Skip to main content
NCBI home page
Journal of Virology logoLink to Journal of Virology
. 2006 May;80(10):4748–4757. doi: 10.1128/JVI.80.10.4748-4757.2006

In Vivo Interactions of Ecotropic and Polytropic Murine Leukemia Viruses in Mixed Retrovirus Infections

Leonard H Evans 1,*, Marc Lavignon 1,†, Karin Peterson 1,‡, Kim Hasenkrug 1, Shelly Robertson 1, Frank Malik 1, Kimmo Virtaneva 1
PMCID: PMC1472087  PMID: 16641268

Abstract

Mixed retrovirus infections are the rule rather than the exception in mice and other species, including humans. Interactions of retroviruses in mixed infections and their effects on disease induction are poorly understood. Upon infection of mice, ecotropic retroviruses recombine with endogenous proviruses to generate polytropic viruses that utilize different cellular receptors. Interactions among the retroviruses of this mixed infection facilitate disease induction. Using mice infected with defined mixtures of the ecotropic Friend murine leukemia virus (F-MuLV) and different polytropic viruses, we demonstrate several dramatic effects of mixed infections. Remarkably, inoculation of F-MuLV with polytropic MuLVs completely suppressed the generation of new recombinant viruses and dramatically altered disease induction. Coinoculation of F-MuLV with one polytropic virus significantly lengthened survival times, while inoculation with another polytropic MuLV induced a rapid and severe neurological disease. In both instances, the level of the polytropic MuLV was increased 100- to 1,000-fold, whereas the ecotropic MuLV level remained unchanged. Surprisingly, nearly all of the polytropic MuLV genomes were packaged within F-MuLV virions (pseudotyped) very soon after infection. At this time, only a fractional percentage of cells in the mouse were infected by either virus, indicating that the coinoculated viruses had infected the same small subpopulation of susceptible cells. The profound amplification of polytropic MuLVs in coinfected mice may be facilitated by pseudotyping or, alternatively, by transactivation of the polytropic virus in the coinfected cells. This study illustrates the complexity of the interactions between components of mixed retrovirus infections and the dramatic effects of these interactions on disease processes.

Naturally occurring retrovirus infections are often mixed infections of retroviruses with different properties (5, 6, 17). Mixed retrovirus infections can be generated by infection with a heterogeneous mixture of viruses, by genetic alterations, such as point mutations or recombination, which occur subsequent to infection, or by infection of a host that already harbors retroviruses. Infection of mice with murine leukemia viruses (MuLVs) presents an excellent model to study mixed retrovirus infections. However, relatively few studies have focused on the effects of mixed retrovirus infections in mice, particularly with regard to the interactions of replication-competent retroviruses on pathogenesis or spread of infection in the host (7, 11, 13, 14, 16). Retroviral pseudotyping, which is the packaging of the RNA genome of one retrovirus within a virion of a distinct retrovirus, and retroviral interference, in which an infected cell is rendered refractory to infection by a second retrovirus utilizing the same receptor, are well documented (27, 37, 41, 50). Although both of these phenomena are consequences of in vivo mixed viral interactions, they have rarely been studied in a quantitative fashion.

Infection of mice by exogenous MuLVs such as Friend MuLV (F-MuLV) frequently results in recombination with endogenous env genes to yield variant retroviruses (19, 24, 26) that utilize a different cellular receptor and exhibit a host range distinct from the exogenous MuLV (3, 4, 19, 24, 26, 37, 46, 52). F-MuLV is an ecotropic MuLV infectious for mouse or rat cells but not cells of other species. Retroviruses generated by recombination between an exogenous ecotropic virus and endogenous retroviruses of mice, are termed polytropic MuLVs. These retroviruses, like ecotropic MuLVs, infect mouse and rat cells, but they also infect cells of several other species (35, 37) and are instrumental in the development of retrovirus-induced proliferative diseases (11, 15, 24, 47, 49).

The generation of recombinant polytropic MuLVs during infection of mice with an ecotropic virus results in a mixed retrovirus infection. Previous studies from this laboratory indicated that the interaction of polytropic and ecotropic MuLVs in a mixed infection may play a major role in a stepwise process of leukemogenesis in mice (29). There are multiple recombinant viruses generated in infected mice (2), and although they utilize the same cell-surface receptor for infection, different polytropic MuLVs may exhibit striking quantitative differences in their abilities to infect different cells. Thus, some polytropic MuLVs infect mink cells 3 to 4 orders of magnitude more efficiently than mouse cells, while others infect both cell types with nearly equal efficiently (19). Furthermore, some polytropic MuLVs infect different murine cells with different efficiencies (39). The presence of multiple viruses with different properties in a mixed retrovirus infection greatly complicates the analysis of their interactions.

In the present study, we found that coinoculation of polytropic MuLVs with F-MuLV 57 completely suppressed the appearance of new host-derived polytropic recombinant viruses. Thus, it was possible to examine interactions of the inoculated viruses in the mixed infections in the absence of other retroviruses. The identity of the coinoculated polytropic MuLV remarkably altered the types and tempos of diseases induced by mixtures of polytropic viruses and F-MuLV 57. These included a highly significant delay in the induction of proliferative disease with one polytropic MuLV and a profound synergistic effect resulting in the abrupt development of a neurological disease with another polytropic isolate. In both cases, these effects were accompanied by extensive pseudotyping of the polytropic MuLV genome within ecotropic virions and a striking elevation of polytropic MuLV infection and replication in coinoculated mice. We observed a high degree of pseudotyping of the polytropic MuLV genome within ecotropic virions even at very early times after infection when only small numbers of cells were releasing viruses. Since pseudotyping can occur only in coinfected cells, this observation suggests that very early after infection, both the polytropic and ecotropic viruses infect the same small subpopulation of susceptible cells in the host.

MATERIALS AND METHODS

Cells and viruses.

NIH 3T3 cells were used for the propagation of viruses and assays of MuLVs. The cells were maintained in Dulbecco's modified Eagle's medium supplemented with glutamine (2 mM), penicillin, streptomycin, and 10% heat-inactivated bovine calf serum at 37°C. The ecotropic MuLVs, F-MuLV 57 (34), M-MuLV1387 (21), and B3 (40) and the polytropic MuLVs Fr98 (36) and Fr54 (25) were obtained as virus stocks after transfection of NIH 3T3 cells with plasmids carrying the proviruses. Fr98 and Fr54 contain the 3′ pol and env genes of FMCF98 (36) and FMCF54 (28, 33) in the background of an F-MuLV.

Mice.

Mice utilized in this report were the NFS/N and the IRW strains, both of which are maintained as inbred colonies at Rocky Mountain Laboratories. Mice were infected by intraperitoneal injection within 24 h of birth. They were observed daily and sacrificed when serious clinical signs became apparent or sacrificed at various times after inoculation, and the sera and tissues were removed and prepared for assay. The mice were anesthetized and bled by severing the axillary vessels. The blood was allowed to clot and the serum separated. After the bleeding, the mice were perfused through the left ventricle with approximately 2 ml of phosphate-buffered saline per gram of body weight. Perfusion served to minimize contamination of the tissue samples with infectious cell-free virus present in the blood. Spleens were removed and dissociated by mincing the tissue, suspending the minced tissue in medium (Dulbecco's modified Eagle's medium containing 10% bovine calf serum), and passing through successively smaller syringe needles from 18 to 23 gauge. Brains were removed, weighed, and dissociated in serum-free medium containing 0.5 mg/ml of trypsin by mincing the tissue and being passed through 16- and 20-gauge syringe needles. Trypsin was included to inactivate cell-free viruses present in the tissue. All procedures were done in accordance with the guidelines of the National Institutes of Health and Rocky Mountain Laboratories animal care and use committees.

MAb and virus assays.

The ecotropic and polytropic viruses were quantified using antibodies which specifically recognize each of the MuLVs in a focal immunofluorescence assay (FIA) (42). Infectious cell-free virus was assayed in sera, and infectious centers (IC) were assayed in dissociated tissues as a measure of the extent of productive infection. Briefly, cell cultures were seeded and infected with dilutions of the samples and allowed to grow to confluence. The monolayers were incubated with a monoclonal antibody (MAb), rinsed, and subsequently incubated with a fluorescein isothiocyanate-conjugated goat anti-mouse immunoglobulin to detect bound MAb. Foci of infected cells were detected by fluorescence microscopy. MAb 48 is specific for all substrains of ecotropic F-MuLV (9), MAb 514 reacts specifically with all polytropic MuLVs (8), and MAb 516 (8) and Hy 7 (10) are reactive with different subsets of polytropic MuLVs (31).

Pseudotyping of Fr98 was assessed by comparisons of infectivity on uninfected cells to infectivity on cells infected with F-MuLV 57. Viral interference renders F-MuLV 57-infected cells completely refractory to reinfection by ecotropic virions, including Fr98 pseudotyped by F-MuLV 57. Both ecotropic and polytropic virions score on uninfected cells; thus, a decrease in titer of Fr98 on F-MuLV 57-infected cells compared to uninfected cells reflects the proportion of Fr98 that is pseudotyped.

Diagnostic criteria.

The proliferative disease induced by F-MuLV 57, Fr54, and Fr98 in this study was characterized by gross splenomegaly (<0.5 g), anemia (hematocrit < 30), and thymic atrophy. Neurological disease was evidenced by a rapid onset of tremors, ataxia, and loss of abductor reflex. Histological examination of brains was performed as described previously (38).

RESULTS

Replication and disease induction by polytropic viruses in NFS/N mice.

The investigation of the interactions of polytropic and ecotropic MuLVs in a mixed infection required the identification of polytropic MuLVs that would replicate efficiently. Initial testing of several polytropic MuLVs indicated that many polytropic viruses replicated poorly or not at all in NFS/N mice (data not shown). However, two polytropic MuLVs, Fr54 and Fr98 (25, 36), that replicated to levels that could be accurately quantified were identified. The level of replication of the polytropic MuLVs was assessed from determinations of viremia two weeks after neonatal inoculation of the mice and compared to mice inoculated with the ecotropic F-MuLV 57. Viremia was readily detectable in both polytropic isolates at two weeks of age. Fr54 titers were about 5 × 102 FFU/ml, Fr98 titers about 104 FFU/ml, and F-MuLV 57 titers about 107 FFU/ml.

Infected NFS/N mice were also followed for the appearance of disease (Fig. 1). Similar to other F-MuLV-derived polytropic MuLVs (1, 26, 48), Fr54 induced splenomegaly accompanied by severe anemia in some NFS/N mice. Surprisingly, Fr98, which induces a severe neurological disease in IRW mice (36), did not induce obvious signs of neurological disease in NFS/N mice. Rather, Fr98 induced splenomegaly and anemia similar to that induced by Fr54 but with a higher incidence and significantly shorter incubation period than Fr54. This may have reflected the higher level of replication observed with Fr98. The proliferative disease induced by the polytropic MuLVs closely resembled the disease induced by the ecotropic F-MuLV 57 and was characterized by gross hepatosplenomegaly accompanied by severe anemia and thymic atrophy (43).

FIG. 1.

FIG. 1.

Open in a new tab

Mortality of NFS/N mice inoculated with Fr54 or Fr98. Neonatal NFS/N mice were inoculated with 2 × 104 FFU of Fr54 (n = 71) or Fr98 (n = 59) and monitored for evidence of disease. Survival times were taken as the times of euthanasia upon evidence of severe disease.

Abrogation of de novo-generated polytropic MuLVs by coinoculation of exogenous polytropic MuLVs with F-MuLV.

Several studies suggest that in vivo retroviral infection may suppress infection by other retroviruses through a mechanism of viral interference in the host (13, 14, 16). Polytropic MuLVs are generated in mice infected with F-MuLV 57 but are not detected in mice until about 10 to 14 days after infection (dpi) (21, 20). At 14 days after infection, the replication of both Fr54 and Fr98 was evident; thus, it seemed possible that neonatal administration of the polytropic MuLVs with F-MuLV 57 as a mixed inoculum might hinder the appearance of new host-derived polytropic viruses through in vivo viral interference.

NFS/N neonates were inoculated with F-MuLV 57 alone, mixtures of Fr54 and F-MuLV 57, or mixtures of Fr98 and F-MuLV 57. In order to distinguish the inoculated polytropic MuLVs from those newly generated by recombination with F-MuLV 57, we utilized focal immunofluorescence assays employing monoclonal antibodies which facilitate the identification of various polytropic viruses in mixed infections (Table 1) (31, 42).

TABLE 1.

Reactivities of polytropic MuLVs to monoclonal antibodies

Virus(es) Reactivity with polytropic MuLV-specific monoclonal antibodya
MAb 514 MAb 516 Hy 7
F-MuLV 57-induced polytropic MuLVsb + − +
Fr54 + + −
Fr98 + − −
Open in a new tab
a

+, reactive with antibody; −, not reactive with antibody.

b

The predominant polytropic MuLVs induced by F-MuLV in most NFS/N mice are reactive with Hy 7 but not MAb 516.

F-MuLV 57 recombines predominantly with a group of endogenous retroviruses that are readily detected by reactivity to the monoclonal antibody Hy 7 (30). The polytropic MuLV Fr54 is reactive with MAb 516 but not Hy 7; thus, an inhibition of the generation of polytropic MuLVs in mice coinfected with a mixture of F-MuLV 57 and Fr54 would be evident by a decrease in the level of Hy 7-reactive polytropic MuLVs. Two weeks after infection, mice inoculated with F-MuLV 57 alone exhibited substantial levels of Hy 7-reactive polytropic MuLVs with lower levels of polytropic MuLVs reactive with MAb 516 (Fig. 2A). This observation is typical of most NFS/N mice inoculated with F-MuLV 57 and reflects the predominant recombination of F-MuLV 57 with a structural subgroup of endogenous polytropic proviruses termed the PT proviruses (45). Remarkably, mice inoculated with mixtures of F-MuLV 57 and Fr54 were completely devoid of Hy 7 polytropic MuLVs, indicating that coinfection of the ecotropic MuLV with an exogenous polytropic MuLV completely abrogated the appearance of Hy 7-reactive recombinants (Fig. 2B). The levels of polytropic MuLVs reactive with MAb 516 and MAb 514 were nearly identical in coinoculated mice and corresponded to detection of the inoculated polytropic MuLV Fr54.

FIG. 2.

FIG. 2.

Open in a new tab

The generation of new polytropic MuLVs in NFS/N mice inoculated with F-MuLV 57 and the suppression new polytropic MuLVs by coinoculation with Fr54 or Fr98. Neonatal NFS/N mice were inoculated with 2 × 104 FFU of F-MuLV 57 by itself or with 2 × 104 FFU of Fr54 or Fr98. The mice were sacrificed 14 days after infection and the sera collected and assayed. F-MuLV 57 was measured by the FIA using MAb 48 and polytropic MuLVs measured using MAb 516, Hy 7, and MAb 514. (A) F-MuLV 57 and polytropic MuLV levels in sera of mice inoculated with F-MuLV 57 alone. (B) F-MuLV 57 and polytropic MuLV levels in sera of mice inoculated with a mixture of F-MuLV 57 and Fr54. (C) F-MuLV 57 and polytropic MuLV levels in sera of mice inoculated with a mixture of F-MuLV 57 and Fr98.

Fr98 is unique among polytropic MuLVs in that it is not reactive with either MAb 516 or Hy 7 but is reactive with MAb 514 (Table 1). Thus, inhibition with a mixture of F-MuLV 57 and Fr98 would result in a decrease of both MAb 516- and Hy 7-reactive recombinants. Mice inoculated with mixtures of F-MuLV 57 and Fr98 were completely devoid of new polytropic MuLVs, including both Hy 7- and MAb 516-reactive recombinants (Fig. 2C). All endogenous polytropic proviruses in NFS/N mice encode an epitope for either Hy 7 or MAb 516 (22); thus, the polytropic MuLVs reactive with MAb 514 but not with either Hy 7 or MAb 516 correspond to the inoculated Fr98.

A surprising observation was the dramatically elevated serum levels of the polytropic MuLVs in coinoculated mice compared to mice inoculated with either of the polytropic MuLVs alone. Both Fr54 and Fr98 exhibited titers very near the levels of F-MuLV 57 in coinoculated mice (Fig. 2B and C). Coinoculation of F-MuLV 57 with Fr54 resulted in a greater than 104-fold increase in the titer of Fr54 (from 5 × 102 to approximately 107) while inoculation with Fr98 resulted in an increase of approximately 103-fold (from 104 to 107). In contrast, the level of F-MuLV 57 in coinfections was nearly the same as that in mice inoculated with F-MuLV 57 alone. Previous studies on the polytropic MuLV infection of mice harboring endogenous ecotropic MuLVs also reported an enhanced replication of polytropic MuLVs by ecotropic MuLVs, although the effect appeared less dramatic than that observed in the present studies (11, 12).

The induction of splenomegaly and anemia by F-MuLV 57 is delayed by coinoculation of Fr54.

Next, it was of obvious interest to determine what effects these alterations might have on the course of disease induced by the viruses. Mice coinoculated with F-MuLV 57 and Fr54 were followed closely for the induction of disease and compared to mice infected with F-MuLV 57 or with Fr54 alone (Fig. 3). Disease induction by the ecotropic F-MuLV 57 was significantly delayed (P < 0.0001) by coinoculation of Fr54, with a median increase in survival times of 31 days, 55% longer than that for mice inoculated with F-MuLV 57 alone. Approximately 10% of the coinoculated mice exhibited neurological disease prior to the earliest onset of proliferative disease (ca. 30 to 40 dpi) (Fig. 4) and were euthanized. Some additional mice developed transient ataxia during the course of infection that did not require euthanasia. Mice sacrificed at latter times after infection exhibited splenomegaly and anemia.

FIG. 3.

FIG. 3.

Open in a new tab

Delay in the induction of splenomegaly and anemia by F-MuLV 57 after coinoculation with Fr54. Neonatal NFS/N mice were inoculated with 2 × 104 FFU of F-MuLV 57 (n = 122) or with a mixture of 2 × 104 FFU of F-MuLV 57 and 2 × 104 FFU of Fr54 (n = 92) and monitored for evidence of disease. Survival times were taken as the times of euthanasia upon evidence of severe disease.

FIG. 4.

FIG. 4.

Open in a new tab

Early mortality due to neurological disease in NFS/N mice inoculated with F-MuLV 57 and Fr98. Neonatal NFS/N mice were inoculated with 2 × 104 FFU of F-MuLV 57 (n = 122), Fr98 (n = 59), or a mixture of 2 × 104 FFU of F-MuLV 57 and 2 × 104 FFU of Fr98 (n = 73) and monitored for evidence of disease. Survival times were taken as times of euthanasia upon evidence of severe disease.

Fr98 and F-MuLV 57 act synergistically in coinfected NFS/N mice to induce neurological disease.

NFS/N mice inoculated as neonates with Fr98 or F-MuLV 57 developed splenomegaly and anemia but showed no signs of neurological disease during the course of infection. However, mice inoculated with a mixture of Fr98 and F-MuLV 57 developed a neurological disease characterized by sudden and severe ataxia and tremors as early as 10 days after inoculation (Fig. 4). The onset of disease was somewhat variable among different litters, generally occurring between 10 and 20 days after inoculation and appeared similar or identical to the disease observed in IRW mice inoculated with Fr98 alone (36). As with IRW mice inoculated with Fr98, there were no obvious histological lesions in the brains of the coinoculated NFS/N mice (data not shown). Similar synergistic effects with Fr98 were observed with other ecotropic MuLVs, including Moloney MuLV, which exhibits a different tissue tropism from that of F-MuLV, and F-MuLV B3, which is an avirulent MuLV closely related to F-MuLV 57 (data not shown).

Fr98 is detected earlier and reaches much higher levels of infection in coinoculated mice.

The rapidly fatal neurological disease in NFS/N mice coinoculated with Fr98 and F-MuLV 57 prompted us to examine this mixed infection in greater detail. Viremia is a measure of virus released into the bloodstream and is dependent on both the extent of infection and the rate of virus production in the mouse tissue. The spleen is an early replicative site of MuLVs (23) and infectious center assays of this organ were taken as an indication of the extent of infection early after inoculation. In addition to examining the spleen and serum, we examined the spread of infection and replication of the viruses in the central nervous system throughout the course of infection.

NFS/N mice were inoculated with the Fr98 or F-MuLV 57 individually or with mixtures of Fr98 and F-MuLV 57. They were sacrificed at various times after infection, and assays were performed on the spleen, serum, and brain of each animal. Infection of the spleen was first detectable at 4 days after inoculation and increased to maximum levels at about 10 days in both coinoculated mice and mice infected with Fr98 or F-MuLV 57 alone (Fig. 5). At the earliest time of detection, the level of infection by Fr98 was not significantly different in coinoculated mice and mice inoculated with Fr98 alone (Fig. 5A). However, by six days and throughout the remainder of the observation period, Fr98 infection of the spleen was 2 to 3 orders of magnitude more extensive in coinoculated mice than in mice inoculated with only Fr98. No difference was observed in the rates or levels of infection of F-MuLV 57 between coinoculated mice and mice infected with only F-MuLV 57 (Fig. 5B). The viremia levels closely paralleled the extent of spleen infection in the mice with a striking elevation of Fr98 in coinoculated mice relative to mice infected with Fr98 alone starting at about 6 days (Fig. 6A). As was the case with spleen infection, no difference was observed in the serum levels of F-MuLV 57 between coinoculated mice and mice infected with only F-MuLV 57 (Fig. 6B).

FIG. 5.

FIG. 5.

Open in a new tab

Spleen infection of NFS/N mice after inoculation of Fr98 or F-MuLV 57 by themselves or as a retrovirus mixture. NFS/N mice were inoculated as neonates with 2 × 104 FFU of Fr98, 2 × 104 FFU of F-MuLV 57 or with a mixture of 2 × 104 FFU of each virus. At the indicated times, mice were sacrificed and their spleens removed and assayed as infectious centers for Fr98 (A) or for F-MuLV 57 (B). Each of the points on the graphs represents the average value ± the standard error of the mean (SEM) obtained from four to nine animals.

FIG. 6.

FIG. 6.

Open in a new tab

Viremia in NFS/N mice after inoculation of Fr98 and F-MuLV 57 by themselves or as a retrovirus mixture. NFS/N mice were inoculated as neonates with 2 × 104 FFU of Fr98, 2 × 104 FFU of F-MuLV 57, or a mixture of 2 × 104 FFU of each virus. At the indicated times, mice were sacrificed and their spleens removed and assayed as infectious centers for Fr98 (A) or for F-MuLV 57 (B). Each of the points on the graphs represents the average value ± SEM obtained from 4 to 9 animals.

Infection of the central nervous system by Fr98 occurred earlier and progressed to much higher levels in coinoculated mice than in mice inoculated with only Fr98. Infection of the brain was detected 2 to 3 days earlier in coinoculated mice and near the terminal stage of infection exceeded the level of infection of age-matched Fr98-infected mice by over 100-fold (Fig. 7A). No difference was observed in infection of the central nervous system (CNS) by F-MuLV 57 between coinoculated mice and mice infected with only F-MuLV 57 (Fig. 7B).

FIG. 7.

FIG. 7.

Open in a new tab

Infection of the CNS in NFS/N mice after inoculation of Fr98 or F-MuLV 57 by themselves or as a retrovirus mixture. NFS/N mice were inoculated as neonates with 2 × 104 FFU of Fr98, 2 × 104 FFU of F-MuLV 57 or with a mixture of 2 × 104 FFU of each virus. At the indicated times, mice were sacrificed and their brains removed and assayed as infectious centers for Fr98 (A) or for F-MuLV 57 (B). Each of the points on the graphs represents the average value ± SEM obtained from 4 to 9 animals.

The results above suggest that the neuropathology observed in NFS/N mice coinoculated with Fr98 and F-MuLV 57 was the result of increased replication of the polytropic MuLV. It seemed possible that the difference in pathology induced by Fr98 in IRW and NFS/N mice might have been due to a difference in the ability of Fr98 to replicate in these mouse strains. We determined the level of Fr98 CNS infection in NFS/N mice inoculated with Fr98 or coinoculated with Fr98 and F-MuLV 57 as well as in IRW mice inoculated with Fr98 near the onset of clinical signs in the animals (12 dpi). The levels of CNS infection in IRW mice infected with Fr98 alone were much higher than the levels observed in NFS/N mice. Indeed, the level of Fr98 in IRW mice was near the highly elevated levels observed in NFS/N mice coinfected with Fr98 and F-MuLV 57 (Fig. 8).

FIG. 8.

FIG. 8.

Open in a new tab

Fr98 infection of the CNS of IRW and NFS/N mice. IRW mice were inoculated as neonates with 2 × 104 FFU of Fr98 (black bar) and NFS/N mice were inoculated with 2 × 104 FFU of Fr98 (white bar) or with a mixture containing 2 × 104 FFU of Fr98 and 2 × 104 FFU of F-MuLV 57 (gray bar). Twelve days after inoculation, the mice were sacrificed; their brains were removed and assayed as infectious centers for Fr98. Each of the bars on the graphs represents the average value ± SEM obtained from 4 to 9 animals.

Fr98 is extensively pseudotyped within F-MuLV 57 virions throughout the course of infection in coinoculated mice.

Extensive pseudotyping of polytropic MuLVs generated in mice after infection by MuLVs has previously been observed (29, 41); however, it was unknown if a polytropic MuLV inoculated as an exogenous virus in a mixture with F-MuLV 57 would also be extensively pseudotyped. We found nearly complete pseudotyping of Fr98 within F-MuLV 57 virions in the spleens, sera, and CNS of coinfected mice, particularly at very early times after infection (Fig. 9). At about 9 to 11 days after infection, some mice did not exhibit extensive pseudotyping in the CNS, while others exhibited nearly complete pseudotyping during this period. No association was seen between the extent of pseudotyping and signs of disease.

FIG. 9.

FIG. 9.

Open in a new tab

Pseudotyping of the Fr98 genome within F-MuLV 57 virions in sera, spleens, and brains of NFS/N mice infected with a mixture of Fr98 and F-MuLV 57. NFS/N mice were inoculated as neonates with a mixture containing 2 × 104 FFU of Fr98 and 2 × 104 FFU of F-MuLV 57. At the times indicated, the mice were sacrificed and the extent of pseudotyping in the sera, spleens, and brains was assessed by comparisons of infectivity on NIH 3T3 cells compared to NIH 3T3 cells chronically infected with F-MuLV 57. Each point on the graph represents an average (±SEM) of values obtained from four to six mice.

Pseudotyping can only occur when the same cell is infected by both viruses. Fr98 RNA genomes released from the spleens of coinoculated mice were nearly completely pseudotyped at 4 days when less than 1% of the susceptible cells in that tissue were infected by either virus (Fig. 5 and 9). Considering the fact that the coinoculation was accomplished by intraperitoneal injection with a mixture of the two viruses, it seemed conceivable that compartmentalization of the infection within the peritoneum may have facilitated the coinfection of a small number of cells that subsequently migrated to the spleen. To examine this possibility, the viruses were inoculated as separate virus stocks at different sites and through different routes of infection. Pseudotyping of Fr98 was determined for viruses released from splenocytes and peripheral blood cells at 4 days after infection. Regardless of the method of infections, Fr98 was found to be extensively pseudotyped within F-MuLV 57 virions (data not shown).

DISCUSSION

Most retrovirus infections can be considered mixed retrovirus infections either due to infection by mixtures of viruses or to genetic alterations such as recombination or point mutations that occur subsequent to infection. Viruses in mixed infections may interact in a direct fashion, such as pseudotyping or interference, or in an indirect manner, such as the expansion of cell populations that serve as additional targets for infection by a companion virus. Also, they may interact by a trans-activation mechanism in which cells are rendered sensitive to receptor-independent infection by polytropic MuLVs upon infection with F-MuLV (51). In mice, the inoculation of exogenous ecotropic viruses frequently results in recombination with endogenous viruses to generate ecotropic/polytropic mixed retrovirus infections. Our previous studies of these mixed infections indicated that the polytropic recombinant viruses were present in the mice at much higher levels and earlier than had previously been reported (18, 29). Furthermore, our studies indicated that interactions of the ecotropic and polytropic MuLVs, including pseudotyping and viral interference, might facilitate the induction of leukemia (29). Infection of mice by ecotropic MuLVs results in the generation of multiple recombinant viruses which may exhibit distinct infectious properties (2, 29) and whose interactions in the host are likely to be quite complex. In this study, we have examined mixed infections that were initiated by coinoculation of an ecotropic MuLV with recombinant polytropic MuLVs. Under these conditions, the appearance of the large heterogeneous population of polytropic MuLVs normally observed after inoculation of an ecotropic virus was suppressed and allowed us to assess the effects of the mixed ecotropic/polytropic infection in the absence of other viruses.

Replication and disease induction by polytropic viruses in NFS/N mice.

Our initial experiments were aimed at identifying polytropic MuLVs that would replicate in NFS/N mice to evaluate their interaction in mixed infections with the ecotropic MuLV. We tested a number of polytropic MuLVs and found that few polytropic MuLVs replicated sufficiently above the level of detection to be useful for our studies. However, two polytropic MuLVs, Fr54 and Fr98, that replicated sufficiently well in the mice were identified and studied. In agreement with other studies of polytropic MuLVs derived by recombination with F-MuLVs (1, 26, 48), both Fr54 and Fr98 resulted in a proliferative disease characterized by gross splenomegaly and anemia when inoculated into neonatal mice. This was a somewhat unexpected result for Fr98 in that this virus had been reported to induce a neurological disorder rather than a proliferative disease in newborn IRW mice (36). The difference in neurovirulence of Fr98 in NFS/N and IRW mice correlated with the ability of the virus to establish a more vigorous infection in the IRW host. Comparisons of Fr98 replication between NFS/N and IRW mice inoculated with Fr98 alone revealed that the level of Fr98 in the CNS was much higher in IRW mice and near the level observed in NFS/N mice coinoculated with Fr98 and F-MuLV 57. Factors that influence the difference in the two mouse strains to support the replication of Fr98 are not known.

Abrogation of de novo-generated polytropic MuLVs by coinoculation of exogenous polytropic MuLVs with F-MuLV 57 and the delay of disease induction.

Previous studies have indicated some degree of in vivo retrovirus interference usually evident by the delay or elimination of a pathogenic phenomenon and/or a decrease in viral replication (13, 14, 16, 32). In this study, we observed the complete suppression of de novo polytropic MuLV recombinants with F-MuLV 57 by coinoculation of exogenous polytropic MuLVs. It seems likely that the suppression of new recombinant viruses was due to in vivo interference rather than an effect on the recombination process. In mice coinfected with F-MuLV and an exogenous polytropic MuLV, infection by the polytropic MuLV reaches high levels prior to the time de novo polytropic MuLVs are observed; thus, the spread of newly generated viruses would likely be impeded by in vivo interference.

The generation of polytropic MuLVs is instrumental in the induction of proliferative diseases by ecotropic MuLVs in mice (15, 24, 47, 49). Coinoculation of F-MuLV 57 with Fr54 resulted in the complete abrogation of new polytropic MuLVs generated by recombination with F-MuLV 57 which was accompanied by a substantial delay in the induction of disease. These results may indicate that the polytropic MuLVs generated by F-MuLV 57 are more virulent than Fr54, which by itself induces disease only after several months. It seems likely that the disease that was observed in coinoculated mice was a result of Fr54 infection that was markedly enhanced compared to mice inoculated with Fr54 alone. Thus, coinfection of F-MuLV 57 with a nonvirulent polytropic MuLV might completely protect the mouse from disease induction by the ecotropic MuLV.

Synergism between Fr98 and F-MuLV 57 to induce neurological disease.

The outcome of mice coinoculated with Fr98 and F-MuLV 57 was strikingly different than those inoculated with Fr54 and F-MuLV 57. NFS/N mice inoculated with Fr98 and F-MuLV 57 exhibited a rapidly fatal neurological disease similar to the disease observed in IRW mice inoculated with Fr98 alone (36). It is likely that this observation is the result of the remarkable elevation of Fr98 replication in coinoculated NFS/N mice. Indeed, our results indicated that Fr98 infection of the CNS in coinoculated mice was similar to IRW mice infected with Fr98. Furthermore, levels of Fr98 viremia in NFS/N mice coinoculated with Fr98 and F-MuLV 57 exceed the levels of viremia observed in IRW mice at 10 days of age after neonatal inoculation (data not shown). It is noteworthy that earlier studies of polytropic and ecotropic expression in mice also reported enhanced replication and/or shortened latency of disease induction (7, 11, 12).

Extensive pseudotyping of Fr98 within F-MuLV 57 virions.

The marked elevation of Fr98 in coinoculated mice could be the result of a direct interaction of the two inoculated viruses or an indirect effect such as the expansion of a cell type by F-MuLV that served as a target for Fr98 infection. We found that Fr98 levels were substantially elevated very early after detection in coinoculated mice. The elevation occurred prior to the widespread infection of the host and also very likely prior to a substantial proliferative effect mediated by the ecotropic MuLV. This observation prompted us to examine the possibility of virus interactions early after infection. One type of interaction in mixed infections is viral pseudotyping, in which the genome of one virus is encapsulated in virions of another. We found that Fr98 was nearly completely pseudotyped within ecotropic virions throughout the course of infection. A similar phenomenon is observed with recombinant polytropic MuLVs generated after infection with ecotropic MuLVs (29, 41).

These observations raise a number of interesting questions. It is unclear why the polytropic genomes are extensively pseudotyped within ecotropic virions, considering the fact that both viruses encode functional env proteins and that both proteins are present on the surface of coinfected cells. Furthermore it is not clear if or why pseudotyping of the polytropic genome within an ecotropic virion facilitates in vivo replication of the polytropic virus. Pseudotyping by the ecotropic MuLV may allow the infection of cells not normally permissive to infection by polytropic MuLVs. It is also possible that ecotropic virions are inherently more infectious than are polytropic virions. In this case, infection and spread of pseudotyped polytropic viruses would occur more rapidly. It must also be considered that pseudotyping, per se, does not result in enhanced infection and spread of the polytropic virus. In vivo amplification of polytropic replication may be the result of trans-activation of the polytropic virus by the ecotropic MuLV within coinfected cells; trans-activation could be independent of pseudotyping of polytropic MuLVs released from the cells.

We observed pseudotyping at times when only a fractional percentage of the susceptible cells were infected by either virus. Considering the fact that pseudotyping requires the coinfection of cells by both viruses, our results indicate that both viruses initially infected the same very small subpopulation of susceptible cells. This is a remarkable observation, in that ecotropic and polytropic viruses utilize different cell-surface receptors for infectious entry and that both viruses, when inoculated by themselves, spread to a large number and variety of cells in the host.

One explanation for early in vivo coinfection of cells by the ecotropic and polytropic MuLVs is that the initial infection by the polytropic MuLV occurs through a receptor-independent trans-activation mechanism mediated by infection of the ecotropic MuLV in coinoculated mice (51). If polytropic MuLV infection occurred largely by this mechanism, it could account for the coinfection of a small population of cells. This possibility seems somewhat unlikely, however, considering the fact that the coinoculated polytropic MuLVs in the present study eliminated the spread of new polytropic recombinants viruses, presumably by a receptor blockade mechanism. If a receptor-independent mechanism were the major route of initial infection, infection by newly generated polytropic MuLVs should not be blocked. An alternative explanation for coinfection of cells by the coinoculated viruses is the possibility that the majority of cells in the host are relatively resistant to infection by cell-free virions, but there exists a unique but very small population of cells that are quite permissive to cell-free infection by both types of MuLVs. Subsequent infectious spread of the viruses to other tissues would occur via cell-to-cell transmission in the animal. This may provide an explanation for the perplexing observation that in vivo retroviral interference is nearly completely established, even when only a small fraction of the susceptible cells are infected (14, 32, 44). Interference would only have to be established within the small susceptible population which would likely occur soon after the initial infection and spread of the inoculated virus. Such a small cell population may have potential as a therapeutic target for resistance to viral infection.

This study has focused on the interactions of retroviruses during in vivo mixed infections in a controlled experimental setting. We have demonstrated in vivo interference resulting in the suppression of new recombinant viruses, specific pseudotyping of one component of the virus mixture, and the profound amplification of that viral component. In addition, we have observed synergistic effects between the inoculated viruses which either delayed the onset of disease in one instance or accelerated and altered the type of disease in another. It is difficult to study such interactions in humans, but it seems likely that the interactions of retroviruses, such as with human immunodeficiency virus or human T-cell lymphotropic virus, in mixed infections may affect the course of infection and disease.

Acknowledgments

This work was supported by the intramural research program of the National Institute of Allergy and Infectious Diseases, National Institutes of Health.

We thank Bruce Chesebro and John Portis for helpful discussions during the course of these studies.

REFERENCES

  • 1.Adachi, A., K. Sakai, N. Kitamura, S. Nakanishi, O. Niwa, M. Matsuyama, and A. Ishimoto. 1984. Characterization of the env gene and long terminal repeat of molecularly cloned Friend mink cell focus-inducing virus DNA. J. Virol. 50:813-821. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Alamgir, A. S., N. Owens, M. Lavignon, F. Malik, and L. H. Evans. 2005. Precise identification of endogenous proviruses of NFS/N mice participating in recombination with Moloney ecotropic murine leukemia virus (MuLV) to generate polytropic MuLVs. J. Virol. 79:4664-4671. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Battini, J. L., J. M. Heard, and O. Danos. 1992. Receptor choice determinants in the envelope glycoproteins of amphotropic, xenotropic, and polytropic murine leukemia viruses. J. Virol. 66:1468-1475. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 4.Battini, J. L., J. E. Rasko, and A. D. Miller. 1999. A human cell-surface receptor for xenotropic and polytropic murine leukemia viruses: possible role in G protein-coupled signal transduction. Proc. Natl. Acad. Sci. USA 96:1385-1390. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Beilke, M. A., D. L. Greenspan, A. Impey, J. Thompson, and P. J. Didier. 1994. Laboratory study of HIV-1 and HTLV-I/II coinfection. J. Med. Virol. 44:132-143. [DOI] [PubMed] [Google Scholar]
  • 6.Beilke, M. A., K. P. Theall, M. O'Brien, J. L. Clayton, S. M. Benjamin, E. L. Winsor, and P. J. Kissinger. 2004. Clinical outcomes and disease progression among patients coinfected with HIV and human T lymphotropic virus types 1 and 2. Clin. Infect. Dis. 39:256-263. [DOI] [PubMed] [Google Scholar]
  • 7.Brightman, B. K., Q. X. Li, D. J. Trepp, and H. Fan. 1991. Differential disease restriction of Moloney and Friend murine leukemia viruses by the mouse Rmcf gene is governed by the viral long terminal repeat. J. Exp. Med. 174:389-396. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.Chesebro, B., W. Britt, L. Evans, K. Wehrly, J. Nishio, and M. Cloyd. 1983. Characterization of monoclonal antibodies reactive with murine leukemia viruses: use in analysis of strains of friend MCF and Friend ecotropic murine leukemia virus. Virology 127:134-148. [DOI] [PubMed] [Google Scholar]
  • 9.Chesebro, B., K. Wehrly, M. Cloyd, W. Britt, J. Portis, J. Collins, and J. Nishio. 1981. Characterization of mouse monoclonal antibodies specific for Friend murine leukemia virus-induced erythroleukemia cells: friend-specific and FMR-specific antigens. Virology 112:131-144. [DOI] [PubMed] [Google Scholar]
  • 10.Cloyd, M. W., B. Chesebro, J. L. Portis, and M. Weir. 1982. MCF-specific murine monoclonal antibodies made against AKR-247 MCF virus recognize a unique determinant associated with the gp70-p-15(E) complex. J. Virol. 41:1112-1117. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 11.Cloyd, M. W., J. W. Hartley, and W. P. Rowe. 1980. Lymphomagenicity of recombinant mink cell focus-inducing murine leukemia viruses. J. Exp. Med. 151:542-552. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 12.Cloyd, M. W., J. W. Hartley, and W. P. Rowe. 1981. Genetic study of lymphoma induction by AKR mink cell focus-inducing virus in AKR × NFS crosses. J. Exp. Med. 154:450-457. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 13.Corbin, A., J. Richardson, C. Denesvre, F. Pozo, H. Ellerbrok, and M. Sitbon. 1994. The envelopes of two ecotropic murine leukemia viruses display distinct efficiencies in retroviral vaccination by interference. Virology 202:70-75. [DOI] [PubMed] [Google Scholar]
  • 14.Corbin, A., and M. Sitbon. 1993. Protection against retroviral diseases after vaccination is conferred by interference to superinfection with attenuated murine leukemia viruses. J. Virol. 67:5146-5152. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Cuypers, H. T., G. Selten, W. Quint, M. Zijlstra, E. R. Maandag, W. Boelens, P. van Wezenbeek, C. Melief, and A. Berns. 1984. Murine leukemia virus-induced T-cell lymphomagenesis: integration of proviruses in a distinct chromosomal region. Cell 37:141-150. [DOI] [PubMed] [Google Scholar]
  • 16.Czub, M., F. J. McAtee, S. Czub, W. P. Lynch, and J. L. Portis. 1995. Prevention of retrovirus-induced neurological disease by infection with a nonneuropathogenic retrovirus. Virology 206:372-380. [DOI] [PubMed] [Google Scholar]
  • 17.Eskild, A., H. H. Samdal, B. Heger, et al. 1996. Co-infection with HIV-1/HTLV-II and the risk of progression to AIDS and death. APMIS 104:666-672. [PubMed] [Google Scholar]
  • 18.Evans, L. H. 1986. Characterization of polytropic MuLVs from three-week-old AKR/J mice. Virology 153:122-136. [DOI] [PubMed] [Google Scholar]
  • 19.Evans, L. H., and M. W. Cloyd. 1985. Friend and Moloney murine leukemia viruses specifically recombine with different endogenous retroviral sequences to generate mink cell focus-forming viruses. Proc. Natl. Acad. Sci. USA 82:459-463. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 20.Evans, L. H., and M. W. Cloyd. 1984. Generation of mink cell focus-forming viruses by Friend murine leukemia virus: recombination with specific endogenous proviral sequences. J. Virol. 49:772-781. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 21.Evans, L. H., and P. H. Duesberg. 1982. Isolation of a transformation-defective deletion mutant of Moloney murine sarcoma virus. J. Virol. 41:735-743. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.Evans, L. H., M. Lavignon, M. Taylor, and A. S. Alamgir. 2003. Antigenic subclasses of polytropic murine leukemia virus (MLV) isolates reflect three distinct groups of endogenous polytropic MLV-related sequences in NFS/N mice. J. Virol. 77:10327-10338. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Evans, L. H., and J. D. Morrey. 1987. Tissue-specific replication of Friend and Moloney murine leukemia viruses in infected mice. J. Virol. 61:1350-1357. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Hartley, J. W., N. K. Wolford, L. J. Old, and W. P. Rowe. 1977. A new class of murine leukemia virus associated with development of spontaneous lymphomas. Proc. Natl. Acad. Sci. USA 74:789-792. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 25.Hasenkrug, K. J., S. J. Robertson, J. Porti, F. McAtee, J. Nishio, and B. Chesebro. 1996. Two separate envelope regions influence induction of brain disease by a polytropic murine retrovirus (FMCF98). J. Virol. 70:4825-4828. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 26.Ishimoto, A., A. Adachi, K. Sakai, T. Yorifuji, and S. Tsuruta. 1981. Rapid emergence of mink cell focus-forming (MCF) virus in various mice infected with NB-tropic friend virus. Virology 113:644-655. [DOI] [PubMed] [Google Scholar]
  • 27.Ishimoto, A., J. W. Hartley, and W. P. Rowe. 1977. Detection and quantitation of phenotypically mixed viruses: mixing of ecotropic and xenotropic murine leukemia viruses. Virology 81:263-269. [DOI] [PubMed] [Google Scholar]
  • 28.Koch, W., W. Zimmermann, A. Oliff, and R. Friedrich. 1984. Molecular analysis of the envelope gene and long terminal repeat of Friend mink cell focus-inducing virus: implications for the functions of these sequences. J. Virol. 49:828-840. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 29.Lavignon, M., and L. Evans. 1996. A multistep process of leukemogenesis in Moloney murine leukemia virus-infected mice that is modulated by retroviral pseudotyping and interference. J. Virol. 70:3852-3862. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Lavignon, M., J. Richardson, and L. H. Evans. 1997. A small region of the ecotropic murine leukemia virus (MuLV) gag gene profoundly influences the types of polytropic MuLVs generated in mice. J. Virol. 71:8923-8927. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 31.Lavignon, M., J. L. Walker, S. M. Perryman, F. G. Malik, A. S. Khan, T. S. Theodore, and L. H. Evans. 1994. Characterization of epitopes defining two major subclasses of polytropic murine leukemia viruses (MuLVs) which are differentially expressed in mice infected with different ecotropic MuLVs. J. Virol. 68:5194-5203. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32.Mitchell, T., and R. Risser. 1992. Interference established in mice by infection with Friend murine leukemia virus. J. Virol. 66:5696-5702. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 33.Oliff, A., L. Collins, and C. Mirenda. 1983. Molecular cloning of Friend mink cell focus-inducing virus: identification of mink cell focus-inducing virus-like messages in normal and transformed cells. J. Virol. 48:542-546. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 34.Oliff, A. I., G. L. Hager, E. H. Chang, E. M. Scolnick, H. W. Chan, and D. R. Lowy. 1980. Transfection of molecularly cloned Friend murine leukemia virus DNA yields a highly leukemogenic helper-independent type C virus. J. Virol. 33:475-486. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 35.Ott, D., and A. Rein. 1992. Basis for receptor specificity of nonecotropic murine leukemia virus surface glycoprotein gp70SU. J. Virol. 66:4632-4638. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36.Portis, J. L., S. Czub, S. Robertson, F. McAtee, and B. Chesebro. 1995. Characterization of a neurologic disease induced by a polytropic murine retrovirus: evidence for differential targeting of ecotropic and polytropic viruses in the brain. J. Virol. 69:8070-8075. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.Rein, A. 1982. Interference grouping of murine leukemia viruses: a distinct receptor for the MCF-recombinant viruses in mouse cells. Virology 120:251-257. [DOI] [PubMed] [Google Scholar]
  • 38.Robertson, S. J., K. J. Hasenkrug, B. Chesebro, and J. L. Portis. 1997. Neurologic disease induced by polytropic murine retroviruses: neurovirulence determined by efficiency of spread to microglial cells. J. Virol. 71:5287-5294. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 39.Rowe, W. P., M. W. Cloyd, and J. W. Hartley. 1980. Status of the association of mink cell focus-forming viruses with leukemogenesis. Cold Spring Harbor Symp. Quant. Biol. 44:1265-1268. [DOI] [PubMed] [Google Scholar]
  • 40.Sitbon, M., L. Evans, J. Nishio, K. Wehrly, and B. Chesebro. 1986. Analysis of two strains of Friend murine leukemia viruses differing in ability to induce early splenomegaly: lack of relationship with generation of recombinant mink cell focus-forming viruses. J. Virol. 57:389-393. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Sitbon, M., J. Nishio, K. Wehrly, and B. Chesebro. 1985. Pseudotyping of dual-tropic recombinant viruses generated by infection of mice with different ecotropic murine leukemia viruses. Virology 140:144-151. [DOI] [PubMed] [Google Scholar]
  • 42.Sitbon, M., J. Nishio, K. Wehrly, D. Lodmell, and B. Chesebro. 1985. Use of a focal immunofluorescence assay on live cells for quantitation of retroviruses: distinction of host range classes in virus mixtures and biological cloning of dual-tropic murine leukemia viruses. Virology 141:110-118. [DOI] [PubMed] [Google Scholar]
  • 43.Sitbon, M., B. Sola, L. Evans, J. Nishio, S. F. Hayes, K. Nathanson, C. F. Garon, and B. Chesebro. 1986. Hemolytic anemia and erythroleukemia, two distinct pathogenic effects of Friend MuLV: mapping of the effects to different regions of the viral genome. Cell 47:851-859. [DOI] [PubMed] [Google Scholar]
  • 44.Stebbings, R., N. Berry, J. Stott, R. Hull, B. Walker, J. Lines, W. Elsley, S. Brown, A. Wade-Evans, G. Davis, J. Cowie, M. Sethi, and N. Almond. 2004. Vaccination with live attenuated simian immunodeficiency virus for 21 days protects against superinfection. Virology 330:249-260. [DOI] [PubMed] [Google Scholar]
  • 45.Stoye, J. P., and J. M. Coffin. 1987. The four classes of endogenous murine leukemia virus: structural relationships and potential for recombination. J. Virol. 61:2659-2669. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 46.Tailor, C. S., A. Nouri, C. G. Lee, C. Kozak, and D. Kabat. 1999. Cloning and characterization of a cell surface receptor for xenotropic and polytropic murine leukemia viruses. Proc. Natl. Acad. Sci. USA 96:927-932. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 47.Thomas, C. Y., R. Khiroya, R. S. Schwartz, and J. M. Coffin. 1984. Role of recombinant ecotropic and polytropic viruses in the development of spontaneous thymic lymphomas in HRS/J mice. J. Virol. 50:397-407. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 48.Troxler, D. H., E. Yuan, D. Linemeyer, S. Ruscetti, and E. M. Scolnick. 1978. Helper-independent mink cell focus-inducing strains of Friend murine type-C virus: potential relationship to the origin of replication-defective spleen focus-forming virus. J. Exp. Med. 148:639-653. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 49.van der Putten, H., W. Quint, J. van Raaij, E. R. Maandag, I. M. Verma, and A. Berns. 1981. M-MuLV-induced leukemogenesis: integration and structure of recombinant proviruses in tumors. Cell 24:729-739. [DOI] [PubMed] [Google Scholar]
  • 50.Vogt, P. K. 1967. Phenotypic mixing in the avian tumor virus group. Virology 32:708-717. [DOI] [PubMed] [Google Scholar]
  • 51.Wensel, D. L., W. Li, and J. M. Cunningham. 2003. A virus-virus interaction circumvents the virus receptor requirement for infection by pathogenic retroviruses. J. Virol. 77:3460-3469. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 52.Yang, Y. L., L. Guo, S. Xu, C. A. Holland, T. Kitamura, K. Hunter, and J. M. Cunningham. 1999. Receptors for polytropic and xenotropic mouse leukaemia viruses encoded by a single gene at Rmc1. Nat. Genet. 21:216-219. [DOI] [PubMed] [Google Scholar]

Articles from Journal of Virology are provided here courtesy of American Society for Microbiology (ASM)

RESOURCES