Summary: A population of mesenchymal stem cells, termed CXC chemokine ligand (CXCL)12‐abundant reticular (CAR) cells or leptin receptor‐expressing cells, are the major cellular component of niches for haematopoietic stem cells (HSCs) in murine bone marrow. CAR cells are characterized by several salient features, including much higher expression of CXCL12, stem cell factor (SCF), forkhead box C1 (FOXC1) and early B‐cell factor 3 (EBF3), which are essential for HSC maintenance, than other cells. However, the human counterpart of CAR cells has not been fully described. Here, we show the presence of cells expressing much higher CXCL12 than other cells in human adult bone marrow using a flow cytometry‐based in situ technique that enables high‐throughput detection of mRNA at single‐cell resolution. Most CXCL12^hi cells expressed high levels of SCF, FOXC1 and EBF3 and had the potential to differentiate into adipocytes and osteoblasts. Histologically, the nuclei of CXCL12^hi cells were identified and quantified by EBF3 expression in fixed marrow sections. CXCL12^hi cells sorted from residual bone marrow aspirates of chronic myeloid leukaemia patients expressed reduced levels of CXCL12, SCF, FOXC1 and EBF3 in correlation with increased leukaemic burden. Together, we identified the human counterpart of CAR cells, enabling the evaluation of their alterations in various haematological disorders by flow cytometric and histological analyses.

The family of signal-transducing adapter proteins (STAPs) has been reported to be involved in a variety of intracellular signaling pathways and implicated as transcriptional factors. We previously cloned STAP-2 as a c-Fms interacting protein and explored its effects on chronic myeloid leukemia (CML) leukemogenesis. STAP-2 binds to BCR-ABL, upregulates BCR-ABL phosphorylation, and activates its downstream molecules. In this study, we evaluated the role of STAP-1, another member of the STAP family, in CML pathogenesis. We found that the expression of STAP-1 is aberrantly upregulated in CML stem cells (LSCs) in patients’ bone marrow. Using experimental model mice, deletion of STAP-1 prolonged the survival of CML mice with inducing apoptosis of LSCs. The impaired phosphorylation status of STAT5 by STAP-1 ablation leads to downregulation of antiapoptotic genes, Bcl-2 and Bcl-xL. Interestingly, transcriptome analyses indicated that STAP-1 affects several signaling pathways related to BCR-ABL, JAK2, and PPARγ. This adapter protein directly binds to not only BCR-ABL, but also STAT5 proteins, showing synergistic effects of STAP-1 inhibition and BCR-ABL or JAK2 tyrosine kinase inhibition. Our results identified STAP-1 as a regulator of CML LSCs and suggested it to be a potential therapeutic target for CML.

Summary: Zandelisib, a selective, potent PI3Kδ inhibitor, demonstrated favourable outcomes in patients with relapsed or refractory follicular lymphoma in a global phase II study. This phase II study evaluated the efficacy and safety of zandelisib for relapsed or refractory follicular lymphoma or marginal zone lymphoma. Sixty‐one patients received zandelisib orally at 60 mg daily continuously in the first two 28‐day cycles, followed by intermittent dosing on Days 1–7 following each cycle until progressive disease or unacceptable toxicity. Objective and complete response rates were 75.4% (95% confidence interval [CI], 62.7%–85.5%) and 24.6% (95% CI, 14.5%–37.3%) respectively. Median time to response was 58 days; 70.5% (43/61) of patients achieved their first response by Week 8. At least one Grade ≥ 3 treatment‐emergent adverse event (TEAE) occurred in 55.7% of patients: transaminase elevation (8.2%); cutaneous reactions (3.3%); and diarrhoea, enterocolitis and lung infection (1.6% each), defined as adverse events of special interest. The discontinuation rate due to any TEAE was 14.8%. No zandelisib‐related death occurred. Zandelisib showed favourable efficacy and tolerability in Japanese patients with relapsed or refractory indolent non‐Hodgkin B‐cell lymphoma. This unique dosing schedule may maintain efficacy while mitigating the safety issues observed with other PI3Kδ inhibitors (ClinicalTrials.gov number, NCT04533581).