Apoptosis induced by tumor necrosis factor in cells chronically infected with feline immunodeficiency virus.

Tumor necrosis factor alpha (TNF-alpha) induced morphologic changes such as chromatin condensation and cell shrinkage in a feline fibroblastic cell line (CRFK) chronically infected with feline immunodeficiency virus (FIV) but not in uninfected CRFK cells. DNA extracted from TNF-alpha-treated CRFK cells infected with FIV showed a ladder of nucleosomal DNA, indicating that this cytocidal effect by TNF-alpha was due to programmed cell death, or apoptosis. These findings may have implications for understanding the pathogenesis of FIV infection and for the design of specific therapeutic strategies for AIDS in humans as well as cats.     

Altered cell tropism and cytopathicity of feline immunodeficiency viruses in two different feline CD4-positive, CD8-negative cell lines.

Two interleukin-2-dependent feline CD4-positive and CD8-negative cell lines, MYA-1 and the newly established FeL-039, were used as host cells for infection with feline immunodeficiency virus (FIV). All FIV strains used, the Petaluma strain and several new isolates, were highly cytopathic to MYA-1. In contrast, the kinetics of FIV replication in FeL-039 differed greatly depending on the strain tested, i.e., noninfectious strain, highly cytopathic strain, and less cytopathic strain producing a persistent state for a long period. It appears, therefore, that cell tropism for FIV differed with each FIV strain tested even in T-cell lines showing similar cell surface phenotypes. Cytopathicity of FIV is evidently due to both the FIV strain and the host T cell.     

Bicyclams, Selective Antagonists of the Human Chemokine Receptor CXCR4, Potently Inhibit Feline Immunodeficiency Virus Replication

Bicyclams are low-molecular-weight anti-human immunodeficiency virus (HIV) agents that have been shown to act as potent and selective CXC chemokine receptor 4 (CXCR4) antagonists. Here, we demonstrate that bicyclams are potent inhibitors of feline immunodeficiency virus (FIV) replication when evaluated in Crandell feline kidney (CRFK) cells. With a series of bicyclam derivatives, 50% inhibitory concentrations (IC50s) against FIV were obtained in this cell system that were comparable to those obtained for HIV-1 IIIB replication in the human CD4(+) MT-4 T-cell line. The bicyclams were also able to block FIV replication in feline thymocytes, albeit at higher concentrations than in the CRFK cells. The prototype bicyclam AMD3100, 1-1'-[1,4-phenylene-bis(methylene)]-bis(1,4,8, 11-tetraazacyclotetradecane), was only fourfold less active in feline thymocytes (IC50, 62 ng/ml) than in CRFK cells (IC50, 14 ng/ml). AMD2763, 1,1'-propylene-bis(1,4,8, 11-tetraazacyclotetradecane), which is a less potent CXCR4 antagonist, was virtually inactive against FIV in feline thymocytes (IC50, >66.5 microgram/ml), while it was clearly active in CRFK cells (IC50, 0.9 microgram/ml). The CXC chemokine stromal-cell-derived factor 1alpha had anti-FIV activity in CRFK cells (IC50, 200 ng/ml) but not in feline thymocytes (IC50, >2.5 microgram/ml). When primary FIV isolates were evaluated for their drug susceptibility in feline thymocytes, the bicyclams AMD3100 and its Zn2+ complex, AMD3479, inhibited all six primary isolates at equal potency. The marked susceptibility of FIV to the bicyclams suggests that FIV predominantly uses feline CXCR4 for entering its target cells.     

Molecular cloning of a novel isolate of feline immunodeficiency virus biologically and genetically different from the original U.S. isolate.

The Japanese isolate (TM1 strain) of feline immunodeficiency virus (FIV) which replicates in a feline CD4 (fCD4)-positive lymphoblastoid cell line (MYA-1 cells) was molecularly cloned from extrachromosomal closed circular DNA. The restriction map of the clone, termed pFTM 191 complete genome (CG), showed a considerable difference from that of the U.S. isolate (Petaluma strain) of FIV. The sequence homology in the long terminal repeat between the TM1 and Petaluma strain was 82%. The pFTM 191 CG was biologically active after transfection into Crandell feline kidney cells which were permissive for replication of FIV Petaluma. However, the progeny virions could not reinfect fCD4-negative Crandell feline kidney cells but could infect fCD4-positive MYA-1 cells. When a specific-pathogen-free cat was inoculated with the virus derived from the pFTM 191 CG, the cat seroconverted within 8 weeks postinoculation and FIV was reisolated at 4, 8, and 20 weeks postinoculation. These results indicate the infectivity of the pFTM 191 CG in vivo.     

A single mutation within the V3 envelope neutralization domain of feline immunodeficiency virus determines its tropism for CRFK cells.

Feline immunodeficiency virus (FIV) isolates differ in the ability to replicate in Crandell feline kidney (CRFK) cells. The difference in tropism between two variants of the Dutch isolate FIV-UT113 was studied by using molecular clones which contained the envelope genes of the variants in a background of the FIV-14 Petaluma sequence. Virus produced from clone pPET-113Th replicated in thymocytes, whereas virus from pPET-113Cr propagated in both thymocytes and CRFK cells, thereby reflecting the phenotypes of the parental variants. Exchange of envelope gene fragments showed that a 464-bp surface protein (SU)-encoding fragment encompassing the third variable region (V3) determines CRFK cell tropism. Sequence analysis of the exchanged fragments demonstrated two amino acid changes that led to an increase of the overall charge of the V3 domain: a G-->R transition at position 397 and a E-->K change at position 407. Mutational analysis of these residues revealed that the E-->K shift was responsible for the change in tropism, while the G-->R mutation improved the replication kinetics in CRFK cells. Mapping of a tropism determinant for FIV to a region which is also a major neutralization domain is reminiscent of human immunodeficiency virus type 1, in which a similar colocation was found.     

In Vivo Monocyte Tropism of Pathogenic Feline Immunodeficiency Viruses

Virus-infected monocytes rarely are detected in the bloodstreams of animals or people infected with immunodeficiency-inducing lentiviruses, yet tissue macrophages are thought to be a major reservoir of virus-infected cells in vivo. We have identified feline immunodeficiency virus (FIV) clinical isolates that are pathogenic in cats and readily transmitted vertically. We report here that five of these FIV isolates are highly monocytotropic in vivo. However, while FIV-infected monocytes were numerous in the blood of experimentally infected cats, viral antigen was not detectable in freshly isolated cells. Only after a short-term (at least 12-h) in vitro monocyte culture were FIV antigens detectable (by immunocytochemical analysis or enzyme-linked immunosorbent assay). In vitro experiments suggested that monocyte adherence provided an important trigger for virus antigen expression. In the blood of cats infected with a prototype monocytotropic isolate (FIV subtype B strain 2542), infected monocytes appeared within 2 weeks, correlating with high blood mononuclear-cell-associated viral titers and CD4 cell depletion. By contrast, infected monocytes could not be detected in the blood of cats infected with a less pathogenic FIV strain (FIV subtype A strain Petaluma). We concluded that some strains of FIV are monocytotropic in vivo. Moreover, this property may relate to virus virulence, vertical transmission, and infection of tissue macrophages.     

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.     

Feline tetherin efficiently restricts release of feline immunodeficiency virus but not spreading of infection

Domestic cats endure infections by all three subfamilies of the retroviridae: lentiviruses (feline immunodeficiency virus [FIV]), gammaretroviruses (feline leukemia virus [FeLV]), and spumaretroviruses (feline foamy virus [FFV]). Thus, cats present an insight into the evolution of the host-retrovirus relationship and the development of intrinsic/innate immune mechanisms. Tetherin (BST-2) is an interferon-inducible transmembrane protein that inhibits the release of enveloped viruses from infected cells. Here, we characterize the feline homologue of tetherin and assess its effects on the replication of FIV. Tetherin was expressed in many feline cell lines, and expression was induced by interferons, including alpha interferon (IFN-α), IFN-ω, and IFN-γ. Like human tetherin, feline tetherin displayed potent inhibition of FIV and HIV-1 particle release; however, this activity resisted antagonism by either HIV-1 Vpu or the FIV Env and "OrfA" proteins. Further, as overexpression of complete FIV genomes in trans could not overcome feline tetherin, these data suggest that FIV lacks a functional tetherin antagonist. However, when expressed stably in feline cell lines, tetherin did not abrogate the replication of FIV; indeed, syncytium formation was significantly enhanced in tetherin-expressing cells infected with cell culture-adapted (CD134-independent) strains of FIV (FIV Fca-F14 and FIV Pco-CoLV). Thus, while tetherin may prevent the release of nascent viral particles, cell-to-cell spread remains efficient in the presence of abundant viral receptors and tetherin upregulation may enhance syncytium formation. Accordingly, tetherin expression in vivo may promote the selective expansion of viral variants capable of more efficient cell-to-cell spread.     

Experimental vaccine protection against homologous and heterologous strains of feline immunodeficiency virus.

More than 90% of cats immunized with inactivated whole infected-cell or cell-free feline immunodeficiency virus (FIV) vaccines were protected against intraperitoneal infection with 10 50% animal infectious doses of either homologous FIV Petaluma (28 of 30 cats) or heterologous FIV Dixon strain (27 of 28 cats). All 15 control cats were readily infected with either strain of FIV. Protection appears to correlate with antiviral envelope antibody levels by a mechanism yet to be determined.     

Passive antibody protection of cats against feline immunodeficiency virus infection.

All six cats passively immunized with sera from either feline immunodeficiency virus (FIV)-vaccinated cats or cats infected with FIV (Petaluma strain) were protected from homologous FIV infection at a challenge dose that infected all six control cats. Passive immunization with sera from cats vaccinated with uninfected allogeneic T cells used to grow the vaccine virus did not protect either of two cats against the same FIV challenge. These results suggest that antiviral humoral immunity, perhaps in synergy with anticellular antibodies, may be responsible for previously reported vaccine protection.     

Differential cell tropism of feline immunodeficiency virus molecular clones in vivo

Independent studies have demonstrated different cell tropisms for molecular clones of feline immunodeficiency virus (FIV). In this report, we examined three clones, FIV-pF34, FIV-14, and FIV-pPPR, for replication in Crandell feline kidney (CrFK) cells, feline peripheral blood mononuclear cells (PBMC), and feline macrophage cultures. Importantly, cell tropism for these three clones was also examined in vivo. FIV-pF34 replication was efficient in CrFK cells but severely restricted in PBMC, whereas replication of FIV-pPPR was vigorous in PBMC but severely restricted in CrFK cells. FIV-14 replication was productive in both CrFK cells and PBMC. Interestingly, all three molecular clones replicated with similar efficiencies in primary feline monocyte-derived macrophages. In vivo, FIV-pF34 proved least efficient for establishing persistent infection, and proviral DNA when detectable, was localized predominately to nonlymphoid cell populations (macrophages). FIV-pPPR proved most efficient for induction of a persistent viremia in vivo, and proviral DNA was localized predominately in CD4(+) and CD8(+) lymphocyte subsets. FIV-14 inoculation of cats resulted in an infection characterized by seroconversion and localization of proviral DNA in CD4(+) lymphocytes only. Results of this study on diverse FIV molecular clones revealed that in vitro replication efficiency of an FIV isolate in PBMC directly correlated with replication efficiency in vivo, whereas proficiency for replication in macrophages in vitro was not predictive for replication potential in vivo. Also, infection of both CD4(+) and CD8(+) lymphocyte subsets was associated with higher virus load in vivo. Results of the studies on these three FIV clones, which exhibited differential cell tropism, indicated a correlation between in vitro and in vivo cell tropism and virus replication.     

Analysis of Aleutian disease virus infection in vitro and in vivo: demonstration of Aleutian disease virus DNA in tissues of infected mink.

Aleutian disease virus (ADV) infection was analyzed in vivo and in vitro to compare virus replication in cell culture and in mink. Initial experiments compared cultures of Crandell feline kidney (CRFK) cells infected with the avirulent ADV-G strain or the highly virulent Utah I ADV. The number of ADV-infected cells was estimated by calculating the percentage of cells displaying ADV antigen by immunofluorescence (IFA), and several parameters of infection were determined. Infected cells contained large quantities of viral DNA (more than 10(5) genomes per infected cell) as estimated by dot-blot DNA-DNA hybridization, and much of the viral DNA, when analyzed by Southern blot hybridization, was found to be of a 4.8-kilobase-pair duplex monomeric replicative form (DM DNA). Furthermore, the cultures contained 7 to 67 fluorescence-forming units (FFU) per infected cell, and the ADV genome per FFU ratio ranged between 2 X 10(3) and 164 X 10(3). Finally, the pattern of viral antigen detected by IFA was characteristically nuclear, although cytoplasmic fluorescence was often found in the same cells. Because no difference was noted between the two virus strains when cultures containing similar numbers of infected cells were compared, it seemed that both viruses behaved similarly in infected cell culture. These data were used as a basis for the analysis of infection of mink by virulent Utah I ADV. Ten days after infection, the highest levels of viral DNA were detected in spleen (373 genomes per cell), mesenteric lymph node (MLN; 750 genomes per cell), and liver (373 genomes per cell). In marked contrast to infected CRFK cells, the predominant species of ADV DNA in all tissues was single-stranded virion DNA; however, 4.8-kilobase-pair DM DNA was found in MLN and spleen. This observation suggested that MLN and spleen were sites of virus replication, but that the DNA found in liver reflected sequestration of virus produced elsewhere. A final set of experiments examined MLN taken from nine mink 10 days after Utah I ADV infection. All of the nodes contained ADV DNA (46 to 750 genomes per cell), and although single-stranded virion DNA was always the most abundant species, DM DNA was observed. All of the lymph nodes contained virus infectious for CRFK cells, but when the genome per FFU ratio was calculated, virus from the lymph nodes required almost 1,000 times more genomes to produce an FFU than did virus prepared from infected cell cultures.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

A neutralizing antibody-inducing peptide of the V3 domain of feline immunodeficiency virus envelope glycoprotein does not induce protective immunity.

Specific-pathogen-free cats, immunized with a 22-amino-acid synthetic peptide designated V3.3 and derived from the third variable region of the envelope glycoprotein of the Petaluma isolate of feline immunodeficiency virus (FIV), developed high antibody titers to the V3.3 peptide and to purified virus, as assayed by enzyme-linked immunoassays, as well as neutralizing antibodies, as assayed by the inhibition of syncytium formation in Crandell feline kidney cells. V3.3-immunized animals and control cats were challenged with FIV and then monitored for 12 months; V3.3 immunization failed to prevent FIV infection, as shown by virus isolation, anti-whole virus and anti-p24 immunoglobulin G antibody responses, and positive PCRs for gag and env gene fragments. Sequence analysis of the V3 region showed no evidence for the emergence of escape mutants that might have contributed to the lack of protection. The sera of the V3.3-hyperimmunized cats and two anti-V3.3 monoclonal antibodies neutralized FIV infectivity for Crandell feline kidney cells at high antibody dilutions but paradoxically failed to completely neutralize FIV infectivity at low dilutions. Moreover, following FIV challenge, V3.3-immunized animals developed a faster and higher antiviral antibody response than control cats. This was probably due to enhanced virus replication, as also suggested by quantitative PCR data.     

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.     

Isolation and partial characterization of infectious molecular clones of feline immunodeficiency virus obtained directly from bone marrow DNA of a naturally infected cat.

Replication-competent molecular clones of feline immunodeficiency virus (FIV) were isolated directly from the DNA of bone marrow cells of a naturally FIV-infected cat. After transfection in a feline kidney cell line (CrFK) and subsequent cocultivation with peripheral blood mononuclear cells (PBMC), the viral progeny of the clones was infectious for PBMC but not for CrFK cells. PBMC infected with these clones showed syncytium formation, a decrease in cell viability, and gradual loss of CD4+ cells. The restriction maps of these clones differed from those obtained for previously described molecular clones of FIV derived from cats in the United States. The predicted amino acid sequence similarity of the envelope genes of the two clones was 99.3%, whereas the similarities of the sequences of the clones to those of two molecular clones from the United States, Petaluma and PPR, were 86 and 88%, respectively. Most of the differences between the amino acid sequences of the two clones and those of the clones from the United States were found in five different hypervariable (HV) regions, HV-1 through HV-5. The viral progeny of one of these clones was inoculated into two specific-pathogen-free cats. The animals seroconverted, and the virus could be reisolated from their PBMC.     

Vaccination with inactivated virus but not viral DNA reduces virus load following challenge with a heterologous and virulent isolate of feline immunodeficiency virus

It has been shown that cats can be protected against infection with the prototypic Petaluma strain of feline immunodeficiency virus (FIV(PET)) using vaccines based on either inactivated virus particles or replication-defective proviral DNA. However, the utility of such vaccines in the field is uncertain, given the absence of consistent protection against antigenically distinct strains and the concern that the Petaluma strain may be an unrepresentative, attenuated isolate. Since reduction of viral pathogenicity and dissemination may be useful outcomes of vaccination, even in the absence of complete protection, we tested whether either of these vaccine strategies ameliorates the early course of infection following challenge with heterologous and more virulent isolates. We now report that an inactivated virus vaccine, which generates high levels of virus neutralizing antibodies, confers reduced virus loads following challenge with two heterologous isolates, FIV(AM6) and FIV(GL8). This vaccine also prevented the marked early decline in CD4/CD8 ratio seen in FIV(GL8)-infected cats. In contrast, DNA vaccines based on either FIV(PET) or FIV(GL8), which induce cell-mediated responses but no detectable antiviral antibodies, protected a fraction of cats against infection with FIV(PET) but had no measurable effect on virus load when the infecting virus was FIV(GL8). These results indicate that the more virulent FIV(GL8) is intrinsically more resistant to vaccinal immunity than the FIV(PET) strain and that a broad spectrum of responses which includes virus neutralizing antibodies is a desirable goal for lentivirus vaccine development.     

Evidence for CD8+ antiviral activity in cats infected with feline immunodeficiency virus.

Human immunodeficiency virus (HIV) causes a long, asymptomatic infection characterized by normal to elevated numbers of circulating CD8+ cells and a progressive decline in CD4+ cells. It has been speculated that HIV-specific antiviral activity driven by CD8+ T cells may control viral replication during this period and maintain the clinically asymptomatic stage of disease. The disease induced in cats by feline immunodeficiency virus (FIV) is similar to HIV in that it is characterized by a long asymptomatic stage with a progressive decline in CD4+ cells, culminating in AIDS. In the present study, we demonstrate that FIV is more readily isolated from CD8+ T-cell-depleted peripheral blood mononuclear cells (PBMC) of FIV-infected cats than from unfractionated PBMC cultures. In addition, CD8+ T cells isolated from FIV-positive cats demonstrating anti-FIV activity in PBMC cultures inhibit FIV infection of FCD4E cells in vitro. Anti-FIV activity is not found in FIV- negative cats and is not characteristic of cats acutely infected with FIV but is present in the majority of chronically infected, clinically asymptomatic and symptomatic cats. Decreases in plasma and cell-associated viremia during the acute-stage FIV infection appears to precede the appearance of CD8+ anti-FIV cells in the circulation. In summary, this study demonstrates a population(s) of CD8+ T cells in chronically FIV-infected cats capable of suppressing FIV replication in cultured PBMC. The significance of anti-FIV CD8+ cells in the immunopathogenesis of the infection and disease progression has yet to be determined.     

CXCR4 Is Required by a Nonprimate Lentivirus: Heterologous Expression of Feline Immunodeficiency Virus in Human, Rodent, and Feline Cells

A heterologous feline immunodeficiency virus (FIV) expression system permitted high-level expression of FIV proteins and efficient production of infectious FIV in human cells. These results identify the FIV U3 element as the sole restriction to the productive phase of replication in nonfeline cells. Heterologous FIV expression in a variety of human cell lines resulted in profuse syncytial lysis that was FIV env specific, CD4 independent, and restricted to cells that express CXCR4, the coreceptor for T-cell-line-adapted strains of human immunodeficiency virus. Stable expression of human CXCR4 in CXCR4-negative human and rodent cell lines resulted in extensive FIV Env-mediated, CXCR4-dependent cell fusion and infection. In feline cells, stable overexpression of human CXCR4 resulted in increased FIV infectivity and marked syncytium formation during FIV replication or after infection with FIV Env-expressing vectors. The use of CXCR4 is a fundamental feature of lentivirus biology independent of CD4 and a shared cellular link to infection and cytopathicity for distantly related lentiviruses that cause AIDS. Their conserved use implicates chemokine receptors as primordial lentivirus receptors.