Differences in feline immunodeficiency virus host cell range correlate with envelope fusogenic properties

Dual-emission fluorescence resonance energy transfer (FRET) real-time PCR differentiates feline immunodeficiency virus subtypes and discriminates infected from vaccinated cats

Feline immunodeficiency virus (FIV) is among the most common infectious agents of cats. Five well-characterized FIV subtypes, A, B, C, D, and E, are recognized worldwide. As in HIV diagnosis, serum antibodies against FIV classically serve as an indicator of infection status. After the introduction of an inactivated FIV vaccine, this approach has become problematic, since antibodies generated by vaccination are indistinguishable from antibodies in response to infection. However, PCR detection of host-cell-integrated FIV DNA will differentiate infection-derived antibody from vaccination-derived positivity because presumably the RNA of inactivated vaccine virus will not integrate into the host genome. In this study, we established a gag gene-based dual-emission fluorescence resonance energy transfer (FRET) real-time PCR that amplifies single-target copies of all known FIV strains and differentiates five FIV subtypes. All blood samples from experimentally FIV-infected cats (n=5) were antibody positive and highly positive in the FIV PCR. In contrast, nine cats became antibody positive after FIV vaccination but remained negative in the FIV PCR. Of 101 FIV antibody-positive feline blood specimens submitted for FIV PCR diagnosis, 61 were positive (60%). A total of 23 of the positive PCRs identified subtype A, 11 identified subtype B1, 11 identified subtype B2/E, and 16 identified subtype C. FIV subtype D was not detected in any submitted specimens even though 13 blood specimens were from cats known to have received the FIV vaccine, which contains FIV subtype A and D inactivated virions. Therefore, this PCR quantitatively identifies FIV subtypes and unambiguously discriminates between FIV-vaccinated and FIV-infected cats.

Feline immunodeficiency virus (FIV) as a model for study of lentivirus infections: parallels with HIV

FIV is a significant pathogen in the cat and is, in addition, the smallest available natural model for the study of lentivirus infections. Although divergent at the amino acid level, the cat lentivirus has an abundance of structural and pathophysiological commonalities with HIV and thus serves well as a model for development of intervention strategies relevant to infection in both cats and man. The following review highlights both the strengths and shortcomings of the FIV/cat model, particular as regards development of antiviral drugs.

Molecular mechanisms of FIV infection

Feline immunodeficiency virus (FIV) is an important viral pathogen worldwide in the domestic cat, which is the smallest animal model for the study of natural lentivirus infection. Thus, understanding the molecular mechanisms by which FIV carries out its life cycle and causes an acquired immune deficiency syndrome (AIDS) in the cat is of high priority. FIV has an overall genome size similar to HIV, the causative agent of AIDS in man, and shares with the human virus genomic features that may serve as common targets for development of broad-based intervention strategies. Specific targets include enzymes encoded by the two lentiviruses, such as protease (PR), reverse transcriptase (RT), RNAse H, and integrase (IN). In addition, both FIV and HIV encode Vif and Rev elements essential for virus replication and also share the use of the chemokine receptor CXCR4 for entry into the host cell. The following review is a brief overview of the current state of characterization of the feline/FIV model and development of its use for generation and testing of anti-viral agents.

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.

Differential utilization of CD134 as a functional receptor by diverse strains of feline immunodeficiency virus

The feline homologue of CD134 (fCD134) is the primary binding receptor for feline immunodeficiency virus (FIV), targeting the virus preferentially to activated CD4+ helper T cells. However, with disease progression, the cell tropism of FIV broadens such that B cells and monocytes/macrophages become significant reservoirs of proviral DNA, suggesting that receptor utilization may alter with disease progression. We examined the receptor utilization of diverse strains of FIV and found that all strains tested utilized CD134 as the primary receptor. Using chimeric feline x human CD134 receptors, the primary determinant of receptor function was mapped to the first cysteine-rich domain (CRD1) of fCD134. For the PPR and B2542 strains, the replacement of CDR1 of fCD134 (amino acids 1 to 64) with human CD134 (hCD134) alone was sufficient to confer nearly optimal receptor function. However, evidence of differential utilization of CD134 was revealed, since strains GL8, CPGammer (CPG41), TM2, 0827, and NCSU1 required determinants in the region spanning amino acids 65 to 85, indicating that these strains may require a more stringent interaction for infection to proceed.

Comparative replication kinetics of two cytopathic feline lentiviruses ex vivo

Feline immunodeficiency virus infection of cats provides a model to elucidate mechanisms of lentiviral pathogenesis. We isolated a non-domestic FIV from a Pallas' cat, FIV-Oma, which replicates in feline PBMCs and CRFK cells. To gain insights into FIV pathogenesis, we compared rates of viral replication and apoptosis of FIV-Oma with FIV-PPR in the MYA-1 T-cell line. To minimize heterogeneity of virus, infections were initiated with virus derived from molecular clones. Viral DNA and RNA levels, assessed by qPCR and qRT-PCR, apoptosis, and supernatant reverse transcriptase were slower in FIV-Oma infections. Immunostaining for cellular Gag showed that few cells were productively infected. The majority of cells infected with either virus instead became apoptotic. Apoptosis was detectable within 6 h PI, suggesting activation of a signaling pathway. We propose that apoptosis is due to interaction of virus with cells, and is the usual outcome of infection by cytopathic FIVs in these cells.

Relative activity of the feline immunodeficiency virus promoter in feline and primate cell lines

The feline immunodeficiency virus (FIV) long terminal repeat (LTR), compared with some primate lentiviral LTRs, is quite a strong basal promoter. However, it seems to be highly species-specific in function and generally not very efficient in cells of non-feline origin. This study systematically explored the function of the FIV LTR in simian Cos cells compared with its activity in feline and human cells. Our studies, using biologically relevant two- and three-plasmid trans complementation assays followed by semi-quantitative reverse transcriptase PCR, show that the FIV LTR is functional in Cos cells. The results of the Cos experiment are different from previously reported literature and suggest that the strain specificity of the FIV LTR is an important determinant of whether the LTR will be functional in a particular cell type.

The envelope glycoprotein E2 is a determinant of cell culture tropism in ruminant pestiviruses

Bovine viral diarrhoea virus (BVDV) isolates infect cultured Madin-Darby bovine kidney (MDBK) cells as efficiently as sheep kidney cells. In contrast, border disease virus (BDV) propagates poorly in MDBK cells but infects sheep cells very efficiently. The envelope glycoprotein E2 has been shown to be essential for virus infectivity. To explore the potential role of E2 in pestivirus host range in cell cultures, we engineered a chimeric BVDV with the E2 coding region from BDV. As expected, the BVDV-E2(bdv) chimera retained the ability of BDV to multiply in sheep cells but experienced a remarkable reduction in its ability to propagate and form plaques in MDBK, a phenotype that is characteristic of the E2 donor, BDV31 virus. Control chimeric BVDV bearing a type II E2 demonstrated that the heterologous E2 does not impair replication in MDBK or lamb cells. These results establish a role for E2 in determining the tropism of a pestivirus in cell culture.

Feline immunodeficiency virus xenoinfection: the role of chemokine receptors and envelope diversity

The use of chemokine receptors as cell recognition signals is a property common to several lentiviruses, including feline, human, and simian immunodeficiency viruses. Previously, two feline immunodeficiency virus (FIV) isolates, V1CSF and Petaluma, were shown to use chemokine receptors in a strain-dependent manner to infect human peripheral blood mononuclear cells (PBMC) (J. Johnston and C. Power, J. Virol. 73:2491-2498, 1999). Since the sequences of these viruses differed primarily in regions of the FIV envelope gene implicated in receptor use and cell tropism, envelope chimeras of V1CSF and Petaluma were constructed to investigate the role of envelope diversity in the profiles of chemokine receptors used by FIV to infect primate cells. By use of a receptor-blocking assay, all viruses were found to infect human and macaque PBMC through a mechanism involving the CXCR4 receptor. However, infection by viruses encoding the V3-to-V5 region of the V1CSF surface unit was also inhibited by blockade of the CCR3 or CCR5 receptor. Similar results were obtained with GHOST cells, human osteosarcoma cells expressing specific combinations of chemokine receptors. CXCR4 was required for infection by all FIV strains, but viruses expressing the V3-to-V5 region of V1CSF required the concurrent presence of either CCR3 or CCR5. In contrast, CXCR4 alone was sufficient to allow infection of GHOST cells by FIV strains possessing the V3-to-V5 region of Petaluma. To assess the role of primate chemokine receptors in productive infection, Crandell feline kidney (CrFK) cells that expressed human CXCR4, CCR3, or CCR5 in addition to feline CXCR4 were generated. Sustained infection by viruses encoding the V3-to-V5 region of V1CSF was detected in CrFK cells expressing human CCR3 or CCR5 but not in cells expressing CXCR4 alone, while all CrFK cell lines were permissive to viruses encoding the V3-to-V5 region of Petaluma. These results indicate that FIV uses chemokine receptors to infect both human and nonhuman primate cells and that the profiles of these receptors are dependent on envelope sequence, and they provide insights into the mechanism by which xenoinfections may occur.

Envelope gene-mediated neurovirulence in feline immunodeficiency virus infection: induction of matrix metalloproteinases and neuronal injury

The release of neurotoxins by activated brain macrophages or microglia is one mechanism proposed to contribute to the development of neurological disease following infection by lentiviruses, including feline immunodeficiency virus (FIV). Since molecular diversity in the lentiviral envelope gene influences the expression of host molecules implicated in neuronal injury, the role of the envelope sequence in FIV neuropathogenesis was investigated by using the neurovirulent FIV strain V1CSF, the nonneurovirulent strain Petaluma, and a chimera (FIVCh) containing the V1CSF envelope gene in a Petaluma background. All three viruses replicated in primary feline macrophages with equal efficiency, but conditioned medium from V1CSF- or FIVCh-infected cells was significantly more neurotoxic than medium from Petaluma-infected cultures (P < 0.001) and could be attenuated in a dose-dependent manner by treatment with either the matrix metalloproteinase (MMP) inhibitor prinomastat (PMT) or function-blocking antibodies to MMP-2. Although FIV sequences were detectable by PCR in brain tissue from neonatal cats infected with each of the viral strains, immunohistochemistry revealed increased astrogliosis and macrophage activation in the brains of V1CSF- and FIVCh-infected cats relative to the other groups, together with elevated markers of neuronal stress that included morphological changes and increased c-fos immunoreactivity. Similarly, MMP-2, but not MMP-9, mRNA and protein expression was increased in brain tissues of V1CSF- and FIVCh-infected cats relative to Petaluma-infected animals (P < 0.01). Infection with V1CSF or FIVCh was also associated with greater CD4(+) cell depletion (P < 0.001) and neurodevelopmental delays (P < 0.005), than in Petaluma-infected animals; these deficits improved following PMT therapy. These findings indicated that diversity in the envelope gene sequence influenced the neurovirulence exhibited by FIV both in vitro and in vivo, possibly through a mechanism involving the differential induction of MMP-2.

Dose response studies of acute feline immunodeficiency virus PPR strain infection in cats

The effects of virus dose on host response were evaluated for the PPR strain of feline immunodeficiency virus (FIV-PPR). Specific pathogen-free cats were inoculated intravenously with 50, 250 or 1250 TCID(50) of FIV-PPR. Two weeks after inoculation, virus was detected in 10(6) peripheral blood mononuclear cells (PBMCs) of all infected animals, and the CD4(+):CD8(+) T lymphocyte ratios fell from greater than 2 to approximately 1 in all infected animals within the first 8 weeks after infection. Provirus detected in all groups using PCR and 10(3) PBMC was biphasic. Nine of 15 animals were positive between weeks 2 and 4 p.i. and 14 of 15 were positive by week 8 p.i. Transient lymphadenopathy was detected in most cats receiving 1250 TCID(50) and the 250 TCID(50) of virus, whereas no lymphadenopathy was detected in the 50 TCID(50) group or the five uninfected cats. Animals that had received the largest dose seroconverted earliest (on average at week 4.0) and those receiving the least seroconverted last (on average at week 5.6). Neither neutropenia nor lymphopenia were detected. FIV-specific CTL responses of memory effector cells could be detected in animals receiving all three doses but was highly variable among individual animals. Neurological manifestations determined after 15 weeks p.i. were observed in most infected cats, including two of the three that had received 50 TCID(50) of virus. However, the observed neurologic abnormalities were markedly less severe in the animals receiving the least amount of virus. Therefore, lymphadenopathy and neurologic signs of illness were less severe and seroconversion was slower in the animals that received the lowest dose compared with those receiving the 250 and 1250 TCID(50) doses of the FIV-PPR strain.

Expanded host cell tropism and cytopathic properties of feline immunodeficiency virus strain PPR subsequent to passage through interleukin-2-independent T cells

A cytopathic variant of feline immunodeficiency virus (FIV) strain PPR emerged after passage of wild-type virus on an interleukin-2-independent cell line. The virus, termed FIV-PPRglial, displayed a phenotype markedly different from the parental virus, including the ability to productively infect previously refractory cell lines, induction of large syncytia, and accelerated kinetic properties. A chimeric molecular clone, FIV-PPRchim42, containing the FIV-PPRglial envelope within the backbone of FIV-PPR, exhibited all the characteristics of the FIV-PPRglial phenotype, demonstrating that the viral envelope was responsible for the acquired traits. Subsequent molecular characterization revealed that the FIV-PPRglial envelope contained five amino acid substitutions relative to wild-type FIV-PPR. Mutagenic analyses further demonstrated that the acquired phenotype was minimally attributable to a combination of three mutations, specifically, a glutamine-to-proline change within the second constant domain of the surface protein (SU); a threonine-to-proline change within the V4 loop, also in the SU; and a premature stop codon in the cytoplasmic tail of the transmembrane protein. All three changes were required to produce the FIV-PPRglial phenotype. Cotransfection studies with mutant viruses in combination with each other and with FIV-PPR indicated that the truncated cytoplasmic tail was responsible for the induction of syncytium formation. Receptor usage analyses were pursued, and distinctions were observed between FIV-PPR and FIV-PPRglial. In vitro infections with FIV-PPR, FIV-PPRglial, and FIV-34TF10 on two adherent cell lines were ablated in the presence of SDF1alpha, the natural ligand for CXCR4. In contrast, viral infection of T cells was not limited to CXCR4 usage, and inhibition studies indicate the potential involvement of a CC chemokine receptor.

Replication rate of feline immunodeficiency virus in astrocytes is envelope dependent: implications for glutamate uptake

Feline immunodeficiency virus (FIV) induces neurological abnormalities in domestic cats. Previously, we demonstrated that two disparate strains of FIV (FIV-34TF10 and FIV-PPR) varied greatly in the ability to replicate in feline cortical astrocytes. To investigate the impact of the env region on the replication efficiency of these strains, we constructed two env chimera viruses, FIV-34TF10-PPRenv and FIV-PPR-34TF10env, to infect feline cortical astrocytes in vitro. Although all of these viruses infected cortical astrocytes, the efficiency of replication depended on strain, and the env region played an essential role. The viruses containing the env of 34TF10, FIV-34TF10, and FIV-PPR-34TF10env had the greatest replication rate, whereas the viruses containing the env of PPR replicated at a lower level. Other viral regions had modulatory effects on the replication rate, with the FIV-PPR genome providing a slight replication advantage over the FIV-34TF10 genome. We also monitored the effects of these viruses on an important astrocyte function, glutamate uptake; all viruses significantly decreased this activity, but only the viruses containing the env of PPR significantly impaired glutamate uptake without altering the culture viability. These results may be particularly relevant in the context of lentivirus-induced central nervous system disease in which a selective breakdown of astroglial function may contribute to neurodegeneration.

Neurophysiologic and immunologic abnormalities associated with feline immunodeficiency virus molecular clone FIV-PPR DNA inoculation

Although direct feline immunodeficiency virus (FIV) proviral DNA inoculation has been shown to be infectious in cats, long-term studies to assess the pathogenic nature of DNA inoculation are lacking. We have recently reported that direct feline leukemia virus (FeLV) DNA inoculation resulted in infection and the development of FeLV-related disease end points with similar temporal expression and virulence to that of cats infected with whole virus. We show in this study that pFIV-PPR DNA inoculation resulted in infection of cats and the development of FIV-related immunologic and neurologic abnormalities. Infected cats demonstrated progressive loss of CD4+ lymphocytes resulting in decreased CD4:CD8 ratios. Neurologic dysfunction was demonstrated by increased bilateral frontal lobe slow-wave activity. Prolongation of the visual evoked potential peak latency onset response pattern also supported a similar progression of abnormal cortical response. Furthermore, histopathologic examination revealed lesions attributed to FIV infection in lymph node, thymus, brain, and lung. Finally, nested polymerase chain reaction detected FIV provirus in brain, bone marrow, mesenteric lymph node, thymus, spleen, tonsil, and liver. These results confirm that FIV DNA inoculation is an efficient model for study of the pathogenic nature of molecular clones in vivo and offers the opportunity to measure temporal genomic stability of a homogeneous challenge material.

Shared usage of the chemokine receptor CXCR4 by primary and laboratory-adapted strains of feline immunodeficiency virus

Strains of the feline immunodeficiency virus (FIV) presently under investigation exhibit distinct patterns of in vitro tropism. In particular, the adaptation of FIV for propagation in Crandell feline kidney (CrFK) cells results in the selection of strains capable of forming syncytia with cell lines of diverse species origin. The infection of CrFK cells by CrFK-adapted strains appears to require the chemokine receptor CXCR4 and is inhibited by its natural ligand, stromal cell-derived factor 1alpha (SDF-1alpha). Here we found that inhibitors of CXCR4-mediated infection by human immunodeficiency virus type I (HIV-1), such as the bicyclam AMD3100 and short peptides derived from the amino-terminal region of SDF-1alpha, also blocked infection of CrFK by FIV. Nevertheless, we observed differences in the ranking order of the peptides as inhibitors of FIV and HIV-1 and showed that such differences are related to the species origin of CXCR4 and not that of the viral envelope. These results suggest that, although the envelope glycoproteins of FIV and HIV-1 are substantially divergent, FIV and HIV-1 interact with CXCR4 in a highly similar manner. We have also addressed the role of CXCR4 in the life cycle of primary isolates of FIV. Various CXCR4 ligands inhibited infection of feline peripheral blood mononuclear cells (PBMC) by primary FIV isolates in a concentration-dependent manner. These ligands also blocked the viral transduction of feline PBMC by pseudotyped viral particles when infection was mediated by the envelope glycoprotein of a primary FIV isolate but not by the G protein of vesicular stomatitis virus, indicating that they act at an envelope-mediated step and presumably at viral entry. These findings strongly suggest that primary and CrFK-adapted strains of FIV, despite disparate in vitro tropisms, share usage of CXCR4.

Productive infection of human peripheral blood mononuclear cells by feline immunodeficiency virus: implications for vector development

Feline immunodeficiency virus (FIV) is a lentivirus causing immune suppression and neurological disease in cats. Like primate lentiviruses, FIV utilizes the chemokine receptor CXCR4 for infection. In addition, FIV gene expression has been demonstrated in immortalized human cell lines. To investigate the extent and mechanism by which FIV infected primary and immortalized human cell lines, we compared the infectivity of two FIV strains, V1CSF and Petaluma, after cell-free infection. FIV genome was detected in infected human peripheral blood mononuclear cells (PBMC) and macrophages at 21 and 14 days postinfection, respectively. Flow cytometry analysis of FIV-infected human PBMC indicated that antibodies to FIV p24 recognized 12% of the cells. Antibodies binding the CCR3 chemokine receptor maximally inhibited infection of human PBMC by both FIV strains compared to antibodies to CXCR4 or CCR5. Reverse transcriptase levels increased in FIV-infected human PBMC, with detection of viral titers of 10(1.3) to 10(2.1) 50% tissue culture infective doses/10(6) cells depending on the FIV strain examined. Cell death in human PBMC infected with either FIV strain was significantly elevated relative to uninfected control cultures. These findings indicate that FIV can productively infect primary human cell lines and that viral strain specificity should be considered in the development of an FIV vector for gene therapy.

The role of the chemokine receptor CXCR4 in infection with feline immunodeficiency virus

Infection with feline immunodeficiency virus (FIV) leads to the development of a disease state similar to AIDS in man. Recent studies have identified the chemokine receptor CXCR4 as the major receptor for cell culture-adapted strains of FIV, suggesting that FIV and human immunodeficiency virus (HIV) share a common mechanism of infection involving an interaction between the virus and a member of the seven transmembrane domain superfamily of molecules. This article reviews the evidence for the involvement of chemokine receptors in FIV infection and contrasts these findings with similar studies on the primate lentiviruses HIV and SIV (simian immunodeficiency virus).

Genetic heterogeneity of env gene of feline immunodeficiency virus obtained from multiple districts in Japan

Feline immunodeficiency virus (FIV) infection is widespread in many countries. FIV isolates have been classified into five distinct subtypes, A, B, C, D and E based on their env gene sequences. Several reports indicate that most of the FIVs isolated in Japan belong to subtype B which includes the first Japanese isolate, TM2 strain. To examine the distribution of FIV subtypes in Japan, proviral DNA sequences of the env gene were directly amplified by nested PCR from FIV-infected cats that had been kept in multiple districts throughout Japan. Phylogenetic analysis of the 11 strains showed that four FIV subtypes, A, B, C and D, were present in Japan. Among these subtypes, subtypes B and D were the two most common subtypes in Japan, and they were mainly distributed in the eastern and western parts of Japan, respectively. The present study provides information that is fundamental for development of a vaccine to protect against FIV infection in cats.

CrFK feline kidney cells produce an RD114-like endogenous virus that can package murine leukemia virus-based vectors

The feline kidney cell line CrFK is used extensively for viral infectivity assays and for study of the biology of various retroviruses and derived vectors. We demonstrate the production of an endogenous, RD114-like, infectious retrovirus from CrFK cells. This virus also is shown to efficiently package Moloney murine leukemia virus vectors.

Delayed infection after immunization with a peptide from the transmembrane glycoprotein of the feline immunodeficiency virus

Recent advances in the quantitative assessment of viral burden, by permitting the extension of criteria applied to assess the efficacy of vaccines from all-or-none protection to diminution of the viral burden, may allow the identification of original immunogens of value in combined vaccines. Peptides corresponding to three domains of the envelope glycoproteins of feline immunodeficiency virus that are recognized during natural infection were used to immunize cats. After challenge with a primary isolate of feline immunodeficiency virus, the development of acute infection was monitored by quantitative assessment of the viral burden in plasma and tissues by competitive reverse transcription-PCR, by measurement of the humoral response developed to viral components, and by lymphocyte subset analysis. Whereas immunization with two peptides derived from the surface glycoprotein had no effect on the early course of infection, immunization with a peptide derived from the transmembrane glycoprotein delayed infection, as reflected by a diminished viral burden in the early phase of primary infection and delayed seroconversion. This peptide, located in the membrane-proximal region of the extracellular domain, has homology to an epitope of human immunodeficiency virus type 1 recognized by a broadly neutralizing monoclonal antibody. These results suggest that lentivirus transmembrane glycoproteins share a determinant in the juxtamembrane ectodomain which could be of importance in the design of vaccines against AIDS.

Enhancement of feline immunodeficiency virus (FIV) infection after DNA vaccination with the FIV envelope

Despite intensive experimentation to develop effective and safe vaccines against the human immunodeficiency viruses and other pathogenic lentiviruses, it remains unclear whether an immune response that does not afford protection may, on the contrary, produce adverse effects. In the present study, the effect of genetic immunization with the env gene was examined in a natural animal model of lentivirus pathogenesis, infection of cats by the feline immunodeficiency virus (FIV). Three groups of seven cats were immunized by intramuscular transfer of plasmid DNAs expressing either the wild-type envelope or two envelopes bearing mutations in the principal immunodominant domain of the transmembrane glycoprotein. Upon homologous challenge, determination of plasma virus load showed that the acute phase of viral infection occurred earlier in the three groups of cats immunized with FIV envelopes than in the control cats. Genetic immunization, however, elicited low or undetectable levels of antibodies directed against envelope glycoproteins. These results suggest that immunization with the FIV env gene may result in enhancement of infection and that mechanisms unrelated to enhancing antibodies underlay the observed acceleration.

Shared usage of the chemokine receptor CXCR4 by the feline and human immunodeficiency viruses

Feline immunodeficiency virus (FIV) induces a disease state in the domestic cat that is similar to AIDS in human immunodeficiency virus (HIV)-infected individuals. As with HIV, FIV can be divided into primary and cell culture-adapted isolates. Adaptation of FIV to replicate and form syncytia in the Crandell feline kidney (CrFK) cell line is accompanied by an increase in the net charge of the V3 loop of the envelope glycoprotein, mirroring the changes observed in the V3 loop of HIV gp120 with the switch from a non-syncytium-inducing phenotype to a syncytium-inducing phenotype. These data suggest a common mechanism of infection with FIV and HIV. In this study, we demonstrate that cell culture-adapted strains of FIV are able to use the alpha-chemokine receptor CXCR4 for cell fusion. Following ectopic expression of human CXCR4 on nonpermissive human cells, the cells are able to fuse with FIV-infected feline cells. Moreover, fusion between FIV-infected feline cells and CXCR4-transfected human cells is inhibited by both anti-CXCR4 and anti-FIV antibodies. cDNAs encoding the feline CXCR4 homolog were cloned from both T-lymphoblastoid and kidney cell lines. Feline CXCR4 displayed 94.9% amino acid sequence identity with human CXCR4 and was found to be expressed widely on cell lines susceptible to infection with cell culture-adapted strains FIV. Ectopic expression of feline CXCR4 on human cells rendered the cells susceptible to FIV-dependent fusion. Moreover, feline CXCR4 was found to be as efficient as human CXCR4 in supporting cell fusion between CD4-expressing murine fibroblast cells and either HIV type 1 (HIV-1) or HIV-2 Env-expressing human cells. Previous studies have demonstrated that feline cells expressing human CD4 are not susceptible to infection with HIV-1; therefore, further restrictions to HIV-1 Env-dependent fusion may exist in feline cells. As feline and human CXCR4 support both FIV- and HIV-dependent cell fusion, these results suggest a close evolutionary link between FIV and HIV and a common mechanism of infection involving an interaction between the virus and a member of the seven-transmembrane domain chemokine receptor family of molecules.

A single amino acid substitution in the transmembrane envelope glycoprotein of feline immunodeficiency virus alters cellular tropism

The cellular tropism of the feline immunodeficiency virus (FIV) is affected by changes in variable region 3 (V3) of the surface (SU) envelope glycoprotein (Verschoor, E. J., et al., J. Virol. 69:4752-4757, 1995). By using high-dose DNA transfection, an FIV molecular clone with a non-CRFK-tropic V3 acquired the ability to replicate in CRFK cells. A single point mutation from a methionine to a threonine in the ectodomain of its transmembrane (TM) envelope glycoprotein was responsible for this change in viral tropism. This substitution is located in the putative SU interactive region, between the fusion peptide and the membrane-spanning region. Our results show that this region of the TM envelope glycoprotein constitutes an additional determinant for cell tropism.

Retention of viral infectivity after extensive mutation of the highly conserved immunodominant domain of the feline immunodeficiency virus envelope

In lentiviruses, including human immunodeficiency virus and feline immunodeficiency virus (FIV), the principal immunodominant domain (PID) of the transmembrane glycoprotein elicits a strong humoral response in infected hosts. The PID is marked by the presence of two cysteines that delimit a sequence, composed of five to seven amino acids in different lentiviruses, which is highly conserved among isolates of the same lentiviral species. While the conservation of the sequence suggests the presence of functional constraints, the conservation of the immunodominance among divergent lentiviruses raises the hypothesis of a selective advantage for the infecting virus conferred by the host humoral response against this domain. We and others have previously shown that an appropriate structure of the PID is required for the production of a functional envelope. In the present work, we analyzed virological functions and immune reactivity of the envelope after random mutagenesis of the PID of FIV. We obtained nine mutant envelopes which were correctly processed and retained fusogenic ability. Mutation of the two C-terminal residues of the PID sequence between the cysteines in a molecular clone of FIV abolished infectivity. In contrast, three molecular clones containing extensive mutations in the four N-terminal amino acids were infectious. However, the mutations affected PID reactivity with sera from infected cats. Our results suggest that functional constraints, although existent, are not sufficient to account for PID sequence conservation. Such conservation may also result from positive selection by anti-PID antibodies which enhance infection.