CD38 ligation in human B cell progenitors triggers tyrosine phosphorylation of CD19 and association of CD19 with lyn and phosphatidylinositol 3-kinase

CD38 is a 45-kDa transmembrane glycoprotein highly expressed in lymphoid progenitors. Ligation of CD38 with specific Abs inhibits growth and induces apoptosis in human immature B cells. CD38 ligation also triggers tyrosine phosphorylation of syk, c-cbl, and phospholipase C-gamma and activates phosphatidylinositol 3-kinase (PI3-K). In the present study, we investigated whether the cell surface membrane molecules used in B cell receptor-mediated signaling, such as Ig alpha, Ig beta, and CD19, could be involved in the CD38-mediated signaling cascade. In the B cell receptor-negative immature B cell lines RS4;11, 380, and REH, Ig alpha and Ig beta were expressed exclusively in the cytoplasm and were not tyrosine phosphorylated after CD38 ligation. By contrast, CD19 was markedly tyrosine phosphorylated and was associated with lyn and PI3-K. PI3-K activation appears to be directly linked to the growth-arresting effects of CD38 ligation, which are reduced by PI3-K inhibitors. Ligation of either CD38 or CD19 resulted in a similar pattern of protein tyrosine phosphorylation; both signaling pathways caused tyrosine phosphorylation of c-cbl. Levels of CD38 surface expression were not affected by prolonged incubation with anti-CD19 Ab, while CD19 expression markedly decreased. These results indicate that CD19 is a major component of the CD38 signaling cascade in B cell precursors, serving as a cell surface membrane docking site for cytoplasmic kinases. CD38 and CD19 are not physically linked, but activate an overlapping set of kinases in human immature B cells.

Prevalence and growth characteristics of malignant stem cells in B-lineage acute lymphoblastic leukemia

We used a stroma-supported culture method to study the prevalence and growth characteristics of malignant stem cells in acute lymphoblastic leukemia (ALL). In 51 of 108 B-lineage ALL samples, bone marrow-derived stroma not only inhibited apoptosis of ALL cells but also supported their proliferation in serum-free medium. When single leukemic cells were placed in the stroma-coated wells of microtiter plates, the percentage of wells with leukemic cell growth after 2 to 5 months of culture ranged from 6% to 20% (median, 15%; 5 experiments). The immunophenotypes and genetic features of cells recovered from these cultures were identical to those noted before culture. All cells maintained their stroma dependency and self-renewal capacity. Leukemic clones derived from single cells contained approximately 10(3) to 10(6) cells after 1 month of culture; other clones became detectable only after prolonged culture. Cell growth in stroma-coated wells correlated with the number of initially seeded cells (1 or 10; r = .87). However, the observed percentages of positive wells seeded with 10 cells always exceeded values predicted from results with single-cell-initiated cultures (P < .003 by paired t-test), suggesting stimulation of leukemic cell growth by paracrine factors. In conclusion, the proportion of ALL cells with clonogenic potential may be considerably higher than previously thought.

Human B-cell progenitors and bone marrow microenvironment

Apoptosis of normal and leukemic immature B-cells in vitro is suppressed by contact with bone marrow-derived stromal layers. In stroma-supported cultures of immature B-cells, we found that ligation of CD38, a type II transmembrane protein, inhibited the cell growth and induced apoptosis. CD38 ligation also induced tyrosine phosphorylation and activation of intracellular substrates, including syk, phospholipase C-gamma, c-cbl, and phosphatidylinositol 3-kinase (PI 3-K). Wortmannin and LY294002, two potent inhibitors of PI 3K, rescued immature B cells from CD38-mediated growth suppression. In vitro culture of leukemic lymphoblasts may have potentially important clinical application. First, stroma-supported cultures of acute lymphoblastic leukemia (ALL) cells can determine the growth potential of leukemic cells. In a series of 70 children enrolled in a single program of chemotherapy, cell growth on stroma was a powerful and independent prognostic indicator. Second, a culture system capable of maintaining the majority of ALL blast cells at high levels of viability is also ideally suited for testing antileukemic drugs. Promising results were obtained with 2-chloro-deoxyadenosine and interleukin-4, leading to clinical trials of these two compounds in children with refractory ALL. In addition, we compared the direct antileukemic activities of dexamethasone and prednisolone and found that dexamethasone is five to six times more cytotoxic (on a molar basis) than prednisolone, in agreement with the anti-inflammatory activities of these drugs. This finding may serve to guide the selection of dexamethasone dosage in the treatment of ALL.

Mutations in the mu heavy-chain gene in patients with agammaglobulinemia

BACKGROUND

Most patients with congenital hypogammaglobulinemia and absent B cells are males with X-linked agammaglobulinemia, which is caused by mutations in the gene for Bruton's tyrosine kinase (Btk); however, there are females with a similar disorder who do not have mutations in this gene. We studied two families with autosomal recessive defects in B-cell development and patients with presumed X-linked agammaglobulinemia who did not have mutations in Btk.

METHODS

A series of candidate genes that encode proteins involved in B-cell signal-transduction pathways were analyzed by linkage studies and mutation screening.

RESULTS

Four different mutations were identified in the mu heavy-chain gene on chromosome 14. In one family, there was a homozygous 75-to-100-kb deletion that included D-region genes, J-region genes, and the mu constant-region gene. In a second family, there was a homozygous base-pair substitution in the alternative splice site of the mu heavy-chain gene. This mutation would inhibit production of the membrane form of the mu chain and produce an amino acid substitution in the secreted form. In addition, a patient previously thought to have X-linked agammaglobulinemia was found to have an amino acid substitution on one chromosome at an invariant cysteine that is required for the intrachain disulfide bond and, on the other chromosome, a large deletion that included the immunoglobulin locus.

CONCLUSIONS

Defects in the mu heavy-chain gene are a cause of agammaglobulinemia in humans. This implies that an intact membrane-bound mu chain is essential for B-cell development.

Comparative cytotoxicity of dexamethasone and prednisolone in childhood acute lymphoblastic leukemia

PURPOSE

The relative cytotoxicity of prednisolone and dexamethasone in acute lymphoblastic leukemia (ALL) is controversial. We therefore compared the direct antileukemic activities of these compounds in stroma-supported cultures of leukemic lymphoblasts.

MATERIALS AND METHODS

Bone marrow samples from children with B-lineage ALL were cultured on allogeneic bone marrow-derived stromal layers and exposed to various concentrations of glucocorticoids. After 4 days of culture, the number of viable leukemic cells was counted by flow cytometry and compared with that in parallel cultures without drugs.

RESULTS

In 28 B-lineage ALL samples tested, the concentration producing 50% cytotoxicity (LC50) of prednisolone ranged from 2.0 to 7,978 nmol/L (median, 43.5 nmol/L), and that of dexamethasone from 0.6 to 327 nmol/L (median, 7.5 nmol/L). Despite the wide variability of responses among samples, there was an excellent correlation between LC50 values obtained with the two drugs (linear r = .99, P < .0001; Spearman rank-order r = .77, P < .0001). The median ratio of dexamethasone to prednisolone LC50 and LC90 values was 1:5.5 (range, 1:1.0 to 1:24.4 for LC50; 1:1.1 to 1:25.5 for LC90). Studies with ALL cell lines demonstrated that both drugs were cytotoxic through induction of apoptosis. Stromal layers did not absorb or inactivate measurable amounts of corticosteroids, which indicates that the assay system did not bias results toward increased drug resistance.

CONCLUSION

In a bone marrow-derived microenvironment, dexamethasone is five to six times more cytotoxic (on a molar basis) than prednisolone, in agreement with the antiinflammatory activities of these drugs. This finding may serve to guide the selection of dexamethasone dosage in the treatment of ALL.

CD38-mediated growth suppression of B-cell progenitors requires activation of phosphatidylinositol 3-kinase and involves its association with the protein product of the c-cbl proto-oncogene

The signalling pathways that arrest the cell cycle and trigger cell death are only partially known. Dimerization of CD38, a 45-kD transmembrane type II glycoprotein highly expressed in immature B cells, inhibits cell growth and causes apoptosis in normal and leukemic B-cell progenitors, but the molecular mechanisms underlying these cellular responses are unknown. In the present study, we found that CD38 ligation in the immature B-cell lines 380, REH, and RS4;11 caused rapid tyrosine phosphorylation of the protein product of the proto-oncogene c-cbl. Dimerization of CD38 was accompanied by the association of cbl with the p85 subunit of phosphatidylinositol 3-kinase (Pl 3-K), resulting in markedly increased Pl 3-K activity in antiphosphotyrosine and anti-cbl immunoprecipitates. Wortmannin and LY294002, two potent inhibitors of Pl 3-K, rescued immature B cells from CD38-mediated growth suppression. This effect was observed not only in model B-cell lines, but also in cultures of leukemic lymphoblasts from patients, and in normal bone marrow B-cell progenitors as well. Concentrations of inhibitors that reversed cellular responses to CD38 significantly decreased Pl 3-K activity. By contrast, rapamycin, a p70 S6-kinase inhibitor, did not rescue immature B cells from CD38-mediated suppression. These results suggest that Pl 3-K activity is essential for CD38-mediated inhibition of lymphopoiesis and that cbl and Pl 3-K are regulatory molecules whose activation can result in suppression of cell proliferation and apoptosis in immature lymphoid cells.

Molecular interactions between human B-cell progenitors and the bone marrow microenvironment

Apoptosis of normal and leukemic immature B-cells in vitro is suppressed when either cell type is grown in direct contact with a feeder layer of bone marrow-derived stromal cells, including fibroblasts, macrophages, endothelial cells, and adipocytes. In this study, our objective was to identify a stromal cell type which is essential for lymphoblast survival and to characterize the molecules involved in lymphoblast adhesion to these cells. In experiments with B-lineage acute lymphoblastic leukemia (ALL) cells (n = 28) and purified CD19(+) cells from normal bone marrow (n = 6) we found that homogeneous populations of bone marrow fibroblasts could sustain survival of normal and leukemic immature B-cells as efficiently as composite bone marrow stromal layers. Electron microscopic studies showed that leukemic lymphoblasts associate with fibroblasts and with the extracellular matrix (ECM) primarily via their specialized cell surface structures. Immunogold labelingsoliduselectron microscopy analysis revealed that the areas of contact between lymphoblasts and fibroblasts contained beta1 integrins (VLA-4 and VLA-5), fibronectin, vascular cell adhesion molecule (VCAM-1), and a cartilage-link protein, CD44. Double immunogold labeling studies disclosed a direct in situ relationship between fibronectin and VLA-4, VLA-5, and CD44. We hypothesize that these molecular interactions either bring lymphoblasts into close physical proximity with other fibroblast-bound or ECM-bound survival factors or provide survival signals themselves.

Prognostic factors in the acute lymphoid and myeloid leukemias of infants

The age boundaries and prognostic factors that define the infant leukemias are still controversial. We therefore analyzed event-free survival according to age group in 96 children treated for acute lymphoblastic leukemia (ALL) and 51 treated for acute myeloid leukemia (AML) before the age of 2 years. The study population was registered in consecutive institutional trials of multiagent chemotherapy conducted between 1980 and 1994. Among infants with ALL, event-free survival was significantly poorer in the 0- to 6-month-old group than in patients treated between 6 and 12 months of age (P = 0.03), whose outcome was in turn inferior to that in the 12- to 18-month and 18- to 24-month age groups (P = 0.013). Leukemic cells from ALL patients younger than 12 months had a significantly higher frequency of 11q23/MLL abnormalities, as well as better growth in stromal cell culture, compared to lymphoblasts from the older groups (P < 0.01). The only independent predictor of adverse prognosis among infants diagnosed with ALL before age 12 months was the presence of an 11q23/MLL rearrangement (P = 0.03). These findings contrast sharply with results for the AML cohort, whose event-free survival did not vary significantly by age group (P = 0.58). Male sex (P = 0.01) and leukocyte count > or = 50 x 10(9/l) (P = 0.04), but not 11q23 abnormalities, were independently associated with a poorer outcome for children with AML younger than 12 months at diagnosis. Thus, in very young children with ALL (but not AML), the rearrangement status of the 11q23/MLL region supersedes age group as a determinant of treatment outcome.

Stroma-supported culture in childhood B-lineage acute lymphoblastic leukemia cells predicts treatment outcome

We developed a stroma cell culture system that suppresses apoptosis of malignant cells from cases of B-lineage acute lymphoblastic leukemia. By multiparameter flow cytometric measurements of cell recovery after culture on stromal layers, we assessed the growth potential of 70 cases of newly diagnosed B-lineage acute lymphoblastic leukemia and related the findings of treatment outcome in a single program of chemotherapy. The numbers of leukemic cells recovered after 7 d of culture ranged from < 1 to 292% (median, 91%). The basis of poor cell recoveries from stromal layers appeared to be a propensity of the lymphoblasts to undergo apoptosis. The probability of event-free survival at 4 yr of follow-up was 50 +/- 9% (SE) among patients with higher cell recoveries ( > 91%), and 94 +/- 6% among those with reduced cell recoveries (+/- 91%; P = 0.0003). The prognostic value of leukemic cell recovery after culture exceeded estimates for all other recognized high-risk features and remained the most significant after adjustment with all competing covariates. Thus, the survival ability of leukemic cells on bone marrow-derived stromal layers reflects aggressiveness of the disease and is a powerful, independent predictor of treatment outcome in children with B-lineage acute lymphoblastic leukemia.

Clinical relevance of BCL-2 overexpression in childhood acute lymphoblastic leukemia

Enforced BCL-2 gene expression in leukemic cell lines suppresses apoptosis and confers resistance to anticancer drugs, but the clinical significance of increased BCL-2 protein levels in acute lymphoblastic leukemia (ALL) is unknown. Among 52 children with newly diagnosed ALL, BCL-2 expression in leukemic lymphoblasts ranged widely, from 4,464 to 59,753 molecules of equivalent soluble fluorochrome per cell (MESF), as determined by flow cytometry. The mean (+/- SD) level of MESF in 43 cases of B-lineage ALL (19,410 +/- 11,834) was higher than that detected in CD10+ B-lymphoid progenitors from normal bone marrow (450 +/- 314; P < .001), and CD19+ peripheral blood B lymphocytes (7,617 +/- 1,731; P = .02). Levels of BCL-2 in T-ALL cases (17,909 +/- 18,691) were also generally higher than those found in normal CD1a+ thymocytes (1,762 +/- 670), or in peripheral blood T lymphocytes (9,687 +/- 3,019). Although higher levels of BCL-2 corresponded to higher leukemic cell recoveries after culture in serum-free medium, they did not correlate with higher cell recoveries after culture on stromal layers, or with in vitro resistance to vincristine, dexamethasone, 6-thioguanine, cytarabine, teniposide, daunorubicin or methotrexate. BCL-2 protein levels did not correlate with presenting clinical features. Unexpectedly, however, lower-than-median MESF values were significantly associated with the presence of chromosomal translocations (P = .010). Notably, all six cases with the Philadelphia chromosome, a known high-risk feature, had low levels of BCL-2 expression (P = .022). Higher levels of BCL-2 were not associated with poorer responses to therapy among 33 uniformly treated patients, and were not observed in three patients studied at relapse. In conclusion, increased BCL-2 expression in childhood ALL appears to enhance the ability of lymphoblasts to survive without essential trophic factors, and is inversely related to the presence of chromosomal translocations. However, it does not reflect increased disease aggressiveness or resistance to chemotherapy.

CD38 signal transduction in human B cell precursors. Rapid induction of tyrosine phosphorylation, activation of syk tyrosine kinase, and phosphorylation of phospholipase C-gamma and phosphatidylinositol 3-kinase

Ligation of CD38 inhibits proliferation and induces apoptosis of human immature B cells, but the molecular mechanisms underlying this function are unknown. We found that CD38 dimerization with the specific mAbs T16 and IB4 induces rapid and transient tyrosine phosphorylation of several intracellular proteins in the immature B cell lines RS4;11, REH, 380, Nalm6, and OP-1. This effect could be markedly reduced by incubating cells with the tyrosine kinase inhibitors genistein, staurosporine, and herbimycin A. CD38 dimerization induced tyrosine phosphorylation of the protein kinase syk and increased syk kinase activity. CD38 dimerization also induced tyrosine phosphorylation of phospholipase C-gamma and of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K). The latter was accompanied by a distinct increase in PI 3-kinase activity in the immunoprecipitates obtained with an anti-phosphotyrosine Ab. In contrast to the signaling triggered by surface Ig engagement in B lymphocytes, CD38 ligation did not appear to induce tyrosine phosphorylation of the src-like protein tyrosine kinases lyn, fyn, and btk, or of vav- and ras-GTPase-activating protein, nor did it induce detectable changes in cytosolic CA2+ concentrations. CD38 signaling also differed from cytokine-induced signaling in that it did not cause tyrosine phosphorylation of Jak1 and Jak2. Finally, CD38 ligation did not inhibit IL-3-induced tyrosine phosphorylation of Jak2. These results identify CD38 as a cell surface receptor with signal transduction properties activated by dimerization. Induction of signal transduction by CD38 ligation implies the existence of a yet unidentified natural ligand of CD38.

CD38 signal transduction in human B cell precursors. Rapid induction of tyrosine phosphorylation, activation of syk tyrosine kinase, and phosphorylation of phospholipase C-gamma and phosphatidylinositol 3-kinase

Ligation of CD38 inhibits proliferation and induces apoptosis of human immature B cells, but the molecular mechanisms underlying this function are unknown. We found that CD38 dimerization with the specific mAbs T16 and IB4 induces rapid and transient tyrosine phosphorylation of several intracellular proteins in the immature B cell lines RS4;11, REH, 380, Nalm6, and OP-1. This effect could be markedly reduced by incubating cells with the tyrosine kinase inhibitors genistein, staurosporine, and herbimycin A. CD38 dimerization induced tyrosine phosphorylation of the protein kinase syk and increased syk kinase activity. CD38 dimerization also induced tyrosine phosphorylation of phospholipase C-gamma and of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-K). The latter was accompanied by a distinct increase in PI 3-kinase activity in the immunoprecipitates obtained with an anti-phosphotyrosine Ab. In contrast to the signaling triggered by surface Ig engagement in B lymphocytes, CD38 ligation did not appear to induce tyrosine phosphorylation of the src-like protein tyrosine kinases lyn, fyn, and btk, or of vav- and ras-GTPase-activating protein, nor did it induce detectable changes in cytosolic CA2+ concentrations. CD38 signaling also differed from cytokine-induced signaling in that it did not cause tyrosine phosphorylation of Jak1 and Jak2. Finally, CD38 ligation did not inhibit IL-3-induced tyrosine phosphorylation of Jak2. These results identify CD38 as a cell surface receptor with signal transduction properties activated by dimerization. Induction of signal transduction by CD38 ligation implies the existence of a yet unidentified natural ligand of CD38.

Stromal cell-mediated transcriptional regulation of the CD13/aminopeptidase N gene in leukemic cells

CD13/aminopeptidase N (APN) is a cell surface metallopeptidase expressed by normal and leukemic myeloid cells, and by lymphoblasts in 5-10% of acute lymphoid leukemia (ALL) cases, previously classified as 'biphenotypic' or 'mixed-lineage' leukemias. In fresh cells from two early B-lineage, t(9;22)-positive, ALL cases that were CD13/APN-negative at diagnosis, high levels of CD13/APN expression were induced after 3 days of in vitro culture. Similarly, continuously growing cell lines established from these ALLs, KOPN-30bi and KOPN-57bi, expressed CD13/APN, but retained other phenotypic, cytochemical and molecular features of early B-lineage cells. After 7 days of culture on human bone marrow stromal layers or murine S17 stromal cells, levels of CD13/APN expression by the leukemic cell lines decreased by more than 4-fold. After 21 days of culture on stromal cells, CD13/APN became undetectable by flow cytometry; however, the original levels of expression were regained when the cell lines were cultured without stroma. A more moderate decrease in CD13/APN expression was also observed in the myeloid lines KG-1 and HL-60 during culture on stroma. Suppression of CD13/APN expression required contact with stroma, but did not depend on VLA-4-mediated adhesion. Surprisingly, the mechanism through which stromal cells down-regulated CD13/APN expression in leukemic cells involved suppression of transcription from the CD13/APN gene. Contact with stroma resulted in a 2-3-fold decrease in CD13/APN mRNA expression and near ablation of CD13/APN gene transcription in nuclear run-on assays. Thus, CD13/APN expression by leukemic cells is regulated by interactions with the bone marrow microenvironment. CD13/APN expression in some ALL at diagnosis could result from a block in the signal transduction pathways that cause its suppression by bone marrow stromal cells.

Ligation of CD38 suppresses human B lymphopoiesis

CD38 is a transmembrane glycoprotein expressed in many cell types, including lymphoid progenitors and activated lymphocytes. High levels of CD38 expression on immature lymphoid cells suggest its role in the regulation of cell growth and differentiation, but there is no evidence demonstrating a functional activity of CD38 on these cells. We used stroma-supported cultures of B cell progenitors and anti-CD38 monoclonal antibodies (T16 and IB4) to study CD38 function. In cultures of normal bone marrow CD19+ cells (n = 5), addition of anti-CD38 markedly reduced the number of cells recovered after 7 d. Cell loss was greatest among CD19+ sIg- B cell progenitors (mean cell recovery +/- SD = 7.2 +/- 11.7% of recovery in control cultures) and extended to CD19+CD34+ B cells (the most immature subset; 7.6 +/- 2.2%). In contrast, CD38 ligation did not substantially affect cell numbers in cultures of normal peripheral blood or tonsillar B cells. In stroma-supported cultures of 22 B-lineage acute lymphoblastic leukemia cases, anti-CD38 suppressed recovery of CD19+ sIg- leukemic cells. CD38 ligation also suppressed the growth of immature lymphoid cell lines cultured on stroma and, in some cases, in the presence of stroma-derived cytokines (interleukin [IL] 7, IL-3, and/or stem cell factor), but did not inhibit growth in stroma- or cytokine-free cultures. DNA content and DNA fragmentation studies showed that CD38 ligation of stroma-supported cells resulted in both inhibition of DNA synthesis and induction of apoptosis. It is known that CD38 catalyzes nicotinamide adenine dinucleotide (NAD+) hydrolysis into cyclic ADP-ribose (cADPR) and ADPR. However, no changes in NAD+ hydrolysis or cADPR and ADPR production after CD38 ligation were found by high-performance liquid chromatography; addition of NAD+, ADPR, or cADPR to cultures of lymphoid progenitors did not offset the inhibitory effects of anti-CD38. Thus, anti-CD38 does not suppress B lymphopoiesis by altering the enzymatic function of the molecule. In conclusion, these data show that CD38 ligation inhibits the growth of immature B lymphoid cells in the bone marrow microenvironment, and suggest that CD38 interaction with a putative ligand represents a novel regulatory mechanism of B lymphopoiesis.

PITSLRE protein kinase activity is associated with apoptosis

Minimal ectopic expression of a 58-kDa protein kinase (PITSLRE beta 1), distantly related to members of the cdc2 gene family, induces telophase delay, abnormal chromosome segregation, and decreased growth rates in Chinese hamster ovary cells. Here we show that this decrease in cell growth rate is due to apoptosis. Apoptosis is also induced by ectopic expression of an amino-terminal deletion mutant containing the catalytic and C-terminal domains of PITSLRE beta 1 but not by other mutants lacking histone H1 kinase activity or by other members of the cdc2 gene family. However, unlike the wild-type PITSLRE beta 1 over-expressors, ectopic expression of the N-terminal PITSLRE beta 1 mutant does not result in telophase delay or abnormal chromosome segregation. These results suggested that the function of this protein kinase could be linked to apoptotic signaling. To test this hypothesis, we examined levels of PITSLRE mRNA, steady-state protein, and enzyme activity in human T cells undergoing apoptosis after activation with the anti-Fas monoclonal antibody (MAb). All were substantially elevated shortly after Fas MAb treatment. In addition to new transcription and translation, proteolysis contributed to the increased steady-state levels of a novel 50-kDa PITSLRE protein, as suggested by the diminution of larger PITSLRE isoforms observed in the same cells. Indeed, treatment of the Fas-activated T cells with a serine protease inhibitor prevented apoptotic death and led to the accumulation of larger, less active PITSLRE kinase isoforms but not the enzymatically active 50-kDa PITSLRE isoform. Finally, induction of apoptosis by glucocorticoids in the same cell line, as well as by Fas MAb treatment of another T-cell line, led to a similar induction of 50-kDa PITSLRE protein levels over time. These findings suggest that (i) PITSLRE kinase(s) may lie within apoptotic signaling pathway(s), (ii) serine protease activation may be an early event in Fas-activated apoptosis of human T cells, and (iii) some PITSLRE kinase isoforms may be targets of apoptotic proteases.

Hyper IgM syndrome associated with defective CD40-mediated B cell activation

Recent studies show that most patients with X-linked hyper IgM syndrome have defects in the gene for CD40 ligand. We evaluated 17 unrelated males suspected of having X-linked hyper IgM syndrome. Activated T cells from 13 of the 17 patients failed to bind a soluble CD40 construct. In these patients, the sequence of CD40 ligand demonstrated mutations. By contrast, T cells from the remaining four patients exhibited normal binding to the CD40 construct. Sequencing of the cDNA for CD40 ligand from these patients did not show mutations. The possibility that hyper IgM syndrome in these four patients was due to abnormalities in the B cell response to CD40-mediated signals was examined. Peripheral blood lymphocytes were stimulated with anti-CD40 alone, IL4 alone or anti-CD40 plus IL4. In comparison with B cells from controls or patients with hyper IgM syndrome and mutant CD40 ligand, B cells from the patients with hyper IgM syndrome and normal CD40 ligand were defective in their ability to secrete IgE (P < 0.02) or express activation markers, CD25 and CD23 (P < 0.02) in response to stimulation with anti-CD40. The failure of these B cells to respond to CD40-mediated activation could not be attributed to a generalized deficiency in B cell activation because IL4 induced normal up-regulation of CD23 and CD25 expression. These findings indicate that hyper IgM syndrome may result from defects in expression of CD40 ligand by activated T cells or defects in CD40-mediated signal transduction in B cells.

Monitoring minimal residual disease in acute leukemia: expectations, possibilities and initial clinical results

Therapy of acute leukemia may be improved by a more accurate assessment of the effects of treatment on tumor burden and by anticipating relapse with greater precision. The sensitivity limit of assessing residual disease by morphology is usually 5%. Several alternative approaches are available to study minimal residual disease, defined as the presence of leukemic cells not detectable by morphology. These include studies of chromosomal abnormalities by conventional karyotyping, flow cytometry, in situ hybridization and polymerase chain reaction (PCR), investigation of gene rearrangements by Southern blotting and PCR, and immunological methods. Some of these techniques enable the detection of 1 leukemic cells among 10,000 or more normal cells. In the following, the advantages and limitations of sensitive methods for detecting small numbers of leukemic cells are reviewed. The rationale for monitoring residual disease in acute leukemia and the initial results of studies correlating minimal residual disease and clinical outcome are discussed.

Use of stroma-supported cultures of leukemic cells to assess antileukemic drugs. II. Potent cytotoxicity of 2-chloro-deoxyadenosine in acute lymphoblastic leukemia

We used a recently established stroma-supported tissue culture technique that allows long-term culture of acute lymphoblastic leukemia (ALL) cells to study 2-chloro-2'-deoxyadenosine (2CdA) cytotoxicity to leukemic lymphoblasts. In the 20 cases of ALL studied, the number of cells recovered after 7 days of culture on allogeneic stromal layers were 58-192% (median, 95.5%) of those originally seeded. In parallel cultures with 2CdA (100 nM), 74- > 99% (median, 97.5%) of leukemic lymphoblasts were killed. The cytotoxicity of 2CdA extended to all ten samples with either the t(9;22) (q34;q11) or 11q23 chromosomal abnormalities, karyotypes associated with an extremely poor outcome, as well as to two samples collected at the time of relapse. The effects of 2CdA were dose-dependent, and were due to triggering of apoptosis as shown by typical morphologic changes and occurrence of DNA fragmentation. Stromal layers were apparently not affected by 2CdA treatment, even when used at 1000 nM. We also tested 2CdA cytotoxicity to multidrug resistant subclones of the CCRF-CEM ALL cell line. CEM/VLB100 expresses P-glycoprotein, whereas CEM/VM-1 and CEM/VM-1-5 have topoisomerase II mutations that are associated with resistance to topoisomerase II inhibitors. Overexpression of P-glycoprotein or alterations in topoisomerase II did not protect cells from 2CdA cytotoxicity. We conclude that 2CdA is cytotoxic in most cases of ALL. The method used in this study may be applied to evaluate leukemic blast cell sensitivity to compounds with potential anti-leukemic activity, and to select patients for entry into clinical trials.

Applications of cytometry to study acute leukemia: in vitro determination of drug sensitivity and detection of minimal residual disease

Modern clinical applications of cytometry include the determination of the most powerful antileukemic drugs in each patient at the time of diagnosis and the monitoring of residual disease during and off treatment. The precision of in vitro assays to test the susceptibility of cancer cells to cytotoxic drugs depends on the ability to maintain the cells' viability in culture. We found that bone marrow-derived allogeneic stromal cells are critical to prevent death by apoptosis of acute lymphoblastic leukemia (ALL) cells. Thus, we devised an in vitro drug sensitivity assay in which ALL cells are seeded onto stromal cells and viable leukemic cells are counted at the end of cultures by flow cytometry. Our preliminary results indicate that this assay is suitable for evaluating the drug sensitivity of leukemic lymphoblasts and testing the antileukemic activity of potentially effective compounds which have not yet been administered to patients with ALL. The identification of immunophenotypes expressed on leukemic cells but absent or extremely rare among normal hematopoietic progenitors allows close monitoring of the effects of drug treatment in vivo. Phenotypes that afford a detection level of 1 leukemic cell among 10,000 normal bone marrow cells have been identified in 90% of cases of T-ALL, 25% of B-lineage ALL, and 40% of acute myeloid leukemia (AML). In several studies, residual disease emerging during continuation therapy or off treatment almost invariably anticipated overt relapse by 1-7 months. These data indicate the reliability of immunologic techniques to detect occult leukemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth requirements of normal and leukemic human B cell progenitors

Leukemic lymphoblasts in B-lineage acute lymphoblastic leukemia (ALL) express morphologic, phenotypic and genotypic features which resemble those of B lymphocyte progenitors in normal bone marrow. Normal immature B cells and cells from most cases of B-lineage ALL rapidly die in vitro unless they are supported by bone marrow-derived stromal feeder layers. Techniques suitable for maintaining normal and leukemic immature B cells in culture have been developed. Thus, the stromal cell types and growth factors that generate a milieu suitable for immature B-cell development can now be elucidated. In addition, the similarities and discrepancies in survival requirements of normal and leukemic B cell precursors can be studied. We postulate that leukemic B cell precursors can survive and expand in microenvironments incapable of supporting their normal counterparts, and that the study of the survival requirements of ALL cells will provide indications about the aggressivity of the disease in vivo. In this review, we discuss the culture conditions that support in vitro survival of human immature B cells, some of the factors that influence their expansion, and the putative molecular basis for the prolonged life-span of leukemic lymphoblasts.