Characteristics and long-term outcome in a large series of chronic myelomonocytic leukaemia patients including 104 formerly referred to as oligomonocytic.

Recently modified diagnostic criteria for chronic myelomonocytic leukaemia (CMML) have lowered the cut-off for absolute monocytosis. In the largest series to date, we have analysed 313 CMML patients, including 104 with oligomonocytic (OM)-CMML. Five-year survival was longer for OM-CMML than for other patients (p < 0.001). Multivariate analysis identified OM-CMML as a favourable prognostic factor (HR 0.58; p = 0.002). The 5-year cumulative incidence of progression to classical CMML was 47%. Older age and transfusion dependence were adverse prognostic factors for OM-CMML. Our results support the inclusion of OM-CMML in the CMML category as a subtype with superior outcomes.

International Consensus Classification of Myeloid Neoplasms and Acute Leukemias: integrating morphologic, clinical, and genomic data.

The classification of myeloid neoplasms and acute leukemias was last updated in 2016 within a collaboration between the World Health Organization (WHO), the Society for Hematopathology, and the European Association for Haematopathology. This collaboration was primarily based on input from a clinical advisory committees (CACs) composed of pathologists, hematologists, oncologists, geneticists, and bioinformaticians from around the world. The recent advances in our understanding of the biology of hematologic malignancies, the experience with the use of the 2016 WHO classification in clinical practice, and the results of clinical trials have indicated the need for further revising and updating the classification. As a continuation of this CAC-based process, the authors, a group with expertise in the clinical, pathologic, and genetic aspects of these disorders, developed the International Consensus Classification (ICC) of myeloid neoplasms and acute leukemias. Using a multiparameter approach, the main objective of the consensus process was the definition of real disease entities, including the introduction of new entities and refined criteria for existing diagnostic categories, based on accumulated data. The ICC is aimed at facilitating diagnosis and prognostication of these neoplasms, improving treatment of affected patients, and allowing the design of innovative clinical trials.

BCL3 rearrangements in B-cell lymphoid neoplasms occur in two breakpoint clusters associated with different diseases.

The t(14;19)(q32;q13) often juxtaposes BCL3 with immunoglobulin heavy chain (IGH) resulting in overexpression of the gene. In contrast to other oncogenic translocations, BCL3 rearrangement (BCL3-R) has been associated with a broad spectrum of lymphoid neoplasms. Here we report an integrative whole-genome sequence, transcriptomic, and DNA methylation analysis of 13 lymphoid neoplasms with BCL3-R. The resolution of the breakpoints at single base-pair revealed that they occur in two clusters at 5' (n=9) and 3' (n=4) regions of BCL3 associated with two different biological and clinical entities. Both breakpoints were mediated by aberrant class switch recombination of the IGH locus. However, the 5' breakpoints (upstream) juxtaposed BCL3 next to an IGH enhancer leading to overexpression of the gene whereas the 3' breakpoints (downstream) positioned BCL3 outside the influence of the IGH and were not associated with its expression. Upstream BCL3-R tumors had unmutated IGHV, trisomy 12, and mutated genes frequently seen in chronic lymphocytic leukemia (CLL) but had an atypical CLL morphology, immunophenotype, DNA methylome, and expression profile that differ from conventional CLL. In contrast, downstream BCL3-R neoplasms were atypical splenic or nodal marginal zone lymphomas (MZL) with mutated IGHV, complex karyotypes and mutated genes typical of MZL. Two of the latter four tumors transformed to a large B-cell lymphoma. We designed a novel fluorescence in situ hybridization assay that recognizes the two different breakpoints and validated these findings in 17 independent tumors. Overall, upstream or downstream breakpoints of BCL3-R are mainly associated with two subtypes of lymphoid neoplasms with different (epi)genomic, expression, and clinicopathological features resembling atypical CLL and MZL, respectively.

Spanish cohort of VEXAS syndrome: clinical manifestations, outcome of treatments and novel evidences about UBA1 mosaicism.

BACKGROUND: The vacuoles, E1-enzyme, X linked, autoinflammatory and somatic (VEXAS) syndrome is an adult-onset autoinflammatory disease (AID) due to postzygotic UBA1 variants. OBJECTIVES: To investigate the presence of VEXAS syndrome among patients with adult-onset undiagnosed AID. Additional studies evaluated the mosaicism distribution and the circulating cytokines. METHODS: Gene analyses were performed by both Sanger and amplicon-based deep sequencing. Patients' data were collected from their medical charts. Cytokines were quantified by Luminex. RESULTS: Genetic analyses of enrolled patients (n=42) identified 30 patients carrying UBA1 pathogenic variants, with frequencies compatible for postzygotic variants. All patients were male individuals who presented with a late-onset disease (mean 67.5 years; median 67.0 years) characterised by cutaneous lesions (90%), fever (66.7%), pulmonary manifestations (66.7%) and arthritis (53.3%). Macrocytic anaemia and increased erythrocyte sedimentation rate and ferritin were the most relevant analytical abnormalities. Glucocorticoids ameliorated the inflammatory manifestations, but most patients became glucocorticoid-dependent. Positive responses were obtained when targeting the haematopoietic component of the disease with either decitabine or allogeneic haematopoietic stem cell transplantation. Additional analyses detected the UBA1 variants in both haematopoietic and non-haematopoietic tissues. Finally, analysis of circulating cytokines did not identify inflammatory mediators of the disease. CONCLUSION: Thirty patients with adult-onset AID were definitively diagnosed with VEXAS syndrome through genetic analyses. Despite minor interindividual differences, their main characteristics were in concordance with previous reports. We detected for the first time the UBA1 mosaicism in non-haematopoietic tissue, which questions the previous concept of myeloid-restricted mosaicism and may have conceptual consequences for the disease mechanisms.

Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase fusion genes: A workshop report with focus on novel entities and a literature review including paediatric cases.

Myeloid/lymphoid neoplasms with eosinophilia (M/LN-eo) and tyrosine kinase (TK) gene fusions are a rare group of haematopoietic neoplasms with a broad range of clinical and morphological presentations. Paediatric cases have increasingly been recognised. Importantly, not all appear as a chronic myeloid neoplasm and eosinophilia is not always present. In addition, standard cytogenetic and molecular methods may not be sufficient to diagnose M/LN-eo due to cytogenetically cryptic aberrations. Therefore, additional evaluation with fluorescence in-situ hybridisation and other molecular genetic techniques (array-based comparative genomic hybridisation, RNA sequencing) are recommended for the identification of specific TK gene fusions. M/LN-eo with JAK2 and FLT3-rearrangements and ETV6::ABL1 fusion were recently added as a formal member to this category in the International Consensus Classification (ICC) and the 5th edition of the WHO classification (WHO-HAEM5). In addition, other less common defined genetic alterations involving TK genes have been described. This study is an update on M/LN-eo with TK gene fusions with focus on novel entities, as illustrated by cases submitted to the Bone Marrow Workshop, organised by the European Bone Marrow Working Group (EBMWG) within the frame of the 21st European Association for Haematopathology congress (EAHP-SH) in Florence 2022. A literature review was performed including paediatric cases of M/LN-eo with TK gene fusions.

Clinico-biological features and outcome of patients with splenic marginal zone lymphoma with histological transformation.

We describe 36 patients with splenic marginal zone lymphoma (SMZL) with transformation (SMZL-T), including 15 from a series of 84 patients with SMZL diagnosed at the Hospital Clinic of Barcelona (HCB) and 21 diagnosed with SMZL-T in other centres. In the HCB cohort, the cumulative incidence of transformation at 5 years was 15%. Predictors for transformation were cytopenias, hypoalbuminaemia, complex karyotype (CK) and both the Intergruppo Italiano Linfomi (ILL) and simplified Haemoglobin, Platelet count, lactate dehydrogenase (LDH) and extrahilar Lymphadenopathy (HPLL)/ABC scores (P < 0·05). The only independent predictor for transformation in multivariate analysis was CK [hazard ratio (HR) 4·025, P = 0·05]. Patients with SMZL-T had a significantly higher risk of death than the remainder (HR 3·89, P < 0·001). Of the 36 patients with SMZL-T, one developed Hodgkin lymphoma and 35 a diffuse large B-cell lymphoma, 71% with a non-germinal centre phenotype. The main features were B symptoms, lymphadenopathy, and high serum LDH. CK was observed in 12/22 (55%) SMZL-T and fluorescence in situ hybridisation detected abnormalities of MYC proto-oncogene, basic helix-loop-helix transcription factor (MYC), B-cell leukaemia/lymphoma 2 (BCL2) and/or BCL6 in six of 14 (43%). In all, 21 patients received immunochemotherapy, six chemotherapy, one radiotherapy and three splenectomy. The complete response (CR) rate was 61% and the median survival from transformation was 4·92 years. Predictors for a worse survival in multivariate analysis were high-risk International Prognostic Index (HR 5·294, P = 0·016) and lack of CR (HR 2·67, P < 0·001).

Results of ARI-0001 CART19 cell therapy in patients with relapsed/refractory CD19-positive acute lymphoblastic leukemia with isolated extramedullary disease.

We evaluated outcomes of 18 patients with isolated extramedullary disease (iEMD) relapsed/refractory (R/R) B-cell acute lymphoblastic leukemia (B-ALL) treated with the CD19-directed CAR T cells ARI-0001 in two centers (adult and pediatric), including patients treated in the CART19-BE-01 trial and the consecutive compassionate use program. iEMD was detected by PET-CT in 78% (14/18), and/or by cerebrospinal fluid analysis in 28% (5/18). Patients received cyclophosphamide and fludarabine followed by 1 × 106 ARI-0001 cells/kg, initially as a single dose (first patient) and later split into three fractions (10%, 30%, and 60%). Cytokine release syndrome (CRS) occurred in 50% (9/18) of patients, with no cases of grade ≥3 CRS, and 1 case (6%) of grade 1 neurotoxicity. Tocilizumab was used in 6% of patients (1/18). Procedure-related mortality was 0% at 2 years. Objective responses were seen in 94% (95% confidence interval [CI]: 73%-99%) of patients, with complete responses (CR) seen in 78% (95% CI: 52%-94%) of them. Progression-free and overall survival were 49% (95% CI: 30%-79%) and 61% (95% CI: 40%-92%) at 2 years. In conclusion, the use of ARI-0001 cells in patients with R/R ALL and iEMD was associated with a safety and efficacy profile that is comparable with what is observed in patients with marrow involvement and in line with other CART19 products.

Non-coding recurrent mutations in chronic lymphocytic leukaemia.

Chronic lymphocytic leukaemia (CLL) is a frequent disease in which the genetic alterations determining the clinicobiological behaviour are not fully understood. Here we describe a comprehensive evaluation of the genomic landscape of 452 CLL cases and 54 patients with monoclonal B-lymphocytosis, a precursor disorder. We extend the number of CLL driver alterations, including changes in ZNF292, ZMYM3, ARID1A and PTPN11. We also identify novel recurrent mutations in non-coding regions, including the 3' region of NOTCH1, which cause aberrant splicing events, increase NOTCH1 activity and result in a more aggressive disease. In addition, mutations in an enhancer located on chromosome 9p13 result in reduced expression of the B-cell-specific transcription factor PAX5. The accumulative number of driver alterations (0 to ≥4) discriminated between patients with differences in clinical behaviour. This study provides an integrated portrait of the CLL genomic landscape, identifies new recurrent driver mutations of the disease, and suggests clinical interventions that may improve the management of this neoplasia.

Mutations in the RAS-BRAF-MAPK-ERK pathway define a specific subgroup of patients with adverse clinical features and provide new therapeutic options in chronic lymphocytic leukemia.

Mutations in genes of the RAS-BRAF-MAPK-ERK pathway have not been fully explored in patients with chronic lymphocytic leukemia. We, therefore, analyzed the clinical and biological characteristics of chronic lymphocytic leukemia patients with mutations in this pathway and investigated the in vitro response of primary cells to BRAF and ERK inhibitors. Putative damaging mutations were found in 25 of 452 patients (5.5%). Among these, BRAF was mutated in nine patients (2.0%), genes upstream of BRAF (KITLG, KIT, PTPN11, GNB1, KRAS and NRAS) were mutated in 12 patients (2.6%), and genes downstream of BRAF (MAPK2K1, MAPK2K2, and MAPK1) were mutated in five patients (1.1%). The most frequent mutations were missense, subclonal and mutually exclusive. Patients with these mutations more frequently had increased lactate dehydrogenase levels, high expression of ZAP-70, CD49d, CD38, trisomy 12 and unmutated immunoglobulin heavy-chain variable region genes and had a worse 5-year time to first treatment (hazard ratio 1.8, P=0.025). Gene expression analysis showed upregulation of genes of the MAPK pathway in the group carrying RAS-BRAF-MAPK-ERK pathway mutations. The BRAF inhibitors vemurafenib and dabrafenib were not able to inhibit phosphorylation of ERK, the downstream effector of the pathway, in primary cells. In contrast, ulixertinib, a pan-ERK inhibitor, decreased phospho-ERK levels. In conclusion, although larger series of patients are needed to corroborate these findings, our results suggest that the RAS-BRAF-MAPK-ERK pathway is one of the core cellular processes affected by novel mutations in chronic lymphocytic leukemia, is associated with adverse clinical features and could be pharmacologically inhibited.

Genomic characterization in triple-negative primary myelofibrosis and other myeloid neoplasms with bone marrow fibrosis.

Triple-negative primary myelofibrosis (TN-PMF) and other myeloid neoplasms with associated bone marrow fibrosis such as the myelodysplastic syndromes (MDS-F) or the myelodysplastic/myeloproliferative neoplasms (MDS/MPN-F) are rare entities, often difficult to distinguish from each other. Thirty-four patients previously diagnosed with TN-PMF (n = 14), MDS-F (n = 18), or MDS/MPN-F (n = 2) were included in the present study. After central revision of the bone marrow histology, diagnoses according to the 2016-WHO classification were TN-PMF (n = 6), MDS-F (n = 19), and MDS/MPN-F (n = 9), with TN-PMF genotype representing only 4% of a cohort of 141 molecularly annotated PMF. Genomic classification according to next-generation sequencing and cytogenetic study was performed in 28 cases. Median number of mutations was 4 (range 1-7) in cases with TP53 disruption/aneuploidy or with chromatin-spliceosome mutations versus 1 mutation (range 0-2) in other molecular subgroups (p < 0.0001). The number of mutations and the molecular classification were better than PMF and MDS conventional scoring systems to predict survival and progression to acute leukemia. In conclusion, TN-PMF is an uncommon entity when the 2016 WHO criteria are strictly applied. Genomic classification may help in the prognostic assessment of patients with myeloid neoplasms with bone marrow fibrosis.

Clinical impact of clonal and subclonal TP53, SF3B1, BIRC3, NOTCH1, and ATM mutations in chronic lymphocytic leukemia.

Genomic studies have revealed the complex clonal heterogeneity of chronic lymphocytic leukemia (CLL). The acquisition and selection of genomic aberrations may be critical to understanding the progression of this disease. In this study, we have extensively characterized the mutational status of TP53, SF3B1, BIRC3, NOTCH1, and ATM in 406 untreated CLL cases by ultra-deep next-generation sequencing, which detected subclonal mutations down to 0.3% allele frequency. Clonal dynamics were examined in longitudinal samples of 48 CLL patients. We identified a high proportion of subclonal mutations, isolated or associated with clonal aberrations. TP53 mutations were present in 10.6% of patients (6.4% clonal, 4.2% subclonal), ATM mutations in 11.1% (7.8% clonal, 1.3% subclonal, 2% germ line mutations considered pathogenic), SF3B1 mutations in 12.6% (7.4% clonal, 5.2% subclonal), NOTCH1 mutations in 21.8% (14.2% clonal, 7.6% subclonal), and BIRC3 mutations in 4.2% (2% clonal, 2.2% subclonal). ATM mutations, clonal SF3B1, and both clonal and subclonal NOTCH1 mutations predicted for shorter time to first treatment irrespective of the immunoglobulin heavy-chain variable-region gene (IGHV) mutational status. Clonal and subclonal TP53 and clonal NOTCH1 mutations predicted for shorter overall survival together with the IGHV mutational status. Clonal evolution in longitudinal samples mainly occurred in cases with mutations in the initial samples and was observed not only after chemotherapy but also in untreated patients. These findings suggest that the characterization of the subclonal architecture and its dynamics in the evolution of the disease may be relevant for the management of CLL patients.

Leukoerythroblastosis in a Young Child with Severe Malaria and Superimposed Gram Negative Infection.

Background: Leukoerythroblastosis, a non-specific and often short-lasting response of the bone marrow to different diseases such as malignancies or infections, is characterized by the presence in the peripheral blood of immature red and white cells. Methods: We present a case of leukoerythoblastosis occurring in a 24 months old Mozambican girl, in the context of a severe malaria episode and an associated urinary tract infection. Peripheral blood smear was used for diagnosis of malaria and leukoerythroblastosis. Enterobacter cloacae isolation and antibiotic susceptibility testing were performed by conventional microbiology. Results: Peripheral blood smear was positive for Plasmodium falciparum and showed a leukoerythroblastosis with red cell anisopoikilocytosis and left shifted neutrophils. Urine culture confirmed the presence of a multi-resistant E. cloacae. Treatment of underlying conditions resolved the leukoerythroblastic reaction. Conclusions: Leukoerythroblastosis may be related to different infectious diseases and may also appear in the context of severe malaria. Bacterial superinfection needs to be investigated.

Transcriptome characterization by RNA sequencing identifies a major molecular and clinical subdivision in chronic lymphocytic leukemia.

Chronic lymphocytic leukemia (CLL) has heterogeneous clinical and biological behavior. Whole-genome and -exome sequencing has contributed to the characterization of the mutational spectrum of the disease, but the underlying transcriptional profile is still poorly understood. We have performed deep RNA sequencing in different subpopulations of normal B-lymphocytes and CLL cells from a cohort of 98 patients, and characterized the CLL transcriptional landscape with unprecedented resolution. We detected thousands of transcriptional elements differentially expressed between the CLL and normal B cells, including protein-coding genes, noncoding RNAs, and pseudogenes. Transposable elements are globally derepressed in CLL cells. In addition, two thousand genes-most of which are not differentially expressed-exhibit CLL-specific splicing patterns. Genes involved in metabolic pathways showed higher expression in CLL, while genes related to spliceosome, proteasome, and ribosome were among the most down-regulated in CLL. Clustering of the CLL samples according to RNA-seq derived gene expression levels unveiled two robust molecular subgroups, C1 and C2. C1/C2 subgroups and the mutational status of the immunoglobulin heavy variable (IGHV) region were the only independent variables in predicting time to treatment in a multivariate analysis with main clinico-biological features. This subdivision was validated in an independent cohort of patients monitored through DNA microarrays. Further analysis shows that B-cell receptor (BCR) activation in the microenvironment of the lymph node may be at the origin of the C1/C2 differences.

Whole-genome sequencing identifies recurrent mutations in chronic lymphocytic leukaemia.

Chronic lymphocytic leukaemia (CLL), the most frequent leukaemia in adults in Western countries, is a heterogeneous disease with variable clinical presentation and evolution. Two major molecular subtypes can be distinguished, characterized respectively by a high or low number of somatic hypermutations in the variable region of immunoglobulin genes. The molecular changes leading to the pathogenesis of the disease are still poorly understood. Here we performed whole-genome sequencing of four cases of CLL and identified 46 somatic mutations that potentially affect gene function. Further analysis of these mutations in 363 patients with CLL identified four genes that are recurrently mutated: notch 1 (NOTCH1), exportin 1 (XPO1), myeloid differentiation primary response gene 88 (MYD88) and kelch-like 6 (KLHL6). Mutations in MYD88 and KLHL6 are predominant in cases of CLL with mutated immunoglobulin genes, whereas NOTCH1 and XPO1 mutations are mainly detected in patients with unmutated immunoglobulins. The patterns of somatic mutation, supported by functional and clinical analyses, strongly indicate that the recurrent NOTCH1, MYD88 and XPO1 mutations are oncogenic changes that contribute to the clinical evolution of the disease. To our knowledge, this is the first comprehensive analysis of CLL combining whole-genome sequencing with clinical characteristics and clinical outcomes. It highlights the usefulness of this approach for the identification of clinically relevant mutations in cancer.

CD49d (ITGA4) expression is a predictor of time to first treatment in patients with chronic lymphocytic leukaemia and mutated IGHV status.

We investigated CD49d (also termed ITGA4) expression and its biological and clinical correlations in 415 patients with chronic lymphocytic leukaemia. CD49d expression was stable over the course of the disease. A high expression of CD49d (>30%) was found in 142/415 (34%) patients and was associated with progressive disease (advanced clinical stage, high serum lactate dehydrogenase or ß2 -microglobulin levels; all p < 0·05) and aggressive disease biology (increased ZAP70 or CD38, unmutated IGHV, trisomy 12, mutations of NOTCH1 and SF3B1; all P < 0·05). A higher CD49d expression was also associated with a lower blood lymphocyte count and a higher number of lymphoid areas involved by the disease. Patients with high CD49d expression were treated more frequently (55% vs. 27%; P < 0·001) and earlier (median time to treatment [TTT] 65·4 months vs. not reached; P < 0·001) than those with low CD49d expression. However, no significant differences in response rates were observed. In the subgroup of patients with mutated IGHV, high CD49d expression was predictive of a shorter TTT while other markers, such as ZAP70 and CD38, were not. In conclusion, in this study CD49d expression correlated with high-risk CLL biomarkers and proved to be useful for separating patients with mutated IGHV into two different prognostic groups.

Mutations in TLR/MYD88 pathway identify a subset of young chronic lymphocytic leukemia patients with favorable outcome.

Mutations in Toll-like receptor (TLR) and myeloid differentiation primary response 88 (MYD88) genes have been found in chronic lymphocytic leukemia (CLL) at low frequency. We analyzed the incidence, clinicobiological characteristics, and outcome of patients with TLR/MYD88 mutations in 587 CLL patients. Twenty-three patients (3.9%) had mutations, 19 in MYD88 (one with concurrent IRAK1 mutation), 2 TLR2 (one with concomitant TLR6 mutation), 1 IRAK1, and 1 TLR5. No mutations were found in IRAK2 and IRAK4. TLR/MYD88-mutated CLL overexpressed genes of the nuclear factor κB pathway. Patients with TLR/MYD88 mutations were significantly younger (83% age ≤50 years) than those with no mutations. TLR/MYD88 mutations were the most frequent in young patients. Patients with mutated TLR/MYD88 CLL had a higher frequency of mutated IGHV and low expression of CD38 and ZAP-70. Overall survival (OS) was better in TLR/MYD88-mutated than unmutated patients in the whole series (10-year OS, 100% vs 62%; P = .002), and in the subset of patients age ≤50 years (100% vs 70%; P = .02). In addition, relative OS of TLR/MYD88-mutated patients was similar to that in the age- and gender-matched population. In summary, TLR/MYD88 mutations identify a population of young CLL patients with favorable outcome.

Landscape of somatic mutations and clonal evolution in mantle cell lymphoma.

Mantle cell lymphoma (MCL) is an aggressive tumor, but a subset of patients may follow an indolent clinical course. To understand the mechanisms underlying this biological heterogeneity, we performed whole-genome and/or whole-exome sequencing on 29 MCL cases and their respective matched normal DNA, as well as 6 MCL cell lines. Recurrently mutated genes were investigated by targeted sequencing in an independent cohort of 172 MCL patients. We identified 25 significantly mutated genes, including known drivers such as ataxia-telangectasia mutated (ATM), cyclin D1 (CCND1), and the tumor suppressor TP53; mutated genes encoding the anti-apoptotic protein BIRC3 and Toll-like receptor 2 (TLR2); and the chromatin modifiers WHSC1, MLL2, and MEF2B. We also found NOTCH2 mutations as an alternative phenomenon to NOTCH1 mutations in aggressive tumors with a dismal prognosis. Analysis of two simultaneous or subsequent MCL samples by whole-genome/whole-exome (n = 8) or targeted (n = 19) sequencing revealed subclonal heterogeneity at diagnosis in samples from different topographic sites and modulation of the initial mutational profile at the progression of the disease. Some mutations were predominantly clonal or subclonal, indicating an early or late event in tumor evolution, respectively. Our study identifies molecular mechanisms contributing to MCL pathogenesis and offers potential targets for therapeutic intervention.

Treatment with G-CSF reduces acute myeloid leukemia blast viability in the presence of bone marrow stroma.

BACKGROUND: The resulting clinical impact of the combined use of G-CSF with chemotherapy as a chemosensitizing strategy for treatment of acute myeloid leukemia (AML) patients is still controversial. In this study, the effect of ex vivo treatment with G-CSF on AML primary blasts was studied. METHODS: Peripheral blood mononuclear cells from AML patients were treated with G-CSF at increasing doses, alone or in co-culture with HS-5 stromal cells. Cell viability and surface phenotype was determined by flow cytometry 72 h after treatment. For clonogenicity assays, AML primary samples were treated for 18 h with G-CSF at increasing concentrations and cultured in methyl-cellulose for 14 days. Colonies were counted based on cellularity and morphology criteria. RESULTS: The presence of G-CSF reduced the overall viability of AML cells co-cultured with bone marrow stroma; whereas, in absence of stroma, a negligible effect was observed. Moreover, clonogenic capacity of AML cells was significantly reduced upon treatment with G-CSF. Interestingly, reduction in the AML clonogenic capacity correlated with the sensitivity to chemotherapy observed in vivo. CONCLUSIONS: These ex vivo results would provide a biological basis to data available from studies showing a clinical benefit with the use of G-CSF as a priming agent in patients with a chemosensitive AML and would support implementation of further studies exploring new strategies of chemotherapy priming in AML.