Genes affecting mink cell focus-inducing (MCF) murine leukemia virus infection and spontaneous lymphoma in AKR F1 hybrids

Distribution of endogenous gammaretroviruses and variants of the Fv1 restriction gene in individual mouse strains and strain subgroups

Inbred laboratory mouse strains carry endogenous retroviruses (ERVs) classed as ecotropic, xenotropic or polytropic mouse leukemia viruses (E-, X- or P-MLVs). Some of these MLV ERVs produce infectious virus and/or contribute to the generation of intersubgroup recombinants. Analyses of selected mouse strains have linked the appearance of MLVs and virus-induced disease to the strain complement of MLV E-ERVs and to host genes that restrict MLVs, particularly Fv1. Here we screened inbred strain DNAs and genome assemblies to describe the distribution patterns of 45 MLV ERVs and Fv1 alleles in 58 classical inbred strains grouped in two ways: by common ancestry to describe ERV inheritance patterns, and by incidence of MLV-associated lymphomagenesis. Each strain carries a unique set of ERVs, and individual ERVs are present in 5–96% of the strains, often showing lineage-specific distributions. Two ERVs are alternatively present as full-length proviruses or solo long terminal repeats. High disease incidence strains carry the permissive Fv1ⁿ allele, tested strains have highly expressed E-ERVs and most have the Bxv1 X-ERV; these three features are not present together in any low-moderate disease strain. The P-ERVs previously implicated in P-MLV generation are not preferentially found in high leukemia strains, but the three Fv1 alleles that restrict inbred strain E-MLVs are found only in low-moderate leukemia strains. This dataset helps define the genetic basis of strain differences in spontaneous lymphomagenesis, describes the distribution of MLV ERVs in strains with shared ancestry, and should help annotate sequenced strain genomes for these insertionally polymorphic and functionally important proviruses.

Recombinant Origins of Pathogenic and Nonpathogenic Mouse Gammaretroviruses with Polytropic Host Range

Ecotropic, xenotropic, and polytropic mouse leukemia viruses (E-, X-, and P-MLVs) exist in mice as infectious viruses and endogenous retroviruses (ERVs) inserted into mouse chromosomes. All three MLV subgroups are linked to leukemogenesis, which involves generation of recombinants with polytropic host range. Although P-MLVs are deemed to be the proximal agents of disease induction, few biologically characterized infectious P-MLVs have been sequenced for comparative analysis. We analyzed the complete genomes of 16 naturally occurring infectious P-MLVs, 12 of which were typed for pathogenic potential. We sought to identify ERV progenitors, recombinational hot spots, and segments that are always replaced, never replaced, or linked to pathogenesis or host range. Each P-MLV has an E-MLV backbone with P- or X-ERV replacements that together cover 100% of the recombinant genomes, with different substitution patterns for X- and P-ERVs. Two segments are always replaced, both coding for envelope (Env) protein segments: the N terminus of the surface subunit and the cytoplasmic tail R peptide. Viral gag gene replacements are influenced by host restriction genes Fv1 and Apobec3 Pathogenic potential maps to the env transmembrane subunit segment encoding the N-heptad repeat (HR1). Molecular dynamics simulations identified three novel interdomain salt bridges in the lymphomagenic virus HR1 that could affect structural stability, entry or sensitivity to host immune responses. The long terminal repeats of lymphomagenic P-MLVs are differentially altered by recombinations, duplications, or mutations. This analysis of the naturally occurring, sometimes pathogenic P-MLV recombinants defines the limits and extent of intersubgroup recombination and identifies specific sequence changes linked to pathogenesis and host interactions.IMPORTANCE During virus-induced leukemogenesis, ecotropic mouse leukemia viruses (MLVs) recombine with nonecotropic endogenous retroviruses (ERVs) to produce polytropic MLVs (P-MLVs). Analysis of 16 P-MLV genomes identified two segments consistently replaced: one at the envelope N terminus that alters receptor choice and one in the R peptide at the envelope C terminus, which is removed during virus assembly. Genome-wide analysis shows that nonecotropic replacements in the progenitor ecotropic MLV genome are more extensive than previously appreciated, covering 100% of the genome; contributions from xenotropic and polytropic ERVs differentially alter the regions responsible for receptor determination or subject to APOBEC3 and Fv1 restriction. All pathogenic viruses had modifications in the regulatory elements in their long terminal repeats and differed in a helical segment of envelope involved in entry and targeted by the host immune system. Virus-induced leukemogenesis thus involves generation of complex recombinants, and specific replacements are linked to pathogenesis and host restrictions.

Cerebellum-specific and age-dependent expression of an endogenous retrovirus with intact coding potential

Background

Endogenous retroviruses (ERVs), including murine leukemia virus (MuLV) type-ERVs (MuLV-ERVs), are presumed to occupy ~10% of the mouse genome. In this study, following the identification of a full-length MuLV-ERV by in silico survey of the C57BL/6J mouse genome, its distribution in different mouse strains and expression characteristics were investigated.

Results

Application of a set of ERV mining protocols identified a MuLV-ERV locus with full coding potential on chromosome 8 (named ERV_mch8). It appears that ERV_mch8 shares the same genomic locus with a replication-incompetent MuLV-ERV, called Emv2; however, it was not confirmed due to a lack of relevant annotation and Emv2 sequence information. The ERV_mch8 sequence was more prevalent in laboratory strains compared to wild-derived strains. Among 16 different tissues of ~12 week-old female C57BL/6J mice, brain homogenate was the only tissue with evident expression of ERV_mch8. Further ERV_mch8 expression analysis in six different brain compartments and four peripheral neuronal tissues of C57BL/6J mice revealed no significant expression except for the cerebellum in which the ERV_mch8 locus' low methylation status was unique compared to the other brain compartments. The ERV_mch8 locus was found to be surrounded by genes associated with neuronal development and/or inflammation. Interestingly, cerebellum-specific ERV_mch8 expression was age-dependent with almost no expression at 2 weeks and a plateau at 6 weeks.

Conclusions

The ecotropic ERV_mch8 locus on the C57BL/6J mouse genome was relatively undermethylated in the cerebellum, and its expression was cerebellum-specific and age-dependent.

Mink epithelial cell killing by pathogenic murine leukemia viruses involves endoplasmic reticulum stress

We previously demonstrated that mink cells undergo apoptosis after MCF13 murine leukemia virus (MLV) infection. In this study, we observed that virus-infected mink epithelial cells had significantly larger amounts of steady-state levels of MCF13 MLV envelope precursor protein (gPr80(env)) than did Mus dunni fibroblasts, which are resistant to virus-induced cytopathicity. Infection of mink cells with the noncytopathic NZB-9 MLV did not result in the accumulation of gPr80(env). MCF13 MLV infection of mink cells produced low cell surface expression of envelope glycoprotein and less efficient spread of infectious virus. Western blot analysis of mink epithelial cells infected with MCF13 MLV showed an increase in GRP78/BiP, which was not observed for either mink cells infected with NZB-9 MLV or M. dunni fibroblasts infected with MCF13 MLV. MCF13 MLV infection of mink cells also resulted in a significant upregulation of CHOP/GADD153. These results indicate that the accumulation of MCF13 MLV gPr80(env) triggers endoplasmic reticulum stress, which may mediate apoptosis in mink epithelial cells.

A virus-virus interaction circumvents the virus receptor requirement for infection by pathogenic retroviruses

During ongoing C-type retrovirus infection, the probability of leukemia caused by insertional gene activation is markedly increased by the emergence of recombinant retroviruses that repeatedly infect host cells. The murine mink cell focus-inducing (MCF) viruses with this property have acquired characteristic changes in the N-terminal domain of their envelope glycoprotein that specify binding to a different receptor than the parental ecotropic virus. In this report, we show that MCF virus infection occurs through binding to this receptor (termed Syg1) and, remarkably, by a second mechanism that does not utilize the Syg1 receptor. By the latter route, the N-terminal domain of the ecotropic virus glycoprotein expressed on the cell surface in a complex with its receptor activates the fusion mechanism of the MCF virus in trans. The rate of MCF virus spread through a population of permissive human cells was increased by establishment of trans activation, indicating that Syg1 receptor-dependent and -independent pathways function in parallel. Also, trans activation shortened the interval between initial infection and onset of cell-cell fusion associated with repeated infection of the same cell. Our findings indicate that pathogenic retrovirus infection may be initiated by virus binding to cell receptors or to the virus envelope glycoprotein of other viruses expressed on the cell surface. Also, they support a broader principle: that cooperative virus-virus interactions, as well as virus-host interactions, shape the composition and properties of the retrovirus quasispecies.

Accelerated appearance of multiple B cell lymphoma types in NFS/N mice congenic for ecotropic murine leukemia viruses

Spontaneous lymphomas occur at high frequency in NFS x V+ mice, strains congenic for ecotropic murine leukemia virus (MuLV) proviral genes and expressing virus at high titer. In the present study, a total of 703 NFS x V+ lymphomas were studied by histopathology, immunophenotypic analysis, immunoglobulin heavy chain or T cell receptor beta chain rearrangements, and somatic ecotropic MuLV integrations; 90% of the lymphomas tested were of B cell lineage. Low-grade tumors included small lymphocytic, follicular, and splenic marginal zone lymphomas, while high-grade tumors comprised diffuse large-cell (centroblastic and immunoblastic types), splenic marginal zone, and lymphoblastic lymphomas. Comparison of mice of similar genetic background except for presence (NFS x V+) or absence (NFS x V-) of functional ecotropic MuLV genomes showed that NFS x V-clonal lymphomas developed at about one-half the rate of those occurring in NFS x V+ mice, and most were low-grade B cell lymphomas with extended latent periods. In NFS x V+ mice, clonal outgrowth, defined by Ig gene rearrangements, was associated with acquisition of somatic ecotropic proviral integrations, suggesting that, although generation of B cell clones can be virus independent, ecotropic virus may act to increase the rate of generation of clones and speed their evolution to lymphoma. The mechanism remains undefined, because only rare rearrangements were detected in several cellular loci previously associated with MuLV insertional mutagenesis.

A direct demonstration of recombination between an injected virus and endogenous viral sequences, resulting in the generation of mink cell focus-inducing viruses in AKR mice

We analyzed viral recombination events that occur during the preleukemic period in AKR mice. We tagged a molecular chimera between the nonleukemogenic virus Akv and the leukemogenic mink cell focus-inducing (MCF) virus MCF 247 with an amber suppressor tRNA gene, supF. We injected the supF-tagged chimeric virus that contains all of the genes of MCF 247 except the envelope gene, which in turn is derived from Akv, into newborn AKR mice to evaluate its pathogenic potential. Approximately the same percentage of animals developed leukemia with similar latent periods when injected with either the tagged or nontagged virus. DNA from tumors induced in AKR mice by the tagged chimeric virus was analyzed by Southern blotting with the supF gene as a probe. One set of tumors contained the injected supF-tagged virus. Two kinds of supF-tagged proviruses were found in a second set of tumors. One group of supF-tagged viruses had a restriction map consistent with that of the injected virus, while the other group of proviruses had restriction maps that suggested that the proviruses had acquired an MCF virus-like envelope gene by recombination with endogenous viral sequences. These results demonstrate that injected viruses recombine in vivo with endogenous viral sequences. Furthermore, the progression to leukemia was accelerated in mice that develop tumors containing proviruses with an MCF virus env gene, emphasizing the importance of the role of the MCF virus env gene product in transformation.

Susceptibility of AKXD recombinant inbred mouse strains to lymphomas

We analyzed the susceptibility of 10 AKXD recombinant inbred (RI) mouse strains to lymphomas. These strains were derived from crosses of AKR/J, a highly lymphomatous strain, and DBA/2J, a weakly lymphomatous strain. Of the 10 strains analyzed, nine showed a high incidence of lymphoma development. As with the other 13 AKXD strains analyzed previously (M. L. Mucenski, B. A. Taylor, N. A. Jenkins, and N. G. Copeland, Mol. Cell. Biol. 6:4236-4243, 1986), the mean age at onset of lymphomas and lymphoma types varied among the strains. Whereas some strains were susceptible to T-cell lymphomas, as was the AKR/J parent, other strains were susceptible to B-cell lymphomas or to a combination of T- and B-cell lymphomas. Somatic mink cell focus-forming proviruses appeared causally associated with T-cell lymphomas, whereas somatic ecotropic proviruses appeared causally associated with B-cell lymphomas. Mice with T-cell lymphomas died significantly earlier than mice with other lymphoma types (stem, pre-B, or B cell and myeloid). The numbers of effective loci influencing the mean age at onset of lymphomas, the presence or absence of mink cell focus-forming viruses in tumors, and the frequency of T-cell lymphomas were estimated to be 3.9, 1.8, and 2.7, respectively. Tests of association with marker loci already typed in the AKXD RI strains suggested that two loci, Rmcf and Pmv-25 (or a locus linked to Pmv-25), influence all three trait variables. Finally, D21S16h, a marker locus on distal chromosome 16, showed 50% probability of linkage to a locus that influences the mean age at onset of lymphomas. Additional studies in combination with classical genetic crosses should be helpful in confirming these linkages and in identifying other loci influencing tumor susceptibility in AKXD RI strains.

Differential disease restriction of Moloney and Friend murine leukemia viruses by the mouse Rmcf gene is governed by the viral long terminal repeat

Neonatal CxD2 (Rmcfr) and Balb/c (Rmcfs) mice inoculated with Moloney murine leukemia virus (M-MuLV) exhibited approximately equivalent time course and pathology for disease. CxD2 mice showed only slightly reduced presence of Moloney mink cell focus-forming virus (M-MCF) provirus as seen by Southern blot analysis compared to Balb/c mice. This lack of restriction for disease and spread of MCF was in sharp contrast to that seen for CxD2 mice inoculated with Friend murine leukemia virus (F-MuLV), where incidence of disease and propagation of MCFs were severely restricted, as previously reported. Inoculation of CxD2 mice with FM-MuLV, a recombinant F-MuLV virus containing M-MuLV LTR sequences (U3 and R), resulted in T cell disease of time course equal to that seen in Balb/c mice; there also was little restriction for propagation of MCFs. This indicated that presence of the M-MuLV long terminal repeat (LTR) was sufficient for propagation of MCFs in CxD2 mice. Differing restriction for F-MuLV vs. M-MuLV in CxD2 mice was explained on the basis of different "MCF propagator cells" for the two viruses. It was suggested that cells propagating F-MCF (e.g., erythroid progenitors) are blocked by endogenous MCF-like gp70env protein, whereas cells propagating M-MCF (e.g., lymphoid) do not express this protein on their surface. F-MuLV disease in CxD2 mice was greatly accelerated when neonates were inoculated with a F-MuLV/F-MCF pseudotypic mixture. However, F-MCF provirus was not detectable or only barely detectable in F-MuLV/F-MCF-induced tumors, suggesting that F-MCF acted indirectly in induction of these tumors.

Retrovirus-induced murine acquired immunodeficiency syndrome: natural history of infection and differing susceptibility of inbred mouse strains

C57BL mice (Fv-1b) develop a severe immunodeficiency disease following inoculation as adults with LP-BM5 murine leukemia virus (MuLV), a derivative of Duplan-Laterjet virus which contains B-tropic ecotropic and mink cell focus-inducing MuLVs and a putative defective genome which may be the proximal cause of disease. The stages of development of this disease were defined for C57BL mice on the basis of lymphadenopathy and splenomegaly; histopathological changes consistent with B-cell activation; and alterations in expression of cell surface antigens affected by proliferation of T cells, B cells, and macrophages. By using this disease profile as a standard, the response of adult mice of various inbred strains and selected F1 hybrids was compared. We show that although the strains which are highly sensitive are of the Fv-1b genotype (i.e., permissive for B-tropic MuLVs), certain Fv-1b strains, e.g., BALB/c and A/J, are resistant to murine acquired immunodeficiency syndrome, whereas certain Fv-1n strains (permissive for N-tropic MuLVs but restrictive for B-tropic MuLVs), notably P/N, BDP, and AKR, show moderate sensitivity and (C57BL/6 x CBA/N)F1 mice (Fv-1n/b and thus dually restrictive) are of relatively high susceptibility. The results of virus recovery tests suggest that apparently anomalous sensitivity, based on predicted Fv-1 restriction, may reflect MuLV induction and/or mutation to provide a helper virus for which the host is permissive.

A population of murine hematopoietic progenitors expresses an endogenous retroviral gp70 linked to the Rmcf gene and associated with resistance to erythroleukemia

Multiple copies of retroviral sequences are stably integrated in the genomes of many higher organisms, and are thus transmitted vertically to offspring via the germline (1). Most of these heritable viral genes are not expressed, and expression, when observed, is commonly limited to envelope (env) genes as demonstrated by the presence of cell surface and serum envelope glycoprotein (gp70) in mice. Studies of the mouse have shown that certain tissues such as the reproductive tract and lymphoid organs are common sites for the expression of endogenous env genes, suggesting that the transcription of at least some endogenous sequences is tissue specific. The transcription of endogenous viral genes is regulated by both cis and trans mechanisms (2-5) and their expression can be temporally linked to differentiation and development (6-8). The consequences to the host of endogenous retroviral genes are varied. At one extreme, expression of endogenous virus can result in the development of leukemia and death. Another potentially detrimental effect is that of insertional mutagenesis, seen when the integration of retroviral sequences interrupts the functioning of a cellular gene (9, 10). However, it is now clear that expression of endogenous retroviral genes may also have a beneficial effect for the host: namely, mediating resistance to retroviral leukemias as has been demonstrated for the Fv-4 gene in mice (11) and some ea loci in chickens (12). This form of resistance is due to the blockage of cellular viral receptors by the expression of envelope glycoprotein on the cell surface. The Rmcf locus of the mouse is another resistance gene that may exert its effect by the expression of an endogenous env gene. A summary of our current state of knowledge concerning the Rmcf gene is shown in Table I. The Rmcf gene was originally described when it was observed that fibroblast cell cultures derived from certain strains of mice restricted the replication of recombinant mink cell focus-forming(MCF)1 viruses (13). As detailed in Table I, DBA/2 mice are the prototypic strain exhibiting the Rmcf resistance (Rmcf(r)) phenotype. Cell cultures from other strains, such as C57BL/6 and IRW, are permissive for MCF viral replication and are termed Rmcf sensitive (Rmcf(s)). Previously, we described two allelic forms of an endogenous env gene, whose expression is linked to the Rmcf gene (14). Cell cultures from Rmcf(r) mice express gp70 related to that of MCF viruses, whereas cultures derived from Rmcf(s) mice either express no gp70 (IRW) or express an endogenous xenotropic gp70 (C57BL/6). These two gp70 alleles are detectable by type-specific mAbs.

Ecotropic virus involvement in spontaneous B-cell lymphomas of CWD/LeAgl mice

The inbred mouse strain CWD/LeAgl, which has a high incidence of spontaneous B-cell lymphomas, expresses both ecotropic MuLV and MCF viruses. Studies indicated that the MCF viruses expressed in CWD tumors were characteristic of nononcogenic MCF viruses and that ecotropic MuLV may be the etiological agent in spontaneous B-cell lymphomagenesis. Somatically acquired proviruses of approximately the same size were detected in several tumor DNAs suggesting that integration of proviral sequences into specific regions of the mouse genome may be an important step in lymphomagenesis of this strain.

The endogenous mink cell focus-forming (MCF) gp70 linked to the Rmcf gene restricts MCF virus replication in vivo and provides partial resistance to erythroleukemia induced by Friend murine leukemia virus

The Rmcf locus restricts the in vitro replication of mink cell focus-forming (MCF) viruses in cell cultures derived from mice carrying the resistance allele. Previously we reported that in cell cultures from first backcross progeny, this Rmcf-linked restriction segregates with the expression of an endogenous retroviral gp70 serologically related to that of MCF viruses. The current report details the results of genetic studies designed to examine the possible association of this endogenous gp70 with resistance of mice to Friend murine leukemia virus (F-MuLV)-induced erythroleukemia. This env gene segregates as a single dominant trait in (DBA/2 X IRW) X IRW progeny, in which the expression of the gene can be detected by serological techniques. Results indicated that the gp70- progeny developed leukemia at the same rate as the susceptible IRW parent, whereas the tempo of disease among the gp70+ progeny was significantly slower. However, the resistance mediated by this gene was only partial, since most of the gp70+ offspring eventually developed erythroleukemia when followed for 6 mo. This endogenous gp70 also segregated with a restriction to the expression of recombinant MCF viruses after infection with F-MuLV. Since in this study all unlinked genes segregated independently, this is direct evidence that MCF viruses participate in the induction of erythroleukemia.

AKXD recombinant inbred strains: models for studying the molecular genetic basis of murine lymphomas

We analyzed the lymphoma susceptibility of 13 AKXD recombinant inbred mouse strains derived from AKR/J, a highly lymphomatous strain, and DBA/2J, a weakly lymphomatous strain. Of the 13 strains used, 12 showed a high incidence of lymphoma development. However, the average age at onset of lymphoma varied considerably among the different AKXD strains, suggesting that they have segregated several loci that affect lymphoma susceptibility. A relatively unambiguous classification of lymphomas was made possible by using histopathology in addition to detailed molecular characterization of rearrangements in immunoglobulin heavy and kappa light genes and in T-cell receptor beta-chain genes. Among the 12 highly lymphomatous strains, only 2 were identified that, like the parental AKR/J strain, died primarily of T-cell lymphomas. Three strains died primarily of B-cell lymphomas, and one strain primarily of myeloid lymphomas. Six strains were susceptible to both T-cell and B-cell lymphomas. Thus, these strains have segregated genes that affect both lymphoma susceptibility and lymphoma type and should prove to be useful models for studying the molecular genetic basis of murine lymphomas.

AKXD recombinant inbred strains: models for studying the molecular genetic basis of murine lymphomas

We analyzed the lymphoma susceptibility of 13 AKXD recombinant inbred mouse strains derived from AKR/J, a highly lymphomatous strain, and DBA/2J, a weakly lymphomatous strain. Of the 13 strains used, 12 showed a high incidence of lymphoma development. However, the average age at onset of lymphoma varied considerably among the different AKXD strains, suggesting that they have segregated several loci that affect lymphoma susceptibility. A relatively unambiguous classification of lymphomas was made possible by using histopathology in addition to detailed molecular characterization of rearrangements in immunoglobulin heavy and kappa light genes and in T-cell receptor beta-chain genes. Among the 12 highly lymphomatous strains, only 2 were identified that, like the parental AKR/J strain, died primarily of T-cell lymphomas. Three strains died primarily of B-cell lymphomas, and one strain primarily of myeloid lymphomas. Six strains were susceptible to both T-cell and B-cell lymphomas. Thus, these strains have segregated genes that affect both lymphoma susceptibility and lymphoma type and should prove to be useful models for studying the molecular genetic basis of murine lymphomas.

Genetic and epigenetic factors that influence the occurrence of spontaneous lymphoid tumors in crosses of mice of high- and low-incidence strains

AKR mice, which spontaneously develop greater than 90% incidence of lymphocytic leukemia (LL), crossed with SJL mice, which show greater than 80% incidence of Hodgkin's-like reticulum-cell sarcoma (RCS), produced F1 progeny showing incidences of 30% LL and 0% RCS. Thus, each strain possesses one or more dominant genes capable of interfering with the emergence of the tumor type typical of the other strain. Although mice of reciprocal F1 crosses showed a profound difference in expression of endogenous ecotropic murine leukemia virus (E-MuLV) due to a maternal resistance factor transmitted by SJL females but not males, the two populations did not differ detectably in LL incidence. Like AKR mice, mice of 5 other strains studied (C58, DBA/2, PL, RF and ST/b) possessed one or more genes conferring resistance to RCS in F1 crosses with SJL. Analysis of LL incidences in F1 generations of all possible crosses among these 7 strains revealed 4 different categories of strains with respect to susceptibility/resistance to LL; only ST/b mice, which show no significant incidence of spontaneous LL, lacked genes that could suppress the disease in crosses with high- or moderate-incidence strains. SJL mice treated topically with 3-methylcholanthrene (MCA) developed a 50% incidence of LL, mostly before one year of age; treated mice surviving after one year of age developed a high incidence of RCS.

Multiparameter analyses of spontaneous nonthymic lymphomas occurring in NFS/N mice congenic for ecotropic murine leukemia viruses

Mouse strains congenic for ecotropic retrovirus genes have a much higher frequency of spontaneous lymphomas than the background NFS/N strain. In this study, most of these lymphomas have been identified as B-cell in origin by morphologic features, identification of immunoglobulin class, and cell-surface antigens. The classification suggested by Pattengale and Taylor proved to be applicable to the lymphomas studied. Most were of large follicular center cells and are considered typical of the type formerly designated as "reticulum cell sarcoma, type B." Many lymphomas contained a large proportion of nonneoplastic cells which partially obscured their neoplastic component. The role of ecotropic murine leukemia viruses as etiologic agents for B-cell lymphomas remains equivocal. However, because the only difference between the NFS/N and congenic mice is the expression of viruses in the latter, it appears that these viruses are somehow involved in induction of B-cell lymphomas.

Susceptibility of BALB/c mice carrying various DBA/2 genes to development of Friend murine leukemia virus-induced erythroleukemia

Using a series of BALB/c mice congenic for various DBA/2 genes, we were able to establish that DBA/2 mice carry a gene on chromosome 5, at or near the Rmcfr locus, that plays a major role in resistance to early erythroleukemia induced by injection of Friend murine leukemia virus (F-MuLV) into newborn mice. The fact that this gene controls the replication of mink cell focus-inducing (MCF) viruses strengthens the case for these viruses playing a crucial role in the development of erythroleukemia, since failure to replicate MCF viruses results in resistance to early erythroleukemia. The expression of the Rmcfr gene is correlated with the constitutive expression of an MCF virus-related envelope glycoprotein that apparently blocks the receptor for MCF viruses, preventing their spread. Thus, the Rmcfr gene is either a structural gene for this unique protein, which can block the receptor for MCF viruses, or is a regulatory gene that controls expression of such a structural gene. Although the Rmcfr gene is clearly involved in resistance to the early erythroleukemia induced by F-MuLV, it appears to have no effect on the late myeloid, lymphoid or erythroid diseases that appear in DBA/2 and other strains of mice after injection of F-MuLV, consistent with data indicating that replication of MCF viruses is not required for the development of these late diseases. Our studies with congenic and backcross mice also indicate that, in addition to the Rmcfr gene, other genes of DBA/2 origin may contribute to resistance to F-MuLV-induced early erythroleukemia by mechanisms other than blocking the replication of MCF viruses.

At least four viral genes contribute to the leukemogenicity of murine retrovirus MCF 247 in AKR mice

Nucleotide sequences encoding gp70, Prp15E, and the U3 region of the long terminal repeat (LTR) distinguish mink cell focus-forming (MCF) retroviruses that can induce leukemia in AKR mice from closely related MCF and ecotropic murine retroviruses that are nonleukemogenic in all inbred mouse strains tested (Lung et al., Cold Spring Harbor Symp. Quant. Biol. 44:1269-1274, 1979; Lung et al., J. Virol. 45:275-290, 1983). We used a set of recombinants constructed in vitro from molecular clones of leukemogenic MCF 247 and nonleukemogenic ecotropic Akv to separate and thereby directly test the role of these genetic elements in disease induction. Leukemogenicity tests of recombinants in AKR mice show that introduction of fragments containing either an MCF LTR or MCF gp70 coding sequences can confer only a very low incidence of disease induction on Akv virus, whereas an MCF type Prp15E alone is completely ineffective. Recombinants with an MCF 247 LTR in combination with MCF Prp15E are moderately oncogenic, whereas those with an MCF 247 LTR plus MCF gp70 coding segment are quite highly leukemogenic. Mice infected with the latter virus show a substantial increase in latent period of disease induction relative to MCF 247; this delay can be reduced when Prp15E, and hence the entire 3' half of the genome, is from MCF 247. Surprisingly, sequences in the 5' half of the genome can also contribute to disease induction. We found a good correlation between oncogenicity and recovery of MCF viruses from thymocytes of injected mice, with early recovery and high titers of MCF in the thymus being correlated with high oncogenicity. This correlation held for recombinants with either an MCF or ecotropic type gp70. Together, these results (i) demonstrate that at least four genes contribute to the oncogenicity of MCF viruses in AKR mice and (ii) suggest that recombinants with only some of the necessary MCF type genes induce leukemia because they recombine to generate complete MCF genomes. Although neither Akv nor MCF 247 is leukemogenic in NFS mice, recombinant viruses whose gp70 gene was derived from Akv but whose LTRs were derived from MCF 247 induced a low incidence of leukemia in this mouse strain.

Expression of murine leukemia viruses in B-cell lymphomas of CWD/Agl mice

The inbred mouse strain CWD/Agl has a high incidence of spontaneous B-cell lymphomas characterized by gross splenomegaly and lymph node enlargement. The endogenous ecotropic retrovirus of CWD/Agl mice is expressed in the spleen within the first 2 weeks of age and in the thymus by 1 month of age. Endogenous xenotropic virus is expressed in the spleen and bone marrow of the earliest age group examined (4 months). Restriction enzyme analysis of DNA extracted from tumorous tissues suggests that mink cell focus-forming viruses are not required for B-cell lymphomagenesis in CWD/Agl mice. CWD/Agl mice provide an important new experimental model for the study of B-cell lymphoma.

Differences in lymphomagenic properties of AKR mouse retroviruses

Long-term studies on lymphomagenicity of several AKR mouse retroviruses have shown that the biologically cloned ecotropic SL3-3c virus is the most lymphomagenic of all viruses tested. This fact was demonstrated by lymphomagenicity in five mouse strains SJL, C3Hf/Bi, C3H/HeJ, CBA/H, and NFS, and lymphoma acceleration in AKR mice. The incidence was higher and latent periods shorter than that found with the other retroviruses tested (SL3-1c, SL3-2c, MCFc, and GMuLVc). In addition, it was the only retrovirus found to be highly oncogenic in the C3H/HeJ and CBA/H strains. Lack of lymphomagenicity of MCFc in CBA/H strain was shown to be due to a block in viral replication. Addition of nononcogenic Akv ecotropic virus did not affect this lack of oncogenicity. The lymphomas developing in CBA/H and SJL mice after neonatal inoculation of SL3-3c virus only produced lymphomagenic ecotropic virus. Thus, SL3-3c lymphomagenesis is most likely due solely to the action of that virus. These studies indicate that pure ecotropic AKR viruses can be highly leukemogenic.

Role of mink cell focus-inducing virus in leukemias induced by Friend ecotropic virus

Recombinant viruses have been implicated in the pathogenesis of murine leukemias induced by a variety of long-latency retroviruses. Neonatal mice of several strains were inoculated with Friend ecotropic virus (F-Eco) and analyzed for the presence of mink cell focus-inducing (MCF) virus or DNA restriction enzyme fragments which were specific for Friend MCF virus (F-MCF). MCF virus was detected within 2 weeks of inoculation in NFS /N mice and at about 2 months after inoculation in BALB/c mice. Both of these strains developed erythroblastosis after inoculation with F-Eco. In contrast, MCF virus was not detected in F-Eco-inoculated C57BL mice. These mice were resistant to erythroblastosis but developed lymphoma or myelogenous leukemia or both at about 5 months after inoculation. Thus, although MCF viruses were associated with F-Eco erythroblastosis in NFS /N and BALB/c mice, they were not necessary for F-Eco-induced lymphoid or myeloid leukemias in C57BL mice. To investigate the association between resistance to erythroblastosis and absence of MCF virus, C57BL mice were inoculated with pseudotypic mixtures of F-Eco plus F-MCF; MCF virus replicated well in these mice, but the mice remained resistant to erythroblastosis. Furthermore, in genetic crosses between C57BL and NFS /N or BALB/c, some mice inherited resistance to F-Eco erythroblastosis without inheriting the C57BL resistance to the generation of MCF viruses. These results indicate that C57BL mice carry a gene for resistance to F-Eco erythroblastosis which is distinct from the C57BL genes which interfere with the generation of MCF viruses.