Biology Of Interleukin-6 Research Articles

Making sense of IL-6 signalling cues in pathophysiology.

Unravelling the molecular mechanisms that account for functional pleiotropy is a major challenge for researchers in cytokine biology. Cytokine-receptor cross-reactivity and shared signalling pathways are considered primary drivers of cytokine pleiotropy. However, reports epitomized by studies of Jak-STAT cytokine signalling identify interesting biochemical and epigenetic determinants of transcription factor regulation that affect the delivery of signal-dependent cytokine responses. Here, a regulatory interplay between STAT transcription factors and their convergence to specific genomic enhancers support the fine-tuning of cytokine responses controlling host immunity, functional identity, and tissue homeostasis and repair. In this review, we provide an overview of the signalling networks that shape the way cells sense and interpret cytokine cues. With an emphasis on the biology of interleukin-6, we highlight the importance of these mechanisms to both physiological processes and pathophysiological outcomes.

ADAM17 orchestrates Interleukin-6, TNFα and EGF-R signaling in inflammation and cancer

It was realized in the 1990s that some membrane proteins such as TNFα, both TNF receptors, ligands of the EGF-R and the Interleukin-6 receptor are proteolytically cleaved and are shed from the cell membrane as soluble proteins. The major responsible protease is a metalloprotease named ADAM17. So far, close to 100 substrates, including cytokines, cytokine receptors, chemokines and adhesion molecules of ADAM17 are known. Therefore, ADAM17 orchestrates many different signaling pathways and is a central signaling hub in inflammation and carcinogenesis. ADAM17 plays an important role in the biology of Interleukin-6 (IL-6) since the generation of the soluble Interleukin-6 receptor (sIL-6R) is needed for trans-signaling, which has been identified as the pro-inflammatory activity of this cytokine. In contrast, Interleukin-6 signaling via the membrane-bound Interleukin-6 receptor is mostly regenerative and protective. Probably due to its broad substrate spectrum, ADAM17 is essential for life and most of the few human individuals identified with ADAM17 gene defects died at young age. Although the potential of ADAM17 as a therapeutic target has been recognized, specific blockade of ADAM17 is not trivial since the metalloprotease domain of ADAM17 shares high structural homology with other proteases, in particular matrix metalloproteases. Here, the critical functions of ADAM17 in IL-6, TNFα and EGF-R pathways and strategies of therapeutic interventions are discussed.

Targeting the Tumor Microenvironment by Intervention in Interleukin-1 Biology.

The importance of anti-tumor immunity in the outcome of cancer is now unequivocally established and recent achivements in the field have stimulated the development of new immunotherapeutical approaches. In invasive tumors, widespread inflammation promotes invasiveness and concomitantly also inhibits anti-tumor immune responses. We suggest that efficient tumor treatment should target both the malignant cells and the tumor microenvironment. Interleukin-1 (IL-1) is a pro-inflammatory as well as an immunostimulatory cytokine that is abundant in the tumor microenvironment. Manipulation of IL-1 can thus serve as an immunotherapeutical approach to reduce inflammation/immunosuppression and thus enhance anti-tumor immunity. The two major IL-1 agonistic molecules are IL-1α and IL-1β, which bind to the same IL-1 signaling receptor and induce the same array of biological activities. The IL-1 receptor antagonist (IL-Ra) is a physiological inhibitor of IL-1 that binds to its receptor without transmition of activation signals and thus serves as a decoy target. We have demonstrated that IL-1α and IL-1β are different in terms of the producing cells and their compartmentalization and the amount. IL-1α is mainly expressed intracellularly, in the cytosol, in the nucleus or exposed on the cell membrane, however, it is rarely secreted. IL-1β is active only as a secreted molecule that is mainly produced by activated myeloid cells. We have shown different functions of IL-1α and IL-1β in the malignant process. Thus, in its membrane- associated form, IL-1α is mainly immunostimulatory, while IL-1β that is secreted into the tumor microenvironment is mainly pro-inflammatory and promotes tumorigenesis, tumor invasiveness and immunosuppression. These distinct functions of the IL-1 agonistic molecules are mainly manifested in early stages of tumor development and the patterns of their expression dictate the direction of the malignant process. Here, we suggest that IL-1 modulation can serve as an effective mean to tilt the balance between inflammation and immunity in tumor sites, towards the latter. Different agents that neutralize IL-1, mainly the IL-Ra and specific antibodies, exist. They are safe and FDA-approved. The IL-1Ra has been widely and successfully used in patients with Rheumatoid arthritis, autoinflammatory diseases and various other diseases that have an inflammatory component. Here, we provide the rationale and experimental evidence for the use of anti-IL-1 agents in cancer patients, following first line therapy to debulk the major tumor's mass. The considerations and constraints of using anti-IL-1 treatments in cancer are also discussed. We hope that this review will stimulate studies that will fasten the application of IL-1 neutralization at the bedside of cancer patients.

Interleukin-1 in Stroke: From Bench to Bedside.

Inflammation is a host defense response to infection that normally culminates in pathogen removal and tissue repair. In the absence of a pathogen, sterile inflammation occurs, which is now recognized as a major contributor to noncommunicable disease (ie, cardiovascular diseases, cancers, chronic respiratory diseases, and diabetes mellitus). Although several regulatory molecules are implicated in inflammation, a key central role is provided by members of the interleukin-1 (IL-1) family, important mediators of the innate immune response.1 Here, we provide an overview of the basic biology of IL-1, before going on to describe both the preclinical and the clinical evidence that suggest IL-1 to be a major therapeutic target in all forms of stroke, including details of completed and ongoing clinical trials. It should be noted that in compiling this review, a full systematic search strategy was not used and that any negative data are likely limited by publication bias. Although 11 members of the IL-1 family have been described, the most important of these and most studied in the context of stroke are the agonists IL-1α and IL-1β, and the IL-1 receptor antagonist (IL-1Ra).2 Both IL-1α and IL-1β are created as 31-kDa precursor proteins that undergo enzymatic cleavage to bioactive 17-kDa forms,3 although they differ in that pro–IL-1α possesses some biological activity, whereas pro–IL-1β is completely inactive.4 Cleavage of IL-1β is classically achieved via caspase-1 (or IL-1–converting enzyme), activity of which is triggered via the activation of the inflammasome, an intracellular protein complex.5 Inflammasome activation in sterile inflammation is triggered by damage-associated molecular patterns, which are released by necrotic cells within the ischemic brain.1 Pro–IL-1β can also be cleaved to the active 17-kDa form by the action of neutrophil serine proteases, including proteinase 3 and elastase.4 Given the contribution of neutrophils to ischemic brain …

ID: 13: The biology of Interleukin-6 and strategies of blockade

Cytokines receptors exist in membrane bound and soluble form. The IL-6/soluble IL-6R complex stimulates target cells not stimulated by IL-6 alone, since they do not express the membrane bound IL-6R. We have named this process ’trans-signaling’. The soluble IL-6R is generated via ectodomain shedding by the membrane bound metalloprotease ADAM17. Soluble gp130 is the natural inhibitor of IL-6/soluble IL-6R complex responses. The dimerized recombinant soluble gp130Fc fusion protein is a molecular tool to discriminate between gp130 responses via membrane bound and soluble IL-6R responses. Interestingly, depending on the animal model used, global blockade of IL-6 signaling by neutralizing monoclonal antibodies and selective blockade of IL-6 trans-signaling can lead to different consequences. We used neutralizing monoclonal antibodies for global blockade of IL-6 signaling and the sgp130Fc protein for selective blockade of IL-6 trans-signaling in several animal models of human diseases. Inhibition of IL-6 trans-signaling but not global IL-6 blockade was beneficial in the cecal ligation and puncture sepsis model. Defense against bacterial infections rely on the membrane bound IL-6R. Acute pancreatitis often results in subsequent acute lung injury, which is an inflammatory disease with high mortality. IL-6 is necessary for the inflammatory process to reach the lung and blockade of IL-6 trans-signaling is sufficient to block the disease. The extent of inflammation is controlled by trans-signaling via the soluble IL-6R. Using the sgp130Fc protein or sgp130Fc transgenic mice we demonstrate in animal models of inflammatory bowel disease, peritonitis, rheumatoid arthritis, atherosclerosis, pancreatitis, lupus erythematodes, colon cancer, ovarian cancer, pancreatic cancer, nephrotoxic nephritis and high fat diet induced obesity that IL-6 trans-signaling via the soluble IL-6R is the crucial step in the development and the progression of the disease. Therefore, sgp130Fc is a novel therapeutic agent for the treatment of chronic inflammatory diseases and cancer and it underwent phase I clinical trials as an anti-inflammatory in 2013/2014. Phase II clinical trials in patients with autoimmune diseases such as inflammatory bowel disease will follow in 2015/16.

The biology of interleukin-1: emerging concepts in the regulation of the actin cytoskeleton and cell junction dynamics.

Interleukin (IL)-1 is a proinflammatory cytokine with important roles in innate immunity, as well as in normal tissue homeostasis. Interestingly, recent studies have also shown IL-1 to function in the dynamics of the actin cytoskeleton and cell junctions. For example, treatment of different epithelia with IL-1α often results in the restructuring of the actin network and cell junctions, thereby leading to junction disassembly. In this review, we highlight new and interesting findings that show IL-1 to be a critical player of restructuring events in the seminiferous epithelium of the testis during spermatogenesis.

The Many Faces of Interleukin-6: The Role of IL-6 in Inflammation, Vasculopathy, and Fibrosis in Systemic Sclerosis

Interleukin-6 is currently attracting significant interest as a potential therapeutic target in systemic sclerosis (SSc). In this paper, the biology of interleukin-6 is reviewed, and the evidence for interleukin-6 dysregulation in SSc is explored. The role of inteleukin-6 classical and trans signalling pathways in SSc relevant phenomena such as chronic inflammation, autoimmunity, endothelial cell dysfunction, and fibrogenesis is discussed. The existing evidence that interventions designed to block interleukin-6 signalling are of therapeutic relevance in SSc is evaluated.

Editorial: Interleukin-1: the paradigm for multifunctional inflammatory cytokines

Cytokines affect nearly all organ systems whether Cytokines affect nearly all organ systems whether in health or disease. In disease or any challenge to the survival of the host, the sub-class of cytokines, classified as “pro-inflammatory cytokines” evokes rapid and profound inflammatory responses. Interleukin-1 (IL-1) was first studied for its ability to cause fever and acute phase changes in response to infection and in many ways established the paradigm for the redundancy of similar inflammatory cytokines that were to be discovered in subsequent years. The paradigm change was based on the ability of IL-1 to induce multiple biological effects and as a result, established the concept that a single molecule evoked a myriad of seemingly unrelated changes in responding cells and in animals. In general, the explanation for the multiple activities lies in the ubiquitous presence of the IL-1 receptor on nearly all cells. Another explanation for these multiple activities is the induction of a set of genes, which are not expressed in health but rather in disease. For example, the induction of prostaglandins via the inducible cyclooxygenase-2, the death of the insulin-producing beta cells in the islets of Langerhans due to the inducible nitric oxide and the release of neutrophils from the bone marrow via the induction of granulocyte colony stimulating factor. Each activity was initially viewed as seemingly unrelated, although clearly IL-1-inducible events. Today, following the identification of IL-1 receptors and post-receptor IL-1 signaling kinases, these events are far less unrelated. Nevertheless, since the initial studies on IL-1, the multiple effects of pro-inflammatory cytokines are readily accepted as part of biology. The biology of IL-1 took a major leap when humans were injected with IL- 1 and multiple systemic effects were observed as well as a set of symptoms unknown from animal studies. In addition, the potency of the cytokine for humans was established to be in the low picomolar range.

Dual functionality of interleukin‐1 family cytokines: implications for anti‐interleukin‐1 therapy

Dysregulated inflammation contributes to disease pathogenesis in both the periphery and the brain. Cytokines are coordinators of inflammation and were originally defined as secreted mediators, released from expressing cells to activate plasma membrane receptors on responsive cells. However, a group of cytokines is now recognized as having dual functionality. In addition to their extracellular effects, these cytokines act inside the nuclei of cytokine-expressing or cytokine-responsive cells. Interleukin-1 (IL-1) family cytokines are key pro-inflammatory mediators, and blockade of the IL-1 system in inflammatory diseases is an attractive therapeutic goal. All current therapies target IL-1 extracellular actions. Here we review evidence that suggests IL-1 family members have dual functionality. Several IL-1 family members have been detected inside the nuclei of IL-1-expressing or IL-1-responsive cells, and intranuclear IL-1 is reported to regulate gene transcription and mRNA splicing. However, further work is required to determine the impact of IL-1 intranuclear actions on disease pathogenesis. The intranuclear actions of IL-1 family members represent a new and potentially important area of IL-1 biology and may have implications for the future development of anti-IL-1 therapies.

45 Key role of the protease ADAM17 in the biology of interleukin-6, TNFα and the EGF-receptor

45 Key role of the protease ADAM17 in the biology of interleukin-6, TNFα and the EGF-receptor

The role of interleukin-1 in neuroinflammation and Alzheimer disease: an evolving perspective

Elevation of the proinflammatory cytokine Interleukin-1 (IL-1) is an integral part of the local tissue reaction to central nervous system (CNS) insult. The discovery of increased IL-1 levels in patients following acute injury and in chronic neurodegenerative disease laid the foundation for two decades of research that has provided important details regarding IL-1's biology and function in the CNS. IL-1 elevation is now recognized as a critical component of the brain's patterned response to insults, termed neuroinflammation, and of leukocyte recruitment to the CNS. These processes are believed to underlie IL-1's function in the setting of acute brain injury, where it has been ascribed potential roles in repair as well as in exacerbation of damage. Explorations of IL-1's role in chronic neurodegenerative disease have mainly focused on Alzheimer disease (AD), where indirect evidence has implicated it in disease pathogenesis. However, recent observations in animal models challenge earlier assumptions that IL-1 elevation and resulting neuroinflammatory processes play a purely detrimental role in AD, and prompt a need for new characterizations of IL-1 function. Potentially adaptive functions of IL-1 elevation in AD warrant further mechanistic studies, and provide evidence that enhancement of these effects may help to alleviate the pathologic burden of disease.

Interleukin-1 in Rheumatoid Arthritis: Its Inhibition by IL-1Ra and Anakinra

Objective: To review the biology of interleukin-1 (IL-1) in the pathogenesis of rheumatoid arthritis (RA), as well as the biology of its natural inhibitor, IL receptor antagonist (IL-1Ra), and the clinical efficacy and safety of the recombinant form, anakinra. Data Sources: A MEDLINE search (1966–January 2007) of English-language articles was conducted using the key words anakinra, arthritis, clinical trial, interleukin-1 receptor antagonist, and Kineret. Study Selection and Data Extraction: Over 79 research articles and literature reviews were used to compile a discussion of the biology of IL-1 and IL-1Ra. Ten of these articles were selected to discuss the clinical safety and efficacy of anakinra. Data Synthesis: In RA, IL-1 primarily acts locally to mediate erosion of cartilage and bone. IL-1Ra serves to modulate its activity through competitive inhibition of cellular receptors. Administration of anakinra to animals with experimental arthritis reduced inflammation and joint damage. In clinical trials, anakinra was reasonably well tolerated; however, injection site reactions were frequent and there was a slight increased risk of serious infection. Alone or in combination with methotrexate, anakinra significantly reduced the symptoms and clinical signs of RA at the American College of Rheumatology 20% response level. However, no additive benefit was observed following coadministration with etanercept, a soluble tumor necrosis factor antagonist, and anakinra had no beneficial effect in patients that failed treatment with etanercept. Conclusions: Laboratory studies have indicated that IL-1 is primarily responsible for cartilage destruction and bone erosion in RA. The selective inhibition of IL-1 through administration of anakinra may offer symptomatic relief of RA in some patients.

Interleukin-6 biology is coordinated by membrane-bound and soluble receptors: role in inflammation and cancer

Cytokine receptors, which exist in membrane-bound and soluble forms, bind their ligands with comparable affinity. Although most soluble receptors are antagonists and compete with their membrane-associated counterparts for the ligands, certain soluble receptors are agonists. In these cases, complexes of ligand and soluble receptor bind on target cells to second receptor subunits and initiate intracellular signaling. The soluble receptors of the interleukin (IL)-6 family of cytokines (sIL-6R, sIL-11R, soluble ciliary neurotrophic factor receptor) are agonists capable of transmitting signals through interaction with the universal signal-transducing receptor for all IL-6 family cytokines, gp130. In vivo, the IL-6/sIL-6R complex stimulates several types of cells, which are unresponsive to IL-6 alone, as they do not express the membrane IL-6R. We have named this process trans-signaling. The generation of soluble cytokine receptors occurs via two distinct mechanisms-limited proteolysis and translation-from differentially spliced mRNA. We have demonstrated that a soluble form of the IL-6 family signaling receptor subunit gp130, which is generated by differential splicing, is the natural inhibitor of IL-6 trans-signaling responses. We have shown that in many chronic inflammatory diseases, including chronic inflammatory bowel disease, peritonitis, rheumatoid arthritis, asthma, as well as colon cancer, IL-6 trans-signaling is critically involved in the maintenance of a disease state, by promoting transition from acute to chronic inflammation. Moreover, in all these models, the course of the disease can be disrupted by specifically interfering with IL-6 trans-signaling using the soluble gp130 protein. The pathophysiological mechanisms by which the IL-6/sIL-6R complex regulates the inflammatory state are discussed.

Interleukin-6 biology is coordinated by membrane bound and soluble receptors.

Cytokine receptors exist in membrane bound and soluble form. Both forms bind their ligands with comparable affinity. While most soluble receptors are antagonists in that they compete for the ligands with their membrane counterparts, some soluble receptors are agonists. In this case, the complex of ligand and soluble receptor binds on target cells to a second receptor subunit and initiates signal transduction. Soluble receptors of the IL-6 family of cytokines are agonists. In vivo, the IL-6/soluble IL-6R complex stimulates several types of target cells not stimulated by IL-6 alone, since they do not express the membrane bound IL-6R. This process has been named transsignaling. We have shown that in several chronic inflammatory diseases like chronic inflammatory bowl disease, peritonitis and rheumatoid arthritis, transsignaling via the soluble IL-6R complexed to IL-6 is a crucial point in the transition from the acute to the chronic state of the disease. The mechanism by which the IL-6/ soluble IL-6R complex regulates the inflammatory state is discussed.

Interleukin-6 biology is coordinated by membrane bound and soluble receptors.

Cytokine receptors exist in membrane bound and soluble form. Both forms bind their ligands with comparable affinity. While most soluble receptors are antagonists in that they compete for the ligands with their membrane counterparts, some soluble receptors are agonists. In this case, the complex of ligand and soluble receptor binds on target cells to a second receptor subunit and initiates signal transduction. Soluble receptors of the IL-6 family of cytokines are agonists. In vivo, the IL-6/soluble IL-6R complex stimulates several types of target cells not stimulated by IL-6 alone, since they do not express the membrane bound IL-6R. This process has been named transsignaling. We have shown that in several chronic inflammatory diseases like chronic inflammatory bowl disease, peritonitis and rheumatoid arthritis, transsignaling via the soluble IL-6R complexed to IL-6 is a crucial point in the transition from the acute to the chronic state of the disease. The mechanism by which the IL-6/ soluble IL-6R complex regulates the inflammatory state is discussed.

Interleukin-6 functions as an autocrine growth factor in human bladder carcinoma cell lines in vitro.

Interleukin (IL)-6 is reported to function as a growth factor for renal and prostatic carcinomas. We conducted the present study to define the role of IL-6 in the growth of normal and neoplastic urothelial cells. Human bladder carcinoma cell lines (253J, RT4 and T24) and primary cultured human urothelial cells derived from normal ureters were used. Recombinant human IL-6 stimulated the growth of bladder carcinoma cell lines far better than that of normal urothelial cells (p < 0.001). All carcinoma cell lines tested produced and released IL-6, whereas normal urothelial cells did so only at marginal levels. Furthermore, treatment with lipopolysaccharide derived from Escherichia coli, tumor necrosis factor-alpha or IL-1 increased IL-6 secretion by bladder carcinoma cell lines but not by normal urothelial cells. Growth of bladder carcinoma cells was significantly inhibited by anti-IL-6 neutralizing antibody or the anti-sense oligonucleotide for IL-6 cDNA. We conclude that IL-6 functions as an autocrine growth factor for bladder carcinoma cells but not for normal urothelial cells and that it may be a factor accounting for the marked enhancement of inflammation-associated bladder carcinogenesis and tumor growth.

Investigative Urology: Hypercalcemia and Cosecretion of Interleukin-6 and Parathyroid Hormone Related Peptide by a Human Renal Cell Carcinoma Implanted Into Nude Mice

Humoral hypercalcemia of malignancy is a paraneoplastic syndrome believed to be due to production by the tumor of substances that stimulate osteoclastic bone resorption primarily. The human renal cell carcinoma cell line RC-8, grown in nude mice, was investigated for factors involved in renal cancer-induced hypercalcemia. At a tumor load of 200 to 400 mm.3 the mice developed hypercalcemia and hypophosphatemia associated with a rise in serum 1,25-dihydroxyvitamin D concentration and cachexia. The tumor released 1) significant amounts of human interleukin-6 (IL-6) and 2) parathyroid hormone-related peptide (PTHrP) into the circulation. Cancer cells further expressed mRNA for both human IL-6 and PTHrP. No secretion of human tumor necrosis factor-alpha or interleukin-1 beta could be demonstrated in the circulation of the host. Antibodies to IL-6 caused a significant (p = 0.043) inhibition of tumor growth and decreased serum calcium concentrations compared with control animals. Our data suggest that IL-6 is involved, either directly or indirectly, in the development of hypercalcemia in renal cell carcinoma.

The biology of interleukin-1.

The biology of interleukin-1.

Interleukin-1 and Interleukin-1 Antagonism

AbstractThe polypeptide cytokine interleukin-1 (IL-1) affects nearly every tissue and organ system. IL-1 is the prototype of the pro-inflammatory cytokines in that it induces the expression of a variety of genes and the synthesis of several proteins that, in turn, induce acute and chronic inflammatory changes. IL-1 is also the prototypic “alarm” cytokine in that it brings about increases in a variety of defense mechanisms, particularly immunologic and hematologic responses. Most studies on the biology of IL-1 have been performed in animals, but human subjects have recently been injected with recombinant IL-1 and the results confirm the two fundamental properties of IL-1 as being both a mediator of disease as well as of host defense. However, in either situation, over or continued production of IL-1 leads to debilitation of normal host functions; therefore, reduction of IL-1 synthesis or its effects becomes a target of therapy in many diseases. In this review, the structure, gene expression, synthesis, and secretion of IL-1 are described. In addition, the two IL-1 surface receptors, possible signal transduction mechanisms, various biologic activities, and production of IL-1 during disease states are discussed. Similarities and differences between IL-1, tumor necrosis factor, and IL-6 are presented. Although various agents for reducing the synthesis and/or for antagonizing the effects of IL-1 have been proposed, the recent cloning of a naturally occurring IL-1 receptor antagonist (IL-Ira) has opened new experimental and clinical approaches. The ability of this IL-Ira to block the triggering of IL-1 receptors in animals without agonist effects has reduced the severity of diseases such as hemodynamic shock, lethal sepsis, inflammatory bowel disease, experimental arthritis, and the spontaneous proliferation of human leukemic cells.

Interleukin-1 and interleukin-1 antagonism

The polypeptide cytokine interleukin-1 (IL-1) affects nearly every tissue and organ system. IL-1 is the prototype of the pro-inflammatory cytokines in that it induces the expression of a variety of genes and the synthesis of several proteins that, in turn, induce acute and chronic inflammatory changes. IL-1 is also the prototypic “alarm' cytokine in that it brings about increases in a variety of defense mechanisms, particularly immunologic and hematologic responses. Most studies on the biology of IL-1 have been performed in animals, but human subjects have recently been injected with recombinant IL-1 and the results confirm the two fundamental properties of IL-1 as being both a mediator of disease as well as of host defense. However, in either situation, over or continued production of IL-1 leads to debilitation of normal host functions; therefore, reduction of IL-1 synthesis or its effects becomes a target of therapy in many diseases. In this review, the structure, gene expression, synthesis, and secretion of IL-1 are described. In addition, the two IL-1 surface receptors, possible signal transduction mechanisms, various biologic activities, and production of IL-1 during disease states are discussed. Similarities and differences between IL-1, tumor necrosis factor, and IL-6 are presented. Although various agents for reducing the synthesis and/or for antagonizing the effects of IL-1 have been proposed, the recent cloning of a naturally occurring IL-1 receptor antagonist (IL- 1ra) has opened new experimental and clinical approaches. The ability of this IL-1ra to block the triggering of IL-1 receptors in animals without agonist effects has reduced the severity of diseases such as hemodynamic shock, lethal sepsis, inflammatory bowel disease, experimental arthritis, and the spontaneous proliferation of human leukemic cells.