Hemolytic anemia and erythroleukemia, two distinct pathogenic effects of Friend MuLV: mapping of the effects to different regions of the viral genome

Determining the antiviral mechanism of MARCH2

Membrane-associated RING-CH (MARCH) 2 protein is a member of the MARCH protein family of RING-CH finger E3 ubiquitin ligases that have important functions in regulating the levels of proteins found on the cell surface. MARCH1, 2 and 8 inhibit HIV-1 infection by preventing the incorporation of the envelope glycoproteins in nascent virions. However, a better understanding on the mechanism utilized by MARCH proteins to restrict HIV-1 is needed. In this report, we identify an amino acid in human MARCH2, that is absent in mouse MARCH2, critical for its antiretroviral function. Moreover, we map the domains of human MARCH2 critical for restricting as well as binding to the HIV-1 envelope glycoproteins. Our findings reveal important new aspects of the antiviral mechanism utilized by human MARCH2 to restrict HIV-1 that have potential implications to all MARCH proteins with antiviral functions.

A detailed analysis of F-MuLV- and SFFV-infected cells in Friend virus-infected mice reveals the contribution of both F-MuLV- and SFFV-infected cells to the interleukin-10 host response

BACKGROUND

Friend virus (FV) is a complex of the Friend murine leukemia virus (F-MuLV) and the replication-defective, pathogenic spleen focus forming virus (SFFV). In the past, we used a fluorescently labeled F-MuLV to analyze FV target cells. To build on these findings, we have now created a double-labeled FV that contains a Katushka-labeled F-MuLV and an mTagBFP-labeled SFFV, which we have used to study the infection by the two individual viruses in the FV infection of highly susceptible BALB/c mice.

RESULTS

Our data show that the target cells of SFFV largely mirror those of F-MuLV, with the highest virus loads in erythroblasts, B cells and myeloid cells. The early phase of infection was dominated by cells infected by either SFFV or F-MuLV, whereas double-infected cells became dominant later in the course of infection with increasing viral loads. In the late phase of infection, the frequency of double-infected cells was similarly high as the frequencies of SFFV or F-MuLV single-infected cells, and single- and double-infected cells outnumbered the uninfected cells in the most highly infected cell populations such as erythroblasts. FV and retroviruses in general have been shown to induce interleukin 10 (IL-10) as a means of suppressing immune responses. Interestingly, we found in infected IL-10-eGFP reporter mice that SFFV-infected cells contributed to the IL-10-producing cell pool much more significantly than F-MuLV-infected cells, suggesting that the truncated SFFV envelope protein gp55 might play a role in IL-10 induction. Even though BALB/c mice mount notoriously weak immune responses against FV, infection of mice with an ablation of IL-10 expression in T cells showed transiently lower viral loads and stronger T cell activation, suggesting that IL-10 induction by FV and by SFFV in particular may contribute to a suppressed immune response in BALB/c mice.

CONCLUSION

Our data provide detailed information about both F-MuLV- and SFFV-infected cells during the course of FV infection in highly susceptible mice and imply that the pathogenic SFFV contributes to immune suppression.

Elucidating the Antiviral Mechanism of Different MARCH Factors

The membrane-associated RING-CH (MARCH) proteins belong to a family of E3 ubiquitin ligases, whose main function is to remove transmembrane proteins from the plasma membrane. Recent work has shown that the human MARCH1, 2, and 8 are antiretroviral factors that target the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins by reducing their incorporation in the budding virions. Nevertheless, the dearth of information regarding the antiviral mechanism of this family of proteins necessitates further examination. In this study, using both the human MARCH proteins and their mouse homologues, we provide a comprehensive analysis of the antiretroviral mechanism of this family of proteins. Moreover, we show that human MARCH proteins restrict to various degrees the envelope glycoproteins of a diverse number of viruses. This report sheds light on the important antiviral function of MARCH proteins and their significance in cell intrinsic immunity.IMPORTANCE This study examines the mechanism utilized by different MARCH proteins to restrict retrovirus infection. MARCH proteins block the incorporation of envelope glycoproteins to the budding virions. In this report, by comparing the human and mouse MARCH genes and using murine leukemia virus (MLV) and HIV-1, we identify differences in the mechanism of restriction among MARCH proteins. Furthermore, we perform a comprehensive analysis on a number of envelope glycoproteins and show that MARCH proteins have broad antiviral functions.

SERINC5 Potently Restricts Retrovirus Infection In Vivo

This study examined for the first time the in vivo function of the serine incorporator (SERINC) proteins during retrovirus infection. SERINC3 and SERINC5 (SERINC3/5) restrict a number of retroviruses, including human immunodeficiency virus 1 (HIV-1) and murine leukemia virus (MLV), by blocking their entry into cells. Nevertheless, HIV-1 and MLV encode factors, Nef and glycosylated Gag, respectively, that counteract SERINC3/5 in vitro. We recently developed SERINC3 and SERINC5 knockout mice to examine the in vivo function of these genes. We found that SERINC5 restriction is dependent on the absence of glycosylated Gag and the expression of a specific viral envelope glycoprotein. On the other hand, SERINC3 had no antiviral function. Our findings have implications for the development of therapeutics that target SERINC5 during retrovirus infection.

The serine incorporator (SERINC) proteins are multipass transmembrane proteins that affect sphingolipid and phosphatidylserine synthesis. Human SERINC5 and SERINC3 were recently shown to possess antiretroviral activity for a number of retroviruses, including human immunodeficiency virus (HIV), murine leukemia virus (MLV), and equine infectious anemia virus (EIAV). In the case of MLV, the glycosylated Gag (glyco-Gag) protein was shown to counteract SERINC5-mediated restriction in in vitro experiments and the viral envelope was found to determine virion sensitivity or resistance to SERINC5. However, nothing is known about the in vivo function of SERINC5. Antiretroviral function of a host factor in vitro is not always associated with antiretroviral function in vivo. Using SERINC5^−/− mice that we had generated, we showed that mouse SERINC5 (mSERINC5) restriction of MLV infection in vivo is influenced not only by glyco-Gag but also by the retroviral envelope. Finally, we also examined the in vivo function of the other SERINC gene with known antiretroviral functions, SERINC3. By using SERINC3^−/− mice, we found that the murine homologue, mSERINC3, had no antiretroviral role either in vivo or in vitro. To our knowledge, this report provides the first data showing that SERINC5 restricts retrovirus infection in vivo and that restriction of retrovirus infectivity in vivo is dependent on the presence of both glyco-Gag and the viral envelope.

Mouse APOBEC3 interferes with autocatalytic cleavage of murine leukemia virus Pr180gag-pol precursor and inhibits Pr65gag processing

Mouse APOBEC3 (mA3) inhibits murine leukemia virus (MuLV) replication by a deamination-independent mechanism in which the reverse transcription is considered the main target process. However, other steps in virus replication that can be targeted by mA3 have not been examined. We have investigated the possible effect of mA3 on MuLV protease-mediated processes and found that mA3 binds both mature viral protease and Pr180gag-pol precursor polyprotein. Using replication-competent MuLVs, we also show that mA3 inhibits the processing of Pr65 Gag precursor. Furthermore, we demonstrate that the autoprocessing of Pr180gag-pol is impeded by mA3, resulting in reduced production of mature viral protease. This reduction appears to link with the above inefficient Pr65gag processing in the presence of mA3. Two major isoforms of mA3, exon 5-containing and -lacking ones, equally exhibit this antiviral activity. Importantly, physiologically expressed levels of mA3 impedes both Pr180gag-pol autocatalysis and Pr65gag processing. This blockade is independent of the deaminase activity and requires the C-terminal region of mA3. These results suggest that the above impairment of Pr180gag-pol autoprocessing may significantly contribute to the deaminase-independent antiretroviral activity exerted by mA3.

Soon after the identification of the polynucleotide cytidine deaminase APOBEC3 as a host restriction factor against vif-deficient HIV, it was noticed that deamination-independent mechanisms are involved in the inhibition of viral replication in addition to the deaminase-dependent mechanism. We previously showed that mouse APOBEC3 (mA3) physiologically restricted mouse retrovirus replication in their natural hosts without causing significant G-to-A hypermutations. Inhibition of reverse transcription is reported to be the most plausible mechanism for the deamination-independent antiretroviral function. However, it remains unknown whether the inhibition of reverse transcription is the only way to explain the whole picture of deamination-independent antiviral activity exerted by APOBEC3. Here we show that mA3 targets the autoprocessing of Pr180gag-pol polyprotein. This activity does not require the deaminase catalytic center and mainly exerted by the C-terminal half of mA3. mA3 physically interacts with murine retroviral protease and its precursor Pr180gag-pol. mA3-induced disruption of the autocatalytic Pr180gag-pol cleavage leads to a significant reduction of mature viral protease, resulting in the inhibition of Pr65gag processing to mature Gag proteins. As the Pr180gag-pol autoprocessing is necessary for the maturation of other viral enzymes including the reverse transcriptase, its inhibition by host APOBEC3 may precede the previously described impairment of reverse transcription. Our discovery may lead to the development of novel antiretroviral drugs through the future identification of detailed molecular interfaces between retroviral Gag-Pol polyprotein and APOBEC3.

Clinical Signs and Blood Test Results Among Humans Infected With Zoonotic Simian Foamy Virus: A Case-Control Study

Background

A spillover of simian foamy virus (SFV) to humans, following bites from infected nonhuman primates (NHPs), is ongoing in exposed populations. These retroviruses establish persistent infections of unknown physiological consequences to the human host.

Methods

We performed a case-control study to compare 24 Cameroonian hunters infected with gorilla SFV and 24 controls matched for age and ethnicity. A complete physical examination and blood test were performed for all participants. Logistic regression and Wilcoxon signed rank tests were used to compare cases and controls.

Results

The cases had significantly lower levels of hemoglobin than the controls (median, 12.7 vs 14.4 g/dL; P = .01). Basophil levels were also significantly lower in cases than controls, with no differences for other leukocyte subsets. Cases had significantly higher urea, creatinine, protein, creatinine phosphokinase, and lactate dehydrogenase levels and lower bilirubin levels than controls. Cases and controls had similar frequencies of general, cutaneous, gastrointestinal, neurological, and cardiorespiratory signs.

Conclusions

The first case-control study of apparently healthy SFV-infected Cameroonian hunters showed the presence of hematological abnormalities. A thorough clinical and laboratory workup is now needed to establish the medical relevance of these observations because more than half of cases had mild or moderate anemia.

Clinical Trials Registration

NCT03225794.

Infection of B Cell Follicle-Resident Cells by Friend Retrovirus Occurs during Acute Infection and Is Maintained during Viral Persistence

B cell follicles of the spleen and lymph nodes are immune privileged sites and serve as sanctuaries for infected CD4⁺ cells in HIV infection. It is assumed that CD8⁺ T cell responses promote the establishment of the reservoir, as B cell follicles do not permit CD8⁺ T cell entry. Here we analyzed the infected cell population in the Friend retrovirus (FV) infection and investigated whether FV can similarly infect follicular cells. For analysis of FV-infected cells, we constructed a recombinant FV encoding the bright fluorescent protein mWasabi and performed flow cytometry with cells isolated from spleens, lymph nodes and bone marrow of FV-mWasabi-infected mice. Using t-stochastic neighbor embedding for data exploration, we demonstrate how the target cell population changes during the course of infection. While FV was widely distributed in erythrocytes, myeloid cells, B cells, and CD4⁺ T cells in the acute phase of infection, the bulk viral load in the late phase was carried by macrophages and follicular B and CD4⁺ T cells, suggesting that FV persists in cells that are protected from CD8⁺ T cell killing. Importantly, seeding into follicular cells was equally observed in CD8⁺ T cell-depleted mice and in highly FV-susceptible mice that mount a very weak immune response, demonstrating that infection of follicular cells is not driven by immune pressure. Our data demonstrate that infection of cells in the B cell follicle is a characteristic of the FV infection, making this murine retrovirus an even more valuable model for development of retrovirus immunotherapy approaches.IMPORTANCE Human immunodeficiency virus is notorious for its ability to avoid clearance by therapeutic interventions, which is partly attributed to the establishment of reservoirs in latently infected cells and cells that reside in immunologically privileged B cell follicles. In the work presented here, we show that cells of the B cell follicle are equally infected by a simple mouse gammaretrovirus. Using fluorescently labeled Friend retrovirus, we found that B cells and T cells in the B cell follicle, while not carrying the bulk of the virus load, were indeed infected by Friend virus in the early acute phase of the infection and persisted in the chronic infection. Our results suggest that infection of follicular cells may be a shared property of lymphotropic viruses and propose the FV infection of mice as a useful model to study strategies for follicular reservoir elimination.

Fcγ Receptor Type I (CD64)-Mediated Impairment of the Capacity of Dendritic Cells to Activate Specific CD8 T Cells by IgG-opsonized Friend Virus

Dendritic cells (DCs) express Fcγ receptors (FcγRs) for the binding immune complexes (ICs) consisting of IgG and antigens (Ags). IC–FcγR interactions have been demonstrated to enhance activation and antigen-presenting functions of DCs. Utilizing Friend virus (FV), an oncogenic mouse retrovirus, we investigated the effect of IgG-opsonization of retroviral particles on the infection of DCs and the subsequent presentation of viral antigens by DCs to virus-specific CD8 T cells. We found that opsonization by virus-specific non-neutralizing IgG abrogated DC infection and as a consequence significantly reduced the capacity of DCs to activate virus-specific CD8 T cells. Effects of IgG-opsonization were mediated by the high-affinity FcγR type I, CD64, expressed on DCs. Our results suggest that different opsonization patterns on the retroviral surface modulate infection and antigen-presenting functions of DCs, whereby, in contrast to complement, IgG reduces the capacity of DCs to activate cytotoxic T cell (CTL) responses.

Analysis of two monoclonal antibodies reactive with envelope proteins of murine retroviruses: one pan specific antibody and one specific for Moloney leukemia virus

Many monoclonal antibodies (MAbs) reactive with various proteins of murine leukemia viruses (MuLVs) have been developed. In this report two additional MAbs with differing and unusual specificities are described. MAb 573 is reactive with the envelope protein of all MuLVs tested including viruses in the ecotropic, xenotropic, polytropic and amphotropic classes. Notably, MAb 573 is one of only two reported MAbs that react with the envelope protein of amphotropic MuLVs. This MAb appears to recognize a conformational epitope within the envelope protein, as it reacts strongly with live virus and live infected cells, but does not react with formalin-fixed or alcohol-fixed infected cells or denatured viral envelope protein in immunoblots. In contrast, Mab 538 reacts only with an epitope unique to the envelope protein of the Moloney (Mo-) strain of MuLV, a prototypic ecotropic MuLV that is the basis for many retroviral tools used in molecular biology. MAb 538 can react with live cells and viruses, or detergent denatured or fixed envelope protein. The derivation of these antibodies as well as their characterization with regard to their isotype, range of reactivity with different MuLVs and utility in different immunological procedures are described in this study.

Differential requirements of cellular and humoral immune responses for Fv2-associated resistance to erythroleukemia and for regulation of retrovirus-induced myeloid leukemia development

To assess the possible contribution of host immune responses to the exertion of Fv2-associated resistance to Friend virus (FV)-induced disease development, we inoculated C57BL/6 (B6) mice that lacked various subsets of lymphocytes with FV containing no lactate dehydrogenase-elevating virus. Fv2(r) B6 mice lacking CD4(+) T cells developed early polycythemia and fatal erythroleukemia, while B6 mice lacking CD8(+) T cells remained resistant. Erythroid progenitor cells infected with spleen focus-forming virus (SFFV) were eliminated, and no polycythemia was observed in B cell-deficient B6 mice, but they later developed myeloid leukemia associated with oligoclonal integration of ecotropic Friend murine leukemia virus. Additional depletion of natural killer and/or CD8(+) T cells from B cell-deficient B6 mice resulted in the expansion of SFFV proviruses and the development of polycythemia, indicating that SFFV-infected erythroid cells are not only restricted in their growth but are actively eliminated in Fv2(r) mice through cellular immune responses.

Narrowing down the critical region within env gene for determining neuropathogenicity of murine leukemia virus A8

Friend murine leukemia virus clone A8 causes spongiform neurodegeneration in the rat brain, and the env gene of A8 is a primary determinant of neuropathogenicity. In order to narrow down the critical region within the env gene that determines neuropathogenicity, we constructed chimeric viruses having chimeric env between A8 and non-neuropathogenic 57 on the background of A8 virus. After replacement of the BamHI (at nucleotide 5715)-AgeI (at nucleotide 6322) fragment of A8 virus with the corresponding fragment of 57, neuropathogenicity was lost. In contrast, the chimeric viruses that have the BamHI (5715)-AgeI (6322) fragment of A8 induced spongiosis in 100% of infected rats at the same or slightly lower intensity than A8 virus. These results indicate that the BamHI (5715)-AgeI (6322) fragment of A8, which contains the signal sequence and the N-terminal half of RBD, is crucial for the induction of spongiform neurodegeneration. In the BamHI (5715)-AgeI (6322) fragment, seven amino acids differed between A8 and 57, one in the signal sequence and six in RBD, which suggests that these amino acids significantly contribute to the neuropathogenicity of A8.

Defective eosinophil hematopoiesis ex vivo in inbred Rocky Mountain White (IRW) mice

We examine the proliferation and differentiation of bone marrow (BM) progenitors from inbred Rocky Mountain White (IRW) mice, a strain used primarily for retrovirus infection studies. In contrast to findings with BALB/c and C57BL/6 strains, IRW BM cells cannot proliferate or generate pure eosinophil cultures ex vivo in response to a defined cytokine regimen. Analysis of IRW BM at baseline was unremarkable, including 0.08 ± 0.03% Lin(-)Sca-1(+)c-kit(+) (LSK) hematopoietic stem cells and 5.2 ± 0.3% eosinophils; the percentage of eosinophil progenitors (EoPs; Lin(-)Sca-1(-)c-kit(+)CD34(+)IL-5Rα(+)) was similar in all three mouse strains. Transcripts encoding GM-CSFRα and the IL-3/IL-5/GM-CSF common β chain were detected at equivalent levels in IRW and BALB/c BM, whereas expression of transcripts encoding IL-5Rα, IL-3Rα, and GATA-2 was diminished in IRW BM compared with BALB/c. Expression of membrane-bound IL-5Rα and intracellular STAT5 proteins was also diminished in IRW BM cells. Diminished expression of transcripts encoding IL-5Rα and GATA-2 and immunoreactive STAT5 in IRW BM persisted after 4 days in culture, along with diminished expression of GATA-1. Western blot revealed that cells from IRW BM overexpress nonsignaling soluble IL-5Rα protein. Interestingly, OVA sensitization and challenge resulted in BM and airway eosinophilia in IRW mice; however, the responses were significantly blunted. These results suggest that IRW mice have diminished capacity to generate eosinophils in culture and in vivo, likely as a result of diminished signaling via IL-5Rα.

A crucial role for infected-cell/antibody immune complexes in the enhancement of endogenous antiviral immunity by short passive immunotherapy

Antiviral monoclonal antibodies (mAbs) represent promising therapeutics. However, most mAbs-based immunotherapies conducted so far have only considered the blunting of viral propagation and not other possible therapeutic effects independent of virus neutralization, namely the modulation of the endogenous immune response. As induction of long-term antiviral immunity still remains a paramount challenge for treating chronic infections, we have asked here whether neutralizing mAbs can, in addition to blunting viral propagation, exert immunomodulatory effects with protective outcomes. Supporting this idea, we report here that mice infected with the FrCas^E murine retrovirus on day 8 after birth die of leukemia within 4–5 months and mount a non-protective immune response, whereas those rapidly subjected to short immunotherapy with a neutralizing mAb survive healthy and mount a long-lasting protective antiviral immunity with strong humoral and cellular immune responses. Interestingly, the administered mAb mediates lysis of infected cells through an antibody-dependent cell cytotoxicity (ADCC) mechanism. In addition, it forms immune complexes (ICs) with infected cells that enhance antiviral CTL responses through FcγR-mediated binding to dendritic cells (DCs). Importantly, the endogenous antiviral antibodies generated in mAb-treated mice also display the same properties, allowing containment of viral propagation and enhancement of memory cellular responses after disappearance of the administered mAb. Thus, our data demonstrate that neutralizing antiviral mAbs can act as immunomodulatory agents capable of stimulating a protective immunity lasting long after the end of the treatment. They also show an important role of infected-cells/antibody complexes in the induction and the maintenance of protective immunity through enhancement of both primary and memory antiviral T-cell responses. They also indicate that targeting infected cells, and not just viruses, by antibodies can be crucial for elicitation of efficient, long-lasting antiviral T-cell responses. This must be considered when designing antiviral mAb-based immunotherapies.

Monoclonal antibodies (mAbs) constitute the largest class of bio-therapeutic proteins and are increasingly being considered as drugs to fight both acute and chronic severe human viral diseases. Most antiviral mAb-based treatments conducted so far, whether in humans or in animal models, have only considered the blunting of viral propagation through direct virus neutralization. However, mAbs might also operate via complementary mechanisms owing to their ability to interact with various components of the immune system. Using a lethal mouse model of retrovirally-induced leukemia, we report here that a neutralizing mAb administered to infected mice for a short period of time can, in addition to its direct effect on viral spread, induce a strong, long-lasting antiviral immune response protecting mice from disease development long after the end of the treatment. Although the initiation and maintenance of this long-term immunity is multi-factorial, we demonstrate a crucial role for the immune complexes formed between antiviral antibodies and infected cells in this process. Our work reveals a thus far underappreciated vaccine-like effect of antiviral neutralizing mAbs, which will have to be considered for future treatment of life-threatening viral infections.

Persistence of viremia and production of neutralizing antibodies differentially regulated by polymorphic APOBEC3 and BAFF-R loci in friend virus-infected mice

Several host genes control retroviral replication and pathogenesis through the regulation of immune responses to viral antigens. The Rfv3 gene influences the persistence of viremia and production of virus-neutralizing antibodies in mice infected with Friend mouse retrovirus complex (FV). This locus has been mapped within a narrow segment of mouse chromosome 15 harboring the APOBEC3 and BAFF-R loci, both of which show functional polymorphisms among different strains of mice. The exon 5-lacking product of the APOBEC3 allele expressed in FV-resistant C57BL/6 (B6) mice directly restricts viral replication, and mice lacking the B6-derived APOBEC3 exhibit exaggerated pathology and reduced production of neutralizing antibodies. However, the mechanisms by which the polymorphisms at the APOBEC3 locus affect the production of neutralizing antibodies remain unclear. Here we show that the APOBEC3 genotypes do not directly affect the B-cell repertoire, and mice lacking B6-derived APOBEC3 still produce FV-neutralizing antibodies in the presence of primed T helper cells. Instead, higher viral loads at a very early stage of FV infection caused by either a lack of the B6-derived APOBEC3 or a lack of the wild-type BAFF-R resulted in slower production of neutralizing antibodies. Indeed, B cells were hyperactivated soon after infection in the APOBEC3- or BAFF-R-deficient mice. In contrast to mice deficient in the B6-derived APOBEC3, which cleared viremia by 4 weeks after FV infection, mice lacking the functional BAFF-R allele exhibited sustained viremia, indicating that the polymorphisms at the BAFF-R locus may better explain the Rfv3-defining phenotype of persistent viremia.

Protection against retrovirus pathogenesis by SR protein inhibitors

Indole derivatives compounds (IDC) are a new class of splicing inhibitors that have a selective action on exonic splicing enhancers (ESE)-dependent activity of individual serine-arginine-rich (SR) proteins. Some of these molecules have been shown to compromise assembly of HIV infectious particles in cell cultures by interfering with the activity of the SR protein SF2/ASF and by subsequently suppressing production of splicing-dependent retroviral accessory proteins. For all replication-competent retroviruses, a limiting requirement for infection and pathogenesis is the expression of the envelope glycoprotein which strictly depends on the host splicing machinery. Here, we have evaluated the efficiency of IDC on an animal model of retroviral pathogenesis using a fully replication-competent retrovirus. In this model, all newborn mice infected with a fully replicative murine leukemia virus (MLV) develop erythroleukemia within 6 to 8 weeks of age. We tested several IDC for their ability to interfere ex vivo with MLV splicing and virus spreading as well as for their protective effect in vivo. We show here that two of these IDC, IDC13 and IDC78, selectively altered splicing-dependent production of the retroviral envelope gene, thus inhibiting early viral replication in vivo, sufficiently to protect mice from MLV-induced pathogenesis. The apparent specificity and clinical safety observed here for both IDC13 and IDC78 strongly support further assessment of inhibitors of SR protein splicing factors as a new class of antiretroviral therapeutic agents.

Murine APOBEC1 is a powerful mutator of retroviral and cellular RNA in vitro and in vivo

Mammalian APOBEC molecules comprise a large family of cytidine deaminases with specificity for RNA and single-stranded DNA (ssDNA). APOBEC1s are invariably highly specific and edit a single residue in a cellular mRNA, while the cellular targets for APOBEC3s are not clearly established, although they may curtail the transposition of some retrotransposons. Two of the seven member human APOBEC3 enzymes strongly restrict human immunodeficiency virus type 1 in vitro and in vivo. We show here that ssDNA hyperediting of an infectious exogenous gammaretrovirus, the Friend-murine leukemia virus, by murine APOBEC1 and APOBEC3 deaminases occurs in vitro. Murine APOBEC1 was able to hyperdeaminate cytidine residues in murine leukemia virus genomic RNA as well. Analysis of the edited sites shows that the deamination in vivo was due to mouse APOBEC1 rather than APOBEC3. Furthermore, murine APOBEC1 is able to hyperedit its primary substrate in vivo, the apolipoprotein B mRNA, and a variety of heterologous RNAs. In short, murine APOBEC1 is a hypermutator of both RNA and ssDNA in vivo, which could exert occasional side effects upon overexpression.

Mouse APOBEC3 restricts friend leukemia virus infection and pathogenesis in vivo

Several members of the apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like complex 3 (APOBEC3) family in primates act as potent inhibitors of retroviral replication. However, lentiviruses have evolved mechanisms to specifically evade host APOBEC3. Likewise, murine leukemia viruses (MuLV) exclude mouse APOBEC3 from the virions and cleave virion-incorporated APOBEC3. Although the betaretrovirus mouse mammary tumor virus has been shown to be susceptible to mouse APOBEC3, it is not known if APOBEC3 has a physiological role in restricting more widely distributed and long-coevolved mouse gammaretroviruses. The pathogenicity of Friend MuLV (F-MuLV) is influenced by several host genes: some directly restrict the cell entry or integration of the virus, while others influence the host immune responses. Among the latter, the Rfv3 gene has been mapped to chromosome 15 in the vicinity of the APOBEC3 locus. Here we have shown that polymorphisms at the mouse APOBEC3 locus indeed influence F-MuLV replication and pathogenesis: the APOBEC3 alleles of F-MuLV-resistant C57BL/6 and -susceptible BALB/c mice differ in their sequences and expression levels in the hematopoietic tissues and in their abilities to restrict F-MuLV replication both in vitro and in vivo. Furthermore, upon infection with the pathogenic Friend virus complex, (BALB/c x C57BL/6)F(1) mice displayed an exacerbated erythroid cell proliferation when the mice carried a targeted disruption of the C57BL/6-derived APOBEC3 allele. These results indicate, for the first time, that mouse APOBEC3 is a physiologically functioning restriction factor to mouse gammaretroviruses.

Neurovirulence of polytropic murine retrovirus is influenced by two separate regions on opposite sides of the envelope protein receptor binding domain

Changes in the envelope proteins of retroviruses can alter the ability of these viruses to infect the central nervous system (CNS) and induce neurological disease. In the present study, nine envelope residues were found to influence neurovirulence of the Friend murine polytropic retrovirus Fr98. When projected on a three-dimensional model, these residues were clustered in two spatially separated groups, one in variable region B of the receptor binding site and the other on the opposite side of the envelope. Further studies indicated a role for these residues in virus replication in the CNS, although the residues did not affect viral entry.

APRIL facilitates viral-induced erythroleukemia but is dispensable for T cell immunity and lymphomagenesis

The TNF family member, a proliferation-inducing ligand (APRIL), has been suggested to act as a costimulatory molecule in T cell responses. However, studies addressing this role in vivo are largely lacking. Here, we evaluated the effects of APRIL on physiological T cell responses in vivo. Although receptors for APRIL are expressed on a subset of T cells, neither TCR transgenic (Tg) T cell responses nor endogenous TCR responses were affected by Tg APRIL expression in vivo. Moreover, APRIL did not significantly enhance the induction of T cell lymphomas upon Moloney murine leukemia virus (MLV) infection. This clearly contrasts current belief and indicates that APRIL does not serve a major role in T cell immunity or lymphomagenesis. However, we did observe a strong increase in erythroleukemia formation after MLV inoculation of APRIL Tg mice. Strikingly, this erythroleukemia-facilitating property of APRIL was confirmed using the erythroleukemogenic Friend-MLV. Erythroleukemia in APRIL Tg mice was characterized by low hematocrits and grossly enlarged spleens with an increased percentage of erythroid precursors. Altogether, these results unveil new proerythroleukemogenic properties of APRIL.

Endogenous cytotoxic T-cell response contributes to the long-term antiretroviral protection induced by a short period of antibody-based immunotherapy of neonatally infected mice

Neutralizing monoclonal antibodies (MAbs) are increasingly being considered for blunting human viral infections. However, whether they can also exert indirect effects on endogenous antiviral immune responses has been essentially overlooked. We have recently shown that a short (several-day) period of immunotherapy with the neutralizing 667 MAb of mouse neonates shortly after infection with the lethal FrCas(E) retrovirus not only has an immediate effect on the viral load but also permits an endogenous antiviral immunity to emerge. Even though passive immunotherapy was administered during the particular period of immunocompetence acquisition, the endogenous response eventually arising was protective and persisted long (>1 year) after the MAb has disappeared. As very high levels of anti-FrCas(E) antibodies, predominantly of the immunoglobulin G2a (IgG2a) isotype and showing strong neutralization activity, were found in the sera of MAb-treated mice, it was necessary to address whether this humoral immunity was sufficient on its own to confer full protection against FrCas(E) or whether a cytotoxic T-lymphocyte (CTL) response was also necessary. Using a variety of in vivo assays in young and adult animals previously infected by FrCas(E) and treated by 667, we show here that transient 667 immunotherapy is associated with the emergence of a CTL response against virus-infected cells. This cytotoxic activity is indispensable for long-term antiviral protective immunity, as high neutralizing antibody titers, even enhanced in in vivo CD8(+) cell depletion experiments, cannot prevent the FrCas(E)-induced death of infected/treated mice. Our work may have important therapeutic consequences, as it indicates that a short period of MAb-based immunotherapy conducted at a stage where the immune system is still developing can be associated with the mounting of a functional Th1-type immune response characterized by both CTL and IgG2a-type humoral contributions, the cooperation of which is known to be essential for the containment of chronic infections by a variety of viruses.

In vivo interactions of ecotropic and polytropic murine leukemia viruses in mixed retrovirus infections

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. Co-inoculation 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 co-inoculated 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.

Linkage of reduced receptor affinity and superinfection to pathogenesis of TR1.3 murine leukemia virus

TR1.3 is a Friend murine leukemia virus (MLV) that induces selective syncytium induction (SI) of brain capillary endothelial cells (BCEC), intracerebral hemorrhage, and death. Syncytium induction by TR1.3 has been mapped to a single tryptophan-to-glycine conversion at position 102 of the envelope glycoprotein (Env102). The mechanism of SI by TR1.3 was examined here in comparison to the non-syncytium-inducing, nonpathogenic MLV FB29, which displays an identical BCEC tropism. Envelope protein expression and stability on both infected cells and viral particles were not statistically different for TR1.3 and FB29. However, affinity measurements derived using purified envelope receptor binding domain (RBD) revealed a reduction of >1 log in the K(D) of TR1.3 RBD relative to FB29 RBD. Whole-virus particles pseudotyped with TR1.3 Env similarly displayed a markedly reduced binding avidity compared to FB29-pseudotyped viral particles. Lastly, decreased receptor affinity of TR1.3 Env correlated with the failure to block superinfection following acute and chronic infection by TR1.3. These results definitively show that acquisition of a SI phenotype can be directly linked to amino acid changes in retroviral Env that decrease receptor affinity, thereby emphasizing the importance of events downstream of receptor binding in the cell fusion process and pathology.

Transduction of nondividing human macrophages with gammaretrovirus-derived vectors

It is commonly accepted that infection of nondividing cells by gammaretroviruses such as the murine leukemia viruses is inefficient due to their inability to cross the nuclear envelope barrier. Challenging this notion, we now show that human nondividing macrophages display a specific window of susceptibility to transduction with a Friend murine leukemia virus (F-MLV)-derived vector during their differentiation from monocytes. This finding suggests that factors other than the nuclear membrane govern permissiveness to gammaretroviral infection and raises the possibility of using the macrophage tropism of F-MLV in gene therapy.

Neuropathogenic murine leukemia virus TR1.3 induces selective syncytia formation of brain capillary endothelium

Exposure of newborn BALB/c mice to murine leukemia virus (MLV) TR1.3 induces fusion of brain capillary endothelial cells (BCEC), loss of cerebral vessel integrity, hemorrhagic stroke, and death. Although TR1.3 infects endothelial cells in multiple organs, syncytia are only observed in BCEC. To determine if viral and cellular factors are responsible for selective syncytia formation, capillary endothelial cells (CEC) from multiple organs were assayed in vitro for MLV infection and cell fusion. Following incubation with virus, all CEC were infected to an equal extent as determined by expression of MLV envelope and infectious virus production; however, MLV-induced syncytia were only observed in TR1.3-infected BCEC cultures. These in vitro results mirror the in vivo pattern of TR1.3 MLV infection and neuropathology, and definitively show that selective fusion and pathology of BCEC by MLV is determined by properties unique to BCEC as contrasted to other endothelial cell types.

Early dissemination rates of Friend murine leukaemia virus variants correlate with late pathogenesis

FIS-2, a less oncogenic, immunosuppressive variant of the Friend murine leukaemia virus (F-MuLV), was used to explore whether the differences in biological features were related to early virus dissemination rates or sites of replication. We found that erythroblasts were the primary target cells for both F-MuLV and FIS-2, while B- and T-cells were infected later in the infection. Although FIS-2 replicated to similar titres as F-MuLV, we observed a delay in peak viraemia titre and in the number of virus-positive cells in bone marrow and spleen. Studies including the chimeric viruses RE3 (FIS-2LTR with a F-MuLV background) and RE4 (F-MuLV LTR with a FIS-2 background) indicated that the delay in dissemination was due to mutations in FIS-2 LTR. The kinetics for early virus replication correlated with previously reported mean latency time for virus-induced erythroleukaemia in mice inoculated as newborns and with the onset of immunosuppression in adult mice. In addition, F-MuLV-induced late immunosuppression coincided with signs of erythroleukaemia and persistent viraemia. FIS-2 induced a more moderate late immunosuppression without persistent viraemia or signs of erythroleukaemia. Overall, our findings indicated that early viral replication is a prognostic factor in murine retrovirus-induced pathogenesis.

Retroviral interleukin 1alpha gene transfer in bone marrow stromal cells in a primate model: induction of myelopoiesis stimulation

Effects of interleukin 1-alpha (IL-1alpha), a proinflammatory cytokine with pleiotropic activity, in the myelopoietic setting, is mainly linked to its ability to increase haematopoietic growth factor production by bone marrow stromal cells. In order to minimize systemic effects of IL-1alpha therapy, we proposed a model of retroviral IL-1alpha gene transfer within bone marrow stromal cells in the macaque cynomolgus. Invitro, 10-15% of bone marrow stromal cells was effectively transduced by retroviral vector (murine Moloney leukaemia virus-derived) expressing IL-1alpha/LacZ, or LacZ alone as control marker, as assessed by betaGal staining. IL-1alpha gene expression was upregulated [semiquantitative reverse transcription polymerase chain reaction (RT-PCR)] within the transduced cells and the cell supernatant showed an increased production of granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage (GM)-CSF (enzyme-linked immunosorbent assay) and an increased clonogenic activity (colony-forming cell assay). Ex vivo autologous expanded IL-1alpha/LacZ transduced bone marrow stromal cells were reinfused in two macaques (and two control animals for LacZ alone as controls), without clinical systemic toxicity; LacZ expression by RT-PCR was detected in one animal of each group between d 4 and 9. A slight increase of the peripheral blood leucocyte counts (both polymorphonuclear cells and monocytes) of the two animals transduced with IL-1alpha/LacZ was observed within 10 d, indicating stimulation of myelopoiesis.

Reexamination of amphotropic murine leukemia virus neurovirulence: neural stem cell-mediated microglial infection fails to induce acute neurodegeneration

The 4070A amphotropic murine leukemia virus (A-MuLV) has been variably reported to harbor neurovirulence determinants within its env gene. In this report we reexamined this issue by applying two approaches previously demonstrated to amplify murine leukemia virus neurovirulence. The first approach involved introducing the 4070A env gene into the background of Friend virus clone FB29 to enhance peripheral virus replication kinetics and central nervous system entry. The resulting chimeric virus, FrAmE, exhibited widespread vascular infection throughout the central nervous system (CNS); however, parenchymal infection was quite limited. Neither clinical neurological signs nor spongiform neurological changes accompanied FrAmE CNS infection. To overcome this CNS entry limitation, 4070A and FrAmE were delivered directly into the CNS via transplantation of infected C17.2 neural stem cells (NSCs). Significantly, NSC dissemination of either 4070A or FrAmE resulted in widespread, high-level amphotropic virus expression within the CNS parenchyma, including the infection of microglia, the critical target required for inducing neurodegeneration. Despite the extensive CNS infection, no associated clinical neurological signs or acute neuropathological changes were observed. Interestingly, we observed the frequent appearance of circulating polytropic (MCF) virus in the serum of amphotropic virus-infected animals. However, neither peripheral inoculation of an amphotropic/MCF virus mixture nor transplantation of NSCs expressing both amphotropic and MCF viruses induced acute clinical neurological signs or spongiform neuropathology. Thus, the results generated in this study suggest that the 4070A env gene is not inherently neurovirulent. However, the frequent appearance of endogenous MCF viruses suggests the possibility that the interactions of amphotropic viruses with endogenous retroviral elements could contribute to the development of retrovirus-induced neurodegenerative disease.

Germ-line deletion of p53 reveals a multistage tumor progression in spi-1/PU.1 transgenic proerythroblasts

Activation of the spi-1/PU.1 proto-oncogene and loss of p53 function are genetic alterations associated with the emergence of Friend malignant erythroleukemic cells. To address the role of p53 during erythroleukemogenesis, spi-1 transgenic mice (spi-1-Tg) which develop erythroleukemia were bred with p53-deficient mice. Three classes of spi-1 transgenic mice differing in their p53 functional status (p53(+/+), p53(+/-) and p53(-/-)) were generated. These mice developed a unique pattern of erythroleukemia. In wild-type p53 spi-1-Tg mice, none of the primary erythroleukemic spleen cells displayed autonomous growth in vitro and in vivo. In contrast, in p53(+/-) spi-1-Tg mice, erythroleukemic cells gave rise to growth factor-independent cell lines and generated tumors in vivo. Malignancy was associated with loss of the wild-type p53 allele. The p53(-/-) spi-1-Tg mice developed erythroleukemia with a total incidence and a reduced latency compared to the two other genotypes. Unexpectedly, 50% of p53(-/-) spi-1-Tg erythroleukemic spleens generated cell lines that were strictly dependent upon erythropoietin (Epo) for proliferation, whereas the remainder proliferated independently of cytokines. Moreover, only 70% of these spleen cells were tumorigenic. These findings indicate that p53 germ-line deletion did not confer malignancy to spi-1-transgenic proerythroblasts. Moreover Epo independence and tumorigenicity appear as separable phenotypic characteristics revealing that the spi-1-Tg proerythroblasts progress towards malignancy through multiple oncogenic events.

Direct transformation of rodent fibroblasts by jaagsiekte sheep retrovirus DNA

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary carcinoma, a unique animal model for human bronchioalveolar carcinoma. We previously isolated a JSRV proviral clone and showed that it was both infectious and oncogenic. Thus JSRV is necessary and sufficient for the development of ovine pulmonary carcinoma, but no data are available on the mechanisms of transformation. Inspection of the JSRV genome reveals standard retroviral genes, but no evidence for a viral oncogene. However, an alternate ORF in pol (orf-x) might be a candidate for a transforming gene. We tested whether the JSRV genome might encode a transforming gene by transfecting an expression plasmid for JSRV [pCMVJS21, driven by the cytomegalovirus (CMV) immediate early promoter] into mouse NIH 3T3 cells. Foci of transformed cells appeared in the transfected cultures 2-3 weeks posttransfection; cloned transformants showed anchorage independence for growth, and they expressed JSRV RNA. These results indicate that the JRSV genome contains information with direct transforming potential for NIH 3T3 cells. Transfection of a mutated version of pCMVJS21 in which the orf-x protein was terminated by two stop codons also gave transformed foci. Thus, orf-x was eliminated as the candidate transforming gene. In addition, another derivative of pCMVJS21 (pCMVJS21DeltaGP) in which the gag, pol (and orf-x) coding sequences were deleted also gave transformed foci. These results indicate that the envelope gene carries the transforming potential. This is an unusual example of a native retroviral structural protein with transformation potential.

A preclinical model of hepatocyte gene transfer: the in vivo, in situ perfused rat liver

Delivering retroviruses targeted to hepatocytes in vivo involves the injection of retroviruses directly into the portal vein. The aim of this work was to establish a clinically relevant system for retrovirus-mediated gene transfer in a new model of in vivo, in situ perfused rat liver and to study the transgene expression. At 24 h after partial hepatectomy, the liver was completely excluded from the splanchnic circulation using an extracorporeal shunt. Two independent normothermal, oxygenated perfusion systems were used. First, liver perfusion was carried out with a recirculating system (1 h). Culture supernatant containing retroviruses (1.5 x 10(8) ffu/ml, beta-galactosidase gene) was used as perfusate. Then the liver perfusion was maintained for more 30 min in a single liver passage system using culture medium without retroviruses as perfusate. High hepatocyte transduction rates (up to 34.4%) were obtained. PCR analysis showed no provirus in extrahepatic organs. Viral titrations performed simultaneously (inflow and outflow liver lines) showed that after 1 h of perfusion (up to 30 successive liver passages) retroviruses were still detected in the liver outflow perfusate (up to 2.0 x 10(7) ffu/ml). Washing the liver for 30 min dramatically decreased the leakage of retroviruses in the outflow. In order to be of clinical use, the injection of retroviruses targeted to hepatocytes in vivo should be done while the liver is completely excluded from the splanchnic circulation to avoid any extrahepatic retrovirus diffusion.

Brain infection by neuroinvasive but avirulent murine oncornaviruses

The chimeric murine oncornavirus FrCas(E) causes a rapidly progressive noninflammatory spongiform encephalomyelopathy after neonatal inoculation. The virus was constructed by the introduction of pol-env sequences from the wild mouse virus CasBrE into the genome of a neuroinvasive but nonneurovirulent strain of Friend murine leukemia virus (FMuLV), FB29. Although the brain infection by FrCas(E) as well as that by other neurovirulent murine retroviruses has been described in detail, little attention has been paid to the neuroinvasive but nonneurovirulent viruses. The purpose of the present study was to compare brain infection by FrCas(E) with that by FB29 and another nonneurovirulent virus, F43, which contains pol-env sequences from FMuLV 57. Both FB29 and F43 infected the same spectrum of cell types in the brain as that infected by FrCas(E), including endothelial cells, microglia, and populations of neurons which divide postnatally. Viral burdens achieved by the two nonneurovirulent viruses in the brain were actually higher than that of FrCas(E). The widespread infection of microglia by the two nonneurovirulent viruses is notable because it is infection of these cells by FrCas(E) which is thought to be a critical determinant of its neuropathogenicity. These results indicate that although the sequence of the envelope gene determines neurovirulence, this effect appears to operate through a mechanism which does not influence either viral tropism or viral burden in the brain. Although all three viruses exhibited similar tropism for granule neurons in the cerebellar cortex, there was a striking difference in the distribution of envelope proteins in those cells in vivo. The FrCas(E) envelope protein accumulated in terminal axons, whereas those of FB29 and F43 remained predominantly in the cell bodies. These observations suggest that differences in the intracellular sorting of these proteins may exist and that these differences appear to correlate with neurovirulence.

Introduction of a cis-acting mutation in the capsid-coding gene of moloney murine leukemia virus extends its leukemogenic properties

Inoculation of newborn mice with the retrovirus Moloney murine leukemia virus (MuLV) results in the exclusive development of T lymphomas with gross thymic enlargement. The T-cell leukemogenic property of Moloney MuLV has been mapped to the U3 enhancer region of the viral promoter. However, we now describe a mutant Moloney MuLV which can induce the rapid development of a uniquely broad panel of leukemic cell types. This mutant Moloney MuLV with synonymous differences (MSD1) was obtained by introduction of nucleotide substitutions at positions 1598, 1599, and 1601 in the capsid gene which maintained the wild-type (WT) coding potential. Leukemias were observed in all MSD1-inoculated animals after a latency period that was shorter than or similar to that of WT Moloney MuLV. Importantly, though, only 56% of MSD1-induced leukemias demonstrated the characteristic thymoma phenotype observed in all WT Moloney MuLV leukemias. The remainder of MSD1-inoculated animals presented either with bona fide clonal erythroid or myelomonocytic leukemias or, alternatively, with other severe erythroid and unidentified disorders. Amplification and sequencing of U3 and capsid-coding regions showed that the inoculated parental MSD1 sequences were conserved in the leukemic spleens. This is the first report of a replication-competent MuLV lacking oncogenes which can rapidly lead to the development of such a broad range of leukemic cell types. Moreover, the ability of MSD1 to transform erythroid and myelomonocytic lineages is not due to changes in the U3 viral enhancer region but rather is the result of a cis-acting effect of the capsid-coding gag sequence.

Gene transfer into canine myoblasts

We have developed and characterized cultures of healthy and dystrophic canine myoblasts for the evaluation of various gene transfer protocols. The number of desmin-positive myoblasts was elevated (>>80%) in cultures of myoblasts obtained from different muscle territories, the diaphragm muscle giving rise to the purest cultures. Myoblasts from dogs turned out to be a very convenient source of well transfectable and transducible cells. Transfection with plasmid DNA allowed efficient transgene expression (50% of beta-galactosidase positive cells and about 375 ng luciferase/mg protein after transfection with a calcium phosphate-precipitated plasmid). Infection with high concentrations of adenoviral and retroviral vectors allowed transgene (beta-galactosidase or mini-dystrophin) detection in about 75 to 90% of the canine cells. Therefore, primary dog myoblast cultures represent a useful in vitro model for viral and non-viral gene delivery, as well as for functional evaluation and cell grafting with applications in genetic diseases, vaccination or production of circulating therapeutic proteins.

In vivo hepatocyte retrovirus-mediated gene transfer through the rat biliary tract

Delivering retroviruses targeted to hepatocytes in vivo involves the injection of retroviruses directly into the blood stream of the portal vein. The aim of this work was to delineate the conditions for delivering retroviruses in vivo by perfusing in situ the bile duct of the regenerating rat liver, and to study the hepatocyte transgene expression. At 24 hr after partial hepatectomy, during the S phase of the cell cycle, regenerating livers were perfused for 2.8+/-0.5 hr through the bile duct with 36.2+/-6.8 ml (0.3+/-01 ml/min) of fresh culture supernatant containing amphotropic recombinant retroviruses encoding the beta-galactosidase gene. The virus total titer was 1.5 x 10(8) ffu (group I) or 6.5 x 10(8) ffu (groups II and III). The hepatic artery blood flow was either maintained (groups I and II) or interrupted (group III) during bile duct perfusion. Liver biopsies taken 7 days later showed that 31.4+/-24.2% (group I), 58.7+/-23.6% (group II), and 45.1+/-21.4% (group III) of hepatocytes expressed beta-galactosidase activity, predominantly in the periportal and mediolobular zones. This study demonstrates that hepatocytes of regenerating rat livers that have entered the S phase of the cell cycle as a result of partial hepatectomy can be transduced in vivo by retroviral vectors delivered in situ by bile duct perfusion. Furthermore, the number of transduced hepatocytes closely correlated with the viral total titer and was diminished by hepatic artery blood flow occlusion during perfusion.

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.

Suppression of rat bone marrow cells by Friend murine leukemia virus envelope proteins

In a retroviral rat model, we have investigated the nontransforming effects of murine leukemia virus FB29 on the bone marrow. Upon intraperitoneal inoculation with murine leukemia virus FB29 of either neonatal or adult rats, bone marrow cells became massively infected within the first 12 days postinoculation. In neonatally inoculated rats, a persistent productive bone marrow infection was established, whereas in rats inoculated as adults, no infected bone marrow cells could be detected beyond 12 days postinoculation. Retroviral infection was most likely cleared by an antiviral immune response (Hein et al., 1995, Virology 211, 408-417). Exposure to virus irreversibly decreased numbers of bone marrow cells staining with monoclonal antibody OX7 by 10-30%. Reduction of OX7+ bone marrow cells by 20% was also observed in vitro, after bone marrow cells from uninfected adult rats had been co-incubated with virus. FB29-envelope proteins were sufficient alone to reduce numbers of OX7+ bone marrow cells, both in vivo and in vitro. According to results on incorporation of propidium iodide, decreased numbers of OX7+ cells were due to cell death. By flow cytometric analyses OX7+ bone marrow cells as well as monocytes/macrophages were identified to be major target cells for infection with FB29 within the bone marrow. Thus, the mechanism(s) responsible for death of OX7+ bone marrow cells might be due to direct toxicity of viral envelope proteins and/or to interactions of viral envelope proteins with cells of the monocytic lineage.

Sequence flexibility in the polytropic env gp70-derived region of the membrane glycoprotein (gp55) of Friend spleen focus-forming virus affects its biological activity

We previously reported (N. Watanabe, M. Nishi, Y. Ikawa, and H. Amanuma, J. Virol. 65:132-137, 1991) that the mutant Friend spleen focus-forming virus (F-SFFV(MS)), which encodes a mutant gp55 membrane glycoprotein with an ecotropic env gp70 sequence, was nonpathogenic. Here we injected the F-SFFV(MS)-Friend murine leukemia virus (F-MuLV) clone 57 complex into newborn DBA/2 mice. We obtained four groups of pathogenic variant F-SFFV complexes, each showing a different degree of pathogenicity in adult mice and a different gp55 profile. Of these, group 1 variant F-SFFV was particularly interesting, because it was the most frequently obtained and because it produced doublet bands of gp55 (59 and 57 kDa), neither of which reacted with the nonecotropic gp70-specific monoclonal antibody, and because its DNA intermediate did not hybridize with the nonecotropic env-specific probe. Cloning and DNA sequence analysis of the env region of one isolate of the group 1 variant F-SFFV revealed that this virus consisted of two distinct F-SFFV genomes; one (clone 117) differed from the other (clone 118) due to the presence of a 39-bp in-frame deletion. Reconstitution to full-length F-SFFV genomes and a pathogenicity assay showed that each reconstituted F-SFFV was pathogenic, with clone 117 showing a higher degree of pathogenicity than clone 118. Both reconstituted F-SFFVs caused activation of the mouse erythropoietin receptor in the factor-independent cell proliferation assay, although much less efficiently than the wild-type polycythemia-inducing isolate F-SFFVp. Clone 118 produced a gp55 of 59 kDa, while clone 117 produced one of 57 kDa. Clone 118 had a substitution by the F-MuLV clone 57 gp70 sequence, indicating that it was derived from the F-SFFV(MS) env gene by a homologous recombination with the F-MuLV clone 57 env gene. The site of the 39-bp deletion in clone 117 corresponded to the portion of the clone 118 sequence which was unique to the ecotropic env genes. These results indicated the importance for the biological activity of gp55 of the sequences in the gp70 differential region, which are contained in both polytropic and ecotropic env genes.

Efficient transduction of hemopoietic CD34+ progenitors of human origin using an original retroviral vector derived from Fr-MuLV-FB29: in vitro assessment

A novel retroviral vector has been designed based on a Friend-murine leukemia virus (Fr-MuLV) FB29 strain. The latter has been selected according to characteristics of pathogenicity in mice where it induces a disease of the haemopoietic system affecting all lineages. Higher infectivity has also been demonstrated as compared to other strains. In accordance with these findings, the amphotropic producer clone used in this study carrying along the neomycine resistance gene (FOCH-Neo), harbors viral titers over 10(7) cfu/ml. To investigate the potential of genetically engineering hematopoietic precursors, CD34+ progenitors were selected from cord blood, bone marrow, and peripheral blood mobilized stem cells (patients + solid tumors) and transduced with FOCH-Neo. High transduction rates were achieved using virus supernatant and minimal doses of hematopoietic growth factors during pretransduction and transduction steps. A polymerase chain reaction (PCR) assay investigating the presence of both neomycin-encoding and viral vector sequences tested positive in 45-90% of granulocyte-macrophage colony-forming units (CFU-GM) generating cells (bone marrow and peripheral blood derived cells) following transduction. An average of 35% colonies showed resistance to G418. Such levels of transduction proved reproducible using only supernatants harboring over 10(7) cfu/ml. In those experiments where long-term in vitro cultures could be maintained over 5 weeks (all cord blood and 5 among 23 PBSC), efficient transduction of long-term culture initiating cell (LTC-IC) hematopoietic progenitors was demonstrated on the basis of both resistance to G418 and virus integration. In the latter case, the PCR assay tested positive in as much as 35-60% of late unselected CFU-colonies. This novel retroviral vector harbors interesting features toward genetic modification of hematopoietic progenitors.

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.

Neurologic disease induced by polytropic murine retroviruses: neurovirulence determined by efficiency of spread to microglial cells

Several murine leukemia viruses (MuLV) induce neurologic disease in susceptible mice. To identify features of central nervous system (CNS) infection that correlate with neurovirulence, we compared two neurovirulent MuLV, Fr98 and Fr98/SE, with a nonneurovirulent MuLV, Fr54. All three viruses utilize the polytropic receptor and are coisogenic, each containing a different envelope gene within a common genetic background. Both Fr98 and Fr98/SE induce a clinical neurologic disease characterized by hyperexcitability and ataxia yet differ in incubation period, 16 to 30 and 30 to 60 days, respectively. Fr54 infects the CNS but fails to induce clinical signs of neurologic disease. In this study, we compared the histopathology, regional virus distribution, and cell tropism in the brain, as well as the relative CNS viral burdens. All three viruses induced similar histopathologic effects, characterized by intense reactive astrogliosis and microglial activation associated with minimal vacuolar degeneration. The infected target cells for each virus consisted primarily of endothelial and microglial cells, with rare oligodendrocytes. Infection localized predominantly in white matter tracts of the cerebellum, internal capsule, and corpus callosum. The only feature that correlated with relative neurovirulence was viral burden as measured by both viral CA protein expression in cerebellar homogenates and quantification of infected cells. Interestingly, Fr54 (nonneurovirulent) and Fr98/SE (slow disease) had similar viral burdens at 3 weeks postinoculation, suggesting that they entered the brain with comparable efficiencies. However, spread of Fr98/SE within the brain thereafter exceeded that of Fr54, reaching levels of viral burden comparable to that seen for Fr98 (rapid disease) at 3 weeks. These results suggest that the determinants of neurovirulence in the envelope gene may influence the efficiency of virus spread within the brain and that a critical number of infected cells may be required for induction of clinical neurologic disease.

Late virus replication events in microglia are required for neurovirulent retrovirus-induced spongiform neurodegeneration: evidence from neural progenitor-derived chimeric mouse brains

CasBrE is a neurovirulent murine retrovirus which induces a spongiform myeloencephalopathy in susceptible mice. Genetic mapping studies have indicated that sequences responsible for neurovirulence reside within the env gene. To address the question of direct envelope protein neuroxicity in the central nervous system (CNS), we have generated chimeric mice expressing the CasBrE envelope protein in cells of neuroectodermal origin. Specifically, the multipotent neural progenitor cell line C17.2 was engineered to express the CasBrE env gene as either gp70/p15E (CasE) or gp70 alone (CasES). CasE expression in these cells resulted in complete (>10(5)) interference of superinfection with Friend murine leukemia virus clone FB29, whereas CasES expression resulted in a 1.8-log-unit decrease in FB29 titer. Introduction of these envelope-expressing C17.2 cells into the brains of highly susceptible IRW mice resulted in significant engraftment as integral cytoarchitecturally correct components of the CNS. Despite high-level envelope protein expression from the engrafted cells, no evidence of spongiform neurodegeneration was observed. To examine whether early virus replication events were necessary for pathogenesis, C17.2 cells expressing whole virus were transplanted into mice in which virus replication in the host was specifically restricted by Fv-1 to preintegration events. Again, significant C17.2 cell engraftment and infectious virus expression failed to precipitate spongiform lesions. In contrast, transplantation of virus-expressing C17.2 progenitor cells in the absence of the Fv-1 restriction resulted in extensive spongiform neurodegeneration by 2 weeks postengraftment. Cytological examination indicated that infection had spread beyond the engrafted cells, and in particular to host microglia. Spongiform neuropathology in these animals was directly correlated with CasBrE env expression in microglia rather than expression from neural progenitor cells. These results suggest that the envelope protein of CasBrE is not itself neurotoxic but that virus infectious events beyond binding and fusion in microglia are necessary for the induction of CNS disease.

Modulation of ecotropic murine retroviruses by N-linked glycosylation of the cell surface receptor/amino acid transporter

The cell surface receptor for ecotropic host-range (infection limited to mice or rats) murine leukemia viruses (MuLVs) is the widely expressed system y+ transporter for cationic amino acids (CAT-1). Like other retroviruses, ecotropic MuLV infection eliminates virus-binding sites from cell surfaces and results in complete interference to superinfection. Surprisingly, infection causes only partial (ca 40 to 60%) loss of mouse CAT-1 transporter activity. The NIH/Swiss mouse CAT-1 (mCAT-1) contains 622 amino acids with 14 hydrophobic potential membrane-spanning sequences, and it is known that the third extracellular loop from the amino terminus is required for virus binding. Although loop 3 is hypervariable in different species and mouse strains, consistent with its proposed role in virus-host coevolution, loop 3 sequences of both susceptible and resistant species contain consensus sites for N-linked glycosylation. Both of the consensus sites in loop 3 of mCAT-1 are known to be glycosylated and to contain oligosaccharides with diverse sizes (J. W. Kim and J. M. Cunningham, J. Biol. Chem. 268:16316-16320, 1993). We confirmed by several lines of evidence that N-linked glycosylation occludes a potentially functional virus-binding site in the CAT-1 protein of hamsters, thus contributing to resistance of that species. To study the role of receptor glycosylation in animals susceptible to infection, we eliminated loop 3 glycosylation sites by mutagenesis of an mCAT-1 cDNA clone, and we expressed wild-type and mutant receptors in mink fibroblasts and Xenopus oocytes. These receptors had indistinguishable transport properties, as determined by kinetic and voltage-jump electrophysiological studies of arginine uptake in oocytes and by analyses Of L-[3H]arginine uptake in mink cells. Bindings of ecotropic envelope glycoprotein gp7O to the accessible receptor sites on surfaces of mink cells expressing wild-type or mutant mCAT-1 were not significantly different in kinetics or in equilibrium affinities (i.e., K(D) approximately 3.7 X 10(-10) to 7.5 X 10(-10) M). However, when values were normalized to the same levels of mCAT-1 transporter expression, cells with wild-type glycosylated mCAT-1 had only approximately 50% as many sites for gp70 binding as cells with unglycosylated mCAT-1. Although infection with ecotropic MuLV had no effect on activity of the mink CAT-1 transporter that does not bind virus, it caused partial down-modulation of wild-type mCAT-1 and complete down-modulation of unglycosylated mutant mCAT-1. These results suggest that N-linked glycosylation causes wild-type mCAT-1 heterogeneity and that a significant proportion is inaccessible to virus. In part because only the interactive fraction of mCAT-1 can be down-modulated, infected murine cells conserve an amino acid transport capability that supports their viability.

The zinc finger of nucleocapsid protein of Friend murine leukemia virus is critical for proviral DNA synthesis in vivo

Nucleocapsid protein NCp10 of murine leukemia virus (MuLV) is encoded by the 3' domain of gag and contains a zinc finger of the form Cys-X2-Cys-X4-His-X4-Cys flanked by basic amino acids. In the course of virus assembly, NCp10 is necessary for core formation, and the zinc finger flanked by the basic residues is required for the packaging of the genomic RNA dimer. In vitro, NCp10 exhibits strong nucleic acid binding and annealing activities that appear to be critical for virus infectivity since NCp10 promotes dimerization of the viral RNA containing the E/DLS packaging-dimerization signal and annealing of replication primer tRNA(Pro) to the initiation site of reverse transcription (PBS). Recent in vitro studies have suggested that NCp10 may also play a role in proviral DNA synthesis. To investigate the function of NCp10 in proviral DNA synthesis in vivo, we developed a simple and convenient genetic packaging system consisting of two DNA constructs expressing the packaging components gag-pol and env of Friend MuLV and a Moloney MuLV-based lacZ vector with either the MuLV E+ or a rat VL30 E packaging signal. This system allowed us to examine the consequences of a set of mutations in NCp10 on a single round of recombinant virus replication. Most mutations in the N- or C-terminal domain of NCp10 do not significantly alter infectivity, while those in the zinc finger drastically impair infectivity. Analysis of the viral RNA content in virions showed that all mutations in the zinc finger decrease but do not abolish packaging of the recombinant genome. Interestingly enough, mutation of Y-28 to S (mutation Y28S) in the zinc finger results in RNA packaging at a level similar to that observed upon deletion of three prolines and three arginines in the C-terminal domain of NCp10 (mutant delta PR3). However, mutant Y28S is noninfectious while mutant delta PR3 is only threefold less infectious than the wild-type virus, which prompted us to examine the role of NCp10 protein in proviral DNA synthesis in vivo using these nucleocapsid mutants. PCR amplification was used to analyze viral DNA synthesized in newly infected cells, and results indicate that the Y28S zinc finger mutation impairs reverse transcription, thus suggesting that the nucleocapsid protein zinc finger plays a key role in proviral DNA synthesis in vivo.

Isolation of the equine granulocytic ehrlichiosis agent, Ehrlichia equi, in tick cell culture

The equine granulocytic ehrlichiosis agent, Ehrlichia equi, is closely related or identical to the human granulocytic ehrlichiosis (HGE) agent. Both are suspected of being transmitted by ticks. We have successfully isolated E. equi in a cell line, IDE8, derived from a putative vector, the tick Ixodes scapularis. Peripheral blood leukocytes from an experimentally infected horse were inoculated onto IDE8 monolayers. Cultures were incubated in a candle jar at 34 degrees C in tick cell culture medium with NaHCO3 and an organic buffer [3-(N-morpholino)-propanesulfonic acid] (MOPS). Within 2 weeks, infected cells were detected in Giemsa-stained culture samples, and the organisms subsequently spread to uninfected cells in the cultures. E. equi was passaged serially by transferring a portion of an infected culture to new cell layers every 2 to 3 weeks. The identity of the organisms was confirmed by PCR using oligonucleotide primers specific for E. equi and the HGE agent and by immunocytology. Homologous equine antibodies and human anti-HGE convalescent serum recognized E. equi grown in tick cell culture. Electron microscopy revealed electron-lucent and -dense ehrlichia-like forms developing within host cell endosomes. E. equi passaged twice in tick cell culture retained infectivity and pathogenicity for the equine host, as demonstrated by intravenous inoculation of a suspension of infected tick cells and subsequent reisolation from peripheral blood, in fulfillment of Koch's postulates. The horse developed severe clinical signs, i.e., fever, inappetence, thrombocytopenia, icterus, and limb edema, typical of granulocytic equine ehrlichiosis, within 1 week.

Functions of the 5' leader of murine leukemia virus genomic RNA in virion structure, viral replication and pathogenesis, and MLV-derived vectors

Retroviruses are a family of widespread small animal viruses that can cause a variety of neoplastic and immunosuppressive diseases. Murine leukemia viruses (MuLV) have been used as model systems to investigate virion and genomic RNA structure, viral replication and variability, and pathogenesis. Detailed knowledge of the genetic structure of MuLV and of the viral life cycle has led to the development of MuLV-derived retroviral vectors for gene transfer with potential applications in human gene therapy. In this review we have summarized the properties and functions of the 5' domain, called the leader, of MuLV genomic RNA. The 5' leader is formed of small interspersed and superimposed genetic elements involved in every step of the viral life cycle. In addition, the 3' domain of the leader encodes the N-terminal part of glycosylated forms of the GAG polyprotein, also named Gross cell surface antigen (GCSA or glycoGAG) which is essential for full spreading and pathogenic abilities of the virus in the animal. Therefore, the 5' leader of MuLV genomic RNA appears to be a very attractive model to study structure-function relationships of a small and multifunctional genetic domain in vitro, in cell culture and in the animal.

Characterization of a neurologic disease induced by a polytropic murine retrovirus: evidence for differential targeting of ecotropic and polytropic viruses in the brain

A variety of ecotropic murine leukemia viruses cause neurodegenerative disease. We describe here the clinical and histopathological features of a neurologic disease induced by a polytropic murine leukemia virus, FMCF98. Clinical disease was dominated by hyperexcitability and ataxia, and the histopathology was characterized primarily by astrocytosis and astrocytic degeneration. The viral envelope gene harbored the determinants of neurovirulence, since the chimeric virus Fr98E, which contained the envelope gene of FMCF98 on a background of the nonneurovirulent virus FB29, caused a similar disease. The disease caused by Fr98E differed from that induced by the coisogenic neurovirulent ecotropic virus FrCasE in clinical presentation, histopathology, and distribution of virus in the central nervous system. Since Fr98E contains a polytropic envelope gene and FrCasE contains an ecotropic envelope gene, these phenotypic differences appeared to be determined by envelope sequences and may reflect differences in virus receptor usage in the central nervous system.

Protective efficacy of nonneutralizing monoclonal antibodies in acute infection with murine leukemia virus

We have used an experimental retrovirus infection to study the roles played by different antibodies in resistance to both infection and disease. A molecularly cloned chimeric murine leukemia virus was used to induce acute lethal neurological disease in neonatal mice. A panel of monoclonal antibodies directed against the Gag and Env proteins was tested for protective efficacy. In vitro neutralization assays demonstrated that anti-Env antibodies gave different degrees of neutralization, while no anti-Gag neutralized the virus. In vivo experimental endpoints were onset of clinical signs and premoribund condition. As expected, different anti-Env antibodies demonstrated different degrees of protection which correlated with their neutralizing abilities. Surprisingly, anti-Gag antibodies directed against both p15 (MA protein) and p30 (CA protein) were also protective, significantly delaying the onset of disease. No protection was seen with either of two control antibodies. The protection with anti-Gag was dose related and time dependent and was also produced with Fab fragments. Treatment with anti-Gag did not prevent viremia but resulted in a slight slowing in viremia kinetics and decreased levels of virus in the central nervous systems of mice protected from disease. These data indicate that nonneutralizing antiretroviral antibodies can influence the outcome of retroviral disease. The data also suggest a functional role for cell surface expression of Gag proteins on murine leukemia virus-infected cells.

Influence of transmembrane domains on the fusogenic abilities of human and murine leukemia retrovirus envelopes

The envelopes of two highly divergent oncoviruses, human T-cell leukemia virus type 1 (HTLV-1) and Friend murine leukemia virus (F-MuLV), have distinct patterns of cellular receptor recognition, fusion, and syncytium formation. To analyze the influence of the transmembrane envelope subunit (TM) on fusogenic properties, we substituted either the entire TM or distinct domains from F-MuLV for the corresponding domains in the HTLV-1 envelope. Parental, chimeric, and truncated envelopes cloned into a eukaryotic expression vector were monitored for fusogenic potential in human, rat, and murine indicator cell lines by using a quantitative assay. This highly sensitive assay allowed us to assess the fusogenic properties and syncytium-forming abilities of the HTLV-1 envelope in murine NIH 3T3 cells. All chimeric envelopes containing extracellular sequences of the F-MuLV TM were blocked in their maturation process. Although deletions of the HTLV-1 cytoplasmic domain, alone and in combination with the membrane-spanning domain, did not prevent envelope cell surface expression, they impaired and suppressed fusogenic properties, respectively. In contrast, envelopes carrying substitutions of membrane-spanning and cytoplasmic domains were highly fusogenic. Our results indicate that these two domains in F-MuLV and HTLV-1 constitute structural entities with similar fusogenic properties. However, in the absence of a cytoplasmic domain, the F-MuLV membrane-spanning domain appeared to confer weaker fusogenic properties than the HTLV-1 membrane-spanning domain.

Isolation of new oncogenic forms of the murine c-fms gene

The c-fms gene encodes the receptor for the macrophage colony-stimulating factor, which plays a key role in the proliferation and differentiation of cells of the myelomonocytic lineage. In order to study the effects of overexpression of the macrophage colony-stimulating factor receptor in hematopoietic cells, a Harvey sarcoma virus-derived retroviral vector containing the murine c-fms cDNA was pseudotyped with Friend murine leukemia virus and inoculated into newborn DBA/2 mice. This viral complex induced monoclonal or oligoclonal leukemias with a shorter latency than that for Friend murine leukemia virus alone. Unexpectedly, 60% of the integrated fms proviruses had deletions at the 5' end of the c-fms gene. Sequence analysis of seven mutant proviruses indicated that the deletions always included the c-fms ligand binding domain and either occurred within the c-fms sequences, leaving the fms open reading frame unchanged, or joined VL30 sequences located at the 5' end of the parental retroviral vector to internal c-fms sequences, resulting in truncated fms proteins devoid of the canonical signal peptide. In contrast to all tyrosine kinase receptors transduced in retroviruses, no helper gag- or env-derived sequences were fused to the rearranged fms sequences. Viral supernatants isolated from hematopoietic tumors with viruses with deletions were able to transform NIH 3T3 cells as efficiently as parental fms virus, indicating that deletions resulted in constitutive activation of the c-fms gene. These oncogenic variants differ from those transduced in the Suzan McDonough strain of feline sarcoma viruses (L. Donner, L. A. Fedele, C. F. Garon, S. J. Anderson, and C. J. Sherr, J. Virol. 41:489-500, 1982). The high rate of c-fms rearrangement and its relevance in the occurrence of hematopoietic tumors are discussed.