The host range and interference properties of two closely related feline leukemia variants suggest that they use distinct receptors

What's New in Feline Leukemia Virus Infection

Feline leukemia virus (FeLV) is a retrovirus with global impact on the health of domestic cats that causes tumors (mainly lymphoma), bone marrow disorders, and immunosuppression. The importance of FeLV is underestimated due to complacency associated with previous decline in prevalence. However, with this comes lowered vigilance, which, along with potential for regressively infected cats to reactivate viremia and shed the virus or develop clinical signs, can pose a risk to feline health. This article summarizes knowledge on FeLV pathogenesis, courses of infection, and factors affecting prevalance, infection outcome, and development of FeLV-associated diseases, with special focus on regressive FeLV infection.

Disruption of thiamine uptake and growth of cells by feline leukemia virus subgroup A

Feline leukemia virus (FeLV) is still a major cause of morbidity and mortality in domestic cats and some wild cats despite the availability of relatively effective vaccines against the virus. FeLV subgroup A (FeLV-A) is transmitted in natural infections, and FeLV subgroups B, C, and T can evolve directly from FeLV-A by mutation and/or recombination with endogenous retroviruses in domestic cats, resulting in a variety of pathogenic outcomes. The cell surface entry receptor for FeLV-A is a putative thiamine transporter (THTR1). Here, we have addressed whether FeLV-A infection might disrupt thiamine uptake into cells and, because thiamine is an essential nutrient, whether this disruption might have pathological consequences. First, we cloned the cat ortholog of the other of the two known thiamine transporters in mammals, THTR2, and we show that feline THTR1 (feTHTR1) and feTHTR2 both mediate thiamine uptake, but feTHTR2 does not function as a receptor for FeLV-A. We found that feTHTR1 is widely expressed in cat tissues and in cell lines, while expression of feTHTR2 is restricted. Thiamine uptake mediated by feTHTR1 was indeed blocked by FeLV-A infection, and in feline fibroblasts that naturally express feTHTR1 and not feTHTR2, this blockade resulted in a growth arrest at physiological concentrations of extracellular thiamine. The growth arrest was reversed at high extracellular concentrations of thiamine. Our results show that FeLV-A infection can indeed disrupt thiamine uptake with pathological consequences. A prediction of these experiments is that raising the plasma levels of thiamine in FeLV-infected cats may ameliorate the pathogenic effects of infection.

Identification of a feline leukemia virus variant that can use THTR1, FLVCR1, and FLVCR2 for infection

The pathogenic subgroup C feline leukemia virus (FeLV-C) arises in infected cats as a result of mutations in the envelope (Env) of the subgroup A FeLV (FeLV-A). To better understand emergence of FeLV-C and potential FeLV intermediates that may arise, we characterized FeLV Env sequences from the primary FY981 FeLV isolate previously derived from an anemic cat. Here, we report the characterization of the novel FY981 FeLV Env that is highly related to FeLV-A Env but whose variable region A (VRA) receptor recognition sequence partially resembles the VRA sequence from the prototypical FeLV-C/Sarma Env. Pseudotype viruses bearing FY981 Env were capable of infecting feline, human, and guinea pig cells, suggestive of a subgroup C phenotype, but also infected porcine ST-IOWA cells that are normally resistant to FeLV-C and to FeLV-A. Analysis of the host receptor used by FY981 suggests that FY981 can use both the FeLV-C receptor FLVCR1 and the feline FeLV-A receptor THTR1 for infection. However, our results suggest that FY981 infection of ST-IOWA cells is not mediated by the porcine homologue of FLVCR1 and THTR1 but by an alternative receptor, which we have now identified as the FLVCR1-related protein FLVCR2. Together, our results suggest that FY981 FeLV uses FLVCR1, FLVCR2, and THTR1 as receptors. Our findings suggest the possibility that pathogenic FeLV-C arises in FeLV-infected cats through intermediates that are multitropic in their receptor use.

A putative thiamine transport protein is a receptor for feline leukemia virus subgroup A

Feline leukemia virus (FeLV) is a horizontally transmitted virus that causes a variety of proliferative and immunosuppressive diseases in cats. There are four subgroups of FeLV, A, B, C, and T, each of which has a distinct receptor requirement. The receptors for all but the FeLV-A subgroup have been defined previously. Here, we report the identification of the cellular receptor for FeLV-A, which is the most transmissible form of FeLV. The receptor cDNA was isolated using a gene transfer approach, which involved introducing sequences from a feline cell line permissive to FeLV-A into a murine cell line that was not permissive. The feline cDNA identified by this method was approximately 3.5 kb, and included an open reading frame predicted to encode a protein of 490 amino acids. This feline cDNA conferred susceptibility to FeLV-A when reintroduced into nonpermissive cells, but it did not render these cells permissive to any other FeLV subgroup. Moreover, these cells specifically bound FeLV-A-pseudotyped virus particles, indicating that the cDNA encodes a binding receptor for FeLV-A. The feline cDNA shares approximately 93% amino acid sequence identity with the human thiamine transport protein 1 (THTR1). The human THTR1 receptor was also functional as a receptor for FeLV-A, albeit with reduced efficiency compared to the feline orthologue. On the basis of these data, which strongly suggest the feline protein is the orthologue of human THTR1, we have named the feline receptor feTHTR1. Identification of this receptor will allow more detailed studies of the early events in FeLV transmission and may provide insights into FeLV pathogenesis.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

Reevaluation of host ranges of feline leukemia virus subgroups

We reevaluated the host ranges of feline leukemia virus (FeLV) subgroups A, B and C using pseudotype assays based on recombinant NB-tropic murine leukemia virus, which is not usually blocked after viral entry in mammalian cells. Pseudotype viruses of FeLV-B and -C infected a variety of cell lines from many mammalian species. Unexpectedly, FeLV-A pseudotype viruses of two independent isolates from the UK and US also infected a variety of non-feline cell lines including cells from humans, rabbits, pigs and minks. Moreover, both isolates of FeLV-A productively infected human embryonic kidney 293 and mink Mv-1-Lu cells. We conclude that FeLV-A is not strictly ecotropic.

Feline leukemia virus-associated myelopathy in cats

Feline leukemia virus (FeLV) infection is associated with distinct neoplastic, hematologic, and immunosuppressive diseases. Here we report on a novel neurologic syndrome in 16 cats infected with FeLV for more than 2 years. Clinical signs consisted of abnormal vocalization, hyperesthesia, and paresis progressing to paralysis. The clinical course of affected cats involved gradually progressive neurologic dysfunction invariably resulting in euthanasia. Microscopically, white-matter degeneration with dilation of myelin sheaths and swollen axons was identified in the spinal cord and brain stem of affected animals. Neither neoplastic nor hematologic diseases commonly associated with FeLV infection were present. Fungal and protozoal infection in one animal was suggestive of impaired immune competence. Immunohistochemical staining of affected tissues revealed consistent expression of FeLV p27 antigens in neurons, endothelial cells, and glial cells. Furthermore, proviral DNA was amplified from multiple sections of spinal cord as well as intestine, spleen, and lymph nodes. These findings suggest that in a proportion of chronically FeLV-infected cats, a virus evolved with cytopathic potential for cells in the central nervous system.

Host range and receptor binding properties of vectors bearing feline leukemia virus subgroup B envelopes can be modulated by envelope sequences outside of the receptor binding domain

To evaluate host range differences between two different strains of feline leukemia virus subgroup B (FeLV-B), we compared the binding and infectivity patterns of retrovirus vectors bearing either FeLV-B-90Z or FeLV-B-GA envelopes. We report here that the ability of these envelopes to utilize different Pit1 orthologs is mediated primarily by the receptor binding domain; however, in the case of FeLV-B-90Z, the C terminus also contributes to the recognition of certain Pit1 orthologs.

Genetic and biochemical analyses of receptor and cofactor determinants for T-cell-tropic feline leukemia virus infection

Entry by retroviruses is mediated through interactions between the viral envelope glycoprotein and the host cell receptor(s). We recently identified two host cell proteins, FeLIX and Pit1, that are necessary for infection by cytopathic, T-cell-tropic feline leukemia viruses (FeLV-T). Pit1 is a classic multiple transmembrane protein used as a receptor by several other simple retroviruses, including subgroup B FeLV (FeLV-B), and FeLIX is a secreted cellular protein expressed from endogenous FeLV-related sequences (enFeLV). FeLIX is nearly identical to FeLV-B envelope sequences that encode the N-terminal half of the viral surface unit (SU), because these FeLV-B sequences are acquired by recombination with enFeLV. FeLV-B SUs can functionally substitute for FeLIX in mediating FeLV-T infection. Both of these enFeLV-derived cofactors can efficiently facilitate FeLV-T infection only of cells expressing Pit1, not of cells expressing the related transport protein Pit2. We therefore have used chimeric Pit1/Pit2 receptors to map the determinants for cofactor binding and FeLV-T infection. Three distinct determinants appear to be required for cofactor-dependent infection by FeLV-T. We also found that Pit1 sequences within these same domains were required for binding by FeLIX to the Pit receptor. In contrast, these determinants were not all required for receptor binding by the FeLV-B SU cofactors used in this study. These data indicate that cofactor binding is not sufficient for FeLV-T infection and suggest that there may be a direct interaction between FeLV-T and the Pit1 receptor.

Altering retroviral tropism using a random-display envelope library

Tissue-specific gene delivery is an important aspect of many gene therapy applications. The experiments reported here constitute the first successful demonstration that cell-specific entry can be obtained by screening a random library of retroviral envelope proteins produced from a mammalian cell system. The library consisted of 10(6) different subgroup A feline leukemia virus envelope protein variants with 10 randomly substituted amino acids in the receptor-determining region. Selecting the library for fully functional envelope proteins able to mediate stable gene transfer resulted in the identification of a single envelope protein variant (EF). Subsequent examination of the host range of EF revealed that it was highly specific for D17 canine osteosarcoma cells. This was in contrast to the host ranges of the parental subgroup A and closely related subgroup C envelope proteins. Interference assays on D17 cells further indicated that receptor usage by EF was also altered compared with the A and C envelope proteins. The EF envelope protein thus isolated should be useful for studying gene therapy treatments of osteosarcoma in a large-animal model.

Feline Pit2 functions as a receptor for subgroup B feline leukemia viruses

Different subgroups of feline leukemia virus (FeLV) use different host cell receptors for entry. Subgroup A FeLV (FeLV-A) is the virus that is transmitted from cat to cat, suggesting that cells expressing the FeLV-A receptor are important targets at the earliest stages of infection. FeLV-B evolves from FeLV-A in the infected cat through acquisition of cellular sequences that are related to the FeLV envelope gene. FeLV-Bs have been shown to infect cells using the Pit1 receptor, and some variants can infect cells at a lower efficiency using Pit2. Because these observations were made using receptor proteins of human or rodent origin, the role that Pit1 and Pit2 may play in FeLV-B replication in the cat is unclear. In this study, the feline Pit receptors were cloned and tested for their ability to act as receptors for different FeLV-Bs. Some FeLV-Bs infected cells expressing feline Pit2 and feline Pit1 with equal high efficiency. Variable region A (VRA) in the putative receptor-binding domain (RBD) was a critical determinant for both feline Pit1 and feline Pit2 binding, although other domains in the RBD appear to influence how efficiently the FeLV-B surface unit can bind to feline Pit2 and promote entry via this receptor. An arginine residue at position 73 in VRA was found to be important for envelope binding to feline Pit2 but not feline Pit1. Interestingly, this arginine is not found in endogenous FeLV sequences or in recombinant viruses recovered from feline cells infected with FeLV-A. Thus, while FeLV-Bs that are able to use feline Pit2 can evolve by recombination with endogenous sequences, a subsequent point mutation during reverse transcription may be needed to generate a virus that can efficiently enter the cells using the feline Pit2 as its receptor. These studies suggest that cells expressing the feline Pit2 protein are likely to be targets for FeLV-B infection in the cat.

Specificity in receptor usage by T-cell-tropic feline leukemia viruses: implications for the in vivo tropism of immunodeficiency-inducing variants

Cytopathic, T-cell-tropic feline leukemia viruses (FeLV-T) evolve from FeLV-A in infected animals and demonstrate host cell specificities that are distinct from those of their parent viruses. We recently identified two cellular proteins, FeLIX and Pit1, required for productive infection by these immunodeficiency-inducing FeLV-T variants (M. M. Anderson, A. S. Lauring, C. C. Burns, and J. Overbaugh, Science 287:1828-1830, 2000). FeLV-T is the first example of a naturally occurring type C retrovirus that requires two proteins to gain entry into target cells. FeLIX is an endogenous protein that is highly related to the N-terminal portion of the FeLV envelope protein, which includes the receptor-binding domain. Pit1 is a multiple-transmembrane phosphate transport protein that also functions as a receptor for FeLV-B. The FeLV-B envelope gene is derived by recombination with endogenous FeLV-like sequences, and its product can functionally substitute for FeLIX in facilitating entry through the Pit1 receptor. In the present study, we tested other retrovirus envelope surface units (SUs) with their cognate receptors to determine whether they also could mediate infection by FeLV-T. Cells were engineered to coexpress the transmembrane form of the envelope proteins and their cognate receptors, or SU protein was added as a soluble protein to cells expressing the receptor. Of the FeLV, murine leukemia virus, and gibbon ape leukemia virus envelopes tested, we found that only those with receptor-binding domains derived from endogenous FeLV could render cells permissive for FeLV-T. We also found that there is a strong preference for Pit1 as the transmembrane receptor. Specifically, FeLV-B SUs could efficiently mediate infection of cells expressing the Pit1 receptor but could only inefficiently mediate infection of cells expressing the Pit2 receptor, even though these SUs are able to bind to Pit2. Expression analysis of feline Pit1 and FeLIX suggests that FeLIX is likely the primary determinant of FeLV-T tropism. These results are discussed in terms of current models for retrovirus entry and the interrelationship among FeLV variants that evolve in vivo.

Receptors and entry cofactors for retroviruses include single and multiple transmembrane-spanning proteins as well as newly described glycophosphatidylinositol-anchored and secreted proteins

In the past few years, many retrovirus receptors, coreceptors, and cofactors have been identified. These molecules are important for some aspects of viral entry, although in some cases it remains to be determined whether they are required for binding or postbinding stages in entry, such as fusion. There are certain common features to the molecules that many retroviruses use to gain entry into the cell. For example, the receptors for most mammalian oncoretroviruses are multiple membrane-spanning transport proteins. However, avian retroviruses use single-pass membrane proteins, and a sheep retrovirus uses a glycosylphosphatidylinositol-anchored molecule as its receptor. For some retroviruses, particularly the lentiviruses, two cell surface molecules are required for efficient entry. More recently, a soluble protein that is required for viral entry has been identified for a feline oncoretrovirus. In this review, we will focus on the various strategies used by mammalian retroviruses to gain entry into the cell. The choice of receptors will also be discussed in light of pressures that drive viral evolution and persistence.

Identification of envelope determinants of feline leukemia virus subgroup B that permit infection and gene transfer to cells expressing human Pit1 or Pit2

The retroviral vector systems that are in common use for gene therapy are designed to infect cells expressing either of two widely expressed phosphate transporter proteins, Pit1 or Pit2. Subgroup B feline leukemia viruses (FeLV-Bs) use the gibbon ape leukemia virus receptor, Pit1, as a receptor for entry. Our previous studies showed that some chimeric envelope proteins encoding portions of FeLV-B could also enter cells by using a related receptor protein, Pit2, which serves as the amphotropic murine leukemia virus receptor (S. Boomer, M. Eiden, C. C. Burns, and J. Overbaugh, J. Virol. 71:8116--8123, 1997). Here we show that an arginine at position 73 within variable region A (VRA) of the FeLV-B envelope surface unit (SU) is necessary for viral entry into cells via the human Pit2 receptor. However, C-terminal SU sequences have a dominant effect in determining human Pit2 entry, even though this portion of the protein is outside known receptor binding domains. This suggests that a combination of specific VRA sequences and C-terminal sequences may influence interactions between FeLV-B SU and the human Pit2 receptor. Binding studies suggest that the C-terminal sequences may affect a postbinding step in viral entry via the Pit2 receptor, although in all cases, binding of FeLV-B SU to human Pit2 was weak. In contrast, neither the arginine 73 nor specific C-terminal sequences are required for efficient binding or infection with Pit1. Taken together, these data suggest that different residues in SU may interact with these two receptors. The specific FeLV-Bs described here, which can enter cells using either human Pit receptor, may be useful as envelope pseudotypes for viruses used in gene therapy.

Identification of a cellular cofactor required for infection by feline leukemia virus

Retroviral infection involves continued genetic variation, leading to phenotypic and immunological selection for more fit virus variants in the host. For retroviruses that cause immunodeficiency, pathogenesis is linked to the emergence of T cell-tropic, cytopathic viruses. Here we show that an immunodeficiency-inducing, T cell-tropic feline leukemia virus (FeLV) has evolved such that it cannot infect cells unless both a classic multiple membrane-spanning receptor molecule (Pit1) and a second coreceptor or entry factor are present. This second receptor component, which we call FeLIX, was identified as an endogenously expressed protein that is similar to a portion of the FeLV envelope protein. This cellular protein can function either as a transmembrane protein or as a soluble component to facilitate infection.