Feline leukemia virus subgroup B uses the same cell surface receptor as gibbon ape leukemia virus

Fungal phosphate transporter serves as a receptor backbone for gibbon ape leukemia virus

Pit1, the receptor for gibbon ape leukemia virus (GALV), is proposed to be an integral membrane protein with five extracellular loops. Chimeras made between Pit1 homologs differing in permissivity for infection and between Pit1 and the related protein Pit2 have shown that the fourth extracellular loop plays a critical role in infection. However, further elucidation of the roles of the extracellular loops in infection is hampered by the high level of sequence similarity among these proteins. The sodium-dependent phosphate transporter, Pho-4, from the filamentous fungus Neurospora crassa is distantly related to Pit1 and -2, showing an amino acid identity of only 35% to Pit1 in the putative extracellular loops. We show here that Pho-4 itself does not function as a receptor for GALV. Introduction of 12 Pit1-specific amino acid residues in the putative fourth extracellular loop of Pho-4 resulted in a functional GALV receptor. Therefore, the presence of a Pit1 loop 4-specific sequence is sufficient to confer receptor function for the mammalian retrovirus GALV on the fungal phosphate transporter Pho-4.

Mutational analysis of the proposed gibbon ape leukemia virus binding site in Pit1 suggests that other regions are important for infection

Region A of Pit1 (residues 550 to 558 in domain IV) and related receptors has remained the only sequence implicated in gibbon ape leukemia virus (GALV) infection, and an acidic residue at the first position appeared indispensable. The region has also been proposed to be the GALV binding site, but this lacks empirical support. Whether an acidic residue at the first position in this sequence is a definitive requirement for GALV infection has also remained unclear; certain receptors retain function even in the absence of this acidic residue. We report here that in Pit1 an acidic residue is dispensable not only at position 550 but also at 553 alone and at both positions. Further, the virus requires no specific residue at either position. Mutations generated a collection of region A sequences, often with fundamentally different physicochemical properties (overall hydrophobicity or hydrophilicity and net charge of -1, or 0, or +1), and yet Pit1 remained an efficient GALV receptor. A comparison of these sequences and a few previously published ones from highly efficient GALV receptors revealed that every position in region A can vary without affecting GALV entry. Even Pit2 is nonfunctional for GALV only because it has lysine at the first position in its region A, which is otherwise highly diverse from region A of Pit1. We propose that region A itself is not the GALV binding motif and that other sequences are required for virus entry. Indeed, certain Pit1/Pit2 chimeras revealed that sequences outside domain IV are specifically important for GALV infection.

FeLV envelope protein (gp70) variable region 5 causes alterations in calcium homeostasis and toxicity of neurons

In humans and animals, retroviruses have been implicated in nervous system disease. Our objective was to characterize the neurotoxicity of a peptide sequence derived from an animal retrovirus, the feline leukemia virus (FeLV). Using a peptide sequence from the subtype FeLV-C envelope protein variable region 5 (VR5), cytotoxicity was demonstrated in studies that evaluated neuronal survival, neurite outgrowth, and alterations in intracellular calcium ion concentration. The FeLV subtype isolate FeLV-CSarma possesses an envelope protein VR5 amino acid sequence that varies by four amino acids from the VR5 amino acid sequence of subtype FeLV-AGlasgow. The polypeptide representing the VR5 of FeLV-CSarma (FeLV-CVR5) is significantly more neurotoxic than the polypeptide sequence representing the VR5 of FeLV-AGlasgow (FeLV-AVR5). FeLV-CVR5 (> or = 3 microM) exposure resulted in significant dose-dependent neurotoxicity. Antibodies to FeLV-CVR5 blocked this effect. Neurite outgrowth was significantly reduced at all tested concentrations (3-12 microM) of FeLV-CVR5, with a 92% reduction in neurite length at 12 microM. FeLV-AVR5 was significantly less neurotoxic with respect to neurite outgrowth than was FeLV-CVR5. The significant reduction in neurotoxicity for FeLV-AVR5 illustrates the importance of the 4-amino-acid difference between it and FeLV-CVR5. Alterations in intracellular calcium ion concentration were associated with this neurotoxicity.

Comparison of T-cell subpopulations in cats naturally infected with feline leukaemia virus or feline immunodeficiency virus

T-cell subsets were studied by flow cytometry in 58 feline leukaemia virus (FeLV)-positive cats with naturally acquired FeLV infection to determine whether the changes in CD4+ or CD8+ T cell populations differed from those observed in 55 feline immunodeficiency virus (FIV)-positive cats with naturally acquired FIV infection. The sole criterion for inclusion into the study was seropositivity. Mean (SD) CD4+ T cell values of FeLV positive cats were decreased to 31.1 (8.0) per cent and their CD8+ T cell values were increased to 22.8 (6.3) per cent in comparison with uninfected control cats (37.9 [9.5] per cent CD4+; 15.2 [6.3] per cent CD8+). The CD4+/CD8+ ratio was reduced to 1.5 (0.7), compared with 3.0 (1.5) in 39 FeLV-and FIV-negative control cats. Differences from control values were significant, but there was no significant difference between CD4+ and CD8+ lymphocytes of FeLV-versus FIV-infected cats. These findings indicate that FeLV and FIV have similar effects on T lymphocyte subsets. Both retrovirus infections can induce immunodeficiency, both viruses infect a broad range of lymphohaemopoietic cells, despite having different primary target cells, and can induce the killing of lymphocytic cells in vitro. It is concluded that a decreased CD4+/CD8+ ratio is not restricted to FIV infections but may also occur in FeLV infection.

Comparison of efficiency of infection of human gene therapy target cells via four different retroviral receptors

The relative efficiency of transduction of gene therapy target cells was measured for retroviruses bearing the envelopes of amphotropic murine leukemia virus (MLV-A), xenotropic murine leukemia virus (MLV-X), gibbon ape leukemia virus (GALV), feline leukemia virus subgroup B (FeLV-B), and the feline endogenous virus RD114. These viruses use various cell-surface receptors. Activated peripheral blood lymphocytes (PBL) and primary melanoma cultures were infected relatively poorly by MLV-X pseudotypes. RD114 pseudotypes infected PBL relatively well, whereas bone marrow progenitor cells were efficiently infected by all viruses. Helper-free virus bearing the envelopes of MLV-A, RD114, or GALV was similarly tested. All infected melanoma or bone marrow progenitor cells efficiently, whereas MLV-A was relatively inefficient for infection of PBL. The general utility of RD114 pseudotyped virus for gene delivery coupled with its resistance to inactivation by human serum makes this envelope the most suitable choice for in vivo gene therapy.

Substitution of a single amino acid residue is sufficient to allow the human amphotropic murine leukemia virus receptor to also function as a gibbon ape leukemia virus receptor

We have previously reported the unique properties of a receptor for amphotropic murine leukemia viruses (A-MuLVs) expressed on Chinese hamster E36 cells (C.A. Wilson, K.B. Farrell, and M.V. Eiden, J. Virol. 68:7697-7703, 1994). This receptor, HaPiT2 (formerly designated EAR), in contrast to the human form of the A-MuLV receptor (PiT2), functions as a receptor not only for A-MuLVs but also for gibbon ape leukemia virus (GALV). Comparison of the deduced amino acid sequences of the HaPiT2 and PiT2 proteins suggested that differences in the amino acid composition of the extracellular region(s) of the hamster and human proteins account for their functional differences. We substituted extracellular regions of HaPiT2 for those of PiT2 to map the region of the HaPiT2 protein required for GALV receptor function. Only those PiT2-HaPiT2 chimeric receptors containing the fourth and fifth extracellular regions of HaPiT2 functioned as GALV receptors. We have now determined that the substitution of a single amino acid residue, glutamic acid, for the lysine residue at position 522 in the fourth extracellular region of the PiT2 protein is sufficient to render PiT2 functional as a GALV receptor.

Analysis of the murine ecotropic leukemia virus receptor reveals a common biochemical determinant on diverse cell surface receptors that is essential to retrovirus entry

Two residues, tyrosine 235 and glutamic acid 237, of the ecotropic murine leukemia virus receptor (ATRC1) have been shown to be essential for receptor-mediated virus envelope binding and entry. We performed genetic analyses to examine the biochemical contribution of these residues in a productive virus-receptor interaction. Altered ATRC1 receptors bearing either a phenylalanine, a tryptophan, a histidine, or a methionine at position 235 mediated ecotropic virus entry comparable to that mediated by ATRC1. In contrast, altered ATRC1 receptors bearing alanine, threonine, serine, or proline at position 235 exhibited a 300- to 10,000-fold decrease in receptor capability. Furthermore, substitution of tyrosine or phenylalanine into the corresponding position (242) of the homologous human protein that lacks ecotropic virus receptor capability resulted in acquisition of ecotropic virus receptor function comparable to that of ATRC1. Substitution of a tryptophan or a histidine at that position of the human protein, however, resulted in a much-reduced receptor capability, suggesting a preference for a benzene ring in the hydrophobic side chain. A similar analysis of proteins substituted at position 237 revealed that aspartic acid, but not arginine or lysine, can functionally substitute for glutamic acid 237 in ATRC1 or at the corresponding position in the human protein. These results suggest a requirement for an acidic and a nearby hydrophobic amino acid for efficient ecotropic virus entry. Similar motifs have been identified in the virus binding sites of other retrovirus receptors, suggesting that the initial step of retrovirus entry may be governed by a common mechanism.

Retroviral retargeting by envelopes expressing an N-terminal binding domain

We have engineered ecotropic Moloney murine leukemia virus-derived envelopes targeted to cell surface molecules expressed on human cells by the N-terminal insertion of polypeptides able to bind either Ram-1 phosphate transporter (the first 208 amino acids of amphotropic murine leukemia virus surface protein) or epidermal growth factor receptor (EGFR) (the 53 amino acids of EGF). Both envelopes were correctly processed and incorporated into viral particles. Virions carrying these envelopes could specifically bind the new cell surface receptors. Virions targeted to Ram-1 could infect human cells, although the efficiency was reduced compared with that of virions carrying wild-type amphotropic murine leukemia virus envelopes. The infectivity of virions targeted to EGFR was blocked at a postbinding step, and our results suggest that EGFR-bound virions were rapidly trafficked to lysosomes. These data suggest that retroviruses require specific properties of cell surface molecules to allow the release of viral cores into the correct cell compartment.

Endogenous env elements: partners in generation of pathogenic feline leukemia viruses

Feline leukemia viruses (FeLVs), which are replication-competent oncoretroviruses of the domestic cat species, are contagiously transmitted in natural environments. They are capable of inducing either acute antiproliferative disease or, after prolonged latency, lymphoid malignancies in this animal population. Current knowledge of the recombinational events between infectious FeLV and noninfectious endogenously inherited FeLV-like elements is reviewed, and the potential role of the derived recombinant viruses in pathogenesis is discussed. Major observations made are as follows: (1) Up to three fourths of the exogenous FeLV envelope glycoprotein (SU), beginning from the N-terminal end, can be replaced by sequences from an endogenous FeLV to produce biologically active chimeric FeLVs. The in vitro replication efficiency or cell tropism of the recombinants appears to be influenced by the amount of SU sequences replaced by the endogenous partner, as well as by the locus of origin of the endogenous sequences. (2) Generation of FeLV recombinants in tissue culture cells corresponds closely to the findings from natural tumors. There is direct evidence, based on molecular genetic analysis, for the prevalence of recombinant proviruses in naturally arising FeLV-induced lymphomas. (3) Certain recombinants harboring an altered primary neutralizing epitope in the middle of SU corresponding to the endogenous FeLV sequence can evade immunity developed against common FeLV infection. In several other recombinants, the epitope sequence is found to be frequently mutated during the process of recombination. (4) FeLV variants with altered epitope, although they may not be efficient in replication in vivo, apparently are capable of causing focal infection in target organs. Evidence is also presented that when coinfected with an exogenous FeLV, the epitope sequence in the variants is reverted to the exogenous type, providing an explanation why this sequence is found to be conserved in all natural isolates of FeLV. (5) A prototype chimeric polyprotein containing most of the SU from the endogenous source is abnormally processed and becomes trapped in the endoplasmic reticulum. A functional consequence of such trapping is interference with specific FeLV infection. (6) Some recombinants, while only poorly replicating in the host, may have the ability to infect target erythroid progenitor cells for the induction of strong cytopathic effect. (7) Some other recombinants appear to potentiate lymphomagenesis by exogenous FeLV and others to acquire properties to infect CNS endothelial cells, an event that could potentially be related to FeLV-induced neuropathogenicity. (8) Of multiple recombinant viruses, a specific recombinant species was found to occur in each of the three cats examined in which lymphoma was experimentally induced, and it was exclusively seen in one of these cats. This recombinant FeLV may potentially be a candidate for strong leukemogenic function. In addition to commonly encountered virus envelope changes, another prominent viral factor involved in tumorigenesis is mutated FeLV transcription regulatory sequences, most frequently with enhancer duplication or triplication. Although only a limited amount of information is available in the area of insertional mutagenesis in FeLV neoplastic disease, activation of certain key nuclear transcription factor genes has been documented.

The envelope glycoprotein of an amphotropic murine retrovirus binds specifically to the cellular receptor/phosphate transporter of susceptible species

A rat cDNA (rRam-1), which was cloned on the basis that it enables Chinese hamster ovary (CHO) cells to be infected by amphotropic host range murine retroviruses, was recently found to encode a widely expressed Na(+)-phosphate symporter (M. P. Kavanaugh, D. G. Miller, W. Zhang, W. Law, S. L. Kozak, D. Kabat, and A. D. Miller, Proc. Natl. Acad. Sci. USA 91:7071-7075, 1994). CHO cells express the hamster homolog of Ram-1 but are resistant to amphotropic retroviruses. Although the amphotropic envelope glycoprotein gp70 bound weakly onto control CHO cells, CHO/rRam-1 cells had novel high-affinity binding sites, and the resulting strongly adsorbed gp70 was only slowly removed from cell surfaces, with a half-life of greater than 6 h. CHO/rRam-1 cells were also specifically and efficiently killed by exposure to amphotropic gp70 followed by antiserum to gp70 in the presence of complement. Infection with an appropriately pseudotyped form of amphotropic retrovirus 4070A did not perturb control CHO cells or inhibit their phosphate transport. In contrast, 4070A infection of CHO/rRam-1 cells caused major alterations including cell-cell fusions, a specific 40% down-modulation of the rRam-1 component of phosphate transport, and complete interference to super-infection by amphotropic viruses. The 4070A virus-infected CHO/rRam-1 cells retained a substantial cell surface pool of rRam-1 that functioned as a phosphate transporter but not as a viral receptor. We conclude that amphotropic gp70 binds more strongly to rRam-1 than to the homologous hamster protein and that this stable attachment is necessary for infection, interference, membrane fusion, and pathogenesis.

Chimeras of receptors for gibbon ape leukemia virus/feline leukemia virus B and amphotropic murine leukemia virus reveal different modes of receptor recognition by retrovirus

Glvr1 encodes the human receptor for gibbon ape leukemia virus (GALV) and feline leukemia virus subgroup B (FeLV-B), while the related gene Glvr2 encodes the human receptor for amphotropic murine leukemia viruses (A-MLVs). The two proteins are 62% identical in their amino acid sequences and are predicted to have 10 transmembrane domains and five extracellular loops. A stretch of nine amino acids (region A) in the predicted fourth extracellular loop was previously shown to be critical for the function of Glvr1 as receptor for GALV and FeLV-B. Glvr1 and -2 show clusters of amino acid differences in several of their predicted extracellular loops, with the highest degree of divergence in region A. Chimeras were made between the two genes to further investigate the role of Glvr1 region A in defining receptor specificity for GALV and FeLV-B and to map which regions of Glvr2 control receptor specificity for A-MLVs. Region A from Glvr1 was sufficient to confer receptor specificity for GALV upon Glvr2, with the same chimera failing to act as a receptor for FeLV-B. However, introduction of additional N- or C-terminal Glvr1-encoding sequences in addition to Glvr1 region A-encoding sequences resulted in functional FeLV-B receptors. Therefore, FeLV-B is dependent on Glvr1 sequences outside region A for infectivity. The receptor specificity of Glvr2 for A-MLV could not be mapped to a single critical region; rather, N-terminal as well as C-terminal Glvr2-encoding sequences could confer specificity for A-MLV infection upon Glvr1. Surprisingly, though GALV/FeLV-B and A-MLV belong to different interference groups, some chimeras functioned as receptors for all three viruses.

Virus receptors: implications for pathogenesis and the design of antiviral agents

A virus initiates infection by attaching to its specific receptor on the surface of a susceptible host cell. This prepares the way for the virus to enter the cell. Consequently, the expression of the receptor on specific cells and tissues of the host is a major determinant of the route of entry of the virus into the host and of the patterns of virus spread and pathogenesis in the host. This review emphasizes the virus-receptor interactions of human immunodeficiency virus, the rhinoviruses, the herpesviruses, and the coronaviruses. These interactions are often found to be complex and dynamic, involving multiple sites or factors on both the virus and the host cell. Also, the receptor may play an important role in virus entry per se in addition to its role in virus binding. In the cases of human immunodeficiency virus and the rhinoviruses, ingenious approaches to therapeutic strategies based on inhibiting virus attachment and entry are under development and in clinical trials.

Haematological disorders associated with feline retrovirus infections

Feline oncornavirus and lentivirus infections have provided useful models to characterize the virus and host cell factors involved in a variety of marrow suppressive disorders and haematological malignancies. Exciting recent progress has been made in the characterization of the viral genotypic features involved in FeLV-associated diseases. Molecular studies have clearly defined the causal role of variant FeLV env gene determinants in two disorders: the T-lymphocyte cytopathicity and the clinical acute immunosuppression induced by the FeLV-FAIDS variant and the pure red cell aplasia induced by FeLV-C/Sarma. Variant or enFeLV env sequences also appear to play a role in FeLV-associated lymphomas. Additional studies are required to determine the host cell processes that are perturbed by these variant env gene products. In the case of the FeLV-FAIDS variant, the aberrant env gene products appear to impair superinfection interference, resulting in accumulation of unintegrated viral DNA and cell death. In other cases it is likely that the viral env proteins interact with host products that are important in cell viability and/or proliferation. Understanding of these mechanisms will therefore provide insights to factors involved in normal lymphohaematopoiesis. Similarly, studies of FeLV-induced haematological neoplasms should reveal recombination or rearrangement events involving as yet unidentified host gene sequences that encode products involved in normal cell growth regulation. These sequences may include novel protoncogenes or sequences homologous to genes implicated in human haematological malignancies. The haematological consequences of FIV are quite similar to those associated with HIV. As with HIV, FIV does not appear to directly infect myeloid or erythroid precursors, and the mechanisms of marrow suppression likely involve virus, viral antigen, and/or infected accessory cells in the marrow microenvironment. Studies using in vitro experimental models are required to define the effects of each of these microenvironmental elements on haematopoietic progenitors. As little is known about the molecular mechanisms of FIV pathogenesis, additional studies of disease-inducing FIV strains are needed to identify the genotypic features that correlate with virulent phenotypic features. Finally, experimental FIV infection in cats provides the opportunity to correlate in vivo virological and haematological changes with in vitro observations in a large animal model that closely mimics HIV infection in man.

Properties of a unique form of the murine amphotropic leukemia virus receptor expressed on hamster cells

Identification and cloning of the receptors for amphotropic murine leukemia virus (A-MuLV) and gibbon ape leukemia virus (GaLV) have both enabled the determination of the normal function of these virus receptors in cells and initiated experimental examination of how these receptors interact with their respective viruses. GaLV and A-MuLV have distinct host ranges and use different receptors to infect human cells. It was therefore surprising to find that the human GaLV and A-MuLV receptors were not only structurally similar but performed similar cellular functions (B. O'Hara, S. V. Johann, H. P. Klinger, D. G. Blair, H. Rubinson, K. J. Dunn, P. Sass, S. M. Vitek, and T. Robbins, Cell Growth Differ. 1:119-127, 1990; M. van Zeijl, S. V. Johann, E. Closs, J. Cunningham, R. Eddy, T. B. Shows, and B. O'Hara, Proc. Natl. Acad. Sci. USA 91:1168-1172, 1994; M. P. Kavanaugh, D. G. Miller, W. Zhang, W. Law, S. L. Kozak, D. Kabat, and A. D. Miller, Proc. Natl. Acad. Sci. USA 91:7071-7075, 1994; and Z. Olah, C. Lehel, W. B. Anderson, M. V. Eiden, and C. A. Wilson, J. Biol. Chem., in press). We have now determined that the murine retrovirus 10A1 can use both the human GaLV receptor and the human A-MuLV receptor to infect cells. Furthermore, we have cloned and functionally characterized a unique form of the amphotropic receptor homolog expressed in E36 hamster cells. This receptor (EAR) can serve as both a GaLV receptor and an A-MuLV receptor, and it therefore differs from the receptors expressed in human cells, which function exclusively as either GaLV or A-MuLV receptors.

Type C retrovirus inactivation by human complement is determined by both the viral genome and the producer cell

The inactivation of type C retroviruses by human serum may be a considerable impediment to the use of retroviral vectors in vivo for gene therapy. Here we show that virus inactivation is dependent both on the virus and on the cell line used to produce the virus. All viruses produced from murine NIH 3T3 or dog Cf2ThS+L- cells are sensitive to human serum. In contrast, those produced from mink Mv-1-Lu and human HOS or TE671 cells are at least partially resistant, with the exception of murine leukemia viruses. In particular, the feline endogenous virus RD114 is completely resistant to a panel of eight human sera when produced from Mv-1-Lu or HOS cells. This differential resistance is controlled by the viral envelope proteins. Virus inactivation can be correlated with the ability of the producer cells to be lysed by human serum. Inactivation of sensitive viruses requires the classical pathway of complement but does not require virion lysis.

A family of retroviruses that utilize related phosphate transporters for cell entry

The amphotropic murine retrovirus receptor Ram-1 shows significant sequence similarity to the gibbon ape leukemia virus (GALV) receptor Glvr-1, and both of these cell surface virus receptors normally function as sodium-dependent phosphate symporters. However, Ram-1 from humans or rats does not serve as a receptor for GALV, and Glvr-1 from humans does not serve as a receptor for amphotropic virus. Here we show that the murine retrovirus 10A1 can enter cells by using either Glvr-1 or Ram-1. Furthermore, we have constructed Ram-1/Glvr-1 hybrid receptors that allow entry of both GALV and amphotropic virus. While GALV and amphotropic virus are in separate interference groups when assayed on human cells, they do interfere with each other in cells expressing the hybrid receptor. These results indicate a close functional relationship between retroviruses that utilize members of this newly defined receptor family and provide a molecular explanation for nonreciprocal and cell type-specific interference observed for some retrovirus classes.

Early interaction of feline calicivirus with cells in culture

The kinetics and biochemical properties of feline calicivirus (FCV) attachment to Crandell-Reese feline kidney cells were determined. Maximum binding was observed at pH 6.5. Cells in suspension at 4 degrees C bound virus more efficiently than cells in monolayers at 4 degrees C or 37 degrees C. High initial binding rate was observed in monolayers or cells in suspension and proceeded to a maximum at 90 min, although half maximal binding was observed as early as 15 min. Binding was specific and competitively blocked by serotypically homologous or heterologous FCV as well as by San Miguel sea lion virus. Treatment of cells with proteases increased FCV binding, whereas phospholipase had no effect on virus attachment. Conversely, cells treated with neuraminidase followed by O-glycanase treatment showed a decreased binding ability. Cells of feline origin bound FCV very efficiently, and non-permissive cells showed a poor binding ability. Following transfection of viral RNA, infectious virus could be recovered from all non-permissive cells, except from Madin-Darby canine kidney cells. These results suggest that FCV binds to a receptor in which carbohydrates may be an important component and that FCV replication in non-permissive cells is primarily restricted by the absence of appropriate receptors on the cell surface.

Recombination between feline exogenous and endogenous retroviral sequences generates tropism for cerebral endothelial cells

Brain tissues of domestic cats that died of aplastic anemia from infection with either parental feline leukemia virus (FeLV), subgroup C, or a mixture of FeLV-C and recombinants between FeLV-C and an endogenous FeLV provirus were examined by the immunoperoxidase staining technique using a monoclonal antibody (C11D8) directed against an epitope of the viral surface glycoprotein (SU). Positive staining of the central nervous system (CNS) capillary endothelial cells with no labeling on neuronal or glial cells was observed in cats that were inoculated with the virus mixture. This was in contrast to brain tissue of cats infected with FeLV-C alone, which showed no such staining. While non-CNS endothelial cells derived from human umbilical vein (HUVEC) could be readily infected in culture by FeLV-C, endothelial cells derived from human retina (REC) or brain (BEC) were resistant to infection by this parental virus. These latter cells in culture, however, could be infected by the viral mixture. The data suggested that at least one or more of the presumptive recombinant viruses could specifically infect CNS-derived endothelial cells. Using polymerase chain reaction and DNA sequencing strategies to amplify and analyze DNA fragments of the proviral SU region from cells infected with REC-selected viruses, we found the occurrence of a single recombinant in which two-thirds of the SU gene from the N-terminus of FeLV-C was replaced by the endogenous FeLV element. This recombinant virus, when molecularly cloned, should be useful in determining its potential in vivo neuropathogenicity.

A human amphotropic retrovirus receptor is a second member of the gibbon ape leukemia virus receptor family

Retrovirus infection is initiated by binding of the viral envelope glycoprotein to a cell-surface receptor. The envelope proteins of type C retroviruses of mammals demonstrate similarities in structural organization and protein sequence. These similarities suggest the possibility that retroviruses from different interference groups might use related proteins as receptors, despite the absence of any relationship between retrovirus receptors isolated to date. To investigate this possibility, we have identified a human cDNA clone encoding a protein closely related to the receptor for gibbon ape leukemia virus and have found that it functions as the receptor for the amphotropic group of murine retroviruses. Expression of this protein (GLVR-2) is likely to be a requirement for infection of human cells by amphotropic retroviral vectors for purposes of gene therapy.

Cloning of the cellular receptor for amphotropic murine retroviruses reveals homology to that for gibbon ape leukemia virus

The host and tissue specificity of retrovirus infection is largely determined by specific cellular receptors that mediate virus entry. Genes encoding these receptors are widely distributed in the genome, and the receptors identified to date show no sequence similarity. We have identified the cellular receptor for amphotropic murine retroviruses, Ram-1, by screening a rat cDNA expression library introduced into amphotropic virus-resistant hamster cells. The 656-amino acid receptor is homologous to the gibbon ape leukemia virus receptor at both hydrophobic termini but is highly divergent in the central hydrophilic region. Both receptors appear to be integral membrane proteins having multiple membrane-spanning regions. Identification of this family of receptors will help define the evolutionary relationship between retroviruses and their cellular receptors.

Human immunodeficiency virus type 1-associated CD4 downmodulation

This chapter discusses human immunodeficiency virus type 1 (HIV-1) associated with CD4 downmodulation. It also discusses the structure and function of CD4 and p56^lck and factors involved in hiv-1-associated cd4 downmodulation. There are, at present, at least three HIV-1 gene products known to be involved in cell surface CD4 downmodulation. These are Nef, Vpu, and gp160. Whereas Nef is expressed during the early phase of HIV-1 gene expression, both Vpu and gp160, which appear to act coordinately, are expressed during the late phase. This functional convergence of HIV-1 proteins on cell surface CD4 downmodulation, whether specific or nonspecific in activity, suggests that this event is of critical importance in the life cycle of HIV-1. Further elucidation of the mechanisms that underlie CD4 cell surface downmodulation may lead to the development of novel strategies aimed at preventing such events, and potentially to the development of new therapeutic approaches.

Induction of interferon by virus glycoprotein(s) in lymphoid cells through interaction with the cellular receptors via lectin-like action: an alternative interferon induction mechanism

When animals and cells are infected with a virus, interferon is produced. Viral-nucleic acid is considered to be one of actual components for interferon induction. In addition, viral glycoproteins trigger interferon induction in lymphoid cells by membrane-membrane interaction via a lectin-like activity. A biological significance of lectin-like activity of viral glycoproteins is discussed.

Mutation of amino acids within the gibbon ape leukemia virus (GALV) receptor differentially affects feline leukemia virus subgroup B, simian sarcoma-associated virus, and GALV infections

The three type C retroviruses, gibbon ape leukemia virus (GALV), simian sarcoma-associated virus (SSAV), and feline leukemia virus subgroup B (FeLV-B), infect human cells by interacting with the same cell surface receptor, GLVR1. Using LacZ retroviral pseudotypes and murine cells transfected with mutant GLVR1 expression vectors, we show that the same 9-amino-acid region of human GLVR1 is critical for infection by the three viruses. Rat cells were not susceptible to infection by LacZ (FeLV-B) pseudotypes because of a block at the receptor level. We found multiple amino acid differences from human GLVR1 in the 9-amino-acid critical region of rat GLVR1. Expression of a human-rat chimeric GLVR1 in murine cells demonstrated that rat GLVR1 could function as a receptor for GALV and SSAV but not for FeLV-B. Substitution of human GLVR1 amino acids in the critical region of rat GLVR1 identified three amino acids as responsible for resistance to FeLV-B infection; two of these affect SSAV infection, but none affects GALV infection.

Definition of a domain of GLVR1 which is necessary for infection by gibbon ape leukemia virus and which is highly polymorphic between species

Expression of human GLVR1 in mouse cells confers susceptibility to infection by gibbon ape leukemia virus (GALV), while the normally expressed mouse Glvr-1 does not. Since human and murine GLVR1 proteins differ at 64 positions in their sequences, some of the residues differing between the two proteins are critical for infection. To identify these, a series of hybrids and in vitro-constructed mutants were tested for the ability to confer susceptibility to infection. The results indicated that human GLVR1 residues 550 to 551, located in a cluster of seven of the sites that differ between the human and mouse proteins, are the only residues differing between the two which must be in the human protein form to allow infection. Sequencing of a portion of GLVR1 from the rat (which is infectible) confirmed the importance of this cluster in that it contained the only notable differences between the rat and mouse proteins. This region, which also differs substantially between the rat and the human proteins, therefore exhibits a pronounced tendency for polymorphism.

Isolation and characterization of a 2.3-kilobase-pair cDNA fragment encoding the binding domain of the bovine leukemia virus cell receptor

An immunoscreening strategy was used to isolate a cDNA clone encoding the binding domain for the external glycoprotein gp51 of the bovine leukemia virus (BLV). Three recombinant phages demonstrating BLV binding activity and containing 2.3-kbp cDNA inserts with identical nucleotide sequences were isolated from a lambda gt11 cDNA library of bovine kidney cells (MDBK). One clone, BLVRcp1, hybridized with a 4.8-kb mRNA from cells of bovine origin and was also found to be conserved as a single-copy gene in murine, bovine, ovine, primate, canine, feline, and porcine DNAs. The same gene is amplified in caprine DNA isolated from a BLV-induced tumor. The longest open reading frame of BLVRcp1 encodes a protein fragment of 729 amino acids with a putative receptor structure. BLVRcp1 cDNA was cloned in the eucaryotic expression vector pXT-1 and transfected into murine NIH 3T3 and human HEp-2 cells. Cells expressing BLVRcp1 mRNA became susceptible to BLV infection. BLVRcp1 has no known physiological function and has no significant homology with sequences registered in the GenBank and EMBL data libraries (31 July 1992). Expression of deleted constructs of BLVRcp1 indicates that the BLV binding region is encoded at the 5' side of the receptor clone.

Characterization of lymphocytic choriomeningitis virus-binding protein(s): a candidate cellular receptor for the virus

The attachment of lymphocytic choriomeningitis virus (LCMV) to murine and primate cell lines was quantitated by a fluorescence-activated cell sorter assay in which binding of biotinylated virus was detected with streptavidin-fluorescein isothiocyanate. Cell lines that were readily infected by LCMV (e.g., MC57, Rin, BHK, Vero, and HeLa) bound virus in a dose-dependent manner, whereas no significant binding was observed to lymphocytic cell lines (e.g., RMA and WIL 2) that were not readily infected. Binding was specific and competitively blocked by nonbiotinylated LCMV. It was also blocked by LCMV-specific antiserum and a neutralizing monoclonal antibody to the virus glycoprotein GP-1 but not by antibodies specific for GP-2, indicating that attachment was likely mediated by GP-1. Treatment of cells with any of several proteases abolished LCMV binding, whereas phospholipases including phosphatidylinositol-specific phospholipase C had no effect, indicating that one or more membrane proteins were involved in virus attachment. These proteins were characterized with a virus overlay protein blot assay. Virus bound to protein(s) with a molecular mass of 120 to 140 kDa in membranes from cell lines permissive for LCMV but not from nonpermissive cell lines. Binding was specific, since unlabeled LCMV, but not the unrelated enveloped virus herpes simplex virus type 1, competed with 125I-labeled LCMV for binding to the 120- to 140-kDa band. The proteinaceous nature of the LCMV-binding substance was confirmed by the lack of virus binding to proteinase K-treated membrane components. By contrast, glycosidase treatment of membranes did not abolish virus binding. However, in membranes treated with endoglycosidase F/N-glycosidase F, and/or neuraminidase and in membranes from cells grown in tunicamycin, the molecular mass of the LCMV-binding entity was reduced. Hence, LCMV attachment to rodent fibroblastic cell lines is mediated by a glycoprotein(s) with a molecular mass of 120 to 140 kDa, with complex N-linked sugars that are not involved in virus binding.

Conserved structural features in the interaction between retroviral surface and transmembrane glycoproteins?

Among the retroviruses, the surface (SU) and transmembrane (TM) glycoproteins of lentiviruses are linked exclusively by noncovalent bonds. For some C-type retroviruses, however, a small proportion of the SU proteins has been shown to be linked to their TM proteins by a disulfide bond, with the remainder being noncovalently associated. A region near the carboxyl terminus of the HIV-1 SU glycoprotein has been implicated in contacting the TM glycoprotein. Computer modelling indicates that this region of divergent lentivirus and oncovirus SU glycoproteins forms a structurally conserved "pocket" which could accommodate a "knob"-like protrusion formed by an immunodominant region in the TM protein containing the CxxxxxC (lentiviruses) or CxxxxxxCC (C- and D-type viruses) motif. An anti-idiotypic monoclonal antibody, raised against a monoclonal antibody reacting with a sequence in the "pocket" of HIV-1 gp120, was found to bind to synthetic peptides close to the CxxxxxC motif. It is suggested that part of the SU-TM linkage mechanism for the lentiviruses and oncoviruses is a 'knob and socket' structure and that the interaction between SU and TM proteins is similar in one region for lentiviruses and C-type as well as D-type viruses. The conserved knob and socket linkage may be relevant to a mechanism for viral-cell membrane fusion that is broadly common to all of these retroviruses.

Hematopoietic target cells of anemogenic subgroup C versus nonanemogenic subgroup A feline leukemia virus

Feline leukemia viruses (FeLVs) belonging to interference subgroup C induce fatal anemia resembling human pure red cell aplasia (PRCA). Subgroup A FeLVs, although closely related genetically to FeLVs of subgroup C, do not induce PRCA. The determinants for PRCA induction by a molecularly cloned prototype subgroup C virus (FeLV-Sarma-C [FSC]) have been localized to the N-terminal 241 amino acids of the surface glycoprotein (SU) gp70. To investigate whether the anemogenic activity of FSC reflects a unique capacity to infect erythroid progenitor cells, we used correlative immunogold, immunofluorescence, and cytological staining to study prospectively the hemopoietic cell populations infected by either FSC or FeLV-FAIDS-61E-A (F6A), a prototype of subgroup A virus. The results demonstrated that although only FSC-infected animals developed erythrocyte aplasia, the env SU and the major core protein (p27) were expressed in a surprisingly large fraction of the lymphoid, erythroid, and myeloid lineage marrow cells in both FSC- and F6A-infected cats. Between days 8 and 17 postinoculation, gp70 and p27 were detected in 43 to 73% of erythroid, 25 to 75% of lymphoid, and 35 to 50% of myeloid lineage cells, regardless of whether the cats were infected with FSC or F6A. Thus, anemogenic subgroup C and nonanemogenic subgroup A FeLVs have similar hemopoietic cell tropism and infection kinetics, despite their divergent effects on erythroid progenitor cell function. Acute anemia induction by subgroup C FeLV, therefore, does not reflect a unique tropism for marrow erythroid cells but rather indicates a unique cytopathic effect of the SU on erythroid progenitor cells.

Identification of a generalised packaging sequence for D-type retroviruses and generation of a D-type retroviral vector

In order to construct vectors based upon D-type, rather than C-type, retroviruses, we have identified a 624-bp fragment of Mason-Pfizer monkey virus (MPMV) which constitutes a packaging sequence for at least two D-type retroviruses. When this fragment was included in an extensively deleted D-type vector genome, the D-type viruses MPMV and SRV-5, but not the C-type viruses MLV-A or MLV-E, rescued the vector RNA from HeLa cells. The recombinant virus stocks have the host range of the rescuing D-type virus as shown by expression of an internal (SV40-puromycin) cassette replacing the retroviral structural genes. The recombinant MPMV was specifically neutralized by anti-MPMV serum and receptor interference was demonstrated when it was plated on cells productively infected with wild type MPMV. When the putative D-type packaging sequence was removed from the vector genome, even though the other sequence elements required for efficient reverse transcription remained, the vector was no longer rescued from HeLa cells. These results complement the recent demonstration of broad specificity of rescue of a C-type vector (carrying only the packaging sequence of Mo-MLV) by several different C-type, but not D-type, viruses. Replacement of the D-type packaging sequence by most of the extended packaging sequence of Mo-MLV prevented the otherwise D-type vector from being rescued by D-type viruses and did not allow it to be rescued by C-type viruses. This was probably because of the incompatibility of the D-type vector sequences with the C-type retroviral proteins involved in viral reverse transcription and integration. Hence, we have localized a packaging sequence that is recognized by D-type, but not by C-type, retroviruses and have constructed a D-type vector which may be useful in gene transfer experiments.