The feline lymphoid cell line MBM and its use for feline immunodeficiency virus isolation and quantitation

Feline Adenovirus Isolate Shows Silent Nucleotide Alterations, Alternative Receptor/Coreceptor Binding, High Resistance to Disinfectants and Antiviral Drugs, as Well as Immunomodulation

Adenovirus (AdV) infection has been rarely documented in cats and other felids. Partial sequences of the hexon and fiber genes of a Hungarian feline adenovirus isolate (FeAdV isolate) showed a close relationship to human AdV (HAdV) type C1. Further molecular and biological characterization is reported here. Whole-genome sequencing revealed two silent mutations in the genome of the FeAdV isolate compared to a HAdV-C1 reference strain (at positions 14,096 and 15,082). Competitive antibody binding to the Coxsackie-adenovirus receptor and αvβ3 and αvβ5 integrin coreceptors inhibited the binding of the FeAdV isolate in different cell lines, but residual infections suggested alternative entry routes. The FeAdV isolate was found to be more sensitive to heat, low pH and detergents, but more resistant to alkaline and free chlorine treatments, as well as to ribavirin, stavudine and cidofovir treatments, than other human AdV types. We observed a suppression of IL-10 and TGF-β1 production during the entire course of viral replication. This immunomodulation may restore intratumoral immunity; thus, the FeAdV isolate could serve as an alternative oncolytic vector. Collectively, our results support that the Hungarian FeAdV isolate is a variant of common HAdV-C1. The cohabitation of cats with humans might result in reverse zoonotic infection. Felids appear to be susceptible to persistent and productive adenovirus infection, but further studies are needed to better understand the clinical and epidemiological implications.

Nonhuman Primates and Humanized Mice for Studies of HIV-1 Integrase Inhibitors: A Review

Since the discovery of the first inhibitors of HIV replication, drug resistance has been a major problem in HIV therapy due in part to the high mutation rate of HIV. Therefore, the development of a predictive animal model is important to identify impending resistance mutations and to possibly inform treatment decisions. Significant advances have been made possible through use of nonhuman primates infected by SIV, SHIV, and simian-tropic HIV-1 (stHIV-1), and use of humanized mouse models of HIV-1 infections. In this review, we describe some of the findings from animal models used for the preclinical testing of integrase strand transfer inhibitors. These models have led to important findings about the potential role of integrase strand transfer inhibitors in both the prevention and treatment of HIV-1 infection.

Pharmacological inhibition of feline immunodeficiency virus (FIV)

Feline immunodeficiency virus (FIV) is a member of the retroviridae family of viruses and causes an acquired immunodeficiency syndrome (AIDS) in domestic and non-domestic cats worldwide. Genome organization of FIV and clinical characteristics of the disease caused by the virus are similar to those of human immunodeficiency virus (HIV). Both viruses infect T lymphocytes, monocytes and macrophages, and their replication cycle in infected cells is analogous. Due to marked similarity in genomic organization, virus structure, virus replication and disease pathogenesis of FIV and HIV, infection of cats with FIV is a useful tool to study and develop novel drugs and vaccines for HIV. Anti-retroviral drugs studied extensively in HIV infection have targeted different steps of the virus replication cycle: (1) inhibition of virus entry into susceptible cells at the level of attachment to host cell surface receptors and co-receptors; (2) inhibition of fusion of the virus membrane with the cell membrane; (3) blockade of reverse transcription of viral genomic RNA; (4) interruption of nuclear translocation and viral DNA integration into host genomes; (5) prevention of viral transcript processing and nuclear export; and (6) inhibition of virion assembly and maturation. Despite much success of anti-retroviral therapy slowing disease progression in people, similar therapy has not been thoroughly investigated in cats. In this article we review current pharmacological approaches and novel targets for anti-lentiviral therapy, and critically assess potentially suitable applications against FIV infection in cats.

Feline immunodeficiency virus (FIV) neutralization: a review

One of the major obstacles that must be overcome in the design of effective lentiviral vaccines is the ability of lentiviruses to evolve in order to escape from neutralizing antibodies. The primary target for neutralizing antibodies is the highly variable viral envelope glycoprotein (Env), a glycoprotein that is essential for viral entry and comprises both variable and conserved regions. As a result of the complex trimeric nature of Env, there is steric hindrance of conserved epitopes required for receptor binding so that these are not accessible to antibodies. Instead, the humoral response is targeted towards decoy immunodominant epitopes on variable domains such as the third hypervariable loop (V3) of Env. For feline immunodeficiency virus (FIV), as well as the related human immunodeficiency virus-1 (HIV-1), little is known about the factors that lead to the development of broadly neutralizing antibodies. In cats infected with FIV and patients infected with HIV-1, only rarely are plasma samples found that contain antibodies capable of neutralizing isolates from other clades. In this review we examine the neutralizing response to FIV, comparing and contrasting with the response to HIV. We ask whether broadly neutralizing antibodies are induced by FIV infection and discuss the comparative value of studies of neutralizing antibodies in FIV infection for the development of more effective vaccine strategies against lentiviral infections in general, including HIV-1.

Env-expressing autologous T lymphocytes induce neutralizing antibody and afford marked protection against feline immunodeficiency virus

The envelope (Env) glycoproteins of HIV and other lentiviruses possess neutralization and other protective epitopes, yet all attempts to induce protective immunity using Env as the only immunogen have either failed or afforded minimal levels of protection. In a novel prime-boost approach, specific-pathogen-free cats were primed with a plasmid expressing Env of feline immunodeficiency virus (FIV) and feline granulocyte-macrophage colony-stimulating factor and then boosted with their own T lymphocytes transduced ex vivo to produce the same Env and interleukin 15 (3 x 10(6) to 10 x 10(6) viable cells/cat). After the boost, the vaccinees developed elevated immune responses, including virus-neutralizing antibodies (NA). Challenge with an ex vivo preparation of FIV readily infected all eight control cats (four mock vaccinated and four naïve) and produced a marked decline in the proportion of peripheral CD4 T cells. In contrast, five of seven vaccinees showed little or no traces of infection, and the remaining two had reduced viral loads and underwent no changes in proportions of CD4 T cells. Interestingly, the viral loads of the vaccinees were inversely correlated to the titers of NA. The findings support the concept that Env is a valuable immunogen but needs to be administered in a way that permits the expression of its full protective potential.

Establishment of a feline astrocyte-derived cell line (G355-5 cells) expressing feline CD134 and a rapid quantitative assay for T-lymphotropic feline immunodeficiency viruses

Few laboratory strains of feline immunodeficiency virus (FIV) can infect Crandell feline kidney cells (an epithelial-type of cells), however, most primary isolates are T-lymphotropic. T-lymphotropic FIV requires both feline CD134 (an activation marker of helper T-lymphocytes) and CXCR4 (a chemokine receptor) in infection as primary and secondary receptors, respectively. Using feline T-lymphoblastoid cell lines, titration of primary FIV isolates was carried out, however the titration assay was laborious and time-consuming. In this study, using G355-5 cells (a feline astrocyte-derived cell line) transduced with a cDNA of feline CD134 as target cells, an assay system was developed to quantitate primary FIV isolates. With a previous method using a feline T-lymphoblastoid cell line (MYA-1 cells) highly sensitive to FIV, it took 12 days to complete the assay, however, it took only 2 days with the new method. The FIV-infected cells became in a state of persistent infection, producing a large amount of FIV, indicating that the cells will be useful for propagation of T-lymphotropic FIV strains.

Evaluation of feline monocyte-derived dendritic cells loaded with internally inactivated virus as a vaccine against feline immunodeficiency virus

Dendritic cells are the only antigen-presenting cells that can present exogenous antigens to both helper and cytolytic T cells and prime Th1-type or Th2-type cellular immune responses. Given their unique immune functions, dendritic cells are considered attractive "live adjuvants" for vaccination and immunotherapy against cancer and infectious diseases. The present study was carried out to assess whether the reinjection of autologous monocyte-derived dendritic cells loaded with an aldithriol-2-inactivated primary isolate of feline immune deficiency virus (FIV) was able to elicit protective immune responses against the homologous virus in naive cats. Vaccine efficacy was assessed by monitoring immune responses and, finally, by challenge with the homologous virus of vaccinated, mock-vaccinated, and healthy cats. The outcome of challenge was followed by measuring cellular and antibody responses and viral and proviral loads and quantitating FIV by isolation and a count of CD4(+)/CD8(+) T cells in blood. Vaccinated animals exhibited clearly evident FIV-specific peripheral blood mononuclear cell proliferation and antibody titers in response to immunization; however, they became infected with the challenge virus at rates comparable to those of control animals.

Human immunodeficiency virus integrase inhibitors efficiently suppress feline immunodeficiency virus replication in vitro and provide a rationale to redesign antiretroviral treatment for feline AIDS

BACKGROUND

Treatment of feline immunodeficiency virus (FIV) infection has been hampered by the absence of a specific combination antiretroviral treatment (ART). Integrase strand transfer inhibitors (INSTIs) are emerging as a promising new drug class for HIV-1 treatment, and we evaluated the possibility of inhibiting FIV replication using INSTIs.

METHODS

Phylogenetic analysis of lentiviral integrase (IN) sequences was carried out using the PAUP* software. A theoretical three-dimensional structure of the FIV IN catalytic core domain (CCD) was obtained by homology modeling based on a crystal structure of HIV-1 IN CCD. The interaction of the transferred strand of viral DNA with the catalytic cavity of FIV IN was deduced from a crystal structure of a structurally similar transposase complexed with transposable DNA. Molecular docking simulations were conducted using a genetic algorithm (GOLD). Antiviral activity was tested in feline lymphoblastoid MBM cells acutely infected with the FIV Petaluma strain. Circular and total proviral DNA was quantified by real-time PCR.

RESULTS

The calculated INSTI-binding sites were found to be nearly identical in FIV and HIV-1 IN CCDs. The close similarity of primate and feline lentivirus IN CCDs was also supported by phylogenetic analysis. In line with these bioinformatic analyses, FIV replication was efficiently inhibited in acutely infected cell cultures by three investigational INSTIs, designed for HIV-1 and belonging to different classes. Of note, the naphthyridine carboxamide INSTI, L-870,810 displayed an EC50 in the low nanomolar range. Inhibition of FIV integration in situ was shown by real-time PCR experiments that revealed accumulation of circular forms of FIV DNA within cells treated with L-870,810.

CONCLUSION

We report a drug class (other than nucleosidic reverse transcriptase inhibitors) that is capable of inhibiting FIV replication in vitro. The present study helped establish L-870,810, a compound successfully tested in human clinical trials, as one of the most potent anti-FIV agents ever tested in vitro. This finding may provide new avenues for treating FIV infection and contribute to the development of a small animal model mimicking the effects of ART in humans.

Effects of feline immunodeficiency virus on feline monocyte-derived dendritic cells infected by spinoculation

During type 1 human immunodeficiency virus infection, not only can dendritic cells (DCs) prime T cells against the virus, but they can also infect them in trans. Feline AIDS is caused by feline immunodeficiency virus (FIV) and is considered a model for the human illness because the two diseases have many features in common. Little is known about the interaction of feline DCs with FIV; therefore, this study attempts to tackle such an issue. Infection of feline monocyte-derived DCs (MDDCs) was attempted by spinoculation with FIV strains Petaluma (FIV-Pet) and M2. FIV-Pet was released rapidly in the supernatants of both infected MDDCs and activated T cells after spinoculation. It is shown that FIV-Pet was produced by MDDCs by monitoring viral content in the supernatants of infected MDDCs, by intracellular staining for p25 and by showing its cytopathic effect. Although activated T cells were better substrates for FIV replication, leading to prolonged viral shedding, both immature MDDCs and MDDCs matured with lipopolysaccharide supported virus production, mostly during the first 2 days after infection. At later times, FIV induced syncytium formation by MDDCs. Concerning the FIV receptors, MDDCs were shown to be CD134-negative and CXCR4-positive, a phenotype compatible with permissiveness to FIV-Pet. These results also suggest that maturation is not hampered by FIV infection and that virus exposure itself does not induce MDDC maturation. It is also shown that infected MDDCs can infect activated PBMCs efficiently in trans. It is concluded that MDDCs can be infected by FIV, although infection does not appear to influence their functionality.

Antibodies generated in cats by a lipopeptide reproducing the membrane-proximal external region of the feline immunodeficiency virus transmembrane enhance virus infectivity

The immunogenicity of a lipoylated peptide (lipo-P59) reproducing the membrane-proximal external region (MPER) of the transmembrane glycoprotein of feline immunodeficiency virus (FIV) was investigated with cats. In the attempt to mimic the context in which MPER is located within intact virions, lipo-P59 was administered in association with membrane-like micelles. Analyses showed that in this milieu, lipo-P59 had a remarkable propensity to be positioned at the membrane interface, displayed a large number of ordered structures folded in turn helices, and was as active as lipo-P59 alone at inhibiting FIV infectivity in vitro. The antibodies developed differed from the ones previously obtained by immunizing cats with the nonlipoylated version of the peptide (G. Freer, S. Giannecchini, A. Tissot, M. F. Bachmann, P. Rovero, P. F. Serres, and M. Bendinelli, Virology 322:360-369, 2004) in epitope specificity and in the fact that they bound FIV virions. However, they too lacked virus-neutralizing activity and actually enhanced FIV infectivity for lymphoid cell cultures. It is concluded that the use of MPER-reproducing oligopeptides is not a viable approach for vaccinating against FIV.

Feline Immunodeficiency Virus Plasma Load Reduction by a Retroinverso Octapeptide Reproducing the Trp-Rich Motif of the Transmembrane Glycoprotein

The Trp-rich motif (TrpM) of the transmembrane glycoprotein (TM) of lentiviruses is an attractive domain on which to design new potential cell entry peptide inhibitors. We recently demonstrated that an octapeptide reproducing the TrpM of feline immunodeficiency virus (FIV), designated C8, broadly inhibited this virus in vitro and that the retroinverso analogue of this peptide (riC8) was almost as inhibitory and exhibited features suggestive of a much increased stability. Here, we demonstrated that riC8 is indeed highly stable, maintaining its concentration unchanged for at least 24 h in cat serum in vitro. Furthermore, once inoculated into cats, riC8 produced no major acute toxic effects and exhibited satisfactory pharmacokinetic properties. Finally, we report the results of a short-term monotherapy experiment in chronically FIV-infected cats showing that riC8 is well tolerated and also has substantial antiviral activity in vivo. In particular, the mean viral load of riC8-treated animals declined progressively with increasing time of treatment, whereas that of control animals given C8 or solvent alone did not. These results provide the first evidence that clinically useful inhibition of virus replication with a small peptide derived from a functional domain of the TM of a lentivirus can be achieved in vivo.

Evolution of two amino acid positions governing broad neutralization resistance in a strain of feline immunodeficiency virus over 7 years of persistence in cats

Fresh isolates of lentiviruses are characterized by an outstanding resistance to antibody-mediated neutralization. By investigating the changes that occurred in a neutralization-sensitive tissue culture-adapted strain of feline immunodeficiency virus after it was reinoculated into cats, a previous study had identified two amino acid positions of the surface glycoprotein (residues 481 and 557) which govern broad neutralization resistance (BNR) in this virus. By extending the follow-up of six independently evolving in vivo variants of such virus for up to 92 months, we now show that the changes at the two BNR-governing positions not only were remarkably stereotyped but also became fixed in an ordered sequential fashion with the duration of in vivo infection. In one variant, the two positions were also seen to slowly alternate at determining BNR. Evidence that evolution at the BNR-governing positions was accompanied, and possibly driven, by changes in the antigenic makeup of the viral surface brought about by the mutations at such positions is also presented.

Dissection of seroreactivity against the tryptophan-rich motif of the feline immunodeficiency virus transmembrane glycoprotein

Immunogenicity of the tryptophan-rich motif (TrpM) in the membrane-proximal ectodomain of the transmembrane (TM) glycoprotein of feline immunodeficiency virus (FIV) was investigated. Peptide 59, a peptide containing the TrpM of the TM of FIV, was covalently coupled to Qbeta phage virus-like particles (Qbeta-59) in the attempt to induce potent anti-TrpM B cell responses in cats. All Qbeta-59 immunized cats, but not cats that received a mixture of uncoupled Qbeta and peptide 59, developed antibodies that reacted with a same epitope in extensive binding and binding competition assays. The epitope recognized was composed of three amino acids, two of which are adjacent. However, Qbeta-59-immune sera failed to recognize whole FIV in all binding and neutralization assays performed. Furthermore, no reactivity against the TrpM was detected by screening sera from FIV-infected cats that had reacted with TM peptides, confirming that this epitope does not seem to be serologically functional in the FIV virion. The data suggest that TrpM may not be a suitable target for antiviral vaccine design.

HIV-1 viral load determination based on reverse transcriptase activity recovered from human plasma

We describe a procedure (ExaVir Load) to carry out human immunodeficiency virus-1 (HIV-1) viral load testing using reverse transcriptase (RT) recovered from HIV-1 virions in plasma. Samples from individuals infected with HIV-1 were treated with a sulphydryl-reactive agent to inactivate endogenous polymerases. Virions were then immobilised on a gel and washed in individual mini columns to remove RT-inhibiting antibodies, antiviral drugs, and other RT inhibitors. Immobilised virions were lysed finally, and the viral RT eluted. The amount of RT recovered was quantified by a sensitive RT activity assay using either colorimetry or fluorimetry to detect DNA produced by RT. The "RT load" values of 390 samples from 302 HIV-1 patients living in Sweden were compared to results obtained with an HIV-1 RNA viral load assay. The correlation between the two tests was r = 0.90, P < 0.0001. Four of 202 samples from healthy blood donors gave low positive values in the RT test. All samples in a panel with 10 HIV-1 subtypes were positive by the RT load. The RT load test provides a technically less demanding and cost-effective alternative to methods based on nucleic acid amplification. Being insensitive to genetic drift occurring in HIV, the assay should be of particular use in resource-limited settings, where different subtypes and recombinant HIV strains occur.

Development of feline immunodeficiency virus ORF-A (tat) mutants: in vitro and in vivo characterization

A functional ORF-A is essential for efficient feline immunodeficiency virus replication in lymphocytes. We have characterized a series of mutants of the Petaluma strain, derived from p34TF10 and having different combinations of stop codons and increasingly long deletions in ORF-A. Six clones proved fully replicative in fibroblastoid Crandell feline kidney cells and monocyte-derived macrophage cultures but failed to replicate in T cell lines and primary lymphoblasts. Cats inoculated with three selected mutants had considerably milder infections than controls given intact ORF-A virus. In vivo, the mutants maintained growth properties similar to those in vitro for at least 7 months, except that replication in lymphoid cells was strongly reduced but not ablated. One mutant underwent extensive ORF-A changes without, however, reverting to wild-type. Antiviral immune responses were feeble in all cats, suggesting that viral loads were too low to represent a sufficiently powerful antigenic stimulus.

Immunogenicity of an anti-clade B feline immunodeficiency fixed-cell virus vaccine in field cats

Attempts at vaccine development for feline immunodeficiency virus (FIV) have been extensive, both because this is a significant health problem for cats and because FIV may be a useful vaccine model for human immunodeficiency virus. To date, only modest success, producing only short-term protection, has been achieved for vaccine trials in controlled laboratory settings. It is unclear how relevant such experiments are to prevention of natural infection. The current study used a vaccine that employs cell-associated FIV-M2 strain fixed with paraformaldehyde. Subject cats were in a private shelter where FIV was endemic, a prevalence of 29 to 58% over an 8-year observation period. Cats roamed freely from the shelter through the surrounding countryside but returned for food and shelter. After ensuring that cats were FIV negative, they were immunized using six doses of vaccine over a 16-month period and observed for 28 months after the initiation of immunization. Twenty-six cats (12 immunized and 14 nonimmunized controls) were monitored for a minimum of 22 months. Immunized cats did not experience significant adverse effects from immunization and developed both antibodies and cellular immunity to FIV, although individual responses varied greatly. At the conclusion of the study, 0 of 12 immunized cats had evidence of FIV infection, while 5 of 14 control cats were infected. Thus, the vaccine was safe and immunogenic and did not transmit infection. Furthermore, vaccinated cats did not develop FIV infection in a limited clinical trial over an extended time period. Thus, the data suggest that a fixed, FIV-infected cell vaccine has potential for preventing natural FIV infection in free-roaming cats.

Monoclonal antibodies to native HIV type 1 reverse transcriptase and their interaction with enzymes from different subtypes

Recombinant reverse transcriptase (RT) from HIV-1 subtype B was used to produce mouse anti-RT monoclonal antibodies (MAbs). Immunization was done by mixing RT with the ISCOM matrix-forming adjuvant saponin (Quil A). Two different assays, both based on the interaction of native RT and antibodies, were used to monitor the immune response in mice and for screening, selection, and characterization of the MAbs. The first assay measures the capacity of antibodies to inhibit the polymerase activity of the RT and the second assay measures the ability of antibodies to capture enzymatically active RT. Twelve clones with the capacity to inhibit at least 50% of the RT activity and 34 clones with high RT-capturing capacity were found. The MAb panel was utilized to evaluate the immunological properties of 18 different RTs representing 9 different HIV1 subtypes. The RT-inhibitory MAbs could be divided into two groups based on their pattern of cross-reactivity toward the different HIV-1 RTs. The degree of diversity recorded among MAbs with RT-capturing capacity was larger. At least seven groups of MAbs with distinct cross-reactivity patterns were identified. Thus, the degree of isoenzyme specificity varied greatly, from MAbs that were quite specific for subtype B RT to one MAb that was able to capture the RTs from all HIV-1 isolates tested except one of the two group O isolates. In conclusion, our study revealed that there exist surprisingly large immunological differences between RTs from different HIV-1 subtypes as well as from the same subtype.

Kinetics of replication of a partially attenuated virus and of the challenge virus during a three-year intersubtype feline immunodeficiency virus superinfection experiment in cats

The effects of preinfecting cats with a partially attenuated feline immunodeficiency virus (FIV) on subsequent infection with a fully virulent FIV belonging to a different subtype were investigated. Eight specific-pathogen-free cats were preinfected with graded doses of a long-term in vitro-cultured cell-free preparation of FIV Petaluma (FIV-P, subtype A). FIV-P established a low-grade or a silent infection in the inoculated animals. Seven months later, the eight preinfected cats and two uninfected cats were challenged with in vivo-grown FIV-M2 (subtype B) and periodically monitored for immunological and virological status. FIV-P-preinfected cats were not protected from acute infection by FIV-M2, and the sustained replication of this virus was accompanied by a reduction of FIV-P viral loads in the peripheral blood mononuclear cells and plasma. However, from 2 years postchallenge (p.c.) until 3 years p.c., when the experiment was terminated, preinfected cats exhibited reduced total viral burdens, and some also exhibited a diminished decline of circulating CD4(+) T lymphocytes relative to control cats infected with FIV-M2 alone. Interestingly, most of the virus detected in challenged cats at late times p.c. was of FIV-P origin, indicating that the preinfecting, attenuated virus had become largely predominant. By the end of follow-up, two challenged cats had no FIV-M2 detectable in the tissues examined. The possible mechanisms underlying the interplay between the two viral populations are discussed.

AIDS vaccination studies using an ex vivo feline immunodeficiency virus model: detailed analysis of the humoral immune response to a protective vaccine

The feline immunodeficiency virus (FIV) cat model is extensively used to investigate possible vaccination approaches against AIDS in humans. Although consistent levels of protection have been achieved with FIV, as with other model systems, by immunizing with whole inactivated virus or fixed infected cells, the mechanisms responsible for protection are elusive. In previous studies we showed that cats immunized with a vaccine consisting of fixed infected cells were protected or unprotected against cell-free or cell-associated FIV challenge depending on the time interval between completion of vaccination and challenge. In an attempt to define possible humoral immune correlates of protection, selected sera harvested at the times of challenge from such cats were examined for anti-FIV-antibody titers and properties by using binding and functional immunological assays. Binding assays included quantitative Western blotting, enzyme-linked tests for antibodies to FIV glycoproteins and immunodominant linear epitopes, and tests for measuring conformation dependence and avidity of anti-viral-envelope antibodies. Functional assays included virus neutralization performed with two different cell substrates, complement- and antibody-dependent virolysis, blocking of reverse transcriptase, and an assay that measured the ability of sera to prevent FIV growth in cocultures of infected and uninfected cells. Despite the wide spectrum of parameters investigated, no correlation between vaccine-induced protection and the humoral parameters measured was noted.

A colorimetric reverse transcriptase assay optimized for Moloney murine leukemia virus, and its use for characterization of reverse transcriptases of unknown identity

A non-radioactive reverse transcriptase (RT) assay, reported as useful for lentivirus RTs, was optimized for the measurement of Moloney murine leukemia virus (MMuLV) RT. The optimized assay could detect 0.3 microU of MMuLV RT. The specificities of the MMuLV and lenti RT assays were demonstrated using the RTs of human immunodeficiency virus type 1, simian immunodeficiency virus, feline immunodeficiency virus (FIV), visna virus, human T-cell lymphotropic virus type 1, MMuLV and feline leukemia virus (FeLV). An RT activity blocking antibody (RTb-ab) assay was standardized for Mn2+ dependent MuLV-related RTs. The assay was used to demonstrate the distinct antigenic properties of RTs from mammalian MuLV-related retroviruses and lentiviruses. Cross-reactivity between MMuLV RTb-ab and FeLV RT but not between MMuLV RTb-ab and e.g. FIV RT was demonstrated. An RT activity found in the murine myeloma cell line SP2/0 was found to have similar assay preferences as MMuLV RT, and the MMuLV-RT hyperimmune sera reacted strongly against this RT, indicating the RT to be of MuLV-related etiology. The use of the RT and RTb-ab assays for detection and characterization of RTs of known or unknown identity is discussed.

Detection of feline immunodeficiency proviral sequences in lymphoid tissues and the central nervous system by in situ gene amplification

The availability of sensitive methods for detecting and localising the feline immunodeficiency virus (FIV) may help shed light on its role in generating tissue damage observed during infection. As immunohistochemical and in situ hybridisation techniques might not be sufficiently sensitive for this type of study, we adapted to FIV PCR-in situ hybridisation (PCR-ISH) that combine the extreme sensitivity of PCR with the precise localisation provided by ISH. The steps important for the success of PCR-ISH, such as sample preparation, permeabilisation, amplification profile, type of labels, and hybridisation conditions were optimised using paraformaldehyde-fixed and formalin-fixed paraffin-embedded sections of cells infected in vitro with FIV. As controls for amplification, the feline tumor necrosis factor-alpha gene (TNF-alpha) and the non-related EBNA-1 gene of the human Epstein-Barr virus were used. Once the method proved sufficiently sensitive and specific with these cells, the PCR-ISH assay was applied to paraffin sections of the lymph nodes, spleen and central nervous system of a 2-year FIV infected cat that, at the time of challenge, harboured low copy numbers of proviral genomes. Comparison of the results of PCR-ISH, competitive PCR and immunohistochemical analysis are described.

Autologous and heterologous neutralization analyses of primary feline immunodeficiency virus isolates

Feline immunodeficiency virus (FIV) provides a model system with which the significance of neutralizing antibody (NA) in immunosuppressive lentivirus infections may be studied. To date, no detailed analysis of the neutralization properties of primary FIV isolates has been reported. In this study, we have conducted the first comprehensive study of the sensitivity to autologous and heterologous neutralization in a lymphoid cell-based assay of 15 primary FIV isolates and, for comparison, of one tissue culture-adapted strain. Primary isolates in general proved highly NA resistant, although there was considerable individual variation. Variation was also observed in the capacity of immune sera to neutralize heterologous FIV isolates. The ability of sera to neutralize isolates or for isolates to be neutralized by sera did not correlate with epidemiological and genetic relatedness or with the quasispecies complexity of the isolates. From the study of specific-pathogen-free cats experimentally infected with viral isolates associated with NA of different breadths, it appears that the development of FIV vaccines cannot rely on the existence of viral strains inherently capable of inducing especially broad NA responses.

Quantitation of feline immunodeficiency proviruses in doubly infected cats using competitive PCR and a fluorescence-based RFLP

A nested polymerase chain reaction assay, which amplifies a region of the gag gene, was developed for the direct detection of feline immunodeficiency virus (FIV) DNA sequences in the blood of infected cats. This method detects as few as ten copies of a plasmid containing the whole genome of the FIV-Pet isolate on agarose gel. To distinguish two FIV isolates in double infected cats, we devised an RFLP analysis on PCR amplified products exploiting sequence differences in the gag gene of the two strains. To quantitate the two strains, a fluorescent inner-sense primer was used in the second amplification step. Amplicons were subsequently digested, heat-denatured and loaded on a polyacrylamide gel in an automated DNA sequencer. The proportion of the two isolates was determined using the laser-excited fluorescence of labelled strain specific fragments. These data were used to extrapolate the numbers of proviral genomes from the total viral load as estimated by using a competitive PCR assay. These sensitive and specific assays complement virological detection of FIV and enable superinfection studies to be evaluated; a prerequisite for the testing of live attenuated immunodeficiency virus vaccines.

Immunohistochemical demonstration of cellular antigens of the cat defined by anti-human antibodies

A panel of monoclonal antibodies (mAb) and some polyclonal rabbit sera directed against human antigens were studied on cryostat tissue sections of three cats using immunohistochemistry. Reactivity of the antibodies was tested on feline tonsil, intestine, thymus, lymph node and spleen with a three-step immunoperoxidase technique and compared with reactions on human thymus, lymph node and spleen. From a total of 95 antibodies, 28 gave reactivity comparable with that in human tissues. The remaining antibodies gave none or miscellaneous results. The positive reactions in the cat included antibodies directed to adhesion molecules (VLA-2 and VLA-4), to natural killer (NK) cells (CD56, CD57 and NCAM), to complement receptor CR1, to proliferation marker Ki-67 (MIB-1), to endothelial antigens (EN-4, PAL-E and von Willebrand factor) and to structural proteins like vimentin, desmin, collagen type IV and cytokeratin. The identification of these cross-reacting antibodies extends the spectrum of immunological reagents that are now available for the cat, and will thus contribute to the study of the feline immune system.

Incubation time for feline immunodeficiency virus cultures

In a recent report, Fiscus et al. (S. A. Fiscus, S. L. Welles, S. A. Spector, and J. L. Lathey, J. Clin. Microbiol. 33:246-247, 1995) have shown that qualitative human immunodeficiency virus cultures can be terminated at day 21 with minimal false-negative results. We have evaluated a large number of qualitative and quantitative feline immunodeficiency virus (FIV) isolations to determine how long FIV cultures should be incubated to obtain reasonably certain results. The rate at which FIV cultures became positive was influenced by whether the cats under study were naturally or experimentally infected, the duration of in vivo infection, and the number of infected peripheral blood mononuclear cells seeded. The results show that cultures for FIV isolation should be kept for 5 to 6 weeks.

Vaccination protects against in vivo-grown feline immunodeficiency virus even in the absence of detectable neutralizing antibodies

So far, vaccination experiments against feline immunodeficiency virus have used in vitro-grown virus to challenge the vaccinated hosts. In this study, cats were vaccinated with fixed feline immunodeficiency virus-infected cells and challenged with plasma obtained from cats infected with the homologous virus diluted to contain 10 cat 50% infectious doses. As judged by virus culture, PCRs, and serological analyses performed over an 18-month period after the challenge, all of the vaccinated cats were clearly protected. Interestingly, prior to challenge most vaccines lacked antibodies capable of neutralizing a fresh isolate of the homologous virus.