• We have developed a biomimetic microfluidic platelet bioreactor that recapitulates bone marrow and blood vessel microenvironments.• Application of shear stress in this bioreactor triggers physiological proplatelet production, and platelet release.Platelet transfusions total >2.17 million apheresis-equivalent units per year in the United States and are derived entirely from human donors, despite clinically significant immunogenicity, associated risk of sepsis, and inventory shortages due to high demand and 5-day shelf life. To take advantage of known physiological drivers of thrombopoiesis, we have developed a microfluidic human platelet bioreactor that recapitulates bone marrow stiffness, extracellular matrix composition, micro-channel size, hemodynamic vascular shear stress, and endothelial cell contacts, and it supports high-resolution live-cell microscopy and quantification of platelet production. Physiological shear stresses triggered proplatelet initiation, reproduced ex vivo bone marrow proplatelet production, and generated functional platelets. Modeling human bone marrow composition and hemodynamics in vitro obviates risks associated with platelet procurement and storage to help meet growing transfusion needs. (Blood. 2014;124(12):1857-1867) IntroductionAlthough platelets (PLTs) play critical roles in hemostasis, 1 angiogenesis, 2 and innate immunity, 3 PLT production remains poorly understood. Consequently, PLT units are derived entirely from human donors, despite serious clinical concerns owing to their immunogenicity and associated risk of sepsis. 4 More than 2.17 million apheresisequivalent PLT units are transfused yearly in the United States 5,6 at a cost of .$1 billion per year. Although demand for PLT transfusions has increased markedly in the past decade, a near-static pool of donors and a 5-day PLT unit shelf life resulting from bacterial contamination 7 and storage-related PLT deterioration, 8 have resulted in significant PLT shortages. 9 Furthermore, artificial platelet substitutes have failed to replace physiological platelet products. 10 An efficient, donorindependent PLT bioreactor capable of generating clinically significant numbers of functional human PLTs is necessary to obviate risks associated with PLT procurement and storage, and help meet growing transfusion needs. In vivo, megakaryocytes (MKs) PLT progenitors sit outside blood vessels in the bone marrow (BM) and extend long, branching cellular structures designated proPLTs into the circulation from which PLTs are released. 11-15 Nearly 100% of human adult MKs must produce ;10 3 PLTs each to account for circulating PLT counts. 16 Although functional human PLTs were first grown in vitro in 1995, 17 to date only ;10% of human MKs initiate proPLT production in culture. This results in yields of 10 122 PLTs per CD34 1 cord blood-derived or embryonic stem cell-derived MK, 18 which are themselves of limited availability, constituting a significant bottleneck in the ex vivo production of a PLT transfusion unit. Although second-generation c...

Key Points• Dynein-dependent microtubule sliding drives proplatelet elongation under static and physiological shear stress conditions. • Proplatelet formation is a process that can be divided into repetitive phases: extension, pause, and retraction.Bone marrow megakaryocytes produce platelets by extending long cytoplasmic protrusions, designated proplatelets, into sinusoidal blood vessels. Although microtubules are known to regulate platelet production, the underlying mechanism of proplatelet elongation has yet to be resolved. Here we report that proplatelet formation is a process that can be divided into repetitive phases (extension, pause, and retraction), as revealed by differential interference contrast and fluorescence loss after photoconversion time-lapse microscopy. Furthermore, we show that microtubule sliding drives proplatelet elongation and is dependent on cytoplasmic dynein under static and physiological shear stress by using fluorescence recovery after photobleaching in proplatelets with fluorescence-tagged b1-tubulin. A refined understanding of the specific mechanisms regulating platelet production will yield strategies to treat patients with thrombocythemia or thrombocytopenia. (Blood. 2015;125(5):860-868)

Key Points• CCL5 increases MK ploidy and subsequent proplatelet formation in a CCR5-dependent manner.• CCL5 may act to increase platelet counts during physiological stress.In times of physiological stress, platelet count can transiently rise. What initiates this reactive thrombocytosis is poorly understood. Intriguingly, we found that treating megakaryocytes (MKs) with the releasate from activated platelets increased proplatelet production by 47%. Platelets store inflammatory cytokines, including the chemokine ligand 5 (CCL5, RANTES); after TRAP activation, platelets release over 25 ng/mL CCL5. We hypothesized that CCL5 could regulate platelet production by binding to its receptor, CCR5, on MKs. Maraviroc (CCR5 antagonist) or CCL5 immunodepletion diminished 95% and 70% of the effect of platelet releasate, respectively, suggesting CCL5 derived from platelets is sufficient to drive increased platelet production through MK CCR5. MKs cultured with recombinant CCL5 increased proplatelet production by 50% and had significantly higher ploidy. Pretreating the MK cultures with maraviroc prior to exposure to CCL5 reversed the augmented proplatelet formation and ploidy, suggesting that CCL5 increases MK ploidy and proplatelet formation in a CCR5-dependent manner. Interrogation of the Akt signaling pathway suggested that CCL5/CCR5 may influence proplatelet production by suppressing apoptosis. In an in vivo murine acute colitis model, platelet count significantly correlated with inflammation whereas maraviroc treatment abolished this correlation. We propose that CCL5 signaling through CCR5 may increase platelet counts during physiological stress. (Blood. 2016;127(7):921-926) IntroductionCirculating blood platelets are specialized cells that function to minimize bleeding and blood vessel injury. As such, platelets play a critical role in both normal and disease physiology. Large progenitor cells in the bone marrow called megakaryocytes (MKs) release platelets by extending long processes, designated proplatelets, into sinusoidal blood vessels.1 Despite the importance of platelets in thrombosis and hemostasis, the mechanism by which MKs complete differentiation and release platelets is poorly understood. Specifically, very little is known about what triggers mature, resting MKs to form proplatelets. Platelet counts rise transiently in the setting of physiological stress, such as myocardial infarction, infection, inflammation, and malignancy. [2][3][4] What initiates this upregulation is not well understood and has largely been attributed to an inflammatory response and increased cytokine release. [5][6][7] One cytokine that is highly expressed in inflammatory states is CCL5 (RANTES).8 CCL5, which is abundant in human platelets, signals predominantly through CCR5, a 7-transmembrane G-protein-coupled receptor that mediates diverse signaling cascades. 9 Methods Platelet purification and activationBlood collection was performed with institutional review board/institutional animal care and use committee approval and in accordance wi...

Selinexor is the first oral selective inhibitor of nuclear export compound tested for cancer treatment. Selinexor has demonstrated a safety therapy profile with broad antitumor activity against solid and hematological malignancies in phases 2 and 3 clinical trials (#NCT03071276, #NCT02343042, #NCT02227251, #NCT03110562, and #NCT02606461). Although selinexor shows promising efficacy, its primary adverse effect is high-grade thrombocytopenia. Therefore, we aimed to identify the mechanism of selinexor-induced thrombocytopenia to relieve it and improve its clinical management. We determined that selinexor causes thrombocytopenia by blocking thrombopoietin (TPO) signaling and therefore differentiation of stem cells into megakaryocytes. We then used both in vitro and in vivo models and patient samples to show that selinexor-induced thrombocytopenia is indeed reversible when TPO agonists are administered in the absence of selinexor (drug holiday). In sum, these data reveal (1) the mechanism of selinexor-induced thrombocytopenia, (2) an effective way to reverse the dose-limiting thrombocytopenia, and (3) a novel role for XPO1 in megakaryopoiesis. The improved selinexor dosing regimen described herein is crucial to help reduce thrombocytopenia in selinexor patients, allowing them to continue their course of chemotherapy and have the best chance of survival. This trial was registered at www.clinicaltrials.gov as #NCT01607905.