Animal model of fetal and neonatal immune thrombocytopenia: role of neonatal Fc receptor in the pathogenesis and therapy

Current Insights Into K-associated Fetal Anemia and Potential Treatment Strategies for Sensitized Pregnancies

K-associated anemic disease of the fetus and newborn (K-ADFN) is a rare but life-threatening disease in which maternal alloantibodies cross the placenta and can mediate an immune attack on fetal red blood cells expressing the K antigen. A considerably more common disease, D-associated hemolytic disease of the fetus and newborn (D-HDFN), can be prophylactically treated using polyclonal α-D antibody preparations. Currently, no such prophylactic treatment exists for K-associated fetal anemia, and disease is usually treated with intrauterine blood transfusions. Here we review current understanding of the biology of K-associated fetal anemia, how the maternal immune system is sensitized to fetal red blood cells, and what is understood about potential mechanisms of prophylactic HDFN interventions. Given the apparent challenges associated with preventing alloimmunization, we highlight novel strategies for treating sensitized mothers to prevent fetal anemia that may hold promise not only for K-mediated disease, but also for other pathogenic alloantibody responses.

The therapeutic age of the neonatal Fc receptor

IgGs are essential soluble components of the adaptive immune response that evolved to protect the body from infection. Compared with other immunoglobulins, the role of IgGs is distinguished and enhanced by their high circulating levels, long half-life and ability to transfer from mother to offspring, properties that are conferred by interactions with neonatal Fc receptor (FcRn). FcRn binds to the Fc portion of IgGs in a pH-dependent manner and protects them from intracellular degradation. It also allows their transport across polarized cells that separate tissue compartments, such as the endothelium and epithelium. Further, it is becoming apparent that FcRn functions to potentiate cellular immune responses when IgGs, bound to their antigens, form IgG immune complexes. Besides the protective role of IgG, IgG autoantibodies are associated with numerous pathological conditions. As such, FcRn blockade is a novel and effective strategy to reduce circulating levels of pathogenic IgG autoantibodies and curtail IgG-mediated diseases, with several FcRn-blocking strategies on the path to therapeutic use. Here, we describe the current state of knowledge of FcRn-IgG immunobiology, with an emphasis on the functional and pathological aspects, and an overview of FcRn-targeted therapy development.

Dual roles of fucoidan-GPIbα interaction in thrombosis and hemostasis: implications for drug development targeting GPIbα

BACKGROUND

Platelet GPIbα-von Willebrand factor (VWF) interaction initiates platelet adhesion, activation, and thrombus growth, especially under high shear conditions. Therefore, the GPIb-VWF axis has been suggested as a promising target against arterial thrombosis. The polysaccharide fucoidan has been reported to have opposing prothrombotic and antithrombotic effects; however, its binding mechanism with platelets has not been adequately studied.

OBJECTIVE

The objective of this study was to explore the mechanism of fucoidan and its hydrolyzed products in thrombosis and hemostasis.

METHODS

Natural fucoidan was hydrolyzed by using hydrochloric acid and was characterized by using size-exclusion chromatography, UV-visible spectroscopy, and fluorometry techniques. The effects of natural and hydrolyzed fucoidan on platelet aggregation were examined by using platelets from wild-type, VWF and fibrinogen-deficient, GPIbα-deficient, and IL4Rα/GPIbα-transgenic and αIIb-deficient mice and from human beings. Platelet activation markers (P-selectin expression, PAC-1, and fibrinogen binding) and platelet-VWF A1 interaction were measured by using flow cytometry. GPIbα-VWF A1 interaction was evaluated by using enzyme-linked immunosorbent assay. GPIb-IX-induced signal transduction was detected by using western blot. Heparinized whole blood from healthy donors was used to test thrombus formation and growth in a perfusion chamber.

RESULTS

We found that GPIbα is critical for fucoidan-induced platelet activation. Fucoidan interacted with the extracellular domain of GPIbα and blocked its interaction with VWF but itself could lead to GPIbα-mediated signal transduction and, subsequently, αIIbβ3 activation and platelet aggregation. Conversely, low-molecular weight fucoidan inhibited GPIb-VWF-mediated platelet aggregation, spreading, and thrombus growth at high shear.

CONCLUSION

Fucoidan-GPIbα interaction may have unique therapeutic potential against bleeding disorders in its high-molecular weight state and protection against arterial thrombosis by blocking GPIb-VWF interaction after fucoidan is hydrolyzed.

Fetal and neonatal alloimmune thrombocytopenia: Current pathophysiological insights and perspectives for future diagnostics and treatment

FNAIT is a pregnancy-associated condition caused by maternal alloantibodies against paternally-inherited platelet antigens, most frequently HPA-1a on integrin β3. The clinical effects range from no symptoms to fatal intracranial hemorrhage, but underlying pathophysiological determinants are poorly understood. Accumulating evidence suggests that differential antibody-Fc-glycosylation, activation of complement/effector cells, and integrin function-blocking effects contribute to clinical outcome. Furthermore, some antibodies preferentially bind platelet integrin αIIbβ3, but others bind αvβ3 on endothelial cells and trophoblasts. Defects in endothelial cells and angiogenesis may therefore contribute to severe anti-HPA-1a associated FNAIT. Moreover, anti-HPA-1a antibodies may cause placental damage, leading to intrauterine growth restriction. We discuss current insights into diversity and actions of HPA-1a antibodies, gathered from clinical studies, in vitro studies, and mouse models. Assessment of all factors determining severity and progression of anti-HPA-1a-associated FNAIT may importantly improve risk stratification and potentially reveal novel treatment strategies, both for FNAIT and other immunohematological disorders.

Chronic Inflammatory Placental Disorders Associated With Recurrent Adverse Pregnancy Outcome

Chronic inflammatory placental disorders are a group of rare but devastating gestational syndromes associated with adverse pregnancy outcome. This review focuses on three related conditions: villitis of unknown etiology (VUE), chronic histiocytic intervillositis (CHI) and massive perivillous fibrin deposition (MPFD). The hallmark of these disorders is infiltration of the placental architecture by maternal immune cells and disruption of the intervillous space, where gas exchange between the mother and fetus occurs. Currently, they can only be detected through histopathological examination of the placenta after a pregnancy has ended. All three are associated with a significant risk of recurrence in subsequent pregnancies. Villitis of unknown etiology is characterised by a destructive infiltrate of maternal CD8+ T lymphocytes invading into the chorionic villi, combined with activation of fetal villous macrophages. The diagnosis can only be made when an infectious aetiology has been excluded. VUE becomes more common as pregnancy progresses and is frequently seen with normal pregnancy outcome. However, severe early-onset villitis is usually associated with fetal growth restriction and recurrent pregnancy loss. Chronic histiocytic intervillositis is characterised by excessive accumulation of maternal CD68+ histiocytes in the intervillous space. It is associated with a wide spectrum of adverse pregnancy outcomes including high rates of first-trimester miscarriage, severe fetal growth restriction and late intrauterine fetal death. Intervillous histiocytes can also accumulate due to infection, including SARS-CoV-2, although this infection-induced intervillositis does not appear to recur. As with VUE, the diagnosis of CHI requires exclusion of an infectious cause. Women with recurrent CHI and their families are predisposed to autoimmune diseases, suggesting CHI may have an alloimmune pathology. This observation has driven attempts to prevent CHI with a wide range of maternal immunosuppression. Massive perivillous fibrin deposition is diagnosed when >25% of the intervillous space is occupied by fibrin, and is associated with fetal growth restriction and late intrauterine fetal death. Although not an inflammatory disorder per se, MPFD is frequently seen in association with both VUE and CHI. This review summarises current understanding of the prevalence, diagnostic features, clinical consequences, immune pathology and potential prophylaxis against recurrence in these three chronic inflammatory placental syndromes.

Successful prenatal therapy for anti-CD36-mediated severe FNAIT by deglycosylated antibodies in a novel murine model

Recent studies have shown that maternal anti-CD36 antibodies represent a frequent cause of fetal/neonatal alloimmune thrombocytopenia (FNAIT) in Asian and African populations. However, little is known about the pathomechanism and antenatal treatment of anti-CD36-mediated FNAIT. Here, we established a novel animal model to examine the clinical features of pups from immunized Cd36-/- female mice after breeding with wild-type male mice. Mild thrombocytopenia was observed, but high pup mortality was also documented (40.26%). Administration of intravenous immunoglobulin (IVIG) (1 g/kg) on days 7, 12, and 17 to immunized Cd36-/- mothers after breeding reduced fetal death (12.70%). However, delaying the IVIG administration series on days 10, 15, and 20 did not reduce fetal death (40.00%). In contrast, injection of deglycosylated anti-CD36 (deg-anti-CD36) polyclonal antibodies (5 mg/kg) on days 10, 15, and 20 significantly reduced fetal death (5.26%). Subsequently, monoclonal antibodies (mAbs) against mouse CD36 were developed, and one clone producing high-affinity anti-CD36 (termed 32-106) effectively inhibited maternal antibody binding and was therefore selected. Using the same approach of deg-anti-CD36, the administration of deg-32-106 significantly reduced fetal death (2.17%). Furthermore, immunized Cd36-/- mothers exhibited placental deficiency. Accordingly, maternal anti-CD36 antibodies inhibited angiogenesis of placenta endothelial cells, which could be restored by deg-32-106. In summary, maternal anti-CD36 antibodies caused a high frequency of fetal death in our animal model, associated with placental dysfunction. This deleterious effect could be diminished by the antenatal administration of IVIG and deg-mAb 32-106. Interestingly, treatment with deg-32-106 seems more beneficial considering the lower dose, later start of treatment, and therapy success.

Blocking Placental Class G Immunoglobulin Transfer Prevents NMDA Receptor Antibody Effects in Newborn Mice

BACKGROUND AND OBJECTIVES

To determine in a mouse model whether neonatal Fc receptor (FcRn) blockade prevents the placental transfer of class G immunoglobulin (IgG) derived from patients with anti-NMDA receptor (NMDAR) encephalitis and their pathogenic effects on the fetuses and offspring.

METHODS

Pregnant C57BL/6J mice were administered via tail vein FcRn antibody (FcRn-ab) or saline solution 6 hours before administration of patients' or controls' IgG on days 14, 15, and 16 of gestation. Three experimental groups were established: mice receiving controls' IgG, patients' IgG, or patients' IgG along with pretreatment with FcRn-ab. Immunohistochemical staining, confocal microscopy, hippocampal long-term potentiation, and standardized developmental and behavioral tasks were used to assess the efficacy of treatment with FcRn-ab.

RESULTS

In pregnant mice that received patients' IgG, treatment with FcRn-ab prevented the IgG from reaching the fetal brain, abrogating the decrease of NMDAR clusters and the reduction of cortical plate thickness that were observed in fetuses from untreated pregnant mice. Moreover, among the offspring of mothers that received patients' IgG, those whose mothers were treated with FcRn-ab did not develop the alterations that occurred in offspring of untreated mothers, including impairment in hippocampal plasticity, delay in innate reflexes, and visuospatial memory deficits.

DISCUSSION

FcRn blockade prevents placental transfer of IgG from patients with anti-NMDAR encephalitis and abrogates the synaptic and neurodevelopmental alterations caused by patients' antibodies. This model has potential therapeutic implications for other antibody-mediated diseases of the CNS during pregnancy.

Preclinical evaluation of immunotherapeutic regimens for fetal/neonatal alloimmune thrombocytopenia

Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a life-threatening bleeding disorder caused by maternal antibodies directed against paternally inherited antigens present on the surface of fetal platelets. The human platelet alloantigen HPA-1a (formerly known as the PlA1 alloantigen), is the most frequently implicated HPA for causing FNAIT in Whites. A single Leu33Pro amino acid polymorphism residing within the ∼50-amino-acid plexin-semaphorin-integrin domain near the N-terminus of the integrin β3 subunit (platelet membrane glycoprotein IIIa [GPIIIa]) is responsible for generating the HPA-1a and HPA-1b epitopes in human GPIIIa and serves as the central target for alloantibody-mediated platelet destruction. To simulate the etiology of human FNAIT, wild-type female mice were pre-immunized with platelets derived from transgenic mice engineered to express the human HPA-1a epitope on a murine GPIIIa backbone. These mice developed a strong alloimmune response specific for HPA-1a, and when bred with HPA-1a+ males, gave birth to severely thrombocytopenic pups that exhibited an accompanying bleeding phenotype. Administering either polyclonal intravenous immunoglobulin G or a human monoclonal blocking antibody specific for the HPA-1a epitope into pregnant female mice resulted in significant elevation of the neonatal platelet count, normalized hemostasis, and prevented bleeding. The establishment of an alloantigen-specific murine model that recapitulates many of the clinically important features of FNAIT should pave the way for the preclinical development and testing of novel therapeutic and prophylactic modalities to treat or prevent FNAIT in humans.

Updated Understanding of Platelets in Thrombosis and Hemostasis: The Roles of Integrin PSI Domains and their Potential as Therapeutic Targets

Platelets are small blood cells known primarily for their ability to adhere and aggregate at injured vessels to arrest bleeding. However, when triggered under pathological conditions, the same adaptive mechanism of platelet adhesion and aggregation may cause thrombosis, a primary cause of heart attack and stroke. Over recent decades, research has made considerable progress in uncovering the intricate and dynamic interactions that regulate these processes. Integrins are heterodimeric cell surface receptors expressed on all metazoan cells that facilitate cell adhesion, movement, and signaling, to drive biological and pathological processes such as thrombosis and hemostasis. Recently, our group discovered that the plexin-semaphorin-integrin (PSI) domains of the integrin β subunits exert endogenous thiol isomerase activity derived from their two highly conserved CXXC active site motifs. Given the importance of redox reactions in integrin activation and its location in the knee region, this PSI domain activity may be critically involved in facilitating the interconversions between integrin conformations. Our monoclonal antibodies against the β3 PSI domain inhibited its thiol isomerase activity and proportionally attenuated fibrinogen binding and platelet aggregation. Notably, these antibodies inhibited thrombosis without significantly impairing hemostasis or causing platelet clearance. In this review, we will update mechanisms of thrombosis and hemostasis, including platelet versatilities and immune-mediated thrombocytopenia, discuss critical contributions of the newly discovered PSI domain thiol isomerase activity, and its potential as a novel target for anti-thrombotic therapies and beyond.

In Translation: FcRn across the Therapeutic Spectrum

As an essential modulator of IgG disposition, the neonatal Fc receptor (FcRn) governs the pharmacokinetics and functions many therapeutic modalities. In this review, we thoroughly reexamine the hitherto elucidated biological and thermodynamic properties of FcRn to provide context for our assessment of more recent advances, which covers antigen-binding fragment (Fab) determinants of FcRn affinity, transgenic preclinical models, and FcRn targeting as an immune-complex (IC)-clearing strategy. We further comment on therapeutic antibodies authorized for treating SARS-CoV-2 (bamlanivimab, casirivimab, and imdevimab) and evaluate their potential to saturate FcRn-mediated recycling. Finally, we discuss modeling and simulation studies that probe the quantitative relationship between in vivo IgG persistence and in vitro FcRn binding, emphasizing the importance of endosomal transit parameters.

Targeting the neonatal Fc receptor (FcRn) to treat autoimmune diseases and maternal-fetal immune cytopenias

FcRn, a non-classical Fc gamma (γ) receptor (FcγR) with near ubiquitous expression, plays key roles in disease pathogenesis and progression though immunoglobulin G (IgG) transport, IgG recycling, and IgG-immune complex clearance. FcRn function can be inhibited using IgG-based and non-IgG-based antagonists, by exploiting the pH-dependent binding affinity of FcRn for the IgG Fc region. FcRn therapeutics have shown promise in murine models and human clinical trials for autoimmune diseases and maternal-fetal immune cytopenias; they appear safe, well-tolerated, and reduce circulating IgG levels. Compared to traditional therapeutics, inhibiting FcRn has fewer adverse side effects and represents a new approach that is less invasive, time-consuming, and costly.

Current perspective and scope of fetal therapy: part 2

Fetal therapy has been defined as any therapeutic intervention either invasive or noninvasive for correcting or treating any fetal malformation or condition. Invasive fetal therapy have its own set of maternal and fetal complications and invasive approach is not feasible in many of fetal conditions that are candidate for fetal therapy. Many such fetal conditions have been treated successfully by medical or noninvasive management. In medical fetal therapy, mothers are treated with medications which are transferred to fetus through placenta and exert positive effect on the fetus, thus avoiding complications that are seen secondary to invasive fetal therapy. The fetal conditions that have been managed with medical therapy includes fetal and neonatal alloimmune thrombocytopenia, neural tube defect, congenital adrenal hyperplasia, perinatal infections, respiratory distress syndrome, inborn error of metabolism, and congenital cystic adenomatoid malformation. This review will cover the medical or noninvasive aspect of fetal therapy and will highlight the progress made in the management of these fetal conditions.

Multidrug Therapy for Refractory Immune Thrombocytopenia in Pregnancy

BACKGROUND

Severe immune thrombocytopenia complicating pregnancy may require treatment beyond first-line medications (intravenous immunoglobulins or corticosteroids), but there is a paucity of literature on the use of such second-line agents in pregnancy.

CASE

The patient is a 29-year-old woman with early-onset severe immune thrombocytopenia at 13 weeks of gestation. Maternal platelet counts reached a nadir of less than 5×10/L. The thrombocytopenia persisted despite first-line medications. Romiplostim, rituximab, and azathioprine were added to the therapeutic regimen. Platelet counts eventually stabilized at greater than 150×10/L before delivery. After delivery at term, the neonate had transient B-cell suppression, which was presumed to be secondary to rituximab, but was otherwise doing well and meeting all milestones at 7 months of age.

CONCLUSION

The addition of second-line agents was associated with sustained elevation in maternal platelet counts and may have obviated the need for splenectomy.

Medical therapy to attenuate fetal anaemia in severe maternal red cell alloimmunisation

Haemolytic disease of the fetus and newborn (HDFN) remains an important cause of fetal mortality with potential neonatal and longer-term morbidity. HDFN is caused by maternal red cell alloimmunisation, with IgG antibodies crossing the placenta to destroy fetal erythroid cells expressing the involved antigen. Intrauterine fetal blood transfusion is the therapy of choice for severe fetal anaemia. Despite a strong evidence base and technical advances, invasive fetal therapy carries risk of miscarriage and preterm birth. Procedure-related risks are increased when invasive, in utero transfusion is instituted prior to 22 weeks to treat severe early-onset fetal anaemia. This review focuses upon this cohort of HDFN and discusses intravenous immunoglobin (IVIg) and novel monoclonal antibody (M281, nipocalimab) treatments which, if started at the end of the first trimester, may attenuate the transplacental passage and fetal effects of IgG antibodies. Such therapy has the ability to improve fetal survival in this severe presentation of HDFN when early in utero transfusion may be required and may have wider implications for the perinatal management in general.

Crosstalk Between Platelets and Microbial Pathogens

Platelets, small anucleate cells circulating in the blood, are critical mediators in haemostasis and thrombosis. Interestingly, recent studies demonstrated that platelets contain both pro-inflammatory and anti-inflammatory molecules, equipping platelets with immunoregulatory function in both innate and adaptive immunity. In the context of infectious diseases, platelets are involved in early detection of invading microorganisms and are actively recruited to sites of infection. Platelets exert their effects on microbial pathogens either by direct binding to eliminate or restrict dissemination, or by shaping the subsequent host immune response. Reciprocally, many invading microbial pathogens can directly or indirectly target host platelets, altering platelet count or/and function. In addition, microbial pathogens can impact the host auto- and alloimmune responses to platelet antigens in several immune-mediated diseases, such as immune thrombocytopenia, and fetal and neonatal alloimmune thrombocytopenia. In this review, we discuss the mechanisms that contribute to the bidirectional interactions between platelets and various microbial pathogens, and how these interactions hold relevant implications in the pathogenesis of many infectious diseases. The knowledge obtained from "well-studied" microbes may also help us understand the pathogenesis of emerging microbes, such as SARS-CoV-2 coronavirus.

Dysregulated megakaryocyte distribution associated with nestin+ mesenchymal stem cells in immune thrombocytopenia

Impaired megakaryocyte (MK) maturation and reduced platelet production are important causes of immune thrombocytopenia (ITP). However, MK distribution and bone marrow (BM) niche alteration in ITP are unclear. To investigate the maturation and distribution of MKs in the BM niche and examine the components of BM niche regulation of MK migration, BM and peripheral blood were obtained from 30 ITP patients and 28 healthy donors. Nestin⁺ mesenchymal stem cells (MSCs) and CD41⁺ MKs were sorted by fluorescence-activated cell sorting. The components of the BM niche and related signaling were analyzed via immunofluorescence, flow cytometry, enzyme-linked immunosorbent assay, reverse transcription polymerase chain reaction, and western blot analysis. The number of MKs in the BM vascular niche was reduced in ITP. Moreover, the concentrations of CXCL12 and CXCR4⁺ MKs in the BM were decreased in ITP. Further investigation demonstrated that nestin⁺ MSCs and CXCL12 messenger RNA (mRNA) in nestin⁺ MSCs were both reduced whereas the apoptosis of nestin⁺ MSCs was significantly increased in ITP. Sympathetic nerves, Schwann cells, the proportion of β3-adrenoreceptor (β3-AR)⁺ nestin⁺ MSCs, and β3-AR mRNA in nestin⁺ MSCs were all markedly reduced in ITP. Moreover, matrix metalloproteinase 9, vascular endothelial growth factor (VEGF), and VEGF receptor 1 were significantly reduced in ITP. Our data show that impaired MK distribution mediated by an abnormal CXCL12/CXCR4 axis is partially involved in reduced platelet production in ITP. Moreover, sympathetic neuropathy and nestin⁺ MSC apoptosis may have an effect on the alterations of BM CXCL12 in ITP.

The use of IVIg in fetal and neonatal alloimmune thrombocytopenia- Principles and mechanisms

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare neonatal disorder that is caused by alloimmunization against platelet antigens during pregnancy. Although rare, affecting only 1 in 1000 live births, it can cause intracranial hemorrhage and other bleeding complications that can lead to miscarriage, stillbirth and life-long neurological complications. One of the gold-standard therapies for at risk pregnancies is the administration of IVIg. Although IVIg has been used in a variety of different disorders for over 40 years, its exact mechanism of action is still unknown. In FNAIT, the majority of its therapeutic effect is thought the be mediated through the neonatal Fc receptor, however other mechanisms cannot be excluded. Due to safety, supply and other concerns that are associated with IVIg use, alternative therapies that could replace IVIg are additionally being investigated. This includes the possibility of a prophylaxis regimen for FNAIT, similarly to what has been successfully used in hemolytic disease of the fetus and newborn for over 50 years.

Murine models for studying treatment, prevention and pathogenesis of FNAIT

Maternal alloimmunization to paternally inherited antigens on fetal/neonatal platelets can cause fetal/neonatal alloimmune thrombocytopenia (FNAIT) after antibody-mediated removal of platelets from the fetal circulation. The complications vary from mild bleeding symptoms to severe intracranial hemorrhage and subsequent neurological impairment or death. Studies on in vivo mechanisms are challenging to measure directly in pregnant women, rendering murine models as valuable and attractive alternatives, despite some critical differences between mice and men affecting the translational value. Here we present and discuss, the different murine models that substantially have increased our knowledge and understanding of FNAIT pathogenesis - as well as pre-clinical evaluation of therapeutic and preventive strategies.

Epidemiology and management of fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a disease in pregnancy characterized by maternal alloantibodies directed against the human platelet antigen (HPA). These antibodies can cause intracranial hemorrhage (ICH) or other major bleeding resulting in lifelong handicaps or death. Optimal fetal care can be provided by timely identification of pregnancies at risk. However, this can only be done by routinely antenatal screening. Whether nationwide screening is cost-effective is still being debated. HPA-1a alloantibodies are estimated to be found in 1 in 400 pregnancies resulting in severe burden and fetal ICH in 1 in 10.000 pregnancies. Antenatal treatment is focused on the prevention of fetal ICH and consists of weekly maternal IVIg administration. In high-risk FNAIT treatment should be initiated at 12-18 weeks gestational age using high dosage and in standard-risk FNAIT at 20-28 weeks gestational age using a lower dosage. Postnatal prophylactic platelet transfusions are often given in case of severe thrombocytopenia to prevent bleedings. The optimal threshold and product for postnatal transfusion is not known and international consensus is lacking. In this review practical guidelines for antenatal and postnatal management are offered to clinicians that face the challenge of reducing the risk of bleeding in fetuses and infants affected by FNAIT.

The Neonatal Fc Receptor (FcRn): A Misnomer?

Antibodies are essential components of an adaptive immune response. Immunoglobulin G (IgG) is the most common type of antibody found in circulation and extracellular fluids. Although IgG alone can directly protect the body from infection through the activities of its antigen binding region, the majority of IgG immune functions are mediated via proteins and receptors expressed by specialized cell subsets that bind to the fragment crystallizable (Fc) region of IgG. Fc gamma (γ) receptors (FcγR) belong to a broad family of proteins that presently include classical membrane-bound surface receptors as well as atypical intracellular receptors and cytoplasmic glycoproteins. Among the atypical FcγRs, the neonatal Fc receptor (FcRn) has increasingly gained notoriety given its intimate influence on IgG biology and its ability to also bind to albumin. FcRn functions as a recycling or transcytosis receptor that is responsible for maintaining IgG and albumin in the circulation, and bidirectionally transporting these two ligands across polarized cellular barriers. More recently, it has been appreciated that FcRn acts as an immune receptor by interacting with and facilitating antigen presentation of peptides derived from IgG immune complexes (IC). Here we review FcRn biology and focus on newer advances including how emerging FcRn-targeted therapies may affect the immune responses to IgG and IgG IC.

Expression of the neonatal Fc-receptor in placental-fetal endothelium and in cells of the placental immune system

INTRODUCTION

Starting from the second trimester of pregnancy, passive immunity is provided to the human fetus by transplacental transfer of maternal IgG. IgG transfer depends on the neonatal Fc receptor, FcRn. While FcRn localization in the placental syncytiotrophoblast (STB) has been demonstrated unequivocally, FcRn expression in placental-fetal endothelial cells (pFECs), which are part of the materno-fetal barrier, is still unclear. Therefore, this study aimed to elucidate the spatio-specific expression pattern of FcRn in placental tissue.

METHODS

FcRn expression was investigated by western blotting in term placentas and in isolated human placental arterial and venous endothelial cells (HPAEC, HPVEC) using a validated affinity-purified polyclonal anti-peptide antibody against the cytoplasmic tail of FcRn α-chain. In situ localization of FcRn and IgG was studied by immunofluorescence microscopy on tissue sections of healthy term placentas.

RESULTS

FcRn expression was demonstrated in placental vasculature particularly, in HPAEC, and HPVEC. FcRn was localized in cytokeratin 7⁺ STB and in CD31⁺ pFECs in terminal as well as stem villi in situ. Additionally, CD68⁺ placental macrophages exhibited FcRn expression in situ. Endogenous IgG partially co-localized with FcRn in STB, pFECs, and in placental macrophages.

DISCUSSION

Placental FcRn expression in endothelial cells and macrophages is analogous to the expression pattern in other organs. FcRn expression in pFECs suggests an involvement of FcRn in IgG transcytosis and/or participation in recycling/salvaging of maternal IgG present in the fetal circulation. FcRn expression in placental macrophages may account for recycling of monomeric IgG and/or processing and presentation of immune complexes.

IVIG-mediated effector functions in autoimmune and inflammatory diseases

Intravenous immunoglobulin (IVIG) is a pooled preparation of normal IgG obtained from several thousand healthy donors. It is widely used in the immunotherapy of a large number of autoimmune and inflammatory diseases. The mechanisms of action of IVIG are complex and, as discussed in this review, experimental and clinical data provide an indicator that the therapeutic benefit of IVIG therapy is due to several mutually non-exclusive mechanisms affecting soluble mediators as well as cellular components of the immune system. These mechanisms depend on Fc and/or F(ab')2 fragments. A better understanding of the effector functions of IVIG should help in identification of biomarkers of responses to IVIG in autoimmune patients.

M281, an Anti-FcRn Antibody: Pharmacodynamics, Pharmacokinetics, and Safety Across the Full Range of IgG Reduction in a First-in-Human Study

M281 is a fully human, anti-neonatal Fc receptor (FcRn) antibody that inhibits FcRn-mediated immunoglobulin G (IgG) recycling to decrease pathogenic IgG while preserving IgG production. A randomized, double-blind, placebo-controlled, first-in-human study with 50 normal healthy volunteers was designed to probe safety and the physiological maximum for reduction of IgG. Intravenous infusion of single ascending doses up to 60 mg/kg induced dose-dependent serum IgG reductions, which were similar across all IgG subclasses. Multiple weekly doses of 15 or 30 mg/kg achieved mean IgG reductions of ≈85% from baseline and maintained IgG reductions ≥75% from baseline for up to 24 days. M281 was well tolerated, with no serious or severe adverse events (AEs), few moderate AEs, and a low incidence of infection-related AEs similar to placebo treatment. The tolerability and consistency of M281 pharmacokinetics and pharmacodynamics support further evaluation of M281 in diseases mediated by pathogenic IgG.

GPIbα is required for platelet-mediated hepatic thrombopoietin generation

Thrombopoietin (TPO), a hematopoietic growth factor produced predominantly by the liver, is essential for thrombopoiesis. Prevailing theory posits that circulating TPO levels are maintained through its clearance by platelets and megakaryocytes via surface c-Mpl receptor internalization. Interestingly, we found a two- to threefold decrease in circulating TPO in GPIbα^-/- mice compared with wild-type (WT) controls, which was consistent in GPIbα-deficient human Bernard-Soulier syndrome (BSS) patients. We showed that lower TPO levels in GPIbα-deficient conditions were not due to increased TPO clearance by GPIbα^-/- platelets but rather to decreased hepatic TPO mRNA transcription and production. We found that WT, but not GPIbα^-/-, platelet transfusions rescued hepatic TPO mRNA and circulating TPO levels in GPIbα^-/- mice. In vitro hepatocyte cocultures with platelets or GPIbα-coupled beads further confirm the disruption of platelet-mediated hepatic TPO generation in the absence of GPIbα. Treatment of GPIbα^-/- platelets with neuraminidase caused significant desialylation; however, strikingly, desialylated GPIbα^-/- platelets could not rescue impaired hepatic TPO production in vivo or in vitro, suggesting that GPIbα, independent of platelet desialylation, is a prerequisite for hepatic TPO generation. Additionally, impaired hepatic TPO production was recapitulated in interleukin-4/GPIbα-transgenic mice, as well as with antibodies targeting the extracellular portion of GPIbα, demonstrating that the N terminus of GPIbα is required for platelet-mediated hepatic TPO generation. These findings reveal a novel nonredundant regulatory role for platelets in hepatic TPO homeostasis, which improves our understanding of constitutive TPO regulation and has important implications in diseases related to GPIbα, such as BSS and auto- and alloimmune-mediated thrombocytopenias.

Activated NK cells cause placental dysfunction and miscarriages in fetal alloimmune thrombocytopenia

Miscarriage and intrauterine growth restriction (IUGR) are devastating complications in fetal/neonatal alloimmune thrombocytopenia (FNAIT). We previously reported the mechanisms for bleeding diatheses, but it is unknown whether placental, decidual immune cells or other abnormalities at the maternal-fetal interface contribute to FNAIT. Here we show that maternal immune responses to fetal platelet antigens cause miscarriage and IUGR that are associated with vascular and immune pathologies in murine FNAIT models. Uterine natural killer (uNK) cell recruitment and survival beyond mid-gestation lead to elevated NKp46 and CD107 expression, perforin release and trophoblast apoptosis. Depletion of NK cells restores normal spiral artery remodeling and placental function, prevents miscarriage, and rescues hemorrhage in neonates. Blockade of NK activation receptors (NKp46, FcɣRIIIa) also rescues pregnancy loss. These findings shed light on uNK antibody-dependent cell-mediated cytotoxicity of invasive trophoblasts as a pathological mechanism in FNAIT, and suggest that anti-NK cell therapies may prevent immune-mediated pregnancy loss and ameliorate FNAIT.Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a gestational disease caused by maternal immune responses against fetal platelets. Using a FNAIT mouse model and human trophoblast cell lines, here the authors show that uterine natural killer cell-mediated trophoblast apoptosis contributes to FNAIT pathogenesis.

Phosphatidylserine-mediated platelet clearance by endothelium decreases platelet aggregates and procoagulant activity in sepsis

The mechanisms that eliminate activated platelets in inflammation-induced disseminated intravascular coagulation (DIC) in micro-capillary circulation are poorly understood. This study explored an alternate pathway for platelet disposal mediated by endothelial cells (ECs) through phosphatidylserine (PS) and examined the effect of platelet clearance on procoagulant activity (PCA) in sepsis. Platelets in septic patients demonstrated increased levels of surface activation markers and apoptotic vesicle formation, and also formed aggregates with leukocytes. Activated platelets adhered were and ultimately digested by ECs in vivo and in vitro. Blocking PS on platelets or αvβ3 integrin on ECs attenuated platelet clearance resulting in increased platelet count in a mouse model of sepsis. Furthermore, platelet removal by ECs resulted in a corresponding decrease in platelet-leukocyte complex formation and markedly reduced generation of factor Xa and thrombin on platelets. Pretreatment with lactadherin significantly increased phagocytosis of platelets by approximately 2-fold, diminished PCA by 70%, prolonged coagulation time, and attenuated fibrin formation by 50%. Our results suggest that PS-mediated clearance of activated platelets by the endothelium results in an anti-inflammatory, anticoagulant, and antithrombotic effect that contribute to maintaining platelet homeostasis during acute inflammation. These results suggest a new therapeutic target for impeding the development of DIC.

[Thirty years of platelet immunology in fetal and neonatal alloimmune thrombocytopenia management, current situation]

Fetal and neonatal allo-immune thrombocytopenia (FNAIT) is considered as a rare disease due to the incidence (1/1000-1/2000 births). The major complication of severe thrombocytopenia is bleeding and particularly intra-cranial hemorrhage and neurologic sequelae following. Serology and molecular biology developments have reconfigured the platelet immunology diagnosis. Anti-HPA-1a allo-immunisation is responsible for more than 80% FNAIT cases with a high recurrence rate of severe bleeding complications. Therapeutic management has changed over the coming years from an invasive concept associating fetal blood sampling and in utero platelet transfusion to a non invasive treatment by intravenous immunoglobulins injection (IVIg). The purpose of this article is to provide an update on FNAIT management in the light of current developments over the past 30years.

Fetal and neonatal alloimmune thrombocytopenia: evidence based antenatal and postnatal management strategies

INTRODUCTION

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a relatively rare but potentially lethal disease, leading to severe bleeding complications in 1 in 11.000 newborns. It is the leading cause of thrombocytopenia in healthy term-born neonates. Areas covered: This review summarizes the antenatal as well as postnatal treatment, thus creating a complete overview of all possible management strategies for FNAIT. Expert commentary: The optimal antenatal therapy in order to prevent bleeding complications in pregnancies complicated by FNAIT is non-invasive treatment with weekly intravenous immunoglobulin (IVIG). Based on risk stratification, weekly doses of IVIG of 0.5 or 1.0g/kg should be administered started early in the second in high risk cases or at the end of the second trimester in low risk cases. The optimal postnatal treatment depends on the platelet count and the clinical condition of the newborn. Prompt administration of compatible platelet transfusion is the first treatment of choice in case of severe thrombocytopenia or active bleeding. In case matched platelets are not directly available, random platelets can also be administered initially to gain time until matched platelets are available. In case of persistent thrombocytopenia despite transfusions, IVIG 1.0-2.0g/kg can be administered.

The integrin PSI domain has an endogenous thiol isomerase function and is a novel target for antiplatelet therapy

Integrins are a large family of heterodimeric transmembrane receptors differentially expressed on almost all metazoan cells. Integrin β subunits contain a highly conserved plexin-semaphorin-integrin (PSI) domain. The CXXC motif, the active site of the protein-disulfide-isomerase (PDI) family, is expressed twice in this domain of all integrins across species. However, the role of the PSI domain in integrins and whether it contains thiol-isomerase activity have not been explored. Here, recombinant PSI domains of murine β3, and human β1 and β2 integrins were generated and their PDI-like activity was demonstrated by refolding of reduced/denatured RNase. We identified that both CXXC motifs of β3 integrin PSI domain are required to maintain its optimal PDI-like activity. Cysteine substitutions (C13A and C26A) of the CXXC motifs also significantly decreased the PDI-like activity of full-length human recombinant β3 subunit. We further developed mouse anti-mouse β3 PSI domain monoclonal antibodies (mAbs) that cross-react with human and other species. These mAbs inhibited αIIbβ3 PDI-like activity and its fibrinogen binding. Using single-molecular Biomembrane-Force-Probe assays, we demonstrated that inhibition of αIIbβ3 endogenous PDI-like activity reduced αIIbβ3-fibrinogen interaction, and these anti-PSI mAbs inhibited fibrinogen binding via different levels of both PDI-like activity-dependent and -independent mechanisms. Importantly, these mAbs inhibited murine/human platelet aggregation in vitro and ex vivo, and murine thrombus formation in vivo, without significantly affecting bleeding time or platelet count. Thus, the PSI domain is a potential regulator of integrin activation and a novel target for antithrombotic therapies. These findings may have broad implications for all integrin functions, and cell-cell and cell-matrix interactions.

Omission of fetal sampling in treatment of subsequent pregnancies in fetal-neonatal alloimmune thrombocytopenia

BACKGROUND

Fetal-neonatal alloimmune thrombocytopenia affects approximately 1 of 1000 live births, most of which are not severely thrombocytopenic. Despite effective treatment with intravenous gammaglobulin and/or prednisone, antenatal management of a subsequent affected pregnancy is complicated by the risks associated with fetal blood sampling. Furthermore, there are no biomarker(s) of high risk other than the occurrence of intracranial hemorrhage in a previous sibling. Management of these high-risk pregnancies requires intensive treatment initiated at 12 weeks of gestation.

OBJECTIVE

The objective of the study was to evaluate whether empiric escalation of therapy at 32 weeks allows the omission of fetal blood sampling in all fetal-neonatal alloimmune thrombocytopenia-affected patients. Specifically, we sought to determine whether intensive intravenous gammaglobulin-based regimens for the treatment of a subsequent fetal-neonatal alloimmune thrombocytopenia-affected pregnancy followed by empirically escalated intravenous gammaglobulin and prednisone treatment would increase the fetal platelet count and thus safely allow omission of fetal blood sampling in the antepartum management of these patients.

STUDY DESIGN

In this prospective, multicenter, randomized controlled study, 99 women with fetal-neonatal alloimmune thrombocytopenia whose prior affected child did not have an intracranial hemorrhage were randomized to receive an intensive intravenous gammaglobulin-based regimen: 2 g/kg per week or intravenous gammaglobulin 1 g/kg per week plus prednisone 0.5 mg/kg per day, starting at 20-30 weeks of gestation. Escalated therapy (intravenous gammaglobulin 2 g/kg per week plus prednisone 0.5 mg/kg per day) was recommended and usually initiated at 32 weeks when fetal counts were <50,000/mL(3) or when fetal blood sampling was not performed. The preliminary report of this study from 2007 demonstrated the efficacy of both intravenous gammaglobulin-based regimens in most patients. Most patients who underwent fetal sampling had adequate fetal counts and therefore did not have their treatment escalated. This post hoc analysis describes the 29 fetuses who had their treatment escalated either because they had low counts at 32 weeks or when sampling was not performed. This study explored whether the empiric escalation of treatment at 32 weeks was sufficiently effective in increasing fetal platelet counts in these patients.

RESULTS

Mean fetal and birth counts of fetuses randomized to each of the 2 initial treatment groups were all >100,000/mL(3). Three neonates had an intracranial hemorrhage; all 3 were grade 1 and all had birth platelet counts >130,000/mL(3). In a post hoc analysis, 19 fetuses undergoing fetal blood sampling at 32 weeks had fetal platelet counts <50,000/mL(3) despite their initial treatment. Of these 19, birth platelet counts were >50,000/mL(3) in 11 of 13 fetuses who received escalated treatment compared with only 1 of 6 of those who did not (P = .01); only 3 fetuses that received initial therapy followed by escalated treatment had birth platelet counts <50,000/mL(3) and none had an intracranial hemorrhage. The platelet counts of 14 of 15 fetuses that received empirically escalated treatment without sampling were >50,000/mL(3) at birth. In addition, none of these had an intracranial hemorrhage.

CONCLUSION

The 2 recommended protocols of intensive initial treatment followed by empiric escalation of therapy at 32 weeks of gestation are reasonably safe, effective in increasing fetal platelet counts, and allow omission of fetal blood sampling by increasing the fetal platelet count in almost all cases.

Platelets are versatile cells: New discoveries in hemostasis, thrombosis, immune responses, tumor metastasis and beyond

Platelets are small anucleate blood cells generated from megakaryocytes in the bone marrow and cleared in the reticuloendothelial system. At the site of vascular injury, platelet adhesion, activation and aggregation constitute the first wave of hemostasis. Blood coagulation, which is initiated by the intrinsic or extrinsic coagulation cascades, is the second wave of hemostasis. Activated platelets can also provide negatively-charged surfaces that harbor coagulation factors and markedly potentiate cell-based thrombin generation. Recently, deposition of plasma fibronectin, and likely other plasma proteins, onto the injured vessel wall has been identified as a new "protein wave of hemostasis" that may occur even earlier than the first wave of hemostasis, platelet accumulation. Although no experimental evidence currently exists, it is conceivable that platelets may also contribute to this protein wave of hemostasis by releasing their granule fibronectin and other proteins that may facilitate fibronectin self- and non-self-assembly on the vessel wall. Thus, platelets may contribute to all three waves of hemostasis and are central players in this critical physiological process to prevent bleeding. Low platelet counts in blood caused by enhanced platelet clearance and/or impaired platelet production are usually associated with hemorrhage. Auto- and allo-immune thrombocytopenias such as idiopathic thrombocytopenic purpura and fetal and neonatal alloimmune thrombocytopenia may cause life-threatening bleeding such as intracranial hemorrhage. When triggered under pathological conditions such as rupture of an atherosclerotic plaque, excessive platelet activation and aggregation may result in thrombosis and vessel occlusion. This may lead to myocardial infarction or ischemic stroke, the major causes of mortality and morbidity worldwide. Platelets are also involved in deep vein thrombosis and thromboembolism, another leading cause of mortality. Although fibrinogen has been documented for more than half a century as essential for platelet aggregation, recent studies demonstrated that fibrinogen-independent platelet aggregation occurs in both gene deficient animals and human patients under physiological and pathological conditions (non-anti-coagulated blood). This indicates that other unidentified platelet ligands may play important roles in thrombosis and might be novel antithrombotic targets. In addition to their critical roles in hemostasis and thrombosis, emerging evidence indicates that platelets are versatile cells involved in many other pathophysiological processes such as innate and adaptive immune responses, atherosclerosis, angiogenesis, lymphatic vessel development, liver regeneration and tumor metastasis. This review summarizes the current knowledge of platelet biology, highlights recent advances in the understanding of platelet production and clearance, molecular and cellular events of thrombosis and hemostasis, and introduces the emerging roles of platelets in the immune system, vascular biology and tumorigenesis. The clinical implications of these basic science and translational research findings will also be discussed.

Clinically significant anti-KEL RBC alloantibodies are transferred by breast milk in a murine model

BACKGROUND AND OBJECTIVES

Fetuses affected by maternal RBC alloantibodies may have prolonged anaemia after birth, leading one to question whether maternal alloantibody transfer may occur outside the placenta. In response to a recent publication describing breast milk transfer of clinically significant amounts of maternal antiplatelet IgA antibodies from mother to nursing infant, we hypothesized that maternal RBC alloantibodies may also be capable of being transferred in breast milk.

MATERIALS AND METHODS

The presence and clinical significance of breast milk alloantibody transfer were tested through a series of pregnancy, fostering and transfusion experiments, using a murine model in which transgenic RBCs express the human KEL glycoprotein.

RESULTS

Maternal anti-KEL immunoglobulins, induced through transfusion or pregnancy, were detected in the aqueous phase of breast milk. Further, efficient transfer of maternal anti-KEL IgG and IgA to nursing pups was observed in fostering experiments. The breast milk-acquired alloantibodies were clinically significant in wild-type pups in a transfusion setting, binding to 'incompatible' KEL RBCs and leading to premature clearance from the circulation. Although breast milk-acquired alloantibodies also bound to the RBCs of transgenic KEL-positive fostered pups, no anaemia resulted.

CONCLUSIONS

Taking these murine data in combination with recently published human data of maternal antiplatelet IgA antibodies in breast milk leading to sequelae in some infants, it is theoretically possible that maternal anti-RBC IgA alloantibodies may also be transferred in human breast milk and may lead to sequelae in some infants under some circumstances.

Fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is an alloimmune disorder resulting from platelet opsonization by maternal antibodies that destroy fetal platelets. The major risk of FNAIT is severe bleeding, particularly intracranial hemorrhage. Miscarriage has also been reported but the incidence requires further study. Analogous to adult autoimmune thrombocytopenia (ITP), the major target antigen in FNAIT is the platelet membrane glycoprotein (GP)IIbIIIa. FNAIT caused by antibodies against platelet GPIbα or other antigens has also been reported, but the reported incidence of the anti-GPIbα-mediated FNAIT is far lower than in ITP. To date, the maternal immune response to fetal platelet antigens is still not well understood and it is unclear why bleeding is more severe in FNAIT than in ITP. In this review, we introduce the pathogenesis of FNAIT, particularly those new discoveries from animal models, and discuss possible improvements for the diagnosis, therapy, and prevention of this devastating disease.

Fetal/Neonatal Alloimmune Thrombocytopenia: Pathogenesis, Diagnostics and Prevention

Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a relatively rare condition (1/1000–1/2000) that was granted orphan status by the European Medicines Agency in 2011. Clinical consequences of FNAIT, however, may be severe. A thrombocytopenic fetus or new-born is at risk of intracranial hemorrhage that may result in lifelong disability or death. Preventing such bleeding is thus vital and requires a solution. Anti-HPA1a antibodies are the most frequent cause of FNAIT in Caucasians. Its pathogenesis is similar to hemolytic disease of the newborn (HDN) due to anti-RhD antibodies, but is characterized by platelet destruction and is more often observed in the first pregnancy. In 75 % of these women, alloimmunization by HPA-1a antigens, however, occurs at delivery, which enables development of antibody-mediated immune suppression to prevent maternal immunization. As for HDN, the recurrence rate of FNAIT is high. For advancing diagnostic efforts and treatment, it is thereby crucial to understand the pathogenesis of FNAIT, including cellular immunity involvement. This review presents the current knowledge on FNAIT. Also described is a program for HPA-1a screening in identifying HPA-1a negative pregnant women at risk of immunization. This program is now performed at the Institute of Hematology and Transfusion Medicine in cooperation with the Department of Obstetrics and Gynecology of the Medical Centre of Postgraduate Education in Warsaw as well as the UiT The Arctic University of Norway.

Platelets in hemostasis and thrombosis: Novel mechanisms of fibrinogen-independent platelet aggregation and fibronectin-mediated protein wave of hemostasis

Platelets are small anucleate cells generated from megakaryocytes in the bone marrow. Although platelet generation, maturation, and clearance are still not fully understood, significant progress has been made in the last 1-2 decades. In blood circulation, platelets can quickly adhere and aggregate at sites of vascular injury, forming the platelet plug (i.e. the first wave of hemostasis). Activated platelets can also provide negatively charged phosphatidylserinerich membrane surface that enhances cell-based thrombin generation, which facilitates blood coagulation (i.e. the second wave of hemostasis). Platelets therefore play central roles in hemostasis. However, the same process of hemostasis may also cause thrombosis and vessel occlusion, which are the most common mechanisms leading to heart attack and stroke following ruptured atherosclerotic lesions. In this review, we will introduce the classical mechanisms and newly discovered pathways of platelets in hemostasis and thrombosis, including fibrinogen-independent platelet aggregation and thrombosis, and the plasma fibronectin-mediated "protein wave" of hemostasis that precedes the classical first wave of hemostasis. Furthermore, we briefly discuss the roles of platelets in inflammation and atherosclerosis and the potential strategies to control atherothrombosis.

Recent progress in understanding the pathogenesis of fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs in c. 1 in 1000 births and is caused by maternal antibodies against human platelet alloantigens that bind incompatible fetal platelets and promote their clearance from the circulation. Affected infants can experience bleeding, bruising and, in severe cases, intracranial haemorrhage and even death. As maternal screening is not routinely performed, and first pregnancies can be affected, most cases are diagnosed at delivery of a first affected pregnancy. Unlike its erythrocyte counterpart, Haemolytic Disease of the Fetus and Newborn, there is no prophylactic treatment for FNAIT. This report will review recent advances made in understanding the pathogenesis of FNAIT: the platelet alloantigens involved, maternal exposure and sensitization to fetal platelet antigens, properties of platelet Immunoglobulin G antibodies, maternal-fetal antibody transport mechanisms and efforts to develop an effective FNAIT prophylaxis.

CD8+ T cells are predominantly protective and required for effective steroid therapy in murine models of immune thrombocytopenia

Immune thrombocytopenia (ITP) is a common autoimmune bleeding disorder characterized by autoantibodies targeting platelet surface proteins, most commonly GPIIbIIIa (αIIbβ3 integrin), leading to platelet destruction. Recently, CD8(+) cytotoxic T-lymphocytes (CTLs) targeting platelets and megakaryocytes have also been implicated in thrombocytopenia. Because steroids are the most commonly administered therapy for ITP worldwide, we established both active (immunized splenocyte engraftment) and passive (antibody injection) murine models of steroid treatment. Surprisingly, we found that, in both models, CD8(+) T cells limited the severity of the thrombocytopenia and were required for an efficacious response to steroid therapy. Conversely, CD8(+) T-cell depletion led to more severe thrombocytopenia, whereas CD8(+) T-cell transfusion ameliorated thrombocytopenia. CD8(+) T-regulatory cell (Treg) subsets were detected, and interestingly, dexamethasone (DEX) treatment selectively expanded CD8(+) Tregs while decreasing CTLs. In vitro coculture studies revealed CD8(+) Tregs suppressed CD4(+) and CD19(+) proliferation, platelet-associated immunoglobulin G generation, CTL cytotoxicity, platelet apoptosis, and clearance. Furthermore, we found increased production of anti-inflammatory interleukin-10 in coculture studies and in vivo after steroid treatment. Thus, we uncovered subsets of CD8(+) Tregs and demonstrated their potent immunosuppressive and protective roles in experimentally induced thrombocytopenia. The data further elucidate mechanisms of steroid treatment and suggest therapeutic potential for CD8(+) Tregs in immune thrombocytopenia.

Platelets and platelet alloantigens: Lessons from human patients and animal models of fetal and neonatal alloimmune thrombocytopenia

Platelets play critical roles in hemostasis and thrombosis. Emerging evidence indicates that they are versatile cells and also involved in many other physiological processes and disease states. Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a life threatening bleeding disorder caused by fetal platelet destruction by maternal alloantibodies developed during pregnancy. Gene polymorphisms cause platelet surface protein incompatibilities between mother and fetus, and ultimately lead to maternal alloimmunization. FNAIT is the most common cause of intracranial hemorrhage in full-term infants and can also lead to intrauterine growth retardation and miscarriage. Proper diagnosis, prevention and treatment of FNAIT is challenging due to insufficient knowledge of the disease and a lack of routine screening as well as its frequent occurrence in first pregnancies. Given the ethical difficulties in performing basic research on human fetuses and neonates, animal models are essential to improve our understanding of the pathogenesis and treatment of FNAIT. The aim of this review is to provide an overview on platelets, hemostasis and thrombocytopenia with a focus on the advancements made in FNAIT by utilizing animal models.

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.

Regulation of immune responses by the neonatal fc receptor and its therapeutic implications

As a single receptor, the neonatal Fc receptor (FcRn) is critically involved in regulating albumin and IgG serum concentrations by protecting these two ligands from degradation. In addition to these essential homeostatic functions, FcRn possesses important functions in regulating immune responses that are equally as critical and are increasingly coming to attention. During the first stages of life, FcRn mediates the passive transfer of IgG across the maternal placenta or neonatal intestinal walls of mammals, thereby conferring passive immunity to the offspring before and after birth. In fact, FcRn is one of the very few molecules that are known to move from luminal to serosal membranes of polarized cells that form epithelial barriers of the lung and intestines. Together with FcRn's recently explored critical role in eliciting MHC II presentation and MHC class I cross-presentation of IgG-complexed antigen, this renders FcRn capable of exerting broad and potent functions in regulating immune responses and immunosurveillance at mucosal sites. Further, it is now clear that FcRn dependent mucosal absorption of therapeutic molecules is a clinically feasible and potent novel route of non-invasive drug delivery, and the interaction between FcRn and IgG has also been utilized for the acquisition of humoral immunity at mucosal sites. In this review, we begin by briefly summarizing the basic knowledge on FcRn expression and IgG binding, then describe more recent discoveries pertaining to the mechanisms by which FcRn orchestrates IgG related mucosal immune responses and immunosurveillance at host-environment interfaces within the adult organism. Finally, we outline how the knowledge of actions of FcRn at mucosal boundaries can be capitalized for the development and engineering of powerful mucosal vaccination strategies and novel routes for the non-invasive delivery of Fc-based therapeutics.

Is dosing of therapeutic immunoglobulins optimal? A review of a three-decade long debate in europe

The consumption of immunoglobulins (Ig) is increasing due to better recognition of antibody deficiencies, an aging population, and new indications. This review aims to examine the various dosing regimens and research developments in the established and in some of the relevant off-label indications in Europe. The background to the current regulatory settings in Europe is provided as a backdrop for the latest developments in primary and secondary immunodeficiencies and in immunomodulatory indications. In these heterogeneous areas, clinical trials encompassing different routes of administration, varying intervals, and infusion rates are paving the way toward more individualized therapy regimens. In primary antibody deficiencies, adjustments in dosing and intervals will depend on the clinical presentation, effective IgG trough levels and IgG metabolism. Ideally, individual pharmacokinetic profiles in conjunction with the clinical phenotype could lead to highly tailored treatment. In practice, incremental dosage increases are necessary to titrate the optimal dose for more severely ill patients. Higher intravenous doses in these patients also have beneficial immunomodulatory effects beyond mere IgG replacement. Better understanding of the pharmacokinetics of Ig therapy is leading to a move away from simplistic "per kg" dosing. Defective antibody production is common in many secondary immunodeficiencies irrespective of whether the causative factor was lymphoid malignancies (established indications), certain autoimmune disorders, immunosuppressive agents, or biologics. This antibody failure, as shown by test immunization, may be amenable to treatment with replacement Ig therapy. In certain immunomodulatory settings [e.g., idiopathic thrombocytopenic purpura (ITP)], selection of patients for Ig therapy may be enhanced by relevant biomarkers in order to exclude non-responders and thus obtain higher response rates. In this review, the developments in dosing of therapeutic immunoglobulins have been limited to high and some medium priority indications such as ITP, Kawasaki' disease, Guillain-Barré syndrome, chronic inflammatory demyelinating polyradiculoneuropathy, myasthenia gravis, multifocal motor neuropathy, fetal alloimmune thrombocytopenia, fetal hemolytic anemia, and dermatological diseases.

On the perplexingly low rate of transport of IgG2 across the human placenta

The neonatal receptor, FcRn, mediates both serum half–life extension as well as active transport of maternal IgG to the fetus during pregnancy. Therefore, transport efficiency and half-life go hand-in-hand. However, while the half-life of the human IgG2 subclass is comparable to IgG1, the placental transport of IgG2 is not, with the neonatal IgG1 levels generally exceeding maternal levels at birth, but not for IgG2. We hypothesized that the unique short-hinged structure of IgG2, which enables its κ-, but not λ-isotype to form at least three different structural isoforms, might be a contributing factor to these differences. To investigate whether there was any preference for either light chain, we measured placental transport of IgG subclasses as well as κ/λ-light chain isotypes of IgG1 and IgG2 in 27 matched mother-child pairs. We also studied the half-life of IgG1 and IgG2 light chain isotypes in mice, as well as that of synthesized IgG2 structural isotypes κA and κB. In order to investigate serum clearance of IgG1 and IgG2 light-chain isotypes in humans, we quantified the relative proportions of IgG1 and IgG2 light chains in hypogammaglobulinemia patients four weeks after IVIg infusion and compared to the original IVIg isotype composition. None of our results indicate any light chain preference in either of the FcRn mediated mechanisms; half-life extension or maternal transport.

Evaluating the Murine Anti-Human Antibody Response and Assessment of General Activity and Cognition after Treatment with Human Intravenous Immunoglobulins in Healthy Adult C57/B6J Mice

Human intravenous immunoglobulin (IVIG) is a fractionated blood product that is used for the treatment of several autoimmune and immunodeficiency disorders. Recently, IVIG has been suggested for the treatment of Alzheimer's disease (AD). However, the molecular mode of action is still largely unknown. Therefore, preclinical assessment of the therapeutic efficacy of IVIG in animals may provide valuable information of the function of IVIG in vivo. However, it is recommended to determine the murine-anti-human antibody (MAHA) response in those animals before starting immunotherapy and subsequent assessment of the therapeutic efficacy in animal models for AD. After weekly administration of 400 μg IVIG in C57/B6J mice for the duration of twelve weeks, we found a significant increase of MAHA response against human IgG. Even after increased MAHA levels starting from week nine after treatment, there was no significant change in basic exploratory behavior, anxiety, and cognition. Therefore, it is suitable to study pharmacological and immunological activity, therapeutic efficacy, as well as mode of action of IVIG in animal models only for a short duration to avoid interference with IVIG treatment and neutralize possible therapeutic effects.

Co-stimulation with LPS or Poly I:C markedly enhances the anti-platelet immune response and severity of fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a life-threatening bleeding disorder caused by maternal antibodies against fetal/neonatal platelets. FNAIT is also linked with miscarriages, although the incidence and mechanisms of fetal death have not been well studied. IntegrinαIIbβ3 (GPIIbIIIa) and the GPIbα complex are major glycoproteins expressed on platelets and are also major antigens targeted in autoimmune thrombocytopenia (ITP), but reported cases of anti-GPIb-mediated FNAIT are rare. Bacterial and viral infections have been causally linked with the pathogenesis of immune-mediated thrombocytopenia (ITP); however, it is unknown whether these infections contribute to the severity of FNAIT. Here, immune responses against platelet antigens were examined by transfusing wild-type (WT) mouse platelets into β3-/- or GPIbα-/- mice. To mimic bacterial or viral infections, lipopolysaccharide (LPS) or polyinosinic:polycytidylic acid (Poly I:C) were injected intraperitoneally following platelet transfusions. The FNAIT model was established by breeding the immunised female mice with WT male mice. We demonstrated for the first time that the platelet GPIbα has lower immunogenicity compared to β3 integrin. Interestingly, co-stimulation with LPS or Poly I:C markedly enhanced the immune response against platelet GPIbα and caused severe pathology of FNAIT (i.e. miscarriages). LPS or Poly I:C also enhanced the immune response against platelet β3 integrin. Our data suggest that bacterial and viral infections facilitate the anti-platelet GPIbα response, which may lead to a severe non-classical FNAIT (i.e. miscarriage but not neonatal bleeding) that has not been adequately reported in humans.

VEGFR-3 neutralization inhibits ovarian lymphangiogenesis, follicle maturation, and murine pregnancy

Lymphatic vessels surround follicles within the ovary, but their roles in folliculogenesis and pregnancy, as well as the necessity of lymphangiogenesis in follicle maturation and health, are undefined. We used systemic delivery of mF4-31C1, a specific antagonist vascular endothelial growth factor receptor 3 (VEGFR-3) antibody to block lymphangiogenesis in mice. VEGFR-3 neutralization for 2 weeks before mating blocked ovarian lymphangiogenesis at all stages of follicle maturation, most notably around corpora lutea, without significantly affecting follicular blood angiogenesis. The numbers of oocytes ovulated, fertilized, and implanted in the uterus were normal in these mice; however, pregnancies were unsuccessful because of retarded fetal growth and miscarriage. Fewer patent secondary follicles were isolated from treated ovaries, and isolated blastocysts exhibited reduced cell densities. Embryos from VEGFR-3-neutralized dams developed normally when transferred to untreated surrogates. Conversely, normal embryos transferred into mF4-31C1-treated dams led to the same fetal deficiencies observed with in situ gestation. Although no significant changes were measured in uterine blood or lymphatic vascular densities, VEGFR-3 neutralization reduced serum and ovarian estradiol concentrations during gestation. VEGFR-3-mediated lymphangiogenesis thus appears to modulate the folliculogenic microenvironment and may be necessary for maintenance of hormone levels during pregnancy; both of these are novel roles for the lymphatic vasculature.

Maternofetal transplacental transport of recombinant IgG antibodies lacking effector functions

The neonatal Fc receptor (FcRn) directs the transfer of maternal immunoglobulin G (IgG) antibodies across the placenta and thus provides the fetus and newborn with passive protective humoral immunity. Pathogenic maternal IgG antibodies will also be delivered via the placenta and can cause alloimmunity, which may be lethal. A novel strategy to control pathogenic antibodies would be administration of a nondestructive IgG antibody blocking antigen binding while retaining binding to FcRn. We report on 2 human IgG3 antibodies with a hinge deletion and a C131S point mutation (IgG3ΔHinge) that eliminate complement activation and binding to all classical Fcγ receptors (FcγRs) and to C1q while binding to FcRn is retained. Additionally, 1 of the antibodies has a single point mutation in the Fc (R435H) at the binding site for FcRn (IgG3ΔHinge:R435H). We compared transplacental transport with wild-type IgG1 and IgG3, and found transport across trophoblast-derived BeWo cells and ex vivo placenta perfusions with hierarchies as follows: IgG3ΔHinge:R435H>wild-type IgG1≥IgG3ΔHinge and IgG3ΔHinge:R435H=wild-type IgG1=wild-type IgG3>>>IgG3ΔHinge, respectively. Collectively, IgG3ΔHinge:R435H was transported efficiently from the maternal to the fetal placental compartment. Thus, IgG3ΔHinge:R435H may be a good candidate for transplacental delivery of a nondestructive antibody to the fetus to combat pathogenic antibodies.

Predicting risk severity and response of fetal neonatal alloimmune thrombocytopenia

Fetal neonatal alloimmune thrombocytopenia (FNAIT) is a devastating bleeding disorder in the fetus or neonate caused by transplacental transport of maternal alloantibodies to paternal-derived antigen on fetal platelets. In Caucasians, up to 80% of FNAIT cases result from maternal immunization to human platelet antigen (HPA)-1a. New methods have developed facilitating detection of common and private antibodies against HPAs triggering FNAIT. Understanding the pathogenesis of FNAIT made it possible to develop a novel strategy to treat this disorder. To date, recombinant monoclonal antibodies directed against the β3 integrin and Fc receptors have been tested in a mouse model of FNAIT, and seem to be promising. Whether those novel treatments will eventually replace the conventional high dose immunoglobulin G in women with FNAIT is yet unknown.