Developmental differences between newborn and adult mice in response to romiplostim

Implementing evidence-based restrictive neonatal intensive care unit platelet transfusion guidelines

Platelet transfusions are life-saving treatments for specific populations of neonates. However, recent evidence indicates that liberal prophylactic platelet transfusion practices cause harm to premature neonates. New efforts to better balance benefits and risks are leading to the adoption of more restrictive platelet transfusion guidelines in neonatal intensive care units (NICU). Although restrictive guidelines have the potential to improve outcomes, implementation barriers exist. We postulate that as neonatologists become more familiar with the data on the harm of liberal platelet transfusions, enthusiasm for restrictive guidelines will increase and barriers to implementation will decrease. Thus, we focused this educational review on; (1) the adverse effects of platelet transfusions to neonates, (2) awareness of platelet transfusion "refractoriness" in thrombocytopenic neonates and its association with poor outcomes, and (3) the impetus to find alternatives to transfusing platelets from adult donors to NICU patients.

Absence of Nkx2-3 induces ectopic lymphatic endothelial differentiation associated with impaired extramedullary stress hematopoiesis in the spleen

The red and white pulps as two main parts of the spleen are arranged around distinct types of vasculature, and perform significantly different functions in both humans and mice. Previous observations indicated a profound alteration of the local vessel specialization in mice lacking Nkx2-3 homeodomain transcription factor, including contradictory results suggesting presence of an ectopic lymphatic vascular structure. Furthermore, how the absence of Nkx2-3 and the consequential changes in endothelial components affect the extramedullary hematopoietic activity restricted to the splenic red pulp is unknown. In this work, we investigated the role of Nkx2-3 homeodomain transcription factor as a major morphogenic determinant for vascular specification, and its effect in the extramedullary hematopoiesis following acute blood loss and pharmacological stimulation of megakaryocyte differentiation after treatment with thrombopoietin-receptor mimetic Romiplostim. We found that, in mice lacking Nkx2-3, Prox1-positive lymphatic capillaries containing gp38/CD31 double positive lymphatic endothelial cells develop, arranged into an extensive meshwork, while the Clever1-positive venous segments of red pulp blood vasculature are absent. This lymphatic endothelial shift is coupled with a severely compromised splenic erythropoiesis and a significantly reduced splenic megakaryocyte colony formation following Romiplostim treatment in mice lacking Nkx2-3. These findings indicate that the shift of microvascular patterning in the absence of Nkx2-3 includes the emergence of ectopic Prox1-positive lymphatic vessels, and that this pivoting towards lymph node-like vascular patterning is associated with an impaired reserve hematopoietic capacity of the splenic red pulp.

Fetal vs adult megakaryopoiesis

Fetal and neonatal megakaryocyte progenitors are hyperproliferative compared with adult progenitors and generate a large number of small, low-ploidy megakaryocytes. Historically, these developmental differences have been interpreted as "immaturity." However, more recent studies have demonstrated that the small, low-ploidy fetal and neonatal megakaryocytes have all the characteristics of adult polyploid megakaryocytes, including the presence of granules, a well-developed demarcation membrane system, and proplatelet formation. Thus, rather than immaturity, the features of fetal and neonatal megakaryopoiesis reflect a developmentally unique uncoupling of proliferation, polyploidization, and cytoplasmic maturation, which allows fetuses and neonates to populate their rapidly expanding bone marrow and blood volume. At the molecular level, the features of fetal and neonatal megakaryopoiesis are the result of a complex interplay of developmentally regulated pathways and environmental signals from the different hematopoietic niches. Over the past few years, studies have challenged traditional paradigms about the origin of the megakaryocyte lineage in both fetal and adult life, and the application of single-cell RNA sequencing has led to a better characterization of embryonic, fetal, and adult megakaryocytes. In particular, a growing body of data suggests that at all stages of development, the various functions of megakaryocytes are not fulfilled by the megakaryocyte population as a whole, but rather by distinct megakaryocyte subpopulations with dedicated roles. Finally, recent studies have provided novel insights into the mechanisms underlying developmental disorders of megakaryopoiesis, which either uniquely affect fetuses and neonates or have different clinical presentations in neonatal compared with adult life.

Iron status influences the response of cord blood megakaryocyte progenitors to eltrombopag in vitro

Eltrombopag (ELT) is a thrombopoietic agent approved for immune thrombocytopenia and also a potent iron chelator. Here we found that ELT exhibited dose-dependent opposing effects on in vitro megakaryopoiesis: low concentrations (≤6 µM, ELT6) stimulated megakaryopoiesis, but high concentrations (30 µM, ELT30) suppressed megakaryocyte (MK) differentiation and proliferation. The suppressive effects of ELT30 were reproduced by other iron chelators, supporting iron chelation as a likely mechanism. During MK differentiation, committed MK progenitors (CD34+/CD41+ and CD34-/CD41+ cells) were significantly more sensitive than undifferentiated progenitors (CD34+/CD41- cells) to the suppressive effects of ELT30, which resulted from both decreased proliferation and increased apoptosis. The antiproliferative effects of ELT30 were reversed by increased iron in the culture, as were the proapoptotic effects when exposure to ELT30 was short. Because committed MK progenitors exhibited the highest proliferative rate and the highest sensitivity to iron chelation, we tested whether their iron status influenced their response to ELT during rapid cell expansion. In these studies, iron deficiency reduced the proliferation of CD41+ cells in response to all ELT concentrations. Severe iron deficiency also reduced the number of MKs generated in response to high thrombopoietin concentrations by ∼50%, compared with iron-replete cultures. Our findings support the hypothesis that although iron deficiency can stimulate certain cells and steps in megakaryopoiesis, it can also limit the proliferation of committed MK progenitors, with severity of iron deficiency and degree of thrombopoietic stimulation influencing the ultimate output. Further studies are needed to clarify how megakaryopoiesis, iron deficiency, and ELT stimulation are clinically interrelated.

Developmental differences of in vitro cultured murine bone marrow- and fetal liver-derived megakaryocytes

Multiple lines of evidence support differences in the megakaryopoiesis during development. Murine in vitro models to study megakaryopoiesis employ cultured megakaryocytes MKs derived from adult bone marrow (BM) or fetal livers (FL) of mouse embryos. Mouse models allow to study the molecular basis for cellular changes utilizing conditional or knock-out models and permit further in vitro genetic or pharmacological manipulations. Despite being extensively used, MKs cultured from these two sources have not been systematically compared. In the present study, we compared BM- and FL-derived MKs, assessing their size, proplatelet production capacity, expression of common MK markers (αIIb, β3, GPIb α, β) and cytoskeletal proteins (filamin A, β1-tubulin, actin), the subcellular appearance of α-granules (VWF), membranes (GPIbβ) and cytoskeleton (F-actin) throughout in vitro development. We demonstrate that FL MKs although smaller in size, spontaneously produce more proplatelets than BM MKs and at earlier stages express more β1-tubulin. In addition, early FL MKs show increased internal GPIbβ staining and present higher GPIbβ (early and late) and VWF (late stages) total fluorescence intensity (TFI)/cell size than BM MKs. BM MKs have up-regulated TPO signaling corresponding to their bigger size and ploidy, without changes in c-Mpl. Expressing endogenous β1-tubulin or the presence of heparin improves BM MKs ability to produce proplatelets. These data suggest that FL MKs undergo cytoplasmic maturation earlier than BM MKs and that this, in addition to higher β1-tubulin levels and GPIb, supported with an extensive F-actin network, could contribute to more efficient proplatelet formation in vitro.

The Fetal-to-Adult Hematopoietic Stem Cell Transition and its Role in Childhood Hematopoietic Malignancies

As with most organ systems that undergo continuous generation and maturation during the transition from fetal to adult life, the hematopoietic and immune systems also experience dynamic changes. Such changes lead to many unique features in blood cell function and immune responses in early childhood. The blood cells and immune cells in neonates are a mixture of fetal and adult origin due to the co-existence of both fetal and adult types of hematopoietic stem cells (HSCs) and progenitor cells (HPCs). Fetal blood and immune cells gradually diminish during maturation of the infant and are almost completely replaced by adult types of cells by 3 to 4 weeks after birth in mice. Such features in early childhood are associated with unique features of hematopoietic and immune diseases, such as leukemia, at these developmental stages. Therefore, understanding the cellular and molecular mechanisms by which hematopoietic and immune changes occur throughout ontogeny will provide useful information for the study and treatment of pediatric blood and immune diseases. In this review, we summarize the most recent studies on hematopoietic initiation during early embryonic development, the expansion of both fetal and adult types of HSCs and HPCs in the fetal liver and fetal bone marrow stages, and the shift from fetal to adult hematopoiesis/immunity during neonatal/infant development. We also discuss the contributions of fetal types of HSCs/HPCs to childhood leukemias.

Development of gates to measure the immature platelet fraction in C57BL/6J mice using the Sysmex XN-V series multispecies hematology analyzer

The immature platelet fraction (IPF) is a measure of newly released platelets, which has been used as a marker of platelet production in multiple human studies but is not widely available in multispecies analyzers. We developed gates to measure the IPF in diluted and undiluted murine blood samples on the Sysmex XN-1000V multispecies hematology analyzer. IPF gates were created using undiluted and diluted (1/10) blood samples obtained from adult and newborn (postnatal day 10, P10) C57BL/6J wild-type (WT) mice, and from 3 murine models of thrombocytopenia: c-MPL^-/- mice, which lack the thrombopoietin receptor (hyporegenerative); antibody-mediated thrombocytopenia; and acute inflammation-induced thrombocytopenia. P10 mice were chosen because, at their size, we could consistently obtain (by terminal phlebotomy) the blood volume needed to run an undiluted sample. The undiluted blood IPF gate successfully differentiated between mechanisms of thrombocytopenia in both adult and P10 mice. For diluted samples, 2 IPF gates were generated: a thrombocytopenic (T) gate, which performed well in samples with platelet counts (PCs) <800 × 10⁹/L in adult mice and <500 × 10⁹/L in newborn mice, and a non-thrombocytopenic (NT) gate, which performed well in samples with PCs above these thresholds. PCs and IPFs measured in diluted blood using these gates agreed well with those measured in undiluted blood and had good reproducibility. These diluted gates allow for the accurate measurement of PCs and IPFs in small (10 µL) blood volumes, which can be obtained easily from adult and newborn mice as small as P1 to assess platelet production serially.

Thrombopoietin receptor agonist for treatment of immune thrombocytopenia in pregnancy: a narrative review

The treatment of immune thrombocytopenia (ITP) in adults has evolved rapidly over the past decade. The second-generation thrombopoietin receptor agonists (TPO-RAs), romiplostim, eltrombopag, and avatrombopag are approved for the treatment of chronic ITP in adults. However, their use in pregnancy is labeled as category C by the United States Food and Drug Administration (FDA) due to the lack of clinical data on human subjects. ITP is a common cause of thrombocytopenia in the first and second trimester of pregnancy, which not only affects the mother but can also lead to thrombocytopenia in the neonatal thrombocytopenia secondary to maternal immune thrombocytopenia (NMITP). Corticosteroids, intravenous immunoglobulins (IVIGs) are commonly used for treating acute ITP in pregnant patients. Drugs such as rituximab, anti-D, and azathioprine that are used to treat ITP in adults, are labeled category C and seldom used in pregnant patients. Cytotoxic chemotherapy (vincristine, cyclophosphamide), danazol, and mycophenolate are contraindicated in pregnant women. In such a scenario, TPO-RAs present an attractive option to treat ITP in pregnant patients. Current evidence on the use of TPO-RAs in pregnant women with ITP is limited. In this narrative review, we will examine the preclinical and the clinical literature regarding the use of TPO-RAs in the management of ITP in pregnancy and their effect on neonates with NMITP.

Aurka loss in CD19+ B cells promotes megakaryocytopoiesis via IL-6/STAT3 signaling-mediated thrombopoietin production

Rationale: Aurora kinase A (Aurora-A), which is required for mitosis, is a therapeutic target in various tumors. Targeting Aurora-A led to an increase in the differentiation and polyploidization of megakaryocytes both in vivo and in vitro. However, the mechanisms involved in controlling megakaryocyte differentiation have not been fully elucidated.

Methods: Conditional Aurka knockout mice were generated. B cell development, platelet development and function were subsequently examined. Proplatelet formation, in vivo response to mTPO, post-transfusion experiment, colony assay, immunofluorescence staining and quantification, and ChIP assay were conducted to gain insights into the mechanisms of Aurka loss in megakaryocytopoiesis.

Results: Loss of Aurka in CD19⁺ B cells impaired B cell development in association with an increase in the number of platelets in peripheral blood (PB). Surprisingly, thrombopoietin (TPO) production and IL-6 were elevated in the plasma in parallel with an increase in the number of differentiated megakaryocytes in the bone marrow (BM) of Aurka^f/f;Cd19^Cre/+ mice. Interestingly, compared with that of the Aurka^f/f mice, a higher number of CD19⁺ B cells close to megakaryocytes was observed in the BM of the Aurka^f/f;Cd19^Cre/+ mice. Moreover, Aurka loss in CD19⁺ B cells induced signal transducer and activator of transcription-3 (STAT3) activation. Inhibition of STAT3 reduced the Tpo mRNA levels. ChIP assays revealed that STAT3 bound to the TPO promoter. Additionally, STAT3-mediated TPO transcription was an autocrine effect provoked by IL-6, at least partially.

Conclusions: Deletion of Aurka in CD19⁺ B cells led to an increase in IL-6 production, promoting STAT3 activation, which in turn contributed to TPO transcription and megakaryocytopoiesis.

Changes in megakaryopoiesis over ontogeny and their implications in health and disease

A growing body of research has made it increasingly clear that there are substantial biological differences between fetal/neonatal and adult megakaryopoiesis. Over the last decade, studies revealed a developmentally unique uncoupling of proliferation, polyploidization, and cytoplasmic maturation in neonatal MKs that results in the production of large numbers of small, low ploidy, but mature MKs during this period of development, and identified substantial molecular differences between fetal/neonatal and adult MKs. This review will summarize our current knowledge on the developmental differences between fetal/neonatal and adult MKs, and recent advances in our understanding of the underlying molecular mechanisms, including newly described developmentally regulated pathways and miRNAs. We will also discuss the implications of these findings on the ways MKs interact with the environment, the response of neonates to thrombocytopenia, the pathogenesis of Down syndrome-transient myeloproliferative disorder (TMD), and the developmental stage specific-manifestations of congenital amegakaryocytic thrombocytopenia.

Romiplostim for the management of pediatric immune thrombocytopenia: drug development and current practice

Since successful cloning of thrombopoietin (TPO) in 1994, significant advances have been made in the development of recombinant TPO receptor agonists. The US Food and Drug Administration (FDA) has approved 2 agents for use in patients with immune thrombocytopenia (ITP): eltrombopag and romiplostim. Romiplostim is a once-weekly subcutaneous injection that has been shown to increase the platelet count, lessen bleeding, and reduce concurrent medication use in adults with ITP. In December 2018, the US FDA approved romiplostim for use in pediatric patients ≥1 year of age with ITP of >6 months' duration and insufficient response to corticosteroids, immunoglobulins, or splenectomy, based on similarly favorable clinical trial data. In addition, romiplostim is well tolerated, making it an attractive option for the treatment of children. Expansion of off-label romiplostim use is being reported in children for ITP <6 months, neonatal thrombocytopenia, hereditary thrombocytopenias, and chemotherapy- and bone marrow transplant-associated thrombocytopenia. We review here the development of romiplostim with a focus on pediatric use.

Medicinal Uses of Hematopoietic Growth Factors in Neonatal Medicine

This review focuses on certain hematopoietic growth factors that are used as medications in clinical neonatology. It is important to note at the chapter onset that although all of the pharmacological agents mentioned in this review have been approved by the US Food and Drug administration for use in humans, none have been granted a specific FDA indication for neonates. Thus, in a sense, all of the agents mentioned in this chapter could be considered experimental, when used in neonates. However, a great many of the pharmacological agents utilized routinely in neonatology practice do not have a specific FDA indication for this population of patients. Consequently, many of the agents reviewed in this chapter are considered by some practitioners to be nonexperimental and are used when they judge such use to be "best practice" for the disorders under treatment.The medicinal uses of the agents in this chapter vary considerably, between geographic locations, and sometimes even within an institutions. "Consistent approaches" aimed at using these agents in uniform ways in the practice of neonatology are encouraged. Indeed some healthcare systems, and some individual NICUs, have developed written guidelines for using these agents within the practice group. Some such guidelines are provided in this review. It should be noted that these guidelines, or "consistent approaches," must be viewed as dynamic and changing, requiring adjustment and refinement as additional evidence accrues.

Cytokine control of megakaryopoiesis

Platelets are anuclear blood cells required for haemostasis and are implicated in other processes including inflammation and metastasis. Platelets are produced by megakaryocytes, specialized cells that are themselves generated by a process of controlled differentiation and maturation of bone-marrow stem and progenitor cells. This process of megakaryopoiesis involves the coordinated interplay of transcription factor-controlled cellular programming with extra-cellular cues produced locally in supporting niches or as circulating factors. This review focuses on these external cues, the cytokines and chemokines, that drive production of megakaryocytes and support the terminal process of platelet release. Emphasis is given to thrombopoietin (Tpo), the major cytokine regulator of steady-state megakaryopoiesis, and its specific cell surface receptor, the Mpl protein, including normal and pathological roles as well as clinical application. The potential for alternative or supplementary regulatory mechanisms for platelet production, particularly in times of acute need, or in states of infection or inflammation are also discussed.

How to perform aggregometry and lumi-aggregometry in mouse platelets

Light transmission aggregometry and lumi-aggregometry are the gold standard platelet assays both clinically and for basic research. The availability of different strains of genetically modified mice and mouse models of human disease means that often laboratories need to use mouse platelets in these assays. Overall, performing aggregometry and lumi-aggregometry with mouse platelets is similar to with human platelets, although methods need be adapted to accommodate their small size, reduced blood volume, and different protein levels. This review aims to highlight these key considerations when planning aggregometry experiments with mouse platelets. These include the method of taking blood, including the use of anticoagulants, as well as the method of platelet preparation, and how to maximize yields. This review also covers how to maximize the number of aggregations that can be performed, both by understanding the minimum requirements of your aggregometer, or by considering new approaches. These include employing high throughput plate-based aggregometry (Optimul), or the use of TPO-mimetics to stimulate platelet production in mice to boost their platelet counts. Finally, phenotypic differences between mouse and human platelets, such as protein expression or sensitivity to agonists are discussed as an important consideration when planning experiments.

Developmental Stage-Specific Manifestations of Absent TPO/c-MPL Signalling in Newborn Mice

Congenital amegakaryocytic thrombocytopaenia (CAMT) is a disorder caused by c-MPL mutations that impair thrombopoietin (TPO) signalling, resulting in a near absence of megakaryocytes (MKs). While this phenotype is consistent in adults, neonates with CAMT can present with severe thrombocytopaenia despite normal MK numbers. To investigate this, we characterized MKs and platelets in newborn c-MPL –/– mice. Liver MKs in c-MPL –/– neonates were reduced in number and size compared with wild-type (WT) age-matched MKs, and exhibited ultrastructural abnormalities not found in adult c-MPL –/– MKs. Platelet counts were lower in c-MPL –/– compared with WT mice at birth and did not increase over the first 2 weeks of life. In vivo biotinylation revealed a significant reduction in the platelet half-life of c-MPL –/– newborn mice (P2) compared with age-matched WT pups, which was not associated with ultrastructural abnormalities. Genetic deletion of the pro-apoptotic Bak did not rescue the severely reduced platelet half-life of c-MPL –/– newborn mice, suggesting that it was due to factors other than platelets entering apoptosis early. Indeed, adult GFP+ (green fluorescent protein transgenic) platelets transfused into thrombocytopenic c-MPL –/– P2 pups also had a shortened lifespan, indicating the importance of cell-extrinsic factors. In addition, neonatal platelets from WT and c-MPL –/– mice exhibited reduced P-selectin surface expression following stimulation compared with adult platelets of either genotype, and platelets from c-MPL –/– neonates exhibited reduced glycoprotein IIb/IIIa (GPIIb/IIIa) activation in response to thrombin compared with age-matched WT platelets. Taken together, our findings indicate that c-MPL deficiency is associated with abnormal maturation of neonatal MKs and developmental stage-specific defects in platelet function.