The development of a liquid culture system for the growth of human bone marrow

A variety of growth conditions for human bone marrow in a liquid culture system are described. The development of an adherent layer of bone marrow stromal cells composed of endothelial cells, fibroblasts, phagocytic mononuclear cells and fat cells is essential for the maintenance of haemopoiesis. Adipocyte development and maintenance is hydrocortisone-dependent, but their role in haemopoiesis is undefined. Maintenance of GM-CFC and limited cellular differentiation occurs for 6–8 weeks in some cultures, with a predominant myeloid morphology for at least 5–6 weeks. There are a number of similarities to the murine system, but the growth conditions for human marrow are suboptimal.     

Effect of a new retinoidal benzoic acid derivative on normal human hematopoietic progenitor cell growth in vitro

13-cis-Retinoic acid (RA) has been demonstrated to alter hemopoiesis in vitro. We compared proliferation and differentiation effects of RA to a synthetic retinoid, (E)-4-[2-(5,6,7, 8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl-1-propenyl]benzoic acid (TTNPB), on human normal bone marrow cells. TTNPB stimulated the in vitro growth of erythroid-granulocyte-macrophage progenitors and erythroid progenitors. Dose-response curves showed that maximal increments of erythroid-granulocyte-macrophage progenitor growth [173 +/- 17% (SE)] and erythroid progenitor growth [210 +/- 40%] occurred with TTNPB at 10(-9) M. With RA, there was maximal increment (173 +/- 21%) for erythroid progenitors only, and that at 10(-8) M. Evaluating clonogenicity of marrow cells in the presence of the two retinoic acid derivatives demonstrated that TTNPB and RA enhanced myeloid colony (CFU-C) growth; maximal stimulation occurred at 10(-6) M (130 +/- 8% and 161 +/- 5% increment for TTNPB and RA, respectively). The two retinoic acid analogues did not alter the differentiation pattern of myeloid colonies (macrophage colonies, granulocytic colonies, or granulocyte-macrophage colonies). Replating studies showed that the formation of secondary hemopoietic colonies was not altered following incubation of hemopoietic progenitors with TTNPB or RA. These data demonstrate that TTNPB is more active than RA in stimulating the growth of hemopoietic progenitors from normal marrows. Such findings may have therapeutic implications for various hemopoietic disorders.     

Maintenance of Philadelphia-chromosome-positive progenitors in long-term marrow cultures from patients with advanced chronic myeloid leukemia.

We investigated the marrows of 19 patients with advanced Philadelphia chromosome positive (Ph+) chronic myeloid leukemia (CML) in long-term marrow culture (LTMC) to determine the frequency of loss of clonogenic leukemic cells in vitro. Sixteen patients were in first chronic phase at a median of 24 months from diagnosis and had received prior therapy with busulphan and/or hydroxyurea. The effect of interferon therapy on loss of Ph+ clonogenic cells in LTMC was also investigated. Of 16 patients who had not previously received interferon, complete loss of Ph+ progenitors was documented by 4-5 weeks in the LTMCs from two (12.5%). Ph+ progenitors persisted at 4-5 weeks in the LTMC derived from 12 patients. Marrows from nine patients treated with interferon were also established in LTMC. Cultures from four patients did not yield colonies with detectable metaphases at 3-5 weeks, while Ph+ clones were present in the cultures initiated with marrows from five patients. Mean hematopoietic colony yields from the adherent layer at 2-4 weeks, and from the supernatant layer at 1-3 weeks, of cultures derived from interferon-treated patients were significantly lower than in LTMCs of patients not treated with interferon (p less than 0.05). The results indicate that in previously treated patients with late chronic phase CML there is a low frequency of conversion of Ph-negative hematopoiesis in long-term culture. Interferon therapy is associated with impaired progenitor yields in LTMC and does not improve selective loss of Ph+ progenitors.     

Retinoic acid induction of CD38 antigen expression on normal and leukemic human myeloid cells: relationship with cell differentiation

Differentiation in the hematopoietic system involves, among other changes, altered expression of antigens, including the CD34 and CD38 surface antigens. In normal hematopoiesis, the most immature stem cells have the CD34+CD34 -phenotype. In acute myeloid leukemia (AML), although blasts from most patients are CD38+, some are CD38 -. AML blasts are blocked at early stages of differentiation; in some leukemic cells this block can be overcome by a variety of agents, including retinoids, that induce maturation into macrophages and granulocytes both in vitro and in vivo . Retinoids can also induce CD38 expression. In the present study, we investigated the relationship between induction of CD38 expression and induction of myeloid differentiation by retinoic acid (RA) in normal and leukemic human hematopoietic cells. In the promyelocytic (PML) CD34 -cell lines, HL60 and CB-1, as well as in normal CD34+CD34 -hematopietic progenitor cells RA induced both CD38 expression as well as morphological and functional myeloid differentiation that resulted in loss of self-renewal. In contrast, in the myeloblastic CD34+ leukemic cell lines, ML-1 and KG-1a, as well as in primary cultures of cells derived from CD34+-AML (M 0 and M 1 ) patients, RA caused an increase in CD38+ that was not associated with significant differentiation. Yet, long exposure of ML-1, but not KG-1, cells to RA resulted in loss of self-renewal. The results suggest that while in normal hematopoietic cells and in PML CD34 -cells induction of CD38 antigen expression by RA results in terminal differentiation along the myeloid lineage, in early myeloblastic leukemic CD34+ cells, induction of CD38 and differentiation are not functionally related. Since, several lines of evidence suggest that the CD38 -cells are the targets of leukemic transformation, transition of these cells into CD38+ phenotype by RA or other drugs may have therapeutic effect, either alone or in conjunction with cytotoxic drugs, regardless the ability of the cells to undergo differentiation.     

In vitro characterization of the hematopoietic system in pediatric patients with acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) has been recognized as a hematologic neoplasia that originates at the level of a primitive lymphoid stem/progenitor cell. To date, however, the biology of the hematopoietic system in this disorder is still not fully understood. In the present study, we have determined the progenitor cell content (including myeloid, erythroid and multipotent progenitors) in 14 children with ALL and followed the proliferation kinetics of these cells in Dexter-type long-term marrow cultures. We have also characterized some aspects related to the composition and function of the hematopoietic microenvironment developed in vitro. All patients included in this study showed extremely reduced levels of progenitor cells (median of 6.2% of the levels found in normal marrow). Proliferation of these cells in long-term cultures was markedly deficient, since they showed very low numbers - compared to normal cultures - and reached undetectable levels after only a few weeks. Regarding the microenvironment developed in vitro, whereas normal marrow samples contained a median of 8 fibroblastic progenitors/10(5) marrow cells and the stromal cell layers developed in culture contained a median of 341000 adherent cells per well, ALL marrow samples showed no fibroblastic progenitors and the numbers of adherent cells were 21% of those in normal cultures. Interestingly, the levels of TNFalpha and IL-6 in ALL culture supernatants were significantly increased, compared to normal cultures. Bone marrow samples from all 14 children were also analyzed once they reached a complete clinical and hematological remission. Myeloid, erythroid and multipotent progenitor cell levels were significantly increased, compared to patients at diagnosis, and proliferation of myeloid progenitors in long-term cultures was also improved. In contrast, proliferation of erythroid progenitors showed no difference to that in cultures from patients at diagnosis. The numbers of fibroblastic progenitors and adherent cells were significantly increased, compared to patients at diagnosis, and TNFalpha and IL-6 levels returned to normal. In summary, in the present study, we have demonstrated significant in vitro alterations of the hematopoietic system, both in terms of its composition and function, in pediatric patients with ALL. Importantly, most of these alterations are corrected, at least partially, after chemotherapy.     

Constitutive and induced expression of growth factors in normal and chronic phase chronic myelogenous leukemia Ph1 bone marrow stroma

Study of growth factor RNA levels in the stromal cells derived from the adherent layer of long-term bone marrow culture demonstrated constitutive expression of transforming growth factor beta 1 (TGF-beta 1) and macrophage colony-stimulating factor. These cells did not express granulocyte colony-stimulating factor, granulocyte-monocyte colony-stimulating factor, interleukin (IL) 1 alpha, IL-1 beta, IL-3, and IL-6. However, granulocyte colony-stimulating factor expression could be induced by recombinant human IL-1 beta; while IL-6 could be induced by both IL-1 beta and tumor necrosis factor-alpha. No differences could be detected between adherent layers established from normal and benign phase Ph1 chronic myelogenous leukemia bone marrow. The uninduced expression of TGF-beta 1, a potent hematopoietic cell growth inhibitor, suggests that stromal cells play an inherent role in regulating the proliferation of adjacent bone marrow hematopoietic progenitor cells. However, a defect in stromal TGF-beta 1 production cannot account for the profoundly expanded myeloid compartment in chronic phase chronic myelogenous leukemia. In contrast to the constitutive expression of TGF-beta 1 and macrophage colony-stimulating factor, hematopoietic growth factors are only expressed following a proper stimulation.     

Hematopoietic progenitor cells from patients with myelodysplastic syndromes: in vitro colony growth and long-term proliferation

It is known that the levels of hematopoietic progenitor cells (HPC) are greatly reduced in the majority of patients with myelodysplastic syndromes (MDS). To date, however, only limited information exists on the growth kinetics of these cells in long-term marrow cultures (LTMC), particularly in terms of erythroid and multipotent progenitors. In the present study, we have determined the HPC content in the bone marrow of 12 MDS patients and followed the proliferation kinetics of myeloid (including granulocyte, macrophage and granulocyte macrophage), erythroid (including early and late) and multipotent progenitor cells in LTMC throughout a 7-week culture period. Both the non-adherent and adherent fractions of the cultures were analyzed, so we were able to look at progenitor cells in suspension and those that physically associated to the stromal cell layer developed in culture. All 12 patients were grouped based on their FAB subtype and the in vitro growth of the HPC was analyzed accordingly. The results presented here indicate that in the majority of MDS patients, pronounced deficiencies exist both in the content and the long-term proliferation of marrow HPC. Such deficiencies were particularly evident for multipotent progenitors and those committed to the erythroid lineage, in which alterations in the maturation process also seem to be present. Our results suggest that, at least in some patients, HPC--besides showing an impaired proliferative capacity--lose their ability to adhere to the stromal cell layers developed in culture. RA patients showed the less affected in vitro HPC growth, whereas HPC from RAEB and RAEB-t showed a markedly deficient growth in culture. Interestingly, myelopoiesis was significantly increased in cultures of CMML patients. These results give some new insights into the biology of MDS-derived HPC.     

Myelopoiesis following phorbol ester exposure in human long-term bone marrow cell culture

Phorbol esters have two opposing effects on bone marrow granulocyte/macrophage colony forming cells (CFC-GM): they reduce the number of CFC-GM which respond to colony-stimulating activity (CSA), and they also induce the release of CSA from accessory marrow cells. We questioned whether the direct inhibitory effect of phorbol esters on CFC-GM was reversible and whether phorbol ester-treated accessory bone marrow cells could increase the in vitro recovery of cultured CFC-GM. Normal human bone marrow cells were exposed to phorbol-12,13-dibutyrate (PDB) for 5 days in liquid cultures. Total nucleated cell counts decreased in a dose-dependent fashion to 42 ± 6% of control at 5 × 10^?8 M PDB, and CFC-GM decreased to 20 ± 8% of control. The cultures were then washed to remove PDB and continued for up to 8 weeks. Total cell counts and CFC-GM/ml remained reduced in PDB-pretreated cultures compared to control. To determine whether this was a direct effect of PDB on nonadherent, proliferating cells or whether PDB acted indirectly by affecting adherent, accessory cells, nonadherent bone marrow cells from control and PDB-preincubated cultures were co-cultured for 8 weeks with either control or PDB-preincubated adherent bone marrow cells. Nonadherent cells preincubated with PDB had a similar growth pattern whether or not the co-cultured adherent cells had been preincubated with PDB. In contrast, nonadherent cells preincubated in control medium and co-cultured with PDB-preincubated adherent cells displayed increased numbers of nucleated cells, CFC-GM and adherent hematopoietic islands compared to control. This was associated with increased amounts of CSA present in the culture supernatants. Thus PDB causes an irreversible decrease in CFC-GM in long term marrow cultures but enhances the ability of adherent stromal cells to support normal CFC-GM growth in culture.     

Relation of protein kinase A and protein kinase C to signaling pathways of hematopoietic factors in leukemia cell lines

In terms of growth, differentiation, and signaling pathways of hematopoietic factors, the effects of protein kinase C (PKC) activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and protein kinase A(PKA) activator, dibutyryl cyclic adenosine monophosphate (dbcAMP) were examined in vitro using three factor-responsive myeloid leukemia cell lines. TPA suppressed the growth of OCI/AML-1 and OCI/AML-5 cells but stimulated the proliferation of OCI/AML-4 cells. TPA differentiated OCI/AML-4 and OCI/AML-5 cells to macrophage-like cells. dbcAMP suppressed the growth of the three without differentiation. The stimulation of TPA induced tyrosine phosphorylation of some proteins in OCI/AML-4 and OCI/AML-5 cells. Their molecular weights were the same as those phosphorylated by hematopoietic factors. The patterns of phosphorylated proteins were different between the two. TPA induced the phosphorylation of MAP kinase, but not that of JAK2 protein in three cell lines. The stimulation of dbcAMP did not induce tyrosine phosphorylation in three cell lines. Overnight exposure of TPA inhibited the tyrosine phosphorylation induced by hematopoietic factors, although that of dbcAMP did not. We suggest a close relation of PKC to signaling pathways of hematopoietic factors, however, the ways of relation were diverse among the cell lines and the clear mechanism explaining its effects on growth and differentiation was not elucidated.     

Recombinant human interleukin-3 opposes the effects of vitamins A and D on HL-60 human myeloid leukaemia cells.

Interleukin-3 (IL-3), retinoic acid (RA) and 1,25-dihydroxyvitamin D3 (calcitriol) (vitamin D) have achieved objective responses in phase I/II clinical trials in myeloid leukaemia and preleukaemia patients. In an effort to explore any additive or synergistic interactions between these biological agents which could provide the basis for improved therapeutic regimens for myeloid leukaemia patients, we have investigated the effects of IL-3 on both RA- and vitamin D-induced actions in HL-60 human myeloid leukaemia cells, which have receptors for these three agents, in both clonogenic microassays and liquid suspension cultures. The proliferative stimulus of IL-3 overrode and reversed the antiproliferative actions of both RA and vitamin D, the latter being more sensitive to the counter-acting effect of IL-3. Whereas only high concentrations of IL-3: 1000 or more U/ml were able to oppose the antiproliferative effect of RA, all concentrations of IL-3 including the low concentration of 10 U/ml, significantly counteracted the anti-proliferative action of vitamin D. RA restrained the proliferative stimulus of IL-3 partly at low physiological concentrations and almost completely at high pharmacological concentrations. The same results were reproduced in liquid suspension culture. Whereas RA restrained the proliferative stimulus of IL-3, vitamin D failed to restrain it. However, IL-3 did not induce differentiation or affect RA- or vitamin D-induced differentiation of HL-60 cells. The present results suggest that the clinical use of the combination of low concentration of IL-3 with RA and not with vitamin D might restrain the progression of myeloid leukaemia incurred during the treatment with IL-3 in some preleukaemic patients.     

Increased glucocorticoid receptor concentration in macrophage differentiation of myeloid leukemia cells with 12-O-tetradecanoylphorbol-13-acetate

HL60 cells, human promyelocytic leukemia cells, can be induced to differentiate into more mature myeloid forms by dimethyl sulfoxide (DMSO) or retinoic acid (RA) and into macrophage-like cells by 12-O-tetradecanoylphorbol-13-acetate (TPA) or related compounds. Macrophage differentiation of HL60 cells by TPA treatment induced a 3- to 4-fold increase in glucocorticoid receptor concentration per cell and a 2- to 3-fold increase in glucocorticoid receptor concentration per mg of protein. The ability of TPA derivatives to increase glucocorticoid receptor concentration paralleled their ability to induce macrophage differentiation. Macrophage differentiation of other myeloid leukemia cells by TPA treatment induced a 2- to 3-fold increase in glucocorticoid receptor concentration per cell. Exposure of T-lymphoblasts or erythroleukemia cells to TPA did not affect glucocorticoid receptor concentration. Myeloid differentiation of myeloid leukemia cells by DMSO or RA induced no significant change in glucocorticoid receptor concentration. The increase in glucocorticoid receptor concentration in macrophage differentiation of myeloid leukemia cells with TPA was considered to depend, not on TPA treatment, but on the process of macrophage differentiation. Further, glucocorticoid receptor concentration can be a sensitive marker of macrophage differentiation of myeloid leukemia cells.     

Alteration in hematopoietic stem cell seeding and proliferation by both high and low dose rate irradiation of bone marrow stromal cells in vitro

The mechanism of physiologic alteration by high (HDR) or low dose rate (LDR) (5 or 120 cGy/min) irradiation of plateau-phase bone marrow stromal cell cultures was investigated using a technique of in vitro bone marrow transplantation. Purified stromal cell cultures from C57BL/6J, C3H/HeJ, or (C57BL/6J X DBA2/J)F1 (B6D2F1) mouse marrow were irradiated to doses of 2.5 to 10 Gy at LDR or 10-100 Gy at HDR and were then engrafted in vitro with nonadherent hematopoietic cells from murine continuous bone marrow cultures. Parameters of engraftment quantitated included: (1) numbers of adherent proliferating hematopoietic cell colonies, "cobblestone islands" (2) cumulative production of nonadherent hematopoietic cells over 8 weeks after engraftment, (3) M-CSF, GM-CSF and multi-CSF (IL-3) dependent hematopoietic progenitor cells forming greater than or equal to 50 cell colonies in semisolid medium, (4) cumulative production of CFUs, and (5) number of adherent stromal cells positive for detectable extracellular laminin or collagen type IV (markers of endothelial cells, reticular adventitial cells, or sinus lining cells). There was a decrease in cobblestone island formation between 5 and 10 Gy and this parameter possibly increased at doses of 50 and 100 Gy. There was no difference between HDR and LDR irradiation to 10 Gy. Irradiation to doses above 10 Gy decreased support of engrafted cells forming CFU-GM and CFU-GEMM. Measures of CFUs after 10 Gy were variable but indicated a possible increase with HDR and no effect of LDR at 1 week and a decrease in both HDR and LDR groups at 3 weeks after engraftment. Thus, LDR and HDR irradiation in vitro alter several specific parameters of marrow stromal cell support for engrafted hematopoietic stem cells.     

Co-resistance to retinoic acid and TRAIL by insertion mutagenesis into RAM

Retinoic acid (RA), used as first-line therapy for acute promyelocytic leukemia (APL), exerts its antileukemic activity by inducing blast differentiation and activating tumor-selective TNF-related apoptosis-inducing ligand (TRAIL) signaling. To identify downstream mediators of RA signaling, we used retrovirus-mediated insertion mutagenesis in PLB985 leukemia cells and established the RA-resistant cell line WY-1. In PLB985, but not WY-1 cells, RA induced TRAIL and its DR4 and DR5 receptors. Knocking down TRAIL expression by RNA interference blocked RA-induced apoptosis. WY-1 cells are defective for RA-induced differentiation, G1 arrest and exhibit co-resistance to TRAIL. In WY-1 cells, a single virus copy is integrated into a novel RA-regulated gene termed RAM (retinoic acid modulator). RAM is expressed in the myelomonocytic lineage and extinguished by RA in PLB985, but not WY-1 cells. Whereas knocking down RAM expression by RNA interference promoted RA-induced differentiation and TRAIL-triggered apoptosis of PLB985 and WY-1 cells, overexpression of the predicted 109 amino-acid RAM open reading frame did not alter RA signaling in PLB985 cells. This indicates that, apart from encoding the putative RAM protein, RAM RNA may exert additional functions that are impaired by the retrovirus insertion. Our study demonstrates that RA induction of the TRAIL pathway is also operative in leukemia cells lacking an RARalpha oncofusion protein and identifies RAM as a novel RA-dependent modulator of myeloid differentiation and death.     

Differentiation of human leukemia cells and its usefulness for clinical diagnosis

Various chemical inducers have effects on the induction of terminal differentiation of human myelogenous leukemia cell lines. We studied morphological and functional changes of human leukemia cells freshly obtained from patients using 12-O-tetradecanoyl phorbol-13-acetate (TPA), retinoic acid (RA) or dimethyl sulphoxide (DMSO). The myeloid leukemia cells cultured with TPA became adherent to plastic culture dishes, and then developed macrophage-like morphology with long filamentous pseudopods within 48 h incubation. They showed marked enhancement of the ability to phagocytose latex particles. But these acquired properties did not always parallel each other, suggesting that the mechanism of functional maturation of leukemic cells induced by chemical agents was not identical with that of morphological changes. On the other hand, the lymphoid leukemia cells did not show morphological and functional changes when cultured with the above inducers. It is suggested that exposure of leukemic cells to TPA for relatively short times (12–24 h) may be useful for determining whether they are of myeloid or lymphoid origin. These characteristic changes were also observed in leukemic cells from the myeloid or lymphoid crisis of chronic myelogenous leukemia.     

Bone marrow stroma from refractory anemia of myelodysplastic syndrome is defective in its ability to support normal CD34-positive cell proliferation and differentiation in vitro

We examined the supportive function of stromal cells from patients with refractory anemia (RA) of myelodysplastic syndrome (MDS) on CD34-positive hematopoietic cell proliferation and differentiation using a long-term bone marrow culture (LTMC) system. Primary marrow stromal cells were obtained from 11 MDS RA patients and 12 healthy volunteers, and freshly prepared CD34-positive bone marrow cells from a normal subject were inoculated onto the stroma. There seems to be three broad patterns of hematopoietic cell growth in the LTMCs. In one group, hematopoietic cells were maintained at near normal levels (type A). In the second group, the number of hematopoietic cells increased within the first 5-10 days of culture, but declined to low levels at 15-20 days of culture as compared with normal control (type B). In the third group, the incidence of hematopoietic cells steadily declined from the beginning of the culture (type C). Furthermore, apoptotic change of hematopoietic cells was very frequently observed in cultures with the type C stroma, which were especially defective for supporting CD34 + cell proliferation and differentiation. The expression of CD95 on hematopoietic cells was induced by the type C stroma, however, production of fas ligand by the stromal cells was not observed. These findings suggest a lack of hematopoietic supportive function in some cases of MDS RA and also indicate that there is heterogeneity of stromal function among MDS RA patients.     

Expression of constitutively active Notch4 (Int-3) modulates myeloid proliferation and differentiation and promotes expansion of hematopoietic progenitors.

The Notch family of transmembrane receptors has been implicated in the regulation of many developmental processes. In this study, we evaluated the role of Notch4 in immature hematopoietic progenitors by inducing, with retroviral transduction, enforced expression of Int-3, the oncogenic and constitutively active form of mouse Notch4. Int-3-transduced human myeloid leukemia (HL-60) cells demonstrated significantly delayed expression of differentiation markers following retinoic acid and 12-0-tetradecanoylphorbol 13-acetate treatment. Furthermore, HL-60 cells expressing Int-3 displayed a slower growth rate than cells infected with void virus, and accumulation in the G0/G1 phases of cell cycle. Transduction with deletion mutants of Int-3 defined the importance of individual domains of the protein (in particular, the ANK domain and the C-terminal domain) in the inhibition of differentiation and growth arrest of HL-60 cells. When mouse bone marrow enriched for stem cells (5-fluorouracil-resistant, lineage negative) was transduced and cultured for two weeks, the Int-3-transduced population displayed a lower expression of differentiation markers and a three- to five-fold higher frequency of colony-forming cells (CFU-GM/BFU-E) than control cultures. These results strongly support the notion that Notch signaling inhibits differentiation and promotes expansion of hematopoietic stem/progenitor cells.     

Differentiation-inducing effect of recombinant human tumor necrosis factor alpha and gamma-interferon in vitro on blast cells from patients with acute myeloid leukemia and myeloid blast crisis of chronic myeloid leukemia

Tumor necrosis factor alpha (TNF-alpha) and gamma-interferon (IFN-gamma) have been shown to suppress clonogenic growth in cultures containing blast cells obtained from patients with acute myeloid leukemia. We report that recombinant human TNF-alpha and IFN-gamma are also able to induce functional and morphological maturation in fresh myeloid leukemic cells in vitro. Assessing suspension cultures containing cells from patients with acute myeloid leukemia (11 patients) or myeloid blast crisis of chronic myeloid leukemia (5 patients), it was found that recombinant human TNF-alpha and IFN-gamma significantly enhanced the number of cells reducing nitroblue tetrazolium, as compared to control cultures containing no cytokine (P less than 0.001 and P less than 0.001, respectively). Cells from responders showed alterations characteristic of monocyte/macrophage differentiation, adherence to plastic surfaces, development of positive staining for alpha-naphthyl acetate esterase, typical morphology, and expression of cell surface antigens detected by the monoclonal antibodies Mo-1, Mo-2, and My-4. Both cytokines decreased the number of viable cells, the number of blast cells, and the number of cluster-forming units in suspension culture, suggesting inhibitory actions on the growth capacity of leukemic cells. Compared to the maximum effects of either factor alone, the combination of recombinant human TNF-alpha and IFN-gamma significantly increased the extent of growth inhibition and cell adherence but did not result in further increases in nitroblue tetrazolium reduction. The presence of Auer rods in IFN-gamma or TNF-alpha differentiation-induced macrophages with cells from a patient with M5 acute myeloid leukemia demonstrates that these cytokines can induce differentiation of a leukemic clone in primary cells from patients with leukemia.