Leukemogenic activity of thymotropic, ecotropic, and xenotropic radiation leukemia virus isolates

Mutants of murine leukemia viruses and retroviral replication

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Generation of mink cell focus-forming viruses by Friend murine leukemia virus: recombination with specific endogenous proviral sequences

A family of recombinant mink cell focus-forming viruses (MCF) was derived by inoculation of NFS mice with a Friend murine leukemia virus, and their genomes were analyzed by RNase T1-resistant oligonucleotide fingerprinting. The viruses were obtained from the thymuses and spleens of preleukemic and leukemic animals and were evaluated for dualtropism and oncogenicity. All these isolates induced cytopathic foci on mink cells but could be classified into two groups based on their relative infectivities for SC-1 (mouse) or mink (ATCC CCL64) cells. One group of Friend MCFs (F-MCFs) (group I) exhibited approximately equal infectivities for SC-1 and mink cells, whereas a second group (group II) infected mink cells 1,000- to 10,000-fold more efficiently than SC-1 cells. Structural analyses of the F-MCFs revealed that group I and group II viruses correlated with recombination of Friend murine leukemia virus with two distinct, but closely related, endogenous NFS proviral sequences. No correlation was found between the type of F-MCF and the tissue of origin or the disease state of the animal. Furthermore, none of the F-MCF isolates were found to be oncogenic in NFS/N or AKR/J mice. F-MCFs of both groups underwent extensive substitution of ecotropic sequences, involving much of the gag and env genes of group I F-MCFs and most of the gag, pol, and env genes of group II F-MCFs. All F-MCF isolates retained the 3' terminal U3 region of Friend murine leukemia virus. Comparison of the RNAs of the F-MCFs with RNAs of MCFs derived from NFS.Akv-1 or NFS.Akv-2 mice indicated that the F-MCFs were derived from NFS proviral sequences which are distinct from the sequences contained in NFS.Akv MCF isolates. This result suggested that recombination with particular endogenous proviral sequences to generate MCFs may be highly specific for a given murine leukemia virus.

Involvement of peritoneal macrophages and spleen stromal cells in X-irradiation-induced reticulum cell neoplasms in C57BL/6 mice

Some correlation was observed between the occurrence of FA-positive PE-MO and spleen stromal cells (removed from X-irradiated RCN-bearing old-adult B6 mice) and the generation of RCN. No significant correlation was found between the viral content of lymphoid organs from the same mice and the occurrence of RCN. The main viral particle detected in lymphoid organs from radiation-induced RCN-bearing mice was the xenotropic virus. Ecotropic viruses were detected in a few spleens and Payer patches from such mice. These ecotropic viruses showed very poor lymphomagenic activity and required 400R X-ray as a cofactor. No dualtropic viruses were detected. However, inoculation of ecotropic (SFA2) helper virus to X-irradiated old-adult B6 mice, resulted in an efficient rescue of lymphomagenic viruses, enriched with phenotypically mixed, dualtropic viruses. Some of these DT viral preparations were cloned and seemed to consist mainly of xenotropic sequences. Thus, inoculation of helper viruses influenced the generation and selection of DT viruses. Such viral preparations, enriched with DT viruses, had a better lymphomagenic activity compared to endogenous ecotropic viruses, isolated from radiation-induced RCN-bearing mice. Indirect evidence suggested the involvement of a defective (xenotropic and possibly adjacent cellular genes) particle in lymphoma induction. To conclude, a possible mechanism for the development of radiation-induced RCN is suggested, emphasizing the role of MO in such a process.

Expression of murine leukemia viruses in the highly lymphomatous BXH-2 recombinant inbred mouse strain

Among 12 recombinant inbred strains of mice derived from crossing two strains, C57BL/6J and C3H/HeJ, which have a low incidence of neoplastic disease, one strain (BXH-2) has been found to have a high incidence of lymphoma, of non-T-cell origin, at an early age. The BXH-2 strain carries the Fv-1b allele and spontaneously expresses a B-tropic murine leukemia virus beginning at as early as 10 days of gestation and continuing throughout their life. No significant differences in ecotropic virus titers were observed at any age tested (16 to 17 days of gestation through 7 months), whereas xenotropic virus was first detected in lymphoid tissues of 2-month-old mice and virus titers increased with age. Dual tropic virus(es), which induced cytopathic changes on mink lung cells, was isolated from BXH-2 lymphomatous tissues. Unlike AKR mink lung focus-forming virus (N-tropic recombinant), BXH-2 dual tropic virus is B tropic and induces cytopathic changes in mouse fibroblast cultures as well. The BXH-2 mouse provides a model system for studying the role of replication-competent viruses in spontaneously occurring leukemias of non-T-cell lineage and neurological disease.

Leukemogenicity and cell transformation mechanisms in vitro by Gross murine leukemia virus: analysis of virus subpopulations

The leukemogenic activity of Gross murine leukemia virus adapted to rats was tested in W/Fu rats and NIH/Swiss mice. All animals infected with this virus developed thymic and nonthymic T-cell leukemia with a short latency period. It was observed that cell-free extracts from thymic lymphoma tissue of mice and rats, induced by either Gross murine leukemia virus or Gross murine leukemia virus adapted to rats, consisted of both small-plaque-forming and large-plaque-forming viruses, as determined by the XC plaque test. MCF-type virus was found in these virus complexes. Transformed cell foci were induced in SC-1 cell layers by double infection of the cloned MCF-type virus and an ecotropic virus. SC-1 cells containing transformed cell foci were shown to be tumorigenic upon inoculation into nude mice. The formation of transformed cell foci in mink lung cells was also observed after double infection with the cloned MCF-type virus and a xenotropic virus. The possible mechanism of leukemogenesis by endogenous viruses is discussed.

Abrogation of radiation leukemia virus-induced lymphomagenesis by antisera to thymotropic but not to ecotropic or dual-tropic viruses

Leukemia induction by culture-grown thymotropic radiation leukemia virus or by tumor-derived virus present in cell-free tumor extracts was abrogated by incubation of either virus with anti-thymotropoc virus serum, but not by antiserum raised against ecotropic or dual-tropic (mink cell focus-inducing type) viruses that were isolated from radiation leukemia virus-induced thymic leukemias. Thus, virus similar or identical to the cultured thymotropic leukemogenic species may also be the major biologically active principle in tumor-derived extracts, even though the latter also contain viruses of the dual-tropic, mink cell focus-inducing type class.

Frequent isolation of ecotropic murine leukemia virus after x-ray irradiation of C57BL/6 mice and establishment of producer lymphoid cell lines from radiation-induced lymphomas

Fractionated whole-body X irradiation of C57BL/Ka mice leads to the development of thymic leukemia in 90% of the treated animals at an average age of 6 months. Using a sensitive high-density cocultivation procedure, we were able to demonstrate the presence of ecotropic murine leukemia virus (MuLV) from 1 month post-irradiation up to leukemia development. These viruses are not specific to any one particular organ, but can be found in at least two of the three lymphoreticular tissues studied, namely, spleen, thymus, and bone marrow. Host range studies on the isolated viruses showed that both N- and B-tropic MuLV could be isolated early after irradiation. However, as mice reached an age where leukemias develop, only the B-tropic MuLV could be recovered. We have established cell lines from primary radiation-induced tumors that are being maintained in continuous culture: except one cell line, all are virus producers. The results clearly indicate that X irradiation induces ecotropic MuLV in C57BL/Ka mice and suggest that B-tropic MuLV might be involved in the disease process.

Cell-surface antigens associated with dualtropic and thymotropic murine leukemia viruses inducing thymic and nonthymic lymphomas

Unique type-specific antigens were detected on cells infected with dualtropic and thymotropic viruses isolated and x-ray-induced T cell- and B cell-malignant lymphomas of C57BL/6 mice. These antigens were defined by membrane fluorescence with antisera made in rabbits against rabbit cells chronically infected with cloned virus. The antisera were qualitatively absorbed with a group of cells chronically infected with related dualtropic, ecotropic, and xenotropic viruses. The absorbed antisera detected type-specific, virus-related cell-surface antigens that were unique for different dualtropic virus isolates. The unabsorbed sera also reacted with antigens found specifically on ecotropic and xenotropic virus-infected cells. These findings support the contention that T cell lymphoma (TCL)-inducing and B cell lymphoma (BCL)-inducing viruses isolated from x-irradiated C57BL/6 mice are env gene recombinants in which ecotropic gene sequences have been substituted by xenotropic sequences. We found that unique antigenicities are associated with each TCL-inducing and BCL-inducing dualtropic virus, and that the thymotropic TCL-inducing virus isolates (e.g., 136.5 adn 136.7 viruses) represent a separate serologic group, different from the dualtropic TCL-inducing viruses. By using a series of absorbed antisera in microimmunofluorescence tests we could perform serologic virus mapping of dualtropic clones isolated by us or by others and relate them serologically to previously isolated clones. These virus mapping experiments indicated that many serologically different recombinant viruses can be isolated from C57BL/6 mice. It is suggested that many distinct recombinant viruses may exist in lymphomagenic C57BL/6 mice, some of which are associated with specific lymphoma induction.

Absence of ecotropic or recombinant murine leukaemia virus in preleukaemic and leukaemic X-irradiated NZB mice

NZB mice X-irradiated with a single dose of 630 R when they were 1-month old developed a high incidence of histologically defined lymphocytic leukaemia 8--25 weeks later. We have screened for murine leukaemia viruses (MuLV) in the lymphoid tissues of 8 of these leukaemic mice, and in 8 "preleukaemic", apparently healthy NZBs killed 1 month post irradiation. Xenotropic, but not ecotropic or recombinant MuLV, was detected by in vitro co-cultivation of bone marrow, spleen and thymus with selectively permissive cell lines, followed by the immunofluorescence test for MuLV gs antigen, and the XC test. Our results are not consistent, therefore, with the concept that the factor causing the leukaemias was an oncogenic virus activated by X-irradiation.

In vitro selection of high-infectious, leukemogenic virus from low-infectious, non-leukemogenic type C virus from a malignant ST/a mouse cell line

Low-infectious, nontransforming type C virus was isolated from an in vitro spontaneously transformed ST/a mouse cell line, ST-L1. The virus released by ST-L1 cells was NB-tropic and XC(-). It gave rise to very small peroxidase antibody plaques (PAP) in cultures which initially were nonproducing. Sodium dodecyl sulfate (SDS)-polyacrylamide gels of the structural proteins of the ST-L1 virus showed an envelope glycoprotein with an apparent mass of 65 kilodaltons (kdal). The mouse cells SC-1, BALB/3T3, and NIH/3T3 could be productively infected with cell-free supernatants from the ST-L1 cell line; however, virus was detected in supernatant fluids only after two to four subcultures of the infected cells. The virus thus produced was XC(+) and a large plaque former. The virus released from infected SC-1 cells was N-tropic, whereas the viruses from infected NIH/3T3 and BALB/3T3 cells were NB-tropic. The structural proteins of the N- and NB-tropic viruses could be distinguished on SDS polyacrylamide gels, the major dissimilarity being a difference in the mobility of the p30. All these viruses had an envelope glycoprotein with an apparent mass of 70 kdal. The infectivity of the viruses, measured as PAP per nanogram of p30, was 30- to 60-fold lower for the virus released from the ST-L1 cell line than that of the viruses after passage in SC-1, NIH/3T3, and BALB/3T3 cells. None of the viruses could infect rabbit or mink cells. Inoculation of the viruses into newborn mice showed that the ST-L1 virus was non-leukemogenic, whereas the NB-tropic virus selected from this after passage in BALB/3T3 or NIH/3T3 cells was highly leukemogenic. Viruses isolated from leukemic animals were indistinguishable with respect to host range and protein mobilities in SDS gels from the ones with which the mice were inoculated. Although the SC-1-selected virus was highly infectious in vitro, it was only weakly, if at all, leukemogenic.

Azathioprine-induced lymphocytic neoplasms of NZB mice lack ecotropic murine leukaemia virus

NZB mice injected intramuscularly throughout a 6-month period with the immunosuppressant azathioprine (Imuran) developed lymphocytic lymphomas 6--7 months after treatment was initiated. These malignancies were quite distinct from the reticulum-cell neoplasia which occurs spontaneously in the strain, and were readily transplantable to NZB or histocompatible BALB/c recipients. Xenotropic, but not ecotropic murine leukaemia virus (MuLV) was detected in leukaemic tissues of some donor and recipient NZBs when tested in vitro by co-cultivation with permissive cell lines, genome rescue, XC and viral polymerase assays. Virus filtrates prepared from donor leukaemic tissues were non-pathogenic when injected into newborn C3H mice. These results are evidence against a mandatory ecotropic MuLV genome in lymphocytic neoplasia.