FcγRIIa and FcγRIIIa polymorphisms in childhood primary immune thrombocytopenia: implications for disease pathogenesis and outcome

The many facets of immune-mediated thrombocytopenia: Principles of immunobiology and immunotherapy

Immune thrombocytopenia (ITP) is a rare autoimmune condition, due to peripheral platelet destruction through antibody-dependent cellular phagocytosis, complement-dependent cytotoxicity, cytotoxic T lymphocyte-mediated cytotoxicity, and megakaryopoiesis alteration. This condition may be idiopathic or triggered by drugs, vaccines, infections, cancers, autoimmune disorders and systemic diseases. Recent advances in our understanding of ITP immunobiology support the idea that other forms of thrombocytopenia, for instance, occurring after immunotherapy or cellular therapies, may share a common pathophysiology with possible therapeutic implications. If a decent pipeline of old and new agents is currently deployed for classical ITP, in other more complex immune-mediated thrombocytopenic disorders, clinical management is less harmonized and would deserve further prospective investigations. Here, we seek to provide a fresh overview of pathophysiology and current therapeutical algorithms for adult patients affected by this disorder with specific insights into poorly codified scenarios, including refractory ITP and post-immunotherapy/cellular therapy immune-mediated thrombocytopenia.

FcγRIIa and FcγRIIIa genes polymorphism in Egyptian children with primary immune thrombocytopenia

INTRODUCTION

Phagocytosis of autoantibody-sensitized coated platelets through Fc gamma receptors on phagocytic cells is an important mechanism of thrombocytopenia in primary immune thrombocytopenia (ITP).

OBJECTIVE

We aimed to investigate the contribution of the FcγRIIa and FcγRIIIa genes polymorphism to the risk of ITP and their association with disease characteristics in Egyptian children.

METHODS

A case control study was conducted on eighty children with primary ITP and eighty age and sex healthy matched subjects as a control group. The FcγRIIa and FcγRIIIa genes polymorphism was detected using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP).

RESULTS

We found that the FcγRIIa-131H and -131R allele frequencies were 51.3 % and 48.7%, respectively, in children with ITP, versus 75% and 25%, respectively, in controls (p = 0.002). The compound heterozygous HR genotype was significantly higher in ITP patients (p < 0.05). The FcγRIIIa-158F and -158V allele frequencies were 46.3% and 53.7%, respectively, in children with ITP, versus 70% and 30%, respectively, in controls (p = 0.002). The compound heterozygous VF genotype was significantly higher in ITP patients (p < 0.05). The combined HR/FV genotype was 47.5% in ITP patients, versus 10% in controls (p < 0.001). No significant difference was found between children with newly diagnosed ITP and those who developed chronic ITP, regarding the frequency distribution of the FcγRIIa and FcγRIIIa alleles and genotypes (p > 0.05).

CONCLUSION

There is a possible association of the FcγRIIa and FcγRIIIa genes polymorphism with the risk for, and genetic susceptibility to ITP in Egyptian children, but large-scale studies are still needed to support our findings.

Transient and chronic childhood immune thrombocytopenia are distinctly affected by Fc-γ receptor polymorphisms

In childhood immune thrombocytopenia (ITP), anti-platelet autoantibodies mediate platelet clearance through Fc-γ receptor (FcγR)-bearing phagocytes. In 75% to 90% of patients, the disease has a transient, self-limiting character. Here we characterized how polymorphisms of FcγR genes affect disease susceptibility, response to intravenous immunoglobulin (IVIg) treatment, and long-term recovery from childhood ITP. Genotyping of the FCGR2/3 locus was performed in 180 children with newly diagnosed ITP, 22 children with chronic ITP, and 180 healthy control children by multiplex ligation-dependent probe amplification. Children with newly diagnosed ITP were randomly assigned to a single administration of IVIg or observation, and followed for 1 year (Treatment With or Without IVIg for Kids With ITP [TIKI] trial). We defined transient ITP as a complete recovery (≥100 × 10⁹/L) 3 months after diagnosis, including both self-limiting disease/IVIg responders and chronic ITP as absence of a complete recovery at 12 months. ITP susceptibility, as well as spontaneous recovery and response to IVIg, was associated with the genetic variants FCGR2C*ORF and FCGR2A*27W and the FCGR2B promoter variant 2B.4. These variants were overrepresented in patients with transient (N = 131), but not chronic (N = 43), disease. The presence of FCGR2C*ORF predisposed to transient ITP with an odds ratio of 4.7 (95% confidence interval, 1.9-14.3). Chronic ITP was associated with a deletion of FCGR2C/FCGR3B (copy number region 1) with an odds ratio of 6.2 (95% confidence interval, 1.8-24.7). Taken together, susceptibility to transient and chronic ITP is distinctly affected by polymorphic variants of FCGR2/3 genes. Our data suggest that genotyping of the FCGR2/3 locus may be useful for prognosis and guidance of treatment decisions in newly diagnosed childhood ITP.

CD4+ T cell phenotypes in the pathogenesis of immune thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disorder characterized by low platelet counts due to enhanced platelet clearance and compromised production. Traditionally, ITP was regarded a B cell mediated disorder as anti-platelet antibodies are detected in most patients. The very nature of self-antigens, evident processes of isotype switching and the affinity maturation of anti-platelet antibodies indicate that B cells in order to mount anti-platelet immune response require assistance of auto-reactive CD4⁺ T cells. For a long time, ITP pathogenesis has been exclusively reviewed through the prism of the disturbed balance between Th1 and Th2 subsets of CD4⁺ T cells, however, more recently new subsets of these cells have been described including Th17, Th9, Th22, T follicular helper and regulatory T cells. In this paper, we review the current understanding of the role and immunological mechanisms by which CD4⁺ T cells contribute to the pathogenesis of ITP.

Emerging Concepts in Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disease defined by low platelet counts which presents with an increased bleeding risk. Several genetic risk factors (e.g., polymorphisms in immunity-related genes) predispose to ITP. Autoantibodies and cytotoxic CD8⁺ T cells (Tc) mediate the anti-platelet response leading to thrombocytopenia. Both effector arms enhance platelet clearance through phagocytosis by splenic macrophages or dendritic cells and by induction of apoptosis. Meanwhile, platelet production is inhibited by CD8⁺ Tc targeting megakaryocytes in the bone marrow. CD4⁺ T helper cells are important for B cell differentiation into autoantibody secreting plasma cells. Regulatory Tc are essential to secure immune tolerance, and reduced levels have been implicated in the development of ITP. Both Fcγ-receptor-dependent and -independent pathways are involved in the etiology of ITP. In this review, we present a simplified model for the pathogenesis of ITP, in which exposure of platelet surface antigens and a loss of tolerance are required for development of chronic anti-platelet responses. We also suggest that infections may comprise an important trigger for the development of auto-immunity against platelets in ITP. Post-translational modification of autoantigens has been firmly implicated in the development of autoimmune disorders like rheumatoid arthritis and type 1 diabetes. Based on these findings, we propose that post-translational modifications of platelet antigens may also contribute to the pathogenesis of ITP.

FcγRIIa defunctioning polymorphism in paediatric patients with renal allograft

Introduction

Fc gamma receptor (FcγR) IIa is considered the most widely distributed of the three classes of Fc receptors, and it expresses an allelic polymorphism. This type of polymorphism may modify the immune response and may be an important factor for some diseases. The aim of the study reported herein was to evaluate the association between the FcγRIIa polymorphism and susceptibility to both end-stage renal disease (ESRD) and acute kidney graft rejection (AR) in children who have undergone renal transplantation.

Material and methods

The study evaluated 70 children who had undergone transplantation and 60 healthy subjects. AR was observed in 25 children.

Results

FcγRIIa genotypes and alleles were significantly different between transplantation patients and the control group. The assessment for FcγR of the groups in which AR was present showed that there was only a risk of having an acute rejection in homozygous genotype RR.

Conclusions

FcγRIIa RR genotype and allele frequency was increased in paediatric renal transplant recipients. The present findings showed that FcγRIIa genotype may be related to ESRD disease susceptibility, and FcγRIIa polymorphisms seemed to affect AR.

Fcγ receptor expression on splenic macrophages in adult immune thrombocytopenia

Splenic macrophages play a key role in immune thrombocytopenia (ITP) pathogenesis by clearing opsonized platelets. Fcγ receptors (FcγR) participate in this phenomenon, but their expression on splenic macrophages and their modulation by treatment have scarcely been studied in human ITP. We aimed to compare the phenotype and function of splenic macrophages between six controls and 24 ITP patients and between ITP patients according to the treatments they received prior to splenectomy. CD86, human leucocyte antigen D-related (HLA-DR) and FcγR expression were measured by flow cytometry on splenic macrophages. The major FcγR polymorphisms were determined and splenic macrophage function was assessed by a phagocytosis assay. The expression of the activation markers CD86 and HLA-DR was higher on splenic macrophages during ITP compared to controls. While the expression of FcγR was not different between ITP and controls, the phagocytic function of splenic macrophages was reduced in ITP patients treated with intravenous immunoglobulin (IVIg) within the 2 weeks prior to splenectomy. The FCGR3A (158V/F) polymorphism, known to increase the affinity of FcγRIII to IgG, was over-represented in ITP patients. Thus, these are the first results arguing for the fact that the therapeutic use of IVIg during human chronic ITP does not modulate FcγR expression on splenic macrophages but decreases their phagocytic capabilities.

Risk factors for heparin-induced thrombocytopenia: Focus on Fcγ receptors

Fcγ receptors have critical roles in the pathophysiology of heparin-induced thrombocytopenia (HIT), a severe immune-mediated complication of heparin treatment. Activation of platelets, monocytes and neutrophils by platelet-activating anti-PF4/heparin IgG antibodies results in thrombocytopenia, hypercoagulability and thrombosis in susceptible patients, effects that depend on FcγRIIA. In addition, FcγRIIIA receptors probably contribute to clearance of platelets sensitised by HIT immune complexes. FcγRI has also been reported to be involved in monocyte activation by HIT IgG antibodies and synthesis of tissue factor. This review focuses on the role of these FcγRs in HIT pathophysiology, including the potential influence of several gene variations associated with variable risk of HIT and related thrombosis. In particular, the 276P and 326Q alleles of CD148, a protein tyrosine phosphatase that regulates FcγRIIA signalling, are associated with a lower risk of HIT, and platelets from healthy donors expressing these alleles are hyporesponsive to anti-PF4/H antibodies. It was also recently demonstrated that the risk of thrombosis is higher in HIT patients expressing the R isoform of the FcγRIIA H131R polymorphism, with HIT antibodies shown to activate RR platelets more efficiently, mainly explained by an inhibitory effect of normal IgG2, which bound to the FcγRIIA 131H isoform more efficiently. Environmental risk factors probably interact with these gene polymorphisms affecting FcγRs, thereby increasing thrombosis risk in HIT.

CD16 and CD32 Gene Polymorphisms May Contribute to Risk of Idiopathic Thrombocytopenic Purpura

BACKGROUND Epidemiological studies have evaluated the associations of CD16 158F>V and CD32 131H>R gene polymorphisms with the risk of idiopathic thrombocytopenic purpura (ITP). MATERIAL AND METHODS Published studies on CD16 158F>V and CD32 131H>R polymorphisms with susceptibility to ITP were systematically reviewed until April 1, 2014. The Cochrane Library Database, Medline, CINAHL, EMBASE, Web of Science, and Chinese Biomedical Database (CBM) were used to search for relevant studies and then a meta-analysis was conducted by using Stata 12.0 software in order to produce consistent statistical results. RESULTS In total, 10 clinical case-control studies with 741 ITP patients and 1092 healthy controls were enrolled for quantitative data analysis. Results of this meta-analysis suggest that CD16 158F>V polymorphism had strong correlations with the susceptibility to ITP under 5 genetic models (all P<0.05). However, no similar associations were found between CD32 131H>R polymorphism and the susceptibility to ITP (all P>0.05). Subgroup analysis by ethnicity revealed that CD16 158F>V polymorphism was associated with the increased risk of ITP among both Caucasian and non-Caucasian populations. Nevertheless, no statistically significant correlations between CD32 131H>R polymorphism and the risk of ITP were observed among Caucasians and non-Caucasians (all P>0.05). CONCLUSIONS Our findings indicate that CD16 158F>V polymorphism may contribute to the increased risk of ITP, whereas CD32 131H>R polymorphism may not be an important risk factor for ITP.

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.

Clinical and laboratory predictors of chronic immune thrombocytopenia in children: a systematic review and meta-analysis

Childhood immune thrombocytopenia (ITP) is a rare autoimmune bleeding disorder. Most children recover within 6 to 12 months, but individual course is difficult to predict. We performed a systematic review and meta-analysis to identify predictors of chronic ITP. We found 1399 articles; after critical appraisal, 54 studies were included. The following predictors of chronic ITP in children, assessed in at least 3 studies, have been identified: female gender (odds ratio [OR] 1.17, 95% confidence interval [CI] 1.04-1.31), older age at presentation (age ≥11 years; OR 2.47, 95% CI 1.94-3.15), no preceding infection or vaccination (OR 3.08, 95 CI 2.19-4.32), insidious onset (OR 11.27, 95% CI 6.27-20.27), higher platelet counts at presentation (≥20 × 10(9)/L: OR 2.15, 95% CI 1.63-2.83), presence of antinuclear antibodies (OR 2.87, 95% 1.57-5.24), and treatment with a combination of methylprednisolone and intravenous immunoglobulin (OR 2.67, 95% CI 1.44-4.96). Children with mucosal bleeding at diagnosis or treatment with intravenous immunoglobulin alone developed chronic ITP less often (OR 0.39, 95% CI 0.28-0.54 and OR 0.71, 95% CI 0.52-0.97, respectively). The protective effect of intravenous immunoglobulin is remarkable and needs confirmation in prospective randomized trials as well as future laboratory studies to elucidate the mechanism of this effect.

The immunophenotype of bone marrow lymphocytes in children with immune thrombocytopenic purpura

PURPOSE

Primary immune thrombocytopenic purpura (ITP), caused by immune system dysfunction, is recognized as the leading cause of thrombocytopenia in pediatric population. Nonetheless, inadequate studies have been performed on bone marrow immunophenotyping of children with ITP. In this study, we aimed to investigate the immunophenotype of bone marrow lymphocytes in these children.

PATIENTS AND METHODS

Between 2008 and 2012, 35 children with ITP and 26 age and sex matched healthy controls were recruited. All participants underwent bone marrow aspiration. Appropriate B-cell, T-cell, and myeloid lineage monoclonal antibodies were employed to determine the immunophenotype of these patients.

RESULTS

CD10, CD19, and CD20, all indicative of premature B-cell markers, were significantly greater in children with ITP. CD22, mainly expressed on mature B cells was slightly, but not significantly reduced in the patients' group (P = .42). On the other hand, T cell markers including CD2, CD3, CD5, and CD7 were underexpressed. CD33, a specific marker for myeloid lineage, was underexpressed in the patients' group (5.6 ± 4.7 vs. 12.9 ± 7.3, P < .001). Noteworthy, the immunophenotype did not significantly differ between acute and persistent cases.

CONCLUSION

Overall, a phenotype characterized by increased pre-B-cell markers along with decreased T cell immunophenotypic markers was observed in bone marrow lymphocytes of children with ITP in the present study. Further larger scale studies are recommended to confirm our findings, as precise mapping of the immunophenotype of lymphocytes in these patients would pave the road to improved diagnosis and treatment.