Involvement of the spleen in preleukemic development of a murine retrovirus-induced promonocytic leukemia

An acute myeloid leukemia can result from the inoculation of Moloney murine leukemia virus into BALB/c mice undergoing a 2,6,10,14-tetramethylpentadecane-induced chronic inflammatory response in the peritoneal cavity. This leukemia is ultimately observed in the peritoneal cavity as an ascites with cells infiltrating the granulomatous tissue. It has been proposed, however, that hematopoietic organs such as the spleen and bone marrow are involved in preleukemic development of Moloney murine leukemia. Therefore, to determine if the spleen plays a role in this development, mice were splenectomized at various times relative to virus inoculation. When splenectomies were performed 3 days before and 2, 4, 6, and 8 weeks after virus inoculation there was, in all cases, a decreased death rate compared to sham-splenectomized controls. The greatest difference in death rate due to promonocytic leukemia was observed when mice were splenectomized at 4 weeks after virus inoculation. The decrease in disease incidence observed as a result of splenectomy was not caused by decreased virus spread in hematopoietic organs or an alteration in the profile of the cellular infiltrate in the granuloma. It was found, however, that the spleens of 2,6,10,14-tetramethylpentadecane-treated mice, relative to those of normal mice, have a significantly increased number of granulocyte-macrophage colony-forming cells and a slightly increased number of multipotential colony-forming cells. These observations suggest that a population of target cells for transformation, consisting of granulocyte-macrophage precursor cells, may reside in the spleen. Alternatively, partially transformed cells may reside temporarily in the spleen during the developmental stages of the disease process.

Deletion of a GC-rich region flanking the enhancer element within the long terminal repeat sequences alters the disease specificity of Moloney murine leukemia virus

Moloney murine leukemia virus (M-MuLV) is a replication-competent retrovirus which induces T-lymphoblastic lymphoma 2 to 4 months after inoculation. Enhancer sequences in the U3 region of the M-MuLV long terminal repeat, primarily the 75-bp tandem repeats, strongly influence the disease specificity and latency of M-MuLV. We investigated the role of GC-rich sequences downstream of the tandem repeats in the disease specificity of M-MuLV. A recombinant M-MuLV lacking 23 bases of a GC-rich sequence (-174 to -151), Delta 27A M-MuLV, was tested for pathogenesis in neonatal NIH Swiss mice. Delta 27A M-MuLV induced disease with a longer latency than did M-MuLV (7 versus 3 months) in greater than 85% of inoculated mice. More interestingly, this virus showed an expanded repertoire of hematopoietic diseases. Molecular analyses and histopathologic examinations indicated that while 39% of mice inoculated with Delta 27A M-MuLV developed T-cell lymphoblastic lymphoma typical of wild-type M-MuLV, the majority developed acute myeloid leukemia, erythroleukemia, or B-cell lymphoma. Viral DNA corresponding to Delta 27A M-MuLV was detectable in most of the tumors analyzed. These findings indicate that the GC-rich region significantly influences the disease specificity and latency of M-MuLV.

Bone marrow depletion by 89Sr complements a preleukemic defect in a long terminal repeat variant of Moloney murine leukemia virus

We previously described a preleukemic state induced by Moloney murine leukemia virus (Mo-MuLV) characterized by hematopoietic hyperplasia in the spleen. Further experiments suggested that splenic hyperplasia results from inhibitory effects in the bone marrow, leading to compensatory extramedullary hematopoiesis. An enhancer variant of Mo-MuLV, Mo + PyF101 Mo-MuLV, fails to induce preleukemic hyperplasia and has greatly reduced leukemogenicity, indicating the importance of this state to efficient leukemogenesis. An alternative method for induction of preleukemic hyperplasia was sought. Treatment of mice with 89Sr causes specific ablation of bone marrow hematopoiesis and compensatory extramedullary hematopoiesis in spleen and nodes. NIH Swiss mice were inoculated neonatally with Mo + PyF101 Mo-MuLV and treated with 89Sr at 6 weeks of age. Approximately 85% developed lymphoid leukemia with a time course resembling that caused by wild-type Mo-MuLV. In contrast, very few animals treated with Mo + PyF101 Mo-MuLV or 89Sr alone developed disease. In approximately one-third of cases, the Mo + PyF101 Mo-MuLV proviruses were found at common sites for wild-type Mo-MuLV-induced tumors (c-myc, pvt-1, and pim-1), indicating that this virus is capable of performing insertional activation in T-lymphoid cells. These results support the proposal that splenic hyperplasia results from inhibitory effects in the bone marrow. They also indicate that Mo + PyF101 Mo-MuLV is blocked in early and not late events in leukemogenesis.

Radiation leukemia virus (RadLV)-induced leukemogenesis is associated with an increased number and activity of thymic macrophages

The radiation leukemia virus (RadLV) is a chronic leukemia retrovirus that induces thymic lymphomas in C57BL/6 mice after a latency of 3 to 6 months. During the pre-leukemic (PL) period, the number of thymic macrophages gradually increased up to 100 fold. Of the cells in a RadLV-induced lymphoma, 0.3% were large macrophages packed with infected lymphoma cells. These thymic lymphoma macrophages (TLM) also ingested RadLV-induced lymphoma cells in vitro. Cultured RadLV-induced lymphoma lines could activate and fix C3 fragments through the alternative complement pathway (ACP). C3-bound lymphoma cells elicited an oxidative burst (OB) response in TLM but not in bone-marrow macrophages (BMM). However, IL4 treatment of BMM rendered them capable of responding with an OB following triggering by C3-opsonized cells. Thymic macrophages (TM) responded moderately with OB to C3-opsonized cells and this response was elevated if the TMs were treated by rIL4. The OB reaction of the TLMs could be partially inhibited by anti-LFA-I or anti-MALA-2 antibodies, and was completely inhibited by anti-CR3 antibodies. These results suggest that IL4 can prime macrophages for triggering an OB reaction and that the interaction between C3-opsonized cells and IL4-primed macrophages is mediated primarily through CR3.

Quantitation, in vitro propagation, and characterization of preleukemic cells induced by radiation leukemia virus

Intrathymic (i.t.) inoculation of radiation leukemia virus into C57BL/6 mice induces a population of preleukemic (PL) cells that can progress into mature thymic lymphomas upon transfer into syngeneic recipients. A minimum of 10(3) PL thymic cells are required to induce lymphomas in the recipient. Most of the individual lymphomas developed in mice which were inoculated with cells of a single PL thymus, derived from different T-cell precursors. PL thymic cells could be grown in vitro on a feeder layer consisting of splenic stromal cells. Growth medium was supplemented with supernatant harvested from an established radiation leukemia virus-induced lymphoma cell line (SR4). The in vitro-grown PL cells were characterized as Thy-1+, CD4+, CD8- T-cells, most of which expressed radiation leukemia virus antigens. Cultured PL cells were found to be nontumorigenic, based on their inability to form s.c. tumors. However, these cells could develop into thymic lymphomas if inoculated i.t. into syngeneic recipients. A culture of PL cells, maintained for 2 mo, showed clonal T-cell receptor arrangement. Lymphomas which developed in several recipient mice upon injection with these PL cells were found to possess the same T-cell receptor arrangement. These results indicate that PL cells can be adapted for in vitro growth while maintaining their preleukemic character.

An enhancer variant of Moloney murine leukemia virus defective in leukemogenesis does not generate detectable mink cell focus-inducing virus in vivo

Moloney murine leukemia virus (Mo-MuLV) induces T-cell lymphoma when inoculated into neonatal mice. This is a multistep process. Early events observed in infected mice include generalized hematopoietic hyperplasia in the spleen and appearance of mink cell focus-inducing (MCF) recombinants; end-stage tumors are characterized by insertional proviral activation of protooncogenes. We previously showed that an Mo-MuLV enhancer variant, Mo+PyF101 Mo-MuLV, has greatly reduced leukemogenicity and is deficient in induction of preleukemic hyperplasia. In this report, we have examined Mo+PyF101 Mo-MuLV-inoculated mice for the presence of MCF recombinants. In contrast to wild-type Mo-MuLV-inoculated mice, Mo+PyF101 Mo-MuLV-inoculated mice did not generate detectable MCF recombinants. This failure was at least partly due to an inability of the MCF virus to propagate in vivo, since a molecularly cloned infectious Mo+PyF101 MCF virus did not replicate, even when inoculated as a Mo+PyF101 Mo-MuLV pseudotype. These results show that the leukemogenic defect of Mo+PyF101 Mo-MuLV is associated with its inability to generate MCF recombinants capable of replication in vivo. This, in turn, is consistent with the view that MCF recombinants play a significant role in Mo-MuLV-induced disease and, in particular, may play a role early in the disease process.

Preleukemic hematopoietic hyperplasia induced by Moloney murine leukemia virus is an indirect consequence of viral infection

We previously showed that neonatal mice inoculated with Moloney murine leukemia virus (M-MuLV) exhibit a preleukemic state characterized by splenomegaly and increased numbers of hematopoietic progenitors. An M-MuLV variant with greatly reduced leukemogenic potential, Mo+PyF101 M-MuLV, does not generally induce this preleukemic state. In order to investigate the mechanism involved in M-MuLV induction of preleukemic hyperplasia, we tested the CFU-mixed myeloid and erythroid (CFUmix) from M-MuLV- and Mo+PyF101 M-MuLV-inoculated mice for the presence of virus by antibody staining and for the release of infectious virus. The majority of CFUmix colonies from both M-MuLV- and Mo+PyF101 M-MuLV-inoculated mice contained infectious virus even though M-MuLV-inoculated mice showed elevated levels of CFUmix while the Mo+PyF101 M-MuLV-inoculated mice did not. This indicates that direct infection of hematopoietic progenitors was not sufficient to induce hyperplasia. Rather, hematopoietic hyperplasia may result indirectly from infection of some other cell type.

Human T-cell lymphotropic virus type I (HTLV-I) antibodies in pre-diagnostic serum of patients with familial adult T-cell leukemia/lymphoma (ATL)

In a study of 7498 American men of Japanese ancestry in Hawaii, 26 incident cases of leukemia or non-Hodgkin's lymphoma were identified after a follow-up period of 19 years. Two of the cases, who were brothers, were diagnosed with adult T-cell leukemia/lymphoma (ATL). Both of these brothers had human T-cell lymphotropic virus type I (HTLV-I) antibodies in their stored serum which were obtained 4 and 18 years before diagnosis. None of the 24 patients with other hematologic malignancies or the 26 matched controls were HTLV-I antibody positive. This finding lends further support for a role of HTLV-I in the etiology of adult T-cell leukemia/lymphoma.

Multiple steps are required for the induction of tumors by Abelson murine leukemia virus

Helper virus-free Abelson murine leukemia virus (A-MuLV) was used to induce monoclonal pre-B-cell tumors in mice. The clonality, patterns of immunoglobulin heavy-chain gene rearrangement, tumorigenicity, and v-abl oncogene expression in individual preleukemic and leukemic colonies were compared. Our results indicate that A-MuLV preleukemic cells with low or undetectable tumorigenic potential give rise to leukemic cells with high tumorigenic potential by a process of subclone selection. The levels of v-abl oncogene product in preleukemic and leukemic cell populations were not significantly different. These results suggest that an additional event(s) unrelated to the level of the v-abl protein product is required for A-MuLV-transformed cells to become fully malignant.

Selective activation of endogenous ecotropic retrovirus in hematopoietic tissues of B6C3F1 mice during the preleukemic phase of 1,3-butadiene exposure

1,3-Butadiene (BD), a comonomer used in the production of synthetic rubber, is a rodent carcinogen. We have observed a marked increase in the incidence of thymic lymphoma in male B6C3F1 relative to NIH Swiss mice chronically exposed to BD in the absence of demonstrable differences in bone marrow (target organ) toxicity. Increased expression of murine leukemia virus (MuLV) antigens was also observed on lymphomas from BD-exposed B6C3F1 mice. Because NIH Swiss mice do not usually express endogenous retroviruses and their ecotropic proviral sequences are not intact, these findings provide presumptive evidence of a role for endogenous retrovirus sequences in BD-induced lymphoma in the B6C3F1 mouse. The present study was conducted to examine the expression and behavior of endogenous retroviruses in these strains during the preleukemic phase of BD exposure. Chronic exposure to BD (1250 ppm) 6 hr/day, 5 days/wk for 3 to 21 weeks increased markedly the quantity of ecotropic retrovirus recoverable from bone marrow, thymus, and spleen of B6C3F1 mice. However, expression of other endogenous retroviruses (xenotropic, MCF-ERV) was not enhanced. No viruses of any type were found in similarly treated NIH Swiss mice. The mechanism of this increase in ecotropic retrovirus in B6C3F1 mice is believed to be de novo activation in greater numbers of cells because changes in the Fv-1 tropism of the replicating viruses or changes in Fv-1 host restriction were not found. Endogenous retroviruses are thus implicated in BD-induced leukemogenesis in B6C3F1 mice. Further studies will examine the role of retrovirus in BD-induced leukemogenesis and the mechanisms of activation of ecotropic proviral sequences in murine cells.

Natural killer cell suppression of Friend virus-induced preleukemic hemopoietic stem cells

To determine whether hemopoietic cells infected with Friend polycythemia-inducing spleen focus-forming virus (SFFVp) are conserved or suppressed via natural surveillance in leukemia-resistant adult mice, we engrafted C57BL/6 recipients with isologous transgenic (donor origin marker) or natural killer (NK) cell-deficient B6 beige marrow cells exposed to SFFVp in vitro. Both groups of primary recipients were viremic and nonleukemic. Spleen cells from primary SFFVp-infected chimeras were engrafted into irradiated leukemia-susceptible secondary recipients to reveal dormant leukemia and grew as tumors of donor origin in 8 of 38 (21%) and 33 of 47 (70%) instances, respectively. Treatment of marrow donors and recipients with anti-asialo GM1 serum resulted in the depression of NK cell activity and the rapid development of dormant leukemia. We conclude that NK cells are an effective surveillance mechanism able to suppress SFFVp-induced preleukemic stem cells.

Class II polytropic murine leukemia viruses (MuLVs) of AKR/J mice: possible role in the generation of class I oncogenic polytropic MuLVs

We examined the frequency of occurrence of polytropic murine leukemia viruses (MuLVs) in the spleens and thymuses of preleukemic AKR/J mice from 1 week to 6 months of age and analyzed the genomic RNAs of several polytropic isolates by RNase T1 oligonucleotide fingerprinting. Polytropic MuLVs were first detected in the spleens of 3-week-old mice and preceded the appearance of polytropic MuLVs in the thymus by over 1 month. At 4 months of age and older, nearly all mice expressed polytropic MuLVs in both organs. In contrast to previous studies which have identified class I polytropic MuLVs in AKR/J mice, fingerprint analysis of polytropic MuLVs from both young (3- to 4-week-old) and older (5- to 6-month-old) preleukemic mice indicated that a large proportion of viruses at both ages were class II polytropic MuLVs. All polytropic viruses (five isolates) analyzed from 3- to 4-week-old mice were recovered from spleen cells and were class II polytropic MuLVs. In older preleukemic mice, five of seven isolates were class II polytropic MuLVs and two were class I polytropic viruses. Class I and class II polytropic MuLVs were recovered from both the spleens and thymuses of older preleukemic mice. A detailed comparison of the class I and class II polytropic MuLVs from 5- to 6-month-old mice revealed that the nonecotropic gp70 sequences of most of the class I and class II MuLVs were identical, consistent with a common origin for these sequences. In contrast, the nonecotropic p15E sequences of class I MuLVs were clearly derived from different endogenous sequences than the nonecotropic p15E sequences of the class II MuLVs. The in vitro host ranges of class I and class II polytropic viruses were clearly distinguishable. Examination of the in vitro host range of several isolates suggested that the predominant polytropic viruses initially identified in the thymus (2 to 3 months of age) were class II polytropic viruses. The order of appearance of the class I and class II polytropic MuLVs and the identity of the gp70 oligonucleotides of these MuLVs suggested a model for the stepwise generation of class I polytropic MuLVs involving a class II polytropic MuLV intermediate.

Tissue-specific replication of Friend and Moloney murine leukemia viruses in infected mice

We have studied the replication of ecotropic murine leukemia viruses (MuLV) in the spleens and thymuses of mice infected with the lymphocytic leukemia-inducing virus Moloney MuLV (M-MuLV), with the erythroleukemia-inducing virus Friend MuLV (F-MuLV), or with in vitro-constructed recombinants between these viruses in which the long terminal repeat (LTR) sequences have been exchanged. At 1 week after infection both the parents and the LTR recombinants replicated predominantly in the spleens with only low levels of replication in the thymus. At 2 weeks after infection, the patterns of replication in the spleens and thymuses were strongly influenced by the type of LTR. Viruses containing the M-MuLV LTR exhibited a remarkable elevation in thymus titers which frequently exceeded the spleen titers, whereas viruses containing the F-MuLV LTR replicated predominantly in the spleen. In older preleukemic mice (5 to 8 weeks of age) the structural genes of M-MuLV or F-MuLV predominantly influenced the patterns of replication. Viruses containing the structural genes of M-MuLV replicated efficiently in both the spleen and thymus, whereas viruses containing the structural genes of F-MuLV replicated predominantly in the spleen. In leukemic mice infected with the recombinant containing F-MuLV structural genes and the M-MuLV LTR, high levels of virus replication were observed in splenic tumors but not in thymic tumors. This phenotypic difference suggested that tumors of the spleen and thymus may have originated by the independent transformation of different cell types. Quantification of polytropic MulVs in late-preleukemic mice infected with each of the ecotropic MuLVs indicated that the level of polytropic MuLV replication closely paralleled the level of replication of the ecotropic MuLVs in all instances. These studies indicated that determinants of tissue tropism are contained in both the LTR and structural gene sequences of F-MuLV and M-MuLV and that high levels of ecotropic or polytropic MuLV replication, per se, are not sufficient for leukemia induction. Our results further suggested that leukemia induction requires a high level of virus replication in the target organ only transiently during an early preleukemic stage of disease.

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.

Effects of nonleukemogenic and wild-type Moloney murine leukemia virus on lymphoid cells in vivo: identification of a preleukemic shift in thymocyte subpopulations

Infection of mice with Moloney murine leukemia virus (M-MuLV) as well as with a nonpathogenic variant, Mo+PyF101 M-MuLV, was studied. Mo+PyF101 M-MuLV differs from wild-type M-MuLV by the addition of enhancer sequences from polyomavirus in the long terminal repeat. Previous experiments indicated that Mo+PyF101 establishes infection in animals, even though it does not induce disease. In vivo infection studies with particular attention to the thymus were performed, since the thymus is the target organ for M-MuLV leukemogenesis. Mice inoculated at birth with wild-type M-MuLV developed maximal levels of thymic infection by 2 to 3 weeks. Animals inoculated with Mo+PyF101 M-MuLV showed considerably less thymic infection at early times (2 to 4 weeks); nevertheless, by 5 to 6 weeks infection equivalent to wild-type M-MuLV-inoculated animals developed. Therefore the nonpathogenicity of Mo+PyF101 M-MuLV did not simply reflect a lack of thymotropism. Furthermore, thymic infection by itself may not be sufficient to induce leukemia. The relative deficit of Mo+PyF101 M-MuLV thymic infection at early versus late times did not reflect a change in the nature of the cells in the thymus, since in vitro infection of primary thymocytes from 2- and 6-week-old animals was equally efficient. One possible explanation is that infected thymocytes normally arise from progenitor cells which were infected in the bone marrow or spleen, and the cells restricted for Mo+PyF101 M-MuLV are located in those organs. Comparison of wild-type and Mo+PyF101 M-MuLV also allowed identification of important preleukemic changes in the thymus of wild-type M-MuLV-inoculated mice. Flow cytometry with monoclonal antibodies specific for thymocyte subpopulations was used. Staining of cells for Thy-1 or Thy-1.2 antigens indicated a shift toward low or negative cells. A concomitant increase in cells positive for antigen Pgp-1 was also observed. This is consistent with an increase in the relative frequency of immature blastlike cells. Importantly, thymuses from mice inoculated with Mo+PyF101 M-MuLV did not show these shifts in thymocyte subpopulations.

Long-term culture of bone marrow-derived preleukemic cells from F-MuLV-infected mice

The replication-competent Friend leukemia virus (F-MuLV) induces leukemias involving three hematopoietic lineages after a latent period of several months. In an attempt to elucidate the early events of the leukemogenic process, we looked for a method allowing the isolation and the long term in vitro maintenance of preleukemic cells. When established as long-term cultures according to the technique described by Dexter et al, bone marrow cells obtained from 7/7 apparently healthy F-MuLV-infected preleukemic mice led to the accumulation of immature myeloblastic cells, and to the generation of permanent myeloblastic cell lines, which in most cases further became tumorigenic in preirradiated recipient animals. The delays required to obtain cell lines were shorter when the duration of the in vivo infection was longer, suggesting that these cells were committed into the leukemogenic pathway before their transfer into culture flasks. The myelomonocytic preleukemic cells exhibited normal sensitivity to purified preparations of CSFs, but acquired the capacity to grow in the absence of exogenous CSF stimulation. Examination of integrated provirus copies demonstrated that the preleukemic cell proliferation involved a single or a few clones which may progress in vitro from a preleukemic to a fully malignant stage without major modifications of the integrated provirus copies.

Association of recombinant murine leukemia viruses of the class II genotype with spontaneous lymphomas in CWD mice

We determined the phenotype and genotype of murine leukemia viruses associated with the development of spontaneous nonthymic lymphomas in the high-leukemia mouse strain CWD/J. By T1 oligonucleotide fingerprint analysis of the viral RNA, the ecotropic viruses recovered from the spleen or thymus of preleukemic CWD/J mice were found to represent the progeny of the two endogenous ecotropic proviruses present in this strain. Polytropic murine leukemia viruses were produced by tissues from one-half of the leukemic mice, and fresh tumor cells from one of the two animals tested expressed recombinant envelope glycoproteins. The genomic structure of the recombinant viruses resembled those of class II polytropic viruses of NFS X Akv mice and differed from those of class I recombinant viruses that are commonly isolated from other high-leukemia strains such as AKR and HRS. Acquired retroviral sequences with the structural features of class II recombinant proviruses were detected in the DNA from each CWD/J tumor by the Southern blot technique. Finally, the injection of a mixture of CWD/J ecotropic and class II recombinant polytropic viruses into neonatal CWD/J mice accelerated the onset of lymphoma, whereas the endogenous ecotropic virus was inactive in these assays.

Influence of genotype and the organ of origin on the subtype of T-cell in Moloney lymphomas induced by transfer of preleukemic cells from athymic and thymus-bearing mice

Thymus, spleen, and bone marrow of 1-month-old neonatally Moloney murine leukemia virus-inoculated mice have been transferred to 400-R-irradiated syngeneic recipients of the opposite sex. The donor or recipient origin of T-cell lymphomas arising in the host animal was identified by the sex chromosome marker. Spleen and bone marrow of athymic BALB-nu/nu mice contain cells with the potential to develop into T-cell lymphomas upon transfer to thymus-bearing BALB/c recipients. Such lymphomas arise from at least two subsets of T-cells, one terminal deoxynucleotidyl transferase (TdT) positive and the other 20 alpha-hydroxysteroid dehydrogenase positive. The enzyme-negative precursor T-cells from the BALB-nu/nu spleen and bone marrow can thus mature to enzyme-positive cells and give rise to lymphoma in the thymus-bearing recipient. Preleukemic spleen and bone marrow, but not thymus, from CBA and BALB/c mice regularly contained cells with the potential to develop lymphoma. The subset of T-cell involved was influenced by the genotype since lymphomas arising after the transfer of CBA and BALB/c spleens were TdT positive and 20 alpha-hydroxysteroid dehydrogenase positive, respectively. In thymus-bearing mice, but not in nude mice, the transfer of preleukemic spleen cells gave lymphomas earlier than did transfer of bone marrow cells. This suggests that the more mature lymphoid cell population in the spleen of thymus-bearing mice may allow leukemic transformation to occur more rapidly than do the less mature cells in the bone marrow. In one-third of the cases, the virus produced by the preleukemic cells transferred induced new lymphomas involving recipient host cells. These de novo-induced lymphomas were all TdT positive. We suggest that leukemic transformation of TdT-positive cells may occur through a different mechanism than does transformation of cells bearing the 20 alpha-hydroxysteroid dehydrogenase marker.

Studies on the specificity of human antibodies reacting with GP70 and P15 antigens of baboon endogenous (BaEV) and gibbon ape leukaemia (GaLV) viruses

Antibodies reacting with gp70 and p15 antigens of baboon endogenous virus (BaEV) and gibbon ape leukaemia virus (GaLV) were detected by radioimmunoprecipitation (RIP) in blood plasma samples of patients with chronic granulocytic leukaemia, acute myeloid leukaemia and with potentially preleukaemic haematological disorders. Anti-gp70 antibodies were found more frequently than anti-p15 antibodies. Digestion of the carbohydrate part of gp70 antigens by glycosidase treatment abrogated the precipitation mediated by IgM antibodies, whereas that mediated by IgG antibodies was not markedly affected. Data suggest that antibodies detected in human plasma samples may have oncovirus specificity, but in considerable part of cases they can be of heterophil nature.

Free and integrated recombinant murine leukemia virus DNAs appear in preleukemic thymuses of AKR/J mice

We studied the appearance and structure of murine leukemia viral genomes in preleukemic AKR/J mice by Southern hybridization. Up to an average of one to two copies per thymocyte of unintegrated murine leukemia virus DNA appears in the thymuses of preleukemic mice beginning at 4 to 5 months of age and disappears in leukemic thymuses. The free viral genomes are absent in the spleens, livers, and brains of preleukemic mice. Using a series of ecotropic and nonecotropic murine leukemia virus hybridization probes, we showed that the unintegrated viral genomes are structurally analogous to those of recombinant mink cell focus-forming viruses that appear as proviruses in leukemic AKR thymocytes, suggesting that these free viral DNAs are the direct precursors to the leukemia-specific proviruses. The mosaic of ecotropic and nonecotropic sequences within these unintegrated viral DNAs varies from one preleukemic thymus to another but often appears structurally homogeneous within individual thymuses, indicating that often each thymus was being infected by a unique mink cell focus-forming virus. Analysis of high-molecular-weight DNA shows that recombinant proviruses reside in the chromosomal DNA of thymocytes within the preleukemic thymus, with the number rising to an average of several copies per thymocyte, but we do not detect any preferred integration sites. These results suggest that, in general, before the development of thymic leukemias in AKR mice there is a massive infection by a unique mink cell focus-forming virus which then integrates into many different sites of individual thymocytes, one of which grows out to become a tumor.

Correlation of alkaline phosphatase activity to normal T-cell differentiation and to radiation leukemia virus-induced preleukemic cells in the C57BL mouse thymus

Cytochemical methods at the light and electron microscopic level were used to define the pattern of alkaline phosphatase (APase) activity in normal thymus and to study its modifications after inoculation with the thymotropic leukemogenic radiation leukemia virus in correlation with the emergence of preleukemic cells and their thymus dependency. APase was found in numerous lymphoblasts of the fetal thymus. The enzyme was also detected in a few lymphoid blast cells of the normal young adult thymus, which were closely associated with thymic nurse cells. The observed distribution of APase in normal thymus suggests that its expression could be limited to an early stage of the T-cell differentiation pathway. After inoculation with radiation leukemia virus, APase activity remained normal for almost the entire latency period, during which virus replication spread to the cortex and thymus-dependent preleukemic cells appeared. An important increase in the number of APase-positive cells occurred later, i.e., at the end of the latency period, in nontumoral thymus, which displayed lymphocytic depletion and contained autonomous thymus-independent preleukemic cells. These latter features obviously reflected the malignant transformation of thymus lymphoblasts, which eventually led to the development of the thymic lymphomas. The results raise the question of the possible filiation between the thymic nurse cell-associated APase-positive lymphoid cells of the normal thymus and the target cells susceptible to productive infection and to neoplastic transformation after radiation leukemia virus infection.

Genetic alterations of RNA leukemia viruses associated with the development of spontaneous thymic leukemia in AKR/J mice

T1-oligonucleotide fingerprinting and mapping were used to study the expression of RNA leukemia viruses in leukemic and preleukemic AKR/J mice, with techniques designed to minimize the loss or inadvertent selection of viruses in vitro before biochemical analysis. In leukemic animals, complex mixtures of ecotropic and mink-tropic viruses were expressed. Unique but similar polytropic virus-like genomes were present in each tumor isolate. In preleukemic mice, viral isolates from the thymus that were grown on NIH3T3 fibroblasts contained genomes with non-Akv polytropic virus-related oligonucleotides. This phenomenon was not evident in fingerprints of viruses from the spleen and bone marrow of the same animals. Remarkably, the non-Akv oligonucleotides located in the 3' portion of the P15E gene, the U3 noncoding region, and the 5' part of the gp70 gene were often expressed independently. Our results suggest the following. (i) Recombinant viruses can be detected in the thymuses of young preleukemic AKR mice and increase in relative abundance with age. (ii) During in vivo generation of the recombinant leukemogenic viruses, the selection of polytropic virus-related sequences in the 3' part of p15E and the U3 region and the 5' portion of gp70 occurs independently. (iii) Independent biological properties encoded in the gp70 and p15E regions of env of the recombinant viruses may mediate viral selection or leukemogenicity. (iv) The leukemogenic polytropic viruses of AKR/J mice arise via genetic recombination involving at least three endogenous viral sequences.

Moloney virus (M-MuLV) leukemogenesis: virus spread, antibody production and antigenic expression in neonatally virus-inoculated young mice

(A X C57BL) and (A X C57L)F1 hybrid mice were inoculated neonatally with M-MuLV. Virus spread, antigenic expression and antibody production were followed during the preleukemic period. M-MuLV was first detectable in the spleen and later in the thymus. Virus spread was faster and the level of viremia higher in A X C57L than in A X C57BL mice. Also, A X C57L mice had no or only low titers of virus neutralizing antibodies, whereas A X C57BL mice had high titers. Anti-MCSA antibodies, reacting with the surface of syngeneic M-MuLV-induced lymphoma cells, were present in a minority of the mice, but disappeared ultimately in all mice. The two groups of mice differed with regard to the length of the preleukemic latency period. High virus load and a low level of virus neutralizing and anti-MCSA antibodies were correlated with an earlier onset of leukemia.

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.

Persistence and pathogenicity of defective Friend spleen focus-forming virus. Decreased transplantability of hemopoietic cells as a marker for preleukemic change

A latent form of persistent infection can be established in susceptible adult mice inoculated with a preparation of defective Friend spleen focus-forming virus (SFFV) purified free from standard leukemia-inducing helper virus (LLV-F). SFFV persistence was initially observed using an in vivo rescue technique in which SFFV could be directly rescued to form splenic foci of malignant erythropoiesis in mice. At approximately 30 d after virus inoculation however, SFFV could not be rescued after inoculation of LLV-F indicating that persistently infected (i.e., SFFV+) mice were either immume to exogenous helper virus or able to express SFFV-associated defective-interfering (DI) function(s). Persistent infection by SFFV was further documented using an in vitro rescue technique and ultimately resulted in the induction by SFFV of erythroleukemia in the absence of polycythemia or overt virus production. However, SFFV rescued by LLV-F from persistently infected normal and transformed hemopoietic cells was able to induce polycythemia in adult mice suggesting that this is a helper controlled property of the Friend virus complex. Transplantable SFFV-induced erythroleukemic cells could be retrieved from persistently infected yet histologically normal mice. The duration of SFFV persistence in normal spleen tissue suggests that the SFFV provirus resides in either a long-lived or pluripotent hemopoietic cell. Further, certain changes occurred, presumably in the membranes of persistently infected cells, which preceded the overt development of Friend leukemia and facilitated the definition of an SFFV preleukemic phase. Cell surface alterations were revealed using cell transfer techniques. Hemopoietic cells harboring a rescuable SFFV failed to proliferate when inoculated into lethally irradiated, syngeneic adult mice. In contrast, the transformed progeny of preleukemic cell populations and spleen cells transformed by FV complex (i.e., cells replicating both SFFV and LLV-F) were not rejected. This result suggests that histologically normal SFFV+ preleukemic cells express an antigen recognition site which is not present on overtly transformed cells and which may be a pertinent surveillance target for host anti-leukemogenic reactions.

Prevalence of non-T-cells in the replication of the N-tropic, type C virus of young AKR mice

The XC infectious center assay was used to study the nature of the lymphoid cells producing N-tropic C-type viruses in preleukemic AKR mice. Viral production by thymic cell suspensions was very low and was possibly due to contaminating cells. Production at least 100-fold higher was found in spleen cells and was probably due to non-T-cells. The significance of these results is discussed briefly, including the possibility that the N-tropic XC syncitia-inducing type C virus of young AKR mice is not the leukemogenic agent.

A combined assay for the rapid preliminary detection of structural retroviruses

This report describes the use of equilibrium gradients, RNA dependent DNA polymerase assays and electron microscopy (EM) in a combined assay for the rapid preliminary detection of intact retroviruses in crude preparations. Positive combined assays of platelets from preleukemic patients corresponded with karyotypic abnormalities found in these patients. Reconstruction experiments with Rauscher Leukemia Virus added to buffer or disrupted mouse spleen demonstrated the ease of detecting 10(9) or greater particles/g crude tissue, and the effects of buffer or added protein.