Feline immunodeficiency virus: quantification in peripheral blood mononuclear cells and isolation from plasma of infected cats

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

Molecular subtyping of feline immunodeficiency virus from domestic cats in Australia

OBJECTIVE

To determine the prevalent subtypes of feline immunodeficiency virus (FIV) present in the domestic cat population of Australia.

METHOD

Blood samples were collected from 41 FIV antibody positive cats from four cities across Australia. Following DNA extraction, polymerase chain reaction (PCR) was performed to amplify the variable V3-V5 region of the envelope (env) gene. Genotypes were assessed by direct sequencing of PCR products and comparison with previously reported FIV sequences. Phylogenetic analysis allowed classification of the Australian sequences into the appropriate subtype.

RESULTS

Of the 41 FIV samples, 40 were found to cluster with previously reported subtype A isolates, whilst the remaining sample grouped within subtype B.

CONCLUSIONS

Subtype A was found to be the predominant FIV subtype present in Australia, although subtype B was also found. These results broaden our knowledge of the genetic diversity of FIV and the associated implications for preventative, diagnostic and therapeutic approaches.

Quantification of viral ribonucleic acid in plasma of cats naturally infected with feline immunodeficiency virus

OBJECTIVE

To assess plasma viral RNA concentration in cats naturally infected with feline immunodeficiency virus (FIV).

ANIMALS

28 FIV-infected cats.

PROCEDURE

Cats were categorized into 1 of the 3 following stages on the basis of clinical signs: asymptomatic (nonclinical) carrier (AC; n = 11), acquired immunodeficiency syndrome-related complex (ARC; 9), or acquired immunodeficiency syndrome (AIDS; 8). Concentration of viral RNA in plasma (copies per ml) was determined by use of a quantitative competitive polymerase chain reaction (QC-PCR) assay. Total lymphocyte count, CD4+ cell and CD8+ cell counts, and the CD4+ cell count-to-CD8+ cell count ratio were determined by use of flow cytometry.

RESULTS

Plasma viral RNA concentration was significantly higher in cats in the AIDS stage, compared with cats in AC and ARC stages. Most (5/7) cats in the AIDS stage had low total lymphocyte, CD4+ cell, and CD8+ cell counts.

CONCLUSIONS AND CLINICAL RELEVANCE

Concentration of plasma viral RNA is a good indicator of disease progression in FIV-infected cats, particularly as cats progress from the ARC to the AIDS stage. Determination of CD4+ and CD8+ cell counts can be used as supportive indicators of disease progression.

Transmission of human and feline immunodeficiency viruses via reused suture material

Several documented cases of human immunodeficiency virus (HIV) infection have involved unconventional or unknown modes of transmission of the virus. Some such cases have occurred within a surgical setting. We investigated the potential for transmission of HIV on suture material that had been reused following passage through an HIV-infected patient. Initial experiments were conducted in vitro using HIV. To provide stronger evidence that HIV could be transmitted via this route, further experiments were undertaken in vivo using a feline immunodeficiency virus (FIV)/cat model. Both methods indicated the possibility of transmission of virus if suture materials were reused.

Development of a simple, rapid and accurate in vitro whole blood technique for the detection and semi-quantification of FIV cellular viremia

A new, simple, rapid and accurate culture technique is described for a semi-quantitative analysis of cellular viremia in FIV-infected cats. This assay can be carried out with small amounts of whole blood, and is based on the detection of FIV core gag antigen, which is released in culture supernatants. The amount of core antigen produced is measured with an enzyme-linked immunoassay using specific monoclonal antibodies. This whole blood technique (WB method) was compared with a culture method using isolated peripheral blood mononuclear cells (PBMC method). FIV could be detected in whole blood of all experimentally infected cats, but not from uninfected cats. This assay offers a number of advantages (small blood samples required, no leukocyte separation and lymphocyte purification procedures) and its reproducibility is very good. It provides a convenient in vitro cellular assay for viral semi-quantitation, well adapted for monitoring efficacy of prototype FIV vaccines or experimental antiviral drugs. Also, it could facilitate the study of the pathogenesis of FIV-related progressive immunodepression. Finally, it offers an alternative to serological techniques for diagnostic purposes in several circumstances: early viremia, maternal antibodies.

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

We report on the development of a feline T lymphoblastoid cell line obtained from the peripheral blood mononuclear cells (PBMC) of a specific pathogen free cat and designated MBM. The cells are pan-T+, CD4- and CD8- and remained interleukin-2-dependent and concanavalin A-dependent throughout the period of observation. MBM cells have proved at least as sensitive as fresh blasts to infection with cell-free stocks of three feline immunodeficiency virus (FIV) isolates. Upon infection, they exhibit a lytic cytopathic effect. Repeated attempts to establish a chronic infection have failed. Using a limiting cell dilution method, it has been shown that MBM cells may be more sensitive than fresh blasts as substrate for isolating FIV from the PBMC of infected cats. These studies have also shown that considerable individual variations exist in the virus loads present in the PBMC of naturally infected cats, and that load size does not appear to correlate with cat age, clinical status, CD4/CD8 ratio and titer of serum neutralizing antibody.

Evaluation of an autologous red cell agglutination test, VetRED FIV, for the presence of FIV antibody in cats

A commercially available whole blood agglutination test, VetRED FIV, used for the detection of antibodies to feline immunodeficiency virus (FIV), was evaluated. The test is based on the use of a synthetic peptide conjugated to a non-agglutinating anti-feline red blood cell monoclonal antibody. The amino acid sequence of the synthetic peptide was derived from the predicted sequence of the transmembrane protein of FIV. The sensitivity and specificity of VetRED FIV was 100% when 34 known FIV-positive and 15 known FIV-negative cats were tested. These cats were part of studies on experimentally induced FIV infection, with their FIV status confirmed by virus isolation. Further, VetRED FIV was compared with another commercially available test for FIV antibody, PetChek in a field trial on 548 feline blood samples received by a diagnostic laboratory. Of the test results 94.2% (516/548) were in agreement: 112 were positive by VetRED FIV and PetChek; 404 were negative by both tests and 32 were discordant. These 5.8% discordant samples producing VetRED FIV-positive/PetChek-negative or VetRED FIV-negative/PetChek-positive were further assessed by Western blot assay. In the field trial, the sensitivity and specificity of VetRED FIV was 97% and 97%, respectively, comparable to the 98% sensitivity and 99% specificity for PetChek. The results from the trial also confirm the relatively high overall prevalence of FIV in Australian cats predominantly among mature male cats in the 9-12 year age group. Given the simplicity of the VetRED FIV procedure, it is concluded that VetRED FIV is a useful addition to the available commercial tests for FIV infection.

Competitive reverse transcription-polymerase chain reaction for quantitation of feline immunodeficiency virus

A competitive reverse transcription-polymerase chain reaction (RT-PCR) was developed to quantify RNA of feline immunodeficiency virus (FIV) in cats. The assay uses in vitro synthesized RNA derived from the gag region of the FIV genome as a competitive internal control. The synthesized RNA has a 22-base deletion with respect to the wild-type sequence. PCR products were quantitated by densitometric analysis of a digitalized image of the ethidium bromide stained gel. The non-radioactive method was evaluated in reconstruction experiments. RNA synthesis in FIV-infected feline thymocytes correlated well with the amount of viral p24 antigen produced. Viral RNA concentrations in the plasma of two cats experimentally infected with FIV strain UT113 were followed for 32 weeks; peak copy numbers (2.3 x 10(4) and 1.3 x 10(4) per ml, respectively) were reached 11 weeks after subcutaneous injection of ten 50% cat infectious doses. With rising antibody titers against FIV-gag and FIV-env gene products, the amount of FIV RNA in plasma decreased. Nine asymptomatic cats that had been experimentally infected 3.5 to 4.5 years earlier had copy numbers between 5.6 x 10(3) and 4.3 x 10(4) per ml. This quantitative competitive RT-PCR will be useful to study the pathogenesis of the FIV infection, to evaluate the effectiveness of vaccines and to monitor antiviral and immunomodulating drugs.

Longitudinal assessment of feline immunodeficiency virus kinetics in plasma by use of a quantitative competitive reverse transcriptase PCR

Cats infected with feline immunodeficiency virus (FIV) develop a disease syndrome similar to that caused by human immunodeficiency virus type 1 (HIV-1) infection in humans. HIV-1 replication has been shown to correlate with the disease stage and progression. To assess replication kinetics and disease progression in early FIV infection, we developed a quantitative competitive reverse transcriptase PCR to measure the plasma virus load at serial time points after virus exposure. We found that an early peak viremia immediately preceded the onset of acute-phase symptoms in infected cats. Plasma virus levels remained high throughout the symptomatic phase of infection, which lasted for 8 to 10 weeks, and then declined as clinical symptoms resolved; however, all cats maintained significant plasma virus titers through 36 weeks postinfection. Early peak viral replication coincided with the initial precipitous decline in circulating CD4+ T lymphocytes. These results indicate that FIV kinetics are similar to those of HIV-1 during the acute and secondary phase of infection and that the plasma FIV load correlates with the disease stage. These results serve to further develop the FIV model and to enhance its usefulness for pathogenesis, vaccine development, and therapeutic studies related to HIV.

Feline immunodeficiency virus: an interesting model for AIDS studies and an important cat pathogen

The lentivirus feline immunodeficiency virus (FIV) is a widespread pathogen of the domestic cat that is mainly transmitted through bites, although other means of transmission are also possible. Its prevalence ranges from 1 to 10% in different cat populations throughout the world, thus representing a large reservoir of naturally infected animals. FIV resembles the human immunodeficiency virus (HIV) in many respects. Similarities include the structural features of the virion, the general organization and great variability of the genome, the life cycle in the infected host, and most importantly, the pathogenic potential. Infection is associated with laboratory signs of immunosuppression as well as with a large variety of superinfections, tumors, and neurological manifestations. Our understanding of FIV is steadily improving and is providing important clues to the pathogenesis of immunodeficiency-inducing lentiviruses. The cellular receptor for FIV is different from the feline equivalent of the human CD4 molecule used by HIV; nevertheless, the major hallmark of infection is a progressive loss of CD4+ T lymphocytes as in HIV infection. The mechanisms by which FIV escapes the host's immune responses are being actively investigated. FIV causes lysis of infected T cells and also appears to predispose these cells to apoptosis. Infection of macrophages and other cell types has also been documented. For reasons yet to be understood, antibody-mediated neutralization of fresh FIV isolates is very inefficient both in vitro and in vivo. Vaccination studies have provided some encouraging results, but the difficulties encountered appear to match those met in HIV vaccine development. FIV susceptibility to antiviral agents is similar to that of HIV, thus providing a valuable system for in vivo preclinical evaluation of therapies. It is concluded that in many respects FIV is an ideal model for AIDS studies.

The induction of in vivo superinfection and recombination using feline immunodeficiency virus as the model

This study investigated the hypothesis that under certain conditions, superinfection of cats with feline immunodeficiency virus (FIV), may occur. One FIV isolate (T91) was used to inoculate three FIV and FeLV-free cats. Blood from an FIV-infected cat (N), which contained two variants and differed from T91 by at least 5% in nucleotide sequence in the env gene, was inoculated into a fourth cat. Both T91 and blood from N were inoculated simultaneously into a fifth cat. After 22 weeks, two of the three cats initially infected with T91 were challenged with blood from N. At 30 weeks following initial infection, peripheral blood mononuclear cells were obtained from all cats, DNA was extracted, and a segment of the env gene was PCR amplified, cloned and sequenced. Nucleotide sequence analysis of the cloned PCR product showed that virus strains used in initial infection were recovered from cats not challenged with a second variant. Challenge of cats with the blood of N following initial infection with T91 resulted in superinfection occurring in one cat and recombination occurring in the other. Furthermore, the use of blood as a source of challenge, in cats where superinfection and simultaneous infections were attempted, may have induced the appearance of variants which more closely resembled the most heterologous strain present in the infectious source.

The detection and quantification of feline immunodeficiency provirus in peripheral blood mononuclear cells using the polymerase chain reaction

The polymerase chain reaction method (PCR) was used to detect feline immunodeficiency virus proviral DNA in peripheral blood mononuclear cells (PBMC) of a group of 8 experimentally infected cats. The proportion of PBMC containing provirus was determined from 6 to 32 weeks post inoculation (p.i.) by performing PCR on serially diluted samples of PBMC. Primers from the p15 and p24 regions of the gag gene were used and Southern hybridization using an end-labelled probe was required to confirm primer-specific products. Provirus was detected in 5 of 8 cats by 6 weeks p.i. in 50000 PBMC, and in all 8 infected cats by 8 weeks p.i. Provirus was not detected in PBMC from any of 3 FIV negative cats. The proportion of PBMC containing provirus in individual cats ranged from 1 in 70 to 1 in 99600 PBMC. There was no significant decline over time in the proportion of PBMC containing provirus. Sequencing of a segment (287 bases) of the gag region of a West Australian FIV isolate (T90) revealed only slight nucleotide divergence from the North American Petaluma and PPR isolates and wider divergence from the Japanese TM2 clone.

Extensive sequence variation of feline immunodeficiency virus env genes in isolates from naturally infected cats

In an investigation of the evolution of feline immunodeficiency virus (FIV) in vivo, sequential isolates from a persistently infected cat were examined by direct sequencing following amplification of selected subgenomic regions by polymerase chain reaction (PCR). Three isolates, T90, T91, and T92, obtained over a three-year period revealed no changes to regions known to be conserved within gag and pol genes. Additionally, no change occurred within gag and pol in an isolate recovered from a second cat which was experimentally infected with T90. Changes were detected within an N-terminal region of the envelope glycoprotein gp 120 (env). These consisted of point mutations, some of which would result in amino acid substitutions and the predicted amino acid changes tended to cluster within variable domains. Inoculation of T90 into a second cat resulted in a different pattern of mutations than that observed for the three isolates from the first cat. In all cases, virus isolates derived from the same cat were much more highly related to each other (extent of env variation was 0.5-1.5%) than to isolates from other cats (10-12% env variation). The rate of change of FIV was estimated to be 3.4 x 10(-3) nucleotide substitutions per site per year for the env gene and less than 10(-4) nucleotide substitutions per site per year for the gag and pol genes, values concordant with that found for human immunodeficiency virus 1. Both nucleotide and amino acid changes in the gp 120 region were found to be directional, suggesting that selective pressures influence FIV envelope gene sequences.

Feline immunodeficiency virus infection: plasma, but not peripheral blood mononuclear cell virus titer is influenced by zidovudine and cyclosporine

The plasma and peripheral blood mononuclear cell (PBMC) titer of feline immunodeficiency virus (FIV) in experimentally infected cats was assessed following administration of either zidovudine or cyclosporine. Treatments were begun 24 h post infection (p.i.) and continued for 4 weeks. Zidovudine treatment did not prevent establishment of infection with FIV, but plasma virus titers were significantly lower than controls at 2 weeks p.i. This reduction of plasma virus titer by zidovudine was not maintained at subsequent sampling times. Similarly, cyclosporine treatment initially lowered plasma virus titers at 2 weeks p.i., but at 4 weeks p.i. the plasma virus titers in cyclosporine-treated cats were significantly higher than in the untreated group. In the untreated group, plasma virus titers declined rapidly to an undetectable level by 14 weeks p.i. Neither zidovudine or cyclosporine treatment significantly influenced the titer of FIV in PBMCs. In all groups (untreated, zidovudine and cyclosporine) the titers in PBMC were high for the duration of the experiment. The decline in plasma virus titers in immunocompetent cats combined with the effect of cyclosporine on plasma titers strongly suggest that the immune system plays a major role in clearing FIV from plasma. In contrast, it appears that the immune response has little impact on PBMC virus titers. This shows that for complete assessment of antiviral agents, both cell-free and cell-associated virus titers must be examined. We also suggest that the limitation of viral titers in PBMC may be of critical importance in the control of lentiviral infection.