Soluble mutant huntingtin drives early human pathogenesis in Huntington's disease

Huntington's disease (HD) is an incurable inherited brain disorder characterised by massive degeneration of striatal neurons, which correlates with abnormal accumulation of misfolded mutant huntingtin (mHTT) protein. Research on HD has been hampered by the inability to study early dysfunction and progressive degeneration of human striatal neurons in vivo. To investigate human pathogenesis in a physiologically relevant context, we transplanted human pluripotent stem cell-derived neural progenitor cells (hNPCs) from control and HD patients into the striatum of new-born mice. Most hNPCs differentiated into striatal neurons that projected to their target areas and established synaptic connexions within the host basal ganglia circuitry. Remarkably, HD human striatal neurons first developed soluble forms of mHTT, which primarily targeted endoplasmic reticulum, mitochondria and nuclear membrane to cause structural alterations. Furthermore, HD human cells secreted extracellular vesicles containing mHTT monomers and oligomers, which were internalised by non-mutated mouse striatal neurons triggering cell death. We conclude that interaction of mHTT soluble forms with key cellular organelles initially drives disease progression in HD patients and their transmission through exosomes contributes to spread the disease in a non-cell autonomous manner.

Efficient and Fast Generation of Relevant Disease Mouse Models by In Vitro and In Vivo Gene Editing of Zygotes

Knockout mice for human disease-causing genes provide valuable models in which new therapeutic approaches can be tested. Electroporation of genome editing tools into zygotes, in vitro or within oviducts, allows for the generation of targeted mutations in a shorter time. We have generated mouse models deficient in genes involved in metabolic rare diseases (Primary Hyperoxaluria Type 1 Pyruvate Kinase Deficiency) or in a tumor suppressor gene (Rasa1). Pairs of guide RNAs were designed to generate controlled deletions that led to the absence of protein. In vitro or in vivo ribonucleoprotein (RNP) electroporation rendered more than 90% and 30% edited newborn animals, respectively. Mice lines with edited alleles were established and disease hallmarks have been verified in the three models that showed a high consistency of results and validating RNP electroporation into zygotes as an efficient technique for disease modeling without the need to outsource to external facilities.

Generation of NKX2.5GFP Reporter Human iPSCs and Differentiation Into Functional Cardiac Fibroblasts

Direct cardiac reprogramming has emerged as an interesting approach for the treatment and regeneration of damaged hearts through the direct conversion of fibroblasts into cardiomyocytes or cardiovascular progenitors. However, in studies with human cells, the lack of reporter fibroblasts has hindered the screening of factors and consequently, the development of robust direct cardiac reprogramming protocols.In this study, we have generated functional human NKX2.5GFP reporter cardiac fibroblasts. We first established a new NKX2.5GFP reporter human induced pluripotent stem cell (hiPSC) line using a CRISPR-Cas9-based knock-in approach in order to preserve function which could alter the biology of the cells. The reporter was found to faithfully track NKX2.5 expressing cells in differentiated NKX2.5GFP hiPSC and the potential of NKX2.5-GFP + cells to give rise to the expected cardiac lineages, including functional ventricular- and atrial-like cardiomyocytes, was demonstrated. Then NKX2.5GFP cardiac fibroblasts were obtained through directed differentiation, and these showed typical fibroblast-like morphology, a specific marker expression profile and, more importantly, functionality similar to patient-derived cardiac fibroblasts. The advantage of using this approach is that it offers an unlimited supply of cellular models for research in cardiac reprogramming, and since NKX2.5 is expressed not only in cardiomyocytes but also in cardiovascular precursors, the detection of both induced cell types would be possible. These reporter lines will be useful tools for human direct cardiac reprogramming research and progress in this field.

CIBERER: Spanish National Network for Research on Rare Diseases: a highly productive collaborative initiative

CIBER (Center for Biomedical Network Research; Centro de Investigación Biomédica En Red) is a public national consortium created in 2006 under the umbrella of the Spanish National Institute of Health Carlos III (ISCIII). This innovative research structure comprises 11 different specific areas dedicated to the main public health priorities in the National Health System. CIBERER, the thematic area of CIBER focused on rare diseases (RDs) currently consists of 75 research groups belonging to universities, research centers, and hospitals of the entire country. CIBERER's mission is to be a center prioritizing and favoring collaboration and cooperation between biomedical and clinical research groups, with special emphasis on the aspects of genetic, molecular, biochemical, and cellular research of RDs. This research is the basis for providing new tools for the diagnosis and therapy of low‐prevalence diseases, in line with the International Rare Diseases Research Consortium (IRDiRC) objectives, thus favoring translational research between the scientific environment of the laboratory and the clinical setting of health centers. In this article, we intend to review CIBERER's 15‐year journey and summarize the main results obtained in terms of internationalization, scientific production, contributions toward the discovery of new therapies and novel genes associated to diseases, cooperation with patients' associations and many other topics related to RD research.

In vivo CRISPR/Cas9 targeting of fusion oncogenes for selective elimination of cancer cells

Fusion oncogenes (FOs) are common in many cancer types and are powerful drivers of tumor development. Because their expression is exclusive to cancer cells and their elimination induces cell apoptosis in FO-driven cancers, FOs are attractive therapeutic targets. However, specifically targeting the resulting chimeric products is challenging. Based on CRISPR/Cas9 technology, here we devise a simple, efficient and non-patient-specific gene-editing strategy through targeting of two introns of the genes involved in the rearrangement, allowing for robust disruption of the FO specifically in cancer cells. As a proof-of-concept of its potential, we demonstrate the efficacy of intron-based targeting of transcription factors or tyrosine kinase FOs in reducing tumor burden/mortality in in vivo models. The FO targeting approach presented here might open new horizons for the selective elimination of cancer cells.

In vivo CRISPR/Cas9 targeting of fusion oncogenes for selective elimination of cancer cells

Fusion oncogenes (FOs) are common in many cancer types and are powerful drivers of tumor development. Because their expression is exclusive to cancer cells and their elimination induces cell apoptosis in FO-driven cancers, FOs are attractive therapeutic targets. However, specifically targeting the resulting chimeric products is challenging. Based on CRISPR/Cas9 technology, here we devise a simple, efficient and non-patient-specific gene-editing strategy through targeting of two introns of the genes involved in the rearrangement, allowing for robust disruption of the FO specifically in cancer cells. As a proof-of-concept of its potential, we demonstrate the efficacy of intron-based targeting of transcription factors or tyrosine kinase FOs in reducing tumor burden/mortality in in vivo models. The FO targeting approach presented here might open new horizons for the selective elimination of cancer cells.

The downregulated membrane expression of CD18 in CD34+ cells defines a primitive population of human hematopoietic stem cells

Background

CD18 is the common beta subunit of β₂ integrins, which are expressed on hematopoietic cells. β₂ integrins are essential for cell adhesion and leukocyte trafficking.

Methods

Here we have analyzed the expression of CD18 in different subsets of human hematopoietic stem and progenitor cells (HSPCs) from cord blood (CB), bone marrow (BM), and mobilized peripheral blood (mPB) samples. CD34⁺ cells were classified into CD18^high and CD18^low/neg, and each of these populations was analyzed for the expression of HSPC markers, as well as for their clonogenity, quiescence state, and repopulating ability in immunodeficient mice.

Results

A downregulated membrane expression of CD18 was associated with a primitive hematopoietic stem cells (HSC) phenotype, as well as with a higher content of quiescent cells and multipotent colony-forming cells (CFCs). Although no differences in the short-term repopulating potential of CD18^low/neg CD34⁺ and CD18^high CD34⁺ cells were observed, CD18^low/neg CD34⁺ cells were characterized by an enhanced long-term repopulating ability in NSG mice.

Conclusions

Overall, our results indicate that the downregulated membrane expression of CD18 characterizes a primitive population of human hematopoietic repopulating cells.

Midkine signaling maintains the self-renewal and tumorigenic capacity of glioma initiating cells

Glioblastoma (GBM) is one of the most aggressive forms of cancer. It has been proposed that the presence within these tumors of a population of cells with stem-like features termed Glioma Initiating Cells (GICs) is responsible for the relapses that take place in the patients with this disease. Targeting this cell population is therefore an issue of great therapeutic interest in neuro-oncology. We had previously found that the neurotrophic factor MIDKINE (MDK) promotes resistance to glioma cell death. The main objective of this work is therefore investigating the role of MDK in the regulation of GICs.

Methods: Assays of gene and protein expression, self-renewal capacity, autophagy and apoptosis in cultures of GICs derived from GBM samples subjected to different treatments. Analysis of the growth of GICs-derived xenografts generated in mice upon blockade of the MDK and its receptor the ALK receptor tyrosine kinase (ALK) upon exposure to different treatments.

Results: Genetic or pharmacological inhibition of MDK or ALK decreases the self-renewal and tumorigenic capacity of GICs via the autophagic degradation of the transcription factor SOX9. Blockade of the MDK/ALK axis in combination with temozolomide depletes the population of GICs in vitro and has a potent anticancer activity in xenografts derived from GICs.

Conclusions: The MDK/ALK axis regulates the self-renewal capacity of GICs by controlling the autophagic degradation of the transcription factor SOX9. Inhibition of the MDK/ALK axis may be a therapeutic strategy to target GICs in GBM patients.

Highly efficient genome editing of human hematopoietic stem cells via a nano-silicon-blade delivery approach

Recently, the clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 bacterial immunity system has opened a promising avenue to treat genetic diseases that affect the human hematopoietic stem cells (HSCs). Therefore, finding a highly efficient delivery method capable of modifying the genome in the hard-to-transfect HSCs, combined with the advanced CRISPR-Cas9 system, may meet the challenges for dissecting the hematologic disease mechanisms and facilitate future clinical applications. Here, we developed an effective HSC-specified delivery microfluidic chip to disrupt the cell membrane transiently by inducing rapid mechanical deformation that allowed the delivery of biomaterials into the cytoplasm from the surrounding matrix. Compared with the previous designs, the new nano-silicon-blade structure was specifically optimized for HSCs. Using the silicon substrate, the sharpness and rigidity of the nano-blade constriction was largely enhanced to improve the biomaterials delivery efficiency. We achieved highly efficient delivery results by transporting various macro-molecules into the HSCs. Moreover, the treated HSCs possess high viability and maintain inherent pluripotency after the delivery via the Nano-Blade Chip (NB-Chip). Subsequently, we disrupted the p42 isoform in C/EBPα on the NB-Chip and induced HSCs into a myeloid proliferation behavior. In conclusion, the NB-Chip provides a harmless, rapid and high-throughput gene editing approach for the HSC study and therapeutics.

Generation of iPSCs from genetically corrected Brca2 hypomorphic cells: implications in cell reprogramming and stem cell therapy

Fanconi anemia (FA) is a complex genetic disease associated with a defective DNA repair pathway known as the FA pathway. In contrast to many other FA proteins, BRCA2 participates downstream in this pathway and has a critical role in homology-directed recombination (HDR). In our current studies, we have observed an extremely low reprogramming efficiency in cells with a hypomorphic mutation in Brca2 (Brca2(Δ) (27/) (Δ27)), that was associated with increased apoptosis and defective generation of nuclear RAD51 foci during the reprogramming process. Gene complementation facilitated the generation of Brca2(Δ) (27/) (Δ27) induced pluripotent stem cells (iPSCs) with a disease-free FA phenotype. Karyotype analyses and comparative genome hybridization arrays of complemented Brca2(Δ) (27/) (Δ27) iPSCs showed, however, the presence of different genetic alterations in these cells, most of which were not evident in their parental Brca2(Δ) (27/) (Δ27) mouse embryonic fibroblasts. Gene-corrected Brca2(Δ) (27/) (Δ27) iPSCs could be differentiated in vitro toward the hematopoietic lineage, although with a more limited efficacy than WT iPSCs or mouse embryonic stem cells, and did not engraft in irradiated Brca2(Δ) (27/) (Δ27) recipients. Our results are consistent with previous studies proposing that HDR is critical for cell reprogramming and demonstrate that reprogramming defects characteristic of Brca2 mutant cells can be efficiently overcome by gene complementation. Finally, based on analysis of the phenotype, genetic stability, and hematopoietic differentiation potential of gene-corrected Brca2(Δ) (27/) (Δ) (27) iPSCs, achievements and limitations in the application of current reprogramming approaches in hematopoietic stem cell therapy are also discussed.

Peritoneal repairing cells: a type of bone marrow derived progenitor cells involved in mesothelial regeneration

The peritoneal mesothelium exhibits a high regenerative ability. Peritoneal regeneration is concomitant with the appearance, in the coelomic cavity, of a free-floating population of cells whose origin and functions are still under discussion. We have isolated and characterized this cell population and we have studied the process of mesothelial regeneration through flow cytometry and confocal microscopy in a murine model lethally irradiated and reconstituted with GFP-expressing bone marrow cells. In unoperated control mice, most free cells positive for mesothelin, a mesothelial marker, are green fluorescent protein (GFP). However, 24 hrs after peritoneal damage, free mesothelin⁺/ GFP⁺ cells appear in peritoneal lavages. Cultured lavage peritoneal cells show colocalization of GFP with mesothelial (mesothelin, cytokeratin) and fibroblastic markers. Immunohistochemical staining of the peritoneal wall also revealed colocalization of GFP with mesothelial markers and with procollagen-1 and smooth muscle α-actin. This was observed in the injured area as well as in the surrounding not-injured peritoneal surfaces. These cells, which we herein call peritoneal repairing cells (PRC), are very abundant 1 week after surgery covering both the damaged peritoneal wall and the surrounding uninjured area. However, they become very scarce 1 month later, when the mesothelium has completely healed. We suggest that PRC constitute a type of monocyte-derived cells, closely related with the tissue-repairing cells known as ‘fibrocytes’ and specifically involved in peritoneal reparation. Thus, our results constitute a synthesis of the different scenarios hitherto proposed about peritoneal regeneration, particularly recruitment of circulating progenitor cells and adhesion of free-floating coelomic cells.

p73 plays a role in erythroid differentiation through GATA1 induction

The TP73 gene gives rise to transactivation domain-p73 isoforms (TAp73) as well as DeltaNp73 variants with a truncated N terminus. Although TAp73alpha and -beta proteins are capable of inducing cell cycle arrest, apoptosis, and differentiation, DeltaNp73 acts in many cell types as a dominant-negative repressor of p53 and TAp73. It has been proposed that p73 is involved in myeloid differentiation, and its altered expression is involved in leukemic degeneration. However, there is little evidence as to which p73 variants (TA or DeltaN) are expressed during differentiation and whether specific p73 isoforms have the capacity to induce, or hinder, this differentiation in leukemia cells. In this study we identify GATA1 as a direct transcriptional target of TAp73alpha. Furthermore, TAp73alpha induces GATA1 activity, and it is required for erythroid differentiation. Additionally, we describe a functional cooperation between TAp73 and DeltaNp73 in the context of erythroid differentiation in human myeloid cells, K562 and UT-7. Moreover, the impaired expression of GATA1 and other erythroid genes in the liver of p73KO embryos, together with the moderated anemia observed in p73KO young mice, suggests a physiological role for TP73 in erythropoiesis.

IGF-I mediates regeneration of endocrine pancreas by increasing beta cell replication through cell cycle protein modulation in mice

AIMS/HYPOTHESIS

Recovery from diabetes requires restoration of beta cell mass. Igf1 expression in beta cells of transgenic mice regenerates the endocrine pancreas during type 1 diabetes. However, the IGF-I-mediated mechanism(s) restoring beta cell mass are not fully understood. Here, we examined the contribution of pre-existing beta cell proliferation and transdifferentiation of progenitor cells from bone marrow in IGF-I-induced islet regeneration.

METHODS

Streptozotocin (STZ)-treated Igf1-expressing transgenic mice transplanted with green fluorescent protein (GFP)-expressing bone marrow cells were used. Bone marrow cell transdifferentiation and beta cell replication were measured by GFP/insulin and by the antigen identified by monoclonal antibody Ki67/insulin immunostaining of pancreatic sections respectively. Key cell cycle proteins were measured by western blot, quantitative RT-PCR and immunohistochemistry.

RESULTS

Despite elevated IGF-I production, recruitment and differentiation of bone marrow cells to beta cells was not increased either in healthy or STZ-treated transgenic mice. In contrast, after STZ treatment, IGF-I overproduction decreased beta cell apoptosis and increased beta cell replication by modulating key cell cycle proteins. Decreased nuclear levels of cyclin-dependent kinase inhibitor 1B (p27) and increased nuclear localisation of cyclin-dependent kinase (CDK)-4 were consistent with increased beta cell proliferation. However, islet expression of cyclin D1 increased only after STZ treatment. In contrast, higher levels of cyclin-dependent kinase inhibitor 1A (p21) were detected in islets from non-STZ-treated transgenic mice.

CONCLUSIONS/INTERPRETATION

These findings indicate that IGF-I modulates cell cycle proteins and increases replication of pre-existing beta cells after damage. Therefore, our study suggests that local production of IGF-I may be a safe approach to regenerate endocrine pancreas to reverse diabetes.

In vitro and in vivo expression of human erythrocyte pyruvate kinase in erythroid cells: a gene therapy approach

Human pyruvate kinase deficiency (PKD), an autosomal recessive disorder produced by mutations in the PKLR gene, is the most common cause of chronic nonspherocytic hemolytic anemia. Transduction of wild-type erythroid (R-type) pyruvate kinase (RPK) cDNA into deficient hematopoietic stem cells could be of potential use as rescue therapy in severe clinical cases. In this study, gammaretroviral vectors expressing human RPK were designed as possible gene therapy candidates for this disease. Through real-time quantitative reverse transcriptase-polymerase chain reaction, Western blotting, and flow cytometric analysis, we demonstrate stable RPK expression in both undifferentiated and differentiated murine erythroleukemia cells. In this in vitro assay, the proportion of transduced cells and the intensity of expression of the transgene remained unaltered after 6 months of culture. Moreover, transplanting human RPK-transduced Lin(-)Sca-1(+) mouse cells in myeloablated primary and secondary recipients rendered high proportions of erythroid precursors and mature erythrocytes expressing RPK, without inducing hematopoietic effects. These findings suggest that retroviral vectors could be useful for the delivery and expression of RPK in erythroid cells, and provide evidence of the potential use of gene therapy strategies to phenotypically correct erythroid PKD.

Functional analysis of gammaretroviral vector transduction by quantitative PCR

BACKGROUND

In a clinical setting of gene therapy, quantitative methods are required to determine recombinant viral titres and transgene mRNA expression, avoiding the use of reporter genes.

METHODS

We describe procedures based on quantitative polymerase chain reaction (qPCR) designed to assess functional titres of murine leukaemia virus (MLV) vectors, determine proviral copy numbers in transduced cells, and estimate retroviral transgene expression in both target cell lines and mice with transduced chimeric haematopoiesis.

RESULTS

Compared to EGFP titration, proviral DNA detection by qPCR was more accurate in assessing the number of infective particles in supernatants, such that average viral titres in terms of proviral copies per cell were two-fold higher. Transgene mRNA expression was directly determined from the vectors used without the need for reporter assays. A new parameter, defined here as the 'transcription index' (TI), served to establish the association between transcribed transgenic mRNA and each proviral insertion. The TI represents the potential expression of every vector or insertion in each cell type, and is thus useful as a control parameter for monitoring preclinical or clinical protocols.

CONCLUSIONS

The practical use of qPCR is demonstrated as a valuable alternative to reporter genes for the assessment and surveillance of insertion numbers and transgene expression. In combination with protein expression, this approach should be capable of establishing safer therapeutic gene doses, avoiding the potential side effects of high transduction and expression levels.

A simplified approach to improve the efficiency and safety of ex vivo hematopoietic gene therapy in fanconi anemia patients

Fanconi anemia (FA) is an inherited DNA repair disorder characterized by genetic instability of cells lacking a functional FA/BRCA pathway. Previous studies have shown that in vitro stimulation of bone marrow cells (BMCs) from FA mice promotes apoptosis, reduces the reconstitution ability of the stem cells, and induces myelodysplasia and myeloid leukemia upon reinfusion of the cells. This suggests the convenience of adapting standard protocols of gene therapy to FA. Here we show that the reserve of BM progenitors in FA patients is generally below 20% of normal values. Because this reduced reserve could activate the cycling of BM progenitors, we developed a simplified protocol to transduce BMCs from FA patients with gammaretroviral vectors. We demonstrate that a short in vitro manipulation (12-24 hr) of fresh mononuclear BMCs is sufficient to transduce 42% of hematopoietic progenitors from FA-A patients, in the absence of in vitro prestimulation. When FANCA-expressing vectors were used, this simple procedure reversed the phenotype of the BM progenitors from these patients. We propose that our approach will be more efficient and safer compared with standard gene therapy protocols for FA.

Efficient engraftment of in utero transplanted mice with retrovirally transduced hematopoietic stem cells

Using an experimental mouse model, we have investigated the kinetics of hematopoietic reconstitution of recipients transplanted during fetal development with fresh and transduced hematopoietic stem cells (HSCs). Total bone marrow (BM) and purified Lin(-)Sca-1(+) cells, either fresh or transduced ex vivo with enhanced green fluorescent protein (EGFP)-encoding retroviral vectors, were in utero transplanted (IUT) into fetal mice. Data obtained 2 months after transplantation showed a similar proportion of engrafted animals, regardless of the fact that samples were purified or not on HSCs, and subjected or not to ex vivo transduction with retroviral vectors. The transplantation of grafts enriched in HSCs, either fresh or transduced, always improved the levels of donor chimerism of IUT mice in comparison with results obtained in mice transplanted with unpurified BM grafts (6.8 and 7.3% versus 1.15% median values, respectively). Significantly, engrafted recipients that were transplanted with the transduced graft always contained transduced EGFP(+) cells in peripheral blood (around 5% of donor cells were EGFP(+) at 2 months post-transplantation). This proportion was essentially maintained at longer times post-transplantation, as well as in secondary recipients transplanted with the BM of IUT mice. Our study describes for the first time a significant and stable engraftment of unconditioned mice subjected to IUT with HSCs transduced with retroviral vectors.

Regulatory elements of the vav gene drive transgene expression in hematopoietic stem cells from adult mice

OBJECTIVE

Previous studies have shown that the HS21/45 promoter of the vav protooncogene drives a predominant expression of exogenous transgenes in mouse hematopoietic cells, including clonogenic bone marrow (BM) progenitors. We investigated the activity of this promoter in the hematopoietic stem cell compartment of adult mice.

MATERIALS AND METHODS

Inbred Ly5.1 transgenic mice expressing a nonfunctional human CD4 marker gene (hCD4) under the control of the HS21/45 promoter were generated. BM cells from these animals were sorted based on the intensity of hCD4 expression. Fractions characterized by high, intermediate, or low/negative expression of the transgene were then assessed for their competitive repopulation ability (CRA), using unfractionated BM cells from Ly5.2 mice as a reference competitor population.

RESULTS

Data showed that BM cells having a low/negative or intermediate expression of hCD4 had a very poor hematopoietic CRA. In contrast, BM cells with high hCD4 expression were characterized by a high CRA. These observations were confirmed in the short- and long-term posttransplantation of primary and secondary recipients when analyzing the lymphoid and myeloid cells of recipient mice.

CONCLUSIONS

Our results demonstrate for the first time that the regulatory HS21/45 sequence of the vav gene constitutes an efficient promoter for driving transgene expression in multipotent hematopoietic stem cells residing in the BM of adult mice. Thus, this promoter is proposed for the development of transgenic mice and gene therapy vectors that require restricted expression of exogenous transgenes in cells of the hematopoietic system, including primitive hematopoietic stem cells.

Purging of leukemia-contaminated bone marrow grafts using suicide adenoviral vectors: an in vivo murine experimental model

Autologous bone marrow transplantation is an alternative therapeutic option for acute myeloid leukemia patients lacking a compatible donor. However, bone marrow from these patients may contain residual leukemic cells that should be ideally eliminated prior to the infusion of the graft. With the aim of developing more efficient protocols of graft purging, adenoviral-mediated gene transfer protocols have been conducted. We studied whether suicide adenoviral vectors expressing the cytosine deaminase gene (AdCD) could be used for selectively killing leukemic WEHI-3B cells. The AdCD transduction followed by the 5-fluorocytosine exposure abrogated the growth of WEHI-3B cells in vitro, with a minimal effect on normal hematopietic progenitors. To test the efficacy of the purging protocol in vivo, bone marrow cells were mixed with syngenic WEHI-3B cells and this chimeric cell population was transduced with AdCD vectors. Infected cells were injected into myeloablated Balb-c mice, which then received a 5-fluorocytosine treatment for 4 days. All mice transplanted with unpurged bone marrow developed leukemia and died. However, 90% of recipients receiving the purging treatment were healthy up to 9 months post-transplantation and had a perfectly re-established hematopoietic system, without any signal of leukemic cell presence. In conclusion, suicide adenoviral vectors are proposed as a tool for the purging of Adenoviral-susceptible myeloid leukemia cells contaminating autologous bone marrow grafts.

In vitro phenotypic correction of hematopoietic progenitors from Fanconi anemia group A knockout mice

Fanconi anemia (FA) is a rare autosomal recessive disease, characterized by bone marrow failure and cancer predisposition. So far, 8 complementation groups have been identified, although mutations in FANCA account for the disease in the majority of FA patients. In this study we characterized the hematopoietic phenotype of a Fanca knockout mouse model and corrected the main phenotypic characteristics of the bone marrow (BM) progenitors using retroviral vectors. The hematopoiesis of these animals was characterized by a modest though significant thrombocytopenia, consistent with reduced numbers of BM megakaryocyte progenitors. As observed in other FA models, the hematopoietic progenitors from Fanca(-/-) mice were highly sensitive to mitomycin C (MMC). In addition, we observed for the first time in a FA mouse model a marked in vitro growth defect of Fanca(-/-) progenitors, either when total BM or when purified Lin(-)Sca-1(+) cells were subjected to in vitro stimulation. Liquid cultures of Fanca(-/-) BM that were stimulated with stem cell factor plus interleukin-11 produced low numbers of granulocyte macrophage colony-forming units, contained a high proportion of apoptotic cells, and generated a decreased proportion of granulocyte versus macrophage cells, compared to normal BM cultures. Aiming to correct the phenotype of Fanca(-/-) progenitors, purified Lin(-)Sca-1(+) cells were transduced with retroviral vectors encoding the enhanced green fluorescent protein (EGFP) gene and human FANCA genes. Lin(-)Sca-1(+) cells from Fanca(-/-) mice were transduced with an efficiency similar to that of samples from wild-type mice. More significantly, transductions with FANCA vectors corrected both the MMC hypersensitivity as well as the impaired ex vivo expansion ability that characterized the BM progenitors of Fanca(-/-) mice.

In vitro and in vivo susceptibility of mouse megakaryocytic progenitors to strain i of parvovirus minute virus of mice

OBJECTIVE

Intranasal inoculation of the i strain of the parvovirus minute virus of mice (MVMi) into immunodeficient SCID mice induces suppression of myeloid and erythroid progenitors in the bone marrow (BM) and lethal leukopenia. In the present study, we investigated whether the mouse megakaryocytic lineage was susceptible to MVMi.

MATERIALS AND METHODS

In vitro and in vivo infections with purified MVMi were conducted and their effects on the megakaryocytic lineage studied.

RESULTS

In vitro infection of BM cells showed a multiplicity of infection-dependent inhibition in the colony-forming ability of megakaryocytic progenitors (colony-forming unit megakaryocyte [CFU-MK]). Neutralization or heat inactivation of the virus abrogated this inhibition. Expression of the MVMi nonstructural-1 protein was detected in the in vitro infected and cultured megakaryocytic cells. In vivo, intranasal inoculation of a lethal dose of virus was incapable of producing significant thrombocytopenia, although an increase in mean platelet volume was observed. Significantly, in the BM of these animals, a progressive decrease in CFU-MK was noted from day 14 postinfection, with survival rates less than 1% by day 35 postinfection. At day 35 postinfection, intermediate megakaryocytic differentiation stages showed maintenance of the proportion and ploidy of cells and a moderate decrease in the total number of these cells per femoral BM.

CONCLUSIONS

The results demonstrate that MVMi is capable of inhibiting the proliferative capacity of megakaryocytic committed progenitors both in vitro and in vivo. Moreover, the in vivo data show that depletion of BM CFU-MK is compensated by the system, and platelet counts in the peripheral blood are maintained close to normal values.

Ex vivo expansion of umbilical cord blood (UCB) CD34(+) cells alters the expression and function of alpha 4 beta 1 and alpha 5 beta 1 integrins

We have investigated the influence of ex vivo expansion of human CD34(+) cord blood cells on the expression and function of adhesion molecules involved in the homing and engraftment of haematopoietic progenitors. Ex vivo expansion of umbilical cord blood CD34(+) cells for 6 d in the presence of interleukin 3 (IL-3), IL-6 and stem cell factor (SCF) or IL-11, SCF and Flt-3L resulted in increased expression of alpha 4, alpha 5, beta 1, alpha M and beta 2 integrins. However, a significant decrease in the adhesion of progenitor cells to fibronectin was observed after the ex vivo culture (adhesion of granulocyte-macrophage colony-forming units (CFU-GM) was 22 +/- 4% in fresh cells versus 5 +/- 2% and 2 +/- 2% in each combination of cytokines). Incubation with the beta 1 integrin-activating antibody TS2/16 restored adhesion to fibronectin. Transplantation of ex vivo expanded umbilical cord blood CD34(+) cells was associated with an early delayed engraftment in non-obese diabetic/severe combined immunodeficient (NOD/SCID) mice. Incubation of cells with the monoclonal antibody TS2/16 before transplantation almost completely abrogated NOD/SCID repopulating ability of both fresh and expanded CD34(+) cells. The seeding efficiency of fresh and expanded CD34(+) cells was similar, but markedly reduced after incubation with the TS2/16 monoclonal antibody. Our results show that functional activation of beta 1 integrins could overcome the decreased very late antigen (VLA)-4- and VLA-5-mediated adhesion observed after ex vivo expansion of haematopoietic progenitors. However, in vivo, these effects induced an almost complete abrogation of the homing and repopulating ability of CD34(+) UCB cells.

A cutaneous gene therapy approach to human leptin deficiencies: correction of the murine ob/ob phenotype using leptin-targeted keratinocyte grafts

Leptin deficiency produces a phenotype of obesity, diabetes, and infertility in the ob/ob mouse. In humans, leptin deficiency occurs in some cases of congenital obesity and in lipodystrophic disorders characterized by reduced adipose tissue and insulin resistance. Cutaneous gene therapy is considered an attractive potential method to correct circulating protein deficiencies, since gene-transferred human keratinocytes can produce and secrete gene products with systemic action. However, no studies showing correction of a systemic defect have been reported. We report the successful correction of leptin deficiency using cutaneous gene therapy in the ob/ob mouse model. As a feasibility approach, skin explants from transgenic mice overexpressing leptin were grafted on immunodeficient ob/ob mice. One month later, recipient mice reached body weight values of lean animals. Other biochemical and clinical parameters were also normalized. In a second human gene therapy approach, a retroviral vector encoding both leptin and EGFP cDNAs was used to transduce HK and, epithelial grafts enriched in high leptin-producing HK were transplanted to immunosuppressed ob/ob mice. HK-derived leptin induced body weight reduction after a drop in blood glucose and food intake. Leptin replacement through genetically engineered HK grafts provides a valuable therapeutic alternative for permanent treatment of human leptin deficiency conditions.

Latent hematopoietic stem cell toxicity associated with protracted drug administration

OBJECTIVE

The protracted administration of near-conventional daily doses of chemotherapeutic agents is a strategy to increase dose intensity and, potentially, efficacy as well. However, protracted therapy carries the risk of damage to stem cells in proliferative tissues that are not targeted by intermittent schedules. Therefore, we have investigated the effects produced by the protracted administration of two anticancer drugs on hematopoietic stem cell function.

MATERIALS AND METHODS

We used the competitive repopulating assay to assess stem cell damage caused by protracted daily drug treatment of mice.

RESULTS

Treatment with acetyldinaline for 10 consecutive days mediated a modest effect on the short-term repopulating cells (STRCs) but spared the long-term repopulating cells (LTRCs). Gemcitabine for 10 days led to a modest decline in both the STRCs and LTRCs. Extending treatment with gemcitabine for 28 days resulted in more severe repopulating cell (RC) damage, which was much worse than in acetyldinaline-treated mice. As expected, melphalan for 10 or 28 days mediated a marked reduction in all of the RCs of treated mice. The analysis of the RCs from mice that were allowed a 1-year recovery period after completing the 28-day treatment with either acetyldinaline or gemcitabine showed normal levels of neutrophils and bone marrow (BM) progenitors. However, a reduction in the RCs was observed in both groups, with larger reductions in gemcitabine-treated mice.

CONCLUSIONS

Our data show that protracted treatment with gemcitabine, but not acetyldinaline, of mice caused severe permanent damage to the stem cell components. Therefore, although 28-day therapy with acetyldinaline or gemcitabine appeared to be well tolerated at the level of peripheral blood and bone marrow progenitors, gemcitabine produces permanent stem cell damage when used in long-term administration regimens that should perhaps only be explored clinically with stem cell support available.

Selective transduction of murine myelomonocytic leukemia cells (WEHI-3B) with regular and RGD-adenoviral vectors

On the basis of the susceptibility of normal myelomonocytic cells to adenoviral vectors, we have studied the possibility of selectively transducing myelomonocytic murine leukemic cells (WEHI-3B) with regular (Reg-Ad) and genetically modified (RGD-Ad) adenoviral vectors. An 8-h incubation of WEHI-3B cells with 100 pfu of Reg-Ad vectors/cell resulted in the whole population becoming positive for transgene expression. Under identical conditions of infection, 20-30% of mouse bone marrow (BM) cells were positive for the transgene. When RGD-Ad vectors were used, a brief exposure (10 min) of WEHI-3B cells to 150 pfu of the virus/cell was enough for 100% of the leukemia cells to become positive for the marker transgene (EGFP). Under these conditions, only 15-20% of BM cells and of primitive hematopoietic progenitors (Lin(-)Sca-1(+) cells) became EGFP(+), indicating an improved selectivity of the vectors for the leukemic cells. The incubation of WEHI-3B but not normal BM cells with soluble fiber protein (FP) inhibited the infection with Reg-Ad. The use of the RGD-Ad bypassed the FP-CAR interaction required for the transduction of WEHI-3B cells with Reg-Ad, suggesting that the abrogation of this requirement accounts for the improved infectivity of these leukemic cells and for the selectivity of RGD-Ad in targeting WEHI-3B leukemia cells.

Systematic analysis of clinically applicable conditions leading to a high efficiency of transduction and transgene expression in human T cells

BACKGROUND

The transduction of human peripheral blood T cells with retroviral vectors constitutes an attractive approach for the correction of a number of genetic diseases. In this study we have conducted a systematic analysis of the relevance of a large number of parameters currently considered to affect the transduction of, and transgene expression in, human T cells.

METHODS

Retroviral vectors encoding the human nerve growth factor receptor (NGFR) were used for transducing human T cells from normal volunteers. The proportion of T cells that expressed the marker transgene was determined by flow cytometry using anti-NGFR antibodies.

RESULTS

Spinoculation and static fibronectin (FN)-assisted infections improved to a similar extent the transduction efficiency of PHA/IL-2 stimulated T cells, when compared with samples subjected to standard static infections. When immobilized anti-CD3 (anti-CD3i) or anti-CD3i/28i-stimulated T cells were considered, static infections in FN-coated plates were reproducibly more efficient than spinoculation infections performed on FN-uncoated plates. Under optimized manipulation conditions (three infection cycles of anti-CD3i/28i-stimulated T cells in FN-coated plates) the total number of NGFR+ T cells harvested after 7 days of incubation represented, on average, twice the total number of T cells seeded at Day 0, and up to 95% of the human T cells efficiently expressed the marker transgene. Similar results were obtained regardless of whether samples were manipulated in medium supplemented with fetal bovine serum or with heat-inactivated autologous serum.

CONCLUSIONS

Our study offers new experimental conditions for the transduction of human T cells, with obvious implications for the development of gene therapy protocols.

Efficient transduction of human hematopoietic repopulating cells generating stable engraftment of transgene-expressing cells in NOD/SCID mice

In an attempt to develop efficient procedures of human hematopoietic gene therapy, retrovirally transduced CD34(+) cord blood cells were transplanted into NOD/SCID mice to evaluate the repopulating potential of transduced grafts. Samples were prestimulated on Retronectin-coated dishes and infected with gibbon ape leukemia virus (GALV)-pseudotyped FMEV vectors encoding the enhanced green fluorescent protein (EGFP). Periodic analyses of bone marrow (BM) from transplanted recipients revealed a sustained engraftment of human hematopoietic cells expressing the EGFP transgene. On average, 33.6% of human CD45(+) cells expressed the transgene 90 to 120 days after transplantation. Moreover, 11.9% of total NOD/SCID BM consisted of human CD45(+) cells expressing the EGFP transgene at this time. The transplantation of purified EGFP(+) cells increased the proportion of CD45(+) cells positive for EGFP expression to 57. 7% at 90 to 120 days after transplantation. At this time, 18.9% and 4.3% of NOD/SCID BM consisted of CD45(+)/EGFP(+) and CD34(+)/EGFP(+) cells, respectively. Interestingly, the transplantation of EGFP(-) cells purified at 24 hours after infection also generated a significant engraftment of CD45(+)/EGFP(+) and CD34(+)/EGFP(+) cells, suggesting that a number of transduced repopulating cells did not express the transgene at that time. Molecular analysis of NOD/SCID BM confirmed the high levels of engraftment of human transduced cells deduced from FACS analysis. Finally, the analysis of the provirus insertion sites by conventional Southern blotting indicated that the human hematopoiesis in the NOD/SCID BM was predominantly oligoclonal.

Transplantation of syngenic bone marrow contaminated with NGFr-marked WEHI-3B cells: an improved model of leukemia relapse in mice

With the aim of developing a model mimicking the relapse of patients transplanted with leukemia-contaminated grafts, myelomonocytic leukemia WEHI-3B D+ cells were first transduced with a retroviral vector encoding the low-affinity human nerve growth factor receptor (NGFr). Clones with a stable and homogeneous expression of the transgene and with a similar in vitro behavior to the parental cell line were selected for further experiments. The analysis of bone marrow (BM) contaminated with WEHI-3B/NGFr cells revealed a linear correlation (r2 = 0.999) between the actual values of BM contamination and the experimental data determined by flow cytometry. Balb/c mice were myeloablated and transplanted with syngenic BM contaminated with graded numbers of leukemic cells; dose-dependent survival curves were obtained, regardless of whether parental or WEHI-3B/NGFr cells were infused. The leukemia dissemination in recipients transplanted with WEHI-3B/NGFr contaminated grafts was easily determined by means of simple flow cytometry analysis of the NGFr marker. A leukemia dose-dependent increase in the number of PB leukocytes was observed in transplanted recipients at 20 days post-transplantation with no changes in myelomonocytic cells. As deduced from our observations, the transplantation of syngenic BM contaminated with WEHI-3B/NGFr cells constitutes an improved model of autograft-mediated leukemia relapse and a good tool for studies of leukemia cell purging.

Preserved long-term repopulation and differentiation properties of hematopoietic grafts subjected to ex vivo expansion with stem cell factor and interleukin 11

BACKGROUND

The ex vivo expansion of hematopoietic grafts has been proposed as an efficient procedure for improving the hematological recovery of recipients. The fate of the long-term repopulating cells during the ex vivo manipulation of the graft is, however, a critical issue in ex vivo expansion protocols and ultimately will define the applicability of this new technology in hematopoietic transplants.

METHODS

The repopulating ability of mouse hematopoietic samples was determined by means of bone marrow (BM*) competition assays, using congenic strains that express the pan-leukocyte Ly-5.1 and Ly-5.2 antigens. The generation of potential changes in the repopulating properties of human hematopoietic samples subjected to ex vivo expansion was determined by comparing the engraftment of fresh and ex vivo-manipulated CD34+ cord blood cells in irradiated nonobese diabetic/severe-combined immunodeficient (NOD/SCID) mice.

RESULTS

Under our optimized conditions of mouse BM incubation (stem cell factor plus interleukin-11, either with or without macrophage inflammatory protein-1alpha or Flt3 ligand), both the short-term and the mid-term repopulating ability of the ex vivo-expanded samples were significantly improved when compared with fresh samples. In the long-term, no changes in the repopulation and differentiation properties of the graft were observed as a result of the ex vivo expansion process. As deduced from the analysis of NOD/SCID mice transplanted with fresh and ex vivo expanded human CD34+ cord blood cells, the in vitro stimulation mediated by SCF/IL-11/FLT3L was capable of preserving the ability of the grafts to repopulate the lympho-hematopoiesis of recipients for at least 3 months.

CONCLUSION

These results indicate that under our optimized conditions of ex vivo expansion, the amplification of the hematopoietic progenitors responsible for the short- and mid-term repopulating properties of the graft can take place without compromising the long-term lympho-hematopoietic repopulating properties.

Severe leukopenia and dysregulated erythropoiesis in SCID mice persistently infected with the parvovirus minute virus of mice

Parvovirus minute virus of mice strain i (MVMi) infects committed granulocyte-macrophage CFU and erythroid burst-forming unit (CFU-GM and BFU-E, respectively) and pluripotent (CFU-S) mouse hematopoietic progenitors in vitro. To study the effects of MVMi infection on mouse hemopoiesis in the absence of a specific immune response, adult SCID mice were inoculated by the natural intranasal route of infection and monitored for hematopoietic and viral multiplication parameters. Infected animals developed a very severe viral-dose-dependent leukopenia by 30 days postinfection (d.p.i.) that led to death within 100 days, even though the number of circulating platelets and erythrocytes remained unaltered throughout the disease. In the bone marrow of every lethally inoculated mouse, a deep suppression of CFU-GM and BFU-E clonogenic progenitors occurring during the 20- to 35-d.p.i. interval corresponded with the maximal MVMi production, as determined by the accumulation of virus DNA replicative intermediates and the yield of infectious virus. Viral productive infection was limited to a small subset of primitive cells expressing the major replicative viral antigen (NS-1 protein), the numbers of which declined with the disease. However, the infection induced a sharp and lasting unbalance of the marrow hemopoiesis, denoted by a marked depletion of granulomacrophagic cells (GR-1(+) and MAC-1(+)) concomitant with a twofold absolute increase in erythroid cells (TER-119(+)). A stimulated definitive erythropoiesis in the infected mice was further evidenced by a 12-fold increase per femur of recognizable proerythroblasts, a quantitative apoptosis confined to uninfected TER-119(+) cells, as well as by a 4-fold elevation in the number of circulating reticulocytes. Therefore, MVMi targets and suppresses primitive hemopoietic progenitors leading to a very severe leukopenia, but compensatory mechanisms are mounted specifically by the erythroid lineage that maintain an effective erythropoiesis. The results show that infection of SCID mice with the parvovirus MVMi causes a novel dysregulation of murine hemopoiesis in vivo.

Delayed engraftment of nonobese diabetic/severe combined immunodeficient mice transplanted with ex vivo-expanded human CD34(+) cord blood cells

The ex vivo expansion of hematopoietic progenitors is a promising approach for accelerating the engraftment of recipients, particularly when cord blood (CB) is used as a source of hematopoietic graft. With the aim of defining the in vivo repopulating properties of ex vivo-expanded CB cells, purified CD34(+) cells were subjected to ex vivo expansion, and equivalent proportions of fresh and ex vivo-expanded samples were transplanted into irradiated nonobese diabetic (NOD)/severe combined immunodeficient (SCID) mice. At periodic intervals after transplantation, femoral bone marrow (BM) samples were obtained from NOD/SCID recipients and the kinetics of engraftment evaluated individually. The transplantation of fresh CD34(+) cells generated a dose-dependent engraftment of recipients, which was evident in all of the posttransplantation times analyzed (15 to 120 days). When compared with fresh CB, samples stimulated for 6 days with interleukin-3 (IL-3)/IL-6/stem cell factor (SCF) contained increased numbers of hematopoietic progenitors (20-fold increase in colony-forming unit granulocyte-macrophage [CFU-GM]). However, a significant impairment in the short-term repopulation of recipients was associated with the transplantation of the ex vivo-expanded versus the fresh CB cells (CD45(+) repopulation in NOD/SCIDs BM: 3. 7% +/- 1.2% v 26.2% +/- 5.9%, respectively, at 20 days posttransplantation; P <.005). An impaired short-term engraftment was also observed in mice transplanted with CB cells incubated with IL-11/SCF/FLT-3 ligand (3.5% +/- 1.7% of CD45(+) cells in femoral BM at 20 days posttransplantation). In contrast to these data, a similar repopulation with the fresh and the ex vivo-expanded cells was observed at later stages posttransplantation. At 120 days, the repopulation of CD45(+) and CD45(+)/CD34(+) cells in the femoral BM of recipients ranged between 67.2% to 81.1% and 8.6% to 12.6%, respectively, and no significant differences of engraftment between recipients transplanted with fresh and the ex vivo-expanded samples were found. The analysis of the engrafted CD45(+) cells showed that both the fresh and the in vitro-incubated samples were capable of lymphomyeloid reconstitution. Our results suggest that although the ex vivo expansion of CB cells preserves the long-term repopulating ability of the sample, an unexpected delay of engraftment is associated with the transplantation of these manipulated cells.

Does the granulocyte-macrophage colony-forming unit content in ex vivo-expanded grafts predict the recovery of the recipient leukocytes?

We have investigated the leukocyte-repopulating-predictive value of granulocyte-macrophage colony-forming unit (CFU-GM) analyses in ex vivo-expanded versus fresh murine bone marrow (BM) grafts. After the transplantation of graded numbers of normal BM cells (from 15 to 5 x 10(3) CFU-GMs/mice), a dose-dependent increase in the recipient leukocytes was observed between the first and third weeks posttransplantation. During these stages, increases in the graft size of 100-fold improved the leukocyte counts up to 30-fold and shortened the leukopenia period by 5 to 11 days, depending on the leukocyte threshold considered. To investigate whether similar correlations could be established using ex vivo-expanded samples, the size of the CFU-GM population was maximized by means of the preactivation of the BM with 5-fluorouracil (9-day 5FU-BM), followed by 3 days of incubation with interleukin-1 plus stem cell factor. Under these conditions, the CFU-GM content of the ex vivo-expanded grafts was 73-fold higher than that observed in equivalent femoral fractions of normal fresh BM. When equivalent fractions of both graft types were transplanted, an improved leukocyte recovery was observed in mice transfused with the expanded grafts. However, the leukocyte values obtained after the transplantation of the ex vivo-expanded samples were not as high as expected, based on the number of transplanted CFU-GMs. Analyses performed during the second week posttransplantation showed that, in comparison with normal fresh BM, ex vivo-expanded grafts containing 6 to 50 times more CFU-GMs were required to generate a similar number of leukocytes. These results were confirmed in both the peripheral blood leukocytes and the myeloid Gr1+ cells, when similar numbers of CFU-GMs were transfused in the fresh and the ex vivo-expanded BM. The possibility that the preactivation of the ex vivo-expanded grafts with 5FU had a role in this effect was ruled out, because the leukocyte repopulation capacity of fresh 5FU-treated BM was as high as that observed in normal fresh BM which contained a similar number of CFU-GMs. Neither by extending the ex vivo incubation period nor by using other hematopoietic growth factor combinations was the functional capacity of the expanded grafts improved. The results presented in this study are consistent with the belief that ex vivo expansion procedures will be a useful tool for improving the hematologic recovery of patients who receive hematopoietic transplants. However, our data indicate that predicting the leukocyte repopulating capacity of ex vivo-expanded grafts according to correlations established with numbers of fresh CFU-GMs can lead to overestimations of their function, and therefore to unexpected and delayed hematopoietic engraftments.

The expression of Bcl-x is downregulated during differentiation of human hematopoietic progenitor cells along the granulocyte but not the monocyte/macrophage lineage

Expression of the apoptosis inhibitory protein Bcl-x was studied in CD34+ hematopoietic precursor cells and in the promyelocytic leukemia cell line HL-60. The enriched population of CD34+ cells (more than 95%) was cultured in the presence of stem cell factor, interleukin-3 (IL-3), IL-6, and either granulocyte colony-stimulating factor or macrophage colony-stimulating factor to achieve granulocyte or monocyte/macrophage differentiation, respectively. The expression of Bcl-x increased in the early stages of both differentiation pathways. However, by day 21 of culture mature granulocytes were Bcl-x-negative, whereas monocytes/macrophages either maintained or increased the expression of Bcl-x. The pattern of Bcl-x expression in the differentiated CD34+ cells was similar to that observed in HL-60 cells differentiated along the granulocyte lineage (induced by incubation with retinoic acid), or along the monocyte/macrophage lineage (induced by incubation with phorbol diester). The bcl-x transcript predominant in HL-60 and CD34+ cells differentiated into monocytes/macrophages was bcl-xL. Although little is yet known regarding the functional significance of Bcl-x within the granulomonocytic compartment, marked changes in the pattern of its expression, as observed during granulomonocytic differentiation of HL-60 and CD34+ cells, are likely to alter the life span of mature granulocytes and monocytes/macrophages.

Myeloid depression follows infection of susceptible newborn mice with the parvovirus minute virus of mice (strain i)

The in vivo myelosuppressive capacity of strain i of the parovirus minute virus of mice (MVMi) was investigated in newborn BALB/c mice inoculated with a lethal intranasal dose. MVMi infection reached maximum levels of DNA synthesis and infectious titers in lymphohemopoietic organs at 4 to 6 days postinoculation and was restricted by an early neutralizing humoral immune response. After viral control (by 10 days postinoculation), a significant decrease in femoral and splenic cellularity, as well as in granulocyte-macrophage colony-forming unit and erythroid burst-forming unit hemopoietic progenitors, was observed in most inoculated animals. This delayed myeloid depression, although it may be not a major cause of the lethality of the infection, implies indirect pathogenic mechanisms induced by MVMi infection in a susceptible host.

Cytotoxic infection of hematopoietic stem and committed progenitor cells by the parvovirus minute virus of mice. Propagation of an acute myelosuppression in culture

We have investigated the ability of two strains of the parvovirus minute virus of mice to impair mouse hematopoiesis in vitro. We found that the BFU-E and CFU-GM committed progenitors, CFU-Mix pluripotent progenitor, as well as the CFU-S12d, one of the most primitive hematopoietic precursors of the stem cell compartment detectable by colony technique, were similarly inhibited in their proliferative capacity by the immunosuppressive strain MVMi, but not by the prototype virus MVMp. The inhibitory effect correlated with the input of purified MVMi and was reversed by neutralizing MVM antiserum, showing that cytotoxic mechanisms underlying infectious MVMi replication and not operating in MVMp-infected cells were responsible for the reproductive death of hematopoietic precursors. In agreement with this, myeloid nonadherent cells of long-term bone marrow cultures were selectively permissive for MVMi but not for MVMp replication, as judged by viral DNA synthesis, the expression of the nonstructural cytotoxic NS-1 protein, and virus propagation in these cells. Altogether, the suppressive effects mediated by the MVMi cytotoxic infection define a wide lympho-myelotropism not previously reported for this virus. The MVM-mouse model highlights the role that unsuspected virus-hematopoietic compartment interactions may play in bone marrow failures of immunocompromised animal or human hosts.

In vitro myelosuppressive effects of the parvovirus minute virus of mice (MVMi) on hematopoietic stem and committed progenitor cells

The interaction of two strains of the parvovirus minute virus of mice (MVM) with the mouse hematopoietic system has been studied. The immunosuppressive strain MVMi, but not the prototype virus MVMp, inhibited hematopoiesis in vitro, as judged by colony-forming assays of the erythroid burst-forming unit and granulocyte-monocyte colony-forming unit (CFU-GM) progenitors. Interestingly, primitive hematopoietic cells of the stem compartment (CFU-S12d), were equally susceptible to the MVMi cytotoxic infection, unravelling an unprecedented feature of virus-hematopoiesis interactions. The replication of both strains of MVM virus was evaluated in primary myeloid cells of long-term bone marrow cultures. A high viral DNA synthesis and maturation was observed in MVMi-infected myeloid cells, but it was undetectable in MVMp infections; moreover, the expression of the cytotoxic nonstructural NS-1 protein, a more reliable parameter of cell permissiveness to MVM infection, was only detected in MVMi-infected cells. Correspondingly, MVMi was propagated to high titers of infectious virus and it mediated an acute myelosuppression in these cultures. We conclude that MVMi has a wider tropism than was previously suspected and it is proposed that cytotoxic infection of hematopoietic stem cells, besides that of committed progenitors, may provide an additional basis to understand the pathogenesis of certain animal and human bone marrow failures of viral etiology.