Targeting of platelet integrin alphaIIbbeta3 determines systemic reaction and bleeding in murine thrombocytopenia regulated by activating and inhibitory FcgammaR

Novel Mouse Model for Studying Hemostatic Function of Human Platelets

OBJECTIVE

Platelets are critical to the formation of a hemostatic plug and the pathogenesis of atherothrombosis. Preclinical animal models, especially the mouse, provide an important platform to assess the efficacy and safety of antiplatelet drugs. However, these studies are limited by inherent differences between human and mouse platelets and the species-selectivity of many drugs. To circumvent these limitations, we developed a new protocol for the adoptive transfer of human platelets into thrombocytopenic nonobese diabetic/severe combined immune deficiency mice, that is, a model where all endogenous platelets are replaced by human platelets in mice accepting xenogeneic tissues. Approach and Results: To demonstrate the power of this new model, we visualized and quantified hemostatic plug formation and stability by intravital spinning disk confocal microscopy following laser ablation injury to the saphenous vein. Integrin α_IIbβ₃-dependent hemostatic platelet plug formation was achieved within ≈30 seconds after laser ablation injury in humanized platelet mice. Pretreatment of mice with standard dual antiplatelet therapy (Aspirin+Ticagrelor) or PAR1 inhibitor, L-003959712 (an analog of vorapaxar), mildly prolonged the bleeding time and significantly reduced platelet adhesion to the site of injury. Consistent with findings from clinical trials, inhibition of PAR1 in combination with dual antiplatelet therapy markedly prolonged bleeding time in humanized platelet mice.

CONCLUSIONS

We propose that this novel mouse model will provide a robust platform to test and predict the safety and efficacy of experimental antiplatelet drugs and to characterize the hemostatic function of synthetic, stored and patient platelets.

Characterization of a transgenic mouse model of chronic conditional platelet depletion

BACKGROUND

Platelets are widely recognized for their role in maintaining hemostasis. Recently, platelets have become appreciated for their varying roles in immunity, neuroprotection, and other physiological processes. While there are currently excellent methods to transiently deplete platelets and models of thrombocytopenia, studying the roles of platelets in chronic processes can be challenging.

OBJECTIVE

Phenotypic characterization of the PF4-DTR mouse model of conditional platelet depletion compared to antibody depletion.

METHODS

We describe the ability of the PF4-DTR mouse to maintain chronic platelet depletion, along with examining the bleeding phenotype compared to antibody-mediated platelet depletion.

RESULTS

Systemic administration of diphtheria toxin resulted in >99% platelet depletion that can be maintained for >2 weeks. When compared to an antibody depletion model, PF4-DTR mice showed similar phenotypes when challenged with tail bleed and saphenous vein measurements of hemostasis. Mice depleted with diphtheria toxin were also able to undergo adoptive transfer of platelets. If the frequency and amount of diphtheria toxin is reduced, mice can be maintained at >40% depletion for >28 days, showing that this model is tunable.

CONCLUSIONS

When compared to the gold standard of antibody-mediated depletion, PF4-DTR mice showed similar phenotypes and should be considered an important tool for examining the impact of thrombocytopenia over longer periods of time.

Maternal anti-platelet β3 integrins impair angiogenesis and cause intracranial hemorrhage

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a life-threatening disease in which intracranial hemorrhage (ICH) is the major risk. Although thrombocytopenia, which is caused by maternal antibodies against β3 integrin and occasionally by maternal antibodies against other platelet antigens, such as glycoprotein GPIbα, has long been assumed to be the cause of bleeding, the mechanism of ICH has not been adequately explored. Utilizing murine models of FNAIT and a high-frequency ultrasound imaging system, we found that ICH only occurred in fetuses and neonates with anti-β3 integrin-mediated, but not anti-GPIbα-mediated, FNAIT, despite similar thrombocytopenia in both groups. Only anti-β3 integrin-mediated FNAIT reduced brain and retina vessel density, impaired angiogenic signaling, and increased endothelial cell apoptosis, all of which were abrogated by maternal administration of intravenous immunoglobulin (IVIG). ICH and impairment of retinal angiogenesis were further reproduced in neonates by injection of anti-β3 integrin, but not anti-GPIbα antisera. Utilizing cultured human endothelial cells, we found that cell proliferation, network formation, and AKT phosphorylation were inhibited only by murine anti-β3 integrin antisera and human anti-HPA-1a IgG purified from mothers with FNAIT children. Our data suggest that fetal hemostasis is distinct and that impairment of angiogenesis rather than thrombocytopenia likely causes FNAIT-associated ICH. Additionally, our results indicate that maternal IVIG therapy can effectively prevent this devastating disorder.

Adoptive transfer method to study platelet function in mouse models of disease

Platelets play an important role in many physiological and pathological situations. However, the molecular mechanisms by which platelets contribute to health and disease are often ill-defined. One of the limiting factors to these studies is a fast but reliable method to generate animals with platelet-specific signaling defects. We here review recent approaches to establish an adoptive platelet transfer model in mice.

A novel canine model of immune thrombocytopenia: has immune thrombocytopenia (ITP) gone to the dogs?

Canine immune thrombocytopenia (ITP) is analogous to human ITP, with similar platelet counts and heterogeneity in bleeding phenotype among affected individuals. With a goal of ultimately investigating this bleeding heterogeneity, a canine model of antibody-mediated ITP was developed. Infusion of healthy dogs with 2F9, a murine IgG2a monoclonal antibody to the canine platelet glycoprotein GPIIb (a common target of autoantibodies in ITP) resulted in profound, dose-dependent thrombocytopenia. Model dogs developed variable bleeding phenotypes, e.g. petechiae and haematuria, despite similar degrees of thrombocytopenia. 2F9 infusion was not associated with systemic inflammation, consumptive coagulopathy, or impairment of platelet function. Unexpectedly however, evaluation of cytokine profiles led to the identification of platelets as a potential source of serum interleukin-8 (IL8) in dogs. This finding was confirmed in humans with ITP, suggesting that platelet IL8 may be a previously unrecognized modulator of platelet-neutrophil crosstalk. The utility of this model will allow future study of bleeding phenotypic heterogeneity including the role of neutrophils and endothelial cells in ITP.

Fcgamma receptor-mediated antigen uptake by lung DC contributes to allergic airway hyper-responsiveness and inflammation

During asthma, lung DC capture and process antigens to initiate and maintain allergic Th2 cell responses to inhaled allergens. The aim of the study was to investigate whether allergen-specific IgG, generated during sensitization, can potentiate the acute airway inflammation through Fcgamma receptor (FcgammaR)-mediated antigen uptake and enhance antigen presentation resulting in augmented T-cell proliferation. We examined the impact of antigen presentation and T-cell stimulation on allergic airway hyperresponsiveness and inflammation using transgenic and gene-deficient mice. Both airway inflammation and eosinophilia in bronchoalveolar lavage fluid were markedly reduced in sensitized and challenged FcgammaR-deficient mice. Lung DC of WT, but not FcgammaR-deficient mice, induced increased antigen-specific CD4+ T-cell proliferation when pulsed with anti-OVA IgG immune complexes. Intranasal application of anti-OVA IgG immune complexes resulted in enhanced airway inflammation, eosinophilia and Th2 cytokine release, mediated through enhanced antigen-specific T-cell proliferation in vivo. Finally, antigen-specific IgG in the serum of sensitized mice led to a significant increase of antigen-specific CD4+ T-cell proliferation induced by WT, but not FcgammaR-deficient, lung DC. We conclude that FcgammaR-mediated enhanced antigen presentation and T-cell stimulation by lung DC has a significant impact on inflammatory responses following allergen challenge in asthma.

Both FcgammaRIV and FcgammaRIII are essential receptors mediating type II and type III autoimmune responses via FcRgamma-LAT-dependent generation of C5a

FcgammaRIV is a relatively new IgG Fc receptor (FcgammaR) that is reported to contribute to the pathogenesis of autoimmune diseases, although its specific role in relation to FcgammaRIII, complement and IgG2 subclasses remains uncertain. Here we define FcgammaRIV on macrophages as a receptor for soluble IgG2a/b complexes but not for cellular bound IgG2a and show that simultaneous activation of FcgammaRIV and FcgammaRIII is critical to mediate certain type II/III autoimmune responses. FcgammaRIII-deficient mice display compensatory enhanced FcgammaRIV expression, are protected from lung inflammation after deposition of IgG complexes, and show reduced sensitivity to IgG2a/b-mediated hemolytic anemia, indicating that increased FcgammaRIV alone is not sufficient to trigger these diseases in the absence of FcgammaRIII. Importantly, however, blockade of FcgammaRIV is also effective in inhibiting phagocytosis and cytokine production in IgG2b-induced anemia and acute lung injury, processes that display a further dependence on C5a anaphylatoxin receptor. Using gene deletion and functional inhibition studies, we found that FcgammaRIII and FcgammaRIV are each essential to trigger an FcRgamma-linker for activation of T-cell-dependent signal that drives C5a production in the Arthus reaction. Together, the results demonstrate a combined requirement for FcgammaRIII and FcgammaRIV in autoimmune injury, and identify the linker for activation of T cells adaptor as an integral component of linked FcgammaR and C5a anaphylatoxin receptor activation to generate inflammation.

STIM1 is essential for Fcγ receptor activation and autoimmune inflammation

Fcγ receptors (FcγRs) on mononuclear phagocytes trigger autoantibody and immune complex–induced diseases through coupling the self-reactive immunoglobulin G (IgG) response to innate effector pathways, such as phagocytosis, and the recruitment of inflammatory cells. FcRγ-based activation is critical in the pathogenesis of these diseases, although the contribution of FcγR-mediated calcium signaling in autoimmune injury is unclear. Here we show that macrophages lacking the endoplasmic reticulum–resident calcium sensor, STIM1, cannot activate FcγR-induced Ca2+ entry and phagocytosis. As a direct consequence, STIM1 deficiency results in resistance to experimental immune thrombocytopenia and anaphylaxis, autoimmune hemolytic anemia, and acute pneumonitis. These results establish STIM1 as a novel and essential component of FcγR activation and also indicate that inhibition of STIM1-dependent signaling might become a new strategy to prevent or treat IgG-dependent immunologic diseases.

Cell-derived anaphylatoxins as key mediators of antibody-dependent type II autoimmunity in mice

Complement C5a, a potent anaphylatoxin, is a candidate target molecule for the treatment of inflammatory diseases, such as myocardial ischemia/reperfusion injury, RA, and the antiphospholipid syndrome. In contrast, up until now, no specific contribution of C5a and its receptor, C5aR, was recognized in diseases of antibody-dependent type II autoimmunity. Here we identify C5a as a novel key mediator of autoimmune hemolytic anemia (AIHA) and show that mice lacking C5aR are partially resistant to this IgG autoantibody-induced disease model. Upon administration of anti-erythrocyte antibodies, upregulation of activating Fcgamma receptors (FcgammaRs) on Kupffer cells, as observed in WT mice, was absent in C5aR-deficient mice, and FcgammaR-mediated in vivo erythrophagocytosis was impaired. Surprisingly, in mice deficient in FcgammaRI and FcgammaRIII, anti-erythrocyte antibody-induced C5 and C5a production was abolished, demonstrating the existence of a previously unidentified FcgammaR-mediated C5a-generating pathway. These results show that the development of a full-blown antibody-dependent autoimmune disease requires C5a--produced by and acting on FcgammaR--and may suggest therapeutic benefits of C5 and/or C5a/C5aR blockade in AIHA and other diseases closely related to type II autoimmune injury.

Fc receptors and their interaction with complement in autoimmunity

Genetic studies in mice indicate a crucial role for Fc receptors (FcR) in antibody-mediated autoimmune diseases. Like other immune regulatory receptor pairs, the FcR system is constituted by activating and inhibitory receptors that bind the same ligand, the Fc portion of Ig. Analyses of animal models have shown that the inhibitory Fc receptor, FcgammaRIIB can suppress antibody-mediated autoimmunity, whereas activating-type FcR, such as FcgammaRIII promote disease development. This review summarizes recent advances of FcR, as obtained from gene deletion studies in mice, and highlights the importance of factors that interact with FcR in autoimmunity. There is emerging evidence for an indispensable role of the complement component C5a in the regulation of FcR and the sensing of FcR-dependent effector cell responses. On the other hand, FcR might be alternatives to serum complement in the generation of C5a at sites of inflammation. Thus, FcR and complement interact with each other at the level of C5a by linking regulatory events with effector cell activities in autoimmunity. This connecting pathway is now proposed to be a promising new therapeutic target for the treatment of inflammation and autoimmune disease in both mice and humans.

Macrophages induce the inflammatory response in the pulmonary Arthus reaction through G alpha i2 activation that controls C5aR and Fc receptor cooperation

Complement and FcgammaR effector pathways are central triggers of immune inflammation; however, the exact mechanisms for their cooperation with effector cells and their nature remain elusive. In this study we show that in the lung Arthus reaction, the initial contact between immune complexes and alveolar macrophages (AM) results in plasma complement-independent C5a production that causes decreased levels of inhibitory FcgammaRIIB, increased levels of activating FcgammaRIII, and highly induced FcgammaR-mediated TNF-alpha and CXCR2 ligand production. Blockade of C5aR completely reversed such changes. Strikingly, studies of pertussis toxin inhibition show the essential role of G(i)-type G protein signaling in C5aR-mediated control of the regulatory FcgammaR system in vitro, and analysis of the various C5aR-, FcgammaR-, and G(i)-deficient mice verifies the importance of Galpha(i2)-associated C5aR and the FcgammaRIII-FcgammaRIIB receptor pair in lung inflammation in vivo. Moreover, adoptive transfer experiments of C5aR- and FcgammaRIII-positive cells into C5aR- and FcgammaRIII-deficient mice establish AM as responsible effector cells. AM lacking either C5aR or FcgammaRIII do not possess any such inducibility of immune complex disease, whereas reconstitution with FcgammaRIIB-negative AM results in an enhanced pathology. These data suggest that AM function as a cellular link of C5a production and C5aR activation that uses a Galpha(i2)-dependent signal for modulating the two opposing FcgammaR, FcgammaRIIB and FcgammaRIII, in the initiation of the inflammatory cascade in the lung Arthus reaction.

Cytokine-mediated regulation of activating and inhibitory Fcγ receptors in human monocytes

Fc gamma receptors (Fc gammaR) trigger inflammatory reactions in response to immunoglobulin-opsonized pathogens and antigen-antibody complexes. The coordinate expression of activating and inhibitory Fc gammaR ensures the homeostasis of immune complex-driven inflammatory responses. In this study, we used antibodies with preferential binding for activating Fc gammaRIIa and inhibitory Fc gammaRIIb receptors to investigate the expression and regulation of Fc gammaRII isoforms in human monocytes. Cross-linking of Fc gammaRIIa triggered phagocytosis and cytokine production. Cross-linking of Fc gammaRIIb was associated with phosphorylation of the immunoreceptor tyrosine-based inhibitory motif and with a marked reduction in monocyte effector functions. Our study revealed that tumor necrosis factor alpha (TNF-alpha), interleukin (IL)-10, and IL-13 altered the transcriptional activity of the Fc gammaRIIB promoter in transfected cell lines and skewed the balance of activating versus inhibitory Fc gammaR in human monocytes. TNF-alpha decreased the expression of inhibitory Fc gammaRIIb. IL-10 up-regulated all classes of Fc gammaR and induced alternative activation in monocytes, an effect that was synergistic with that of TNF-alpha. In contrast, IL-4 and IL-13, in combination with TNF-alpha, decreased the expression of activating Fc gammaR and markedly down-regulated Fc gammaR-mediated function. Our findings suggest that the cytokine milieu can induce changes in the relative expression of Fc gammaR with opposing function and thus, may regulate the amplitude of Fc gammaR-mediated uptake and inflammation.

Platelet function and immune response

Altered platelet function may cause abnormal bleeding tendency or thrombosis. The goal of this article is to provide insights for understanding how platelet functions are related to immune response. Autoantibodies and drug-induced platelet antibodies have been demonstrated to downregulate or enhance platelet function. Drug-induced immune thrombocytopenia is an important adverse effect of glycoprotein IIb/IIIa antagonists. Platelets respond to binding of glycoprotein IIb/IIIa by partial platelet activation. This includes conformational changes of glycoprotein IIb/IIIa. Membrane changes may expose immunogenic neoantigens capable of abnormally altering immune responses. The presence of drug-dependent antibodies in an unexpectedly high frequency compared with the frequency of overt thrombocytopenia has opened a model for further studies. These may include monitoring of antiplatelet immune responses when new platelet antagonists are developed and comparisons of specific immune responses in other acute thrombocytopenias, such as those induced by quinidine or heparin and that associated with gold therapy or in acute profound thrombocytopenia, which may follow vaccination with live attenuated viruses.

The contributions of the alpha 2 beta 1 integrin to vascular thrombosis in vivo

The alpha 2 beta 1 integrin serves as a receptor for collagens, laminin, and several other nonmatrix ligands. Many studies have suggested that the alpha 2 beta 1 integrin is a critical mediator of platelet adhesion to collagen within the vessel wall after vascular injury and that the interactions of the platelet alpha 2 beta 1 integrin with subendothelial collagen after vascular injury are required for proper hemostasis. We have used the alpha 2 beta 1 integrin-deficient mouse to evaluate the contributions of the alpha 2 beta 1 integrin in 2 in vivo models of thrombosis. Studies using a model of endothelial injury to the carotid artery reveal that the alpha 2 beta 1 integrin plays a critical role in vascular thrombosis at the blood-vessel wall interface under flow conditions. In contrast, the alpha 2 beta 1 integrin is not required for the formation of thrombi and pulmonary emboli following intravascular injection of collagen. Our results are the first to document a critical in vivo role for the alpha 2 beta 1 integrin in thrombus formation at the vessel wall under conditions of shear following vascular injury.