Abstract: Abstract 4970 Background Essential thrombocythaemia (ET) is one of the chronic myeloproliferative neoplasms (MPN), along with polycythaemia vera (PV), primary myelofibrosis (PMF) and chronic myeloid leukaemia (CML). Their common feature is excessive proliferation of a certain stem or progenitor cell in the bone marrow; in the case of ET, the megakaryocytic lineage is affected. Clinical manifestations include thrombotic events and haemorrhage. Diagnosis of ET according to new WHO-criteria requires a sustained high platelet count, bone marrow biopsy showing proliferation of the megakaryocytic lineage with large and mature morphology, demonstration of JAK2 V617F (although only present in about 50% of patients with ET) or another clonal marker and explicit exclusion of other myeloid and myeloproliferative neoplasms as well as signs of reactive thrombocytosis. Additionally, spontaneous proliferation of megakaryocytes obtained from peripheral blood can be detected in in vitro culture assays. Presently, we use agar as a matrix for megakaryocyte cultivation, although this assay has never been validated in connection with ET. The identification of megakaryocytic colonies grown on agar can sometimes be quite difficult. Our aims were therefore to technically evaluate the use of a collagen based matrix and to investigate its suitability to identify patients with ET. Patients and Methods We have examined 63 patients (26 with ET, 21 with PV, 8 with myelofibrosis [MF; including PMF and post-ET/PV-MF], 6 with secondary or idiopathic erythrocytosis and 2 with secondary thrombocytosis; mean age=59.8, male=33, female=30, mean platelet count 457 G/l) and 5 healthy subjects. Following informed consent, both clinical and laboratory data was collected. Medication intake, phlebotomies, smoking habits and regular haemogram results were noted in order to recognise possible confounding factors influencing laboratory results. Results of megakaryocyte cultivation on both agar and collagen matrixes were recorded, considering both spontaneous growth and growth stimulated by megakaryocyte derived growth factor (MDGF). Results Based on our collagen culture results we were able to define 2 or more spontaneously grown megakaryocyte colonies as the most optimal cut-off for the identification of patients with MPN (sensitivity 71%, specificity 100% with positive and negative predictive values of 100% and 45%, respectively). Compared to the agar culture results (where a specificity and a positive predictive value of 100% were demonstrated at a cut-off value of ≥ 10 CFU-Mega) we found a higher accuracy and better reproducibility. In addition, we observed an improved negative predictive value (45% with collagen versus 25% with agar cultures) reducing false negative results. Healthy subjects and patients with secondary thrombocytosis showed no significant spontaneous megakaryocyte proliferation. In patients with MF, we observed strong spontaneous and MDGF-stimulated growth of megakaryocytic colonies. At a cut-off value of ≥ 50 CFU-Mega (after stimulation with MDGF), the collagen assay showed a sensitivity of 100% and a specifity of 70% for this special form of MPN, resulting in a negative predictive value of 100%. We found no confounding clinical or laboratory parameters such as medication intake (particularly cytoreductive treatment with hydroxyurea) or phlebotomies influencing our culture results, and no significant effect of the Jak2-V617F mutation on the growth behaviour of megakaryocytic colonies. Conclusion The results of this ongoing study imply that the collagen based assay is more sensitive, specific, time efficient and user friendly regarding the detection of spontaneous proliferation of megakaryocytes than the currently used agar based culture assay. In addition, the collagen based assay also has the great advantage that it allows isolation of single megakaryocytic colonies for further analyses, for example PCR-based identification of a JAK2 mutation. Furthermore, the collagen based assay facilitates the diagnosis of patients with MPN, especially in cases where conventional diagnostic criteria are lacking, such as in ET without a JAK2 mutation. Ultimately, the new assay may well be able to detect transformation from PV/ET to MF. Disclosures No relevant conflicts of interest to declare.

Abstract: Background/Aims: Myeloproliferative neoplasms (MPN), including essential thrombocythemia (ET), polycythemia vera (PV) and myelofibrosis (MF), are clonal disorders characterized by unlimited proliferation of hematopoietic cells. Besides the dominant clone, in the majority of cases defined by a driver mutation (JAK2V617F, CALR or MPL mutation), sub-clones carrying additional mutations (e. g. in the genes ASXL1, CBL, DNMT3A, EZH2, IDH1, U2AF1, SF3B1, TET2, TP53) and/or normal polyclonal hematopoiesis coexist. Due to the loss of telomere sequences with each cell division, the length of telomeres can be used to describe clonal dynamics of hematopoietic cells. In neoplastic cells with a high mitotic rate shorter telomeres are typically found compared to the telomere length (TL) in normal counterparts. In the ET phase 2 study with the telomerase inhibitor imetelstat we demonstrated rapid and durable hematologic and molecular responses (Baerlocher et al. N Engl J Med 2015) and suppression of clones with non-driver mutations (Oppliger Leibundgut et al, ASH 2015). Here, our aims were 1) to evaluate the TL in subsets of leukocytes from patients (pts) with MPN compared to healthy individuals and 2) to study the TL dynamics in ET pts treated with imetelstat and to correlate those with hematologic and molecular responses. Patients and Methods: Blood samples from 63 standard of care (SOC)-treated or untreated pts with MPN (15 ET, 36 PV, 12 MF) diagnosed according to WHO 2008 criteria as well as from 14 ET pts treated with imetelstat after failure or intolerance to ≥1 prior therapy were analyzed for TL. TL percentiles from over 400 healthy individuals served as reference. Leukocytes were extracted from the peripheral blood and TL was measured in subsets of leukocytes by automated multicolor flow-FISH (Baerlocher et al, Nat Protoc. 2006). Results: 81% of the 62 MPN pts (11/15 ET, 32/35 PV, 7/12 MF) carried a JAK2 mutation, 6 (1 ET, 5 MF) a CALR mutation, 1 ET pt had a MPL mutation, 2 ET pts were triple negative and 3 PV pts were JAK2 wild type. 23 pts were treated with phlebotomy, 32 with hydroxyurea, 3 with anagrelide and 3 with IFN-alpha. The median age at study entry was 62 yrs (23-89, ET/PV/MF 64/58/66 yrs) and the median time since diagnosis was 2.7 yrs (0.3-14.2) in the ET SOC cohort. In the imetelstat cohort, 9/14 ET pts carried a JAK2V617F mutation and 5 a CALR mutation. The median age at study entry was 60.5 yrs (21-80) and the median time since diagnosis was 6.7 yrs (0.3-21). The telomere length values (TLV) in granulocytes from the 63 MPN pts on SOC were around the 10th percentile for pts with ET and PV and below the 1st percentile for most pts with MF. In the imetelstat cohort, 8 of 14 ET pts with a median of 2 prior therapies demonstrated TLV in granulocytes 〈 1st percentile before the imetelstat treatment, which is significantly lower than in ET pts on SOC (p 〈 0.01). In 6 of 9 ET pts on imetelstat the TLV were higher after 9 months. In addition, this difference in TL correlated significantly with the maximum reduction of the JAK2V617F mutational burden (p = 0.0137). Of interest, the 3 pts with lower or steady TLV after 9 months of treatment had the highest number of additional mutations before imetelstat treatment and despite a good suppression of the clone with the driver mutation these sub-clones were only partially responsive to imetelstat (i.e. 3 to 5 additional mutations in ASXL1, CBL, DNMT3A, EZH2, TET2, TP53, SF3B1 and U2AF1). Overall, pts with high level additional mutational burden had shorter TLV (age-adjusted mean difference from 50th percentile ±STD -3.6 ± 0.5 kb) at baseline than pts with no or low level additional mutational burden (age-adjusted mean difference from 50th percentile ±STD -2.9 ± 1 kb), and in both sets of pts mean TLV were higher after 9 months of imetelstat treatment. Conclusions: The lower TLV found in granulocytes of pts with MPN and especially with MF compared to healthy individuals reflect the higher mitotic history of malignant clones. In the ET imetelstat cohort, the very low TLV found at baseline might result from longer disease duration and resistance and/or intolerance to prior therapies. The higher TLV observed after 9 months of treatment with imetelstat and correlation with the reduction in the driver mutational burden suggests that imetelstat treatment in ET pts may suppress neoplastic clones and, in the absence of a high additional mutational burden, favor recovery of normal hematopoiesis. Disclosures Oppliger Leibundgut: Geron: Research Funding; Novartis: Research Funding. Burington:Geron Corporation: Employment. Ottmann:Fusion Pharma: Consultancy, Honoraria; Ariad: Consultancy, Honoraria; Pfizer: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Amgen: Consultancy, Honoraria. Spitzer:Trovagene Inc: Consultancy; Moldx Palmetto GBA: Consultancy; Incyte: Consultancy. Odenike:CTI/Baxter: Honoraria, Membership on an entity's Board of Directors or advisory committees; Spectrum Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; Algeta Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi-Aventis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; Suneisis Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; Geron: Research Funding. McDevitt:Salamandra Inc: Consultancy; Alexion: Membership on an entity's Board of Directors or advisory committees. Röth:Alexion Pharmaceuticals: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Geron: Research Funding. Snyder:Ariad: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees. Baerlocher:Geron: Research Funding; Janssen: Research Funding; Novartis: Research Funding.

Abstract: Abstract 893 Introduction: A European collaborative harmonization study involving 61 laboratories is being conducted under the auspices of the European Treatment and Outcome Study (EUTOS) for CML that aims to facilitate reporting of molecular BCR-ABL quantification results according to the International Scale (IS). The aim of this analysis was to investigate the effectiveness of this process and specifically the stability of conversion factors (CF) over time. Methods: The currently accepted way of adopting the IS is to establish and validate a laboratory-specific CF which is then used to convert local results to the IS. For round 1, preliminary CFs were calculated by centrally distributing standard samples containing 10–20 million WBC approximating to 10%, 1%, 0.1%, and 0.01% BCR-ABL IS. Rounds 2 and 3 were employed to refine the CF calculations using 25–30 CML patient samples from each participating laboratories covering a range of BCR-ABL levels between 0.01% and 10%. Log BCR-ABL values for the same samples were compared between reference and local laboratories applying the Bland-Altman bias plot. In order to judge the stability of each laboratory`s methodology, a CF index (ratio of round 3 CF divided by round 2 CF) was calculated and evaluated according to its capability to achieve optimum concordance of results. Results: Of the 61 laboratories participating in round 1, evaluable patient samples have been provided to date by 56 and 30 laboratories in rounds 2 and 3, respectively. Of the 30 laboratories with complete data, 12 had stable CFs (defined as a CF index within 0.75–1.33) whereas 18 laboratories were outside this range. Comparison of the CFs derived from round 2 with those derived from round 3 revealed better and more consistent concordance between laboratories with stable CFs compared to those with unstable CFs. For the 12 stable laboratories, 79% (round 3 CF) vs 79% (round 2 CF) of the samples were within a 2-fold range (0.5–2.0) and 93% vs 89% were within a 3-fold range (0.33–3.0). For the 18 unstable laboratories, 74% vs 55% of the samples were within a 2-fold range (0.5–2.0), p=0.0005 and 92% vs 77% were within a 3-fold range (0.33–3.0), p=0.0005. 2 of 12 laboratories with stable CFs and 8 of 18 laboratories with unstable CFs indicated changes in either one or more components of their procedures (cDNA synthesis, PCR platform, RQ-PCR protocol) that may have impacted on their CFs. Conclusion: These data indicate that CFs may be unstable in some laboratories even in the absence of significant changes to laboratory protocols. Further, it supports the need for continuous revalidation of CFs. In laboratories with unstable CFs we suggest revalidation within 3 to 6 months whereas those with stable CFs should be assessed on a yearly basis. We also suggest that laboratories with unstable CFs need to rigorously examine their internal processes to identify potential sources of variation. Disclosures: Müller: Novartis: Honoraria, Research Funding. Schnittger:MLL Munich Leukemia Laboratory: Employment, Equity Ownership.

Abstract: Practical guidance on diagnostic and prognostic use of next generation sequencing (NGS) in multiple myeloma (MM) and other plasma cell neoplasms (PCN) has yet to be developed. To this end, we analyzed the correlation of NGS data with the degree of bone marrow (BM) plasma cell (PC) involvement obtained by cytomorphology (CM), histopathology (HP), and multiparameter flow cytometry (MFC) from patients with PCN/MM. To perform this correlation, we analyzed 90 PCN cases, that included MM (n=77), MGUS (n=7), AL-amyloidosis (ALA) (n=4) and solitary plasmocytoma (SP) (n=2). The degree of BM infiltration (percent of PC) was defined by MFC as grade I & lt;1%, grade II 1-3%, grade III & gt;3%; by CM as grade I & lt;10%; grade II, 10-30%; grade III & gt;30% and by HP as grade I & lt;10%, grade II 10-30%, grade III & gt;30%. The newly designed NGS panel consisted of 15 genes including splice sites or hotspots: BRAF (exons 11, 15), CCND1, DIS3, EGR1, TENT5C (FAM46C), FGFR3, IDH1 (exon 4), IDH2 (exon 4), IRF4 (exon 3), KRAS (exons 2, 3), MYD88 (L265P), NRAS (exons 2, 3) PRDM1, TP53 and TRAF3. Genes and hotspots were chosen according to the frequency in the literature, prognostic impact, and possible function as therapeutic targets (Chapman et al., Nature 2011; Walker et al., JCO 2015). PC enrichment based on CD138+ magnetic cell sorting of marrow samples was successfully performed before the NGS procedure in all cases. In total, 102 mutations were detected by NGS in 64/90 (71%) cases analysed, 26 cases (29%) showed no mutations. In 41 cases (45%) one mutation/sample was detected and in 23 (26%) two to five mutations/sample. The proportion of cases affected by mutations was 59/77 patients (77%) in MM, 2/7 (29%) in MGUS and 1/4 (25%) in ALA. The most frequent mutations across all PCNs types were KRAS 22/90 (24%) and NRAS 14/90 (16%), followed by DIS3 and TENT5C in 11/90 cases each or 12% (Table 1). Mutational load correlated with degree of bone marrow infiltration: BM samples with no mutation had the lowest plasma cell infiltration: mean 1.6% by MFC (range, 0.003 - 11.7%), 20% by CM (1 - 90%), and 24% by HP (1 - 80%). Presence of at least one mutation/sample corresponded to a higher degree of BM involvement with a mean of 11% pathologic PC by MFC (range, 0.002 - 62%), and ~50% (3 - 100%) as defined by both CM and HP (Figure 1). The majority of samples without mutation (26 cases, 29%) were obtained from patients with initial BM evaluation for PCN suspicion (19/26 cases or 73% of this subgroup), whereas BM samples with & gt;1 mutation (23 cases, 26%) were mostly acquired from patients with relapsed/refractory disease (15/23 cases, or 65% of this subgroup). TP53 mutated cases had the highest percentage of PC infiltration by all methods and were mostly correlated to relapsed/refractory MM (rrMM). In conclusion, the probability to detect a mutation by NGS in the BM was highest in samples with & gt;10% clonal PC by flow cytometry, or & gt;20% PC by CM/HP. We propose further evaluation of these thresholds as a practical cut-off for processing of samples by NGS at initial PCN/MM diagnosis, whereas disease progression is commonly associated to higher rates of mutations. TP53 mutation was detected in all but one samples from rrMM and was associated with the highest degree of BM involvement. Further large-scale studies are warranted. Disclosures No relevant conflicts of interest to declare.

Abstract: The most frequent chromosomal aberrations in B-cell chronic lymphocytic leukemia (B-CLL) are deletions on 13q, 11q, and 17p, and trisomy 12, all of which are of prognostic significance. Conventional cytogenetic analysis and fluorescence in situ hybridization (FISH) are used for their detection, but cytogenetic analysis is hampered by the low mitotic index of B-CLL cells, and FISH depends on accurate information about candidate regions. We used a set of 400 highly informative microsatellite markers covering all chromosomal arms (allelotyping) and automated polymerase chain reaction (PCR) protocols to screen 46 patients with typical B-CLL for chromosomal aberrations. For validation, we compared data with our conventional karyotype results and fine mapping with conventional single-site PCR. All clonal cytogenetic abnormalities potentially detectable by our microsatellite PCR (eg, del13q14 and trisomy 12) were picked up. Allelotyping revealed additional complex aberrations in patients with both normal and abnormal B-CLL karyotypes. Aberrations detectable in the samples with our microsatellite panel were found on almost all chromosomal arms. We detected new aberrant loci in typical B-CLL, such as allelic losses on 1q, 9q, and 22q in up to 25% of our patients, and allelic imbalances mirroring chromosomal duplications, amplifications, or aneuploidies on 2q, 10p, and 22q in up to 27% of our patients. We conclude that allelotyping with our battery of informative microsatellites is suitable for molecular screening of B-CLL. The technique is well suited for analyses in clinical trials, it provides a comprehensive view of genetic alterations, and it may identify new loci with candidate genes relevant in the molecular biology of B-CLL.

Abstract: Background: Imetelstat, a first in class specific telomerase inhibitor, induced hematologic responses in all patients (pts) with essential thrombocythemia (ET) in a recent phase-2 study, and molecular responses were seen within 1-12 months in the majority of pts carrying JAK2 V617F (JAK2m) and CALR mutations (CALRm) (Baerlocher et al., ASH 2014). It has been reported that the treatment response to imetelstat in pts with myelofibrosis (Tefferi et al., ASH 2014) was negatively influenced by mutations in ASXL1 and favorably impacted by mutations in SF3B1 and U2AF1. In pts with CALRm ET treated with Interferon-alpha (INFa) the response rate was lower if the patient carried more than one mutation in ASXL1, TET2, IDH2, CSF3R and SH2B3 (Kiladjian et al., ASH 2014). In addition, in JAK2m pts with polycythemia vera, TET2-mutated clones have been demonstrated to be resistant to IFNa therapy (Kiladjian et al., Leukemia 2010). Aims: Our aim was to assess the dynamics of additional mutations besides JAK2 V617F, CALR and MPL mutations in pts with ET treated with imetelstat, and to investigate their association with hematologic and molecular response. Methods: The study enrolled 18 pts with ET who had failed or were intolerant to ≥1 prior therapy, or refused standard therapy. Pts were treated with imetelstat 7.5 mg/kg or 9.4 mg/kg IV weekly until attainment of platelet count ~250-300x109/L followed by a maintenance phase with dosing titrated according to platelet count. DNA was extracted from granulocytes or leukocytes. Mutation analysis was performed by high-throughput sequencing of selectively amplified target sequences on a PGM Ion Torrent instrument covering the coding and adjacent intronic sequences of 15 genes known for mutations in MPN (ASXL1, CBL, DNMT3A, EZH2, IDH1, IDH2, JAK2, MPL, SF3B1, SRSF2, SOCS1, TET2, TP53, U2AF1 and ZRSR2). Samples were taken at baseline and up to 8 time points during treatment through cycle 26, with approximately 3 cycles between samples. Results: As a driver mutation, at baseline, 9 pts had JAK2V617F, 5 pts had CALR and 2 pts had MPL mutations (MPLm; one L and one K). Two pts were triple negative. A partial molecular response (according to Barosi et al., Blood 2009) was seen in 7/8 JAK2m pts and reductions in CALRm allele burden were between 15% and 55%. At baseline, 19 additional somatic mutations (11 missense, 4 frameshift, 3 nonsense, 1 splice site) were detected in 6/9 JAK2m and 2/5 CALRm pts, affecting the genes ASXL1 (n=3), DNMT3A (n=5), EZH2(n=1), SF3B1 (n=1), SOCS1 (n=2), TET2 (n=4) and TP53 (n=3). Two mutations in DNMT3A and ZRSR2 were detected in 1/2 MPLm pts and none were found in the triple negative pts. Of note, all but one mutated pts carried at least 2 mutations in addition to their driver mutation (up to 5 additional mutations). ASXL1 and SOCS1 mutations were only present in JAK2m pts, and these pts reached hematologic and partial molecular response. At time of best molecular response, a reduction of mutant allele burden corresponding to the reduction of the driver mutations was observed for mutations in ASXL1, EZH2, SOCS1 and some DNMT3A, TET2 and TP53 mutations, but not for SF3B1 and ZRSR2 mutations. Sequential analysis in a JAK2m patient with 4 additional mutations showed that all 4 mutated clones were sensitive to imetelstat treatment and followed the dynamics of the JAK2 mutation, and in a patient with 5 mutations in addition to the CALR mutation, 3/5 mutated clones were responsive. Three pts with a weaker molecular response had TP53 mutations which persisted over time, and 2 were accompanied by additional mutations. Conclusions: 9/18 (50%) pts in this study carried no additional mutations at baseline, and 50% carried 1-5 mutations in addition to the driver mutation, suggesting genetic instability in a subset of pts. Genetic instability might be enhanced in this pretreated patient cohort with a median time since diagnosis of 7.2 years (range 0.3-24.9). Clones with ASXL1 mutations, a known poor prognostic marker in MPN, appear to be sensitive to imetelstat treatment, and pts with 2-5 additional mutations had both hematologic and molecular responses. TP53 mutations were an exception, being associated with weaker molecular responses to imetelstat treatment. Additional analyses of associations between mutations, disease characteristics and response will be presented. . Disclosures Oppliger Leibundgut: Novartis: Research Funding; Geron: Research Funding. Burington:Geron Corporation: Employment. Ottmann:Astra Zeneca: Honoraria, Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees; Ariad: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Spitzer:Moldx Palmetto GBA: Consultancy; Incyte: Consultancy; Trovagene Inc: Consultancy. Odenike:Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; Suneisis Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi-Aventis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Algeta Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; Spectrum Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees; CTI/Baxter: Honoraria, Membership on an entity's Board of Directors or advisory committees; Geron: Research Funding. McDevitt:Salamandra Inc: Consultancy; Alexion: Membership on an entity's Board of Directors or advisory committees. Roeth:Alexion Pharmaceuticals: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Geron: Research Funding. Snyder:Ariad: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees. Baerlocher:Geron: Research Funding; Janssen: Research Funding; Novartis: Research Funding.

Abstract: BACKGROUND: Previous observations suggested recruitment of platelets (PLTs) and white blood cells (WBCs) during plateletpheresis and recruitment of hematopoietic progenitor cells (HPCs) by HPC apheresis. Quantification of recruitment helps to optimize yields and safety of these procedures; detection of WBC or HPC recruitment during plateletpheresis may further elucidate the mechanisms. STUDY DESIGN AND METHODS: This study was a prospective cohort study with 68 single‐needle plateletpheresis donations (23 double, PLT yield ≥4.8 × 10 11 ; 23 triple, ≥7.2 × 10 11 ; 22 quadruple, ≥9.6 × 10 11 ). We measured PLTs, WBCs, WBC subpopulations, and circulating HPCs ( CD34 mRNA) before, during, and after apheresis; calculated PLT recruitment and ratio of recruited to yielded PLT; and propose a novel concept to optimize the prediction of the PLT counts after apheresis (PLT post ). RESULTS: PLT recruitment (mean ± SD, 1.56 ± 0.31) caused a higher PLT post than predicted by the instrument (164 × 10 9  ± 23 × 10 9 vs. 111 × 10 9  ± 31 × 10 9 /L; p  〈  0.0001) in all three groups. The ratio of recruited to yielded PLT was 0.36 ± 0.15. The WBC count decreased by 9% to the time before rinse‐back and returned to the baseline thereafter. No changes in circulating HPCs occurred. CONCLUSIONS: PLT recruitment during high‐yield plateletpheresis facilitates the harvest of multiple PLT units in a single donation. The use of the ratio of recruited to yielded PLT may optimize the algorithm to predict PLT post . There was no recruitment of WBCs and HPCs during plateletpheresis. Therefore, the previously observed recruitment of HPCs during HPC apheresis seems to be related to other factors than the apheresis procedure itself.

Abstract: Group 2 innate lymphoid cells (ILC2s) are involved in human diseases, such as allergy, atopic dermatitis and nasal polyposis, but their function in human cancer remains unclear. Here we show that, in acute promyelocytic leukaemia (APL), ILC2s are increased and hyper-activated through the interaction of CRTH2 and NKp30 with elevated tumour-derived PGD2 and B7H6, respectively. ILC2s, in turn, activate monocytic myeloid-derived suppressor cells (M-MDSCs) via IL-13 secretion. Upon treating APL with all-trans retinoic acid and achieving complete remission, the levels of PGD2, NKp30, ILC2s, IL-13 and M-MDSCs are restored. Similarly, disruption of this tumour immunosuppressive axis by specifically blocking PGD2, IL-13 and NKp30 partially restores ILC2 and M-MDSC levels and results in increased survival. Thus, using APL as a model, we uncover a tolerogenic pathway that may represent a relevant immunosuppressive, therapeutic targetable, mechanism operating in various human tumour types, as supported by our observations in prostate cancer.