Resistance genes to murine leukemia in the I immune response gene region of the H-2 complex.

A resistance locus to leukemogenesis in mice by A-RadLV (a variant of the radiation leukemia virus) is described. This locus, Rrv-1, was mapped to subregions I-A, I-B, and I-J of the H-2 complex. It is suggested that Rrv-1 may be in complementation with a second locus to the right of it, between Rrv-1 and H-2D. This localization and the complementation of the two loci for resistance are characteristics similar to Ir genes, and indicate a possible relationship between the genetic regulation of immune responsiveness and susceptibility to leukemia.

High incidence of B cell lymphomas derived from thymectomized AKR mice expressing TL.4 antigen.

AKR mice, 6-12 mo after birth, display a high incidence of spontaneous T cell lymphomas that can be prevented by thymus removal at the age of 1-3 mo. We report here the presence of dormant preleukemic cells among bone marrow cells of 8-12-mo-old AKR mice that have been thymectomized when 40-60 d old. Transplantation of bone marrow cells from these thymectomized AKR donors into syngeneic or hybrid (AKR X DBA/2)F1 intact or thymectomized recipients resulted in lymphoma development of AKR origin in 80-100% of the recipients. Analysis, by flow microfluorometry, of the antigenic cell surface phenotypes of the developing lymphomas revealed that all tumors were B cell lymphomas, since the cells stained with class-specific anti-IgM reagents and other reagents specific for B cells (RA3-2C2, RA3-6B2, anti-I-A, and anti-Fc receptor), and were Thy-1-. All these B cell tumors also expressed two T cell differentiation antigens, TL.4, found exclusively on T cell lymphomas, and Lyt-1 antigen, previously shown (11) to be expressed on some B cell neoplasms. The surface markers mu, I-A, RA3-2C2, and TL.4 identified by immunofluorescence, were shown to be integral membrane components synthesized by the tumor cells, rather than passively acquired proteins.

Spontaneous and induced preleukemia cells in C57BL/6 mice:brief communication.

A transplantation bioassay method was used to verify the presence of preleukemia cells in C57BL/6 mice shortly after leukemogenic treatment or in relation to age increase. Preleukemia cells were identified mainly among bone marrow cells of old C57BL/6 mice or within 10 to 30 days after leukemogenic treatment of young mice with radiation-induced leukemia virus variants, fractionated doses of irradiation, or 7,12-dimethylbenz[a]anthracene (DMBA), although the overt disease did not occur until many months later. Mice could carry preleukemia cells without necessarily developing overt leukemia. Since the leukemogenic agents used in the present studies induced T-leukemias, the role of the thymus in the induction of preleukemia cells was tested. Thymectomy affected viral transformation but did not diminish the number of preleukemia cells induced by DMBA or X-ray.

Studies on leukemia development in the SJL/J strain of mice.

Spontaneous reticulum cell neoplasms developed in 70-80% of intact male and female SJL/J mice at a mean age of 380 days. The neoplasms had a basic histological pattern of a multicellular type-B reticulum cell neoplasm, as designated by Dunn. The tumors were transplanted in isogenic mice. Serial passages of cell suspensions of these tumors were carried on through at least 4 tumor-inducing generations, and the growing transplanted tumors maintained the structure of a reticulum cell neoplasm similar to the original tumor. A histological survey of spontaneous lesions in SJL/J mice revealed different characteristics of reticulum cell neoplasms, some similar to Hodgkin's lesions. Five or ten feedings of 7,12-dimethylbenz-[alpha]anthracene (DMBA) in polyethylene glycol 400 (1 mg DMBA per feeding) to SJL/J mice induced lymphosarcomas in 74-83% of the mice at a mean age of 157-188 days. A single feeding of DMBA induced lymphosarcomas in only 27% of the mice at a mean age of 246 days. These DMBA-induced lymphatic leukemias do not appear to depend on the thymus for development--a 60% incidence of lymphosarcomas was obtained in adult thymectomized mice treated with DMBA, and 27% of the thymectomized DMBA-treated mice developed myeloid leukemia at a mean age of 180 days. Urethan was only slightly leukemogenic in SJL/J mice.

Enhancement and retardation of spontaneous reticulum cell neoplasm development in SJL/J mice.

The enhancement or delay in the appearance of spontaneous reticulum cell neoplasms (RCN-B) in SJL/J mice was tested. Immunosuppressive treatment with antithymocyte serum and prolonged antigenic stimulation by repeated injections of sheep erythrocytes, leukemogenic cell-free centrifugates, and multichain synthetic polypeptides had no effect on the incidence or latency of spontaneous tumor appearance. Treatment with mineral oil or incomplete Freund adjuvant markedly enhanced tumor appearance (though the abundance of plasma cells induced by these treatments did not evoke the development of plasmacytomas, nor did it affect the incidence of serum paraproteinemia among mice with spontaneous tumors). Treatment with cortisone acetate, however, markedly retarded spontaneous tumor development. Tumors occurring early due to mineral oil and adjuvant treatment all had the characteristics of "RCN-B plasma cell type," and tumors occurring late due to cortisone treatment were all of the "RCN-B reticulum cell type." The possible involvement of the stem cell pool size in the enhancement or delay of spontaneous tumor development was discussed.

Chromosome changes (trisomies #15 and 17) associated with tumor progression in leukemias induced by radiation leukemia virus.

An experimental system was developed that permitted nonrandom chromosome changes that occur in radiation leukemia virus (RadLV)-induced lymphomas to be followed during tumor progression. RadLV variant-induced preleukemia and leukemia cells originating from female inbred C57BL/6 mice were injected into male animals of the same strain. Since all donors were females and all recipients were males, the sex chromosome complements (XX and XY) were used to distinguish the preleukemia and leukemia cells from those of host origin. The G-banding analysis revealed that more than 50% of animals that were inoculated with preleukemia cells and that developed leukemia possessed tumor stem-lines of 41 chromosomes with a tristomy of chromosome #15. In animals inoculated with overt leukemia cells and in which tumor progression occurred, the G-banding an additional trisomy of chromosome #17. The cytogenic data strongly suggested that the trisomy of chromosome #15 was the first specific tumor-associated chromosome change that occurred in the process of conversion of RadLV-induced preleukemia cells to fully autonomous tumor cells.

Characteristics of potential lymphoma-inducing cells in mice sensitive or resistant to lymphomagenesis by radiation leukemia virus variants.

The relationship between the H-2-associated responsiveness of mice to radiation leukemia virus variants (A-RadLV and D-RadLV) lymphomagenesis and the characteristics of early occurring potential lymphoma-inducing cells (PLC) among thymus and bone marrow cells of these virus-infected mice was investigated. Sensitivity to virus-induced T-cell lymphomagenesis was shown to involve early occurrence of Thy-positive PLC, found predominantly among thymocytes, whereas resistance was rather related with early identification of PLC-Thy-negative cells mostly among bone marrow cells. PLC were further characterized in the sensitive (BL/6 + A-RadLV) and resistant (BL/6 + D-RadLV) situations by testing in parallel the tumorigenic potential (using the transplantation bioassay method) and type of thymus and bone marrow cell populations separated by different methods such as size fractionation by centrifugal elutriation, cytotoxic elimination of lymphocytes, or panning. The early occurring PLC among thymocytes of BL/6 mice 10-20 days following infection with A-RadLV were shown to be cortisone-resistant, Thy+, CD4+, and/or CD8+ medium size dividing thymocytes. PLC among thymocytes of BL/6 mice + D-RadLV, identified among the medium and large cell fractions, were shown to be cortisone-resistant Thy-, CD4-CD8- lymphocytes. High tumorigenic potential of PLC was demonstrated only among unseparated or separated (on size basis) bone marrow cells of BL/6 + D-RadLV (72-84%), whereas unseparated or separated fractions of bone marrow from BL/6 + A-RadLV had a low lymphomagenic potential (15-20%). The parallelism between the bone marrow fractions that induced optimal thymus cellularity following reconstitution of lethally irradiated mice and optimal lymphomagenicity stress the prothymocyte characteristics of PLC among bone marrow cells of BL/6 mice infected with D-RadLV. It is suggested that in resistant and sensitive haplotypes RadLV variants infect different cell populations and thereby induce PLC which differ in their capacity to present associative MuLV antigens with self H-2.

Analysis of Ly-1+ B-cell populations and IgH rearrangements in "normal" spleens and in lymphomas of AKR/J and AKR Fv-1b mice.

AKR mice are highly susceptible to development of spontaneous T-cell lymphoma. Thymus removal at the age of 1-3 months greatly reduces T-cell lymphoma. Lymphomas that have the characteristics of T- and/or B-cells occur sporadically in peripheral lymphoid tissues of old thymectomized AKR/J mice. These thymectomized mice were shown to carry dormant potential lymphoma cells. Transplantation of lymphoid cells from 8-12-month-old AKR/J mice, thymectomized at the age of 6 to 8 weeks, into intact or thymectomized young recipients yielded 80-100% Ly-1+ pre-B or B-cell lymphomas. In the AKR-Fv-1b congenic strain the Fv-1n allele of AKR/J mice was substituted with the Fv-1b allele, thereby limiting viral replication and spread of the endogenous N-tropic murine leukemia virus. As a result of this restriction in virus spread, AKR-Fv-1b mice develop a low spontaneous incidence (7%) of T-cell lymphomas and about 28% of Ly-1+ B-cell lymphomas at old age. In spleens of 15-18-month-old thymectomized AKR/J mice and intact AKR-Fv-1b mice, 30-60% of the B-cells were of the Ly-1+ B type. Analysis of the IgH locus in these normal old spleens and Ly-1+ B lymphomas indicated mono- or oligoclonality. One particular IgH rearrangement was identified in many individual old spleens and tumors. A second specific IgH rearrangement was found in some tumors. Possible mechanisms involved in the expansion of Ly-1+ clones and their progression into tumors are discussed.

Lymphomagenesis in AKR.Fv-1b congenic mice.

In the AKR.Fv-1b congenic strain the Fv-1n allele of the AKR/J mice was substituted with the Fv-1b allele, thereby limiting viral replication and spread of the endogenous N-tropic murine leukemia virus. As a result of this genetic change AKR.Fv-1b mice develop a low spontaneous incidence (7%) of T-cell lymphomas and about 28% of Ly-1+ B-cell lymphomas are observed in old mice. Characteristic changes in thymus subpopulations of AKR/J mice (related to the formation of the dual tropic mink cell focus inducing (MCF) type virus in the thymus) were not observed in the thymus of AKR.Fv-1b mice. In contrast to the low susceptibility to spontaneous T-cell lymphoma development, these mice were highly sensitive to fractionated irradiation or to radiation leukemia virus (a mixture of N- and B-tropic viruses) induced T-cell lymphoma. Potential lymphoma cells (that would ultimately develop into Ly-1+ B-cell lymphomas) were demonstrated in bone marrow and spleens of 16-24-month-old mice. Analysis of the Ly-1+ IgM+ B-cell population in spleens of 18-month-old mice revealed a significant increase in this population (35% versus 2% in young spleens). The spontaneous Ly-1+ B-cell lymphoma incidence could be enhanced (up to 77%) by in vivo administration of anti-CD8 monoclonal antibody or IL-4 to 18-month-old mice. Virological analysis of T/B-cell lymphomas for class I MCF viruses indicated that Class I MCF development was tightly correlated with T-lymphoma development (except radiation induced tumors that showed no MCF provirus involvement). In contrast, Ly-1+ B-cell lymphoma development was independent of Class I MCF pathogenic virus involvement.

Nonrandom chromosomal changes in thy-1-positive and thy-1-negative lymphomas induced by 7,12-dimethylbenzanthracene in SJL mice.

Leukemias were induced by 7,12-dimethylbenzanthracene feeding of intact, thymectomized, or Freund's adjuvant-pretreated SJL mice. Four of six Thy-1-positive thymomas that arose in intact mice had a pseudodiploid stemline with one morphologically similar or identical marker. Banding analysis showed that the marker had arisen by the translocation of the distal part of one chromosome 15 to one X chromosome [t(X;ter 15)]. Two normal No. 15 chromosomes were also present in the same metaphase plates. These four Thy-1-positive lymphomas were thus trisomic for the distal part of chromosome 15. All 8 Thy-1-negative lymphomas, originating in the spleen or lymph nodes of thymectomized or adjuvant-pretreatment mice, had a trisomy of chromosome 12 and also a trisomy of either chromosome 3 or chromosome 18. These results further stress the importance of gene dosage effects, related to the distal part of chromosome 15, in Thy-1-positive T-cell leukemogenesis. The cytogenetic difference between the Thy-1-positive and -negative leukemias supports our hypothesis that nonrandom chromosomal changes in murine leukemias are dependent on the target cell type, rather than the inducing agent.

Validity of the in vitro system as a correlate of the in vivo model of RadLV lymphomagnesis.

Adult BL/6 mice are highly sensitive to lymphomagnesis by the radiation leukemia virus variant A-RadLV (80-100% T-cell lymphoma incidence after a latency of 70-110 days). This study shows that the in vivo elimination of T-cell subsets (including suppressor and cytotoxic T-cells) achieved by the repeated administration of cyclophosphamide, cyclosporin A, anti CD4 or anti CD8 mAb shortly after virus infection did not interfere with the lymphomagenic pathway. No reduction in the high lymphoma incidence or tumor latency was observed following the different treatments. Thus the suggestion on the basis of in vitro studies that the early phase of A-RadLV lymphomagenesis is associated with suppressor T-cells which abrogate a potential anti-tumor immune response has not been confirmed in these studies.

Prevention of T-cell lymphoma in AKR/J mice.

Injection of a dual tropic virus (DTV) isolated from a T cell lymphoma AKR/J mice into the thymus of 14 day old AKR/J puppies accelerates lymphoma development; 90-100% of the injected mice develop the disease within 120 days. In contrast a cell free centrifuge CFC-666 prepared B cell lymphoma of AKR/J origin injected into the thymus of 14 day old AKR/J mice failed to accelerate T cell lymphomagenesis and actually prevented the spontaneous T cell lymphoma development. However, 50% of the treated mice developed B cell lymphoma with a latency of 417 + 18 days. DTV injection induces amplification of thymic expression of MuLV related antigens, besides changes in thymus subpopulation. Such changes emerge spontaneously in 5-6 month preleukemic AKR/J mice (at the time of spontaneous DTV formation in the thymus). These changes in the thymus were not observed following CFC-666 injection. We assume therefore that CFC-666 interferes with spontaneous DTV formation that contributes to T cell lymphomagenesis.

Role of cytokines in termination of the B cell lymphoma dormant state in AKR mice.

The high incidence of spontaneous T cell lymphomas in AKR mice (affected by sustained viremia) can be greatly reduced by experimental manipulations including thymus removal at young age or by genetic manipulation changing the Fv-1 allele that controls replication and spread of viruses (establishing the congenic AKR.Fv-1b mice). Although T cell lymphomagenesis is prevented, all these mice were shown to carry endogenous ecotropic provirus-induced potential lymphoma cells (PLCs) in a dormant state. The termination of the dormant state, leading to a high incidence of CD5+ IgM+ B cell lymphomas, was triggered by interference with T cell functions (optimal effect observed following in vivo administration of anti-CD8 moAb), administration of T cell growth factors or by injecting the MCF-247 recombinant virus isolate (from AKR origin) that affects T cell functions. The assumption that the PLC dormant state is maintained through specific immunological mechanisms (involving T cells or antibodies recognizing PLCs) could not be substantiated experimentally. The results of the present studies suggest that T cells provide immunoregulatory signals or factors that contribute to the maintenance of the B cell lymphoma arrest and/or proliferation. Analysis of cytokine levels produced by splenocytes taken from mice during PLC dormancy or its breakdown indicated reduced levels of IL-2 and IL-4 and marked elevation of IL-1 and IL-6 associated with the termination of the dormant state. The effect of IL-1 and IL-6 on terminating the dormant state was demonstrated by injecting these cytokines into PLC carriers, thymectomized 12-month-old AKR mice, yielding 80-85% CD5+ IgM+ B cell lymphomas. The role of IL-6 on B cell lymphoma proliferation was also indicated in MCF-247 mediated termination of dormancy, by inhibiting significantly its effect via in vivo administration of anti IL-6 moAbs.

Chromosomal mapping of the murine c-abl proto-oncogene by in situ hybridization.

Deletion and rearrangement of chromosome 2 were shown to be major cytogenetic characteristics of radiation-induced murine myeloid leukemias. Analysis of the localization of the murine protooncogene c-abl, previously assigned by Goff et al. to chromosome 2, was done using the in situ hybridization method. The c-abl was located close to the centromere, within bands 2A-2B. This site does not correspond to the common characteristic deleted segments (2C-2D) predominantly observed in radiation induced murine myeloid leukemias.

Deletion of chromosome 2 is an early event in the development of radiation-induced myeloid leukemia in SJL/J mice.

In this study we have analyzed the chromosomal changes in the preleukemic phase in SJL/J mice treated with radiation and acute myeloid leukemias (AMLs) induced by radiation alone or with additional corticosteroid treatment. SJL/J mice exposed to 300 rad whole body irradiation developed a low incidence of AML (20-25%) that could be markedly increased (to 50-70%) by additional coleukemogenic treatment with corticosteroids. Partial deletion in one chromosome 2 was found in 100% of bone marrow and spleen cells of leukemic animals in both treatment modalities, whereas the age-matched controls exhibited a normal karyotype. Five types of deletion were observed according to site and size, but region D through G was the common missing part in all five types of chromosome 2 deletion. The occurrence of chromosome 2 deletion was also tested among bone marrow cells removed from 17 mice, 4 months after exposure to 300 rad whole body irradiation, long before the time when AML development is expected. About 80% of the mice tested had different levels of deleted chromosome 2 among their bone marrow population. Cytological and histological examination of bone marrow and spleen of most tested animals showed a normal hematologic picture. These results suggest that the marker chromosome is related to the process of radiation-induced initiation of AML in SJL/J mice.

Enhanced AKR leukemogenesis by the dual tropic viruses. II. Effect on cell-mediated immune responses.

We examined the relationship of the leukemia-accelerating properties of a dual-tropic virus (DTV-70) (when injected into the thymus of 14-day-old AKR mice) to its ability to impair T cell functions. Splenic lymphocytes from virus-infected AKR mice were found to have reduced T cell mitogenic responses; moreover, these cells suppressed phytohemagglutinin stimulation of cells from normal, uninfected AKR mice. The response to the B cell mitogen lipopolysaccharide was slightly enhanced at 15 days following DTV-70 infection and was unaffected at later ages. AKR mice infected with DTV-70 showed reduced ability to develop delayed-type hypersensitivity reaction and interleukin 2 production. In contrast, spleen cells from the virus-infected mice responded normally to allogeneic stimulation in mixed lymphocyte culture and mounted an almost normal graft versus host reaction. The data suggest that DTV-70 impairs certain T cell functions that could interfere with immune surveillance and thus permit progression of preleukemic cells into overt leukemia. These T cell functions are suppressed normally by 6 months of age, perhaps by spontaneously arising DTV.

Multiphase process involved in radiation induced murine AML.

Exposure of 3 month old SJL/J mice to a single dose of 300 r yielded 15-30% acute myelomonocytic leukemia (AML) development at a mean latency of 1 year. Additional treatment with dexamethasone shortly after irradiation increased leukemia incidence to 50%. All tumors were characterized by a partial deletion of one allele of chromosome 2 and the same deletion was detected in bone marrow and spleen cells of most irradiated mice, irrespective of the development of the disease. The presence of potential leukemic cells (PLC) in mice 4 months after the leukemogenic treatment was confirmed by transplantation studies. In these experiments PLC transition into overt AML seemed to be dependent on their transfer into irradiated recipients. Thus, exposure to 300 r results in the initiation of potential leukemic cells. Experiments were conducted in order to explore the possible role of radiation, cytokines and different hemopoietic growth factors on PLC promotion to overt leukemia. Exposure to 300 r, beside PLC initiation, was found to trigger the production of IL-6 and CSF-1; the additional administration of dexamethasone further increased CSF-1 levels. In vivo administration of CSF-1 into mice carrying radiation-induced PLC was most effective in PLC promotion to overt AML development.

Initiation and promotion in radiation-induced myeloid leukemia.

Acute myelomonocytic leukemia develops in 10-30% of irradiated (300 rad) SJL/J mice, after a lag period of around one year. Additional treatment with dexamethasone shortly after irradiation increased leukemia incidence up to 50%. Experiments were conducted in order to demonstrate the existence of preleukemic cells in irradiated mice and to explore the possible role of dexamethasone, cyclophosphamide, and different hemopoietic growth factors on their promotion to overt leukemia. Transplantation of bone marrow cells from mice exposed to 300 rad plus dexamethasone into appropriate recipients, performed 4-5 months after leukemogenic treatment, resulted in acute myeloid leukemia (AML) development of donor origin in 70% of the recipients. Transfer of fractionated preleukemic bone marrow showed that the highest AML incidence developed in the recipients of fractions enriched in early hemopoietic precursors. The promoting effect of dexamethasone on preleukemic cells was confirmed by demonstrating its similar coleukemogenic effect whether administered within several hours or 130 days after radiation. Treatment with cyclophosphamide shortly after radiation could not replace the dexamethasone effect but was found to be complementary to the coleukemogenic effect of dexamethasone. Early administration of hemopoietic growth factors (starting 14 days after radiation and dexamethasone) showed that colony-stimulating factor (CSF) 1 increased the AML incidence (75%) and reduced its latency. Treatment with recombinant granulocyte-CSF (rG-CSF) had a reduced effect and recombinant granulocyte-macrophage CSF (rGM-CSF) had no promoting effect. However, administration of different factors several months after the leukemogenic treatment revealed that rGM-CSF increased AML incidence (75%) and shortened its latency, whereas rG-CSF and CSF-1 had no effect. In contrast, the late administration of recombinant interleukin 6 reduced AML incidence significantly (23%). The present results indicate that murine radiation induced AML is a multiphase process involving radiation induced preleukemia that can be promoted by different treatments.

The effects of passive antiviral immunotherapy in AKR mice: I. The susceptibility of AKR mice to spontaneous and induced T cell lymphomagenesis.

The AKR inbred mouse strain displays a high incidence of spontaneous T cell lymphomas that arise predominantly in the thymus of 6 to 12-month-old mice. Heterogenous nonacute transforming retroviruses are associated with the etiology of the disease: the endogenous ecotropic viruses (inherited in AKR mice at two non-linked chromosomal loci, Akv-1 and Akv-2), the xenotropic virus and recombinant viruses. Prevention of spontaneous T cell lymphomagenesis in AKR mice by passive anti-viral immunotherapy was accomplished by suppressing endogenous ecotropic virus release. Treatment with monoclonal antibody Hy-72 reacting only with Akv1 type ecotropic viruses, or with mAb 18-5 with specificity for both ecotropic and MCF recombinant virus envelope glycoprotein (administered from birth for 10 days) inhibited similarly T cell lymphoma development. A reduced thymus cellularity observed in these mAb treated mice coincided with reduced level of earliest intrathymic low CD4 precursor population in their thymus. The role of endogenous viruses (MuLV) and presence of potential lymphoma cells (PLCs) (identified among bone marrow cells of untreated AKR mice) in enhanced T cell lymphomagenesis in AKR mice, triggered by different leukemogenic agents, was evaluated. Intrathymic injection of the radiation leukemia virus variant A-RadLV or administration of methylnitrosourea resulted in a high lymphoma incidence within a short latent period of 80-100 days irrespective of the presence or absence of MuLV or PLCs in these treated mice. Thus, a direct action of these agents on thymocytes seems to occur. The high susceptibility of untreated AKR mice to radiation induced T cell lymphomagenesis was not affected by pretreatment with mAb Hy-72; in contrast to markedly reduced sensitivity following pretreatment with mAb 18-5 (15 vs 100%). The mAb 18-5 induced resistance to radiation lymphomagenesis seems to be related to defects in the bone marrow stem cell pool as well as in the thymus microenvironment of mAb 18-5 treated mice. Thus, different developmental pathways are involved in enhanced T cell lymphomagenesis in AKR mice.

Enhanced AKR leukemogenesis by the dual tropic viruses. I. The time and site of origin of potential leukemic cells.

The occurrence of potential leukemia cells (PLC) among bone marrow, spleen, and thymus of AKR mice during the preleukemic period was tested by an in vivo transplantation bioassay. The presence of PLC in 30- and 75-day-old AKR mice was demonstrated mostly among bone marrow cells, less in spleen, and was lacking in thymus. Occurrence of PLC in young AKR mice was shown to be thymus independent. However, progression of PLC from young donors (14-80 days old) into overt leukemia following transplantation into F1 recipients was shown to be dependent on specific host conditions including an intact thymus and an Fv-1nn allele. In contrast, PLC from 7-9-month-old AKR mice or frank leukemic cells when transplanted grew in any intact or thymectomized histocompatible host, thereby indicating their autonomous growth state. Infection of 2-week-old AKR mice with the dual-tropic virus DTV-70 induced characteristic changes in the thymus and accelerated leukemia development. DTV-70 inoculation into 14-day-old AKR mice did not change the spontaneous PLC distribution pattern in the tested host organs within 30 days postinfection, nor did it change PLC-specific host requirements for further progression into leukemic cells; however, it enhanced PLC transition to autonomous leukemic cells. The preferential cell tropism of DTV-70 for target cells (prothymocytes) among bone marrow and young spleen cells rather than for thymocytes was also demonstrated in an in vitro-in vivo test. The dual tropic virus may act as a promoter on preexisting PLC (present mostly among bone marrow cells) by enhancing their ability to progress into autonomous leukemic cells.