Molecular mechanisms of FIV infection

Conformational analysis of a new peptide derived from feline immunodeficiency virus gp36 in SDS micelles: An NMR-MD based investigation

Feline immunodeficiency virus (FIV) shares structural similarities with human immunodeficiency virus (HIV): the surface glycoprotein gp36 corresponds to the HIV gp41, which drives virus-host cell interactions and is targeted by the peptide entry inhibitor enfuvirtide. Following a similar drug design strategy for the development of an anti-FIV therapy, the present study investigates 627-646gp36 NHR, a peptide sequence derived from a region of gp36 that was previously found to interfere with the antiviral activity of the peptide C8, which instead derives from the gp36 MPER. CD, NMR, and MD simulations were employed to probe the conformational characteristics of 627-646gp36 NHR in the membrane-mimicking environment of SDS micelles. Our data show that 627-646gp36 NHR is characterized by three dynamic helix structures. MD simulations involving 627-646gp36 NHR, C8, and a larger protein, including the CHR and MPER regions, suggest that the interaction of C8 with the MPER region, the origin of the antiviral activity of C8, is disfavored in the presence of 627-646gp36 NHR in the simulation. This evidence can be useful for interpreting the molecular mechanism that leads to interference with the activity of C8, providing information on the folding/unfolding mechanism of the viral glycoprotein to design new strategies to inhibit viral entry.

Identification of a putative Gag binding site critical for feline immunodeficiency virus genomic RNA packaging

The retroviral Gag precursor plays a central role in the selection and packaging of viral genomic RNA (gRNA) by binding to virus-specific packaging signal(s) (psi or ψ). Previously, we mapped the feline immunodeficiency virus (FIV) ψ to two discontinuous regions within the 5' end of the gRNA that assumes a higher order structure harboring several structural motifs. To better define the region and structural elements important for gRNA packaging, we methodically investigated these FIV ψ sequences using genetic, biochemical, and structure-function relationship approaches. Our mutational analysis revealed that the unpaired U85CUG88 stretch within FIV ψ is crucial for gRNA encapsidation into nascent virions. High-throughput selective 2' hydroxyl acylation analyzed by primer extension (hSHAPE) performed on wild type (WT) and mutant FIV ψ sequences, with substitutions in the U85CUG88 stretch, revealed that these mutations had limited structural impact and maintained nucleotides 80-92 unpaired, as in the WT structure. Since these mutations dramatically affected packaging, our data suggest that the single-stranded U85CUG88 sequence is important during FIV RNA packaging. Filter-binding assays performed using purified FIV Pr50Gag on WT and mutant U85CUG88 ψ RNAs led to reduced levels of Pr50Gag binding to mutant U85CUG88 ψ RNAs, indicating that the U85CUG88 stretch is crucial for ψ RNA-Pr50Gag interactions. Delineating sequences important for FIV gRNA encapsidation should enhance our understanding of both gRNA packaging and virion assembly, making them potential targets for novel retroviral therapeutic interventions, as well as the development of FIV-based vectors for human gene therapy.

Exploring the link between viruses and cancer in companion animals: a comprehensive and comparative analysis

Currently, it is estimated that 15% of human neoplasms globally are caused by infectious agents, with new evidence emerging continuously. Multiple agents have been implicated in various forms of neoplasia, with viruses as the most frequent. In recent years, investigation on viral mechanisms underlying tumoral transformation in cancer development and progression are in the spotlight, both in human and veterinary oncology. Oncogenic viruses in veterinary medicine are of primary importance not only as original pathogens of pets, but also in the view of pets as models of human malignancies. Hence, this work will provide an overview of the main oncogenic viruses of companion animals, with brief notes of comparative medicine.

Equal contributions of feline immunodeficiency virus and coinfections to morbidity in African lions

Feline immunodeficiency virus (FIV) is a pathogenic lentivirus related to human and simian immunodeficiency viruses that has been associated with AIDS-like pathologies in domestic and wild cats, as well as in hyenas. Despite known pathologies, progressive immunosuppression and ill health effects driven by these lentiviruses in association with other secondary infections remain understudied in free-ranging species. Here, the role of coinfections by gastrointestinal parasites and tick-borne hemoparasites for FIV disease progression was explored in 195 free-ranging African lions (Panthera leo) living in Kruger National Park (KNP), South Africa. Using statistical methodology, we evaluated the effects of FIV on a range of health indicators to explore how direct and indirect effects of FIV and associated coinfections align to determine lion health outcomes. Findings show direct negative effects of FIV on host immunity and nutritional status, and exacerbation of aggressive behaviors, conditions which may increase exposure/susceptibility to other secondary infections. When taken together, the contribution of coinfecting parasites to morbidity in lions is of similar magnitude as direct effects of FIV infection alone, suggesting that the particular coinfection assemblage may play a role in mediating disease progression within natural lion populations.

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Immunosuppression by FIV increases richness and abundance of secondary parasites.

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Infection by gastrointestinal parasites drives severe malnourishment in FIV hosts.

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Hemoparasite infection contributed to liver pathology and clinical wasting.

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Contributions of secondary infections to morbidity equal the direct effects of FIV.

Structural and Mechanistic Studies of the Rare Myristoylation Signal of the Feline Immunodeficiency Virus

All retroviruses encode a Gag polyprotein containing an N-terminal matrix domain (MA) that anchors Gag to the plasma membrane and recruits envelope glycoproteins to virus assembly sites. Membrane binding by the Gag protein of HIV-1 and most other lentiviruses is dependent on N-terminal myristoylation of MA by host N-myristoyltransferase enzymes (NMTs), which recognize a six-residue "myristoylation signal" with consensus sequence: M1GXXX[ST]. For unknown reasons, the feline immunodeficiency virus (FIV), which infects both domestic and wild cats, encodes a non-consensus myristoylation sequence not utilized by its host or by other mammals (most commonly: M1GNGQG). To explore the evolutionary basis for this sequence, we compared the structure, dynamics, and myristoylation properties of native FIV MA with a mutant protein containing a consensus feline myristoylation motif (MANOS) and examined the impact of MA mutations on virus assembly and ability to support spreading infection. Unexpectedly, myristoylation efficiency of MANOS in Escherichia coli by co-expressed mammalian NMT was reduced by ~70% compared to the wild-type protein. NMR studies revealed that residues of the N-terminal myristoylation signal are fully exposed and mobile in the native protein but partially sequestered in the MANOS chimera, suggesting that the unusual FIV sequence is conserved to promote exposure and efficient myristoylation of the MA N terminus. In contrast, virus assembly studies indicate that the MANOS mutation does not affect virus assembly, but does prevent virus spread, in feline kidney cells. Our findings indicate that residues of the FIV myristoylation sequence play roles in replication beyond NMT recognition and Gag-membrane binding.

Purification and Functional Characterization of a Biologically Active Full-Length Feline Immunodeficiency Virus (FIV) Pr50Gag

The feline immunodeficiency virus (FIV) full-length Pr50^Gag precursor is a key player in the assembly of new viral particles. It is also a critical component of the efficient selection and packaging of two copies of genomic RNA (gRNA) into the newly formed virus particles from a wide pool of cellular and spliced viral RNA. To understand the molecular mechanisms involved during FIV gRNA packaging, we expressed the His₆-tagged and untagged recombinant FIV Pr50^Gag protein both in eukaryotic and prokaryotic cells. The recombinant Pr50^Gag-His₆-tag fusion protein was purified from soluble fractions of prokaryotic cultures using immobilized metal affinity chromatography (IMAC). This purified protein was able to assemble in vitro into virus-like particles (VLPs), indicating that it preserved its ability to oligomerize/multimerize. Furthermore, VLPs formed in eukaryotic cells by the FIV full-length Pr50^Gag both in the presence and absence of His₆-tag could package FIV sub-genomic RNA to similar levels, suggesting that the biological activity of the recombinant full-length Pr50^Gag fusion protein was retained in the presence of His₆-tag at the carboxy terminus. Successful expression and purification of a biologically active, recombinant full-length Pr50^Gag-His₆-tag fusion protein will allow study of the intricate RNA-protein interactions involved during FIV gRNA encapsidation.

FIV vaccine with receptor epitopes results in neutralizing antibodies but does not confer resistance to challenge

Feline immunodeficiency virus (FIV) is the feline analogue to human immunodeficiency virus (HIV) and utilizes parallel modes of receptor-mediated entry. The FIV surface glycoprotein (SU) is an important target for induction of neutralizing antibodies, and autoantibodies to the FIV binding receptor (CD134) block infection ex vivo; thus highlighting the potential for immunotherapies which utilize anti-receptor antibodies to block viral infection. To determine whether vaccination with CD134-SU complexes could induce protection against FIV infection, cats (n = 5 per group) were immunized with soluble CD134, recombinant FIV-SU protein, and/or CD134+SU complexes. Two trials were performed with different antigen combinations and vaccination schedules. In vivo generation of anti-CD134 and anti-SU IgG antibodies was measured, and in vitro neutralization assays were conducted. Immunization induced production of anti-CD134 and anti-SU antibodies that significantly inhibited FIV infection in vitro. However, no vaccine combination protected cats from FIV infection, and neat serum from vaccinated cats enhanced FIV growth in vitro. CD134+SU vaccinated cats exhibited increased CD4:CD8 ratio immediately prior to challenge, and antibodies were much more efficiently generated against vaccine by-products versus target antigens. Results suggest vaccination against viral and cryptic receptor epitopes yields neutralizing antibodies that synergistically inhibit FIV infection in vitro. Factors contributing to vaccine failure may include: (1) Heat-labile serum factors that enhance viral replication, (2) changes in circulating target cell populations induced by vaccination, and (3) weak immunogenicity of neutralizing epitopes compared to off-target vaccine components. Results reinforce the need to monitor vaccine preparation components and avoid non-specific immune stimulation during vaccination.

A vaccine candidate for feline immunodeficiency virus elicits strong immunological reaction in vitro, but no protection to live cats. The feline analog to human immunodeficiency virus, FIV shares a similar infection paradigm and has only one partially effective vaccine. A US team, led by Colorado State University’s Susan VandeWoude, immunized cats using a complex of an FIV surface protein and a feline cell-surface protein known to facilitate FIV’s entry into immune cells. Tissue culture assays yielded promising results; however, this did not translate to live-animal protection. The researchers highlighted multiple factors that could explain the lack of success, including circulatory pro-infection factors, and immune responses generated against vaccine by-products rather than intended targets. While the vaccine candidate failed, the research provides invaluable guidance for future efforts into FIV vaccination with implications for HIV vaccine trials.

Applications of the FIV Model to Study HIV Pathogenesis

Feline immunodeficiency virus (FIV) is a naturally-occurring retrovirus that infects domestic and non-domestic feline species, producing progressive immune depletion that results in an acquired immunodeficiency syndrome (AIDS). Much has been learned about FIV since it was first described in 1987, particularly in regard to its application as a model to study the closely related lentivirus, human immunodeficiency virus (HIV). In particular, FIV and HIV share remarkable structure and sequence organization, utilize parallel modes of receptor-mediated entry, and result in a similar spectrum of immunodeficiency-related diseases due to analogous modes of immune dysfunction. This review summarizes current knowledge of FIV infection kinetics and the mechanisms of immune dysfunction in relation to opportunistic disease, specifically in regard to studying HIV pathogenesis. Furthermore, we present data that highlight changes in the oral microbiota and oral immune system during FIV infection, and outline the potential for the feline model of oral AIDS manifestations to elucidate pathogenic mechanisms of HIV-induced oral disease. Finally, we discuss advances in molecular biology, vaccine development, neurologic dysfunction, and the ability to apply pharmacologic interventions and sophisticated imaging technologies to study experimental and naturally occurring FIV, which provide an excellent, but often overlooked, resource for advancing therapies and the management of HIV/AIDS.

Differences in Env and Gag protein expression patterns and epitope availability in feline immunodeficiency virus infected PBMC compared to infected and transfected feline model cell lines

Env and Gag are key components of the FIV virion that are targeted to the plasma membrane for virion assembly. They are both important stimulators and targets of anti-FIV immunity. To investigate and compare the expression pattern and antigenic changes of Gag and Env in various research models, infected PBMC (the natural FIV host cells) and GFox, and transfected CrFK were stained over time with various Env and Gag specific MAbs. In FIV infected GFox and PBMC, Env showed changes in epitope availability for antibody binding during processing and trafficking, which was not seen in transfected CrFK. Interestingly, epitopes exposed on intracellular Env and Env present on the plasma membrane of CrFK and GFox seem to be hidden on plasma membrane expressed Env of FIV infected PBMC. A kinetic follow up of Gag and Env expression showed a polarization of both Gag and Env expression to specific sites at the plasma membrane of PBMC, but not in other cell lines. In conclusion, mature trimeric cell surface expressed Env might be antigenically distinct from intracellular monomeric Env in PBMC and might possibly be unrecognizable by feline humoral immunity. In addition, Env expression is restricted to a small area on the plasma membrane and co-localizes with a large moiety of Gag, which may represent a preferred FIV budding site, or initiation of virological synapses with direct cell-to-cell virus transmission.

Label-free capacitance-based identification of viruses

This study was undertaken to quantitate a single virus suspension in culture medium without any pre-processing. The electrical capacitance per virus particle was used to identify the kind of virus present by measuring the suspension (virus plus medium) capacitance, de-embedding the medium contribution, and dividing by the virus count. The proposed technique is based on finding the single virus effective dielectric constant which is directly related to the virus composition. This value was used to identify the virus type accordingly. Two types of viruses thus tested were further quantified by a biochemical technique to validate the results. Furthermore, non-organic nanoparticles with known concentration and capacitance per particle were identified using the proposed method. The selectivity of the method was demonstrated by performing electrical measurements on a third virus, revealing that the proposed technique is specific and sensitive enough to permit detection of a few hundred virus particles per milliliter within a few minutes.

FIV in cats--a useful model of HIV in people?

The feline immunodeficiency virus (FIV) shares genomic organization, receptor usage, lymphocyte tropism, and induction of immunodeficiency and increased susceptibility to cancer with the human immunodeficiency virus (HIV). Global distribution, marked heterogeneity and variable host adaptation are also properties of both viruses. These features render the FIV-cat model suitable to explore many aspects of lentivirus-host interaction and adaptation, and to explore treatment and prevention of infection. Examples of fundamental discoveries that have emerged from study in the FIV-cat model concern two-receptor entrance strategies that target memory T-lymphocytes, host factors that restrict retroviral infection, viral strategies for replication in non-dividing cells, and identification of correlates of immunity to the virus. This article provides a brief overview of strengths and limitations of the FIV-cat model for comparative biology and medicine.

Immunochemistry of adenoviruses: limitations and new horizons of gene therapy

Adenoviruses have increasingly been recognized as significant viral pathogens causing high morbidity and mortality especially among immunocompromised individuals such as transplant recipients and AIDS patients. Through the infection process, after the adenovirus fiber and penton are bonded to cell surface receptors through special amino acid moieties, secondary messengers activate protein kinases, pro-inflammatory cytokines and chemokines. Serotype and species specific antibodies also are induced. Recombinant human adenoviruses have been pivotal in the development of gene therapy strategies and have shown a great promise for the treatment of genetic disorders and malignancies. Recent studies have enlightened their harmful immunological effects dependent on fiber and hexon polypeptide structure and receptor binding. Pre-existing antibodies or those elicited by vectors neutralize input recombinant adenovirus particles rendering them ineffective. Mediators induce serious even lethal side effects and cytotoxic reactions which extinguish transgene expression. To overcome these difficulties new strategies are required in the application of recombinant adenoviruses to redirect vector entry from the natural receptors to alternative binding sites or using rare human or animal adenovirus fiber molecules to modify the native fiber structure by altering amino acid structure and creating chimeric fibers. This requires searching for, isolating and characterizing new serotypes, mutants or variants for new generation vectors. Human adenovirus 1 feline isolate (feline adenovirus) might fulfil these criteria.

Mutations in the feline immunodeficiency virus envelope glycoprotein confer resistance to a dominant-negative fragment of Tsg101 by enhancing infectivity and cell-to-cell virus transmission

The Pro-Ser-Ala-Pro (PSAP) motif in the p2 domain of feline immunodeficiency virus (FIV) Gag is required for efficient virus release, virus replication, and Gag binding to the ubiquitin-E2-variant (UEV) domain of Tsg101. As a result of this direct interaction, expression of an N-terminal fragment of Tsg101 containing the UEV domain (referred to as TSG-5') inhibits FIV release. In these respects, the FIV p2(Gag) PSAP motif is analogous to the PTAP motif of HIV-1 p6(Gag). To evaluate the feasibility of a late domain-targeted inhibition of virus replication, we created an enriched Crandell-Rees feline kidney (CRFK) cell line (T5'(hi)) that stably expresses high levels of TSG-5'. Here we show that mutations in either the V3 loop or the second heptad repeat (HR2) domain of the FIV envelope glycoprotein (Env) rescue FIV replication in T5'(hi) cells without increasing FIV release efficiency. TSG-5'-resistance mutations in Env enhance virion infectivity and the cell-cell spread of FIV when diffusion is limited using a semi-solid growth medium. These findings show that mutations in functional domains of Env confer TSG-5'-resistance, which we propose enhances specific infectivity and the cell-cell transmission of virus to counteract inefficient virus release. This article is part of a Special Issue entitled: Viral Membrane Proteins-Channels for Cellular Networking.

CXCR4 expression in feline mammary carcinoma cells: evidence of a proliferative role for the SDF-1/CXCR4 axis

BACKGROUND

Mammary tumours frequently develop in female domestic cats being highly malignant in a large percentage of cases. Chemokines regulate many physiological and pathological processes including organogenesis, chemotaxis of inflammatory cells, as well as tumour progression and metastasization. In particular, the chemokine/receptor pair SDF-1/CXCR4 has been involved in the regulation of metastatic potential of neoplastic cells, including breast cancer. The aim of this study was the immunohistochemical defininition of the expression profile of CXCR4 in primary and metastatic feline mammary carcinomas and the evaluation of the role of SDF-1 in feline mammary tumour cell proliferation.

RESULTS

A total of 45 mammary surgical samples, including 33 primary tumours (31 carcinomas and 2 adenomas), 6 metastases, and 4 normal mammary tissues were anlyzed. Tumor samples were collected from a total number of 26 animals, as in some cases concurrent occurrence of neoplasm in more than one mammary gland was observed. Tissues were processed for standard histological examination, and all lesions were classified according to the World Health Organization criteria. CXCR4 expression in neoplastic cells was evaluated by immunohistochemistry. The level of CXCR4 immunoreactivity was semi-quantitatively estimated as CXCR4 score evaluating both the number of positive cells and the intensity of staining. Six primary, fibroblast-free primary cultures were obtained from fresh feline mammary carcinomas and characterized by immunofluorescence for CXCR4 and malignant mammary cell marker expression. SDF-1-dependent in vitro proliferative effects were also assayed. CXCR4 expression was observed in 29 out of 31 malignant tissues with a higher CXCR4 score observed in 4 out of 6 metastatic lesions than in the respective primary tumours. In 2 benign lesions analyzed, only the single basaloid adenoma showed a mild positive immunostaining against CXCR4. Normal tissue did not show CXCR4 immunoreactivity. CXCR4 score was statistically significantly associated with the histological features of the samples, showing an increase accordingly with the degree of neoplastic transformation (from normal tissue to metastatic lesions). Finally, in the primary cultures obtained from 6 primary feline mammary carcinomas CXCR4 expression was detected in all cells and its activation by SDF-1 in vitro treatment caused a significant increase in the proliferation rate in 5 out of 6 tumours.

CONCLUSIONS

These results indicate that malignant feline mammary tumours commonly express CXCR4, with a higher level in malignant tumours, and, in most of the cases analysed, metastatic cells display stronger immunoreactivity for CXCR4 than the corresponding primary tumours. Moreover, CXCR4 activation in primary cultures of feline mammary carcinomas causes increase in the proliferative rate. Thus, SDF-1/CXCR4 system seems to play a tumorigenic in feline mammary gland malignancy and in vitro cultures from these tumour samples may represent an experimental model to investigate the biological and pharmacological role of this chemokinergic axis.

Viral transcriptome analysis of feline immunodeficiency virus infected cells using second generation sequencing technology

Feline immunodeficiency virus (FIV) is a widespread pathogen causing immunodeficiency in domestic cats and related wild cat species. The virus genome includes the main structural genes common to all retroviruses as well as accessory genes displaying essential functions during the viral life cycle. Expression of viral genes involves transcription of provirus genomes into full-length transcripts, which are partially processed into several spliced mRNA variants for the translation of particular proteins. Among several FIV isolates derived from domestic cats, notable differences in pathogenicity could be observed leading to identification of low and high pathogenic virus isolates. This study investigates the viral transcriptome of two differentially virulent FIV strains using second generation sequencing (SGS) technology. The expression levels of viral genes as detected by SGS were additionally determined by reverse transcription quantitative PCR analysis in order to compare two methods of mRNA quantification. The different properties of both methods, especially regarding normalization between samples, had to be considered when comparing the resulting data. SGS turned out to be a suitable technique for comparing mRNA transcription between both FIV infected cell lines and the identification of spliced viral transcripts. In contrast to this, the quantification of these spliced isoforms using SGS data was impeded by the short length of sequencing reads. In summary, SGS analysis revealed very consistent mRNA levels for the majority of viral genes between the low pathogenic Petaluma and the more highly pathogenic Glasgow 8 isolate. Notable differences among the two FIV strains could be observed in the viral mRNA splicing where Glasgow 8 displays similarities to the transcription pattern seen in the early stages of natural lentivirus infections. Thus, divergences in the regulation of post-transcriptional RNA processing might represent an additional contributor to the diverse pathogenic effects of individual FIV isolates. Taken together, this study aims to investigate the viral transcriptome as one part of the complex network of virus-host interactions, which will contribute to gaining deeper insights into FIV pathogenesis.

Structural basis for drug and substrate specificity exhibited by FIV encoding a chimeric FIV/HIV protease

A chimeric feline immunodeficiency virus (FIV) protease (PR) has been engineered that supports infectivity but confers sensitivity to the human immunodeficiency virus (HIV) PR inhibitors darunavir (DRV) and lopinavir (LPV). The 6s-98S PR has five replacements mimicking homologous residues in HIV PR and a sixth which mutated from Pro to Ser during selection. Crystal structures of the 6s-98S FIV PR chimera with DRV and LPV bound have been determined at 1.7 and 1.8 Å resolution, respectively. The structures reveal the role of a flexible 90s loop and residue 98 in supporting Gag processing and infectivity and the roles of residue 37 in the active site and residues 55, 57 and 59 in the flap in conferring the ability to specifically recognize HIV PR drugs. Specifically, Ile37Val preserves tertiary structure but prevents steric clashes with DRV and LPV. Asn55Met and Val59Ile induce a distinct kink in the flap and a new hydrogen bond to DRV. Ile98Pro→Ser and Pro100Asn increase 90s loop flexibility, Gln99Val contributes hydrophobic contacts to DRV and LPV, and Pro100Asn forms compensatory hydrogen bonds. The chimeric PR exhibits a comparable number of hydrogen bonds, electrostatic interactions and hydrophobic contacts with DRV and LPV as in the corresponding HIV PR complexes, consistent with IC(50) values in the nanomolar range.

Identification of amino acid residues important for heparan sulfate proteoglycan interaction within variable region 3 of the feline immunodeficiency virus surface glycoprotein

Heparan sulfate proteoglycans (HSPGs) act as binding receptors or attachment factors for the viral envelope of many viruses, including strains of HIV and feline immunodeficiency virus (FIV). The FIV gp95 glycoprotein (SU) from laboratory-adapted strains (tissue culture adapted [TCA]) such as FIV-34TF10 can bind to HSPG, whereas SU from field strains (FS) such as FIV-PPR cannot. Previous studies indicate that SU-HSPG interactions occur within the V3 loop. We utilized a series of nested V3 peptides to further map the HSPG binding sites and found that both sides of the predicted V3 loop stem were critical for the binding but not the CXCR4 binding domain near the predicted tip of the V3 loop. Neutralization assays for TCA strain entry using the same set of V3 peptides showed that peptides targeting CXCR4 or HSPG binding sites can block infection, supporting the V3 loop as a critical neutralization target. Site-directed mutagenesis identified two highly conserved arginines, R379 and R389, on the N-terminal side of the V3 stem as critical for the contact between SU and HSPG. Residues K407, K409, K410, and K412 on the C-terminal side of the V3 stem form a second nonconserved domain necessary for HSPG binding, consistent with the observed specificity distinctions with FS FIV. Our findings discriminate structural determinants important for HSPG and CXCR4 binding by FIV SU and thus further define the importance of the V3 loop for virus entry and infection.

Inflammation and epithelial cell injury in AIDS enteropathy: involvement of endoplasmic reticulum stress

Immunosuppressive lentivirus infections, including human, simian, and feline immunodeficiency viruses (HIV, SIV, and FIV, respectively), cause the acquired immunodeficiency syndrome (AIDS), frequently associated with AIDS enteropathy. Herein, we investigated the extent to which lentivirus infections affected mucosal integrity and intestinal permeability in conjunction with immune responses and activation of endoplasmic reticulum (ER) stress pathways. Duodenal biopsies from individuals with HIV/AIDS exhibited induction of IL-1β, CD3ε, HLA-DRA, spliced XBP-1(Xbp-1s), and CHOP expression compared to uninfected persons (P<0.05). Gut epithelial cells exposed to HIV-1 Vpr demonstrated elevated TNF-α, IL-1β, spliced Xbp-1s, and CHOP expression (P<0.05) together with calcium activation and disruption of epithelial cell monolayer permeability. In addition to reduced blood CD4(+) T lymphocyte levels, viral loads in the gut and plasma were high in FIV-infected animals (P<0.05). FIV-infected animals also exhibited a failure to gain weight and increased lactulose/mannitol ratios compared with uninfected animals (P<0.05). Proinflammatory and ER stress gene expression were activated in the ileum of FIV-infected animals (P<0.05), accompanied by intestinal epithelial damage with loss of epithelial cells and leukocyte infiltration of the lamina propria. Lentivirus infections cause gut inflammation and ensuing damage to intestinal epithelial cells, likely through induction of ER stress pathways, resulting in disruption of gut functional integrity.

Interplay between HIV entry and transportin-SR2 dependency

BACKGROUND

Transportin-SR2 (TRN-SR2, TNPO3, transportin 3) was previously identified as an interaction partner of human immunodeficiency virus type 1 (HIV-1) integrase and functions as a nuclear import factor of HIV-1. A possible role of capsid in transportin-SR2-mediated nuclear import was recently suggested by the findings that a chimeric HIV virus, carrying the murine leukemia virus (MLV) capsid and matrix proteins, displayed a transportin-SR2 independent phenotype, and that the HIV-1 N74D capsid mutant proved insensitive to transportin-SR2 knockdown.

RESULTS

Our present analysis of viral specificity reveals that TRN-SR2 is not used to the same extent by all lentiviruses. The DNA flap does not determine the TRN-SR2 requirement of HIV-1. We corroborate the TRN-SR2 independent phenotype of the chimeric HIV virus carrying the MLV capsid and matrix proteins. We reanalyzed the HIV-1 N74D capsid mutant in cells transiently or stably depleted of transportin-SR2 and confirm that the N74D capsid mutant is independent of TRN-SR2 when pseudotyped with the vesicular stomatitis virus glycoprotein (VSV-G). Remarkably, although somewhat less dependent on TRN-SR2 than wild type virus, the N74D capsid mutant carrying the wild type HIV-1 envelope required TRN-SR2 for efficient replication. By pseudotyping with envelopes that mediate pH-independent viral uptake including HIV-1, measles virus and amphotropic MLV envelopes, we demonstrate that HIV-1 N74D capsid mutant viruses retain partial dependency on TRN-SR2. However, this dependency on TRN-SR2 is lost when the HIV N74D capsid mutant is pseudotyped with envelopes mediating pH-dependent endocytosis, such as the VSV-G and Ebola virus envelopes.

CONCLUSION

Here we discover a link between the viral entry of HIV and its interaction with TRN-SR2. Our data confirm the importance of TRN-SR2 in HIV-1 replication and argue for careful interpretation of experiments performed with VSV-G pseudotyped viruses in studies on early steps of HIV replication including the role of capsid therein.

Optimal packaging of FIV genomic RNA depends upon a conserved long-range interaction and a palindromic sequence within gag

The feline immunodeficiency virus (FIV) is a lentivirus that is related to human immunodeficiency virus (HIV), causing a similar pathology in cats. It is a potential small animal model for AIDS and the FIV-based vectors are also being pursued for human gene therapy. Previous studies have mapped the FIV packaging signal (ψ) to two or more discontinuous regions within the 5' 511 nt of the genomic RNA and structural analyses have determined its secondary structure. The 5' and 3' sequences within ψ region interact through extensive long-range interactions (LRIs), including a conserved heptanucleotide interaction between R/U5 and gag. Other secondary structural elements identified include a conserved 150 nt stem-loop (SL2) and a small palindromic stem-loop within gag open reading frame that might act as a viral dimerization initiation site. We have performed extensive mutational analysis of these sequences and structures and ascertained their importance in FIV packaging using a trans-complementation assay. Disrupting the conserved heptanucleotide LRI to prevent base pairing between R/U5 and gag reduced packaging by 2.8-5.5 fold. Restoration of pairing using an alternative, non-wild type (wt) LRI sequence restored RNA packaging and propagation to wt levels, suggesting that it is the structure of the LRI, rather than its sequence, that is important for FIV packaging. Disrupting the palindrome within gag reduced packaging by 1.5-3-fold, but substitution with a different palindromic sequence did not restore packaging completely, suggesting that the sequence of this region as well as its palindromic nature is important. Mutation of individual regions of SL2 did not have a pronounced effect on FIV packaging, suggesting that either it is the structure of SL2 as a whole that is necessary for optimal packaging, or that there is redundancy within this structure. The mutational analysis presented here has further validated the previously predicted RNA secondary structure of FIV ψ.

Domain- and nucleotide-specific Rev response element regulation of feline immunodeficiency virus production

Computational analysis of feline immunodeficiency virus (FIV) RNA sequences indicated that common FIV strains contain a rev response element (RRE) defined by a long unbranched hairpin with 6 stem-loop sub-domains, termed stem-loop A (SLA). To examine the role of the RNA secondary structure of the RRE, mutational analyses were performed in both an infectious FIV molecular clone and a FIV CAT-RRE reporter system. These studies disclosed that the stems within SLA (SA1, 2, 3, 4, and 5) of the RRE were critical but SA6 was not essential for FIV replication and CAT expression. These studies also revealed that the secondary structure rather than an antisense protein (ASP) mediates virus expression and replication in vitro. In addition, a single synonymous mutation within the FIV-RRE, SA3/45, reduced viral reverse transcriptase activity and p24 expression after transfection but in addition also showed a marked reduction in viral expression and production following infection.

Fine definition of the CXCR4-binding region on the V3 loop of feline immunodeficiency virus surface glycoprotein

The chemokine receptor CXCR4 is shared by primary and laboratory-adapted strains of feline immunodeficiency virus (FIV) for viral entry. Our previous studies implicated a contiguous nine-amino-acid region of the V3 loop of the FIV envelope surface as important in CXCR4 binding and virus entry. The binding is specific for CXCR4 since it can be inhibited by AMD3100, a selective CXCR4 inhibitor. Additional site-directed mutagenesis was used to further reveal the key residues. Binding studies indicated that basic residues R395, K397, R399 as well as N398 are critical for CXCR4 binding. The effect of other amino acid residues on receptor binding depends on the type of amino acid residue substituted. The binding study results were confirmed on human CXCR4-expressing SupT1 cells and correlated with entry efficiency using a virus entry assay. Amino acid residues critical for CXCR4 are not critical for interactions with the primary binding receptor CD134, which has an equivalent role as CD4 for HIV-1 binding. The ELISA results show that W394 and W400 are crucial for the recognition by neutralizing anti-V3 antibodies. Since certain strains of HIV-1 also use CXCR4 as the entry receptor, the findings make the feline model attractive for development of broad-based entry antagonists and for study of the molecular mechanism of receptor/virus interactions.

Generation of infectious feline immunodeficiency virus (FIV) encoding FIV/human immunodeficiency virus chimeric protease

Feline immunodeficiency virus (FIV) and human immunodeficiency virus type 1 (HIV-1) proteases (PRs) share only 23% amino acid identity and exhibit distinct specificities yet have very similar 3-dimensional structures. Chimeric PRs in which HIV residues were substituted in structurally equivalent positions in FIV PR were prepared in order to study the molecular basis of PR specificity. Previous in vitro analyses showed that such substitutions dramatically altered the inhibitor specificity of mutant PRs but changed the rate and specificity of Gag cleavage so that chimeric FIVs were not infectious. Chimeric PRs encoding combinations of the I37V, N55M, M56I, V59I, L97T, I98P, Q99V, and P100N mutations were cloned into FIV Gag-Pol, and those constructs that best approximated the temporal cleavage pattern generated by wild-type FIV PR, while maintaining HIV-like inhibitor specificity, were selected. Two mutations, M56I and L97T, were intolerant to change and caused inefficient cleavage at NC-p2. However, a mutant PR with six substitutions (I37V, N55M, V59I, I98P, Q99V, and P100N) was selected and placed in the context of full-length FIV-34TF10. This virus, termed YCL6, had low-level infectivity ex vivo, and after passage, progeny that exhibited a higher growth rate emerged. The residue at the position of one of the six mutations, I98P, further mutated on passage to either P98H or P98S. Both PRs were sensitive to the HIV-1 PR inhibitors lopinavir (LPV) and darunavir (DRV), as well as to the broad-based inhibitor TL-3, with 50% inhibitory concentrations (IC(50)) of 30 to 40 nM, consistent with ex vivo results obtained using mutant FIVs. The chimeras offer an infectivity system with which to screen compounds for potential as broad-based PR inhibitors, define structural parameters that dictate specificity, and investigate pathways for drug resistance development.

FIV Gag: virus assembly and host-cell interactions

Infection of domestic cats with virulent strains of the feline immunodeficiency virus (FIV) leads to an acquired immunodeficiency syndrome (AIDS), similar to the pathogenesis induced in humans by infection with human immunodeficiency virus type 1 (HIV-1). Thus, FIV is a highly relevant model for anti-HIV therapy and vaccine development. FIV is not infectious in humans, so it is also a potentially effective non-toxic gene therapy vector. To make better use of this model, it is important to define the cellular machinery utilized by each virus to produce virus particles so that relevant similarities can be identified. It is well understood that all replication-competent retroviruses encode gag, pol, and env genes, which provide core elements for virus replication. As a result, most antiretroviral therapy targets pol-derived enzymes (protease, reverse transcriptase, and integrase) orenv-derived glycoproteins that mediate virus attachment and entry. However, resistance to drugs against these targets is a persistent problem, and novel targets must be identified to produce more effective drugs that can either substitute or be combined with current therapy. Elements of the gag gene (matrix, capsid, nucleocapsid, and "late" domains) have yet to be exploited as antiviral targets, even though the Gag precursor polyprotein is self-sufficient for the assembly and release of virus particles from cells. This process is far better understood in primate lentiviruses, especially HIV-1. However, there has been significant progress in recent years in defining how FIV Gag is targeted to the cellular plasma membrane, assembles into virions, incorporates FIV Env glycoproteins, and utilizes host cell machinery to complete virus release. Recent discoveries of intracellular restriction factors that target HIV-1 and FIV capsids after virus entry have also opened exciting new areas of research. This review summarizes currently known interactions involving HIV-1 and FIV Gag that affect virus release, infectivity, and replication.

Regulation of lentivirus neurovirulence by lipopolysaccharide conditioning: suppression of CXCL10 in the brain by IL-10

Lentivirus infections including HIV and feline immunodeficiency virus (FIV) cause neurovirulence, which is largely mediated by innate immunity. To investigate the interactions between neurovirulence and repeated conditioning by innate immune activation, models of lentivirus infection were exposed to LPS. Gene expression in HIV-infected (HIV+) and control (HIV-) patient brains was compared by real time RT-PCR and immunocytochemistry. Supernatants from mock and HIV-infected monocyte-derived macrophages exposed to LPS were applied to human neurons. FIV-infected (FIV+) and control (FIV-) animals were exposed repeatedly to LPS postinfection together with concurrent neurobehavioral testing, viral load, and host gene analyses. Brains from HIV+ individuals exhibited induction of CD3epsilon, CXCL10, and granzyme A expression (p < 0.05). Supernatants from HIV+ monocyte-derived macrophages induced CXCL10 expression in neurons, which was diminished by IL-10 treatment (p < 0.05). LPS-exposed FIV+ animals demonstrated lower plasma and brain viral loads (p < 0.05). Neuronal CXCL10 expression was increased in FIV+ animals but was suppressed by LPS exposure, together with reduced brain CD3epsilon and granzyme A expression (p < 0.05). In conjunction with preserved NeuN-positive neuronal counts in parietal cortex (p < 0.05), FIV+ animals exposed to LPS also showed less severe neurobehavioral deficits (p < 0.05). Repeated LPS exposures suppressed CXCL10 in the brain and ensuing T cell infiltration with a concomitant reduction in neurovirulence. Thus, innate immune chronic conditioning exerted beneficial effects on neurovirulence through suppression of a specific chemotactic factor, CXCL10, mediated by IL-10, leading to reduced leukocyte infiltration and release of neurotoxic factors.

Quantitative TaqMan real-time PCR assays for gene expression normalisation in feline tissues

Background

Gene expression analysis is an important tool in contemporary research, with real-time PCR as the method of choice for quantifying transcription levels. Co-analysis of suitable reference genes is crucial for accurate expression normalisation. Reference gene expression may vary, e.g., among species or tissues; thus, candidate genes must be tested prior to use in expression studies. The domestic cat is an important study subject in both medical research and veterinary medicine. The aim of the present study was to develop TaqMan^® real-time PCR assays for eight potential reference genes and to test their applicability for feline samples, including blood, lymphoid, endocrine, and gastrointestinal tissues from healthy cats, and neoplastic tissues from FeLV-infected cats.

Results

RNA extraction from tissues was optimised for minimal genomic DNA (gDNA) contamination without use of a DNase treatment. Real-time PCR assays were established and optimised for v-abl Abelson murine leukaemia viral oncogene homolog (ABL), β-actin (ACTB), β-2-microglobulin (B2M), β-glucuronidase (GUSB), hydroxymethyl-bilane synthase (HMBS), hypoxanthine phosphoribosyltransferase (HPRT), ribosomal protein S7 (RPS7), and tryptophan 5-monooxygenase activation protein, zeta polypeptide (YWHAZ). The presence of pseudogenes was confirmed for four of the eight investigated genes (ACTB, HPRT, RPS7, and YWHAZ). The assays were tested together with previously developed TaqMan^® assays for feline glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the universal 18S rRNA gene. Significant differences were found among the expression levels of the ten candidate reference genes, with a ~10⁶-fold expression difference between the most abundant (18S rRNA) and the least abundant genes (ABL, GUSB, and HMBS). The expression stability determined by the geNorm and NormFinder programs differed significantly. Using the ANOVA-based NormFinder program, RPS7 was the most stable gene in the tissues studied, followed by ACTB and ABL; B2M, HPRT, and the 18S rRNA genes were the least stable ones.

Conclusion

The reference gene expression stability varied considerably among the feline tissues investigated. No tested gene was optimal for normalisation in all tissues. For the majority of the tissues, two to three reference genes were necessary for accurate normalisation. The present study yields essential information on the correct choice of feline reference genes depending on the tissues analysed.

An EIAV field isolate reveals much higher levels of subtype variability than currently reported for the equine lentivirus family

Background

Equine infectious anemia virus (EIAV), a lentivirus that infects horses, has been utilized as an animal model for the study of HIV. Furthermore, the disease associated with the equine lentivirus poses a significant challenge to veterinary medicine around the world. As with all lentiviruses, EIAV has been shown to have a high propensity for genomic sequence and antigenic variation, especially in its envelope (Env) proteins. Recent studies have demonstrated Env variation to be a major determinant of vaccine efficacy, emphasizing the importance of defining natural variation among field isolates of EIAV. To date, however, published EIAV sequences have been reported only for cell-adapted strains of virus, predominantly derived from a single primary virus isolate, EIAV_Wyoming (EIAV_WY).

Results

We present here the first characterization of the Env protein of a natural primary isolate from Pennsylvania (EIAV_PA) since the widely utilized and referenced EIAV_WY strain. The data demonstrated that the level of EIAV_PA Env amino acid sequence variation, approximately 40% as compared to EIAV_WY, is much greater than current perceptions or published reports of natural EIAV variation between field isolates. This variation did not appear to give rise to changes in the predicted secondary structure of the proteins. While the EIAV_PA Env was serologically cross reactive with the Env proteins of the cell-adapted reference strain, EIAV_PV (derivative of EIAV_WY), the two variant Envs were shown to lack any cross neutralization by immune serum from horses infected with the respective virus strains.

Conclusion

Taking into account the significance of serum neutralization to universal vaccine efficacy, these findings are crucial considerations towards successful EIAV vaccine development and the potential inclusion of field isolate Envs in vaccine candidates.

Restrictions to cross-species transmission of lentiviral infection gleaned from studies of FIV

More than 40 species of primates and over 20 species of cats harbor antibodies that sero-react to lentiviral antigens. In nearly all cases where viral genetic analysis has been conducted, each host species is infected with a unique lentivirus. Though lentivirus clades within a species can be substantially divergent, they are typically monophyletic within that species. A notable significant departure from this observation is apparent cross-species transmission of FIV between bobcats (Lynx rufus) and pumas (Puma concolor) in Southern California that has occurred at least three times; evidence from one bobcat sequence suggests this cross-over may have also occurred in Florida between bobcats and the endangered Florida panther. Several other isolated reports demonstrate cross-species transmission of FIV isolates among captive animals housed in close proximity, and it is well established that HIV-1 and HIV-2 arose from human contact with SIV-infected non-human primates. Using an experimental model, we have determined that domestic cats (Felis catus) are susceptible to FIVs originating from pumas or lions. While infections are initially replicative, and animals seroconvert, within a relatively short period of time circulating virus is reduced to nearly undetectable levels in a majority of animals. This diminution of viral load is proportional to initial viral peak. Although viral reservoirs can be identified in gastrointestinal tissues, most viral genomes recovered peripherally are highly mutated, suggesting that the non-adapted host successfully inhibits normal viral replication, leading to replication incompetent viral progeny. Mechanisms possible for such restriction of cross-species infections in natural settings include: (1) Lack of contact conducive to lentiviral transmission between infected and shedding animals of different species; (2) Lack of suitable receptor repertoire to allow viral entry to susceptible cells of a new species; (3) Cellular machinery in the new host sufficiently divergent from the primary host to support viral replication (i.e. passive unfacilitated viral replication); (4) Intracellular restriction mechanisms present in the new host that is able to limit viral replication (i.e. active interrupted viral replication. These include factors that limit uncoating, replication, packaging, and virion release); (5) Unique ability of new host to raise sterilizing adaptive immunity, resulting in aborted infection and inability to spread infections among con-specifics; or (6) Production of defective or non-infectious viral progeny that lack cellular cofactors to render them infectious to con-specifics (i.e. particles lacking appropriate cellular components in viral Env to render them infectious to other animals of the same species). Data to support or refute the relative importance of each of these possibilities is described in this review. Insights based on our in vivo cross-species model suggest intracellular restriction mechanisms effectively inhibit rapid inter-specific transmission of lentiviruses. Further, limited contact both within and between species in natural populations is highly relevant to limiting the opportunity for spread of FIV strains. Studies of naturally occurring SIV and innate host restriction systems suggest these same two mechanisms are significant factors inhibiting widespread cross-species transmission of lentiviruses among primate species as well.

Design and synthesis of membrane fusion inhibitors against the feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a pathogenic virus that causes an AIDS-like syndrome in the domestic cats. For viral entry and infection, fusion between the virus and the cell membrane is the critical process and this process is mediated by an envelope glycoprotein gp40. We have identified fusion inhibitory peptides from the heptad repeat-2 (HR2) of gp40. Remodeling of the original sequences using alpha-helix-inducible motifs revealed the interactive residues of gp40. Comparative analysis of HR2 peptides derived from four FIV strains demonstrated that the interactive surface of the Shizuoka strain-derived HR2 peptides provides the highest affinity of all the FIV strains examined.

CXCR3 activation by lentivirus infection suppresses neuronal autophagy: neuroprotective effects of antiretroviral therapy

Previous studies have implicated CXCL12 in the neuropathogenesis of HIV infection. Proteolysis of CXCL12 generates a neurotoxic molecule, CXCL12(5-67), which engages and activates CXCR3, in addition to exhibiting increased expression in the brains of patients with HIV-associated dementia (HAD). Herein, we investigated CXCR3-mediated neuronal injury, particularly, its contribution to autophagy suppression and the concomitant effects of antiretroviral therapy using human brain samples and models of HIV neuropathogenesis. Neurons in the brains of HAD patients and feline immunodeficiency virus (FIV)-infected animals, as well as cultured human neurons, expressed CXCR3, which was modulated in a ligand-specific manner. Exposure of human neurons to CXCL12(5-67) caused a reduction in the autophagy-associated molecule LC3 (P<0.05) and neuronal survival (P<0.05), which recapitulated findings in FIV- and HIV-infected brains (P<0.05). Oral didanosine (ddI) treatment of FIV-infected animals reduced neurobehavioral abnormalities in conjunction with diminished plasma viral load (P<0.05). F4/80 transcript abundance and CXCL12(5-67) immunoreactivity were reduced with restored neuronal LC3 expression in the brains of FIV-infected animals after ddI treatment (P<0.05). ddI treatment also prevented microglial activation and depletion of synaptic proteins in the cortex of FIV-infected animals (P<0.05). These findings indicate that the beneficial effects of ddI might be a consequence of a reduced systemic viral burden and concurrent leukocyte activation, leading to diminished neuroinflammation with preservation of neuronal autophagy by regulating CXCR3 activation.

The secondary structure of the 5' end of the FIV genome reveals a long-range interaction between R/U5 and gag sequences, and a large, stable stem-loop

Feline immunodeficiency virus (FIV) is a lentivirus that infects cats and is related to human immunodeficiency virus (HIV). Although it is a common worldwide infection, and has potential uses as a human gene therapy vector and as a nonprimate model for HIV infection, little detail is known of the viral life cycle. Previous experiments have shown that its packaging signal includes two or more regions within the first 511 nucleotides of the genomic RNA. We have undertaken a secondary structural analysis of this RNA by minimal free-energy structural prediction, biochemical mapping, and phylogenetic analysis, and show that it contains five conserved stem-loops and a conserved long-range interaction between heptanucleotide sequences 5'-CCCUGUC-3' in R/U5 and 5'-GACAGGG-3' in gag. This long-range interaction is similar to that seen in primate lentiviruses where it is thought to be functionally important. Along with strains that infect domestic cats, this heptanucleotide interaction can also occur in species-specific FIV strains that infect pumas, lions, and Pallas' cats where the heptanucleotide sequences involved vary. We have analyzed spliced and genomic FIV RNAs and see little structural change or sequence conservation within single-stranded regions of the 5' UTR that are important for viral packaging, suggesting that FIV may employ a cotranslational packaging mechanism.