Passive Immune Thrombocytopenia Model Research Articles

Production of recombinant human IgG1 Fc with beneficial N-glycosylation pattern for anti-inflammatory activity using genome-edited chickens

Intravenous immunoglobulin (IVIG) is a plasma-derived polyclonal IgG used for treatment of autoimmune diseases. Studies show that α-2,6 sialylation of the Fc improves anti-inflammatory activity. Also, afucosylation of the Fc efficiently blocks FcγRIIIA by increasing monovalent affinity to this receptor, which can be beneficial for treatment of refractory immune thrombocytopenia (ITP). Here, we generated genome-edited chickens that synthesize human IgG1 Fc in the liver and secrete α-2,6 sialylated and low-fucosylated human IgG1 Fc (rhIgG1 Fc) into serum and egg yolk. Also, rhIgG1 Fc has higher affinity for FcγRIIIA than commercial IVIG. Thus, rhIgG1 Fc efficiently inhibits immune complex-mediated FcγRIIIA crosslinking and subsequent ADCC response. Furthermore, rhIgG1 Fc exerts anti-inflammatory activity in a passive ITP model, demonstrating chicken liver derived rhIgG1 Fc successfully recapitulated efficacy of IVIG. These results show that genome-edited chickens can be used as a production platform for rhIgG1 Fc with beneficial N-glycosylation pattern for anti-inflammatory activities.

A Novel Allogeneic Cell Therapy That Targets Pathogenic Autoantibodies for the Treatment of Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disorder characterized by low platelet counts (<100x10 9/L) and increased risk of bleeding. ITP is predominantly driven by immunoglobulin G (IgG)-autoantibodies directed against platelet surface antigens resulting in platelet destruction by the spleen and/or impaired platelet production by bone marrow megakaryocytes. Approximately 60% of ITP patients have measurable levels of anti-platelet autoantibodies in plasma and about 70% of them have autoantibodies directed against the fibrinogen receptor glycoprotein (GP) IIbIIIa.Current treatment strategies for ITP include non-specific immunosuppression (steroids, rituximab), inhibition of platelet clearance (immunoglobulins, splenectomy, anti-D immunoglobulin, and the Syk inhibitor fostamatinib) and stimulation of platelet production (thrombopoietin receptor agonists). In addition, therapeutics targeting the neonatal Fc receptor, responsible for IgG recycling, are under clinical investigation for ITP. Despite these treatment options, some patients with ITP are refractory or have inadequate responses to existing therapy.Here we describe a novel allogeneic cellular therapy, entitled platelet-like cells (PLC), that specifically targets pathogenic IgG for the treatment of thrombocytopenia in ITP. PLC are produced by differentiating a human induced pluripotent stem cell (hiPSC) line into megakaryocyte-like cells (MLC) which are further processed in a proprietary bioreactor that mimics the bone marrow environment and induces the release of anucleate PLC which are then cryopreserved. Analyses of PLC demonstrate sizes ranging between 65 nm and 10 μm. Expression of GPIIbIIIa protein as measured by enzyme-linked immunosorbent assay (ELISA) is evident in all particle sizes. PLC were administered intravenously (IV) into NOD-scid IL2Rgamma null (NSG) mice to evaluate their pharmacokinetics and biodistribution. PLC were rapidly cleared from the mouse circulation and predominantly distributed to the mouse liver where they colocalized primarily with Kupffer cells. PLC clearance through the liver was significantly inhibited by pre-treating mice with liposomes expressing phosphatidylserine (PS), indicating that one mechanism of PLC clearance is dependent on PS exposure on the PLC outer plasma membrane.The ability of PLC to bind anti-GPIIbIIIa autoantibodies was evaluated in an in vitro assay where PLC were incubated with ITP patient plasmas and subsequently removed by centrifugation and filtration. The concentration of anti-GPIIbIIIa autoantibodies remaining in the plasmas decreased following PLC treatment (96.6% ± 2.4%).The ability of PLC to clear anti-human GPIIbIIIa antibodies in circulation was evaluated by using a passive mouse model of ITP. Here, a fluorescently labeled mouse anti-human GPIIbIIIa monoclonal antibody (PAB-1) was dosed intravenously (IV) into NSG mice to achieve a circulating level of antibody comparable to those observed in ITP patients. The GPIIbIIIa antibody persisted in circulation for over 24 hours following a single dose in untreated mice and did not induce clearance of mouse platelets. Following administration of PLC in the passive ITP model, a decrease in antibody fluorescence in blood was observed within 2 minutes and full antibody clearance was observed at 3 hours post dosing. Furthermore, clearance of the antibody was observed to be both dose responsive and specific since PLC treatment did not affect the antibody concentration of an isotype matched control antibody. PLC activity in clearing the anti-GPIIbIIIa antibody was observed in all particle sizes. Fluorescence quantification of liver and spleen tissue demonstrated that PLC driven anti-GPIIbIIIa antibody clearance occurred preferentially in the liver. Taken together, these data indicate that by specifically binding and rapidly removing anti-platelet antibodies from circulation, PLC may disrupt the platelet degradation mechanisms underlying ITP pathogenesis and restore platelet counts. DisclosuresPatel-Hett: PlateletBio: Current Employment. Giannini: Platelet Biogenesis: Current Employment. Peters: Platelet Biogenesis: Current Employment. Dykstra: Platelet Biogenesis: Current Employment. Lehmann: Platelet Biogenesis: Current Employment. Russo: Platelet Biogenesis: Current Employment. Kohnke: Platelet Biogenesis: Current Employment. Carpenter: Platelet Biogenesis: Current Employment. Tomczak: Platelet Biogenesis: Current Employment. Lazarus: Platelet Biogenesis: Consultancy. Masuko: Platelet Biogenesis: Current Employment. Leung: Platelet Biogenesis: Current Employment. Meredith: Platelet Biogenesis: Current Employment. Pete: Platelet Biogenesis: Current Employment. Lee: Platelet Biogenesis: Current Employment. Falb: Platelet Biogenesis: Current Employment.

Erythrocyte-Specific Antibodies As a Potential New Therapeutic Modality in Autoimmune and Inflammatory Disorders Beyond Immune Thrombocytopenia (ITP)

ITP is an autoimmune bleeding disorder characterized by platelet destruction and suppression of platelet production. In the majority of patients, platelet destruction due to IgG antibodies reactive with platelet GPIIbIIIa results in platelet phagocytosis by phagocytic cells in the mononuclear phagocytic system. Anti-D is an effective first-line therapy for patients with ITP and was originally brought into practice based on the ability of anti-D to competitively inhibit opsonized-platelet clearance by phagocytic cells. Similar to anti-D, an RBC-specific monoclonal antibody (TER-119) is also able to inhibit platelet clearance in a murine model of passive ITP, also ostensibly due to RBC clearance and competitive inhibition of phagocytic cells. Here, we show that TER-119 only caused measurable anemia after 3 hours post-injection while its ability to ameliorate thrombocytopenia in the passive-murine ITP model was clearly evident by 30 minutes post-injection. Further, TER-119 mediated maximal anemia 4 days after its injection, yet displayed no measurable ameliorative effects in passive-ITP at this time point. Based on these observations, we hypothesized that anti-inflammatory activity, rather than simple competitive inhibition of opsonized-platelet clearance, might be involved in this mechanism. To test this hypothesis, the ability of TER-119 to successfully treat inflammatory diseases not involving antibody-mediated splenic cellular sequestration were examined. In the K/BxN serum-transfer model of arthritis, recipient C57BL/6 mice are injected with K/BxN serum and a rapidly progressing arthritis occurs due to antibodies specific to the self-protein glucose-6-phosphate isomerase. Using this model, the same dose of TER-119 (50 μg/mouse) which was effective in murine ITP could significantly prevent the induction of inflammatory arthritis. In separate experiments, mice were allowed to develop significant arthritis for 5 days and then injected with TER-119. These arthritic mice underwent a reversal in disease severity with significant and substantial decreases in joint swelling and clinical score evident 24 hours after injection of TER-119. In fact three days after injection with TER-119, clinical scores and ankle width measurements returned to baseline. The function of the Fc region was important in the anti-inflammatory activity of TER-119 as deglycosylation of the Fc region of the antibody (known to inhibit IgG Fc region effector functions) dramatically reduced anti-inflammatory function in the arthritis model. To determine if TER-119 could also ameliorate disease activity in an unrelated (non-autoimmune) inflammatory disease model, TER-119 was also examined in a murine model of transfusion-related acute lung injury (TRALI). TRALI is a syndrome of respiratory distress primarily triggered by leukocyte-reactive antibodies from blood product transfusions and is the leading cause of transfusion-related mortality. TRALI was induced by injecting mice with the anti-MHC class I antibody (34-1-2s) which induced a rapid decrease in body temperature and induced significant lung damage, assessed by the presence of pulmonary edema. Mice pretreated with TER-119 (50 μg) underwent a rescue from hypothermia and exhibited an almost complete inhibition of lung edema. We conclude that TER-119 has significant anti-inflammatory activity and suggest that RBC specific antibodies such as anti-D may have significant therapeutic potential in inflammatory disorders beyond just ITP. DisclosuresSemple:Novartis: Consultancy; Rigel: Consultancy; Amgen: Consultancy; UCB: Consultancy. Lazarus:CSL pharmaceuticals: Research Funding; Momenta pharmaceuticals: Honoraria; Rigel pharmaceuticals: Research Funding; St.Michael's Hosp/The Canadian Blood Services: Other: Patents on monoclonal antibodies as IVIg and anti-D replacements.

TNF-α Blockade Inhibits Pro-Inflammatory Monocytes/Macrophages in ITP

Primary immune thrombocytopenia (ITP) is an acquired autoimmune inflammatory bleeding disorder with elevated expression of tumor necrosis factor-α (TNF-α) (Pehlivan M et. al. Platelets 2011). In our current study, we isolated the CD14+ monocytes from ITP patients and healthy controls, and performed qPCR assay to test the expression of TNF-α. In ITP patients positive for anti-GPIIbIIIa antibody, monocytes expressed significantly higher levels of TNF-α than that in anti-GPIIbIIIa antibody negative patients or healthy controls, indicating that TNF-α may play an important role in anti-GPIIbIIIa antibody mediated Fc-dependent thrombocytopenia.Monocytes and macrophages are the major cell types involved in Fc-dependent platelet clearance in ITP. Monocytes can be divided into three groups based on their CD16 expressions, including the classical (CD14++CD16−), intermediate (CD14++CD16+) and non-classical (CD14+CD16++) monocytes. Macrophages consist of the classically activated M1 type (TNFα+, iNOS+) and the alternatively activated M2 type (CD206+). The CD16+ monocytes (intermediate and non-classical monocytes) and M1 macrophages are pro-inflammatory with increased TNF-α expression. In our current and previous studies, we discovered a deviation of the monocyte/macrophage system manifesting in an increased percentage of intermediate monocytes in the peripheral blood and elevated M1 macrophages in the spleens of ITP patients (Feng Q et. al. J Thromb Haemost 2017). We carried out in vitro blockade of TNF-α using the monoclonal TNF-α antibody in the peripheral blood mono-nuclear cells (PBMCs) of ITP patients and bone marrow-derived M1 macrophages induced by LPS and IFN-γ. We found that blockade of TNF-α could significantly decrease the percentage of CD16+ monocytes and lower the expression of M1 markers like iNOS, TNF, MCP1. We also isolated the classical monocytes from PBMCs and cultured with M-CSF to generate macrophages. Interestingly, CD16 could be expressed by the classical monocyte-derived macrophagesduring the cultivation, while intervention by TNF-α blockade could reduce the CD16 expression during the macrophage-induction process. This finding further indicates the pro-inflammatory role of TNF-a in the monocyte/macrophage system. Subsequently, we performed western blotting to investigate the mechanisms of TNF-α blockade in regulation of the balance of monocytes/macrophages using cultured bone marrow derived macrophages and found TNF-α blockade could induce autophagy in an Akt/PI3K/mTOR-dependent pathway.To determine the effect of TNF-α blockade on the function of monocytes/macrophages, we performed the phagocytosis assay using FITC-dextran. It was shown that TNF-α blockade could significantly dampen the phagocytic ability of monocyte-derived macrophages from ITP patients and M1 of RAW264.7 cell line. We also isolated the CD4+ T cells from ITP patients and labeled them with CFSE to test the function of monocyte-derived macrophages for stimulating T cell proliferation. The results showed that TNF-α blockade could significantly inhibit the stimulation of T cell proliferation by M1 macrophages.Finally, in the in vivo study, we established the murine passive ITP model by injecting anti-GPIIbIIIa antibody. We then performed flow cytometry to determine the subtypes of monocytes/macrophages after 7 days of injection. The ITP mice were treated with TNF-α antibody at 5mg/kg or isotype IgG control every other day. We observed that TNF-α blockade could ameliorate thrombocytopenia in the murine model by decreasing non-classical monocytes(ly6c-CD11b+CD115+) from peripheral blood and M1 macrophages(F4/80+TNFα+) from abdomen, liver and spleen. Additionally, we injected the DIR-labeled platelets into those ITP mice and found TNF-α blockade could reduce the rentention of platelets in spleen and liver. Our study showed that TNF-α blockade could decrease the antibody-mediated platelet destruction by restoring the monocyte/macrophage subsets in vitro and in vivo. Therefore, TNF-α blockade might be a promising therapeutic strategy for the management of ITP patients especially those positive for anti-GPIIbIIIa auto-antibody. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Anti-inflammatory activity of CD44 antibodies in murine immune thrombocytopenia is mediated by Fcγ receptor inhibition

Monoclonal immunoglobulin G (IgG) antibodies to CD44 (anti-CD44) are anti-inflammatory in numerous murine autoimmune models, but the mechanisms are poorly understood. Anti-CD44 anti-inflammatory activity shows complete therapeutic concordance with IV immunoglobulin (IVIg) in treating autoimmune disease models, making anti-CD44 a potential IVIg alternative. In murine immune thrombocytopenia (ITP), there is no mechanistic explanation for anti-CD44 activity, although anti-CD44 ameliorates disease similarly to IVIg. Here, we demonstrate a novel anti-inflammatory mechanism of anti-CD44 that explains disease amelioration by anti-CD44 in murine ITP. Macrophages treated with anti-CD44 in vitro had dramatically suppressed phagocytosis through FcγRs in 2 separate systems of IgG-opsonized platelets and erythrocytes. Phagocytosis inhibition by anti-CD44 was mediated by blockade of the FcγR IgG binding site without changing surface FcγR expression. Anti-CD44 of different subclasses revealed that FcγR blockade was specific to receptors that could be engaged by the respective anti-CD44 subclass, and Fc-deactivated anti-CD44 variants lost all FcγR-inhibiting activity. In vivo, anti-CD44 functioned analogously in the murine passive ITP model and protected mice from ITP when thrombocytopenia was induced through an FcγR that could be engaged by the CD44 antibody's subclass. Consistent with FcγR blockade, Fc-deactivated variants of anti-CD44 were completely unable to ameliorate ITP. Together, anti-CD44 inhibits macrophage FcγR function and ameliorates ITP consistent with an FcγR blockade mechanism. Anti-CD44 is a potential IVIg alternative and may be of particular benefit in ITP because of the significant role that FcγRs play in human ITP pathophysiology.

Antiplatelet antibody-induced thrombocytopenia does not correlate with megakaryocyte abnormalities in murine immune thrombocytopenia.

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder characterized by increased peripheral immune platelet destruction and megakaryocyte defects in the bone marrow. Although ITP was originally thought to be primarily due to antibody-mediated autoimmunity, it is now clear that T cells also play a significant role in the disease. However, the exact interplay between platelet destruction, megakaryocyte dysfunction and the elements of both humoral and cell-mediated immunity in ITP remains incompletely defined. While most studies have focused on immune platelet destruction in the spleen, an additional possibility is that the antiplatelet antibodies can also destroy bone marrow megakaryocytes. To address this, we negated the effects of T cells by utilizing an invivo passive ITP model where BALB/c mice were administered various anti-αIIb, anti-β3 or anti-GPIb antibodies or antisera and platelet counts and bone marrow megakaryocytes were enumerated. Our results show that after 24hours, all the different antiplatelet antibodies/sera induced variable degrees of thrombocytopenia in recipient mice. Compared with naïve control mice, however, histological examination of the bone marrow revealed that only 2 antibody preparations (mouse-anti-mouse β3 sera and an anti- αIIb monoclonal antibody (MWReg30) could affect bone marrow megakaryocyte counts. Our study shows that while most antiplatelet antibodies induce acute thrombocytopenia, the majority of them do not affect the number of megakaryocytes in the bone marrow. This suggests that other mechanisms may be responsible for megakaryocyte abnormalities seen during immune thrombocytopenia.

Unveiling the Regulatory Role of CD8+ T-Cells in the Pathogenesis and Effective Steroid Treatment in ITP

Background:Immune thrombocytopenia (ITP) is a common bleeding disorder. Autoantibodies against platelet GPIIbIIIa (integrin αIIbβ3, 70-80%) and GPIb-complex (20-40%) are considered to be the major mechanism leading to autologous platelet destruction. Recent studies demonstrated that in addition to autoantibodies, CD8+ cytotoxic T cells (CTLs) also contribute to thrombocytopenia, either through direct cytotoxicity against platelets or megakaryocytes. However, the roles of CD8+ regulatory T cells (Tregs) in ITP have not been adequately explored.Methods and Results: We developed the first animal models of steroid treatment in ITP, encompassing both the passive and active forms. In the passive model, we injected anti-β3 antibodies to induce transient antibody mediated thrombocytopenia. We found that a single intraperitoneal (IP) injection of steroids post-antibody injection was effective at rescuing platelet counts. We also adapted an active model of ITP whereby wild-type (WT) BALB/c mice were transfused with splenocytes from WT platelet immunized β3-/-mice. This model encompasses both antibody and cell-mediated ITP resulting in sustained thrombocytopenia. In this model, we found steroid treatment (prednisone and dexamethasone) administered daily either orally or through IP-injection were equally efficacious at ameliorating thrombocytopenia. Furthermore, immunophenotyping and cytokine analysis reveal a similar profile as reported of human ITP patients responsive to steroid treatments. Thus, successful steroid treatments in these animal models are representative of the therapeutic effects of steroid treatments seen in human ITP patients.To study the role of CD8+ T cells in the pathogenesis and response to steroid treatments in ITP, we depleted CD8+ T cells from splenocytes prior to its transfusion into WT mice. Unexpectedly,we found CD8+ T cell depleted splenocyte (lacking in CTL cells) engrafted mice had lower, but not higher, platelet counts and were less responsive to dexamethasone (DEX) treatment compared to non-depleted engrafted mice. Furthermore, in the passive ITP model, depletion of CD8+ T cells from mice prior to injection of anti-β3 antibodies resulted in more severe thrombocytopenia, compared with non-depleted mice. Conversely, transfusion of either antigen-primed CD8+ (isolated from immunized β3-/- splenocytes) or WT/β3-/- naïve CD8+ T cells alone was sufficient to rescue platelet counts and improve response to DEX in the passive ITP model. These results indicate for the first time that CD8+ T cells from both antigen-primed and naïve populations play a protective role in attenuating platelet clearance. In further support of these observations, we detected significant increased populations of both CTLs and CD8+ Tregs including, CD8+CD25+Foxp3+, CD8+CD103+, CD8+CD122+ and CD8+CD28- in the blood, and spleen of immunized β3-/- mice. Interestingly, the CD8+ Tregs populations were further increased while CTL population decreased following DEX treatment in the active ITP model. In vitro splenocyte cultures were used to explore putative regulatory mechanisms of CD8+ Tregs. It was found that antigen-primed CD8+ Tregs exerted significantly stronger inhibition CD4+ T- and CD19+ B cell proliferation, platelet apoptosis, and platelet associated IgG production in the presence of platelet antigens, while both antigen-primed and naïve CD8+ Tregs could effectively inhibit macrophage mediated phagocytosis of anti-β3 opsonized platelets.Conclusion: To the best of our knowledge, these are the first reported animal models of effective steroid treatment of ITP. Utilizing these models we uncovered a previously unidentified regulatory role of CD8+ T cells in both ITP and steroid treatment. The increased populations of various CD8+ Tregs following β3-/- immunization exerted a significant inhibitory function against other immune-cell mediated anti-platelet responses. In addition, therapeutic administration of both antigen-primed and naïve CD8+ T cells were able to rescue platelet counts in the passive ITP model. This suggests that CD8+ Treg may play a predominantly protective role in ITP. These data provides significant insights into the understanding of immunopathogenesis of ITP, which may be important in designing effective therapy including the potential usage of CD8+ Tregs as a cellular target in the treatment of ITP. DisclosuresNo relevant conflicts of interest to declare.

Megakaryocyte Abnormalities Occur In The Absence Of Cell-Mediated Immunity In a Murine Model Of Passive Immune Thrombocytopenia (ITP)

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder characterized by increased peripheral immune platelet destruction and megakaryocyte defects in the bone marrow. Although ITP was originally thought to be primarily due to humoral mediated autoimmunity it is now evident that T cells can also play a contributing role to the thrombocytopenia. In fact, the exact interplay between platelet destruction, megakaryocyte dysfunction, and the elements of both the humoral and cell mediated immune systems still remain incompletely defined. In murine passive models of ITP, the direct administration of anti-platelet antibodies can result in severe thrombocytopenia which is evident within 24 hours of injection. While most studies have focused on immune platelet destruction in the spleen, an additional possibility is that the anti-platelet antibody also has an effect on megakaryocytes. To unequivocally determine if antiplatelet antibodies have an effect on megakaryocytes in an in vivo model, BALB/c mice were intravenously administered 2 ug of an anti-GPIIbIIIa antibody (MReg30) or 50 uL of a high tittered anti-GPIIIa (anti-β3) serum from BALB/c GPIIIa (CD61) knockout mice immunized with wild type platelets. Platelet counts were assessed over time and the bone marrow and spleens were harvested for histological examination of megakaryocytes. Both preparations of antiplatelet antibodies significantly reduced platelet numbers within 1 day of antibody or serum administration. This thrombocytopenia could be rescued by administration of 2 g/kg of IVIg ip. Compared with naïve control mice, histological (H&E staining) examination of the bone marrow and spleens revealed that megakaryocytes were significantly increased in number and all exhibited abnormalities consistent with apoptosis e.g. pyknotic nuclei. IVIg administration completely prevented these megakaryocyte abnormalities. These results show that passively administered anti-platelet antibodies not only affect platelet counts but also significantly affect megakaryocyte physiology in the absence of cell mediated immunity. Disclosures:No relevant conflicts of interest to declare.

In Vivo Platelet Production Occurs from an Expanding Pool of Non-Apoptotic Megakaryocytes in a Mouse Model of Passive Immune Thrombocytopenia

AbstractAbstract 1214Immune thrombocytopenia (ITP) is the most common acquired thrombocytopenia in children. Typically, external triggers as infections or vaccinations cause the rise of antibodies that crossreact with antigens expressed on the platelet surface. These anti-platelet antibodies are mostly directed against glycoprotein complexes GPIIb/IIIa or GPIb/IX/V, resulting in an increased turnover of antibody-decorated platelets which are then sequestered by the reticuloendothelial system. Recently, it has been suggested that thrombocytopenia might also be due to an insufficient platelet production as serum of some patients with ITP can impair the maturation of CD34+ hematopoietic stem cells to bone marrow megakaryocytes (MKs) in vitro or abrogate the formation of proplatelets in an in vitro culture system. The accelerated platelet turnover demands the generation of platelets de novo. Bone marrow smears often reveal normal or slightly increased MKs, although they seem to be smaller and of altered morphology. However, very little is known about the consequences of anti-platelet antibodies on bone marrow MKs in vivo and in situ. Here, we took advantage of a simple animal model of passive ITP by single or multiple intraperitoneal injections of an anti-GPIb antibody into mice. MKs were evaluated by multi-color immunofluorescence histology on whole femur sections in a modified staining procedure that bypasses decalcification. MK numbers on day 3 were doubled in response to a single injection and tripled on day 8 when mice were injected additionally on day 3 and 7. In these mice platelet counts were up to 2000/nL on day 10, indicating the power to produce platelets. MK area per section was transiently upregulated on day 3 in single injected mice and quadrupled after multiple injections on day 8 before shrinking below norm on day 14. Staining with an anti-rat IgG antibody showed that the antibody was present on MKs within the bone marrow several hours to days after injection. The signal was present for 5 days and no antibody was detected on day 7. MKs had an overall normal morphology and showed no signs of apoptosis or DNA blebbing. All MKs analyzed were negative for TdT in a classical TUNEL assay, indicating that there were no single strand breaks. As platelet counts rose markedly while the antibody was still present on the MK surface, we sought to identify whether the pool of MKs is expanded or formed de novo. To address this, mice where fed with nucleotide analogue EdU for up to 12 days and femur sections stained with Click-It-647 reagent to stain for newly incorporated DNA while mice were treated with anti-platelet antibody or isotype control. We found EdU-positive MKs after 12 days in control isotype-injected mice indicating the de novo formation from hematopoietic stem cells. In antibody-injected mice, newly formed MKs were negative or stained weakly for EdU on day 12, suggesting that they arise partially from an existing pool of progenitors. Finally, we analyzed platelet formation in vivo by imaging of the cranial bone marrow of GPIIb-eYFP-heterozygous mice. The depletion antibody was labeled with Atto-590-fluorophore and injected hours before imaging. Vasculature was counterstained by Quantum dots. We found that MKs residing at the bone marrow were decorated with the antibody and released pre- and proplatelets into the vasculature, indicating that platelet biogenesis can occur in the presence of anti-platelet antibodies on MKs. Our data thus provide novel insight into the pathomechanism of platelet production in patients with ITP. Disclosures:No relevant conflicts of interest to declare.

Enrichment of Sialylated IgG by Lectin Fractionation Does Not Enhance the Efficacy of Immunoglobulin G in a Murine Model of Immune Thrombocytopenia

Intravenous immunoglobulin G (IVIg) is widely used against a range of clinical symptoms. For its use in immune modulating therapies such as treatment of immune thrombocytopenic purpura high doses of IVIg are required. It has been suggested that only a fraction of IVIg causes this anti immune modulating effect. Recent studies indicated that this fraction is the Fc-sialylated IgG fraction. The aim of our study was to determine the efficacy of IVIg enriched for sialylated IgG (IVIg-SA (+)) in a murine model of passive immune thrombocytopenia (PIT). We enriched IVIg for sialylated IgG by Sambucus nigra agglutinin (SNA) lectin fractionation and determined the degree of sialylation. Analysis of IVIg-SA (+) using a lectin-based ELISA revealed that we enriched predominantly for Fab-sialylated IgG, whereas we did not find an increase in Fc-sialylated IgG. Mass spectrometric analysis confirmed that Fc sialylation did not change after SNA lectin fractionation. The efficacy of sialylated IgG was measured by administering IVIg or IVIg-SA (+) 24 hours prior to an injection of a rat anti-mouse platelet mAb. We found an 85% decrease in platelet count after injection of an anti-platelet mAb, which was reduced to a 70% decrease by injecting IVIg (p<0.01). In contrast, IVIg-SA (+) had no effect on the platelet count. Serum levels of IVIg and IVIg-SA (+) were similar, ruling out enhanced IgG clearance as a possible explanation. Our results indicate that SNA lectin fractionation is not a suitable method to enrich IVIg for Fc-sialylated IgG. The use of IVIg enriched for Fab-sialylated IgG abolishes the efficacy of IVIg in the murine PIT model.

Steroids Are Less Effective In Treating Thrombocytopenia Caused by Immune Responses Against Platelet GPIbα: A Comparative Study Using Passive and Active ITP Models.

AbstractAbstract 1433 Introduction:Immune thrombocytopenia (ITP) is a common autoimmune disorder in which autoantibodies are generated against a patient's platelets, leading to decreased platelet counts and bleeding diatheses. Autoantibodies in ITP mainly target platelet receptors GPIIbIIIa (αIIbβ3 integrin) and GPIbα. Current first-line therapy for ITP patients is steroids, while intravenous immunoglobulin (IVIG) is a common second-line therapy. However, not all patients are responsive and we lack useful exclusion criteria for steroid therapy. Appropriate identification of non-responsive patients would facilitate proper treatment and limit side effects. We previously showed anti-GPIbα (versus anti-GPIIbIIIa) mediated ITP is less responsive to IVIG (Blood. 2006;108(3):943-6). Our preliminary data from human patients also suggests that patients with anti-GPIbα antibodies are less responsive to steroids. However, these data need to be confirmed and mechanisms remain to be elucidated. Methods:To examine whether antibody specificity (GPIIbIIIa or GPIbα) correlates with responsiveness to steroids and determine the target immune cells for steroid action, we used two murine ITP models (passive antibody transfer model and active splenocyte transfer model (Blood. 2010;115:1247-53)). In both models, mice are treated with the corticosteroid Dexamethasone (DEX) and platelet counts are monitored. In the passive antibody transfer model: BALB/c or C57 mice were pretreated for 3 days with 0.1mg/kg, 1mg/kg, or 10 mg/kg of DEX prior to injection with our newly developed mouse-anti-mouse monoclonal antibodies (mAbs) targeting GPIIbIIIa or GPIbα. A second group of mice were treated with DEX 1 day following mAb injection (i.e. after becoming thrombocytopenic) and daily thereafter. In both groups, platelet counts were monitored for 4 consecutive days post mAb injection. In the active splenocyte transfer model: β3-/- and GPIbα-/- mice were immunized with wild-type (WT) platelets resulting in an antibody titer of >1:12,800. Purified splenocytes from immunized mice were transferred into syngeneic WT or SCID recipients, such that they developed ITP. Recipient mice were treated with 1mg/kg or 10 mg/kg of DEX starting 1 day after splenocyte transfer, and daily thereafter. Platelet counts were monitored. Results and Discussion:In the passive ITP model, irrespective of DEX dose, route of administration (i.p. or s.c.) or mouse strain, pretreatment with DEX was unable to ameliorate ITP induced by either anti-GPIIbIIIa or anti-GPIbα mAbs. In addition to pretreating with DEX, administration of DEX following ITP induction also appeared to be unable to ameliorate ITP induced by either anti-GPIIbIIIa or anti-GPIbα mAbs. Similar results were obtained in the passive model when using the corticosteroid Prednisolone in a different experimental setting. In the active splenocyte transfer model, we found that varying doses of DEX ameliorated ITP in mice engrafted with anti-GPIIbIIIa reactive splenocytes while untreated mice remained thrombocytopenic. Importantly, DEX failed to show a significant effect on ITP amelioration in mice engrafted with anti-GPIbα reactive splenocytes. Since the passive model involves exogenous administration of mAb, the effect of steroid treatment is largely limited to effects on the reticuloendothelial system (RES) and destruction of opsonized platelets. Conversely, the splenocyte transfer model allows for steroid effects on active immune responses (i.e. it may affect dendritic cells, T cells, B cells, etc.) in addition to the RES. As DEX was efficacious in only the splenocyte transfer model, this suggests steroids primarily affect upstream of the immune responses in ITP. Ongoing experiments involving depletion of CD4+, CD8+, or CD19+ splenocytes will help elucidate specific immune cells as targets of DEX. Conclusions:Our data clearly demonstrated that DEX ameliorated anti-GPIIbIIIa mediated ITP but was not significantly effective for amelioration of anti-GPIbα mediated ITP in the active splenocyte transfer model. We also demonstrated that steroid therapy failed in our passive ITP model. This finding is consistent with our preliminary data in human patients (patients with anti-GPIbα antibodies may be refractory to both IVIG and steroids). This may explain the clinical variability seen in response to steroids and lead to new diagnostic/therapeutic approaches. Disclosures:No relevant conflicts of interest to declare.

The Role of Recipient Platelets In the Prevention of Antibody-Mediated Transfusion Related Acute Lung Injury (TRALI)

AbstractAbstract 3351Transfusion related acute lung injury (TRALI) is a serious complication of transfusion. The pathogenesis of TRALI is not fully understood but previous findings have suggested that platelet depletion can protect mice in a two-hit model of TRALI (Looney et al J Clin Invest 119:3450, 2009). To further understand the role of platelets in preventing antibody-mediated TRALI, two mouse models of immune thrombocytopenia (ITP) were utilized. In the passive ITP model, SCID mice were injected with a monoclonal anti-platelet antibody (MWReg30) intraperitoneally (ip, 18 h before TRALI induction) or intravenously (iv, 2 h before TRALI induction). In the active ITP model, SCID mice were transferred with splenocytes from anti-CD61 immune GPIIIa-knockout mice and thrombocytopenia occurred within 2 weeks post transfer (Chow et al Blood 115;1247, 2010). TRALI induction was performed by injecting the various thrombocytopenic SCID mice with a murine monoclonal MHC class I antibody (mAb, 34-1 -2s) iv and several parameters were observed for up to 2 h post antibody injection. In control, non-thrombocytopenic SCID mice, 34-1 -2s injection caused severe systemic shock as noted by reduced rectal temperatures which was associated with significant lung damage and mortality (45%) within 1 hour of 34-1 -2s infusion as previously shown (Fung et al. Blood DOI 10.1182/blood-2010-05-284570). In contrast, while SCID mice depleted of platelets by the passive ip route had systemic shock, lung damage and a 60% mortality rate, those mice made thrombocytopenic by the iv route were completely protected from mortality. On the other hand, in the active ITP model, where the induced thrombocytopenia is associated with a proinflammatory anti-platelet immune response, no mortality was observed in those mice made thrombocytopenic by antibody-mediated immune mechanisms whereas 80% of mice rendered thrombocytopenic by CD8+ T cell-mediated immunity were dead within 1 hr post 34-1 -2s infusion. These results suggest that thrombocytopenia in itself does not protect against antibody-mediated TRALI severity but the nature of the thrombocytopenia induction (e.g. acute passive iv infusion or active ITP immune transfer) is important. In fact, depending on the inflammatory milieu associated with the thrombocytopenia, platelets may actually increase the severity of TRALI. Disclosures:No relevant conflicts of interest to declare.

Thrombopoietic effect of VPAC1 inhibition during megakaryopoiesis

Megakaryocytes and platelets express the stimulatory G protein (Gs)-coupled VPAC1 receptor, for which the pituitary adenylyl cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP) are agonists. The neuropeptide PACAP and VPAC1 were previously found to negatively regulate megakaryopoiesis, and inhibition of their physiological pathway was found to have a thrombopoietic effect in conditions where megakaryopoiesis and thrombopoiesis were impaired, such as chemotherapy-induced thrombocytopenia and congenital thrombocytopenia. The present study explored the thrombopoietic effect of VPAC1 inhibition in a murine model of syngeneic bone marrow transplantation (BMT) and in passive immune thrombocytopenia. Treatment of donor mice with a neutralizing anti-VPAC1 antibody stimulated the initial, most critical recovery of the platelets in irradiated mice. In the passive immune thrombocytopenia model, we observed a thrombopoietic effect, resulting in a less severe platelet drop after induction of their removal in the spleen by an anti-platelet antibody. We concluded that inhibition of the physiological PACAP/VPAC1 pathway could stimulate in vivo megakaryopoiesis. This inhibition can be applied to attenuate thrombocytopenia in conditions where platelets are destroyed as the major pathogenetic mechanism, e.g. immune thrombocytopenia purpura, or need to be produced de novo, e.g. after irradiation and BMT.