Infection of SCID mice with Montana Myotis leukoencephalitis virus as a model for flavivirus encephalitis

A SCID Mouse Model To Evaluate the Efficacy of Antivirals against SARS-CoV-2 Infection

Ancestral severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) lacks the intrinsic ability to bind to the mouse ACE2 receptor, and therefore establishment of SARS-CoV-2 mouse models has been limited to the use of mouse-adapted viruses or genetically modified mice. Interestingly, some of the variants of concern, such as the Beta B.1.351 variant, show an improved binding to the mouse receptor and hence better replication in different wild-type (WT) mouse species. Here, we describe the establishment of a SARS-CoV-2 Beta B.1.351 variant infection model in male SCID mice as a tool to assess the antiviral efficacy of potential SARS-CoV-2 small-molecule inhibitors. Intranasal infection of male SCID mice with 10⁵ 50% tissue culture infective doses (TCID₅₀) of the Beta B.1.351 variant resulted in high viral loads in the lungs and moderate signs of lung pathology on day 3 postinfection. Treatment of infected mice with the antiviral drugs molnupiravir (200 mg/kg, twice a day [BID]) or nirmatrelvir (300 mg/kg, BID) for 3 consecutive days significantly reduced the infectious virus titers in the lungs by 2 and 3.9 log₁₀ TCID₅₀/mg of tissue, respectively, and significantly improved lung pathology. Together, these data demonstrate the validity of this SCID mouse Beta B.1.351 variant infection model as a convenient preclinical model for assessment of potential activity of antivirals against SARS-CoV-2.

IMPORTANCE Unlike the ancestral SARS-CoV-2 strain, the Beta (B.1.351) variant of concern has been reported to replicate to some extent in WT mice (C57BL/6 and BALB/c). We demonstrate here that infection of SCID mice with the Beta variant resulted in high viral loads in the lungs on day 3 postinfection. Treatment of infected mice with molnupiravir or nirmatrelvir for 3 consecutive days markedly reduced the infectious virus titers in the lungs and improved lung pathology. The SARS-CoV2 SCID mouse infection model, which is ideally suited for antiviral studies, offers an advantage in comparison to other SARS-CoV2 mouse models, in that there is no need for the use of mouse-adapted virus strains or genetically modified mice. Mouse models also have advantages over hamster models because (i) lower amounts of test drugs are needed, (ii) more animals can be housed in a cage, and (iii) reagents to analyze mouse samples are more readily available than those for hamsters.

The anti-viral facet of anti-rheumatic drugs: Lessons from COVID-19

The outbreak of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has posed the world at a pandemic risk. Coronavirus-19 disease (COVID-19) is an infectious disease caused by SARS-CoV-2, which causes pneumonia, requires intensive care unit hospitalization in about 10% of cases and can lead to a fatal outcome. Several efforts are currently made to find a treatment for COVID-19 patients. So far, several anti-viral and immunosuppressive or immunomodulating drugs have demonstrated some efficacy on COVID-19 both in vitro and in animal models as well as in cases series. In COVID-19 patients a pro-inflammatory status with high levels of interleukin (IL)-1B, IL-1 receptor (R)A and tumor necrosis factor (TNF)-α has been demonstrated. Moreover, high levels of IL-6 and TNF-α have been observed in patients requiring intensive-care-unit hospitalization. This provided rationale for the use of anti-rheumatic drugs as potential treatments for this severe viral infection. Other agents, such as hydroxychloroquine and chloroquine might have a direct anti-viral effect. The anti-viral aspect of immunosuppressants towards a variety of viruses has been known since long time and it is herein discussed in the view of searching for a potential treatment for SARS-CoV-2 infection.

A Review of Flaviviruses that Have No Known Arthropod Vector

Most viruses in the genus Flavivirus are horizontally transmitted between hematophagous arthropods and vertebrate hosts, but some are maintained in arthropod- or vertebrate-restricted transmission cycles. Flaviviruses maintained by vertebrate-only transmission are commonly referred to as no known vector (NKV) flaviviruses. Fourteen species and two subtypes of NKV flaviviruses are recognized by the International Committee on Taxonomy of Viruses (ICTV), and Tamana bat virus potentially belongs to this group. NKV flaviviruses have been isolated in nature almost exclusively from bats and rodents; exceptions are the two isolates of Dakar bat virus recovered from febrile humans and the recent isolations of Sokoluk virus from field-collected ticks, which raises questions as to whether it should remain classified as an NKV flavivirus. There is evidence to suggest that two other NKV flaviviruses, Entebbe bat virus and Yokose virus, may also infect arthropods in nature. The best characterized bat- and rodent-associated NKV flaviviruses are Rio Bravo and Modoc viruses, respectively, but both have received limited research attention compared to many of their arthropod-infecting counterparts. Herein, we provide a comprehensive review of NKV flaviviruses, placing a particular emphasis on their classification, host range, geographic distribution, replication kinetics, pathogenesis, transmissibility and molecular biology.

The Role of Bats as Reservoir Hosts of Emerging Neuroviruses

Recent studies have clearly shown that bats are the reservoir hosts of a wide diversity of novel viruses with representatives from most of the known animal virus families. In many respects bats make ideal reservoir hosts for viruses: they are the only mammals that fly, thus assisting in virus dispersal; they roost in large numbers, thus aiding transmission cycles; some bats hibernate over winter, thus providing a mechanism for viruses to persist between seasons; and genetic factors may play a role in the ability of bats to host viruses without resulting in clinical disease. Within the broad diversity of viruses found in bats are some important neurological pathogens, including rabies and other lyssaviruses, and Hendra and Nipah viruses, two recently described viruses that have been placed in a new genus, Henipaviruses in the family Paramyxoviridae. In addition, bats can also act as alternative hosts for the flaviviruses Japanese encephalitis and St Louis encephalitis viruses, two important mosquito-borne encephalitogenic viruses, and bats can assist in the dispersal and over-wintering of these viruses. Bats are also the reservoir hosts of progenitors of SARS and MERS coronaviruses, although other animals act as spillover hosts. This chapter presents the physiological and ecological factors affecting the ability of bats to act as reservoirs of neurotropic viruses, and describes the major transmission cycles leading to human infection.

Immune evasion strategies of flaviviruses

Flavivirus is a genus of the family Flaviviridae. It includes West Nile virus (WNV), dengue virus (DENV), yellow fever virus (YFV), Japanese encephalitis virus (JEV), tick-borne encephalitis virus (TBEV), and several other viruses which lead to extensive morbidity and mortality in humans. To establish infection and replication in the hosts, flaviviruses have evolved a variety of strategies to modulate the host's immune responses. In this review, the strategies employed by flaviviruses to evade the innate and adaptive immunity of host are summarized based on current studies, with a major focus on the inhibition of interferon, complement, natural killer (NK) cell, B cell, and T cell responses. This review aims to provide an overview of the current understanding for the mechanisms used by flaviviruses to escape the host's immune response, which will facilitate the future studies on flavivirus pathogenesis and the development of anti-flavivirus therapeutics.

Role of endoplasmic reticulum stress in cystic fibrosis-related airway inflammatory responses

Chronic airway infection and inflammation are hallmarks of cystic fibrosis (CF) pulmonary disease. The altered airway environment resulting from infection and inflammation can affect the innate defense of the airway epithelia. Luminal bacterial and inflammatory stimuli trigger an adaptation in human airway epithelia, characterized by a hyperinflammatory response to inflammatory mediators, which is mediated by an expansion of the endoplasmic reticulum (ER) and its Ca(2+) stores. Recent studies demonstrated that a form of ER stress, the unfolded protein response (UPR), is activated in airway epithelia by bacterial infection-induced airway inflammation. UPR-dependent signaling is responsible for the ER Ca(2+) store expansion-mediated amplification of airway inflammatory responses. These studies highlight the functional importance of the UPR in airway inflammation and suggest that targeting the UPR may be a therapeutic strategy for airway diseases typified by chronic inflammation. This article reviews the contribution of airway epithelia to airway inflammatory responses, discusses how expansion of the ER Ca(2+) stores in inflamed airway epithelia contributes to airway inflammation, describes the functional role of the UPR in these processes, and discusses how UPR activation might be relevant for CF airways inflammatory disease.

Yet another ten stories on antiviral drug discovery (part D): paradigms, paradoxes, and paraductions

This review article presents the fourth part (part D) in the series of stories on antiviral drug discovery. The stories told in part D focus on: (i) the cyclotriazadisulfonamide compounds; (ii) the {5-[(4-bromophenylmethyl]-2-phenyl-5H-imidazo[4,5-c]pyridine} compounds; (iii) (1H,3H-thiazolo[3,4-a]benzimidazole) derivatives; (iv) T-705 (6-fluoro-3-hydroxy-2-pyrazinecarboxamide) and (v) its structurally closely related analogue pyrazine 2-carboxamide (pyrazinamide); (vi) new strategies for the treatment of hemorrhagic fever virus infections, including, as the most imminent, (vii) dengue fever, (viii) the veterinary use of acyclic nucleoside phosphonates; (ix) the potential (off-label) use of cidofovir in the treatment of papillomatosis, particularly RRP (recurrent respiratory papillomatosis); and (x) finally, the prophylactic use of tenofovir to prevent HIV infections.

Strategies for the treatment of dengue virus infections: a narrative account

In comparison with other virus infections, such as HIV, HBV, HCV, influenza and herpes virus, dengue virus infections have received relatively little attention from a (chemo)therapeutic viewpoint, despite their global incidence (2.5 billion people estimated to be at risk) and absence of an effective prophylactic measure (i.e., vaccination). However, the replicative cycle of the dengue virus offers a wealth of potential target sites, including virus adsorption and fusion, RNA-dependent RNA polymerase (and the associated 2´-O-methyltransferase) and the N-glycosylation process. Numerous compounds have been identified that interfere with dengue virus infections (i.e., ribavirin, interferon, sulfated polysaccharides, 2´-C-methylnucleoside analogs and iminosugar derivatives). For other compounds found to be effective against the dengue virus, including geneticin (G418) and FGI-106, the mechanism of action still remains unresolved.

The role of intracellular calcium signals in inflammatory responses of polarised cystic fibrosis human airway epithelia

Hyperinflammatory host responses to bacterial infection have been postulated to be a key step in the pathogenesis of cystic fibrosis (CF) lung disease. Previous studies have indicated that the CF airway epithelium itself contributes to the hyperinflammation of CF airways via an excessive inflammatory response to bacterial infection. However, it has been controversial whether the hyperinflammation of CF epithelia results from mutations in the CF transmembrane conductance regulator (CFTR) and/or is a consequence of persistent airways infection. Recent studies have demonstrated that intracellular calcium (Ca2+i) signals consequent to activation of apical G protein-coupled receptors (GPCRs) by pro-inflammatory mediators are increased in CF airway epithelia. Because of the relationship between Ca2+i mobilisation and inflammatory responses, the mechanism for the increased Ca2+i signals in CF was investigated and found to result from endoplasmic reticulum (ER) Ca2+ store expansion. The ER Ca2+ store expansion imparts a hyperinflammatory phenotype to chronically infected airway epithelia as a result of the larger Ca2+i mobilisation coupled to an excessive inflammatory response following GPCR activation. The ER expansion is not dependent on ER retention of misfolded DeltaF508 CFTR, but reflects an epithelial response acquired following persistent luminal airway infection. With respect to the mechanism of ER expansion in CF, the current view is that chronic airway epithelial infection triggers an unfolded protein response as a result of the increased flux of newly synthesised inflammatory mediators and defensive factors into the ER compartment. This unfolded protein response is coupled to X-box binding protein 1 (XBP-1) mRNA splicing and transcription of genes associated with the expansion of the protein-folding capacity of the ER (e.g. increases in ER chaperones and ER membranes). These studies have revealed a novel adaptive response in chronically infected airway epithelia, where the increased protein secretory capacity serves to promote epithelial homeostasis by increasing both the secretory and the reparative capacity of the cell. In addition, the increased ER-derived Ca2+i signaling allows the epithelia to amplify its inflammatory responses to infectious agents and exogenous toxicants. This review is devoted to a discussion of these recent findings and their implication for Ca2+i-dependent hyperinflammatory responses in CF airways.

Chronic airway infection/inflammation induces a Ca2+i-dependent hyperinflammatory response in human cystic fibrosis airway epithelia

Hyperinflammatory responses to infection have been postulated as a component of cystic fibrosis (CF) lung disease. Studies have linked intracellular calcium (Ca(2+)(i)) mobilization with inflammatory responses in several systems. We have reported that the pro-inflammatory mediator bradykinin (BK) promotes larger Ca(2+)(i) signals in CF compared with normal bronchial epithelia, a response that reflects endoplasmic reticulum (ER)/Ca(2+) store expansion induced by chronic luminal airway infection/inflammation. The present study investigated whether CF airway epithelia were hyperinflammatory and, if so, whether the hyperinflammatory CF phenotype was linked to larger Ca(2+) stores in the ER. We found that DeltaF508 CF bronchial epithelia were hyperinflammatory as defined by an increased basal and mucosal BK-induced interleukin (IL)-8 secretion. However, the CF hyperinflammation expressed in short-term (6-11-day-old) primary cultures of DeltaF508 bronchial epithelia was lost in long-term (30-40-day-old) primary cultures of DeltaF508 bronchial epithelia, indicating this response was independent of mutant cystic fibrosis transmembrane conductance regulator. Exposure of 30-40-day-old cultures of normal airway epithelia to supernatant from mucopurulent material (SMM) from CF airways reproduced the increased basal and mucosal BK-stimulated IL-8 secretion of short-term CF cultures. The BK-triggered increased IL-8 secretion in SMM-treated cultures was mediated by an increased Ca(2+)(i) mobilization consequent to an ER expansion associated with increases in protein synthesis (total, cytokines, and antimicrobial factors). The increased ER-dependent, Ca(2+)(i)-mediated hyperinflammatory epithelial response may represent a general beneficial airway epithelial adaptation to transient luminal infection. However, in CF airways, the Ca(2+)(i)-mediated hyperinflammation may be ineffective in promoting the eradication of infection in thickened mucus and, consequently, may have adverse effects in the lung.

The predominant mechanism by which ribavirin exerts its antiviral activity in vitro against flaviviruses and paramyxoviruses is mediated by inhibition of IMP dehydrogenase

It is not yet clear to what extent depletion of intracellular GTP pools contributes to the antiviral activity of ribavirin. Therefore, the antiviral activities of (i) ribavirin, (ii) its 5-ethynyl analogue, 5-ethynyl-1-beta-D-ribofuranosylimidazole-4-carboxamide (EICAR), and (iii) mycophenolic acid (MPA) (a compound that inhibits only cellular IMP dehydrogenase activity) were studied on the replication of flaviviruses and paramyxoviruses. In addition, the effects of these three compounds on intracellular GTP pools were assessed. A linear correlation was observed over a broad concentration range between the antiviral activities of ribavirin, EICAR, and MPA and the effects of these compounds on GTP pool depletion. When the 50% effective concentrations (EC50s) for the antiviral activities of ribavirin, EICAR, and MPA were plotted against the respective EC50 values for GTP pool depletion, a linear correlation was calculated. These data provide compelling evidence that the predominant mechanism of action of ribavirin in vitro against flavi- and paramyxoviruses is based on inhibition of cellular IMP dehydrogenase activity.

Exchanging the yellow fever virus envelope proteins with Modoc virus prM and E proteins results in a chimeric virus that is neuroinvasive in SCID mice

A chimeric flavivirus infectious cDNA was constructed by exchanging the premembrane (prM) and envelope (E) genes of the yellow fever virus vaccine strain 17D (YF17D) with the corresponding genes of Modoc virus (MOD). This latter virus belongs to the cluster of the "not-known vector" flaviviruses and is, unlike YF17D, neuroinvasive in SCID mice. Replication of in vitro-transcribed RNA from this chimeric flavivirus was shown by [(3)H]uridine labeling and RNA analysis. Expression of the MOD prM and E proteins was monitored by radioimmunoprecipitation and revealed that the MOD proteins were correctly and efficiently produced from the chimeric precursor protein. The MOD E protein was shown to be N-linked glycosylated, whereas prM, as predicted from the genome sequence, did not contain N-linked carbohydrates. In Vero cells, the chimeric virus replicated with a similar efficiency as the parental viruses, although it formed smaller plaques than YF17D and MOD. In SCID mice that had been infected intraperitoneally with the chimeric virus, the viral load increased steadily until death. The MOD/YF virus, like MOD from which it had acquired the prM and E structural proteins, but unlike YF, proved neuroinvasive in SCID mice. Animals developed neurological symptoms about 15 days after inoculation and died shortly thereafter. The distribution of MOD/YF RNA in the brain of infected mice was similar to that observed in MOD-infected mice. The observations provide compelling evidence that the determinants of neuroinvasiveness of flaviviruses are entirely located in the envelope proteins prM and E.

Cytokine production in brain of mice experimentally infected with dengue virus

Dengue virus encephalopathy was experimentally induced in newborn mice by i.p. inoculation of dengue 2 virus. At 6 and 9 days post-infection, motor incoordination and posterior limb paralysis were observed with focal necrosis, apoptotic cells, perivascular inflammatory cuffing and astrocytosis, mainly in the cortex and hippocampus. Expression of dengue virus genome and mRNA encoding for TNF-alpha, IL-1alpha, IL-2, IL-6, IL-1beta, IL-12 p40, IFNalpha, and IFNbeta; in addition host survival and inflammatory response genes MAC-1, EB22, GFAP, ICAM-1 and A20, were increased, suggesting that experimental dengue encephalitis could be associated with virus replication, inflammatory cytokine production or both. Similar findings have been observed in human dengue virus infection. Therefore, our results can be useful to elucidate and support the physiopathology of the disease.

Innate and adaptive immune responses determine protection against disseminated infection by West Nile encephalitis virus

WNV continues to spread throughout the Western Hemisphere as virus activity in insects and animals has been reported in the United States, Canada, Mexico, and the Caribbean islands. West Nile virus (WNV) infects the central nervous system and causes severe disease primarily in humans who are immunocompromised or elderly. In this review, we discuss the mechanisms by which the immune system limits dissemination of WNV infection. Recent experimental studies in animals suggest important roles for both the innate and the adaptive immune responses in controlling WNV infection. Interferons, antibody, complement components and CD8+ T cells coordinate protection against severe infection and disease. These findings are analyzed in the context of recent approaches to vaccine development and immunotherapy against WNV.

Evasion of innate and adaptive immunity by flaviviruses

After a virus infects an animal, antiviral responses are generated that attempt to prevent dissemination. Interferons, antibody, complement, T and natural killer cells all contribute to the control and eradication of viral infections. Most flaviviruses, with the exception of some of the encephalitic viruses, cause acute disease and do not establish persistent infection. The outcome of flavivirus infection in an animal is determined by a balance between the speed of viral replication and spread, and the immune system response. Although many of the mechanistic details require further elucidation, flaviviruses have evolved specific tactics to evade the innate and adaptive immune response. A more thorough understanding of these principles could lead to improved models for viral pathogenesis and to strategies for the development of novel antiviral agents.

The pathogenesis of spinal cord involvement in dengue virus infection

To investigate the mechanisms of dengue (DEN) virus transmission within the spinal cord, severe combined immunodeficient mice were intracerebrally inoculated with DEN virus type 2. After inoculation, a high virus titer and antigens were detected in the brain and spinal cord. At early stages of the infection, ultrastructural examinations showed that a few virions were present in the cytoplasm of ependymal cells lining the central canal. As the infection progressed, virions were observed in the lumen of the rough endoplasmic reticulum (RER), RER-derived vesicles and the Golgi region of infected neurons. These data suggest that the inoculated DEN virus might spread to the neurons of the spinal cord via the cerebral spinal fluid and cause several neuronal pathological responses. Moreover, DEN virus was also observed in myelinated and unmyelinated nerve fibers and typical neuronal synapses. Some virion-containing vesicles appeared to be fused with the membrane of presynapses, indicating that neuron-to-neuron transport of DEN virus might occur in the spinal cord. Additionally, anterior, lateral and posterior horns of the spinal cord exhibited different numbers of the positive neurons and different staining intensities of the DEN antigen during the infection. This difference likely represents variation of susceptibility to the DEN virus among the neurons of the spinal cord.

Detection, differentiation, and quantitation of pathogenic leishmania organisms by a fluorescence resonance energy transfer-based real-time PCR assay

Real-time technology eliminates many of the pitfalls of diagnostic PCR, but this method has not been applied to differentiation of Leishmania organisms so far. We have developed a real-time PCR that simultaneously detects, quantitates, and categorizes Leishmania organisms into three relevant groups causing distinct clinical pictures. The analytical sensitivity (detection rate of >or=95% at 94.1 parasites/ml of blood) was within a range that has been determined previously to facilitate the confirmation of visceral leishmaniasis from peripheral blood. Parasites were successfully detected in 12 different clinical samples (blood, bone marrow, skin, and liver). The Leishmania donovani complex, the Leishmania brasiliensis complex, and species other than these could be clearly discriminated by means of distinct melting temperatures obtained with fluorescence resonance energy transfer probes (melting points, 72.7, 67.1, and 65.0 degrees C, respectively). All three groups could be quantified within equal ranges. As in other real-time PCRs, the variability in the quantification of DNA was small (coefficient of variation [CV], <2%). However, human samples containing low levels of parasites (100 parasites per ml of blood) showed higher variation (CV, 60.89%). Therefore, despite its superior analytical performance, care must be taken when real-time PCR is utilized for therapy monitoring.

Pathogenesis of flavivirus encephalitis

Within the flavivirus family, viruses that cause natural infections of the central nervous system (CNS) principally include members of the Japanese encephalitis virus (JEV) serogroup and the tick-borne encephalitis virus (TBEV) serocomplex. The pathogenesis of diseases involves complex interactions of viruses, which differ in neurovirulence potential, and a number of host factors, which govern susceptibility to infection and the capacity to mount effective antiviral immune responses both in the periphery and within the CNS. This chapter summarizes progress in the field of flavivirus neuropathogenesis. Mosquito-borne and tickborne viruses are considered together. Flavivirus neuropathogenesis involves both neuroinvasiveness (capacity to enter the CNS) and neurovirulence (replication within the CNS), both of which can be manipulated experimentally. Neuronal injury as a result of bystander effects may be a factor during flavivirus neuropathogenesis given that microglial activation and elaboration of inflammatory mediators, including IL-1β and TNF-α, occur in the CNS during these infections and may accompany the production of nitric oxide and peroxynitrite, which can cause neurotoxicity.

Complete genome sequence of Montana Myotis leukoencephalitis virus, phylogenetic analysis and comparative study of the 3' untranslated region of flaviviruses with no known vector

Montana Myotis leukoencephalitis virus (MMLV), a virus isolated from bats, causes an encephalitis in small rodents reminiscent of flavivirus encephalitis in humans. The complete MMLV genome is 10690 nucleotides long and encodes a putative polyprotein of 3374 amino acids. The virus contains the same conserved motifs in genes that are believed to be interesting antiviral targets (NTPase/helicase, serine protease and RNA-dependent RNA polymerase) as flaviviruses of clinical importance. Phylogenetic analysis of the entire coding region has confirmed the classification of MMLV in the clade of the flaviviruses with no known vector (NKV) and within this clade to the Rio Bravo branch (both viruses have the bat as their vertebrate host). We have provided for the first time a comparative analysis of the RNA folding of the 3' UTR of the NKV flaviviruses (Modoc, Rio Bravo and Apoi viruses, in addition to MMLV). Structural elements in the 3' UTR that are preserved among other flaviviruses have been revealed, as well as elements that distinguish the NKV from the mosquito- and tick-borne flaviviruses. In particular, the pentanucleotide sequence 5' CACAG 3', which is conserved in all mosquito- and tick-borne flaviviruses, is replaced by the sequence 5' C(C/U)(C/U)AG 3' in the loop of the 3' long stable hairpin structure of all four NKV flaviviruses. The availability of this latter sequence motif allows us to designate a virus as either an NKV or a vector-borne flavivirus.