Feline coronavirus replication is affected by both cyclophilin A and cyclophilin B

Broad-spectrum antiviral strategy: Host-targeting antivirals against emerging and re-emerging viruses

Viral infections are amongst the most prevalent diseases that pose a significant threat to human health. Targeting viral proteins or host factors represents two primary strategies for the development of antiviral drugs. In contrast to virus-targeting antivirals (VTAs), host-targeting antivirals (HTAs) offer advantages in terms of overcoming drug resistance and effectively combating a wide range of viruses, including newly emerging ones. Therefore, targeting host factors emerges as an extremely promising strategy with the potential to address critical challenges faced by VTAs. In recent years, extensive research has been conducted on the discovery and development of HTAs, leading to the approval of maraviroc, a chemokine receptor type 5 (CCR5) antagonist used for the treatment of HIV-1 infected individuals, with several other potential treatments in various stages of development for different viral infections. This review systematically summarizes advancements made in medicinal chemistry regarding various host targets and classifies them into four distinct catagories based on their involvement in the viral life cycle: virus attachment and entry, biosynthesis, nuclear import and export, and viral release.

Immunoinformatics-aided rational design of a multi-epitope vaccine targeting feline infectious peritonitis virus

Feline infectious peritonitis (FIP) is a grave and frequently lethal ailment instigated by feline coronavirus (FCoV) in wild and domestic feline species. The spike (S) protein of FCoV assumes a critical function in viral ingress and infection, thereby presenting a promising avenue for the development of a vaccine. In this investigation, an immunoinformatics approach was employed to ascertain immunogenic epitopes within the S-protein of FIP and formulate an innovative vaccine candidate. By subjecting the amino acid sequence of the FIP S-protein to computational scrutiny, MHC-I binding T-cell epitopes were predicted, which were subsequently evaluated for their antigenicity, toxicity, and allergenicity through in silico tools. Our analyses yielded the identification of 11 potential epitopes capable of provoking a robust immune response against FIPV. Additionally, molecular docking analysis demonstrated the ability of these epitopes to bind with feline MHC class I molecules. Through the utilization of suitable linkers, these epitopes, along with adjuvants, were integrated to design a multi-epitope vaccine candidate. Furthermore, the stability of the interaction between the vaccine candidate and feline Toll-like receptor 4 (TLR4) was established via molecular docking and molecular dynamics simulation analyses. This suggests good prospects for future experimental validation to ascertain the efficacy of our vaccine candidate in inducing a protective immune response against FIP.

Targeting Cyclophilin A and CD147 to Inhibit Replication of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) and SARS-CoV-2-Induced Inflammation

Identification and development of effective therapeutics for coronavirus disease 2019 (COVID-19) are still urgently needed. The CD147-spike interaction is involved in the severe acute respiratory syndrome coronavirus (SARS-CoV)-2 invasion process in addition to angiotensin-converting enzyme 2 (ACE2). Cyclophilin A (CyPA), the extracellular ligand of CD147, has been found to play a role in the infection and replication of coronaviruses. In this study, our results show that CyPA inhibitors such as cyclosporine A (CsA) and STG-175 can suppress the intracellular replication of SARS-CoV-2 by inhibiting the binding of CyPA to the SARS-CoV-2 nucleocapsid C-terminal domain (N-CTD), and the IC50 is 0.23 μM and 0.17 μM, respectively. Due to high homology, CsA also had inhibitory effects on SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV), and the IC50 is 3.2 μM and 2.8 μM, respectively. Finally, we generated a formulation of phosphatidylserine (PS)-liposome-CsA for pulmonary drug delivery. These findings provide a scientific basis for identifying CyPA as a potential drug target for the treatment of COVID-19 as well as for the development of broad-spectrum inhibitors for coronavirus via targeting CyPA. Highlights: 1) SARS-CoV-2 infects cells via the binding of its S protein and CD147; 2) binding of SARS-CoV-2 N protein and CyPA is essential for viral replication; 3) CD147 and CyPA are potential therapeutic targets for SARS-CoV-2; and 4) CsA is a potential therapeutic strategy by interrupting CD147/CyPA interactions. SIGNIFICANCE STATEMENT: New severe acute respiratory syndrome coronavirus (SARS-CoV)-2 variants and other pathogenic coronaviruses (CoVs) are continually emerging, and new broad-spectrum anti-CoV therapy is urgently needed. We found that binding sites of cyclophilin A/cyclosporin A (CyPA/CsA) overlap with CyPA/N-CTD (nucleocapsid C-terminal domain), which shows the potential to target CyPA during SARS-CoV-2 infection. Here, we provide new evidence for targeting CyPA in the treatment of coronavirus disease 2019 (COVID-19) as well as the potential of developing CyPA inhibitors for broad-spectrum inhibition of CoVs.

Cyclophilin A-mediated mitigation of coronavirus SARS-CoV-2

Human cyclophilin A (hCypA) is important for the replication of multiple coronaviruses (CoVs), and cyclosporine A inhibitors can suppress CoVs. The emergence of rapidly spreading severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants has sparked concerns that mutations affect the binding ability of the spike (S) protein to the angiotensin-converting enzyme 2 (ACE2) cell receptor, affecting the severity of coronavirus disease (COVID-19). Far-western blotting and surface plasmon resonance (SPR) results revealed that hCypA interacts strongly with the viral SARS-CoV-2 receptor-binding domain (RBD), with a binding affinity of 6.85 × 10-8 M. The molecular interaction between hCypA and the viral protein interface was shown using three-dimensional structural analysis, which revealed the blocking of key residues on the RBD interface by hCypA. The RBD facilitates binding to the ACE2 receptor. The hCypA-S protein complex suppressed the binding of RBD to the ACE2 receptor, which a required event for CoV entry into the host cell. The reliability of this postulated blocking mechanism of the hCypA-SARS-CoV2 RBD complex with ACE was confirmed by SPR and molecular interaction lateral flow (MILF) strip assay, which offers the immunochromatographic signal read-outs. The emergence of new SARS-CoV-2 variants with key mutations in RBD had a negligible effect on the binding of the RBD variants to hCypA, indicating an effective mitigation strategy for SARS-CoV-2 variants. The MILF strip assay results also highlight the neutralizing effect of hCypA by effectively blocking RBD (wild type and its variants) from binding ACE2. Given the importance of hCypA in viral entry regulation, it has the potential to be used as a target for antiviral therapy.

Ionophore Antibiotics Inhibit Type II Feline Coronavirus Proliferation In Vitro

Feline coronaviruses (FCoVs) infect cats worldwide and cause severe systemic diseases, such as feline infectious peritonitis (FIP). FIP has a high mortality rate, and drugs approved by the Food and Drug Administration have been ineffective for the treatment of FIP. Investigating host factors and the functions required for FCoV replication is necessary to develop effective drugs for the treatment of FIP. FCoV utilizes an endosomal trafficking system for cellular entry after binding between the viral spike (S) protein and its receptor. The cellular enzymes that cleave the S protein of FCoV to release the viral genome into the cytosol require an acidic pH optimized in the endosomes by regulating cellular ion concentrations. Ionophore antibiotics are compounds that form complexes with alkali ions to alter the endosomal pH conditions. This study shows that ionophore antibiotics, including valinomycin, salinomycin, and nigericin, inhibit FCoV proliferation in vitro in a dose-dependent manner. These results suggest that ionophore antibiotics should be investigated further as potential broad-spectrum anti-FCoV agents.

Cyclophilin A represses reactive oxygen species generation and death of hypoxic non-small-cell lung cancer cells by degrading thioredoxin-interacting protein

Cyclophilin A (cypA) is overexpressed in many types of carcinomas, including non-small-cell lung cancer (NSCLC). However, the effect of anoxia, a critical feature of the carcinoma cell microenvironment, on cypA expression in NSCLC is unknown. Here, formaldehyde-fixed and paraffin-embedded samples were collected from 60 subjects with NSCLC. The protein expression levels of cypA and hypoxia-inducible factor-1α (HIF-1α) were evaluated using immunohistochemistry. Kaplan-Meier analysis showed that subjects with high cypA expression had remarkably shorter progression-free survival than those with low cypA expression. Furthermore, cypA expression levels were significantly related to HIF-1α expression levels (Spearman's correlation=0.34, P<0.0001). To further assess the effect of cypA, an anoxic carcinoma cell model was established. CypA expression was remarkably upregulated in H1299 and A549 cell lines under hypoxic conditions. Overexpression of cypA restored hypoxia-impaired cell growth and prevented reactive oxygen species (ROS) production and cell death in hypoxic A549 and H1299 cells. However, these phenotypes were not altered by the inactive R55A mutant of cypA. Mechanistic studies demonstrated that cypA can bind to and degrade the tumor suppressor protein TXNIP in H1299 and A549 cells. Restored TXNIP expression in cypA-overexpressed and hypoxic NSCLC cells led to increased ROS levels and apoptotic cell numbers and decreased cell growth compared with cypA-overexpressed and hypoxic NSCLC cells. These findings indicate that anoxia results in an increase in cypA expression in NSCLC. Additionally, cypA served as an oncogene during hypoxia by interacting with TXNIP.

A review of COVID-19: Treatment strategies and CRISPR/Cas9 gene editing technology approaches to the coronavirus disease

The new coronavirus SARS-CoV-2 pandemic has put the world on lockdown for the first time in decades. This has wreaked havoc on the global economy, put additional burden on local and global public health resources, and, most importantly, jeopardised human health. CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats, and the CRISPR associated (Cas) protein (CRISPR/Cas) was identified to have structures in E. coli. The most modern of these systems is CRISPR/Cas. Editing the genomes of plants and animals took several years and cost hundreds of thousands of dollars until the CRISPR approach was discovered in 2012. As a result, CRISPR/Cas has piqued the scientific community's attention, particularly for disease diagnosis and treatment, because it is faster, less expensive, and more precise than previous genome editing technologies. Data from gene mutations in specific patients gathered using CRISPR/Cas can aid in the identification of the best treatment strategy for each patient, as well as other research domains such as coronavirus replication in cell culture, such as SARS-CoV2. The implications of the most prevalent driver mutations, on the other hand, are often unknown, making treatment interpretation difficult. For detecting a wide range of target genes, the CRISPR/Cas categories provide highly sensitive and selective tools. Genome-wide association studies are a relatively new strategy to discovering genes involved in human disease when it comes to the next steps in genomic research. Furthermore, CRISPR/Cas provides a method for modifying non-coding portions of the genome, which will help advance whole genome libraries by speeding up the analysis of these poorly defined parts of the genome.

Therapeutic Effects of Mutian® Xraphconn on 141 Client-Owned Cats with Feline Infectious Peritonitis Predicted by Total Bilirubin Levels

Feline infectious peritonitis (FIP) is a fatal disease caused by feline coronavirus or its variant, referred to as the FIP virus. Recently, favorable treatment outcomes of the anti-viral drug Mutian^® Xraphconn (Mutian X) were noted in cats with FIP. Thus, the therapeutic efficacy of Mutian X in cats with FIP must be explored, although the predictors of therapeutic success remain unknown. In the present study, we administered Mutian X to 141 pet cats with effusive FIP following initial veterinarian examinations. Of these, 116 cats survived but the remaining 25 died during treatment. Pre-treatment signalment, viral gene expression, and representative laboratory parameters for routine FIP diagnosis (i.e., hematocrit, albumin-to-globulin ratio, total bilirubin, serum amyloid-A, and α1-acid glycoprotein) were statistically compared between the survivor and non-survivor groups. The majority of these parameters, including hematocrit, albumin-to-globulin ratio, serum amyloid-A, α1-acid glycoprotein, and viral gene expression, were comparable between the two groups. Interestingly, however, total bilirubin levels in the survivor group were significantly lower than those in the non-survivor group (p < 0.0001). Furthermore, in almost all surviving cats with effusive FIP (96.6%, 28/29), the pre-treatment total bilirubin levels were below 0.5 mg/dL; however, the survival rate decreased drastically (14.3%, 1/7) when the pre-treatment total bilirubin levels exceeded 4.0 mg/dL. Thus, circulating total bilirubin levels may act as a prognostic risk factor for severe FIP and may serve as the predictor of the therapeutic efficacy of Mutian X against this fatal disease.

Current understanding on molecular drug targets and emerging treatment strategy for novel coronavirus-19

SARS-CoV-2 is an enveloped positive-sense RNA virus, contain crown-like spikes on its surface, exceptional of large RNA genome, and a special replication machinery. Common symptoms of SARS-CoV-2 include cough, common cold, fever, sore throat, and a variety of severe acute respiratory disease (SARD) such as pneumonia. SARS-CoV-2 infects epithelial cells, T-cells, macrophages, and dendritic cells and also influences the production and implantation of pro-inflammatory cytokines and chemokines. Repurposing of various drugs during this emergency condition can reduce the rate of mortality as well as time and cost. Two druggable protein and enzyme targets have been selected in this review article due to their crucial role in the viral life cycle. The eukaryotic translation initiation factor (eIF4A), cyclophilin, nucleocapsid protein, spike protein, Angiotensin-converting enzyme 2 (ACE2), 3-chymotrypsin-like cysteine protease (3CLpro), and RNA-dependent RNA polymerase (RdRp) play significant role in early and late phase of SARS-CoV-2 replication and translation. This review paper is based on the rationale of inhibiting of various SARS-CoV-2 proteins and enzymes as novel therapeutic approaches for the management and treatment of patients with SARS-CoV-2 infection. We also discussed the structural and functional relationship of different proteins and enzymes to develop therapeutic approaches for novel coronavirus SARS-CoV-2.

Role of host factors in SARS-CoV-2 entry

The zoonotic transmission of highly pathogenic coronaviruses into the human population is a pressing concern highlighted by the ongoing SARS-CoV-2 pandemic. Recent work has helped to illuminate much about the mechanisms of SARS-CoV-2 entry into the cell, which determines host- and tissue-specific tropism, pathogenicity, and zoonotic transmission. Here we discuss current findings on the factors governing SARS-CoV-2 entry. We first reviewed key features of the viral spike protein (S) mediating fusion of the viral envelope and host cell membrane through binding to the SARS-CoV-2 receptor, angiotensin-converting enzyme 2. We then examined the roles of host proteases including transmembrane protease serine 2 and cathepsins in processing S for virus entry and the impact of this processing on endosomal and plasma membrane virus entry routes. We further discussed recent work on several host cofactors that enhance SARS-CoV-2 entry including Neuropilin-1, CD147, phosphatidylserine receptors, heparan sulfate proteoglycans, sialic acids, and C-type lectins. Finally, we discussed two key host restriction factors, i.e., interferon-induced transmembrane proteins and lymphocyte antigen 6 complex locus E, which can disrupt SARS-CoV-2 entry. The features of SARS-CoV-2 are presented in the context of other human coronaviruses, highlighting unique aspects. In addition, we identify the gaps in understanding of SARS-CoV-2 entry that will need to be addressed by future studies.

Drug repurposing screens reveal cell-type-specific entry pathways and FDA-approved drugs active against SARS-Cov-2

There is an urgent need for antivirals to treat the newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). To identify new candidates, we screen a repurposing library of ∼3,000 drugs. Screening in Vero cells finds few antivirals, while screening in human Huh7.5 cells validates 23 diverse antiviral drugs. Extending our studies to lung epithelial cells, we find that there are major differences in drug sensitivity and entry pathways used by SARS-CoV-2 in these cells. Entry in lung epithelial Calu-3 cells is pH independent and requires TMPRSS2, while entry in Vero and Huh7.5 cells requires low pH and triggering by acid-dependent endosomal proteases. Moreover, we find nine drugs are antiviral in respiratory cells, seven of which have been used in humans, and three are US Food and Drug Administration (FDA) approved, including cyclosporine. We find that the antiviral activity of cyclosporine is targeting Cyclophilin rather than calcineurin, revealing essential host targets that have the potential for rapid clinical implementation.

COVID-19 Pandemic: Time to Revive the Cyclophilin Inhibitor Alisporivir

December 2019 saw the emergence of a new epidemic of pneumonia of varying severity, called COVID-19, caused by a newly identified coronavirus, SARS-CoV-2. No therapeutic option is available to treat this infection that has already killed more than 235,000 people worldwide. This Viewpoint summarizes the strong scientific arguments supporting the use of alisporivir, a non-immunosuppressive analogue of cyclosporine A with potent cyclophilin inhibition properties that has reached Phase 3 clinical development, for the treatment of COVID-19. They include the strong cyclophilin dependency of the lifecycle of many coronaviruses, including SARS-CoV and MERS-CoV, and preclinical data showing strong antiviral and cytoprotective properties of alisporivir in various models of coronavirus infection, including SARS-CoV-2. Alisporivir should be tested without delay on both virological and clinical endpoints in patients with or at-risk of severe forms of SARS-CoV-2 infection.

Drawing Comparisons between SARS-CoV-2 and the Animal Coronaviruses

The COVID-19 pandemic, caused by a novel zoonotic coronavirus (CoV), SARS-CoV-2, has infected 46,182 million people, resulting in 1,197,026 deaths (as of 1 November 2020), with devastating and far-reaching impacts on economies and societies worldwide. The complex origin, extended human-to-human transmission, pathogenesis, host immune responses, and various clinical presentations of SARS-CoV-2 have presented serious challenges in understanding and combating the pandemic situation. Human CoVs gained attention only after the SARS-CoV outbreak of 2002-2003. On the other hand, animal CoVs have been studied extensively for many decades, providing a plethora of important information on their genetic diversity, transmission, tissue tropism and pathology, host immunity, and therapeutic and prophylactic strategies, some of which have striking resemblance to those seen with SARS-CoV-2. Moreover, the evolution of human CoVs, including SARS-CoV-2, is intermingled with those of animal CoVs. In this comprehensive review, attempts have been made to compare the current knowledge on evolution, transmission, pathogenesis, immunopathology, therapeutics, and prophylaxis of SARS-CoV-2 with those of various animal CoVs. Information on animal CoVs might enhance our understanding of SARS-CoV-2, and accordingly, benefit the development of effective control and prevention strategies against COVID-19.

Cyclophilin A and CD147: novel therapeutic targets for the treatment of COVID-19

The outbreak of pneumonia caused by a new coronavirus (SARS-CoV-2) occurred in December 2019, and spread rapidly throughout the world. There have been other severe coronavirus outbreaks worldwide, namely, severe acute respiratory syndrome (SARS-CoV) and Middle East respiratory syndrome (MERS-CoV). Because the genetic diversity of coronaviruses renders the design of vaccines complicated, broad spectrum-anti-coronavirus drugs have become a critical approach to control the coronavirus epidemic. Cyclophilin A is an important protein needed for coronavirus replication, and its inhibitor cyclosporine A has the ability to suppress coronavirus on a broad spectrum. CD147-S protein was found to be one route by which SARS-CoV-2 invades host cells, while CD147 was found to play a functional role in facilitating the infection of host cells by SARS-CoV. The CyPA/CD147 interaction may play a critical role in the ability of the SARS-CoV-2 virus to enter the host cells. However, cyclosporine A has immunosuppressive effects, so the conditions for its use as an antiviral drug are limited. As a result, cyclosporine A analogues without immunosuppressive side effects have attracted lots of interest. This review primarily discusses the drug development prospects of cyclophilin A as a therapeutic target for the treatment of coronavirus infection, especially coronavirus disease 2019 (COVID-19), and non-immunosuppressive cyclosporine analogues.

Inhibition of SARS-CoV-2 Infection by the Cyclophilin Inhibitor Alisporivir (Debio 025)

Cyclophilins play a key role in the life cycle of coronaviruses. Alisporivir (Debio 025) is a nonimmunosuppressive analogue of cyclosporine with potent cyclophilin inhibition properties. Alisporivir reduced SARS-CoV-2 RNA production in a dose-dependent manner in Vero E6 cells, with a 50% effective concentration (EC₅₀) of 0.46 ± 0.04 μM. Alisporivir inhibited a postentry step of the SARS-CoV-2 life cycle. These results justify rapidly conducting a proof-of-concept phase 2 trial with alisporivir in patients with SARS-CoV-2 infection.

Cyclophilins play a key role in the life cycle of coronaviruses. Alisporivir (Debio 025) is a nonimmunosuppressive analogue of cyclosporine with potent cyclophilin inhibition properties. Alisporivir reduced SARS-CoV-2 RNA production in a dose-dependent manner in Vero E6 cells, with a 50% effective concentration (EC₅₀) of 0.46 ± 0.04 μM. Alisporivir inhibited a postentry step of the SARS-CoV-2 life cycle. These results justify rapidly conducting a proof-of-concept phase 2 trial with alisporivir in patients with SARS-CoV-2 infection.

Coronavirus disease 2019 in chronic kidney disease

The clinical spectrum of coronavirus disease 2019 (COVID-19) infection ranges from asymptomatic infection to severe pneumonia with respiratory failure and even death. More severe cases with higher mortality have been reported in older patients and in those with chronic illness such as hypertension, diabetes or cardiovascular diseases. In this regard, patients with chronic kidney disease (CKD) have a higher rate of all-type infections and cardiovascular disease than the general population. A markedly altered immune system and immunosuppressed state may predispose CKD patients to infectious complications. Likewise, they have a state of chronic systemic inflammation that may increase their morbidity and mortality. In this review we discuss the chronic immunologic changes observed in CKD patients, the risk of COVID-19 infections and the clinical implications for and specific COVID-19 therapy in CKD patients. Indeed, the risk for severe COVID-19 is 3-fold higher in CKD than in non-CKD patients; CKD is 12-fold more frequent in intensive care unit than in non-hospitalized COVID-19 patients, and this ratio is higher than for diabetes or cardiovascular disease; and acute COVID-19 mortality is 15-25% for haemodialysis patients even when not developing pneumonia.

Influences of cyclosporin A and non-immunosuppressive derivatives on cellular cyclophilins and viral nucleocapsid protein during human coronavirus 229E replication

The well-known immunosuppressive drug cyclosporin A inhibits replication of various viruses including coronaviruses by binding to cellular cyclophilins thus inactivating their cis-trans peptidyl-prolyl isomerase function. Viral nucleocapsid proteins are inevitable for genome encapsidation and replication. Here we demonstrate the interaction between the N protein of HCoV-229E and cyclophilin A, not cyclophilin B. Cyclophilin inhibitors abolish this interaction. Upon infection, cyclophilin A stays evenly distributed throughout the cell, whereas cyclophilin B concentrates at ER-bleb-like structures. We further show the inhibitory potential of non-immunosuppressive CsA derivatives Alisporivir, NIM811, compound 3 on HCoV-229E-GFP and -Luciferase replication in human Huh-7.5 hepatoma cells at 18 and 48 h time points post infection with EC₅₀ s at low micromolar ranges. Thus, non-immunosuppressive CsA derivatives effectively inhibit HCoV-229E replication suggesting them as possible candidates for the treatment of HCoV infection. The interruption of interaction between CypA and N protein by CsA and its derivatives suggest a mechanism how CypA inhibitors suppress viral replication.

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HCoV-229E replication is inhibited by Alisporivir, NIM811 and other non-immunosuppressive Cyclosporin A derivatives.

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HCoV-229E N protein interacts with cyclophilin A.

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Cyclophilin A is required for coronavirus replication.

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Cyclophilin B concentrates in bleb-like structures of the ER in HCoV-infected Huh7 cells.

Host Factors in Coronavirus Replication

Coronaviruses are pathogens with a serious impact on human and animal health. They mostly cause enteric or respiratory disease, which can be severe and life threatening, e.g., in the case of the zoonotic coronaviruses causing severe acute respiratory syndrome (SARS) and Middle East Respiratory Syndrome (MERS) in humans. Despite the economic and societal impact of such coronavirus infections, and the likelihood of future outbreaks of additional pathogenic coronaviruses, our options to prevent or treat coronavirus infections remain very limited. This highlights the importance of advancing our knowledge on the replication of these viruses and their interactions with the host. Compared to other +RNA viruses, coronaviruses have an exceptionally large genome and employ a complex genome expression strategy. Next to a role in basic virus replication or virus assembly, many of the coronavirus proteins expressed in the infected cell contribute to the coronavirus-host interplay. For example, by interacting with the host cell to create an optimal environment for coronavirus replication, by altering host gene expression or by counteracting the host’s antiviral defenses. These coronavirus–host interactions are key to viral pathogenesis and will ultimately determine the outcome of infection. Due to the complexity of the coronavirus proteome and replication cycle, our knowledge of host factors involved in coronavirus replication is still in an early stage compared to what is known for some other +RNA viruses. This review summarizes our current understanding of coronavirus–host interactions at the level of the infected cell, with special attention for the assembly and function of the viral RNA-synthesising machinery and the evasion of cellular innate immune responses.

Cyclophilins and cyclophilin inhibitors in nidovirus replication

Cyclophilins (Cyps) belong to the family of peptidyl-prolyl isomerases (PPIases). The PPIase activity of most Cyps is inhibited by the immunosuppressive drug cyclosporin A and several of its non-immunosuppressive analogs, which can also block the replication of nidoviruses (arteriviruses and coronaviruses). Cyclophilins have been reported to play an essential role in the replication of several other RNA viruses, including human immunodeficiency virus-1, hepatitis C virus, and influenza A virus. Likewise, the replication of various nidoviruses was reported to depend on Cyps or other PPIases. This review summarizes our current understanding of this class of nidovirus-host interactions, including the potential function of in particular CypA and the inhibitory effect of Cyp inhibitors. Also the involvement of the FK-506-binding proteins and parvulins is discussed. The nidovirus data are placed in a broader perspective by summarizing the most relevant data on Cyp interactions and Cyp inhibitors for other RNA viruses.

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Nidovirus replication is inhibited by cyclophilin inhibitors.

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Arterivirus replication depends on cyclophilin A.

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Cyclosporin A blocks arterivirus RNA synthesis.

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Using cyclophilin inhibitors against nidoviruses in vivo needs more investigation.

Establishment and characterization of a cell line from a feline histiocytic sarcoma

Feline histiocytic sarcoma (HS) is an aggressive and uncommon tumor originating from dendritic cells/macrophages. Here, a feline HS cell line, FHS-1, was established from a case of feline HS and characterized. Immunohistochemically, FHS-1 cells were positive for vimentin and Iba-1, and negative for MHC class II and CD163. FHS-1 cells were positive for α-naphthyl butyrate esterase staining, which was clearly inhibited by sodium fluoride. FHS-1 cells had phagocytic and antigen uptake/processing activities. Moreover, FHS-1 cells were tested for susceptibility to feline infectious peritonitis virus (FIPV) strain 79-1146; however, this cell line was not susceptible to this viral strain. Although FHS-1 cells lost the expression of MHC class II and CD163, our findings indicate that FHS-1 is a feline HS cell line that retains functional properties of dendritic cells/macrophages in terms of phagocytic and antigen uptake/processing activities. While FHS-1 cells are not suitable for in vitro study of FIP using strain 79-1146, they may be applicable for studies aimed at developing new diagnostic and therapeutic strategies for feline HS.

Coronaviruses and arteriviruses display striking differences in their cyclophilin A-dependence during replication in cell culture

Cyclophilin A (CypA) is an important host factor in the replication of a variety of RNA viruses. Also the replication of several nidoviruses was reported to depend on CypA, although possibly not to the same extent. These prior studies are difficult to compare, since different nidoviruses, cell lines and experimental set-ups were used. Here, we investigated the CypA dependence of three distantly related nidoviruses that can all replicate in Huh7 cells: the arterivirus equine arteritis virus (EAV), the alphacoronavirus human coronavirus 229E (HCoV-229E), and the betacoronavirus Middle East respiratory syndrome coronavirus (MERS-CoV). The replication of these viruses was compared in the same parental Huh7 cells and in CypA-knockout Huh7 cells generated using CRISPR/Cas9-technology. CypA depletion reduced EAV yields by ~ 3-log, whereas MERS-CoV progeny titers were modestly reduced (3-fold) and HCoV-229E replication was unchanged. This study reveals that the replication of nidoviruses can differ strikingly in its dependence on cellular CypA.

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Nidoviruses display differences in sensitivity towards cyclophilin A depletion.

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Replication of MERS-coronavirus is reduced modestly in cyclophilin A-knockout cells.

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Equine arteritis virus replication is strongly inhibited by cyclophilin A depletion.

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Chromosomal anomalies complicate CRISPR/Cas9-mediated gene editing in Huh7 cells.

Nanoparticulate vacuolar ATPase blocker exhibits potent host-targeted antiviral activity against feline coronavirus

Feline infectious peritonitis (FIP), caused by a mutated feline coronavirus, is one of the most serious and fatal viral diseases in cats. The disease remains incurable, and there is no effective vaccine available. In light of the pathogenic mechanism of feline coronavirus that relies on endosomal acidification for cytoplasmic entry, a novel vacuolar ATPase blocker, diphyllin, and its nanoformulation are herein investigated for their antiviral activity against the type II feline infectious peritonitis virus (FIPV). Experimental results show that diphyllin dose-dependently inhibits endosomal acidification in fcwf-4 cells, alters the cellular susceptibility to FIPV, and inhibits the downstream virus replication. In addition, diphyllin delivered by polymeric nanoparticles consisting of poly(ethylene glycol)-block-poly(lactide-co-glycolide) (PEG-PLGA) further demonstrates an improved safety profile and enhanced inhibitory activity against FIPV. In an in vitro model of antibody-dependent enhancement of FIPV infection, diphyllin nanoparticles showed a prominent antiviral effect against the feline coronavirus. In addition, the diphyllin nanoparticles were well tolerated in mice following high-dose intravenous administration. This study highlights the therapeutic potential of diphyllin and its nanoformulation for the treatment of FIP.