Polypeptides of feline leukemia virus: a glycosylated gag-related protein is released into culture fluids

Exploring FeLV-Gag-Based VLPs as a New Vaccine Platform-Analysis of Production and Immunogenicity

Feline leukemia virus (FeLV) is one of the most prevalent infectious diseases in domestic cats. Although different commercial vaccines are available, none of them provides full protection. Thus, efforts to design a more efficient vaccine are needed. Our group has successfully engineered HIV-1 Gag-based VLPs that induce a potent and functional immune response against the HIV-1 transmembrane protein gp41. Here, we propose to use this concept to generate FeLV-Gag-based VLPs as a novel vaccine strategy against this retrovirus. By analogy to our HIV-1 platform, a fragment of the FeLV transmembrane p15E protein was exposed on FeLV-Gag-based VLPs. After optimization of Gag sequences, the immunogenicity of the selected candidates was evaluated in C57BL/6 and BALB/c mice, showing strong cellular and humoral responses to Gag but failing to generate anti-p15E antibodies. Altogether, this study not only tests the versatility of the enveloped VLP-based vaccine platform but also sheds light on FeLV vaccine research.

Murine Leukemia Virus Glycosylated Gag Reduces Murine SERINC5 Protein Expression at Steady-State Levels via the Endosome/Lysosome Pathway to Counteract SERINC5 Antiretroviral Activity

Glycosylated Gag (glycoGag) is an accessory protein expressed by most gammaretroviruses, including murine leukemia virus (MLV). MLV glycoGag not only enhances MLV replication and disease progression but also increases human immunodeficiency virus type 1 (HIV-1) infectivity as Nef does. Recently, SERINC5 (Ser5) was identified as the target for Nef, and the glycoGag Nef-like activity has been attributed to the Ser5 antagonism. Here, we investigated how glycoGag antagonizes Ser5 using MLV glycoMA and murine Ser5 proteins. We confirm previous observations that glycoMA relocalizes Ser5 from plasma membrane to perinuclear punctated compartments and the important role of its Y₃₆XXL₃₉ motif in this process. We find that glycoMA decreases Ser5 expression at steady-state levels and identify two other glycoGag crucial residues, P31 and R63, for the Ser5 downregulation. The glycoMA and Ser5 interaction is detected in live cells using a bimolecular fluorescence complementation assay. Ser5 is internalized via receptor-mediated endocytosis and relocalized to Rab5⁺ early, Rab7⁺ late, and Rab11⁺ recycling endosomes by glycoMA. Although glycoMA is not polyubiquitinated, the Ser5 downregulation requires Ser5 polyubiquitination via the K48- and K63-linkage, resulting in Ser5 destruction in lysosomes. Although P31, Y36, L39, and R63 are not required for glycoMA interaction with Ser5, they are required for Ser5 relocalization to lysosomes for destruction. In addition, although murine Ser1, Ser2, and Ser3 exhibit very poor antiviral activity, they are also targeted by glycoMA for lysosomal destruction. We conclude that glycoGag has a broad activity to downregulate SERINC proteins via the cellular endosome/lysosome pathway, which promotes viral replication.IMPORTANCE MLV glycoGag not only enhances MLV replication but also increases HIV-1 infectivity similarly as Nef. Recent studies have discovered that both glycoGag and Nef antagonize a novel host restriction factor Ser5 and promote viral replication. Compared to Nef, the glycoGag antagonism of Ser5 is still poorly understood. MLV glycoGag is a transmembrane version of the structural Gag protein with an extra 88-amino-acid leader region that determines its activity. We now show that glycoGag interacts with Ser5 in live cells and internalizes Ser5 via receptor-mediated endocytosis. Ser5 is polyubiquitinated and relocalized to endosomes and lysosomes for massive destruction. In addition to the previously identified tyrosine-based sorting signal, we find two more important residues for Ser5 relocalization and downregulation. We also find that the Ser5 sensitivity to glycoGag is conserved in the SERINC family. Together, our findings highlight the important role of endosome/lysosome pathway in the enhancement of viral replication by viral proteins.

Barriers to Infection of Human Cells by Feline Leukemia Virus: Insights into Resistance to Zoonosis

The human genome displays a rich fossil record of past gammaretrovirus infections, yet no current epidemic is evident, despite environmental exposure to viruses that infect human cells in vitro. Feline leukemia viruses (FeLVs) rank high on this list, but neither domestic nor workplace exposure has been associated with detectable serological responses. Nonspecific inactivation of gammaretroviruses by serum factors appears insufficient to explain these observations. To investigate further, we explored the susceptibilities of primary and established human cell lines to FeLV-B, the most likely zoonotic variant. Fully permissive infection was common in cancer-derived cell lines but was also a feature of nontransformed keratinocytes and lung fibroblasts. Cells of hematopoietic origin were generally less permissive and formed discrete groups on the basis of high or low intracellular protein expression and virion release. Potent repression was observed in primary human blood mononuclear cells and a subset of leukemia cell lines. However, the early steps of reverse transcription and integration appear to be unimpaired in nonpermissive cells. FeLV-B was subject to G→A hypermutation with a predominant APOBEC3G signature in partially permissive cells but was not mutated in permissive cells or in nonpermissive cells that block secondary viral spread. Distinct cellular barriers that protect primary human blood cells are likely to be important in protection against zoonotic infection with FeLV.

IMPORTANCE Domestic exposure to gammaretroviruses such as feline leukemia viruses (FeLVs) occurs worldwide, but the basis of human resistance to infection remains incompletely understood. The potential threat is evident from the human genome sequence, which reveals many past epidemics of gammaretrovirus infection, and from recent cross-species jumps of gammaretroviruses from rodents to primates and marsupials. This study examined resistance to infection at the cellular level with the most prevalent human cell-tropic FeLV variant, FeLV-B. We found that blood cells are uniquely resistant to infection with FeLV-B due to the activity of cellular enzymes that mutate the viral genome. A second block, which appears to suppress viral gene expression after the viral genome has integrated into the host cell genome, was identified. Since cells derived from other normal human cell types are fully supportive of FeLV replication, innate resistance of blood cells could be critical in protecting against cross-species infection.

AKT capture by feline leukemia virus

Oncogene-containing retroviruses are generated by recombination events between viral and cellular sequences, a phenomenon called "oncogene capture". The captured cellular genes, referred to as "v-onc" genes, then acquire new oncogenic properties. We report a novel feline leukemia virus (FeLV), designated "FeLV-AKT", that has captured feline c-AKT1 in feline lymphoma. FeLV-AKT contains a gag-AKT fusion gene that encodes the myristoylated Gag matrix protein and the kinase domain of feline c-AKT1, but not its pleckstrin homology domain. Therefore, it differs structurally from the v-Akt gene of murine retrovirus AKT8. AKT may be involved in the mechanisms underlying malignant diseases in cats.

A Naturally Occurring Domestic Cat APOBEC3 Variant Confers Resistance to Feline Immunodeficiency Virus Infection

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3; A3) DNA cytosine deaminases can be incorporated into progeny virions and inhibit lentiviral replication. On the other hand, viral infectivity factor (Vif) of lentiviruses antagonizes A3-mediated antiviral activities by degrading A3 proteins. It is known that domestic cat (Felis catus) APOBEC3Z3 (A3Z3), the ortholog of human APOBEC3H, potently suppresses the infectivity of vif-defective feline immunodeficiency virus (FIV). Although a recent report has shown that domestic cat encodes 7 haplotypes (hap I to hap VII) of A3Z3, the relevance of A3Z3 polymorphism in domestic cats with FIV Vif has not yet been addressed. In this study, we demonstrated that these feline A3Z3 variants suppress vif-defective FIV infectivity. We also revealed that codon 65 of feline A3Z3 is a positively selected site and that A3Z3 hap V is subject to positive selection during evolution. It is particularly noteworthy that feline A3Z3 hap V is resistant to FIV Vif-mediated degradation and still inhibits vif-proficient viral infection. Moreover, the side chain size, but not the hydrophobicity, of the amino acid at position 65 determines the resistance to FIV Vif-mediated degradation. Furthermore, phylogenetic analyses have led to the inference that feline A3Z3 hap V emerged approximately 60,000 years ago. Taken together, these findings suggest that feline A3Z3 hap V may have been selected for escape from an ancestral FIV. This is the first evidence for an evolutionary “arms race” between the domestic cat and its cognate lentivirus.

IMPORTANCE Gene diversity and selective pressure are intriguing topics in the field of evolutionary biology. A direct interaction between a cellular protein and a viral protein can precipitate an evolutionary arms race between host and virus. One example is primate APOBEC3G, which potently restricts the replication of primate lentiviruses (e.g., human immunodeficiency virus type 1 [HIV-1] and simian immunodeficiency virus [SIV]) if its activity is not counteracted by the viral Vif protein. Here we investigate the ability of 7 naturally occurring variants of feline APOBEC3, APOBEC3Z3 (A3Z3), to inhibit FIV replication. Interestingly, one feline A3Z3 variant is dominant, restrictive, and naturally resistant to FIV Vif-mediated degradation. Phylogenetic analyses revealed that the ancestral change that generated this variant could have been caused by positive Darwinian selection, presumably due to an ancestral FIV infection. The experimental-phylogenetic investigation sheds light on the evolutionary history of the domestic cat, which was likely influenced by lentiviral infection.

MLV glycosylated-Gag is an infectivity factor that rescues Nef-deficient HIV-1

Optimal infectivity of HIV-1 virions requires synthesis of the HIV-1 regulatory protein Nef in some producer cells but not others. A survey of 18 lymphoid cell lines found that Nef was dispensable in three, each of which harbored gammaretroviruses. Nef-dependent cell lines were rendered Nef-independent by a cell-free supernatant from the independent lines or by transfection of cloned murine leukemia virus (MLV). Analysis of MLV deletion mutations identified glycosylated gag (glycogag) as the factor that rescues Nef-defective HIV-1 virions. Glycogag was also demonstrated to be required for the infectivity of MLV virions produced in lymphoid cells. Direct comparison of Nef and glycogag revealed identical dependence for activity on Env-pseudotype and producer cell type. The two proteins colocalize within cells, and both increase the yield of viral cDNA in target cells. The functional similarity of Nef and glycogag is a compelling example of convergent evolution in which two structurally unrelated proteins provide a function necessary for virion infectivity in lymphoid cells.

N-terminal cleavage fragment of glycosylated Gag is incorporated into murine oncornavirus particles

Glycosylated Gag (Glycogag) is a transmembrane protein encoded by murine and feline oncornaviruses. While the protein is dispensible for virus replication, Glycogag-null mutants of a neurovirulent murine oncornavirus are slow to spread in vivo and exhibit a loss of pathogenicity. The function of this protein in the virus life cycle, however, is not understood. Glycogag is expressed at the plasma membrane of infected cells but has not been detected in virions. In the present study we have reexamined this issue and have found an N-terminal cleavage fragment of Glycogag which was pelleted by high-speed centrifugation and sedimented in sucrose density gradients at the same bouyant density as virus particles. Its association with virions was confirmed by velocity sedimentation through iodixanol, which effectively separated membrane microvesicles from virus particles. Furthermore, the apparent molecular weight of the virion-associated protein was different from that of the protein extracted from the plasma membrane, suggesting some level of specificity or selectivity of incorporation.

The neuroinvasiveness of a murine retrovirus is influenced by a dileucine-containing sequence in the cytoplasmic tail of glycosylated Gag

The tempo and intensity of retroviral neuropathogenesis are dependent on the capacity of the virus to invade the central nervous system. For murine leukemia viruses, an important determinant of neuroinvasiveness is the virus-encoded protein glycosylated Gag, the function of which in the virus life cycle is not known. While this protein is dispensable for virus replication, mutations which prevent its expression slow the spread of virus in vivo and restrict virus dissemination to the brain. To further explore the function of this protein, we compared two viruses, CasFrKP (KP) and CasFrKP41 (KP41), which differ dramatically in neurovirulence. KP expresses high early viremia titers, is neuroinvasive, and induces clinical neurologic disease in 100% of neonatally inoculated mice, with an incubation period of 18 to 23 days. In contrast, KP41 expresses early viremia titers 100- fold lower than those of KP, exhibits attenuated neuroinvasiveness, and induces clinical neurologic disease infrequently, with a relatively long incubation period. The genomes of these two viruses differ by only 10 nucleotides, resulting in differences at five residues, all located within the N-terminal cytoplasmic tail of glycosylated Gag. In this study, using KP as the parental virus, we systematically mutated each of the five amino acid residues to those of KP41 and found that substitution mutation of two membrane-proximal residues, E53 and L56, to K and P, respectively produced the greatest effect on early viremia kinetics and neurovirulence. These mutations disrupted the KP sequence E53FLL56, the leucine dipeptide of which suggests the possibility that it may represent a sorting signal for glycosylated Gag. Supporting this idea was the finding that alteration of this sequence motif increased the level of cell surface expression of the protein, which suggests that analysis of the intracellular trafficking of glycosylated Gag may provide further clues to its function.

Characterization of glycosylated Gag expressed by a neurovirulent murine leukemia virus: identification of differences in processing in vitro and in vivo

The neuroinvasiveness of a chimeric murine retrovirus, CasFrKP (KP), is dependent on the expression of glycosylated Gag (gp85gag). This viral protein is the product of alternate translation initiation 88 codons upstream of and in frame with the initiation codon of pr65gag, the precursor of the viral core proteins. Although expression of glycosylated Gag affects virus spread in the spleen, it appears not to affect virus spread in vitro in fibroblast cell lines (J. L. Portis et al., J. Virol. 68:3879-3887, 1994). The differential effects of this protein in vitro and in vivo have not been explained, and its function is unknown. We have here compared the in vitro processing of this molecule with that expressed in spleens of infected mice. In vitro, gp85gag was cleaved near the middle of the molecule, releasing the C-terminal half (containing capsid and nucleocapsid domains of pr65gag) as a secreted glycoprotein. The N-terminal half of the protein was associated with the plasma membrane as a approximately 55-kDa glycoprotein bearing the matrix domain of pr65gag as well as the N-terminal 88 residue L domain. This processing scheme was also observed in vivo, although two differences were seen. There were differences in N-linked glycosylation of the secreted form of the protein expressed in the spleen. In addition, whereas the membrane-associated species assumed the orientation of a type II integral membrane protein (N(cyto) C(exo)) in fibroblasts in vitro, a subpopulation of spleen cells was detected in which the N terminus of the protein was exposed at the cell surface. These results suggest that the differential effects of glycosylated Gag expression in vivo and in vitro may be related to differences in posttranslational processing of the protein.

Identification of a sequence in the unique 5' open reading frame of the gene encoding glycosylated Gag which influences the incubation period of neurodegenerative disease induced by a murine retrovirus

Neonatal inoculation of the wild-mouse ecotropic retrovirus CasBrE (clone 15-1) causes a noninflammatory spongiform neurodegenerative disease with an incubation period of > or = 6 months. Introduction of sequences from Friend murine leukemia virus (clone FB29) into the genome of CasBrE results in a marked shortening of the incubation period. The FB29 sequences which influence the incubation period were previously localized to the 5' leader sequence of the viral genome (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). In the current study, we constructed a series of chimeric viruses consisting of the genome of CasBrE containing various segments of the leader sequence from FB29. A 41-nucleotide element (positions 481 through 521) near the 3' end of the leader was found to have a strong influence on the incubation period. This element influenced the kinetics of virus replication and/or spread in nonneuronal tissues, a property which was shown previously to determine the extent of central nervous system infection (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). Curiously, this sequence had no demonstrable effect on virus replication in vitro in a fibroblastic cell line from Mus dunni. This segment encodes 14 of the unique 88-amino-acid N terminus of pr75gag, the precursor of a glycosylated form of the gag polyprotein which is expressed at the cell surface. Previous in vitro studies of mutants of Moloney murine leukemia virus lacking expression of glycosylated Gag failed to reveal a function for this protein in virus replication. We mutated the Kozak consensus sequence around the initiation codon for this protein in the chimeric virus CasFrKP, a virus which induces neurologic disease with a short (18- to 23-day) incubation period. M. dunni cells infected with the mutants lacked detectable cell surface Gag, but, compared with CasFrKP, no effect on replication kinetics in vitro was observed. In contrast, there was a marked slowing of the replication kinetics in vivo and a dramatic attenuation of neurovirulence. These studies indicate that glycosylated Gag has an important function in virus replication and/or spread in the mouse and further suggest that the sequence of its N terminus is a critical, though likely indirect, determinant of neurovirulence.

A nonstructural gag-encoded glycoprotein precursor is necessary for efficient spreading and pathogenesis of murine leukemia viruses

In addition to the Gag-Pol and Env precursors whose translation initiates at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag), glycoGag translation is initiated at CUG codons located upstream of the Gag AUG initiation codon. In contrast to Gag, glycoGag is translocated into the endoplasmic reticulum and is absent from virions. Since glycoGag has been described to be dispensable ex vivo, we investigated the in vivo effects of a glycoGag- mutation in the Friend murine leukemia virus (F-MuLV). F-MuLV induces severe early hemolytic anemia and subsequent erythroleukemia within 2 months after inoculation of newborn mice. We obtained a glycoGag- F-MuLV, strain H5, by inserting an octanucleotide linker downstream of the CUG codon leading to the reading of a stop codon in all reading frames upstream of the Gag AUG. F-MuLV H5 did not induce severe early hemolytic anemia, and latency of erythroleukemia was significantly increased most likely because of an approximately 1-week delay in the in vivo spreading. Accordingly, induction of recombinant polytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-MuLV H5. Western blot (immunoblot) performed after inoculation of newborn mice with this glycoGag- virus indicated the emergence of new glycoGag+ viruses. PCR analyses with F-MuLV-specific primers demonstrated in vivo pseudoreversions restoring the glycoGag reading frame. Our results demonstrated that glycoGag expression is positively selected and essential for full spreading and pathogenic abilities.

Strong sequence conservation among horizontally transmissible, minimally pathogenic feline leukemia viruses

We report the first complete nucleotide sequence (8,440 base pairs) of a biologically active feline leukemia virus (FeLV), designated FeLV-61E (or F6A), and the molecular cloning, biological activity, and env-long terminal repeat (LTR) sequence of another FeLV isolate, FeLV-3281 (or F3A). F6A corresponds to the non-disease-specific common-form component of the immunodeficiency disease-inducing strain of FeLV, FeLV-FAIDS, and was isolated from tissue DNA of a cat following experimental transmission of naturally occurring feline acquired immunodeficiency syndrome. F3A clones were derived from a subgroup-A-virus-producing feline tumor cell line. Both are unusual relative to other molecularly cloned FeLVs studied to date in their ability to induce viremia in weanling (8-week-old) cats and in their failure to induce acute disease. The F6A provirus is organized into 5'-LTR-gag-pol-env-LTR-3' regions; the gag and pol open reading frames are separated by an amber codon, and env is in a different reading frame. The deduced extracellular glycoproteins of F6A, F3A, and the Glasgow-1 subgroup A isolate of FeLV (M. Stewart, M. Warnock, A. Wheeler, N. Wilkie, J. Mullins, D. Onions, and J. Neil, J. Virol. 58:825-834, 1986) are 98% homologous, despite having been isolated from naturally infected cats 6 to 13 years apart and from widely different geographic locations. As a group, their envelope gene sequences differ markedly from those of the disease-associated subgroup B and acutely pathogenic subgroup C viruses. Thus, F6A and F3A correspond to members of a highly conserved family and represent prototypes of the horizontally transmitted, minimally pathogenic FeLV present in all naturally occurring infections.

Feline leukemia virus-and feline sarcoma virus-related polypeptides released by virus producer and nonproducer cells

Polypeptides specific for feline leukemia virus (FeLV) have been identified in the media of cells that produce FeLV as well as in nonproducer cells transformed by feline sarcoma viruses (FeSV). Cat fibroblasts that were persistently infected with FELV release the major virus envelope glycoprotein, whereas cultured cat lymphoma cells shed both glycopeptides related to the virus core gene (gag) and glycopeptides related to the virus envelope gene (env). Mink cells and cat cells transformed by FeSV secrete polypeptides of a wide range of sizes that cross-react with the major virus core protein p27. Differences in the classes of p27-related proteins produced may be related to the strain of virus and the cell type. Cat cells transformed by FeSV release a glycopeptide that appears to be processed differently from those identified in the media of FeSV-transformed mink cells. The possibility that such FeLV-related secretory proteins may interfere with the immune response of the host is discussed.

Nucleotide sequence of the gag gene and gag-pol junction of feline leukemia virus

The nucleotide sequence of the gag gene of feline leukemia virus and its flanking sequences were determined and compared with the corresponding sequences of two strains of feline sarcoma virus and with that of the Moloney strain of murine leukemia virus. A high degree of nucleotide sequence homology between the feline leukemia virus and murine leukemia virus gag genes was observed, suggesting that retroviruses of domestic cats and laboratory mice have a common, proximal evolutionary progenitor. The predicted structure of the complete feline leukemia virus gag gene precursor suggests that the translation of nonglycosylated and glycosylated gag gene polypeptides is initiated at two different AUG codons. These initiator codons fall in the same reading frame and are separated by a 222-base-pair segment which encodes an amino terminal signal peptide. The nucleotide sequence predicts the order of amino acids in each of the individual gag-coded proteins (p15, p12, p30, p10), all of which derive from the gag gene precursor. Stable stem-and-loop secondary structures are proposed for two regions of viral RNA. The first falls within sequences at the 5' end of the viral genome, together with adjacent palindromic sequences which may play a role in dimer linkage of RNA subunits. The second includes coding sequences at the gag-pol junction and is proposed to be involved in translation of the pol gene product. Sequence analysis of the latter region shows that the gag and pol genes are translated in different reading frames. Classical consensus splice donor and acceptor sequences could not be localized to regions which would permit synthesis of the expected gag-pol precursor protein. Alternatively, we suggest that the pol gene product (RNA-dependent DNA polymerase) could be translated by a frameshift suppressing mechanism which could involve cleavage modification of stems and loops in a manner similar to that observed in tRNA processing.