CD44v10 in hematopoiesis and stem cell mobilization

A Review of Advances in Hematopoietic Stem Cell Mobilization and the Potential Role of Notch2 Blockade

Hematopoietic stem cell (HSC) transplantation can be a potential cure for hematological malignancies and some nonhematologic diseases. Hematopoietic stem and progenitor cells (HSPCs) collected from peripheral blood after mobilization are the primary source to provide HSC transplantation. In most of the cases, mobilization by the cytokine granulocyte colony-stimulating factor with chemotherapy, and in some settings, with the CXC chemokine receptor type 4 antagonist plerixafor, can achieve high yield of hematopoietic progenitor cells (HPCs). However, adequate mobilization is not always successful in a significant portion of donors. Research is going on to find new agents or strategies to increase HSC mobilization. Here, we briefly review the history of HSC transplantation, current mobilization regimens, some of the novel agents that are under investigation for clinical practice, and our recent findings from animal studies regarding Notch and ligand interaction as potential targets for HSPC mobilization.

Mobilized peripheral blood: an updated perspective

Enforced egress of hematopoietic stem cells (HSCs) out of the bone marrow (BM) into the peripheral circulation, termed mobilization, has come a long way since its discovery over four decades ago. Mobilization research continues to be driven by the need to optimize the regimen currently available in the clinic with regard to pharmacokinetic and pharmacodynamic profile, costs, and donor convenience. In this review, we describe the most recent findings in the field and how we anticipate them to affect the development of mobilization strategies in the future. Furthermore, the significance of mobilization beyond HSC collection, i.e. for chemosensitization, conditioning, and gene therapy as well as a means to study the interactions between HSCs and their BM microenvironment, is reviewed. Open questions, controversies, and the potential impact of recent technical progress on mobilization research are also highlighted.

CD44v10, osteopontin and lymphoma growth retardation by a CD44v10-specific antibody

Blockade of CD44 is considered a therapeutic option for the elimination of leukemia-initiating cells. However, the application of anti-panCD44 can be burdened by severe side effects. We determined whether these side effects could be avoided by replacing anti-panCD44 with CD44 variant isoform (CD44v)-specific antibodies in CD44v-positive hematological malignancies using the EL4 thymoma and CD44v10-transfected EL4 (EL4-v10) as models. Subcutaneous growth of EL4 and EL4-v10 was equally well inhibited by the anti-panCD44 and anti-CD44v10 antibodies, respectively. Ex vivo analysis indicated that natural killer cytotoxicity and antibody-dependent cellular cytotoxicity were the main effector mechanisms. Under local inflammation, the efficacy of anti-CD44v10 prolonged the survival time twofold compared with untreated, EL4-v10 tumor-bearing mice, and this was due to inflammation-induced expression of osteopontin (OPN). A high level of OPN in EL4-v10 tumors supported leukocyte recruitment and tumor-infiltrating T-cell activation. Taken together, in hematological malignancies expressing CD44v, anti-panCD44 can be replaced by CD44v-specific antibodies without a loss in efficacy. Furthermore, CD44v10-specific antibodies appear particularly advantageous in cutaneous leukemia therapy, as CD44v10 binding of OPN drives leukocyte recruitment and activation.

CD44 standard and CD44v10 isoform expression on leukemia cells distinctly influences niche embedding of hematopoietic stem cells

Background

A blockade of CD44 is considered a therapeutic option for the elimination of leukemia initiating cells. However, anti-panCD44 can interfere with hematopoiesis. Therefore we explored, whether a CD44 variant isoform (CD44v)-specific antibody can inhibit leukemia growth without attacking hematopoiesis. As a model we used CD44v10 transfected EL4 thymoma cells (EL4-v10).

Methods

The therapeutic efficacy of anti-panCD44 and anti-CD44v10 was evaluated after intravenous application of EL4/EL4-v10. Ex vivo and in vitro studies evaluated the impact of anti-panCD44 and anti-CD44v10 as well as of EL4 and EL4-v10 on hematopoietic stem cells (HSC) in cocultures with bone marrow stroma cells with a focus on adhesion, migration, cell cycle progression and apoptosis resistance.

Results

Intravenously injected EL4-v10 grow in bone marrow and spleen. Anti-panCD44 and, more pronounced anti-CD44v10 prolong the survival time. The higher efficacy of anti-CD44v10 compared to anti-panCD44 does not rely on stronger antibody-dependent cellular cytotoxicity or on promoting EL4-v10 apoptosis. Instead, EL4 compete with HSC niche embedding. This has consequences on quiescence and apoptosis-protecting signals provided by the stroma. Anti-panCD44, too, more efficiently affected embedding of HSC than of EL4 in the bone marrow stroma. EL4-v10, by catching osteopontin, migrated on bone marrow stroma and did not or weakly interfere with HSC adhesion. Anti-CD44v10, too, did not affect the HSC – bone marrow stroma crosstalk.

Conclusion

The therapeutic effect of anti-panCD44 and anti-CD44v10 is based on stimulation of antibody-dependent cellular cytotoxicity. The superiority of anti-CD44v10 is partly due to blocking CD44v10-stimulated osteopontin expression that could drive HSC out of the niche. However, the main reason for the superiority of anti-CD44v10 relies on neither EL4-v10 nor anti-CD44v10 severely interfering with HSC – stroma cell interactions that, on the other hand, are affected by EL4 and anti-panCD44. Anti-panCD44 disturbing HSC embedding in the osteogenic niche weakens its therapeutic effect towards EL4. Thus, as far as leukemic cells express CD44v isoforms, the therapeutic use of anti-panCD44 should be avoided in favor of CD44v-specific antibodies.

CD44 integrates signaling in normal stem cell, cancer stem cell and (pre)metastatic niches

The stem cell niche provides a regulatory microenvironment for cells as diverse as totipotent embryonic stem cells to cancer stem cells (CSCs) which exhibit stem cell-like characteristics and have the capability of regenerating the bulk of tumor cells while maintaining self-renewal potential. The transmembrane glycoprotein CD44 is a common component of the stem cell niche and exists as a standard isoform (CD44s) and a range of variant isoforms (CD44v) generated though alternative splicing. CD44 modulates signal transduction through post-translational modifications as well as interactions with hyaluronan, extracellular matrix molecules and growth factors and their cognate receptor tyrosine kinases. While the function of CD44 in hematopoietic stem cells has been studied in considerable detail, our knowledge of CD44 function in tissue-derived stem cell niches remains limited. Here we review CD44s and CD44v in both hematopoietic and tissue-derived stem cell niches, focusing on their roles in regulating stem cell behavior including self-renewal and differentiation in addition to cell-matrix interactions and signal transduction during cell migration and tumor progression. Determining the role of CD44 and CD44v in normal stem cell, CSC and (pre)metastatic niches and elucidating their unique functions could provide tools and therapeutic strategies for treating diseases as diverse as fibrosis during injury repair to cancer progression.

Transcatheter injection-induced changes in human bone marrow-derived mesenchymal stem cells

Human mesenchymal stem cells (hMSC) are being administered by direct intramyocardial (IM) injection into patients with myocardial dysfunction with an objective to improve clinical status. However, surprisingly little attention has been directed to qualifying hMSC functionality beyond simple viability. In particular, the transit of hMSCs through a small-caliber needle lumen, the final fluidic pathway for all IM injection devices, may be especially prone to inducing unwarranted effects on cell function. This study evaluated the changes in clonogenicity, gene expression, and cytokine secretion that may be induced in hMSC (20 million/ml) by injection through a 26-gauge Nitinol needle at two different flow rates compared to noninjected control samples. Results indicated that hMSC viability and colony forming unit (CFU) formation was not altered by changes in injection rate, although a trend toward lower titers was noted at the higher flow rate, for the specific batch of hMSCs studied. The gene expression and cytokine analysis data suggest that delivering a suspension of MSCs through narrow lumen needles may marginally alter certain gene expression programs, but that such in vitro effects are transient and not translated into measurable differences in protein production. Gene expression levels of four cytokines (bFGF, SDF-1, SCF, VEGF) were significantly different at 400 microl/min, and that of all cytokines were significantly different at 1600 microl/min when compared to controls (p < 0.05). These changes were less pronounced (statistically insignificant for most cases, p > 0.05) and, in certain instances directionally opposite, at 72 h. However, no differences in the amounts of secreted bFGF, VEGF, or TGF-beta were detectable at either of the two time points or flow rates. We infer that intramyocardial administration by transcatheter techniques is unlikely to interfere with the machinery required for cell replication or secretion of regulatory and other growth factors, which are the mainstays of MSC contribution to cardiac tissue repair and regeneration.

The multi-functional cellular adhesion molecule CD44 is regulated by the 8;21 chromosomal translocation

The 8;21 translocation is a common chromosomal abnormality in acute myeloid leukemia (AML). We recently identified a naturally occurring leukemogenic splice variant, AML1-ETO9a (acute myeloid leukemia-1 transcription factor and the eight-twenty-one corepressor-9a), of t(8;21). To understand the leukemic potential of AML1-ETO9a, we performed microarray analysis with the murine multipotential hematopoietic FDCP-mix A4 cell line. We identified changes in expression of various genes including CD44. CD44 is a type I transmembrane protein and functions as the major cellular adhesion molecule for hyaluronic acid, a component of the extracellular matrix. CD44 is expressed in most human cell types and is implicated in myeloid leukemia pathogenesis. We show that the presence of AML1-ETO9a significantly increased the expression of CD44 at both RNA and protein levels. Furthermore, the CD44 promoter is bound by AML1-ETO9a and AML1-ETO at the chromatin level. In addition, in the AML1-ETO9a leukemia mouse model CD44 is regulated in a cell context-dependent manner. Thus, our observations suggest that AML1-ETO and its splice variant AML1-ETO9a are able to regulate the expression of the CD44 gene, linking the 8;21 translocation to the regulation of a cell adhesion molecule that is involved in the growth and maintenance of the AML blast/stem cells.

Clonal diversity of the stem cell compartment

PURPOSE OF REVIEW

Hematopoietic stem cells are functionally heterogeneous even when isolated as phenotypically homogenous populations. How this heterogeneity is generated is incompletely understood. Several models have been formulated to explain the generation of diversity. All of these assume the existence of a single type of hematopoietic stem cell that generates heterogeneous daughter stem cells in response to extrinsic or intrinsic (stochastic) signals. This view has encouraged the idea that stem cells can be instructed to adapt their function. Newer data, however, challenge this concept. Here, we summarize these findings and discuss their implication for applications of stem cells.

RECENT FINDINGS

Hematopoietic stem cells that differ in function have been documented during development and within the adult stem cell compartment. The differences in function are stably inherited to daughter stem cells when these cells proliferate to self-renew. Collectively, the data show that the adult stem cell compartment consists of a limited number of distinct classes of stem cells.

SUMMARY

The most important stem cell functions, including self-renewal and differentiation capacity, are preprogrammed through epigenetic or genetic mechanisms. Thus, stem cells are much more predictable than previously thought. Changes in the stem cell compartment through disease or aging can be interpreted as shifts in its clonal composition, rather than a modification of individual hematopoietic stem cells.

Cyclical mobilization and gated importation of thymocyte progenitors in the adult mouse: evidence for a thymus-bone marrow feedback loop

It has recently been observed, as in the fetal thymus, that the importation of hematogenous thymocyte progenitors by the adult thymus is a gated phenomenon, whereby saturating numbers of progenitors periodically enter the thymus and occupy a finite number of intrathymic niches. In addition, the mobilization of thymocyte progenitors from the bone marrow appears to be a cyclical process that coincides temporally with the periods of thymic receptivity (open gate). It is proposed that these events are coordinated by a thymus-bone marrow feedback loop in which a wave of developing triple negative (CD3- CD4- CD8-) thymocytes interacts with stromal cells in the stratified regions of the thymus cortex to sequentially induce the release of diffusible cytokines that regulate the production, mobilization, and recruitment of thymocyte progenitors. The likely components of this feedback loop are described here, as are the properties of the intrathymic vascular gates and niches for thymocyte progenitors. The cyclical production and release of thymocyte progenitors from the bone marrow is placed in the context of a general phenomenon of oscillatory feedback regulation involving all lymphohemopoietic cell lineages. Lastly, the question of whether the gated (as opposed to the continuous) entry of thymocyte progenitors is essential for normal thymocytopoiesis in adult life is discussed.

Chronic delayed-type hypersensitivity reaction as a means to treat alopecia areata

The acute phase of alopecia areata (AA) is characterized by an increase in CD44v3+ and CD44v10+ skin-infiltrating leucocytes (SkIL). Induction of a contact eczema, one of the therapeutic options in AA, can be mitigated strongly by a blockade of CD44v10. The observation that induction of a delayed type hypersensitivity (DTH) reaction abrogates an autoimmune reaction, where both responses apparently use similar effector mechanisms, is surprising and prompted us to search for the underlying mechanisms. AA-affected C3H/HeJ mice were treated with the contact sensitizer SADBE (squaric acid dibutylester) and leucocyte subpopulations and their activation state was evaluated in SkIL and draining lymph nodes. AA-affected mice exhibited an increased number of SkIL with a predominance of T lymphocytes. After treatment with the contact sensitizer SADBE recovery of SkIL was reduced and monocytes predominated. However, a significantly increased number of leucocytes was recovered from draining lymph nodes. Draining lymph node cells from untreated and treated AA mice exhibited all signs of recent activation with high-level expression of co-stimulatory and accessory molecules and an increased percentage of CD44v3+ and CD44v10+ leucocytes. In contrast, SkIL of SADBE-treated AA mice contained relatively few activated T cells and reduced numbers of CD44v3+ and CD44v10+ cells. Thus, the activation state and the distribution of leucocyte subsets in SADBE-treated AA mice are consistent with a blockade of leucocyte extravasation. Accordingly, the therapeutic effect of long-term SADBE treatment may rely on impaired leucocyte traffic.

Combining G-CSF with a blockade of adhesion strongly improves the reconstitutive capacity of mobilized hematopoietic progenitor cells

OBJECTIVE

Mobilization of hematopoietic progenitor cells is achieved mainly by application of growth factors and, more recently, by blockade of adhesion. In this report, we describe the advantages of a combined treatment with granulocyte colony-stimulating factor (G-CSF) and anti-VLA4 (CD49d)/anti-CD44 as compared to treatment with the individual components.

MATERIALS AND METHODS

Mobilization by intravenous injection of anti-CD44, anti-VLA4, or G-CSF was controlled in spleen and bone marrow with regard to frequencies of multipotential colony-forming unit (C-CFU), marrow repopulating ability, long-term reconstitution, recovery of myelopoiesis, and regain of immunocompetence.

RESULTS

Mobilization by anti-CD44 had a strong effect on expansion of early progenitor cells in the bone marrow, while the recovery in the spleen was poor. In anti-CD49d-mobilized noncommitted and committed progenitors, progenitor expansion was less pronounced, but settlement in the spleen was quite efficient. Thus, anti-CD44 and anti-CD49d differently influenced mobilization. Accordingly, mobilization and recovery after transfer were improved by combining anti-CD44 with anti-CD49d treatment. Mobilization by G-CSF was most efficient with respect to recovery of progenitor cells in the spleen. However, when transferring G-CSF-mobilized cells, regain of immunocompetence was strongly delayed. This disadvantage could be overridden when progenitor cells were mobilized via blockade of adhesion and when expansion of these mobilized progenitor cells was supported by low-dose G-CSF only during the last 24 hours before transfer.

CONCLUSION

Mobilization of pluripotent progenitor cells via antibody blockade of CD44 or CD49d or via G-CSF relies on distinct mechanisms. Therefore, the reconstitutive capacity of a transplant can be significantly improved by mobilization regimens combining antibody with low-dose G-CSF treatment.

The role of cytokines and adhesion molecules for mobilization of peripheral blood stem cells

CD34(+) hematopoietic stem cells from peripheral blood are commonly used for autologous or allogeneic transplantation following high-dose therapy in malignant diseases. The introduction of hematopoietic growth factors such as G-CSF has greatly facilitated the mobilization of CD34(+) cells. The mechanism of stem cell mobilization is not yet clear. It seems to be a multistep process with a crosstalk between cytokines and adhesion molecules. In this review, the role of hematopoietic growth factors, chemokines, and adhesion molecules for mobilization and homing of CD34(+) cells is summarized. In addition, factors influencing the cytokine-induced mobilization in patients and healthy donors are described. The review closes with an overview of new classes of mobilizing drugs such as monoclonal antibodies, specific peptides, or antisense oligonucleotides targeting adhesion molecules.