Author Correction: Epoxyeicosatrienoic acids enhance embryonic haematopoiesis and adult marrow engraftment.

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

CLEC12A and CD33 coexpression as a preferential target for pediatric AML combinatorial immunotherapy.

Emerging immunotherapies such as chimeric antigen receptor T cells have advanced the treatment of acute lymphoblastic leukemia. In contrast, long-term control of acute myeloid leukemia (AML) cannot be achieved by single lineage-specific targeting while sparing benign hematopoiesis. In addition, heterogeneity of AML warrants combinatorial targeting, and several suitable immunotargets (HAVCR2/CD33 and HAVCR2/CLEC12A) have been identified in adult AML. However, clinical and biologic characteristics of AML differ between children and the elderly. Here, we analyzed 36 bone marrow (BM) samples of pediatric AML patients and 13 age-matched healthy donors using whole RNA sequencing of sorted CD45dim and CD34+CD38-CD45dim BM populations and flow cytometry for surface expression of putative target antigens. Pediatric AML clusters apart from healthy myeloid BM precursors in principal-component analysis. Known immunotargets of adult AML, such as IL3RA, were not overexpressed in pediatric AML compared with healthy precursors by RNA sequencing. CD33 and CLEC12A were the most upregulated immunotargets on the RNA level and showed the highest surface expression on AML detected by flow cytometry. KMT2A-mutated infant AML clusters separately by RNA sequencing and overexpresses FLT3, and hence, CD33/FLT3 cotargeting is an additional specific option for this subgroup. CLEC12A and CD33/CLEC12Adouble-positive expression was absent in CD34+CD38-CD45RA-CD90+ hematopoietic stem cells (HSCs) and nonhematopoietic tissue, while CD33 and FLT3 are expressed on HSCs. In summary, we show that expression of immunotargets in pediatric AML differs from known expression profiles in adult AML. We identify CLEC12A and CD33 as preferential generic combinatorial immunotargets in pediatric AML and CD33 and FLT3 as immunotargets specific for KMT2A-mutated infant AML.

Epoxyeicosatrienoic acids enhance embryonic haematopoiesis and adult marrow engraftment.

Haematopoietic stem and progenitor cell (HSPC) transplant is a widely used treatment for life-threatening conditions such as leukaemia; however, the molecular mechanisms regulating HSPC engraftment of the recipient niche remain incompletely understood. Here we develop a competitive HSPC transplant method in adult zebrafish, using in vivo imaging as a non-invasive readout. We use this system to conduct a chemical screen, and identify epoxyeicosatrienoic acids (EETs) as a family of lipids that enhance HSPC engraftment. The pro-haematopoietic effects of EETs were conserved in the developing zebrafish embryo, where 11,12-EET promoted HSPC specification by activating a unique activator protein 1 (AP-1) and runx1 transcription program autonomous to the haemogenic endothelium. This effect required the activation of the phosphatidylinositol-3-OH kinase (PI(3)K) pathway, specifically PI(3)Kγ. In adult HSPCs, 11,12-EET induced transcriptional programs, including AP-1 activation, which modulate several cellular processes, such as migration, to promote engraftment. Furthermore, we demonstrate that the EET effects on enhancing HSPC homing and engraftment are conserved in mammals. Our study establishes a new method to explore the molecular mechanisms of HSPC engraftment, and discovers a previously unrecognized, evolutionarily conserved pathway regulating multiple haematopoietic generation and regeneration processes. EETs may have clinical application in marrow or cord blood transplantation.

Efficacy, safety and feasibility of fosaprepitant for the prevention of chemotherapy-induced nausea and vomiting in pediatric patients receiving moderately and highly emetogenic chemotherapy - results of a non-interventional observation study.

BACKGROUND: Chemotherapy-induced nausea and vomiting (CINV) belong among the most burdensome side effects in hemato-oncology. Mostly, a combination of ondansetron and dexamethasone is used as antiemetic prophylaxis in pediatric patients undergoing emetogenic chemotherapy. However, dexamethasone is prohibited in different pediatric chemotherapy protocols. Currently, data on the use of ondansetron with the new antiemetic agent fosaprepitant without dexamethasone is not available for pediatric patients. METHODS: In this non-interventional observation study, 79 pediatric patients with a median age of 8.0 years (range 0.5-17.9 years) who received a CINV prophylaxis regimen with either fosaprepitant (4 mg/kg; maximum 150 mg) and ondansetron (as 24-h continuous infusion) (n = 40; fosaprepitant group/FG) or ondansetron only (n = 39; control group/CG) during moderately or highly emetogenic chemotherapy were analyzed. The groups were analyzed and compared for frequency of vomiting, administered doses of on-demand antiemetic dimenhydrinate and adverse events during the acute (0-24 h after chemotherapy administration) and delayed (> 24 h-120 h) CINV phases. RESULTS: A total of 112 and 116 chemotherapy blocks were analyzed in the fosaprepitant and the control group, respectively. The emetogenic potential of the administered chemotherapy did not significantly differ (p = 0.8812) between the two cohorts. In the acute CINV phase, the percentage of patients experiencing vomiting (n = 26 patients) and the vomiting events were significantly higher (p = 0.0005 and p < 0.0001, respectively) in the CG (n = 26 patients (66.7%); 88 events) compared with the FG (n = 10 patients (25.0%); 37 events). In the delayed CINV phase, the percentage of patients experiencing vomiting and the vomiting events were also significantly higher (p = 0.0017 and p < 0.0001, respectively) in the CG (n = 31 patients (79.5%); 164 events) compared with the FG (n = 17 patients (42.5%); 103 events). Additionally, significantly more dimenhydrinate doses were administered in the CG compared with the FG patients (n = 322/n = 198; p < 0.0001). The occurrence of adverse events did not significantly differ between the two groups (p > 0.05). CONCLUSION: Fosaprepitant (4.0 mg/kg) in addition to ondansetron, without application of dexamethasone, was well tolerated, safe, effective and superior to ondansetron only as CINV prophylaxis in pediatric patients during moderately and highly emetogenic chemotherapy.

A new workflow for whole-genome sequencing of single human cells.

Unbiased amplification of the whole-genome amplification (WGA) of single cells is crucial to study cancer evolution and genetic heterogeneity, but is challenging due to the high complexity of the human genome. Here, we present a new workflow combining an efficient adapter-linker PCR-based WGA method with second-generation sequencing. This approach allows comparison of single cells at base pair resolution. Amplification recovered up to 74% of the human genome. Copy-number variants and loss of heterozygosity detected in single cell genomes showed concordance of up to 99% to pooled genomic DNA. Allele frequencies of mutations could be determined accurately due to an allele dropout rate of only 2%, clearly demonstrating the low bias of our PCR-based WGA approach. Sequencing with paired-end reads allowed genome-wide analysis of structural variants. By direct comparison to other WGA methods, we further endorse its suitability to analyze genetic heterogeneity.

Next-generation-sequencing-based risk stratification and identification of new genes involved in structural and sequence variations in near haploid lymphoblastic leukemia.

Near haploidy (23-29 chromosomes) is a numerical cytogenetic aberration in childhood acute lymphoblastic leukemia (ALL) associated with particularly poor outcome. In contrast, high hyperdiploidy (51-67 chromosomes) has a favorable prognosis. Correct classification and appropriate risk stratification of near haploidy is frequently hampered by the presence of apparently high hyperdiploid clones that arise by endoreduplication of the original near haploid clone. We evaluated next-generation-sequencing (NGS) to distinguish between "high hyperdiploid" leukemic clones of near haploid and true high hyperdiploid origin. Five high hyperdiploid ALL cases and the "high hyperdiploid" cell line MHH-CALL-2, derived from a near haploid clone, were tested for uniparental isodisomy. NGS showed that all disomic chromosomes of MHH-CALL-2, but none of the patients, were of uniparental origin, thus reliably discriminating these subtypes. Whole-exome- and whole-genome-sequencing of MHH-CALL-2 revealed homozygous non-synonymous coding mutations predicted to be deleterious for the protein function of 63 genes, among them known cancer-associated genes, such as FANCA, NF1, TCF7L2, CARD11, EP400, histone demethylases, and transferases (KDM6B, KDM1A, PRDM11). Only eight of these were also, but heterozygously, mutated in the high hyperdiploid patients. Structural variations in MHH-CALL-2 include a homozygous deletion (MTAP/CDKN2A/CDKN2B/ANRIL), a homozygous inversion (NCKAP5), and an unbalanced translocation (FAM189A1). Together, the sequence variations provide MHH-CALL-2 with capabilities typically acquired during cancer development, e.g., loss of cell cycle control, enhanced proliferation, lack of DNA repair, cell death evasion, and disturbance of epigenetic gene regulation. Poorer prognosis of near haploid ALL most likely results from full penetrance of a large array of detrimental homozygous mutations.

Human-Level Differentiation of Medulloblastoma from Pilocytic Astrocytoma: A Real-World Multicenter Pilot Study.

Medulloblastoma and pilocytic astrocytoma are the two most common pediatric brain tumors with overlapping imaging features. In this proof-of-concept study, we investigated using a deep learning classifier trained on a multicenter data set to differentiate these tumor types. We developed a patch-based 3D-DenseNet classifier, utilizing automated tumor segmentation. Given the heterogeneity of imaging data (and available sequences), we used all individually available preoperative imaging sequences to make the model robust to varying input. We compared the classifier to diagnostic assessments by five readers with varying experience in pediatric brain tumors. Overall, we included 195 preoperative MRIs from children with medulloblastoma (n = 69) or pilocytic astrocytoma (n = 126) across six university hospitals. In the 64-patient test set, the DenseNet classifier achieved a high AUC of 0.986, correctly predicting 62/64 (97%) diagnoses. It misclassified one case of each tumor type. Human reader accuracy ranged from 100% (expert neuroradiologist) to 80% (resident). The classifier performed significantly better than relatively inexperienced readers (p < 0.05) and was on par with pediatric neuro-oncology experts. Our proof-of-concept study demonstrates a deep learning model based on automated tumor segmentation that can reliably preoperatively differentiate between medulloblastoma and pilocytic astrocytoma, even in heterogeneous data.

Microenvironmental control of hematopoietic stem cell fate via CXCL8 and protein kinase C.

Altered hematopoietic stem cell (HSC) fate underlies primary blood disorders but microenvironmental factors controlling this are poorly understood. Genetically barcoded genome editing of synthetic target arrays for lineage tracing (GESTALT) zebrafish were used to screen for factors expressed by the sinusoidal vascular niche that alter the phylogenetic distribution of the HSC pool under native conditions. Dysregulated expression of protein kinase C delta (PKC-δ, encoded by prkcda) increases the number of HSC clones by up to 80% and expands polyclonal populations of immature neutrophil and erythroid precursors. PKC agonists such as cxcl8 augment HSC competition for residency within the niche and expand defined niche populations. CXCL8 induces association of PKC-δ with the focal adhesion complex, activating extracellular signal-regulated kinase (ERK) signaling and expression of niche factors in human endothelial cells. Our findings demonstrate the existence of reserve capacity within the niche that is controlled by CXCL8 and PKC and has significant impact on HSC phylogenetic and phenotypic fate.

Fatal outcome despite full lympho-hematopoietic reconstitution after allogeneic stem cell transplantation in atypical ataxia telangiectasia.

Allogeneic hematopoietic stem cell transplantation (HSCT) has not been a therapeutic option in ataxia telangiectasia (AT) due to overwhelming toxicity of conditioning in the context of the global DNA repair deficiency. Furthermore HSCT is unable to cure neurological involvement of AT. We report on a Turkish child with a Hyper IgM phenotype disorder, in which clinical aspects of AT were absent and thus, AT not diagnosed. He was transplanted with a reduced toxicity, but full intensity conditioning regimen comprising treosulfan, fludarabine and ATG. The peritransplant period was uneventful and the patient was discharged at day +57. 8 months after HSCT, the patient developed hepatopathy with monoclonal gammopathy of unclear significance and died due to hepatic failure and encephalopathy at the age of 32 months. Post mortem high throughput sequencing revealed a mutation in the ATM gene.

CD34+ gene expression profiling of individual children with very severe aplastic anemia indicates a pathogenic role of integrin receptors and the proapoptotic death ligand TRAIL.

UNLABELLED: BACKGROUND Very severe aplastic anemia is characterized by a hypoplastic bone marrow due to destruction of CD34(+) stem cells by autoreactive T cells. Investigation of the pathomechanism by patient-specific gene expression analysis of the attacked stem cells has previously been impractical because of the scarcity of these cells at diagnosis. DESIGN AND METHODS: Employing unbiased RNA amplification, patient-specific gene expression profiling was carried out for CD34(+) cells from patients newly diagnosed with very severe aplastic anemia (n=13), refractory anemia (n=8) and healthy controls (n=10). These data were compared to profiles of myelodysplastic disease (n=55), including refractory anemia (n=18). To identify possible targets of autoimmune attack, presence of autoreactive antibodies was tested in pre-therapeutic sera of patients with very severe aplastic anemia (n=19). RESULTS: CD34(+) gene expression profiling distinguished between healthy controls, children with aplastic or refractory anemia and clonal disease. Interferon stimulated genes such as the apoptosis inducing death ligand TRAIL were strongly up-regulated in CD34(+) cells of patients with aplastic anemia, in particular in patients responding to immunosuppressive treatment. In contrast, mRNA expression of integrin GPVI and the integrin complexes GPIa/IIa, GPIIb/IIIa, GPIB/GPIX/GPV was significantly down-regulated and corresponding antibodies were detected in 7 of 11 profiled patients and in 11 of 19 aplastic anemia patients. CONCLUSIONS As a potential diagnostic tool, patient-specific gene expression profiling of CD34(+) stem cells made it possible to make the difficult differential diagnosis of most patients with aplastic and refractory anemia. Profiling indicated a prognostic correlation of TRAIL expression and patient benefit from immunosuppressive therapy. Downregulation of integrin expression and concurrent presence of autoreactive anti-integrin-antibodies suggested a previously unrecognized pathological role of integrins in aplastic anemia.

A novel mtDNA large-scale mutation clinically exclusively presenting with refractory anemia: is there a chance to predict disease progression?

Because of the diversity of clinical symptoms, the diagnosis of mitochondrial DNA (mtDNA) deletion disorders can be difficult. Here, we describe an 8-month-old boy presenting clinically exclusively with refractory anemia. Mutation analysis in our patient revealed a large, novel deletion in his mtDNA encompassing ATPase 6, cytochrome oxidase subunit III, NADH dehydrogenase genes ND3 to ND6, and cytochrome b. Comparison with other cases from the literature showed that there is no genotype-phenotype correlation regarding hematologic features. It is not possible to predict whether our patient will develop additional features from Pearson syndrome or Kearns-Sayre syndrome, both syndromic mitochondrial disorders with hematological manifestations.

In vivo inducible reverse genetics in patients' tumors to identify individual therapeutic targets.

High-throughput sequencing describes multiple alterations in individual tumors, but their functional relevance is often unclear. Clinic-close, individualized molecular model systems are required for functional validation and to identify therapeutic targets of high significance for each patient. Here, we establish a Cre-ERT2-loxP (causes recombination, estrogen receptor mutant T2, locus of X-over P1) based inducible RNAi- (ribonucleic acid interference) mediated gene silencing system in patient-derived xenograft (PDX) models of acute leukemias in vivo. Mimicking anti-cancer therapy in patients, gene inhibition is initiated in mice harboring orthotopic tumors. In fluorochrome guided, competitive in vivo trials, silencing of the apoptosis regulator MCL1 (myeloid cell leukemia sequence 1) correlates to pharmacological MCL1 inhibition in patients´ tumors, demonstrating the ability of the method to detect therapeutic vulnerabilities. The technique identifies a major tumor-maintaining potency of the MLL-AF4 (mixed lineage leukemia, ALL1-fused gene from chromosome 4) fusion, restricted to samples carrying the translocation. DUX4 (double homeobox 4) plays an essential role in patients' leukemias carrying the recently described DUX4-IGH (immunoglobulin heavy chain) translocation, while the downstream mediator DDIT4L (DNA-damage-inducible transcript 4 like) is identified as therapeutic vulnerability. By individualizing functional genomics in established tumors in vivo, our technique decisively complements the value chain of precision oncology. Being broadly applicable to tumors of all kinds, it will considerably reinforce personalizing anti-cancer treatment in the future.

Leukemia escape in immune desert: intraocular relapse of pediatric pro-B-ALL during systemic control by CD19-CAR T cells.

BACKGROUND: Relapsed/refractory B-precursor acute lymphoblastic leukemia (BCP-ALL) remains a major therapeutic challenge in pediatric hematology. Chimeric antigen receptor (CAR) T cells targeting CD19 have shown remarkable initial response rates in BCP-ALL patients, while long-term leukemia control rate is only about 50%. So far, main mechanisms of BCP-ALL relapse after CD19-CAR T-cell therapy have been either insufficient CAR T-cell persistence in vivo or loss of surface CD19. CASE REPORT: Here, we report an exceptional presentation of BCP-ALL relapse in the eye during the systemic control through CAR T-cell therapy. We report a case of fatal intraocular relapse in a pediatric patient with pro-B-ALL after initial response to CD19-CAR T-cell therapy. One month after CD19-CAR T-cell therapy, remission was documented by bone marrow aspirate analysis with absence of CD19+ cells and CD19-CAR T cells could be detected in both peripheral blood and bone marrow. At the same time, however, the patient presented with progressive visual disturbance and CD19+ cells were found within the anterior chamber of the eye. Despite local and systemic therapy, ocular relapse led to BCP-ALL dissemination and systemic relapse within weeks. The eye represents a rare site for local manifestation of BCP-ALL, but isolated intraocular relapse is a clinically unreckoned presentation of BCP-ALL in the era of CD19-CAR T cells. CONCLUSION: During systemic control of BCP-ALL through CD19-CAR T cells, relapse can emerge in the eye as an immune-privileged organ. Ocular symptoms after CD19-CAR T-cell therapy should guide the clinician to elucidate the etiology in a timely fashion in order to adjust leukemia treatment strategy. Both, local immune escape as well as insufficient CAR T-cell persistence may have contributed to relapse in the reported patient. Mechanisms of relapse in an immune desert under CAR T-cell therapy require future clinical and experimental attention. In particular, ocular symptoms after CAR T-cell therapy should be considered a potentially early sign of leukemia relapse.

Antiemetic Prophylaxis with Fosaprepitant and 5-HT3-Receptor Antagonists in Pediatric Patients Undergoing Autologous Hematopoietic Stem Cell Transplantation.

BACKGROUND: High-dose myeloablative conditioning prior to autologous hematopoietic stem cell transplantation (autoHSCT) in pediatric patients is usually highly emetogenic. The antiemetic neurokinin-1 receptor antagonist fosaprepitant was safe and effective in children receiving highly emetogenic chemotherapy. Data on fosaprepitant during autoHSCT in children are currently not available. METHODS: A total of 35 consecutive pediatric patients, who received an antiemetic prophylaxis with fosaprepitant (4 mg/kg; single dose, max. 1 x 150 mg/kg BW) and ondansetron (24-hours continuous infusion; 8-32 mg/24h) or granisetron (2 x 40 µg/kg∙d-1) during highly emetogenic conditioning chemotherapy before autoHSCT were retrospectively analyzed, and their results were compared with a control group comprising 35 consecutive pediatric patients, who received granisetron or ondansetron only. The antiemetic efficacy and the safety of the two prophylaxis regimens were compared with respect to three time periods after the first chemotherapy administration (0-24h, >24-120h, >120-240h). RESULTS: Clinical adverse events and clinically relevant increases/decreases of laboratory markers were similarly low and did not significantly differ between the two study groups (p>0.05). The registered number of vomiting events was significantly higher in the control group in the time periods of 0-24h (64 vs 22 events; p<0.01), >24-120h (135 vs 78 events; p<0.0001), >120-240h (268 vs 105 events; p<0.0001), and the whole observation period 0-240h (467 vs 205 events; p<0.0001). The percentage of patients experiencing vomiting was higher in the control group during the time period of >24-120h (100% vs 74.3%) but not the other analyzed time periods (p>0.05). CONCLUSION: The fosaprepitant-based antiemetic prophylaxis was safe, well tolerated and significantly reduced vomiting in children undergoing highly emetogenic chemotherapy prior to autoHSCT. Prospective randomized trials are necessary to confirm these results.

Leukemia-induced dysfunctional TIM-3+CD4+ bone marrow T cells increase risk of relapse in pediatric B-precursor ALL patients.

Interaction of malignancies with tissue-specific immune cells has gained interest for prognosis and intervention of emerging immunotherapies. We analyzed bone marrow T cells (bmT) as tumor-infiltrating lymphocytes in pediatric precursor-B cell acute lymphoblastic leukemia (ALL). Based on data from 100 patients, we show that ALL is associated with late-stage CD4+ phenotype and loss of early CD8+ T cells. The inhibitory exhaustion marker TIM-3 on CD4+ bmT increased relapse risk (RFS = 94.6/70.3%) confirmed by multivariate analysis. The hazard ratio of TIM-3 expression nearly reached the hazard ratio of MRD (7.1 vs. 8.0) indicating that patients with a high frequency of TIM-3+CD4+ bone marrow T cells at initial diagnosis have a 7.1-fold increased risk to develop ALL relapse. Comparison of wild type primary T cells to CRISPR/Cas9-mediated TIM-3 knockout and TIM-3 overexpression confirmed the negative effect of TIM-3 on T cell responses against ALL. TIM-3+CD4+ bmT are increased in ALL overexpressing CD200, that leads to dysfunctional antileukemic T cell responses. In conclusion, TIM-3-mediated interaction between bmT and leukemia cells is shown as a strong risk factor for relapse in pediatric B-lineage ALL. CD200/TIM-3-signaling, rather than PD-1/PD-L1, is uncovered as a mechanism of T cell dysfunction in ALL with major implication for future immunotherapies.

Mutations of JAK2 in acute lymphoblastic leukaemias associated with Down's syndrome.

BACKGROUND: Children with Down's syndrome have a greatly increased risk of acute megakaryoblastic and acute lymphoblastic leukaemias. Acute megakaryoblastic leukaemia in Down's syndrome is characterised by a somatic mutation in GATA1. Constitutive activation of the JAK/STAT (Janus kinase and signal transducer and activator of transcription) pathway occurs in several haematopoietic malignant diseases. We tested the hypothesis that mutations in JAK2 might be a common molecular event in acute lymphoblastic leukaemia associated with Down's syndrome. METHODS: JAK2 DNA mutational analysis was done on diagnostic bone marrow samples obtained from 88 patients with Down's syndrome-associated acute lymphoblastic leukaemia; and 216 patients with sporadic acute lymphoblastic leukaemia, Down's syndrome-associated acute megakaryoblastic leukaemia, and essential thrombocythaemia. Functional consequences of identified mutations were studied in mouse haematopoietic progenitor cells. FINDINGS: Somatically acquired JAK2 mutations were identified in 16 (18%) patients with Down's syndrome-associated acute lymphoblastic leukaemia. The only patient with non-Down's syndrome-associated leukaemia but with a JAK2 mutation had an isochromosome 21q. Children with a JAK2 mutation were younger (mean [SE] age 4.5 years [0.86] vs 8.6 years [0.59], p<0.0001) at diagnosis. Five mutant alleles were identified, each affecting a highly conserved arginine residue (R683). These mutations immortalised primary mouse haematopoietic progenitor cells in vitro, and caused constitutive Jak/Stat activation and cytokine-independent growth of BaF3 cells, which was sensitive to pharmacological inhibition with JAK inhibitor I. In modelling studies of the JAK2 pseudokinase domain, R683 was situated in an exposed conserved region separated from the one implicated in myeloproliferative disorders. INTERPRETATION: A specific genotype-phenotype association exists between the type of somatic mutation within the JAK2 pseudokinase domain and the development of B-lymphoid or myeloid neoplasms. Somatically acquired R683 JAK2 mutations define a distinct acute lymphoblastic leukaemia subgroup that is uniquely associated with trisomy 21. JAK2 inhibitors could be useful for treatment of this leukaemia. FUNDING: Israel Trade Ministry, Israel Science Ministry, Jewish National Fund UK, Sam Waxman Cancer Research Foundation, Israel Science Foundation, Israel Cancer Association, Curtis Katz, Constantiner Institute for Molecular Genetics, German-Israel Foundation, and European Commission FP6 Integrated Project EUROHEAR.

The genotype of the original Wiskott phenotype.

The Wiskott-Aldrich syndrome is an X-linked hereditary disorder associated with combined immunodeficiency, thrombocytopenia, small platelets, eczema, and increased susceptibility to autoimmune disorders and cancers. It is caused by mutations in the gene (WAS) for the Wiskott-Aldrich syndrome protein (WASP). We investigated family members of the patients originally described by Wiskott in 1937 and identified a new frame shift mutation in exon 1 of WAS. This mutation is likely to be the hypothesized genotype that caused the severe form of the Wiskott-Aldrich syndrome in the three brothers described by Wiskott.

CXCR1 remodels the vascular niche to promote hematopoietic stem and progenitor cell engraftment.

The microenvironment is an important regulator of hematopoietic stem and progenitor cell (HSPC) biology. Recent advances marking fluorescent HSPCs have allowed exquisite visualization of HSPCs in the caudal hematopoietic tissue (CHT) of the developing zebrafish. Here, we show that the chemokine cxcl8 and its receptor, cxcr1, are expressed by zebrafish endothelial cells, and we identify cxcl8/cxcr1 signaling as a positive regulator of HSPC colonization. Single-cell tracking experiments demonstrated that this is a result of increases in HSPC-endothelial cell "cuddling," HSPC residency time within the CHT, and HSPC mitotic rate. Enhanced cxcl8/cxcr1 signaling was associated with an increase in the volume of the CHT and induction of cxcl12a expression. Finally, using parabiotic zebrafish, we show that cxcr1 acts HSPC nonautonomously to improve the efficiency of donor HSPC engraftment. This work identifies a mechanism by which the hematopoietic niche remodels to promote HSPC engraftment and suggests that cxcl8/cxcr1 signaling is a potential therapeutic target in patients undergoing hematopoietic stem cell transplantation.