Engineering Precision Medicine to Increase Graft-Versus-Lymphoma Activity: Hematopoietic Stem Cells Modified with Chimeric Antigen Receptors

Abstract Background: Patients with refractory or recurrent B-lineage hematological malignancies have less than 50% of chance of cure. Trials using autologous T-cells engineered with anti-CD19 chimeric antigen receptors (CAR) have demonstrated complete remissions even in chemotherapy-resistant malignancies, but the persistence of the cells is transient, limiting efficacy. Our hypothesis is modification of hematopoietic stem cells (HSC) with CAR will lead to persistent production of target-specific immune cells in multiple lineages, enhancing graft-versus-tumor activity and development of immunological memory. Design/Methods: Using CD19 as target, we generated second-generation CD28- and 4-1BB-costimulated CAR constructs for modification of human HSC for assessment in vitro and in vivo, using third-generation lentiviral vectors. Additionally, co-delivery of suicide gene systems was tested to allow ablation of gene-modified cells. Results: Gene modification of HSC with anti-CD19 CAR using lentiviral vectors did not impair differentiation or proliferation, and led to functional CAR-expressing cell progeny, at 40-50% transduction efficiency and engineered antigen-dependent cytotoxicity in myeloid, NK and T-cells. In vivo studies using humanized NSG engrafted with CAR-modified HSC demonstrated similar levels of humanization to non-modified HSC, with multilineage CAR-expressing cells present in bone marrow, spleen, blood and thymus in stable levels up to 44 weeks of life. No animals engrafted with CAR-modified HSC presented autoimmunity or inflammation. Ex vivo cells presented antigen-dependent cytotoxicity against targets. Mice engrafted with CAR-modified HSC had decreased CD19+ populations and successfully presented tumor growth inhibition and survival advantage at tumor challenge (55-60%). CAR-modified HSC led to development of T-cell effector memory and T-cell central memory phenotypes, confirming the development of long-lasting phenotypes due to directed antigen specificity. Mice humanized with gene-modified HSC presented significant ablation of gene-modified cells after treatment (p=0.002). Remaining gene-modified cells were close to background on flow cytometry and within two logs of decrease of vector copy numbers by ddPCR in mouse tissues. Conclusions: CAR modification of HSC for cancer immunotherapy is feasible. This approach can be applied to different cancers by adjusting target specificity and could be employed in the context of HSC transplantation to augment the anti-lymphoma activity. It also bears the possibility of decreased morbidity and mortality and offers alternative treatment for patients with no available sources for bone marrow transplantation, benefiting ethnic minorities. These results also give proof of principle for CAR-modified HSC regulated by suicide gene; further studies are needed to enable clinical translation. Disclosures No relevant conflicts of interest to declare.

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ZFN-Driven Gene Editing Prevents HLA-A Expression On Hematopoietic Stem Cells -Improving The Chance Of Finding An HLA-Matched Donor

Blood ◽

10.1182/blood.v122.21.1655.1655 ◽

2013 ◽

Vol 122 (21) ◽

pp. 1655-1655

Author(s):

Hiroki Torikai ◽

Tiejuan Mi ◽

Sonny O Ang ◽

Loren Gragert ◽

Martin Maiers ◽

...

Keyword(s):

Stem Cells ◽

Bone Marrow ◽

T Cells ◽

Hematopoietic Stem Cells ◽

Donor Pool ◽

Hematopoietic Stem ◽

Nsg Mice ◽

Immune Mediated ◽

Matched Donor

Abstract Hematopoietic stem cells (HSCs) are administered (i) to restore hematopoiesis and immunity in the course of hematopoietic stem-cell transplantation (HSCT), (ii) as a replacement for inherited blood disorders and bone marrow failure, (iii) to regenerate cells of alternative lineages for restorative medicine, and (iv) as a source for generating specific hematopoietic cells (e.g., T cells, NK cells, and dendritic cells). However, the widespread application of allogeneic HSCs for humans is hampered by their immune-mediated destruction by host T cells recognizing mismatched HLA or by HLA-specific antibodies. Despite pre-banking umbilical cord blood (UCB) units and access to adult donors through the National Marrow Donor Program (NMDP), finding a suitable HLA-matched product is challenging for many recipients, especially those from ethnic minorities who are under-represented in the donor pool. The available donor pool would be markedly increased if donor HSCs were edited to eliminate expression of the HLA-A locus. Indeed, modeling from NMDP shows that the chance of an African American recipient finding a HLA-matched donor increases from 18% to 73% when matched for HLA-B, C and DR, instead of HLA-A, B, C and DR. We have previously shown that engineered zinc finger nucleases (ZFNs) can disrupt HLA-A expression in genetically edited T cells (Blood 2013). To extend this proof-of-concept to HSCs, we sought to disrupt HLA-A expression by introducing ZFNs targeting this locus. CD34+lineageneg HSCs (99% purity) were isolated using paramagnetic beads from UCB. Electro-transfer of in vitro transcribed mRNA encoding the HLA-A-specific ZFN generated 30% HLA-Aneg HSCs after one week ex vivo culture with defined cytokines (FLT3-L, SCF, TPO, and IL-6) and an aryl hydrocarbon receptor antagonist (stem reginin-1, SR-1). As expected, SR1 treatment maintained greater numbers of CD34+ cells (also CD34posCD38neg) in culture compared to controls. DNA sequence analysis revealed that HLA-Aneg HSCs encode the expected nucleotide changes at the ZFN target site. An in vivo engraftment assay, using NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) mice, demonstrated that HLA-Aneg HSCs maintain the capability of engraftment and differentiation into HLA-Aneg hematopoietic cells (Figure). Thus, disruption of HLA-A expression in HSCs provides an appealing approach to increasing the chances for of finding HLA-matched donors and may broaden the clinical application of allogeneic HSCT. Furthermore, the ability to genetically edit HSCs has implications for (i) preventing immune-mediated recognition of HLA-disparate HSC and (ii) preventing immune mediated recognition of self-antigens. Engraftment of HLA-A2neg HSCs was evaluated in vivo. Data shown are flow-cytometry analysis of bone marrow obtained from NSG mice 16 weeks after HSC injection. HSC engraftment and HLA-A2 expression in NSG mice injected with un-modified HSCs (left panel) and HSCs treated with the HLA-A specific ZFNs (right panel) are shown. Data are gated on human CD45 positive cells. Figure Engraftment of HSCs modified by the HLA-A specific ZFNs in NSG mice. Figure. Engraftment of HSCs modified by the HLA-A specific ZFNs in NSG mice. Disclosures: Reik: Sangamo BioSciences: Employment. Holmes:Sangamo BioSciences: Employment. Gregory:Sangamo BioSciences: Employment.

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Reprogrammed Adult Human Endothelium into Hematopoietic Stem Cells Yields Functional T Cells In Vivo

Blood ◽

10.1182/blood-2018-169 ◽

2018 ◽

Vol 132 (Supplement 1) ◽

pp. 169-169

Author(s):

José Gabriel Barcia Durán ◽

Tyler M. Lu ◽

Raphael Lis ◽

Shahin Rafii

Keyword(s):

Stem Cells ◽

Bone Marrow ◽

T Cells ◽

Hematopoietic Stem Cells ◽

Constitutive Expression ◽

Direct Conversion ◽

Hematopoietic Stem ◽

Nsg Mice ◽

Human Endothelium

Abstract During development, the hematopoietic stem cells that go on to populate the bone marrow and give rise to all blood cell lineages emerge from a specialized endothelial subpopulation. We have previously harnessed this vestigial identity to achieve the direct conversion of adult endothelial cells (ECs) into long-term engraftable hematopoietic stem and progenitor cells (rEC-HSPCs); however, to date, we had only detected and characterized functional T cells that result from the transplantation, engraftment, and differentiation of mouse rEC-HSPCs (Lis, R. et al., Conversion of adult endothelium to immunocompetent haematopoietic stem cells. Nature, 545:439-445. 2017). We first reprogrammed adult human endothelium using lentiviral vectors encoding FOSB, GFI1, SPI1, and RUNX1 (FGRS) under constitutive promoters and an inductive vascular niche (Sandler V. et al., Reprogramming of human endothelium into hematopoietic cells requires vascular niche induction. Nature, 511:312-8. 2014). The resulting rEC-HSPCs maintained exogenous expression of the four transcription factors for over 20 weeks post-transplantation into immuno-compromised NSG mice. Constitutive expression of Spi1, however, has been shown to hinder lymphoid differentiation in vivo by blocking T lymphopoiesis (Anderson, M. et al. Constitutive expression of PU.1 in fetal hematopoietic progenitors blocks T cell development at the pro-T cell stage. Immunity 16:285-296. 2002). In addition, mice of the NSG strain cannot educate native B or T cells to maturity. Our system of constitutive exogenous FGRS expression was therefore unable to confer transplanted immuno-compromised mice the ability to generate an adaptive immune response. Here, we obtained human rEC-HSPCs making use of (i) doxycycline-inducible vectors to temporarily overexpress FGRS and (ii) transgenic substrains of NSG mice (one carrying human stem cell factor, granulocyte/macrophage colony-stimulating factor, and interleukin 3; the other, human major histocompatibility complex class I as well as beta-2 microglobulin) for transplantation assays. We show that human rEC-HSPCs engraft primary- and secondary-transplanted mice for over a year at levels of up to 20% in the spleen or bone marrow. Engrafted cells differentiate into all blood lineages including phenotypically and functionally mature T cells in the absence of exogenous FGRS expression in vivo. Notably, the resulting T cells undergo TCR rearrangement and are able to clear viral particles one week post-LCMV infection. Functional and phenotypic analyses are presented in juxtaposition with experiments using cord blood-transplanted mice. These results demonstrate that our present direct conversion strategy generates bona fide human hematopoietic stem cells from adult endothelial cells. Disclosures Rafii: Angiocrine Bioscience: Equity Ownership.

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In vivo depletion of hematopoietic stem cells in the rat by an anti-CD45 (RT7) antibody

Blood ◽

10.1182/blood.v99.10.3566 ◽

2002 ◽

Vol 99 (10) ◽

pp. 3566-3572 ◽

Cited By ~ 18

Author(s):

Marc H. Dahlke ◽

Oliver S. Lauth ◽

Mark D. Jäger ◽

Till Roeseler ◽

Kai Timrott ◽

...

Keyword(s):

Stem Cells ◽

T Cells ◽

T Cell ◽

B Cells ◽

Hematopoietic Stem Cells ◽

Precursor Cells ◽

Severe Thrombocytopenia ◽

Antibody Treatment ◽

Hematopoietic Stem

Anti-CD45 monoclonal antibodies (mAbs) are potentially powerful tools for the depletion of mature leukocytes. As their application for immunotherapy also depends on their effects on bone marrow (BM) progeny, the in vivo effects of an anti-CD45 mAb (anti-RT7a mAb) on BM precursor cells were analyzed in a rat model. Anti-RT7a mAb treatment was performed in LEW.1W (RT1u RT7a) rats with the use of different dosages. In addition, major histocompatibility complex (MHC)–congenic BM transplantation making use of a diallelic polymorphism (RT7a/RT7b) of rat CD45 was applied. Following injection of anti-RT7a mAb into normal LEW.1W rats, T cells were profoundly depleted in blood, lymph nodes, and spleen, whereas B cells were coated only by the antibody. Single injection of anti-RT7a mAb in a high dose induced a lethal aplastic syndrome with severe thrombocytopenia. Rescue of antibody-treated animals with BM from congenic LEW.1W-7B rats (RT1u RT7b) and transplantation of BM from LEW.1W rats pretreated with anti-RT7a mAb into sublethally irradiated LEW.1W-7B recipients revealed a profound effect of the mAb on progeny of myeloid and T-cell lineage. Following repeated antibody treatment of stable mixed chimeras (RT7b/RT7a), very few RT7a-positive B cells were still detectable after 6 months and their number declined during the subsequent year. These observations show that this anti-RT7a mAb effectively depletes mature T cells as well as BM precursor cells of myeloid, T-cell, and thrombocytic lineage after in vivo application. In contrast, mature B cells are not depleted, but precursors also appear to be eliminated. Overall, the findings suggest that the anti-RT7a mAb efficiently depletes early rat hematopoietic stem cells.

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Altered colony-forming activities of bone marrow hematopoietic stem cells in mice following short-term in vivo exposure to parathion

The International Journal Of Cell Cloning ◽

10.1002/stem.5530050307 ◽

1987 ◽

Vol 5 (3) ◽

pp. 231-241 ◽

Cited By ~ 7

Author(s):

Vincent S. Gallicchio ◽

Thomas D. Watts ◽

George P. Casale ◽

Philip M. Bartholomew

Keyword(s):

Stem Cells ◽

Bone Marrow ◽

Hematopoietic Stem Cells ◽

Short Term ◽

Hematopoietic Stem

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Endothelial stroma programs hematopoietic stem cells to differentiate into regulatory dendritic cells through IL-10

Blood ◽

10.1182/blood-2006-01-007187 ◽

2006 ◽

Vol 108 (4) ◽

pp. 1189-1197 ◽

Cited By ~ 88

Author(s):

Hua Tang ◽

Zhenhong Guo ◽

Minghui Zhang ◽

Jianli Wang ◽

Guoyou Chen ◽

...

Keyword(s):

Stem Cells ◽

Dendritic Cells ◽

T Cell ◽

Hematopoietic Stem Cells ◽

Critical Role ◽

Regulatory Function ◽

Hematopoietic Stem ◽

Regulatory Dendritic Cells

Abstract Regulatory dendritic cells (DCs) have been reported recently, but their origin is poorly understood. Our previous study demonstrated that splenic stroma can drive mature DCs to proliferate and differentiate into regulatory DCs, and their natural counterpart with similar regulatory function in normal spleens has been identified. Considering that the spleen microenvironment supports hematopoiesis and that hematopoietic stem cells (HSCs) are found in spleens of adult mice, we wondered whether splenic microenvironment could differentiate HSCs into regulatory DCs. In this report, we demonstrate that endothelial splenic stroma induce HSCs to differentiate into a distinct regulatory DC subset with high expression of CD11b but low expression of Ia. CD11bhiIalo DCs secreting high levels of TGF-β, IL-10, and NO can suppress T-cell proliferation both in vitro and in vivo. Furthermore, CD11bhiIalo DCs have the ability to potently suppress allo-DTH in vivo, indicating their preventive or therapeutic perspectives for some immunologic disorders. The inhibitory function of CD11bhiIalo DCs is mediated through NO but not through induction of regulatory T (Treg) cells or T-cell anergy. IL-10, which is secreted by endothelial splenic stroma, plays a critical role in the differentiation of the regulatory CD11bhiIalo DCs from HSCs. These results suggest that splenic microenvironment may physiologically induce regulatory DC differentiation in situ.

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Dissecting the Immune Cell Progeny of CAR-Expressing Hematopoietic Stem Cells in a Humanized Mouse Model

Blood ◽

10.1182/blood-2019-129746 ◽

2019 ◽

Vol 134 (Supplement_1) ◽

pp. 4640-4640

Author(s):

Heng-Yi Liu ◽

Nezia Rahman ◽

Tzu-Ting Chiou ◽

Satiro N. De Oliveira

Keyword(s):

Stem Cells ◽

T Cells ◽

T Cell ◽

Hematopoietic Stem Cells ◽

Cell Lines ◽

Immune Cells ◽

Research Funding ◽

Humanized Mice ◽

Tumor Challenge ◽

Hematopoietic Stem

Background: Chemotherapy-refractory or recurrent B-lineage leukemias and lymphomas yield less than 50% of chance of cure. Therapy with autologous T-cells expressing chimeric antigen receptors (CAR) have led to complete remissions, but the effector cells may not persist, limiting clinical efficacy. Our hypothesis is the modification of hematopoietic stem cells (HSC) with anti-CD19 CAR will lead to persistent generation of multilineage target-specific immune cells, enhancing graft-versus-cancer activity and leading to development of immunological memory. Design/Methods: We generated second-generation CD28- and 4-1BB-costimulated CD19-specific CAR constructs using third-generation lentiviral vectors for modification of human HSC for assessment in vivo in NSG mice engrafted neonatally with human CD34-positive cells. Cells were harvested from bone marrows, spleens, thymus and peripheral blood at different time points for evaluation by flow cytometry and ddPCR for vector copy numbers. Cohorts of mice received tumor challenge with subcutaneous injection of lymphoma cell lines. Results: Gene modification of HSC with CD19-specific CAR did not impair differentiation or proliferation in humanized mice, leading to CAR-expressing cell progeny in myeloid, NK and T-cells. Humanized NSG engrafted with CAR-modified HSC presented similar humanization rates to non-modified HSC, with multilineage CAR-expressing cells present in all tissues with stable levels up to 44 weeks post-transplant. No animals engrafted with CAR-modified HSC presented autoimmunity or inflammation. T-cell populations were identified at higher rates in humanized mice with CAR-modified HSC in comparison to mice engrafted with non-modified HSC. CAR-modified HSC led to development of T-cell effector memory and T-cell central memory phenotypes, confirming the development of long-lasting phenotypes due to directed antigen specificity. Mice engrafted with CAR-modified HSC successfully presented tumor growth inhibition and survival advantage at tumor challenge with lymphoma cell lines, with no difference between both constructs (62.5% survival for CD28-costimulated CAR and 66.6% for 41BB-costimulated CAR). In mice sacrificed due to tumor development, survival post-tumor injection was directly correlated with tumor infiltration by CAR T-cells. Conclusions: CAR modification of human HSC for cancer immunotherapy is feasible and continuously generates CAR-bearing cells in multiple lineages of immune cells. Targeting of different malignancies can be achieved by adjusting target specificity, and this approach can augment the anti-lymphoma activity in autologous HSC recipients. It bears decreased morbidity and mortality and offers alternative therapeutic approach for patients with no available sources for allogeneic transplantation, benefiting ethnic minorities. Disclosures De Oliveira: National Institute for Health Research Biomedical Research Centre at Great Ormond Street Hospital for Children NHS Foundation Trust and University College London: Research Funding; NIAID, NHI: Research Funding; Medical Research Council: Research Funding; CIRM: Research Funding; National Gene Vector Repository: Research Funding.

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CD34+ Hematopoietic Stem Cells Exert Accessory Function in Lipopolysaccharide-induced T Cell Stimulation and CD80 Expression on Monocytes

Journal of Experimental Medicine ◽

10.1084/jem.189.4.693 ◽

1999 ◽

Vol 189 (4) ◽

pp. 693-700 ◽

Cited By ~ 15

Author(s):

Taila Mattern ◽

Gundolf Girroleit ◽

Hans-Dieter Flad ◽

Ernst T. Rietschel ◽

Artur J. Ulmer

Keyword(s):

Stem Cells ◽

T Cells ◽

Stem Cell ◽

T Cell ◽

Hematopoietic Stem Cells ◽

Cell Stimulation ◽

Hematopoietic Stem ◽

Accessory Function ◽

T Cell Stimulation ◽

Blood Stem Cells

CD34+ hematopoietic stem cells, which circulate in peripheral blood with very low frequency, exert essential accessory function during lipopolysaccharide (LPS)-induced human T lymphocyte activation, resulting in interferon γ production and proliferation. In contrast, stimulation of T cells by “conventional” recall antigens is not controlled by blood stem cells. These conclusions are based on the observation that depletion of CD34+ blood stem cells results in a loss of LPS-induced T cell stimulation as well as reduced expression of CD80 antigen on monocytes. The addition of CD34-enriched blood stem cells resulted in a recovery of reactivity of T cells and monocytes to LPS. Blood stem cells could be replaced by the hematopoietic stem cell line KG-1a. These findings may be of relevance for high risk patients treated with stem cells or stem cell recruiting compounds and for patients suffering from endotoxin-mediated diseases.

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High levels of interleukin 10 production in vivo are associated with tolerance in SCID patients transplanted with HLA mismatched hematopoietic stem cells.

Journal of Experimental Medicine ◽

10.1084/jem.179.2.493 ◽

1994 ◽

Vol 179 (2) ◽

pp. 493-502 ◽

Cited By ~ 288

Author(s):

R Bacchetta ◽

M Bigler ◽

J L Touraine ◽

R Parkman ◽

P A Tovo ◽

...

Keyword(s):

Stem Cells ◽

T Cells ◽

T Cell ◽

Mrna Expression ◽

Fetal Liver ◽

Hla Antigens ◽

Interleukin 10 ◽

Hematopoietic Stem

Transplantation of HLA mismatched hematopoietic stem cells in patients with severe combined immunodeficiency (SCID) can result in a selective engraftment of T cells of donor origin with complete immunologic reconstitution and in vivo tolerance. The latter may occur in the absence of clonal deletion of donor T lymphocytes able to recognize the host HLA antigens. The activity of these host-reactive T cells is suppressed in vivo, since no graft-vs. -host disease is observed in these human chimeras. Here it is shown that the CD4+ host-reactive T cell clones isolated from a SCID patient transplanted with fetal liver stem cells produce unusually high quantities of interleukin 10 (IL-10) and very low amounts of IL-2 after antigen-specific stimulation in vitro. The specific proliferative responses of the host-reactive T cell clones were considerably enhanced in the presence of neutralizing concentrations of an anti-IL-10 monoclonal antibody, suggesting that high levels of endogenous IL-10 suppress the activity of these cells. These in vitro data correlate with observations made in vivo. Semi-quantitative polymerase chain reaction analysis carried out on freshly isolated peripheral blood mononuclear cells (PBMC) of the patient indicated that the levels of IL-10 messenger RNA (mRNA) expression were strongly enhanced, whereas IL-2 mRNA expression was much lower than that in PBMC of healthy donors. In vivo IL-10 mRNA expression was not only high in the T cells, but also in the non-T cell fraction, indicating that host cells also contributed to the high levels of IL-10 in vivo. Patient-derived monocytes were found to be major IL-10 producers. Although no circulating IL-10 could be detected, freshly isolated monocytes of the patient showed a reduced expression of class II HLA antigens. However, their capacity to stimulate T cells of normal donors in primary mixed lymphocyte cultures was within the normal range. Interestingly, similar high in vivo IL-10 mRNA expressions in the T and non-T cell compartment were also observed in three SCID patients transplanted with fetal liver stem cells and in four SCID patients transplanted with T cell-depleted haploidentical bone marrow stem cells. Taken together, these data indicate that high endogenous IL-10 production is a general phenomenon in SCID patients in whom allogenic stem cell transplantation results in immunologic reconstitution and induction of tolerance. Both donor T cells and host accessory cells contribute to these high levels of IL-10, which would suppress the activity of host-reactive T cell in vivo.

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Lentiviral gene transfer regenerates hematopoietic stem cells in a mouse model for Mpl-deficient aplastic anemia

Blood ◽

10.1182/blood-2010-09-308262 ◽

2011 ◽

Vol 117 (14) ◽

pp. 3737-3747 ◽

Cited By ~ 20

Author(s):

Dirk Heckl ◽

Daniel C. Wicke ◽

Martijn H. Brugman ◽

Johann Meyer ◽

Axel Schambach ◽

...

Keyword(s):

Stem Cells ◽

Bone Marrow ◽

Mouse Model ◽

Hematopoietic Stem Cells ◽

Aplastic Anemia ◽

Bone Marrow Cells ◽

Specific Expression ◽

Hematopoietic Stem ◽

Egfp Reporter

AbstractThpo/Mpl signaling plays an important role in the maintenance of hematopoietic stem cells (HSCs) in addition to its role in megakaryopoiesis. Patients with inactivating mutations in Mpl develop thrombocytopenia and aplastic anemia because of progressive loss of HSCs. Yet, it is unknown whether this loss of HSCs is an irreversible process. In this study, we used the Mpl knockout (Mpl−/−) mouse model and expressed Mpl from newly developed lentiviral vectors specifically in the physiologic Mpl target populations, namely, HSCs and megakaryocytes. After validating lineage-specific expression in vivo using lentiviral eGFP reporter vectors, we performed bone marrow transplantation of transduced Mpl−/− bone marrow cells into Mpl−/− mice. We show that restoration of Mpl expression from transcriptionally targeted vectors prevents lethal adverse reactions of ectopic Mpl expression, replenishes the HSC pool, restores stem cell properties, and corrects platelet production. In some mice, megakaryocyte counts were atypically high, accompanied by bone neo-formation and marrow fibrosis. Gene-corrected Mpl−/− cells had increased long-term repopulating potential, with a marked increase in lineage−Sca1+cKit+ cells and early progenitor populations in reconstituted mice. Transcriptome analysis of lineage−Sca1+cKit+ cells in Mpl-corrected mice showed functional adjustment of genes involved in HSC self-renewal.

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Exhaustion of Donor Hematopoietic Stem Cells in Lethally-Irradiated Hosts Can Be Sbstantially Mitigated by Targeting p18INK4C.

Blood ◽

10.1182/blood.v104.11.1200.1200 ◽

2004 ◽

Vol 104 (11) ◽

pp. 1200-1200

Author(s):

Hui Yu ◽

Youzhong Yuan ◽

Xianmin Song ◽

Feng Xu ◽

Hongmei Shen ◽

...

Keyword(s):

Stem Cells ◽

Bone Marrow ◽

Hematopoietic Stem Cells ◽

Kinase Inhibitor ◽

Hematopoietic Cells ◽

Hematopoietic Stem ◽

Total Period ◽

Over Expression

Abstract Hematopoietic stem cells (HSCs) are significantly restricted in their ability to regenerate themselves in the irradiated hosts and this exhausting effect appears to be accelerated in the absence of the cyclin-dependent kinase inhibitor (CKI), p21. Our recent study demonstrated that unlike p21 absence, deletion of the distinct CKI, p18 results in a strikingly positive effect on long-term engraftment owing to increased self-renewing divisions in vivo (Yuan et al, 2004). To test the extent to which enhanced self-renewal in the absence of p18 can persist over a prolonged period of time, we first performed the classical serial bone marrow transfer (sBMT). The activities of hematopoietic cells from p18−/− cell transplanted mice were significantly higher than those from p18+/+ cell transplanted mice during the serial transplantation. To our expectation, there was no detectable donor p18+/+ HSC progeny in the majority (4/6) of recipients after three rounds of sBMT. However, we observed significant engraftment levels (66.7% on average) of p18-null progeny in all recipients (7/7) within a total period of 22 months. In addition, in follow-up with our previous study involving the use of competitive bone marrow transplantation (cBMT), we found that p18−/− HSCs during the 3rd cycle of cBMT in an extended long-term period of 30 months were still comparable to the freshly isolated p18+/+ cells from 8 week-old young mice. Based on these two independent assays and the widely-held assumption of 1-10/105 HSC frequency in normal unmanipulated marrow, we estimated that p18−/− HSCs had more than 50–500 times more regenerative potential than p18+/+ HSCs, at the cellular age that is equal to a mouse life span. Interestingly, p18 absence was able to significantly loosen the accelerated exhaustion of hematopoietic repopulation caused by p21 deficiency as examined in the p18/p21 double mutant cells with the cBMT model. This data directly indicates the opposite effect of these two molecules on HSC durability. To define whether p18 absence may override the regulatory mechanisms that maintain the HSC pool size within the normal range, we performed the transplantation with 80 highly purified HSCs (CD34-KLS) and then determined how many competitive reconstitution units (CRUs) were regenerated in the primary recipients by conducting secondary transplantation with limiting dilution analysis. While 14 times more CRUs were regenerated in the primary recipients transplanted with p18−/−HSCs than those transplanted with p18+/+ HSCs, the level was not beyond that found in normal non-transplanted mice. Therefore, the expansion of HSCs in the absence of p18 is still subject to some inhibitory regulation, perhaps exerted by the HSC niches in vivo. Such a result was similar to the effect of over-expression of the transcription factor, HoxB4 in hematopoietic cells. However, to our surprise, the p18 mRNA level was not significantly altered by over-expression of HoxB4 in Lin-Sca-1+ cells as assessed by real time PCR (n=4), thereby suggesting a HoxB4-independent transcriptional regulation on p18 in HSCs. Taken together, our current results shed light on strategies aimed at sustaining the durability of therapeutically transplanted HSCs for a lifetime treatment. It also offers a rationale for the feasibility study intended to temporarily target p18 during the early engraftment for therapeutic purposes.

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