ALBERT

Description: The immune response relies on the integration of cell-intrinsic processes with cell-extrinsic cues. During infection, B cells vacate the marrow during emergency granulopoiesis but return upon restoration of homeostasis. Here we report a novel glycosylation-mediated crosstalk between marrow B cells and hematopoietic progenitors. Human B cells secrete active ST6GAL1 sialyltransferase that remodels progenitor cell surface glycans to suppress granulopoiesis. In mouse models, ST6GAL1 from B cells alters the sialylation profile of bone marrow populations, and mature IgD+ B cells were enriched in sialylated bone marrow niches. In clinical multiple myeloma, ST6GAL1 abundance in the multiple myeloma cells negatively correlated with neutrophil abundance. These observations highlight not only the ability of medullary B cells to influence blood cell production, but also the disruption to normal granulopoiesis by excessive ST6GAL1 in malignancy.

Description: Arp2/3 complex is believed to induce de novo nucleation of actin filaments at the edge of motile cells downstream of WASp family proteins. In this study, the signaling pathways leading to Arp2/3 complex activation, actin assembly, and shape change were investigated in platelets isolated from patients with Wiskott-Aldrich Syndrome (WAS), that is, who lack WASp, and in WASp-deficient mouse platelets. WASp-deficient human and mouse platelets elaborate filopodia, spread lamellae, and assemble actin, identical to control WASp-expressing platelets. Human platelets contain 2 μM Arp2/3 complex, or 8600 molecules/cell. Arp2/3 complex redistributes to the edge of the lamellae and to the Triton X-100–insoluble actin cytoskeleton of activated WASp-deficient platelets. Furthermore, the C-terminal CA domain of N-WASp, which sequesters Arp2/3 complex, inhibits by half the actin nucleation capacity of octylglucoside-permeabilized and activated WAS platelets, similar to its effect in WASp-expressing cells. Along with WASp, platelets express WAVE-2 as a physiologic activator of Arp2/3 complex and a small amount of N-WASp. Taken together, our findings show that platelets activate Arp2/3 complex, assemble actin, and change shape in the absence of WASp, indicating a more specialized role for WASp in these cells.

Description: Although surface sialic acid is considered a key determinant for the survival of circulating blood cells and glycoproteins, its role in platelet survival is unclear. We investigated the importance of sialic acid for platelet clearance using mice deficient in the ST3GalIV sialyltransferase gene (ST3GalIV−/− mice) and identified a novel clearance mechanism previously unrecognized for platelets. ST3GalIV catalyzes the addition of sialic acid onto exposed galactose residues of cell surface glycoproteins. ST3GalIV−/− mice have increased platelet surface galactose exposure, a 70% reduction in platelet count, and prolonged bleeding times. We report that ST3GalIV−/− platelets transfused into wild-type C57BL/6J mice exhibit markedly reduced recoveries and shortened survivals respectively compared to littermate wild-type platelets. Infusion of asialofetuin, an antagonist of the asialoglycoprotein-receptor (ASGPR) restored platelet recovery time and initial circulation of ST3GalIV−/− platelets to normal values. Immunohistochemical studies of organ specimens harvested shortly after transfusion of biotin-labelled platelets demonstrated the predominant clearance of ST3GalIV−/− platelets by the liver Kupffer cells and, unexpectedly, hepatocytes. Megakaryocytes cultured from ST3GalIV−/− mice produced proplatelets normally compared to megakaryocytes generated from wild-type littermates, indicating that the thrombocytopenia in the ST3GalIV−/− mice is not due to reduced platelet production. Comparison of ST3GalIV+/+ and ST3GalIV−/− platelet surface receptor expression as evidenced by flow cytometry and preliminary in vitro activation studies did not reveal any significant differences in the two genotypes. We conclude that the absence of terminal sialic acid residues on platelet surfaces exposes galactose residues to the lectin domain of ASGPR on both hepatocytes and liver Kupffer cells, resulting in platelet clearance from the circulation.

Description: Key Points DNM2-dependent endocytosis in MKs regulates megakaryopoiesis, thrombopoiesis, and bone marrow homeostasis.

Description: Abstract 706 Thrombocytopenia is a common clinical problem among neonates, affecting 30% of all infants admitted to the neonatal intensive care unit, and up to 70% of those born prematurely. The neonatal predisposition to develop thrombocytopenia has been largely attributed to the increasingly recognized substantial differences between fetal/neonatal and adult megakaryocytes (MKs). However, little is known about overall platelet production rates and platelet survival during neonatal life. To address this question, we evaluated the sites of thrombopoiesis, platelet counts, reticulated platelet percentages (RP%), blood volume, and platelet survival in C57/B6 mice during the first 2 wks of life. During that period, the MK concentration decreased sharply in the liver (the main site of platelet production during fetal life), while it increased steadily in the BM, where it reached nearly adult levels by day 14. During this transition, the spleen helped support thrombopoiesis through a 6-fold increase in MK concentration (which peaked at day 10), prior to returning to baseline levels. Consistent with our recent findings in human MKs (Liu et al., Blood), murine neonatal MKs were small but cytoplasmically mature by immunofluorescence and electron microscopy. In regard to platelet production, platelet counts in one day old pups were approximately 50% of adults, but then increased and reached nearly adult values by day 14. During the same time period, the body weight and total blood volume increased ∼5-fold. Combined, these changes led to a 10-fold increase in the total platelet number over the first 2 wks of life. The RP%, a measure of new platelet release, was ∼2-fold higher in 3- and 10-day old pups compared to adults, suggesting that increased platelet production contributes to the rapid increase in total platelet number during this period of development. To evaluate for potential developmental differences in platelet survival, we conducted in vivo biotinylation studies in newborn and adult mice. These showed that neonatal platelets have a significantly longer half life than adult platelets (90 vs. 50 hrs, respectively; p

Description: The αMβ2 receptor on macrophages, which recognizes clustered β-N-acetylglucosamine (βGlcNAc) residues on glycoprotein (GP) Ib-V-IX, is one phagocytic receptor involved in the removal of transfused refrigerated platelets. Although coverage of the exposed βGlcNAc residues by galactosylation prevents the clearance of murine platelets refrigerated for less than 4h, it does not impact the clearance of platelets refrigerated for 48h, suggesting that further changes occur in platelets during prolonged refrigeration. We have now found that murine GPIbα and GPV are partially shed from the platelet surface after refrigeration for 48h and re-warming. Other members of the complex, GPIX and GPIbβ, were not affected. Platelets isolated from mice with inactive ADAM17 enzyme, a GPIbα and GPV sheddase, had preserved surface GPIbα and decreased GPV shedding compared to WT mice after long-term refrigeration. 48h refrigerated platelets with inactive ADAM17 or platelets treated with the metalloproteinase inhibitor GM 6001, had increased survival when transfused into WT mice. Metalloproteinase inhibitor-treated and galactosylated, 48h refrigerated and re-warmed mouse platelets exhibited survivals equivalent to fresh platelets. We conclude that 48h long-term refrigeration and re-warming of platelets induces shedding of GPIbα and GPV in murine platelets, and that metalloproteinase inhibitors increase the survival of long-term refrigerated platelets in mice. Further changes in GPIb-V-IX in long-term refrigerated platelets may recruit additional phagocytic receptors to promote clearance, in addition to the αMβ2 receptor.

Description: We previously reported that the lectin domain of αMβ2 receptors on hepatic macrophages mediates rapid clearance of washed murine platelets transfused after refrigeration for 2 hours, recognizing exposed βN-acetylglucosamine (βGlcNAc) residues of N-linked glycans on clustered platelet GPIbα molecules and that the same receptors elicit phagocytosis of refrigerated human platelets human macrophages in vitro. A platelet-associated galactosyltransferase catalyzes the covering of βGlcNAc residues with galactose in the presence of UDP-galactose, thereby blocking clearance of cold mouse platelets in vivo and phagocytosis of human platelets in vitro. These intriguing findings contradicted earlier evidence that refrigeration of human platelets procured for transfusion only promotes their rapid clearance after prolonged (〉8h) incubation and also are inconsistent with the well-known recognition system for exposed galactose residues through asialoglycoprotein (ASGP) receptors. Reconciling these contradictions, we report that the absence of plasma during storage accounts for the differences in time of exposure to cold to promote clearance and that mouse platelets cold-stored in plasma also only clear rapidly after long-term (48h) storage. We also found that hepatic clearance of long-term cold-stored (LTCS) mouse platelets occurs in hepatocytes. Streptavidin-POD staining revealed abundant LTCS biotinylated platelets in hepatocyte phagosomes. Furthermore, cells of the hepatocyte HepG2 line avidly ingest fluorescently-labeled LTCS human platelets (7-fold above the baseline of room-temperature-stored platelets), as evidenced by flow cytometry, fluorescent microscopy and by time-laps video microscopy. Long-term cold storage increases by ~1.7-fold platelet binding of the galactose-specific lectin RCA I, implying that with long-term cold storage, exposed galactose residues cluster sufficiently to induce recognition by hepatocyte ASGPR receptors. The results define a new clearance mechanism, representing the first example of blood cell removal by a non-myeloid cell. Since we find that human platelets also express a cell surface sialotransferase that adds sialic acid to galactose residues, we suggest that a combination of sialylation and glactosylation, achievable by addition of sugar substrates alone, might accommodate long-term cold storage of platelets for transfusion.

Description: αMβ2 integrin receptors on myeloid cells mediate the adhesion or uptake of diverse ligands, and particular sequences on this integrin’s alpha subunit are responsible for recognition specificity. Inhibition of neutrophil adhesion to platelet monolayers by αM-derived peptides has implicated an αM sequence (I-domain) that also binds iC3b, fibrinogen, ICAM-1 and other ligands. However, αMβ2 also recognizes βN-acetylglucosamine (βGlcNAc) residues on the platelet von Willebrand factor receptor (GPIbα) that are clustered after cooling, suggesting that the αM lectin domain is important for this recognition. The phagocytosis of chilled platelets can be reconstituted when CHO cells are transfected with αMβ2. Removal of the N-T portion of GPIbα, containing the exposed βGlcNAc residues, from the chilled platelet surface using the snake venom mocarhagin inhibits this phagocytosis, as does the addition of 1 mM β-methyl-N-acetyl-D-glucosamine. Replacement of the I domain, or the lectin domain of the αM-chain with the corresponding domain from the αx-chain (p150), reveals that the activity of the αMβ2 integrin toward chilled platelets resides within the lectin-domain, and does not require the I domain. Other evidences for this conclusion include: (1) Sf9 cells expressing αM lectin domain but not β2 bind 3-4 x more chilled platelets than control cells; βGlcNAc inhibits this binding. (2) Soluble recombinant αM lectin domain inhibits the phagocytosis of chilled platelets by αMβ2-expressing THP-1 cells while I-domain substrates (fibrinogen or LPM19c), recombinant I-domain protein, or a synthetic peptide that inhibits neutrophil adhesion to platelet monolayers, are without inhibitory effect. We conclude that chilled platelets are removed from blood by an interaction between βGlcNAc residues on clustered GPIbα and the lectin domain of αM-chain of the αMβ2 integrin. Since the phagocytosis and clearance of chilled platelets by hepatic macrophages has precluded refrigeration of platelets procured for transfusion, a more precise molecular definition of the recognition mechanism could inform strategies for its inhibition, thereby accommodating cold platelet storage.

Description: Abstract 2027 Filamin A (FlnA) is a large cytoplasmic protein that crosslinks and stabilizes actin filament networks and links membrane glycoproteins and signaling proteins to the underlying cytoskeleton. FlnA loxP-PF4-Cre mice that specifically lack FlnA in their platelets and megakaryocytes (MKs) have a macrothrombocytopenia with increased tail bleeding time. FlnA KO platelets have decreased surface expression of the von Willebrand factor receptor (vWfR) and are cleared rapidly from the circulation of WT mice, indicating an inherent surface defect that leads to detection and removal. FlnA loxP-PF4-Cre mice have a marked increase in MK numbers in bone marrow and spleen. Detailed analysis of platelet production by FlnA KO MKs in vitro reveals an altered maturation program and defects in vWfR stability, compared to WT. Surprisingly, the surface expression of vWfR components (Ibα, Ibβ, IX and V) on FlnA KO MKs is normal or increased compared to WT MKs, although the total amount of GPIbα is decreased and GPIbα is not associated with the actin cytoskeleton. Analysis of the GPIbβ subunit shows increased degradation in FlnA KO MKs and platelets. Although FlnA KO and WT MK cultures contain comparable cell numbers, FlnA KO MKs more rapidly convert their proplatelets into large CD61+ platelet-sized particles. These findings suggest that aberrant platelet maturation by FlnA KO MKs results in enlarged platelets that are cleared rapidly because of altered vWfR expression. Disclosures: No relevant conflicts of interest to declare.