 copies for all 3 variants. In addition, for nucleocapsid markers N1 and N2, 81% and 92% of the analysts detected ≤20 copies in the assays, respectively. The analytical specificity of the evaluated methods was >99%. Participating laboratories were able to assess their current method performance, identify possible limitations, and recognize method strengths as part of a continuous learning environment to support the critical need for the reliable diagnosis of COVID-19 in potentially infected animals and humans.

Serotype I and II Feline Coronavirus Replication and Gene Expression Patterns of Feline Cells-Building a Better Understanding of Serotype I FIPV Biology

Feline infectious peritonitis (FIP) is a disease of domestic cats caused by the genetic variant of the feline coronavirus (FCoV) and feline infectious peritonitis virus (FIPV), currently grouped into two serotypes, I and II. Although serotype I FIPV is more prevalent in cats with FIP, serotype II has been more extensively studied in vitro due to the relative ease in propagating this viral serotype in culture systems. As a result, more is known about serotype II FIPV than the more biologically prevalent serotype I. The primary cell receptor for serotype II has been determined, while it remains unknown for serotype I. The recent development of a culture-adapted feline cell line that more effectively propagates serotype I FIPV, FCWF-4 CU, derived from FCWF-4 cells available through the ATCC, offers the potential for an improved understanding of serotype I FIPV biology. To learn more about FIPV receptor biology, we determined targeted gene expression patterns in feline cells variably permissive to replication of serotype I or II FIPV. We utilized normal feline tissues to determine the immunohistochemical expression patterns of two known coronavirus receptors, ACE2 and DC-SIGN. Lastly, we compared the global transcriptomes of the two closely related FCWF-4 cell lines and identified viral transcripts with potential importance for the differential replication kinetics of serotype I FIPV.

Coronaviruses Associated with the Superfamily Musteloidea

Among the animal superfamily Musteloidea, which includes those commonly known as mustelids, naturally occurring and species-specific alphacoronavirus infections have been observed in both mink (Mustela vison/Neovison vison) and domestic ferrets (Mustela putorius furo). Ferret systemic coronavirus (FRSCV), in particular, has been associated with a rare but fatal systemic disease. In recent months, it has become apparent that both minks and ferrets are susceptible to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a betacoronavirus and the cause of the coronavirus disease 2019 (COVID-19) pandemic. Several mink farms have experienced SARS-CoV-2 outbreaks, and experimental models have demonstrated susceptibility of ferrets to SARS-CoV-2. The potential for pet ferrets to become infected with SARS-CoV-2, however, remains elusive. During the 2002-2003 SARS epidemic, it was also apparent that ferrets were susceptible to SARS-CoV and could be utilized in vaccine development. From a comparative standpoint, understanding the relationships between different infections and disease pathogenesis in the animal superfamily Musteloidea may help elucidate viral infection and transmission mechanisms, as well as treatment and prevention strategies for coronaviruses.

Development of Feline Ileum- and Colon-Derived Organoids and Their Potential Use to Support Feline Coronavirus Infection

Feline coronaviruses (FCoVs) infect both wild and domestic cat populations world-wide. FCoVs present as two main biotypes: the mild feline enteric coronavirus (FECV) and the fatal feline infectious peritonitis virus (FIPV). FIPV develops through mutations from FECV during a persistence infection. So far, the molecular mechanism of FECV-persistence and contributing factors for FIPV development may not be studied, since field FECV isolates do not grow in available cell culture models. In this work, we aimed at establishing feline ileum and colon organoids that allow the propagation of field FECVs. We have determined the best methods to isolate, culture and passage feline ileum and colon organoids. Importantly, we have demonstrated using GFP-expressing recombinant field FECV that colon organoids are able to support infection of FECV, which were unable to infect traditional feline cell culture models. These organoids in combination with recombinant FECVs can now open the door to unravel the molecular mechanisms by which FECV can persist in the gut for a longer period of time and how transition to FIPV is achieved.

MicroRNA expression profiling of caudal fin cell of C. auratus gibelio upon cyprinid herpesvirus 2 infection

As a member of the genus Cyprinivirus in the family Alloherpesviridae, Cyprinid herpesvirus 2 (CyHV-2) has caused great economic loss in the aquaculture industry, mainly in C. auratus gibelio and goldfish. However, the molecular mechanisms underlying the pathogenicity of CyHV-2 remain elusive. In this study, high-throughput sequencing technology was employed to explore the miRNA expression profiles of C. auratus gibelio (GiCF) caudal fin cells in response to Cyprinid Herpesvirus-2 (CyHV-2) infection. A total of 631 novel miRNAs and 409 known miRNAs were identified. The expression levels of 7 miRNAs were found as significantly modulated (5 down-regulation and 2 up-regulation; P < 0.01, |logFC|>1, TPM>10) in CyHV-2 infected cells. 7 miRNA and their potential mRNA targets were validated by Real-time PCR (qRT-PCR), respectively. Targets prediction and functional analysis of these 7 miRNAs revealed significant enrichment for several signaling pathways, including PPAR, p53 and FoxO pathways. These studies provided more valuable basis for further study on the roles of miRNAs in CyHV-2 replication and pathogenesis.

A Tale of Two Viruses: The Distinct Spike Glycoproteins of Feline Coronaviruses

Feline coronavirus (FCoV) is a complex viral agent that causes a variety of clinical manifestations in cats, commonly known as feline infectious peritonitis (FIP). It is recognized that FCoV can occur in two different serotypes. However, differences in the S protein are much more than serological or antigenic variants, resulting in the effective presence of two distinct viruses. Here, we review the distinct differences in the S proteins of these viruses, which are likely to translate into distinct biological outcomes. We introduce a new concept related to the non-taxonomical classification and differentiation among FCoVs by analyzing and comparing the genetic, structural, and functional characteristics of FCoV and the FCoV S protein among the two serotypes and FCoV biotypes. Based on our analysis, we suggest that our understanding of FIP needs to consider whether the presence of these two distinct viruses has implications in clinical settings.

Endocytic Pathway of Feline Coronavirus for Cell Entry: Differences in Serotype-Dependent Viral Entry Pathway

Feline coronavirus (FCoV) is a pathogen causing a lethal infectious disease in cats, feline infectious peritonitis. It has two serotypes (type I FCoV and type II FCoV). According to our previous study, type I FCoV infection is inhibited by compounds inducing intracellular cholesterol accumulation, whereas type II FCoV infection is not inhibited. Intracellular cholesterol accumulation was reported to disrupt late endosome function. Based on these findings, types I and II FCoV are considered to enter the cytosol through late and early endosomes, respectively. We investigated whether the antiviral activities of a late endosome trafficking inhibitor and cholesterol-accumulating agents are different between the FCoV serotypes. The late endosome trafficking inhibitor did not inhibit type II FCoV infection, but it inhibited type I FCoV infection. Type I FCoV infection was inhibited by cholesterol-accumulating triazoles, but not by non-cholesterol-accumulating triazoles. These phenomena were observed in both feline cell lines and feline primary macrophages. This study provides additional information on the differences in intracellular reproductive cycle between type I and type II FCoV.

Entry of Scotophilus Bat Coronavirus-512 and Severe Acute Respiratory Syndrome Coronavirus in Human and Multiple Animal Cells

Bats are natural reservoirs of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome CoV (MERS-CoV). Scotophilus bat CoV-512 demonstrates potential for cross-species transmission because its viral RNA and specific antibodies have been detected in three bat species of Taiwan. Understanding the cell tropism of Scotophilus bat CoV-512 is the first step for studying the mechanism of cross-species transmission. In this study, a lentivirus-based pseudovirus was produced using the spike (S) protein of Scotophilus bat CoV-512 or SARS-CoV as a surface protein to test the interaction between coronaviral S protein and its cell receptor on 11 different cells. Susceptible cells expressed red fluorescence protein (RFP) after the entry of RFP-bound green fluorescence protein (GFP)-fused S protein of Scotophilus bat CoV-512 (RFP-Sco-S-eGFP) or RFP-SARS-S pseudovirus, and firefly luciferase (FLuc) activity expressed by cells infected with FLuc-Sco-S-eGFP or FLuc-SARS-S pseudovirus was quantified. Scotophilus bat CoV-512 pseudovirus had significantly higher entry efficiencies in Madin Darby dog kidney epithelial cells (MDCK), black flying fox brain cells (Pabr), and rat small intestine epithelial cells (IEC-6). SARS-CoV pseudovirus had significantly higher entry efficiencies in human embryonic kidney epithelial cells (HEK-293T), pig kidney epithelial cells (PK15), and MDCK cells. These findings demonstrated that Scotophilus bat CoV-512 had a broad host range for cross-species transmission like SARS-CoV.

Generating and evaluating type I interferon receptor-deficient and feline TMPRSS2-expressing cells for propagating serotype I feline infectious peritonitis virus

Feline coronavirus infection can progress to a fatal infectious peritonitis, which is a widespread feline disease without an effective vaccine. Generating feline cells with reduced ability to respond to interferon (IFN) is an essential step facilitating isolation of new candidate vaccine strains. Here, we describe the use of Crispr/Cas technology to disrupt type I IFN signaling in two feline cell lines, AK-D and Fcwf-4 CU, and evaluate the replication kinetics of a serotype I feline infectious peritonitis virus (FIPV) within these cells. We report that polyclonal cell populations and a clonal isolate, termed Fcwf-4 IRN, exhibited significantly diminished IFN-responsiveness and allowed FIPV replication kinetics comparable to parental cells. Furthermore, we demonstrate that replication of FIPV is enhanced by ectopic expression of a host serine protease, TMPRSS2, in these cells. We discuss the potential of these cells for isolating new clinical strains and for propagating candidate vaccine strains of FIPV.