Friend Spleen Focus-Forming Virus Activates the Tyrosine Kinase sf-Stk and the Transcription Factor PU.1 to Cause a Multi-Stage Erythroleukemia in Mice

A detailed analysis of F-MuLV- and SFFV-infected cells in Friend virus-infected mice reveals the contribution of both F-MuLV- and SFFV-infected cells to the interleukin-10 host response

BACKGROUND

Friend virus (FV) is a complex of the Friend murine leukemia virus (F-MuLV) and the replication-defective, pathogenic spleen focus forming virus (SFFV). In the past, we used a fluorescently labeled F-MuLV to analyze FV target cells. To build on these findings, we have now created a double-labeled FV that contains a Katushka-labeled F-MuLV and an mTagBFP-labeled SFFV, which we have used to study the infection by the two individual viruses in the FV infection of highly susceptible BALB/c mice.

RESULTS

Our data show that the target cells of SFFV largely mirror those of F-MuLV, with the highest virus loads in erythroblasts, B cells and myeloid cells. The early phase of infection was dominated by cells infected by either SFFV or F-MuLV, whereas double-infected cells became dominant later in the course of infection with increasing viral loads. In the late phase of infection, the frequency of double-infected cells was similarly high as the frequencies of SFFV or F-MuLV single-infected cells, and single- and double-infected cells outnumbered the uninfected cells in the most highly infected cell populations such as erythroblasts. FV and retroviruses in general have been shown to induce interleukin 10 (IL-10) as a means of suppressing immune responses. Interestingly, we found in infected IL-10-eGFP reporter mice that SFFV-infected cells contributed to the IL-10-producing cell pool much more significantly than F-MuLV-infected cells, suggesting that the truncated SFFV envelope protein gp55 might play a role in IL-10 induction. Even though BALB/c mice mount notoriously weak immune responses against FV, infection of mice with an ablation of IL-10 expression in T cells showed transiently lower viral loads and stronger T cell activation, suggesting that IL-10 induction by FV and by SFFV in particular may contribute to a suppressed immune response in BALB/c mice.

CONCLUSION

Our data provide detailed information about both F-MuLV- and SFFV-infected cells during the course of FV infection in highly susceptible mice and imply that the pathogenic SFFV contributes to immune suppression.

Inhibition of IL-2 or NF-κB Subunit c-Rel-Dependent Signaling Inhibits Expansion of Regulatory T Cells During Acute Friend Retrovirus Infection

In retroviral infections, different immunological mechanisms are involved in the development of a chronic infection. In the Friend virus (FV) model, regulatory T cells (Tregs) were found to induce CD8⁺ T cell dysfunction before viral clearance is achieved and thus contribute to viral chronicity. Although studied for decades, the exact suppressive mechanisms of Tregs in the FV model remain elusive and an unavailable therapeutic target. However, extracellular IL-2 and intracellular NF-κB signaling were shown to be important pathways for Treg expansion and activation. Therefore, we decided to focus on these two pathways to test therapeutic approaches inhibiting Treg activation during FV infection. In this study, we show that the inhibition of either IL-2 or the NF-κB subunit c-Rel, impaired Treg expansion and activation at 2 weeks post-FV infection. Total numbers of Tregs as well as activated Tregs were reduced in FV-infected mice after treatment with anti-IL-2 antibodies or the c-Rel blocking reagent pentoxifylline. Surprisingly, this did not affect the expansion or function of virus-specific CD8⁺ T cells nor viral loads in the spleen. However, our data suggest that neutralization of IL-2 as well as blocking c-Rel efficiently inhibits virus-induced Treg expansion.

Friend retrovirus studies reveal complex interactions between intrinsic, innate and adaptive immunity

Approximately 4.4% of the human genome is comprised of endogenous retroviral sequences, a record of an evolutionary battle between man and retroviruses. Much of what we know about viral immunity comes from studies using mouse models. Experiments using the Friend virus (FV) model have been particularly informative in defining highly complex anti-retroviral mechanisms of the intrinsic, innate and adaptive arms of immunity. FV studies have unraveled fundamental principles about how the immune system controls both acute and chronic viral infections. They led to a more complete understanding of retroviral immunity that begins with cellular sensing, production of type I interferons, and the induction of intrinsic restriction factors. Novel mechanisms have been revealed, which demonstrate that these earliest responses affect not only virus replication, but also subsequent innate and adaptive immunity. This review on FV immunity not only surveys the complex host responses to a retroviral infection from acute infection to chronicity, but also highlights the many feedback mechanisms that regulate and counter-regulate the various arms of the immune system. In addition, the discovery of molecular mechanisms of immunity in this model have led to therapeutic interventions with implications for HIV cure and vaccine development.

Murine Models of Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a rare but severe form of human cancer that results from a limited number of functionally cooperating genetic abnormalities leading to uncontrolled proliferation and impaired differentiation of hematopoietic stem and progenitor cells. Before the identification of genetic driver lesions, chemically, irradiation or viral infection-induced mouse leukaemia models provided platforms to test novel chemotherapeutics. Later, transgenic mouse models were established to test the in vivo transforming potential of newly cloned fusion genes and genetic aberrations detected in patients' genomes. Hereby researchers constitutively or conditionally expressed the respective gene in the germline of the mouse or reconstituted the hematopoietic system of lethally irradiated mice with bone marrow virally expressing the mutation of interest. More recently, immune deficient mice have been explored to study patient-derived human AML cells in vivo. Unfortunately, although complementary to each other, none of the currently available strategies faithfully model the initiation and progression of the human disease. Nevertheless, fast advances in the fields of next generation sequencing, molecular technology and bioengineering are continuously contributing to the generation of better mouse models. Here we review the most important AML mouse models of each category, briefly describe their advantages and limitations and show how they have contributed to our understanding of the biology and to the development of novel therapies.

The miR-23a~27a~24-2 microRNA cluster buffers transcription and signaling pathways during hematopoiesis

MicroRNA cluster mirn23a has previously been shown to promote myeloid development at the expense of lymphoid development in overexpression and knockout mouse models. This polarization is observed early in hematopoietic development, with an increase in common lymphoid progenitors (CLPs) and a decrease in all myeloid progenitor subsets in adult bone marrow. The pool size of multipotential progenitors (MPPs) is unchanged; however, in this report we observe by flow cytometry that polarized subsets of MPPs are changed in the absence of mirn23a. Additionally, in vitro culture of MPPs and sorted MPP transplants showed that these cells have decreased myeloid and increased lymphoid potential in vitro and in vivo. We investigated the mechanism by which mirn23a regulates hematopoietic differentiation and observed that mirn23a promotes myeloid development of hematopoietic progenitors through regulation of hematopoietic transcription factors and signaling pathways. Early transcription factors that direct the commitment of MPPs to CLPs (Ikzf1, Runx1, Satb1, Bach1 and Bach2) are increased in the absence of mirn23a miRNAs as well as factors that commit the CLP to the B cell lineage (FoxO1, Ebf1, and Pax5). Mirn23a appears to buffer transcription factor levels so that they do not stochastically reach a threshold level to direct differentiation. Intriguingly, mirn23a also inversely regulates the PI3 kinase (PI3K)/Akt and BMP/Smad signaling pathways. Pharmacological inhibitor studies, coupled with dominant active/dominant negative biochemical experiments, show that both signaling pathways are critical to mirn23a’s regulation of hematopoietic differentiation. Lastly, consistent with mirn23a being a physiological inhibitor of B cell development, we observed that the essential B cell transcription factor EBF1 represses expression of mirn23a. In summary, our data demonstrates that mirn23a regulates a complex array of transcription and signaling pathways to modulate adult hematopoiesis.

MicroRNAs (miRNAs) are small ~22 nucleotide long RNA molecules that are involved in regulating multiple cellular processes through inhibiting the expression of target proteins. We previously identified a gene (mirn23a) that codes for 3 miRNAs that control the development of immune cells in the bone marrow. The miRNAs promote the development of innate immune cells, macrophages and granulocytes, while repressing the development of B cells. Here we show that mirn23a miRNAs negatively affect the expression of multiple proteins that are involved in directing blood progenitor cells to become B cells. Additionally, we observed that modulation of FoxO1 and Smad proteins, downstream effectors of two signaling pathways (PI3 kinase/ Akt and BMP/ Smad), is critical to direct immune cell development. This is the first observation that these pathways are potentially coregulated during the commitment of blood progenitors to mature cells of the immune system. Consistent with mirn23a being a critical gene for committing progenitors to innate immune cells at the expense of B cells, we observed that a critical B cell protein represses the expression of mirn23a. In conclusion, we demonstrate the mirn23a regulation of blood development is due to a complex regulation of both transcription factors and signaling pathways.

Serial infection of diverse host (Mus) genotypes rapidly impedes pathogen fitness and virulence

Reduced genetic variation among hosts may favour the emergence of virulent infectious diseases by enhancing pathogen replication and its associated virulence due to adaptation to a limited set of host genotypes. Here, we test this hypothesis using experimental evolution of a mouse-specific retroviral pathogen, Friend virus (FV) complex. We demonstrate rapid fitness (i.e. viral titre) and virulence increases when FV complex serially infects a series of inbred mice representing the same genotype, but not when infecting a diverse array of inbred mouse strains modelling the diversity in natural host populations. Additionally, a single infection of a different host genotype was sufficient to constrain the emergence of a high fitness/high virulence FV complex phenotype in these experiments. The potent inhibition of viral fitness and virulence was associated with an observed loss of the defective retroviral genome (spleen focus-forming virus), whose presence exacerbates infection and drives disease in susceptible mice. Results from our experiments provide an important first step in understanding how genetic variation among vertebrate hosts influences pathogen evolution and suggests that serial exposure to different genotypes within a single host species may act as a constraint on pathogen adaptation that prohibits the emergence of more virulent infections. From a practical perspective, these results have implications for low-diversity host populations such as endangered species and domestic animals.

Biology and Diseases of Mice

Today’s laboratory mouse, Mus musculus, has its origins as the ‘house mouse’ of North America and Europe. Beginning with mice bred by mouse fanciers, laboratory stocks (outbred) derived from M. musculus musculus from eastern Europe and M. m. domesticus from western Europe were developed into inbred strains. Since the mid-1980s, additional strains have been developed from Asian mice (M. m. castaneus from Thailand and M. m. molossinus from Japan) and from M. spretus which originated from the western Mediterranean region.

Differential requirements of cellular and humoral immune responses for Fv2-associated resistance to erythroleukemia and for regulation of retrovirus-induced myeloid leukemia development

To assess the possible contribution of host immune responses to the exertion of Fv2-associated resistance to Friend virus (FV)-induced disease development, we inoculated C57BL/6 (B6) mice that lacked various subsets of lymphocytes with FV containing no lactate dehydrogenase-elevating virus. Fv2(r) B6 mice lacking CD4(+) T cells developed early polycythemia and fatal erythroleukemia, while B6 mice lacking CD8(+) T cells remained resistant. Erythroid progenitor cells infected with spleen focus-forming virus (SFFV) were eliminated, and no polycythemia was observed in B cell-deficient B6 mice, but they later developed myeloid leukemia associated with oligoclonal integration of ecotropic Friend murine leukemia virus. Additional depletion of natural killer and/or CD8(+) T cells from B cell-deficient B6 mice resulted in the expansion of SFFV proviruses and the development of polycythemia, indicating that SFFV-infected erythroid cells are not only restricted in their growth but are actively eliminated in Fv2(r) mice through cellular immune responses.

DNA methylation-mediated silencing of PU.1 in leukemia cells resistant to cell differentiation

In mice, the proviral integration of the Friend Spleen Focus Forming Virus (SFFV) within the PU.1 locus of erythroid precursors results in the development of erythroleukemia. SFFV integrates several kilobases upstream of the PU.1 transcription initiation start site leading to the constitutive activation of the gene which in turn results in a block of erythroid differentiation. In this study we have mapped and sequenced the exact location of the retroviral integration site. We have shown that SFFV integrates downstream of a previously described upstream regulatory element (URE), precisely 2,976 bp downstream of the URE-distal element. We have also found that SFFV persists integrated within the same location in resistant cell lines that have lost their differentiation capacity and in which case PU.1 remains silent. We have examined the methylation status of PU.1 and found that in resistant cells the nearby CpG islands remained methylated in contrast to a non-methylated status of the parental cell lines. Treatment with 5-aza-2′-deoxycytidine caused resistant cells to differentiate yet only when combined with HMBA. Altogether these results strongly suggest that methylation plays a crucial role with regard to PU.1 silencing. However, although demethylation is required, it is not sufficient to overcome the differentiation impasse. We have also showed that activation blockage of the Epo/Epo-R pathway remains despite of the absence of PU.1.

Retrovirus-transformed erythroleukemia cells induce central nervous system failure in a new syngeneic mouse model of meningeal leukemia

Lack of suitable mouse models for central nervous system (CNS)-associated leukemias has hindered mechanism-guided development of therapeutics. By transplanting retrovirus-transformed mouse erythroleukemia cells into syngeneic mice, we developed a new animal model of meningeal leukemia associated with rapid paralysis. Necropsy revealed massive proliferation of the leukemic cells in the bone marrow (BM) followed by pathological angiogenesis and invasion of the leukemic cells into the meninges of the CNS. Further analysis demonstrated that the erythroleukemia cells secreted high levels of VEGF and preferentially adhered in vitro to fibronectin. This unique animal model for meningeal leukemia should facilitate studies of engraftment and proliferation of leukemic cells in the BM and their invasion of the CNS as well as pre-clinical evaluation of experimental therapeutics for CNS-associated leukemias.