Monoclonal antibody targeting of IL-3 receptor α with CSL362 effectively depletes CML progenitor and stem cells

A dual role for the N-terminal domain of the IL-3 receptor in cell signalling

The interleukin-3 (IL-3) receptor is a cell-surface heterodimer that links the haemopoietic, vascular and immune systems and is overexpressed in acute and chronic myeloid leukaemia progenitor cells. It belongs to the type I cytokine receptor family in which the α-subunits consist of two fibronectin III-like domains that bind cytokine, and a third, evolutionarily unrelated and topologically conserved, N-terminal domain (NTD) with unknown function. Here we show by crystallography that, while the NTD of IL3Rα is highly mobile in the presence of IL-3, it becomes surprisingly rigid in the presence of IL-3 K116W. Mutagenesis, biochemical and functional studies show that the NTD of IL3Rα regulates IL-3 binding and signalling and reveal an unexpected role in preventing spontaneous receptor dimerisation. Our work identifies a dual role for the NTD in this cytokine receptor family, protecting against inappropriate signalling and dynamically regulating cytokine receptor binding and function.

The N-terminal domain (NTD) of interleukin-3 receptor α-subunit (IL3Rα) is involved in IL-3 recognition but the underlying mechanism is unknown. Here, the authors present crystal structures of the IL3Rα complex and provide biochemical evidence that the NTD regulates IL-3 binding and signalling complex assembly.

Monoclonal Antibody Therapies for Hematological Malignancies: Not Just Lineage-Specific Targets

Today, monoclonal antibodies (mAbs) are a widespread and necessary tool for biomedical science. In the hematological cancer field, since rituximab became the first mAb approved by the Food and Drug Administration for the treatment of B-cell malignancies, a number of effective mAbs targeting lineage-specific antigens (LSAs) have been successfully developed. Non-LSAs (NLSAs) are molecules that are not restricted to specific leukocyte subsets or tissues but play relevant pathogenic roles in blood cancers including the development, proliferation, survival, and refractoriness to therapy of tumor cells. In consequence, efforts to target NLSAs have resulted in a plethora of mAbs-marketed or in development-to achieve different goals like neutralizing oncogenic pathways, blocking tumor-related chemotactic pathways, mobilizing malignant cells from tumor microenvironment to peripheral blood, modulating immune-checkpoints, or delivering cytotoxic drugs into tumor cells. Here, we extensively review several novel mAbs directed against NLSAs undergoing clinical evaluation for treating hematological malignancies. The review focuses on the structure of these antibodies, proposed mechanisms of action, efficacy and safety profile in clinical studies, and their potential applications in the treatment of hematological malignancies.

Therapeutic antibodies against cancer stem cells: a promising approach

Monoclonal antibodies have been extensively used to treat malignancy along with routine chemotherapeutic drugs. Chemotherapy for metastatic cancer has not been successful in securing long-term remission of disease. This is in part due to the resistance of cancer cells to drugs. One aspect of the drug resistance is the inability of conventional drugs to eliminate cancer stem cells (CSCs) which often constitute less than 1-2% of the whole tumor. In some tumor types, it is possible to identify these cells using surface markers. Monoclonal antibodies targeting these CSCs are an attractive option for a new therapeutic approach. Although administering antibodies has not been effective, when combined with chemotherapy they have proved synergistic. This review highlights the potential of improving treatment efficacy using functional antibodies against CSCs, which could be combined with chemotherapy in the future.

Cellular and Molecular Networks in Chronic Myeloid Leukemia: The Leukemic Stem, Progenitor and Stromal Cell Interplay

The use of imatinib, second and third generation ABL tyrosine kinase inhibitors (TKI) (i.e. dasatinib, nilotinib, bosutinib and ponatinib) made CML a clinically manageable and, in a small percentage of cases, a cured disease. TKI therapy also turned CML blastic transformation into a rare event; however, disease progression still occurs in those patients who are refractory, not compliant with TKI therapy or develop resistance to multiple TKIs. In the past few years, it became clear that the BCRABL1 oncogene does not operate alone to drive disease emergence, maintenance and progression. Indeed, it seems that bone marrow (BM) microenvironment-generated signals and cell autonomous BCRABL1 kinase-independent genetic and epigenetic alterations all contribute to: i. persistence of a quiescent leukemic stem cell (LSC) reservoir, ii. innate or acquired resistance to TKIs, and iii. progression into the fatal blast crisis stage. Herein, we review the intricate leukemic network in which aberrant, but finely tuned, survival, mitogenic and self-renewal signals are generated by leukemic progenitors, stromal cells, immune cells and metabolic microenvironmental conditions (e.g. hypoxia) to promote LSC maintenance and blastic transformation.

Selective surface marker and miRNA profiles of CD34+ blast-derived microvesicles in chronic myelogenous leukemia

The present study aimed at investigating the selective enrichment of surface marker and functional microRNA (miRNA) profiles of cluster of differentiation (CD)34⁺ blast-derived microvesicles (MVs) from parental cells in chronic myelogenous leukemia (CML), thus providing an experimental basis for MVs to be used to predict characteristics of CD34⁺ blasts. Magnetic activated cell sorting and continuous differential centrifugation were used to isolate primary CML CD34⁺ blasts and MVs, in addition to utilizing flow cytometry to identify surface markers of CD34⁺ blasts and blast-derived MVs. Microarray analysis and the reverse transcription-quantitative polymerase chain reaction were performed to analyze miRNA profiles of CD34⁺ blasts and MVs. The results of the present study indicated that primary CML CD34⁺ blasts were able to release MVs, which were selectively enriched with the surface markers CD34 and CD123, and functional miRNAs from parental cells. A total of 15 miRNAs were upregulated in CD34⁺ blast derived-MVs compared with in CD34⁺ cells. Distinct Kyoto Encyclopedia of Genes and Genomes pathways and Gene Ontology terms characterized by altered gene expression and potentially associated miRNA were identified. Upregulated miRNAs in MVs were associated with cell development, tumorigenesis and signaling pathways involving ErbB and phosphoinositide 3-kinase/protein kinase B. The present study provides evidence, which increases the understanding of physiological functions of cancer-derived MVs, and aids the understanding of the roles of CD34⁺ blast-derived MVs in CML-associated processes.

CD123 target validation and preclinical evaluation of ADCC activity of anti-CD123 antibody CSL362 in combination with NKs from AML patients in remission

Despite the heterogeneity of acute myeloid leukemia (AML), overexpression of the interleukin-3 receptor-α (CD123) on both the more differentiated leukemic blast and leukemic stem cells (LSCs) provides a therapeutic target for antibody treatment. Here we present data on the potential clinical activity of the monoclonal antibody CSL362, which binds to CD123 with high affinity. We first validated the expression of CD123 by 100% (52/52) of patient samples and the correlation of NPM1 and FLT3-ITD mutations with the high frequency of CD123 in AML. In vitro studies demonstrated that CSL362 potently induced antibody-dependent cell cytotoxicity (ADCC) of AML blasts including CD34⁺CD38⁻CD123⁺ LSCs by natural killer cells (NKs). Importantly, compared with healthy donor (HD) NKs, NKs drawn from AML patients in remission had a comparable ADCC activity against leukemic cells; of note, during remission, immature NKs were five times higher in AML patients than that in HDs. Significantly, we report a case where leukemic cells were resistant to autologous ADCC; however, the blasts were effectively lysed by CSL362 together with donor-derived NKs after allogeneic hematopoietic stem cell transplantation. These studies highlight CSL362 as a promising therapeutic option following chemotherapy and transplant so as to improve the outcome of AML patients.

Immune Effector Recovery in Chronic Myeloid Leukemia and Treatment-Free Remission

Chronic myeloid leukemia (CML) is a hematological cancer, characterized by a reciprocal chromosomal translocation between chromosomes 9 and 22 [t(9;22)], producing the Bcr-Abl oncogene. Tyrosine kinase inhibitors (TKIs) represent the standard of care for CML patients and exert a dual mode of action: direct oncokinase inhibition and restoration of effector-mediated immune surveillance, which is rendered dysfunctional in CML patients at diagnosis, prior to TKI therapy. TKIs such as imatinib, and more potent second-generation nilotinib and dasatinib induce a high rate of deep molecular response (DMR, BCR-ABL1 ≤ 0.01%) in CML patients. As a result, the more recent goal of therapy in CML treatment is to induce a durable DMR as a prelude to successful treatment-free remission (TFR), which occurs in approximately half of all CML patients who cease TKI therapy. The lack of overt relapse in such patients has been attributed to immunological control of CML. In this review, we discuss an immunological timeline to successful TFR, focusing on the immunology of CML during TKI treatment; an initial period of immune suppression, limiting antitumor immune effector responses in newly diagnosed CML patients, linked to an expansion of immature myeloid-derived suppressor cells and regulatory T cells and aberrant expression of immune checkpoint signaling pathways, including programmed death-1/programmed death ligand-1. Commencement of TKI treatment is associated with immune system re-activation and restoration of effector-mediated [natural killer (NK) cell and T cell] immune surveillance in CML patients, albeit with differing frequencies in concert with differing levels of molecular response achieved on TKI. DMR is associated with maximal restoration of immune recovery in CML patients on TKI. Current data suggest a net balance between both the effector and suppressor arms of the immune system, at a minimum involving mature, cytotoxic CD56^dim NK cells may be important in mediating TFR success. However, a major goal remains in CML to identify the most effective pathways to target to maximize an advantageous immune response and promote TFR success.

Interleukin 3- receptor targeted exosomes inhibit in vitro and in vivo Chronic Myelogenous Leukemia cell growth

Despite Imatinib (IM), a selective inhibitor of Bcr-Abl, having led to improved prognosis in Chronic Myeloid Leukemia (CML) patients, acquired resistance and long-term adverse effects is still being encountered. There is, therefore, urgent need to develop alternative strategies to overcome drug resistance.

According to the molecules expressed on their surface, exosomes can target specific cells. Exosomes can also be loaded with a variety of molecules, thereby acting as a vehicle for the delivery of therapeutic agents.

In this study, we engineered HEK293T cells to express the exosomal protein Lamp2b, fused to a fragment of Interleukin 3 (IL3). The IL3 receptor (IL3-R) is overexpressed in CML blasts compared to normal hematopoietic cells and thus is able to act as a receptor target in a cancer drug delivery system. Here we show that IL3L exosomes, loaded with Imatinib or with BCR-ABL siRNA, are able to target CML cells and inhibit in vitro and in vivo cancer cell growth.

Single-cell molecular analysis defines therapy response and immunophenotype of stem cell subpopulations in CML

Understanding leukemia heterogeneity is critical for the development of curative treatments as the failure to eliminate therapy-persistent leukemic stem cells (LSCs) may result in disease relapse. Here we have combined high-throughput immunophenotypic screens with large-scale single-cell gene expression analysis to define the heterogeneity within the LSC population in chronic phase chronic myeloid leukemia (CML) patients at diagnosis and following conventional tyrosine kinase inhibitor (TKI) treatment. Our results reveal substantial heterogeneity within the putative LSC population in CML at diagnosis and demonstrate differences in response to subsequent TKI treatment between distinct subpopulations. Importantly, LSC subpopulations with myeloid and proliferative molecular signatures are proportionally reduced at a higher extent in response to TKI therapy compared with subfractions displaying primitive and quiescent signatures. Additionally, cell surface expression of the CML stem cell markers CD25, CD26, and IL1RAP is high in all subpopulations at diagnosis but downregulated and unevenly distributed across subpopulations in response to TKI treatment. The most TKI-insensitive cells of the LSC compartment can be captured within the CD45RA^- fraction and further defined as positive for CD26 in combination with an aberrant lack of cKIT expression. Together, our results expose a considerable heterogeneity of the CML stem cell population and propose a Lin^-CD34⁺CD38^-/lowCD45RA^-cKIT^-CD26⁺ population as a potential therapeutic target for improved therapy response.

Codelivery of Ponatinib and SAR302503 by Active Bone-Targeted Polymeric Micelles for the Treatment of Therapy-Resistant Chronic Myeloid Leukemia

Point mutations in the BCR-ABL1 domain and primitive chronic myelogenous leukemia (CML) cells existing in the bone marrow environment insensitive to tyrosine kinase inhibitors (TKIs) have become two major challenges in the CML therapy. In this study, combined TKI ponatinib and JAK2 inhibitor SAR302503 short-term treatment effectively suppressed growth and promoted apoptosis of BaF3/T315I cells in cytokine-containing medium in vitro. SAR302503 prevented cytokine-dependent resistance to ponatinib via inhibition of JAK2/STAT5 phosphorylation. Codelivery of ponatinib and SAR302503 by active bone-targeted polymeric micellar formulation greatly increased the drug accumulation in medullary cavity. The therapeutic efficacy of bone-targeted formulation was demonstrated in BaF3/T315I cells inoculated murine model with no dose-limited toxicity detectable in health mice. Thus, the intravenous injectable bone-homing ponatinib and SAR302503 micellar formulation represents a promising strategy for the treatment of therapy-resistant CML.

Impaired haematopoietic stem cell differentiation and enhanced skewing towards myeloid progenitors in aged caspase-2-deficient mice

The apoptotic cysteine protease caspase-2 has been shown to suppress tumourigenesis in mice and its reduced expression correlates with poor prognosis in some human malignancies. Caspase-2-deficient mice develop normally but show ageing-related traits and, when challenged by oncogenic stimuli or certain stress, show enhanced tumour development, often accompanied by extensive aneuploidy. As stem cells are susceptible to acquiring age-related functional defects because of their self-renewal and proliferative capacity, we examined whether loss of caspase-2 promotes such defects with age. Using young and aged Casp2^−/− mice, we demonstrate that deficiency of caspase-2 results in enhanced aneuploidy and DNA damage in bone marrow (BM) cells with ageing. Furthermore, we demonstrate for the first time that caspase-2 loss results in significant increase in immunophenotypically defined short-term haematopoietic stem cells (HSCs) and multipotent progenitors fractions in BM with a skewed differentiation towards myeloid progenitors with ageing. Caspase-2 deficiency leads to enhanced granulocyte macrophage and erythroid progenitors in aged mice. Colony-forming assays and long-term culture-initiating assay further recapitulated these results. Our results provide the first evidence of caspase-2 in regulating HSC and progenitor differentiation, as well as aneuploidy, in vivo.

ILs-3, 6 and 11 increase, but ILs-10 and 24 decrease stemness of human prostate cancer cells in vitro

Cancer stem cells (CSCs) are associated with cancer recurrence and metastasis. Prostate cancer cells often metastasize to the bone with a complex microenvironment of cytokines favoring cell survival. In this study, the cell stemness influence of a group of interleukins including IL-3, 6, 10, 11 and 24 on human prostate cancer cell lines LNCaP and PC-3 was explored in vitro. Sulforhodamine B(SRB) and 5-ethynyl-2′-deoxyuridine (EdU) assays were applied to examine the effect on cell proliferation, and wound healing and transwell assays were used for migration and invasion studies, in addition to colony formation, Western blotting and flowcytometry for the expression of stemness factors and chemotherapy sensitivity. We observed that ILs-3, 6 and 11 stimulated while ILs-10 and 24 inhibited the growth, invasion and migration of both cell lines. Interestingly, ILs-3, 6 and 11 significantly promoted colony formation and increased the expression of SOX2, CD44 and ABCG2 in both prostate cancer cell lines. However, ILs-10 and 24 showed the opposite effect on the expression of these factors. In line with the above findings, treatment with either IL-3 or IL-6 or IL-11 decreased the chemosensitivity to docetaxel while treatment with either IL-10 or IL-24 increased the sensitivity of docetaxel chemotherapy. In conclusion, our results suggest that ILs-3, 6 and 11 function as tumor promoters while ILs-10 and 24 function as tumor suppressors in the prostate cancer cell lines PC-3 and LNCaP in vitro, and such differences may attribute to their different effect on the stemness of PCa cells.

IL1RAP antibodies block IL-1-induced expansion of candidate CML stem cells and mediate cell killing in xenograft models

Chronic myeloid leukemia (CML) is currently treated with tyrosine kinase inhibitors, but these do not effectively eliminate the CML stem cells. As a consequence, CML stem cells persist and cause relapse in most patients upon drug discontinuation. Furthermore, no effective therapy exists for the advanced stages of the disease. Interleukin-1 receptor accessory protein (IL1RAP; IL1R3) is a coreceptor of interleukin-1 receptor type 1 and has been found upregulated on CML stem cells. Here, we show that primitive (CD34⁺CD38^-) CML cells, in contrast to corresponding normal cells, express a functional interleukin-1 (IL-1) receptor complex and respond with NF-κB activation and marked proliferation in response to IL-1. IL1RAP antibodies that inhibit IL-1 signaling could block these effects. In vivo administration of IL1RAP antibodies in mice transplanted with chronic and blast phase CML cells resulted in therapeutic effects mediated by murine effector cells. These results provide novel insights into the role of IL1RAP in CML and a strong rationale for the development of an IL1RAP antibody therapy to target residual CML stem cells.

CSL311, a novel, potent, therapeutic monoclonal antibody for the treatment of diseases mediated by the common β chain of the IL-3, GM-CSF and IL-5 receptors

The β common-signaling cytokines interleukin (IL)-3, granulocyte-macrophage colony stimulating factor (GM-CSF) and IL-5 stimulate pro-inflammatory activities of haematopoietic cells via a receptor complex incorporating cytokine-specific α and shared β common (βc, CD131) receptor. Evidence from animal models and recent clinical trials demonstrate that these cytokines are critical mediators of the pathogenesis of inflammatory airway disease such as asthma. However, no therapeutic agents, other than steroids, that specifically and effectively target inflammation mediated by all 3 of these cytokines exist. We employed phage display technology to identify and optimize a novel, human monoclonal antibody (CSL311) that binds to a unique epitope that is specific to the cytokine-binding site of the human βc receptor. The binding epitope of CSL311 on the βc receptor was defined by X-ray crystallography and site-directed mutagenesis. CSL311 has picomolar binding affinity for the human βc receptor, and at therapeutic concentrations is a highly potent antagonist of the combined activities of IL-3, GM-CSF and IL-5 on primary eosinophil survival in vitro. Importantly, CSL311 inhibited the survival of inflammatory cells present in induced sputum from human allergic asthmatic subjects undergoing allergen bronchoprovocation. Due to its high potency and ability to simultaneously suppress the activity of all 3 β common cytokines, CSL311 may provide a new strategy for the treatment of chronic inflammatory diseases where the human βc receptor is central to pathogenesis. The coordinates for the βc/CSL311 Fab complex structure have been deposited with the RCSB Protein Data Bank (PDB 5DWU).

T cells expressing CD123 chimeric antigen receptors for treatment of acute myeloid leukemia

PURPOSE OF REVIEW

The purpose of this article is to discuss the rationale of targeting CD123 using chimeric antigen receptor (CAR) T cells for the treatment of leukemia.

RECENT FINDINGS

CD123 is a leukemia-associated antigen that expresses at high levels in leukemic stem cells and leukemic blasts and low level in normal hematopoietic stem/progenitor cells. Immune-based therapies targeting CD123 are being developed. Preclinical data suggest that CD123 CAR T cells exhibit potent antileukemic activity and various impacts on normal hematopoiesis.

SUMMARY

CD123 is an attractive surface target for novel antileukemic therapies. CD123 CAR T-cell-based immunotherapy is a promising treatment for patients with relapsed or refractory acute myeloid leukemia.

SKLB-677, an FLT3 and Wnt/β-catenin signaling inhibitor, displays potent activity in models of FLT3-driven AML

FLT3 has been identified as a valid target for the treatment of acute myeloid leukemia (AML), and some FLT3 inhibitors have shown very good efficacy in treating AML in clinical trials. Nevertheless, recent studies indicated that relapse and drug resistance are still difficult to avoid, and leukemia stem cells (LSCs) are considered one of the most important contributors. Here, we report the characterization of SKLB-677, a new FLT3 inhibitor developed by us recently. SKLB-677 exhibits low nanomolar potency in biochemical and cellular assays. It is efficacious in animal models at doses as low as 1mg/kg when administrated orally once daily. In particular, SKLB-677 but not first-generation and second-generation FLT3 inhibitors in clinical trials has the ability to inhibit Wnt/β-catenin signaling; Wnt/β-catenin signaling is required for the development of LSCs, but not necessary for the development of adult hematopoietic stem cells (HSCs). This compound indeed showed considerable suppression effects on leukemia stem-like cells in in vitro functional assays, but had no influence on normal HSCs. Collectively, SKLB-677 is an interesting lead compound for the treatment of AML, and deserves further investigations.

Characterization and targeting of neoplastic stem cells in Ph+ chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm characterized by the presence of an oncogenic fusion gene, BCR–ABL1. This fusion gene produces a cytoplasmic protein with tyrosine kinase activity that acts as a main driver of oncogenesis and abnormal proliferation of myeloid cells in CML. Targeted therapy with BCR–ABL1 tyrosine kinase inhibitors (TKIs) such as imatinib is followed by long-term responses in most patients. However, despite continuous treatment, relapses occur, suggesting the presence of TKI-resistant neoplastic stem cells in these patients. Here, we discuss potential mechanisms and signaling molecules involved in the prosurvival and self-renewal capacity of CML neoplastic stem cells as well as antigens expressed by these cells. Several of these signaling molecules and cell surface antigens may serve as potential targets of therapy and their use may overcome TKI resistance in CML in the future.

The implication of cancer progenitor cells and the role of epigenetics in the development of novel therapeutic strategies for chronic myeloid leukemia

SIGNIFICANCE

Chronic myeloid leukemia (CML) involves the malignant transformation of hematopoietic stem cells, defined largely by the Philadelphia chromosome and expression of the breakpoint cluster region-Abelson (BCR-ABL) oncoprotein. Pharmacological tyrosine kinase inhibitors (TKIs), including imatinib mesylate, have overcome limitations in conventional treatment for the improved clinical management of CML.

RECENT ADVANCES

Accumulated evidence has led to the identification of a subpopulation of quiescent leukemia progenitor cells with stem-like self renewal properties that may initiate leukemogenesis, which are also shown to be present in residual disease due to their insensitivity to tyrosine kinase inhibition.

CRITICAL ISSUES

The characterization of quiescent leukemia progenitor cells as a unique cell population in CML pathogenesis has become critical with the complete elucidation of mechanisms involved in their survival independent of BCR-ABL that is important in the development of novel anticancer strategies. Understanding of these functional pathways in CML progenitor cells will allow for their selective therapeutic targeting. In addition, disease pathogenesis and drug responsiveness is also thought to be modulated by epigenetic regulatory mechanisms such as DNA methylation, histone acetylation, and microRNA expression, with a capacity to control CML-associated gene transcription.

FUTURE DIRECTIONS

A number of compounds in combination with TKIs are under preclinical and clinical investigation to assess their synergistic potential in targeting leukemic progenitor cells and/or the epigenome in CML. Despite the collective promise, further research is required in order to refine understanding, and, ultimately, advance antileukemic therapeutic strategies.

Myeloid derived suppressor cells in chronic myeloid leukemia

The suppression of the immune system creates a permissive environment for development and progression of cancer. One population of immunosuppressive cells that have become the focus of intense study is myeloid derived suppressor cells (MDSCs), immature myeloid cells able to induce immune-escape, angiogenesis, and tumor progression. Two different subpopulations have been identified and studied: granulocytic and monocytic MDSCs, with a different immunophenotype and immunosuppressive properties. Recently, an accumulation of both Gr-MDSCs and Mo-MDSCs cells has been found in the peripheral blood of chronic myeloid leukemia (CML) patients. They are part of the tumor clone showing BCR/ABL expression. Imatinib therapy decreases both MDSCs and arginase 1 levels to normal ones. This review will focus on actual knowledge for human MDSCs and their immunosuppressive activity in CML patients, with a critical attention to comparison of Gr-MDSCs and polymorphonuclear cells (PMNs). We will then suggest the monitoring of MDSCs in patients who have discontinued tyrosine kinase inhibitors (TKIs) therapy to evaluate if their increase could correlate with disease relapse.

Cancer stem cells in basic science and in translational oncology: can we translate into clinical application?

Since their description and identification in leukemias and solid tumors, cancer stem cells (CSC) have been the subject of intensive research in translational oncology. Indeed, recent advances have led to the identification of CSC markers, CSC targets, and the preclinical and clinical evaluation of the CSC-eradicating (curative) potential of various drugs. However, although diverse CSC markers and targets have been identified, several questions remain, such as the origin and evolution of CSC, mechanisms underlying resistance of CSC against various targeted drugs, and the biochemical basis and function of stroma cell-CSC interactions in the so-called ‘stem cell niche.’ Additional aspects that have to be taken into account when considering CSC elimination as primary treatment-goal are the genomic plasticity and extensive subclone formation of CSC. Notably, various cell fractions with different combinations of molecular aberrations and varying proliferative potential may display CSC function in a given neoplasm, and the related molecular complexity of the genome in CSC subsets is considered to contribute essentially to disease evolution and acquired drug resistance. In the current article, we discuss new developments in the field of CSC research and whether these new concepts can be exploited in clinical practice in the future.

Blast phase in chronic myelogenous leukemia is skewed toward unusual blast types in patients treated with tyrosine kinase inhibitors: a comparative study of 67 cases

OBJECTIVES

To compare the features of the blast phase of chronic myelogenous leukemia (CML) in patients treated with tyrosine kinase inhibitors (TKIs) with those in the pre-TKI era.

METHODS

Sixty-seven patients with blast phase CML were identified in the Duke Pathology database from 1991 to 2011. The morphology and immunophenotype of blasts were evaluated, along with cytogenetic studies and associated findings in the peripheral blood and bone marrow.

RESULTS

In the TKI era, the blasts were more frequently of a type other than the usual myeloid or lymphoid types when compared with the pre-TKI era. Blast phase in TKI-treated patients was associated with a higher peripheral WBC count and a lower blast percentage in the bone marrow. Of the 23 patients with cytogenetic studies during blast phase, additional cytogenetic changes more frequently occurred in patients with an unusual blast type, and some patients showed these changes months before the onset of blast phase.

CONCLUSIONS

Blast phase CML in TKI- and non-TKI-treated patients differs in the morphology and immunophenotype of blasts, cytogenetic findings, and associated findings in the peripheral blood and bone marrow.

Unexpected mechanisms of action for a cytokine receptor-blocking antibody

CSL362 is a humanized interleukin-3 (IL-3)-neutralizing monoclonal antibody with enhanced effector function that binds the α subunit of the IL-3 receptor (IL3Rα). The crystal structure of an IL3Rα:CSL362 complex shows that IL3Rα adopts “open” and “closed” conformations. CSL362 blocks IL-3 function through both IL3Rα conformations but via distinct and unexpected mechanisms.

Crystal structure of the mouse interleukin-3 β-receptor: insights into interleukin-3 binding and receptor activation

The structure of the mouse IL-3-specific β-receptor (βIL-3) is presented giving insights into direct IL-3 binding and receptor activation via the IL-3 receptor α (IL-3Rα) ‘SP2’ isoform, which lacks the N-terminal Ig-like domain. It provides an important reference structure for interpreting mutagenesis and receptor activation studies.

Dual mechanism of interleukin-3 receptor blockade by an anti-cancer antibody

Interleukin-3 (IL-3) is an activated T cell product that bridges innate and adaptive immunity and contributes to several immunopathologies. Here, we report the crystal structure of the IL-3 receptor α chain (IL3Rα) in complex with the anti-leukemia antibody CSL362 that reveals the N-terminal domain (NTD), a domain also present in the granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-5, and IL-13 receptors, adopting unique "open" and classical "closed" conformations. Although extensive mutational analyses of the NTD epitope of CSL362 show minor overlap with the IL-3 binding site, CSL362 only inhibits IL-3 binding to the closed conformation, indicating alternative mechanisms for blocking IL-3 signaling. Significantly, whereas "open-like" IL3Rα mutants can simultaneously bind IL-3 and CSL362, CSL362 still prevents the assembly of a higher-order IL-3 receptor-signaling complex. The discovery of open forms of cytokine receptors provides the framework for development of potent antibodies that can achieve a "double hit" cytokine receptor blockade.

SL-401 and SL-501, targeted therapeutics directed at the interleukin-3 receptor, inhibit the growth of leukaemic cells and stem cells in advanced phase chronic myeloid leukaemia

While imatinib and other tyrosine kinase inhibitors (TKIs) are highly efficacious in the treatment of chronic myeloid leukaemia (CML), some patients become refractory to these therapies. After confirming that interleukin-3 receptor (IL3R, CD123) is highly expressed on CD34(+) /CD38(-) BCR-ABL1(+) CML stem cells, we investigated whether targeting IL3R with diphtheria toxin (DT)-IL3 fusion proteins SL-401 (DT388 -IL3) and SL-501 (DT388 -IL3[K116W]) could eradicate these stem cells. SL-401 and SL-501 inhibited cell growth and induced apoptosis in the KBM5 cell line and its TKI-resistant KBM5-STI subline. Combinations of imatinib with these agents increased apoptosis in KBM5 and in primary CML cells. In six primary CML samples, including CML cells harbouring the ABL1 T315I mutation, SL-401 and SL-501 decreased the absolute numbers of viable CD34(+) /CD38(-) /CD123(+) CML progenitor cells by inducing apoptosis. IL3-targeting agents reduced clonogenic growth and diminished the fraction of primitive long-term culture-initiating cells in samples from patients with advanced phase CML that were resistant to TKIs or harboured an ABL1 mutation. Survival was also extended in a mouse model of primary TKI-resistant CML blast crisis. These data suggest that the DT-IL3 fusion proteins, SL-401 and SL-501, deplete CML stem cells and may increase the effectiveness of current CML treatment, which principally targets tumour bulk.

Activity of SL-401, a targeted therapy directed to interleukin-3 receptor, in blastic plasmacytoid dendritic cell neoplasm patients

This is the first prospective study of treatment of patients with blastic plasmacytoid dendritic cell neoplasm (BPDCN), an aggressive hematologic malignancy derived from plasmacytoid dendritic cells that typically involves the skin and rapidly progresses to a leukemia phase. Despite being initially responsive to intensive combination chemotherapy, most patients relapse and succumb to their disease. Because BPDCN blasts overexpress the interleukin-3 receptor (IL3R), the activity of SL-401, diptheria toxin (DT)388IL3 composed of the catalytic and translocation domains of DT fused to IL3, was evaluated in BPDCN patients in a phase 1-2 study. Eleven patients were treated with a single course of SL-401 at 12.5 μg/kg intravenously over 15 minutes daily for up to 5 doses; 3 patients who had initial responses to SL-401 received a second course in relapse. The most common adverse events including fever, chills, hypotension, edema, hypoalbuminemia, thrombocytopenia, and transaminasemia were transient. Seven of 9 evaluable (78%) BPDCN patients had major responses including 5 complete responses and 2 partial responses after a single course of SL-401. The median duration of responses was 5 months (range, 1-20+ months). Further studies of SL-401 in BPDCN including those involving multiple sequential courses, alternate schedules, and combinations with other therapeutics are warranted. This trial is registered at clinicaltrials.gov as #NCT00397579.

Crystallization and preliminary X-ray diffraction analysis of the interleukin-3 alpha receptor bound to the Fab fragment of antibody CSL362

Interleukin-3 (IL-3) is a member of the beta common family of cytokines that regulate multiple functions of myeloid cells. The IL-3 receptor-specific alpha subunit (IL3Rα) is overexpressed on stem cells/progenitor cells of patients with acute myeloid leukaemia, where elevated receptor expression correlates clinically with a reduced patient survival rate. The monoclonal antibody (MAb) CSL362 is a humanized MAb derived from the murine MAb 7G3, originally identified for its ability to specifically recognize the human IL-3 receptor and for blocking the signalling of IL-3 in myeloid and endothelial cells. In order to elucidate the molecular mechanism of CSL362 antagonism, a preliminary structure of human IL3Rα in complex with the MAb CSL362 has been determined.

CD 123 is a membrane biomarker and a therapeutic target in hematologic malignancies

Recent studies indicate that abnormalities of the alpha-chain of the interleukin-3 receptor (IL-3RA or CD123) are frequently observed in some leukemic disorders and may contribute to the proliferative advantage of leukemic cells. This review analyzes the studies indicating that CD123 is overexpressed in various hematologic malignancies, including a part of acute myeloid and B-lymphoid leukemias, blastic plasmocytoid dendritic neoplasms (BPDCN) and hairy cell leukemia.Given the low/absent CD123 expression on normal hematopoietic stem cells, attempts have been made at preclinical first, and then at clinical level to target this receptor. Since the IL-3R is a membrane receptor there are two relatively simple means to target this molecule, either using its natural ligand or neutralizing monoclonal antibodies. Recent reports using a fusion molecule composed by human IL-3 coupled to a truncated diphteria toxin have shown promising antitumor activity in BPDCN and AML patients.