The Wellcome Foundation lecture, 1986. The molecular control of normal and leukaemic granulocytes and macrophages

Don't Turn Off the Tap! The Importance of Discovery Science to the Australian Cardiovascular Sector and Improving Clinical Outcomes Into the Future

Despite significant advances in interventional and therapeutic approaches, cardiovascular disease (CVD) remains the leading cause of death and mortality. To lower this health burden, cardiovascular discovery scientists need to play an integral part in the solution. Successful clinical translation is achieved when built upon a strong foundational understanding of the disease mechanisms involved. Changes in the Australian funding landscape, to place greater emphasis on translation, however, have increased job insecurity for discovery science researchers and especially early-mid career researchers. To highlight the importance of discovery science in cardiovascular research, this review compiles six science stories in which fundamental discoveries, often involving Australian researchers, has led to or is advancing to clinical translation. These stories demonstrate the importance of the role of discovery scientists and the need for their work to be prioritised now and in the future. Australia needs to keep discovery scientists supported and fully engaged within the broader cardiovascular research ecosystem so they can help realise the next game-changing therapy or diagnostic approach that diminishes the burden of CVD on society.

"Differentiation Induction" culture of human leukemic myeloid cells stimulates high production of macrophage differentiation inducing factor

A suitable procedure for the production of human monokines was defined as 'differentiation-induction' culture. Human monocytic leukemia THP-1 cells were well-differentiated from nonfunctional promonocytes into macrophage-like cells by the induction with a combination of mezerein, retinoic acid, and aMycoplasma fermentans extract. The differentiated THP-1 cells secreted a high amount of macrophage differentiation-inducing factor (DIF) activity and concomitantly produced other known monokines, such as tumor necrosis factor-α (TNF-α), interleukin-1α (IL-1α) and interleukin-1β (IL-1β), into the medium. These results suggest that other novel human monokines may also be found in the conditioned medium of THP-1 cells induced by the 'differentiation-induction' culture conditions defined in this study. Macrophage DIF was purified to homogeneity and NH(2)-terminal amino acid sequence analysis revealed that macrophage DIF is very similar or identical to human leukemia inhibitory factor (LIF). The cDNA encoding human LIF was isolated using the polymerase chain reaction, and a clone producing 3.7 μg/10(6) cells day recombinant LIF was selected from Chinese hamster ovary (CHO) cells which were transfected with the LIF cDNA. The recombinant LIF production in CHO cells was quantified using MTT reduction assay with M1 cells.

Activation of Granulocyte-Macrophage Colony-Stimulating Factor and Interleukin-3 Receptor Subunits in a Multipotential Hematopoietic Progenitor Cell Line Leads to Differential Effects on Development

Activation of specific cytokine receptors promotes survival and proliferation of hematopoietic progenitor cells but their role in the control of differentiation is unclear. To address this issue, the effects of human interleukin-3 (hIL-3) and human granulocyte-macrophage colony-stimulating factor (hGM-CSF) on hematopoietic development were investigated in hematopoietic progenitor cells. Murine multipotent factor-dependent cell-Paterson (FDCP)-mix cells, which can self-renew or differentiate, were transfected with the genes encoding the unique  and/or shared βc human hIL-3 receptor (hIL-3 R) or hGM-CSF receptor (hGM R) subunits by retroviral gene transfer. Selective activation of hIL-3 R,βc or hGM R,βc transfects by hIL-3 and hGM-CSF promoted self-renewal and myeloid differentiation, respectively, over a range of cytokine (0.1 to 100 ng/mL) concentrations. These qualitatively distinct developmental outcomes were associated with different patterns of protein tyrosine phosphorylation and, thus, differential signaling pathway activation. The cell lines generated provide a model to investigate molecular events underlying self-renewal and differentiation and indicate that the  subunits act in combination with the hβc to govern developmental decisions. The role of the  subunit in conferring specificity was studied by using a chimeric receptor composed of the extracellular hIL-3 R and intracellular hGM R subunit domains. This receptor promoted differentiation in response to hIL-3. Thus, the  subunit cytosolic domain is an essential component in determining cell fate via specific signaling events.

Activation of Granulocyte-Macrophage Colony-Stimulating Factor and Interleukin-3 Receptor Subunits in a Multipotential Hematopoietic Progenitor Cell Line Leads to Differential Effects on Development

Activation of specific cytokine receptors promotes survival and proliferation of hematopoietic progenitor cells but their role in the control of differentiation is unclear. To address this issue, the effects of human interleukin-3 (hIL-3) and human granulocyte-macrophage colony-stimulating factor (hGM-CSF) on hematopoietic development were investigated in hematopoietic progenitor cells. Murine multipotent factor-dependent cell-Paterson (FDCP)-mix cells, which can self-renew or differentiate, were transfected with the genes encoding the unique  and/or shared βc human hIL-3 receptor (hIL-3 R) or hGM-CSF receptor (hGM R) subunits by retroviral gene transfer. Selective activation of hIL-3 R,βc or hGM R,βc transfects by hIL-3 and hGM-CSF promoted self-renewal and myeloid differentiation, respectively, over a range of cytokine (0.1 to 100 ng/mL) concentrations. These qualitatively distinct developmental outcomes were associated with different patterns of protein tyrosine phosphorylation and, thus, differential signaling pathway activation. The cell lines generated provide a model to investigate molecular events underlying self-renewal and differentiation and indicate that the  subunits act in combination with the hβc to govern developmental decisions. The role of the  subunit in conferring specificity was studied by using a chimeric receptor composed of the extracellular hIL-3 R and intracellular hGM R subunit domains. This receptor promoted differentiation in response to hIL-3. Thus, the  subunit cytosolic domain is an essential component in determining cell fate via specific signaling events.

The development and magnitude of thermotolerance during chronic hyperthermia in murine granulocyte-macrophage progenitors: II

We have previously reported that murine granulocyte-macrophage progenitors (CFU-GM) are capable of developing thermotolerance during chronic hyperthermia at temperatures of 40 to 42 degrees C. However, a differential profile of intrinsic thermal response and, in particular, the capability of developing thermotolerance during chronic heating was identified between CFU-GM and macrophage colony-forming units (CFU-M) stimulated respectively, by lung conditioned medium (LCM) and L929 cell conditioned medium (CCM). Nucleated marrow cells treated in vitro were cultured in McCoy's 5A medium plus 15% fetal bovine serum (FBS) in semisolid agar with 10% of CCM. Two different treatment protocols were used in this study to determine the kinetics of thermotolerance in CFU-M: (1) nucleated marrow from mouse tibia and femur were chronically heated in vitro at temperatures of 40, 41 and 42 degrees C (up to 480 min) or (2) nucleated marrow cells were heated over a period of 90 min stepwise from 37 to 42 degrees C, at a heating rate of 0.056 degrees C/min, before exposure to 42 degrees C. The amount of thermotolerance developed was analysed at various times after chronic incubation at 40-42 degrees C by a challenge with 15 min at 44 degrees C. In contrast to CFU-GM, the surviving fraction of CFU-M heated with 15 min at 44 degrees C did not increase during chronic hyperthermia at 40 degrees C for up to 480 min indicating failure to develop thermotolerance. However, CFU-M were able to develop thermotolerance during prolonged incubation at 41 and 42 degrees C, although to a much less extent than observed in CFU-GM. In other words, there was much less development of thermotolerance in murine CFU-M compared to that in CFU-GM. Furthermore, a slow temperature transit from 37 to 42 degrees C over 90 min before exposure to 42 degrees C induced CFU-M to develop thermotolerance. The thermotolerance ratio (TTR, the ratio of the surviving fraction at maximum tolerance versus normotolerance) increased from a maximum of 3.5 after 180 min at 42 degrees C (no warm-up) to a maximum of 4.1 after 60 min at 42 degrees C when the cells received a slow warm-up to 42 degrees C. This implies that in the murine bone marrow granulocyte/macrophage lineage, CFU-M does not normally develop thermotolerance during hyperthermia and that the colony forming unit-granulocyte (CFU-G) and CFU-GM play a more critical role than CFU-M in the initiation and promotion of thermotolerance during chronic hyperthermia. However, in a situation that simulates the slow heat-up used clinically in wholebody hyperthermia, e.g., the 90 min slow warm-up from 37 to 42 degrees C, stimulated CFU-M to develop greater thermotolerance more rapidly than during rapid heating.

GM-CSF restoration of a differentiated (growth factor-regulated) phenotype in an anaplastic tumor

GM-CSF (granulocyte-macrophage-derived colony-stimulating factor) is a differentiation agent that stimulates bone marrow activity in patients receiving chemotherapy. GM-CSF (1 microgram/ml daily for 10 days), administered intralesionally, was evaluated to determine whether it would restore a more differentiated phenotype to an anaplastic, rapidly growing, hormone-independent variant (R3327 MAT-LyLu) of the Dunning prostatic adenocarcinoma. Immunohistology was used to quantitate the expression of epithelial growth factor receptors (rEGF) and the tissue testosterone content. GM-CSF therapy significantly (P less than 0.05) restored rEGF expression and tissue testosterone to levels associated with better differentiated, slower growing, androgen-dependent Dunning variants (R3327 H and G). GM-CSF may have a role in treatment of prostatic cancers by promoting androgen and epithelial growth factor regulation.

Differential dose-related haematological effects of GM-CSF in pancytopenia: evidence supporting the advantage of low- over high-dose administration in selected patients

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a multifunctional haematopoietin which can promote production of several blood cell lineages, though the predominant target cells are neutrophils, monocytes, and their precursors. Occasional undesirable clinical effects include eosinophilia, an increase in blasts, or thrombocytopenia. Here, we describe four patients who were treated with GM-CSF, at subcutaneous doses significantly lower than are conventional, and experienced an unusual response pattern. Three patients had severe pancytopenia associated with chronic lymphocytic leukaemia (CLL) or myelodysplastic syndrome (MDS) and exhibited an unexpected switch in the responsive lineage on high- versus very low-dose therapy. The two CLL patients developed marked eosinophilia (up to 10.0 x 10(9) cells/l) without an increase in neutrophils on 125-300 micrograms/m2/d of GM-CSF. In contrast, when the dose was lowered to 10 micrograms/m2/d, the neutrophils rose to physiological levels, without significant eosinophilia. The MDS patient showed a rapid rise in peripheral blasts (baseline level = 0; post-therapy level = 5.0 x 10(9)/l), without a change in other cell types, when receiving 60 micrograms/m2/d of GM-CSF. After GM-CSF was held, blasts returned to baseline levels; reinstituting therapy at the very low dose of 6 micrograms/m2/d was followed by an increase in platelet counts from 50 to 185 x 10(9)/l with only a minor increase in blasts. The fourth patient, who suffered from severe aplastic anaemia complicated by recurrent gastrointestinal haemorrhage, was only treated with the low-dose regimen. He showed a predominant platelet effect with counts rising from 9 to 169 x 10(9)/l. Very low-dose GM-CSF therapy was devoid of constitutional side effects. The biological implications of these GM-CSF responses are discussed. Our results indicate that, in some patients, GM-CSF may stimulate different target cells depending on the dose. Therefore, in contrast to the results of administration of many classical drugs, there may not always be a direct relationship between the amount of GM-CSF given and the optimal effect.

The induction and inhibition of differentiation in normal and leukaemic cells

For granulocytic-macrophage progenitor populations and their progeny, five glycoproteins have been identified: GM-CSF, G-CSF, multi-CSF, M-CSF and IL-6 that can regulate their proliferative activity, maturation and functional activities. The same glycoproteins also have a capacity to induce irreversible differentiation commitment in normal bipotential granulocyte-macrophage progenitors and in some myeloid leukaemic cell lines, which suggests that common cellular processes exist in both situations. The leukaemia inhibitory factor (LIF) is a glycoprotein, with intriguing properties, which can either induce differentiation in some myeloid leukaemic cell lines or prevent differentiation in normal totipotential embryonic stem cells. The data from the LIF studies suggest a genetic mechanism controlling self-generation that is relatively simple and may be common to all cells. However, the actual cellular response observed appears to depend on the nature of the responding cell.

Therapeutic administration of recombinant human granulocyte colony-stimulating factor accelerates hemopoietic regeneration and enhances survival in a murine model of radiation-induced myelosuppression

The primary cause of death after radiation exposure is infection resulting from myelosuppression. Because granulocytes play a critical role in host defense against infection and because granulocyte proliferation and differentiation are enhanced by granulocyte colony-stimulating factor (G-CSF), this agent was evaluated for the ability to accelerate hemopoietic regeneration and to enhance survival in irradiated mice. C3H/HeN mice were irradiated and G-CSF (2.5 micrograms/day, s.c.) or saline was administered on days 3-12, 1-12 or 0-12 post-irradiation. Bone marrow, splenic and peripheral blood cellularity, and bone marrow and splenic granulocyte-macrophage progenitor cell recoveries were evaluated in mice exposed to 6.5 Gy. Mice exposed to 8 Gy were evaluated for multipotent hemopoietic stem cell recovery (using endogenous spleen colony-forming units) and enhanced survival. Results demonstrated that therapeutic G-CSF 1) accelerates hemopoietic regeneration after radiation-induced myelosuppression, 2) enhances survival after potentially lethal irradiation and 3) is most effective when initiated 1 h following exposure.

The role of hemopoietic growth factors in self-renewal and differentiation of IL-3-dependent multipotential stem cells

A multipotent hemopoietic cell line has been employed to assess the influence of the hemopoietic growth factors, IL-3, GM-CSF, G-CSF, and CSF-1 on the processes of self-renewal, the generation of lineage restricted progenitor cells and the production of mature neutrophils and macrophages. At a high concentration of IL-3, the cells undergo self-renewal and demonstrate little or no ability to undergo differentiation in the presence of the other growth factors. In the absence of IL-3, the cells show minimal (GM-CSF) or no (G-CSF or CSF-1) ability to respond to these other growth factors. When combined with a low concentration of IL-3, the ability of the cells to respond to GM-CSF, G-CSF, and CSF-1 is enhanced and a selective preference for the neutrophil or macrophage lineage is seen depending on the combination used, i.e., the presence of CSF-1 preferentially promotes macrophage development and G-CSF preferentially promotes neutrophil development. Conditions optimal for neutrophil development were seen using a combination of low IL-3 concentrations plus GM-CSF plus G-CSF. In such conditions, the cells undergo extensive proliferation and progressively lose their clonogenic potential (i.e., differentiation much greater than self-renewal) and acquire the biochemical markers characteristic of fully mature phagocytes.

Interaction of interferon with other cytokines

Interferons interact with other cytokines to exert their antiviral, cell growth regulatory and immunomodulatory activities. Growth factors, tumor necrosis factors, colony stimulating factors, interleukins and interferons have pleiotropic effects and form a parallel network of intercellular signals. These signals are transduced at the cell surface through specific receptors with intrinsic enzymatic activity or with the capacity to regulate intracellular enzymes through interactive effects with G-proteins. This leads to regulated gene transcription of intracellular and secreted, functional and structural proteins. Although much is known about the interaction of cytokines with their receptors and about the regulation of transcription at the genomic level the various steps linking these two phenomena deserve further research.

A tripartite structure of the signals that determine protein insertion into the endoplasmic reticulum membrane

Multilineage colony stimulating factor is a secretory protein with a cleavable signal sequence that is unusually long and hydrophobic. Using molecular cloning techniques we exchanged sequences NH2- or COOH-terminally flanking the hydrophobic signal sequence. Such modified fusion proteins still inserted into the membrane but their signal sequence was not cleaved. Instead the proteins were now anchored in the membrane by the formerly cleaved signal sequence (signal-anchor sequence). They exposed the NH2 terminus on the exoplasmic and the COOH terminus on the cytoplasmic side of the membrane. We conclude from our results that hydrophilic sequences flanking the hydrophobic core of a signal sequence can determine cleavage by signal peptidase and insertion into the membrane. It appears that negatively charged amino acid residues close to the NH2 terminal side of the hydrophobic segment are compatible with translocation of this segment across the membrane. A tripartite structure is proposed for signal-anchor sequences: a hydrophobic core region that mediates targeting to and insertion into the ER membrane and flanking hydrophilic segments that determine the orientation of the protein in the membrane.

A proposed growth regulatory function for the serologically detectable sex-specific antigen H-Ys

It is widely believed that the serologically detectable sex-specific antigen H-Ys plays a major role in the primary determination of sex. The cellular distribution of the antigen, however, seems to be at odds with its postulated function. Consideration of this apparent paradox has prompted the suggestion that the H-Ys antigen functions as a growth regulator, and that its role in the primary determination of sex can be accounted for on this basis. Circumstantial evidence is adduced that H-Ys is a growth regulator in the embryo, and this is supported by evidence from several sources not immediately related to embryonic growth or development. Genes coding for growth regulators can function as oncogenes in situations involving disordered regulation, and it is suggested that this accounts for the high incidence of ovarian neoplasms in H-Ys positive, but not in H-Ys negative, female patients with 46,XY gonadal dysgenesis (Swyer's syndrome). A postulated growth regulatory function of H-Ys lends weight to the contention of others, not only that the direction of differentiation of the indifferent gonad in the embryo is determined by its growth rate, but also that a common mechanism underlies genotypic sex determination and environmental sex determination.

GM-CSF expression is preferential to multi-CSF (IL-3) expression in murine T lymphocyte clones

The expression of the lymphokines GM-CSF and Multi-CSF (IL-3) has been studied in three IL-2-dependent CD4+ T lymphocyte clones. By contrast with the widely held view that lymphokine genes are coordinately expressed, the present study revealed a marked preference for GM-CSF compared with Multi-CSF expression. Preferential expression of GM-CSF was evident in a number of situations: early after stimulation via the T cell antigen receptor; in a proportion of low-producing cells of a clone; and in response to IL-2. There was a clear hierarchy of the three clones studied, each being ranked in the same order in all situations, suggesting that a common mechanism underlies each phenomenon. The possibility that the GM-CSF gene is responsive to lower doses of intercellular signal than the Multi-CSF gene was rendered unlikely, since in only one clone was GM-CSF preferentially expressed at low doses of stimulus. Since GM-CSF expression occurred more rapidly after stimulation, the possibility that Multi-CSF expression is dependent upon that of GM-CSF was considered. However, GM-CSF production was neither necessary nor sufficient for Multi-CSF expression: A retroviral construct expressing a GM-CSF cDNA was introduced into one of the clones, leading to constitutive GM-CSF expression, but Multi-CSF expression was not induced. The possibility is discussed that Multi-CSF expression is dependent on transcriptional activation of the GM-CSF locus and that positive feedback occurs between these two tightly linked genes at the chromosomal level.

The leucocytosis of exercise. A review and model

Exercise is known to induce an immediate leucocytosis, the magnitude of which is related, in most instances, to the intensity and duration of the work. On finishing exercise, however, the leucocyte count may change in any one of several different ways. The pattern of postexercise changes in the leucocyte count is determined mainly by the time which has elapsed since beginning exercise, rather than the work intensity or the total work done, if, for example, exercise has been intermittent. Consideration of, firstly, the circumstances under which the plasma concentrations of catecholamines and cortisol have been found separately to correlate with the leucocyte count at the finish of exercise, and, secondly, the effects on the leucocyte count of exogenous administration of these substances has led us to develop a model which can satisfactorily account for all of the principal changes in the leucocyte count that have been noted during and after exercise. It is proposed that catecholamines produced during exercise act to increase the ratio of circulating to non-circulating leucocytes, while cortisol acts, by a mechanism which involves a time lag, to increase the total number of leucocytes in the vascular compartment. Examination of previously published reports shows that many contain results which support this model. Using the model as a basis, some predictions are made that can be tested experimentally, and some experiments are suggested which should help elucidate the mode of action of catecholamines and cortisol.

Structural characterization of a murine myeloid leukaemia inhibitory factor

A leukaemia inhibitory factor (LIF) which induces macrophage differentiation in M1 murine myeloid leukaemia cells and suppresses their proliferation in vitro has been isolated in sufficient quantities (30 micrograms) from Krebs ascites tumour cell conditioned medium to permit its partial characterization by amino acid sequence analysis. The combination of sensitive microbore column (1.0 and 2.1 mm internal diameter) HPLC technology and microsequence analysis has enabled the positive identification of 125 of the total 179 amino acid residues (70%) in the molecule. The amino acid sequence data reported here permitted the isolation of a partial cDNA clone encoding LIF [Gearing et al. (1987) EMBO J. 6, 3995-4002]. A candidate C-terminus of the LIF molecule predicted from the amino acid sequence was confirmed by subsequent isolation of a cDNA clone corresponding to the C-terminus of the protein. No strong similarity was revealed when the amino acid sequence of LIF was compared with other haemopoietic growth factors, in particular granulocyte-macrophage colony-stimulating factor, granulocyte colony-stimulating factor and tumour necrosis factor-alpha or interleukins. The protein sequence data reported here indicate three sites of post-translational modification (N-linked glycosylation).

Specific binding of murine leukemia inhibitory factor to normal and leukemic monocytic cells

Leukemia inhibitory factor (LIF), a glycoprotein capable of suppressing the clonogenicity and inducing the differentiation of the murine myeloid leukemia cell line M1, was radioiodinated to a high specific radioactivity with retention of full biological activity. Binding of 125I-labeled LIF to M1 cells reached a steady state at 37 degrees C after approximately equal to 40 min and was in competition with unlabeled LIF but not granulocyte colony-stimulating factor or a range of other cytokines or differentiation-inducing agents. Specific binding was demonstrable to cells from a range of murine hemopoietic tissues including the bone marrow, the spleen, and the peritoneal cavity. Autoradiography revealed macrophages, monocytes, and their precursors to be the major cell types responsible for 125I-labeled LIF binding within these tissues. Receptors on M1 cells were of high affinity (apparent Kd, 100-200 pM) and few in number (300-500 per cell).

Haemopoietic growth factors

Erythropoietin, GM-colony-stimulating factor and G-colony-stimulating factor are the first recombinant haemopoietic growth factors to reach clinical use. There are a number of additional haemopoietic regulators that have now been cloned and are being mass-produced with a view to clinical use. The next decade should witness exciting advances in the clinical treatment of haematological diseases and infections that will be comparable with those that were seen last with the introduction of effective treatments for pernicious anaemia.

Cellular processing of murine colony-stimulating factor (Multi-CSF, GM-CSF, G-CSF) receptors by normal hemopoietic cells and cell lines

The binding, internalization and degradation rates of three different murine colony-stimulating factors (Multi-CSF or interleukin-3, GM-CSF and G-CSF) and their receptor turnover rates were determined for normal bone marrow cells and a number of different cell lines at 37 degrees C. The kinetic parameters were extracted from a curve-fitting analysis of the approach to steady-state of surface-bound and internalized CSFs by methods described by Myers et al. (1987). The primary binding kinetic constants (association and dissociation) for each CSF on different cell types were similar, suggesting a single type of receptor for each CSF. In all cases, CSF binding induced a faster rate of internalization of occupied receptors than unoccupied receptors and resulted in significant accumulation of CSF inside the cell under steady-state conditions. The steady-state constant, determining the relationship between CSF concentration and receptor occupancy, indicated that, in all cases, more receptors were occupied at a given CSF concentration under steady-state conditions than would be under equilibrium conditions. Nevertheless, the data predicted that maximal biological effects of the CSFs were exerted at concentrations that did not result in full receptor occupancy. Comparison of the kinetic constants derived for the same CSF interacting with different types of cells or different CSFs interacting with the same cell type indicated that CSF and receptor processing resulted from a dynamic interplay of receptor-determined and cell-determined events. This resulted in a flexibility of the kinetic parameters that matched the variety of biological responses elicited by CSFs in different cell types.

Molecular cloning and expression of cDNA encoding a murine myeloid leukaemia inhibitory factor (LIF)

Leukaemia inhibitory factor (LIF) can induce macrophage differentiation in M1 murine myeloid leukaemic cells and suppress their proliferation in vitro. It does not stimulate the proliferation of normal progenitor cells and is apparently distinct from known colony-stimulating factors. We have used oligo-nucleotides complementary to partial amino acid sequence of LIF to isolate a LIF clone from a T lymphocyte cDNA library. When this cDNA was coupled to a yeast expression vector (YEpsec1) and introduced into yeast cells, a molecule with the biological properties characteristic of native LIF was secreted into the growth medium. The amino acid sequence of LIF established it to be a unique molecular entity, distinct from the other known haemopoietic growth factors. Since LIF is encoded by a unique gene, two biochemically separable forms of LIF probably represent post-transcriptional or posttranslational variants of the same gene product. In contrast to several other haemopoietic regulators, the 0.8- to 1-kb LIF mRNA was expressed constitutively in two murine T lymphocyte cell lines examined, and its abundance was not enhanced by stimulation with concanavalin A. Cloning, sequencing and expressing LIF has resolved several discrepancies in the literature concerning the identity of factors capable of inducing differentiation of murine myeloid leukaemic cells in vitro.