Dual role of IL-21 in megakaryopoiesis and platelet homeostasis.

Gene profiling studies have indicated that in vitro differentiated human megakaryocytes express the receptor for IL-21 (IL-21R), an immunostimulatory cytokine associated with inflammatory disorders and currently under evaluation in cancer therapy. The aim of this study was to investigate whether IL-21 modulates megakaryopoiesis. We first checked the expression of IL-21 receptor on human bone marrow and in vitro differentiated megakaryocytes. We then investigated the effect of IL-21 on the in vitro differentiation of human blood CD34+ progenitors into megakaryocytes. Finally, we analyzed the consequences of hydrodynamic transfection-mediated transient expression of IL-21, on megakaryopoiesis and thrombopoiesis in mice. The IL-21Rα chain was expressed in human bone marrow megakaryocytes and was progressively induced during in vitro differentiation of human peripheral CD34+ progenitors, while the signal transducing γ chain was down-regulated. Consistently, the STAT3 phosphorylation induced by IL-21 diminished during the later stages of megakaryocytic differentiation. In vitro, IL-21 increased the number of colony-forming unit megakaryocytes generated from CD34+ cells and the number of megakaryocytes differentiated from CD34+ progenitors in a JAK3- and STAT3-dependent manner. Forced expression of IL-21 in mice increased the density of bi-potent megakaryocyte progenitors and bone marrow megakaryocytes, and the platelet generation, but increased platelet clearance with a consequent reduction in blood cell counts. Our work suggests that IL-21 regulates megakaryocyte development and platelet homeostasis. Thus, IL-21 may link immune responses to physiological or pathological platelet-dependent processes.

The P2X1 receptor is required for neutrophil extravasation during lipopolysaccharide-induced lethal endotoxemia in mice.

Extracellular ATP is becoming increasingly recognized as an important regulator of inflammation. However, the known repertoire of P2 receptor subtypes responsible for the proinflammatory effects of ATP is sparse. We looked at whether the P2X1 receptor, an ATP-gated cation channel present on platelets, neutrophils, and macrophages, participates in the acute systemic inflammation provoked by LPS. Compared with wild-type (WT) mice, P2X1(-/-) mice displayed strongly diminished pathological responses, with dampened neutrophil accumulation in the lungs, less tissue damage, reduced activation of coagulation, and resistance to LPS-induced death. P2X1 receptor deficiency also was associated with a marked reduction in plasma levels of the main proinflammatory cytokines and chemokines induced by LPS. Interestingly, macrophages and neutrophils isolated from WT and P2X1(-/-) mice produced similar levels of proinflammatory cytokines when stimulated with LPS in vitro. Intravital microscopy revealed a defect in LPS-induced neutrophil emigration from cremaster venules into the tissues of P2X1(-/-) mice. Using adoptive transfer of immunofluorescently labeled neutrophils from WT and P2X1(-/-) mice into WT mice, we demonstrate that the absence of the P2X1 receptor on neutrophils was responsible for this defect. This study reveals a major role for the P2X1 receptor in LPS-induced lethal endotoxemia through its critical involvement in neutrophil emigration from venules.

A Rab11A/myosin Vb/Rab11-FIP2 complex frames two late recycling steps of langerin from the ERC to the plasma membrane.

A large body of knowledge relating to the constitution of Rab GTPase/Rab effector complexes and their impact on both membrane domain organization and overall membrane trafficking has been built up in recent years. However in the context of the live cell there are still many questions that remain to be answered, such as where and when these complexes assemble and where they perform their primary function(s). We describe here the dynamic processes that take place in the final steps of the Rab11A dependent recycling pathway, in the context of the membrane platform constituted by Myosin Vb, Rab11A, and Rab11-FIP2. We first confirm that a series of previously reported observations obtained during the study of a number of trafficking cargoes also apply to langerin. Langerin is a cargo molecule that traffics through Rab11A-positive membrane domains of the endosomal recycling pathway. In order to explore the relative dynamics of this set of partners, we make extensive use of a combinatory approach of Live-FRET, fast FRAP video, fast confocal and TIRF microscopy modalities. Our data show that the Myosin Vb/Rab11A/Rab11-FIP2 platform is spatially involved in the regulation of langerin trafficking at two distinct sites within live cells, first at the sorting site in the endosomal recycling compartment (ERC) where transport vesicles are formed, and subsequently, in a strict time-defined order, at the very late stage of docking/tethering and fusion of these langerin recycling vesicles to the plasma membrane.

HLA-DQA2 and HLA-DQB2 genes are specifically expressed in human Langerhans cells and encode a new HLA class II molecule.

The precise role of human epidermal Langerhans cells (LCs) in immune response is highly controversial. While studying the gene expression profile of these cells, we were intrigued to identify the HLA-DQB2 gene as potentially expressed in LCs. Despite a strong evolutionary conservation of their sequences, the concomitant expression of the poorly polymorphic HLA-DQA2/HLA-DQB2 genes, paralogous to the HLA-DQA1/HLA-DQB1 genes, has never been detected in any cell type. We confirmed by RT-PCR that the HLA-DQA2 and -DQB2 genes are both expressed in LCs, but not in monocyte-derived dendritic cells, or in blood CD1c(+) or plasmacytoid dendritic cells. The presence of the HLA-DQß2 chain in LCs could be demonstrated by Western blotting, whereas immunofluorescence revealed its localization in early endosomes. As in the case of other HLA class II molecules, the HLA-DQα2 and -DQß2 chains formed heterodimers that had to associate with the invariant chain to reach endosomal compartments. HLA-DQα2/ß2 heterodimers were expressed at the cell surface, where they could mediate staphylococcal superantigen stimulation of T cells. Interestingly, HLA-DQα2 and HLA-DQß1 chains formed mixed heterodimers which efficiently left the endoplasmic reticulum. These observations strongly suggest that the poorly polymorphic HLA-DQA2 and -DQB2 genes should be considered to be of immunological importance. The HLA-DQα2/ß2 molecules could influence the complexity of the repertoire of Ags presented by LCs.

Structural reorganization of the antigen-binding groove of human CD1b for presentation of mycobacterial sulfoglycolipids.

The mechanisms permitting nonpolymorphic CD1 molecules to present lipid antigens that differ considerably in polar head and aliphatic tails remain elusive. It is also unclear why hydrophobic motifs in the aliphatic tails of some antigens, which presumably embed inside CD1 pockets, contribute to determinants for T-cell recognition. The 1.9-Å crystal structure of an active complex of CD1b and a mycobacterial diacylsulfoglycolipid presented here provides some clues. Upon antigen binding, endogenous spacers of CD1b, which consist of a mixture of diradylglycerols, moved considerably within the lipid-binding groove. Spacer displacement was accompanied by F' pocket closure and an extensive rearrangement of residues exposed to T-cell receptors. Such structural reorganization resulted in reduction of the A' pocket capacity and led to incomplete embedding of the methyl-ramified portion of the phthioceranoyl chain of the antigen, explaining why such hydrophobic motifs are critical for T-cell receptor recognition. Mutagenesis experiments supported the functional importance of the observed structural alterations for T-cell stimulation. Overall, our data delineate a complex molecular mechanism combining spacer repositioning and ligand-induced conformational changes that, together with pocket intricacy, endows CD1b with the required molecular plasticity to present a broad range of structurally diverse antigens.

Fine tuning by human CD1e of lipid-specific immune responses.

CD1e is a member of the CD1 family that participates in lipid antigen presentation without interacting with the T-cell receptor. It binds lipids in lysosomes and facilitates processing of complex glycolipids, thus promoting editing of lipid antigens. We find that CD1e may positively or negatively affect lipid presentation by CD1b, CD1c, and CD1d. This effect is caused by the capacity of CD1e to facilitate rapid formation of CD1-lipid complexes, as shown for CD1d, and also to accelerate their turnover. Similar results were obtained with antigen-presenting cells from CD1e transgenic mice in which lipid complexes are assembled more efficiently and show faster turnover than in WT antigen-presenting cells. These effects maximize and temporally narrow CD1-restricted responses, as shown by reactivity to Sphingomonas paucimobilis-derived lipid antigens. CD1e is therefore an important modulator of both group 1 and group 2 CD1-restricted responses influencing the lipid antigen availability as well as the generation and persistence of CD1-lipid complexes.

Crystal structure of human CD1e reveals a groove suited for lipid-exchange processes.

CD1e is the only human CD1 protein existing in soluble form in the late endosomes of dendritic cells, where it facilitates the processing of glycolipid antigens that are ultimately recognized by CD1b-restricted T cells. The precise function of CD1e remains undefined, thus impeding efforts to predict the participation of this protein in the presentation of other antigens. To gain insight into its function, we determined the crystal structure of recombinant CD1e expressed in human cells at 2.90-Å resolution. The structure revealed a groove less intricate than in other CD1 proteins, with a significantly wider portal characterized by a 2 Å-larger spacing between the α1 and α2 helices. No electron density corresponding to endogenous ligands was detected within the groove, despite the presence of ligands unequivocally established by native mass spectrometry in recombinant CD1e. Our structural data indicate that the water-exposed CD1e groove could ensure the establishment of loose contacts with lipids. In agreement with this possibility, lipid association and dissociation processes were found to be considerably faster with CD1e than with CD1b. Moreover, CD1e was found to mediate in vitro the transfer of lipids to CD1b and the displacement of lipids from stable CD1b-antigen complexes. Altogether, these data support that CD1e could have evolved to mediate lipid-exchange/editing processes with CD1b and point to a pathway whereby the repertoire of lipid antigens presented by human dendritic cells might be expanded.

Deciphering the role of CD1e protein in mycobacterial phosphatidyl-myo-inositol mannosides (PIM) processing for presentation by CD1b to T lymphocytes.

Lipids are important antigens that induce T cell-mediated specific immune responses. They are presented to T lymphocytes by a specific class of MHC-I like proteins, termed CD1. The majority of the described CD1-presented mycobacterial antigens are presented by the CD1b isoform. We previously demonstrated that the stimulation of CD1b-restricted T cells by the hexamannosylated phosphatidyl-myo-inositol (PIM(6)), a family of mycobacterial antigens, requires a prior partial digestion of the antigen oligomannoside moiety by α-mannosidase and that CD1e is an accessory protein absolutely required for the generation of the lipid immunogenic form. Here, we show that CD1e behaves as a lipid transfer protein influencing lipid immunoediting and membrane transfer of PIM lipids. CD1e selectively assists the α-mannosidase-dependent digestion of PIM(6) species according to their degree of acylation. Moreover, CD1e transfers only diacylated PIM from donor to acceptor liposomes and also from membranes to CD1b. This study provides new insight into the molecular mechanisms by which CD1e contributes to lipid immunoediting and CD1-restricted presentation to T cells.

Treatment of congenital thrombocytopenia and decreased collagen reactivity in G6b-B-deficient mice.

Mice lacking the immunoreceptor tyrosine-based inhibition motif-containing co-inhibitory receptor G6b-B (Mpig6b, G6b knockout, KO) are born with a complex megakaryocyte (MK) per platelet phenotype, characterized by severe macrothrombocytopenia, expansion of the MK population, and focal myelofibrosis in the bone marrow and spleen. Platelets are almost completely devoid of the glycoprotein VI (GPVI)-FcRγ-chain collagen receptor complex, have reduced collagen integrin α2ß1, elevated Syk tyrosine kinase activity, and a subset has increased surface immunoglobulins. A similar phenotype was recently reported in patients with null and loss-of-function mutations in MPIG6B. To better understand the cause and treatment of this pathology, we used pharmacological- and genetic-based approaches to rescue platelet counts and function in G6b KO mice. Intravenous immunoglobulin resulted in a transient partial recovery of platelet counts, whereas immune deficiency did not affect platelet counts or receptor expression in G6b KO mice. Syk loss-of-function (R41A) rescued macrothrombocytopenia, GPVI and α2ß1 expression in G6b KO mice, whereas treatment with the Syk kinase inhibitor BI1002494 partially rescued platelet count but had no effect on GPVI and α2ß1 expression or bleeding. The Src family kinase inhibitor dasatinib was not beneficial in G6b KO mice. In contrast, treatment with the thrombopoietin mimetic romiplostim rescued thrombocytopenia, GPVI expression, and platelet reactivity to collagen, suggesting that it may be a promising therapeutic option for patients lacking functional G6b-B. Intriguingly, GPVI and α2ß1 expression were significantly downregulated in romiplostim-treated wild-type mice, whereas GPVI was upregulated in romiplostim-treated G6b KO mice, suggesting a cell intrinsic feedback mechanism that autoregulates platelet reactivity depending on physiological needs.

Impact of TCR reactivity and HLA phenotype on naive CD8 T cell frequency in humans.

The impact of MHC phenotype on the shaping of the peripheral naive T cell repertoire in humans remains unknown. To address this, we compared the frequency and antigenic avidity of naive T cells specific for immunodominant self-, viral, and tumor Ags presented by a human MHC class I allele (HLA-A*02, referred to as A2) in individuals expressing or not this allele. Naive T cell frequencies varied from one Ag specificity to another but were restrained for a given specificity. Although A2-restricted T cells showed similar repertoire features and antigenic avidities in A2+ and A2- donors, A2 expression had either a positive, neutral, or negative impact on the frequency of A2-restricted naive CD8 T cells, depending on their fine specificity. We also identified in all donors CD4 T cells specific for A2/peptide complexes, whose frequencies were not affected by MHC class I expression, but nevertheless correlated with those of their naive CD8 T cell counterparts. Therefore, both selection by self-MHC and inherent TCR reactivity regulate the frequency of human naive T cell precursors. Moreover this study also suggests that T cell repertoire shaping by a given self-MHC allele is dispensable for generation of immunodominant T cell responses restricted by this particular allele.

Megakaryocytes form linear podosomes devoid of digestive properties to remodel medullar matrix.

Bone marrow megakaryocytes (MKs) undergo a maturation involving contacts with the microenvironment before extending proplatelets through sinusoids to deliver platelets in the bloodstream. We demonstrated that MKs assemble linear F-actin-enriched podosomes on collagen I fibers. Microscopy analysis evidenced an inverse correlation between the number of dot-like versus linear podosomes over time. Confocal videomicroscopy confirmed that they derived from each-other. This dynamics was dependent on myosin IIA. Importantly, MKs progenitors expressed the Tks4/5 adaptors, displayed a strong gelatinolytic ability and did not form linear podosomes. While maturing, MKs lost Tks expression together with digestive ability. However, those MKs were still able to remodel the matrix by exerting traction on collagen I fibers through a collaboration between GPVI, ß1 integrin and linear podosomes. Our data demonstrated that a change in structure and composition of podosomes accounted for the shift of function during megakaryopoiesis. These data highlight the fact that members of the invadosome family could correspond to different maturation status of the same entity, to adapt to functional responses required by differentiation stages of the cell that bears them.

Shared reactivity of V{delta}2(neg) {gamma}{delta} T cells against cytomegalovirus-infected cells and tumor intestinal epithelial cells.

Long-lasting expansion of Vdelta2(neg) gammadelta T cells is a hallmark of cytomegalovirus (CMV) infection in kidney transplant recipients. The ligands of these cells and their role remain elusive. To better understand their immune function, we generated gammadelta T cell clones from several transplanted patients. Numerous patient Vdelta1(+), Vdelta3(+), and Vdelta5(+) gammadelta T cell clones expressing diverse Vgamma chains, but not control Vgamma9Vdelta2(+) T clones, displayed strong reactivity against CMV-infected cells, as shown by their production of tumor necrosis factor-alpha. Vdelta2(neg) gammadelta T lymphocytes could also kill CMV-infected targets and limit CMV propagation in vitro. Their anti-CMV reactivity was specific for this virus among herpesviridae and required T cell receptor engagement, but did not involve major histocompatibility complex class I molecules or NKG2D. Vdelta2(neg) gammadelta T lymphocytes expressed receptors essential for intestinal homing and were strongly activated by intestinal tumor, but not normal, epithelial cell lines. High frequencies of CMV- and tumor-specific Vdelta2(neg) gammadelta T lymphocytes were found among patients' gammadelta T cells. In conclusion, Vdelta2(neg) gammadelta T cells may play a role in protecting against CMV and tumors, probably through mucosal surveillance of cellular stress, and represent a population that is largely functionally distinct from Vgamma9Vdelta2(+) T cells.

Leukodepletion Filters-Derived CD34+ Cells As a Cell Source to Study Megakaryocyte Differentiation and Platelet Formation.

The in vitro expansion and differentiation of human hematopoietic progenitors into megakaryocytes capable of elongating proplatelets and releasing platelets allows an in-depth study of the mechanisms underlying platelet biogenesis. Available culture protocols are mostly based on hematopoietic progenitors derived from bone marrow or cord blood raising a number of ethical, technical, and economic concerns. If there are already available protocols for obtaining CD34 cells from peripheral blood, this manuscript proposes a straightforward and optimized protocol for obtaining CD34+ cells from leukodepletion filters readily available in blood centers. These cells are isolated from leukodepletion filters used in the preparation of blood transfusion products, corresponding to eight blood donations. These filters are meant to be discarded. A detailed procedure to collect hematopoietic progenitors identified as CD34+ cells from these filters is described. The method to obtain mature megakaryocytes extending proplatelets while discussing their phenotypic evolution is also detailed. Finally, the protocol present a calibrated pipetting method, to efficiently release platelets that are morphologically and functionally similar to native ones. This protocol can serve as a basis for evaluating pharmacological compounds acting at various steps of the process to dissect the underlying mechanisms and approach the in vivo platelet yields.

Platelet FcγRIIA-induced serotonin release exacerbates the severity of transfusion-related acute lung injury in mice.

Transfusion-related acute lung injury (TRALI) remains a major cause of transfusion-related fatalities. The mechanism of human antibody-mediated TRALI, especially the involvement of the Fcγ receptors, is not clearly established. Contrary to mice, human platelets are unique in their expression of the FcγRIIA/CD32A receptor, suggesting that our understanding of the pathogenesis of antibody-mediated TRALI is partial, as the current murine models incompletely recapitulate the human immunology. We evaluated the role of FcγRIIA/CD32A in TRALI using a humanized mouse model expressing the FcγRIIA/CD32A receptor. When challenged with a recombinant chimeric human immunoglobulin G1/mouse anti-major histocompatibility complex class I monoclonal antibody, these mice exhibited exacerbated alveolar edema and higher mortality compared with wild-type (WT) mice. Unlike in WT mice, monocytes/macrophages in CD32A+ mice were accessory for TRALI initiation, indicating the decisive contribution of another cell type. Platelet activation was dramatically increased in CD32A+ animals, resulting in their increased consumption and massive release of their granule contents. Platelet depletion prevented the exacerbation of TRALI in CD32A+ mice but did not affect TRALI in WT animals. By blocking platelet serotonin uptake with fluoxetine, we showed that the severity of TRALI in CD32A+ mice resulted from the serotonin released by the activated platelets. Furthermore, inhibition of 5-hydroxytryptamine 2A serotonin receptor with sarpogrelate, before or after the induction of TRALI, abolished the aggravation of lung edema in CD32A+ mice. Our findings show that platelet FcγRIIA/CD32A activation exacerbates antibody-mediated TRALI and provide a rationale for designing prophylactic and therapeutic strategies targeting the serotonin pathway to attenuate TRALI in patients.

Control of the intracellular pathway of CD1e.

CD1e is a membrane-associated protein predominantly detected in the Golgi compartments of immature human dendritic cells. Without transiting through the plasma membrane, it is targeted to lysosomes (Ls) where it remains as a cleaved and soluble form and participates in the processing of glycolipidic antigens. The role of the cytoplasmic tail of CD1e in the control of its intracellular pathway was studied. Experiments with chimeric molecules demonstrated that the cytoplasmic domain determines a cellular pathway that conditions the endosomal cleavage of these molecules. Other experiments showed that the C-terminal half of the cytoplasmic tail mediates the accumulation of CD1e in Golgi compartments. The cytoplasmic domain of CD1e undergoes monoubiquitinations, and its ubiquitination profile is maintained when its N- or C-terminal half is deleted. Replacement of the eight cytoplasmic lysines by arginines results in a marked accumulation of CD1e in trans Golgi network 46+ compartments, its expression on the plasma membrane and a moderate slowing of its transport to Ls. Fusion of this mutated form with ubiquitin abolishes the accumulation of CD1e molecules in the Golgi compartments and restores the kinetics of their transport to Ls. Thus, ubiquitination of CD1e appears to trigger its exit from Golgi compartments and its transport to endosomes. This ubiquitin-dependent pathway may explain several features of the very particular intracellular traffic of CD1e in dendritic cells compared with other CD1 molecules.

The varicellovirus UL49.5 protein blocks the transporter associated with antigen processing (TAP) by inhibiting essential conformational transitions in the 6+6 transmembrane TAP core complex.

TAP translocates virus-derived peptides from the cytosol into the endoplasmic reticulum, where the peptides are loaded onto MHC class I molecules. This process is crucial for the detection of virus-infected cells by CTL that recognize the MHC class I-peptide complexes at the cell surface. The varicellovirus bovine herpesvirus 1 encodes a protein, UL49.5, that acts as a potent inhibitor of TAP. UL49.5 acts in two ways, as follows: 1) by blocking conformational changes of TAP required for the translocation of peptides into the endoplasmic reticulum, and 2) by targeting TAP1 and TAP2 for proteasomal degradation. At present, it is unknown whether UL49.5 interacts with TAP1, TAP2, or both. The contribution of other members of the peptide-loading complex has not been established. Using TAP-deficient cells reconstituted with wild-type and recombinant forms of TAP1 and TAP2, TAP was defined as the prime target of UL49.5 within the peptide-loading complex. The presence of TAP1 and TAP2 was required for efficient interaction with UL49.5. Using deletion mutants of TAP1 and TAP2, the 6+6 transmembrane core complex of TAP was shown to be sufficient for UL49.5 to interact with TAP and block its function. However, UL49.5-induced inhibition of peptide transport was most efficient in cells expressing full-length TAP1 and TAP2. Inhibition of TAP by UL49.5 appeared to be independent of the presence of other peptide-loading complex components, including tapasin. These results demonstrate that UL49.5 acts directly on the 6+6 transmembrane TAP core complex of TAP by blocking essential conformational transitions required for peptide transport.

The assembly of CD1e is controlled by an N-terminal propeptide which is processed in endosomal compartments.

CD1e displays unique features in comparison with other CD1 proteins. CD1e accumulates in Golgi compartments of immature dendritic cells and is transported directly to lysosomes, where it is cleaved into a soluble form. In these latter compartments, CD1e participates in the processing of glycolipid antigens. In the present study, we show that the N-terminal end of the membrane-associated molecule begins at amino acid 20, whereas the soluble molecule consists of amino acids 32-333. Thus immature CD1e includes an N-terminal propeptide which is cleaved in acidic compartments and so is absent from its mature endosomal form. Mutagenesis experiments demonstrated that the propeptide controls the assembly of the CD1e alpha-chain with beta(2)-microglobulin, whereas propeptide-deleted CD1e molecules are immunologically active. Comparison of CD1e cDNAs from different mammalian species indicates that the CD1e propeptide is conserved during evolution, suggesting that it may also optimize the generation of CD1e molecules in other species.

Rab11A controls the biogenesis of Birbeck granules by regulating Langerin recycling and stability.

The extent to which Rab GTPases, Rab-interacting proteins, and cargo molecules cooperate in the dynamic organization of membrane architecture remains to be clarified. Langerin, a recycling protein accumulating in the Rab11-positive compartments of Langerhans cells, induces the formation of Birbeck granules (BGs), which are membrane subdomains of the endosomal recycling network. We investigated the role of Rab11A and two members of the Rab11 family of interacting proteins, Rip11 and RCP, in Langerin traffic and the biogenesis of BGs. The overexpression of a dominant-negative Rab11A mutant or Rab11A depletion strongly influenced Langerin traffic and stability and the formation of BGs, whereas modulation of other Rab proteins involved in dynamic regulation of the endocytic-recycling pathway had no effect. Impairment of Rab11A function led to a missorting of Langerin to lysosomal compartments, but inhibition of Langerin degradation by chloroquine did not restore the formation of BGs. Loss of RCP, but not of Rip11, also had a modest, but reproducible effect on Langerin stability and BG biogenesis, pointing to a role for Rab11A-RCP complexes in these events. Our results show that Rab11A and Langerin are required for BG biogenesis, and they illustrate the role played by a Rab GTPase in the formation of a specialized subcompartment within the endocytic-recycling system.

Megakaryocytes use in vivo podosome-like structures working collectively to penetrate the endothelial barrier of bone marrow sinusoids.

BACKGROUND: Blood platelets are anucleate cell fragments that prevent bleeding and minimize blood vessel injury. They are formed from the cytoplasm of megakaryocytes located in the bone marrow. For successful platelet production, megakaryocyte fragments must pass through the sinusoid endothelial barrier by a cell biology process unique to these giant cells as compared with erythrocytes and leukocytes. Currently, the mechanisms by which megakaryocytes interact and progress through the endothelial cells are not understood, resulting in a significant gap in our knowledge of platelet production. OBJECTIVE: The aim of this study was to investigate how megakaryocytes interact and progress through the endothelial cells of mouse bone marrow sinusoids. METHODS: We used a combination of fluorescence, electron, and three-dimensional microscopy to characterize the cellular events between megakaryocytes and endothelial cells. RESULTS: We identified protrusive, F-actin-based podosome-like structures, called in vivo-MK podosomes, which initiate the formation of pores through endothelial cells. These structures present a collective and spatial organization through their interconnection via a contractile network of actomyosin, essential to regulate the endothelial openings. This ensures proper passage of megakaryocyte-derived processes into the blood circulation to promote thrombopoiesis. CONCLUSION: This study provides novel insight into the in vivo function of podosomes of megakaryocytes with critical importance to platelet production.

Cell surface expression of HLA I molecules as a marker of young platelets.

BACKGROUND: Accurate identification of the proportion of young platelets is important to distinguish peripheral thrombocytopenia from a deficit in platelet production. Young platelets are defined by their higher RNA content and are often assessed as thiazole orange bright (TObright ) by flow cytometry. In clinical practice, their proportion is estimated by automatic blood counter according to their greater RNA content, which identifies a so-called immature platelet fraction (IPF). However, the detected IPFs are not strictly identical to the young TObright platelet population observed by flow cytometry. OBJECTIVES: The aim of this study was to assess the reliability of HLA I/major histocompatibility I (MHC I) cell surface expression as a marker of young platelets. METHODS: The HLA I/MHC I expression was evaluated by flow cytometry after costaining blood with TO and antibodies directed against HLA I/MHC I molecules. RESULTS: We found that platelets with a higher expression of plasma membrane-localized MHC I molecules displayed an increased TO staining and a higher content in ribosomal P-antigen. Transfusion experiments in mice showed that the number of MHC I molecules expressed on the cell surface of young murine platelets decreased during platelet aging, reaching basal levels within 24 h. Finally, we demonstrated that for patients with thrombocytopenias, the identification of young platelets is better assessed by the flow cytometric determination of the level of HLA I expression than by TO staining or the use of hematological blood counter. CONCLUSION: Overall, our results highlight the relevance of MHC I/HLA I expression as a valuable parameter to identify young platelets.