The leukemogenic potential of an enhancer variant of Moloney murine leukemia virus varies with the route of inoculation

Dual role of commensal bacteria in viral infections

With our abilities to culture and sequence the commensal bacteria that dwell on and within a host, we can now study the host in its entirety, as a supraorganism that must be navigated by the pathogen invader. At present, the majority of studies have focused on the interaction between the host's microbiota and bacterial pathogens. This is not unwarranted, given that bacterial pathogens must compete with commensal organisms for the limited territory afforded by the host. However, viral pathogens also enter the host through surfaces coated with microbial life and encounter an immune system shaped by this symbiotic community. Therefore, we believe that the microbiota cannot be ignored when examining the interplay between the host and viral pathogens. Here, we review work that details mechanisms by which the microbiota either promotes or inhibits viral replication and virally induced pathogenesis. The impact of the microbitota on viral infection promises to be a new and exciting avenue of investigation, which will ultimately lead to better treatments and preventions of virally induced disease.

ZASC1 knockout mice exhibit an early bone marrow-specific defect in murine leukemia virus replication

Background

ZASC1 is a zinc finger-containing transcription factor that was previously shown to bind to specific DNA binding sites in the Moloney murine leukemia virus (Mo-MuLV) promoter and is required for efficient viral mRNA transcription (J. Virol. 84:7473-7483, 2010).

Methods

To determine whether this cellular factor influences Mo-MuLV replication and viral disease pathogenesis in vivo, we generated a ZASC1 knockout mouse model and completed both early infection and long term disease pathogenesis studies.

Results

Mice lacking ZASC1 were born at the expected Mendelian ratio and showed no obvious physical or behavioral defects. Analysis of bone marrow samples revealed a specific increase in a common myeloid progenitor cell population in ZASC1-deficient mice, a result that is of considerable interest because osteoclasts derived from the myeloid lineage are among the first bone marrow cells infected by Mo-MuLV (J. Virol. 73: 1617-1623, 1999). Indeed, Mo-MuLV infection of neonatal mice revealed that ZASC1 is required for efficient early virus replication in the bone marrow, but not in the thymus or spleen. However, the absence of ZASC1 did not influence the timing of subsequent tumor progression or the types of tumors resulting from virus infection.

Conclusions

These studies have revealed that ZASC1 is important for myeloid cell differentiation in the bone marrow compartment and that this cellular factor is required for efficient Mo-MuLV replication in this tissue at an early time point post-infection.

Hematopoietic stem cells and retroviral infection

Retroviral induced malignancies serve as ideal models to help us better understand the molecular mechanisms associated with the initiation and progression of leukemogenesis. Numerous retroviruses including AEV, FLV, M-MuLV and HTLV-1 have the ability to infect hematopoietic stem and progenitor cells, resulting in the deregulation of normal hematopoiesis and the development of leukemia/lymphoma. Research over the last few decades has elucidated similarities between retroviral-induced leukemogenesis, initiated by deregulation of innate hematopoietic stem cell traits, and the cancer stem cell hypothesis. Ongoing research in some of these models may provide a better understanding of the processes of normal hematopoiesis and cancer stem cells. Research on retroviral induced leukemias and lymphomas may identify the molecular events which trigger the initial cellular transformation and subsequent maintenance of hematologic malignancies, including the generation of cancer stem cells. This review focuses on the role of retroviral infection in hematopoietic stem cells and the initiation, maintenance and progression of hematological malignancies.

Enhanced replication and pathogenesis of Moloney murine leukemia virus in mice defective in the murine APOBEC3 gene

Human APOBEC3G (hA3G), a member of the AID/APOBEC family of deaminases, is a restriction factor for human immunodeficiency virus (HIV). In the absence of the viral Vif protein hA3G is packaged into virions and during reverse transcription in a recipient cell it deaminates cytosines, leading to G-->A hypermutation and inactivation of the viral DNA. Unlike humans, who carry seven APOBEC3 genes, mice only carry one, mA3. Thus the role of mA3 in restriction of retroviral infection could be studied in mA3 -/- knockout mice, where the gene is inactivated. M-MuLV-infected mA3 -/- mice showed substantially higher levels of infection at very early times compared to wild-type mice (ca. 2 logs at 0-10 days), particularly in the bone marrow and spleen. Restriction of M-MuLV infection was studied ex vivo in primary bone marrow-derived dendritic cells (BMDCs) that express or lack mA3, using an M-MuLV-based retroviral vector expressing enhanced jellyfish green fluorescent protein (EGFP). The results indicated that mA3 within the virions as well as mA3 in the recipient cell contribute to resistance to infection in BMDCs. Finally, M-MuLV-infected mA3 +/+ mice developed leukemia more slowly compared to animals lacking one or both copies of mA3 although the resulting disease was similar (T-lymphoma). These studies indicate that mA3 restricts replication and pathogenesis of M-MuLV in vivo.

Disruption of hematopoiesis and thymopoiesis in the early premalignant stages of infection with SL3-3 murine leukemia virus

A time course analysis of SL3-3 murine leukemia virus (SL3) infection in thymus and bone marrow of NIH/Swiss mice was performed to assess changes that occur during the early stages of progression to lymphoma. Virus was detectable in thymocytes, bone marrow, and spleen as early as 1 to 2 weeks postinoculation (p.i.). In bone marrow, virus infection was detected predominantly in immature myeloid or granulocytic cells. Flow cytometry revealed significant reductions of the Ter-119(+) and Mac-1(+) populations, and significant expansions of the Gr-1(+) and CD34(+) populations, between 2 and 4 weeks p.i. Analysis of colony-forming potential confirmed these findings. In the thymus, SL3 replication was associated with significant disruption in thymocyte subpopulation distribution between 4 and 7 weeks p.i. A significant thymic regression was observed just prior to the clonal outgrowth of tumor cells. Proviral long terminal repeats (LTRs) with increasing numbers of enhancer repeats were observed to accumulate exclusively in the thymus during the first 8 weeks p.i. Observations were compared to the early stages of infection with a virtually nonpathogenic SL3 mutant, termed SL3DeltaMyb5, which was shown by real-time PCR to be replication competent. Comparison of SL3 with SL3DeltaMyb5 implicated certain premalignant changes in tumorigenesis, including (i) increased proportions of Gr-1(+) and CD34(+) bone marrow progenitors, (ii) a significant increase in the proportion of CD4(-) CD8(-) thymocytes, (iii) thymic regression prior to tumor outgrowth, and (iv) accumulation of LTR enhancer variants. A model in which disrupted bone marrow hematopoiesis and thymopoiesis contribute to the development of lymphoma in the SL3-infected animal is discussed.

Appearance of mink cell focus-inducing recombinants during in vivo infection by moloney murine leukemia virus (M-MuLV) or the Mo+PyF101 M-MuLV enhancer variant: implications for sites of generation and roles in leukemogenesis

One hallmark of murine leukemia virus (MuLV) leukemogenesis in mice is the appearance of env gene recombinants known as mink cell focus-inducing (MCF) viruses. The site(s) of MCF recombinant generation in the animal during Moloney MuLV (M-MuLV) infection is unknown, and the exact roles of MCF viruses in disease induction remain unclear. Previous comparative studies between M-MuLV and an enhancer variant, Mo+PyF101 MuLV, suggested that MCF generation or early propagation might take place in the bone marrow under conditions of efficient leukemogenesis. Moreover, M-MuLV induces disease efficiently following both intraperitoneal (i.p.) and subcutaneous (s.c.) inoculation but leukemogenicity by Mo+PyF101 M-MuLV is efficient following i.p. inoculation but attenuated upon s. c. inoculation. Time course studies of MCF recombinant appearance in the bone marrow, spleen, and thymus of wild-type and Mo+PyF101 M-MuLV i.p.- and s.c.-inoculated mice were carried out by performing focal immunofluorescence assays. Both the route of inoculation and the presence of the PyF101 enhancer sequences affected the patterns of MCF generation or early propagation. The bone marrow was a likely site of MCF recombinant generation and/or early propagation following i.p. inoculation of M-MuLV. On the other hand, when the same virus was inoculated s.c., the primary site of MCF generation appeared to be the thymus. Also, when Mo+PyF101 M-MuLV was inoculated i.p., MCF generation appeared to occur primarily in the thymus. The time course studies indicated that MCF recombinants are not involved in preleukemic changes such as splenic hyperplasia. On the other hand, MCFs were detected in tumors from Mo+PyF101 M-MuLV s. c.-inoculated mice even though they were largely undetectable at preleukemic times. These results support a role for MCF recombinants late in disease induction.

Moloney murine leukemia virus-induced preleukemic thymic atrophy and enhanced thymocyte apoptosis correlate with disease pathogenicity

Moloney murine leukemia virus (M-MuLV) is a replication-competent, simple retrovirus that induces T-cell lymphoma with a mean latency of 3 to 4 months. During the preleukemic period (4 to 10 weeks postinoculation) a marked decrease in thymic size is apparent for M-MuLV-inoculated mice in comparison to age-matched uninoculated mice. We were interested in studying whether the thymic regression was due to an increased rate of thymocyte apoptosis in the thymi of M-MuLV-inoculated mice. Neonatal NIH/Swiss mice were inoculated subcutaneously (s.c.) with wild-type M-MuLV (approximately 10(5) XC PFU). Mice were sacrificed at 4 to 11 weeks postinoculation. Thymic single-cell suspensions were prepared and tested for apoptosis by two-parameter flow cytometry. Indications of apoptosis included changes in cell size and staining with 7-aminoactinomycin D or annexin V. The levels of thymocyte apoptosis were significantly higher in M-MuLV-inoculated mice than in uninoculated control animals, and the levels of apoptosis were correlated with thymic atrophy. To test the relevance of enhanced thymocyte apoptosis to leukemogenesis, mice were inoculated with the Mo+PyF101 enhancer variant of M-MuLV. When inoculated intraperitoneally, a route that results in wild-type M-MuLV leukemogenesis, mice displayed levels of enhanced thymocyte apoptosis comparable to those seen with wild-type M-MuLV. However, in mice inoculated s.c., a route that results in attenuated leukemogenesis, significantly lower levels of apoptosis were observed. This supported a role for higher levels of thymocyte apoptosis in M-MuLV leukemogenesis. To examine the possible role of mink cell focus-forming (MCF) recombinant virus in raising levels of thymocyte apoptosis, MCF-specific focal immunofluorescence assays were performed on thymocytes from preleukemic mice inoculated with M-MuLV and Mo+PyF101 M-MuLV. The results indicated that infection of thymocytes by MCF virus recombinants is not required for the increased level of apoptosis and thymic atrophy.

Moloney murine leukemia virus infects cells of the developing hair follicle after neonatal subcutaneous inoculation in mice

The nature of Moloney murine leukemia virus (M-MuLV) infection after a subcutaneous (s.c.) inoculation was studied. We have previously shown that an enhancer variant of M-MuLV, Mo+PyF101 M-MuLV, is poorly leukemogenic when used to inoculate mice s.c., but not when inoculated intraperitoneally. This attenuation of leukemogenesis correlated with an inability of Mo+PyF101 M-MuLV to establish infection in the bone marrow of mice at early times postinfection. These results suggested that a cell type(s) is infected in the skin by wild-type but not Mo+PyF101 M-MuLV after s.c. inoculation and that this infection is important for the delivery of infection to the bone marrow, as well as for efficient leukemogenesis. To determine the nature of the cell types infected by M-MuLV and Mo+PyF101 M-MuLV in the skin after a s.c. inoculation, immunohistochemistry with an anti-M-MuLV CA antibody was performed. Cells of developing hair follicles, specifically cells of the outer root sheath (ORS), were extensively infected by M-MuLV after s.c. inoculation. The Mo+PyF101 M-MuLV variant also infected cells of the ORS but the level of infection was lower. By Western blot analysis, the level of infection in skin by Mo+PyF101 M-MuLV was approximately 4- to 10-fold less than that of wild-type M-MuLV. Similar results were seen when a mouse keratinocyte line was infected in vitro with both viruses. Cells of the ORS are a primary target of infection in vivo, since a replication defective M-MuLV-based vector expressing beta-galactosidase also infected these cells after a s.c. inoculation.

Identification of directly infected cells in the bone marrow of neonatal moloney murine leukemia virus-infected mice by use of a moloney murine leukemia virus-based vector

Early bone marrow infection of Moloney murine leukemia virus (M-MuLV)-infected mice was studied. Previous experiments indicated that early bone marrow infection is essential for the efficient development of T lymphoma. In order to identify the cellular pathway of infection in the bone marrow, infection of mice with a helper-free replication-defective M-MuLV-based retroviral vector was carried out. Such a vector will undergo only one round of infection, without spreading to other cells; thus, cells infected by the initially injected virus (directly infected cells) can be identified. For these experiments, the BAG vector that expresses bacterial beta-galactosidase was employed. Neonatal NIH/Swiss mice were inoculated intraperitoneally with ca. 10(6) infectious units of a BAG vector pseudotyped with M-MuLV proteins, and bone marrow cells were recovered 2 to 12 days postinfection. Single-cell suspensions were tested for infection by staining with X-Gal (5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside) or by immunofluorescence with an anti-beta-galactosidase antibody. Two sizes of infected cells were evident: large multinucleated cells and small nondescript (presumptively hematopoietic) cells. Secondary stains for lineage-specific markers indicated that the large cells were osteoclasts. Some of the small cells expressed nonspecific esterase, which placed them in the myeloid lineage, but they lacked markers for hematopoietic progenitors (mac-1, gr-1, sca-1, and CD34). These results provide evidence for primary M-MuLV infection of osteoclasts or osteoclast progenitors in the bone marrow, and they suggest that known hematopoietic progenitors are not primary targets for infection. However, the subsequent spread of infection to hematopoietic progenitors was indicated, since bone marrow from mice infected in parallel with replication-competent wild-type M-MuLV showed detectable infection in small cells positive for mac-1 or CD34, as well as in osteoclasts.

Thymic dendritic cells are primary targets for the oncogenic virus SL3-3

The murine retrovirus SL3-3 causes malignant transformation of thymocytes and thymic lymphoma in mice of the AKR and NFS strains when they are inoculated neonatally. The objective of the present study was to identify the primary target cells for the virus in the thymuses of these mice. Immunohistochemical studies of the thymus after neonatal inoculation of the SL3-3 virus showed that cells expressing the viral envelope glycoprotein (gp70(+) cells) were first seen at 2 weeks of age. These virus-expressing cells were found in the cortex and at the corticomedullary junction in both mouse strains. The gp70(+) cells had the morphology and immunophenotype of dendritic cells. They lacked macrophage-specific antigens. Cell separation studies showed that bright gp70(+) cells were detected in a fraction enriched for dendritic cells. At 3 weeks of age, macrophages also expressed gp70. At that time, both gp70(+) dendritic cells and macrophages were found at the corticomedullary junction and in foci in the thymic cortex. At no time during this 3-week period was the virus expressed in cortical and medullary epithelial cells or in thymic lymphoid cells. Infectious cell center assays indicated that cells expressing infectious virus were present in small numbers at 2 weeks after inoculation but increased at 5 weeks of age by several orders of magnitude, indicating virus spread to the thymic lymphoid cells. Thus, at 2 weeks after neonatal inoculation of SL3-3, thymic dendritic cells are the first cells to express the virus. At 3 weeks of age, macrophages also express the virus. In subsequent weeks, the virus spreads to the thymocytes. This pathway of virus expression in the thymus allows the inevitable provirus integration in a thymocyte that results in a clonal lymphoma.

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

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

Differential behavior of the Mo + PyF101 enhancer variant of Moloney murine leukemia virus in rats and mice

The Mo + PyF101 enhancer variant of Moloney murine leukemia virus (M-MuLV) has been very useful in investigating M-MuLV leukemogenesis. When inoculated subcutaneously (s.c.) into neonatal mice, Mo + PyF101 M-MuLV is attenuated for development of disease. Previous studies in mice infected with wild-type M-MuLV have revealed several important preleukemic events, including development of splenic hyperplasia, defects in bone marrow hematopoiesis, and in vivo generation of MCF viruses that arise by recombination in the uninfected mouse. Mo + PyF101 M-MuLV is defective in inducing these effects after s.c. inoculation. In the experiments reported here, a study of Mo + PyF101 M-MuLV infection in rats was carried out. Wild-type M-MuLV is leukemogenic in rats, but infected rats do not form MCF recombinants since they lack the necessary endogenous polytropic envelope sequences. Since Mo + PyF101 M-MuLV's leukemogenic defect is correlated with a failure to generate MCF recombinants, it seemed possible that wild-type M-MuLV might not have a leukemogenic advantage over Mo + PyF101 M-MuLV in rats, where MCF recombinants cannot form. Neonatal Fisher F344 rats were inoculated s.c. or intraperitoneally by wild-type and Mo + PyF101 M-MuLVs. Surprisingly, Mo + PyF101 M-MuLV was completely deficient in leukemogenesis in rats when inoculated by either route while wild-type M-MuLV induced lymphoma with the predicted time course. The leukemogenic defect for Mo + PyF101 M-MuLV resulted from a pronounced defect for establishing infection in rats. Further studies of wild-type M-MuLV in rats indicated that infection was confined almost exclusively to the thymus at early times. In mice wild-type M-MuLV establishes substantial infection in other hematopoietic organs such as spleen and bone marrow as well. Thymic infection was also correlated with a decrease in thymic cellularity at early times.

Tumorigenic potential of a recombinant retrovirus containing sequences from Moloney murine leukemia virus and feline leukemia virus

A recombinant retrovirus, termed MoFe2-MuLV, was constructed in which the U3 region of T-lymphomagenic Moloney murine leukemia virus (Mo-MuLV) was replaced by that of FeLV-945, a provirus of unique long terminal repeat (LTR) structure identified only in non-T-cell, non-B-cell lymphomas of the domestic cat. The LTR of FeLV-945 is unusual in that it contains only a single copy of the transcriptional enhancer followed 25 bp downstream by a 21-bp sequence in triplicate in tandem. Infectivity of MoFe2-MuLV was demonstrated in vitro in SC-1 cells and in vivo in neonatal NIH-Swiss mice. Tumors occurred in MoFe2-MuLV-infected animals following a latency period of 4 to 10 months (average, 6 months). The results of Southern blot analysis of the T-cell receptor beta locus demonstrated that all tumors were lymphomas of T-cell origin. MoFe2-MuLV LTRs were amplified by PCR from tumor DNA and were characterized by nucleotide sequence analysis. LTRs from the tumors that occurred with relatively shorter latency predominantly retained the original MoFe2-MuLV sequence intact and unaltered. Tumors that occurred with relatively longer latency contained LTRs that also retained the 21-bp sequence triplication characteristic of the original virus but had acquired various duplications of enhancer sequences. The repeated identification of enhancer duplications in late-appearing tumors suggests that the duplication affords a selective advantage, although apparently not in the efficient induction of T-cell lymphoma. Proto-oncogenes known to be targets of insertional mutagenesis in the majority of Mo-MuLV-induced tumors or in feline non-T-cell, non-B-cell lymphomas were shown not to be rearranged in any tumor examined. Mink cell focus-inducing (MCF) proviral DNA was readily detectable in some, but not all, tumors. The presence or absence of MCF did not correlate with the kinetics of tumor induction. These studies indicate that the single-enhancer, triplication-containing FeLV LTR, typical of non-T-cell, non-B-cell lymphomas in cats, is competent in the induction of T-cell lymphoma in mice. The findings suggest that the mechanism of MoFe2-MuLV-mediated lymphomagenesis may differ from that of Mo-MuLV-mediated disease, considering the possible involvement of novel oncogenes and the variable presence of MCF recombinants.

Leukemogenesis by Moloney murine leukemia virus: a multistep process

Moloney murine leukemia virus is a prototypical simple retrovirus that has been an extremely useful model for leukemogenesis. Important steps in leukemogenesis include proviral activation of cellular proto-oncogenes, generation of mink cell focus-inducing recombinants, and early (preleukemic) virus-induced changes in hematopoiesis.

Recombinant mink cell focus-inducing virus and long terminal repeat alterations accompany the increased leukemogenicity of the Mo+PyF101 variant of Moloney murine leukemia virus after intraperitoneal inoculation

We recently showed that different routes of inoculation affect the leukemogenicity of the Mo+PyF101 variant of Moloney murine leukemia virus (M-MuLV). Intraperitoneal (i.p.) inoculation of neonatal mice with Mo+PyF101 M-MuLV greatly enhanced its leukemogenicity compared with subcutaneous (s.c.) inoculation. We previously also suggested that the leukemogenicity defect of Mo+PyF101 M-MuLV when inoculated s.c. may result from the inability of this virus to form env gene recombinant (mink cell focus-inducing [MCF]) virus. In this study, virus present in end-stage tumors and in preleukemic animals inoculated i.p. by Mo+PyF101 M-MuLV was characterized. In contrast to s.c. inoculation, all tumors from i.p.-inoculated mice contained high levels of recombinant MCF virus. Furthermore, Southern blot analyses demonstrated that the majority of the tumors contained altered Mo+PyF101 M-MuLV long terminal repeats. The U3 regions from several tumors with altered long terminal repeats were cloned by PCR amplification. Sequence analyses indicated that the M-MuLV 75-bp tandem repeat in the enhancer region was triplicated. This amplification was also previously observed in mice infected s.c. with a pseudotypic mixture of Mo+PyF101 M-MuLV and Mo+PyF101 MCF virus. The enhancer triplication was an early event, and it occurred within 2 weeks postinfection. Recombinant MCF viruses were not detected by Southern blot analyses until 4 weeks postinfection. Thus, the M-MuLV enhancer triplication event was initially important for efficient propagation of ecotropic Mo+PyF101 M-MuLV. The increased leukemogenicity following i.p. inoculation could be explained if the triplication enhances Mo+PyF101 M-MuLV replication in the bone marrow and bone marrow infection is required for recombinant MCF virus formation.