The presence of antibodies in blood (lupus anticoagulant or anticardiolipin antibodies [aCL]) that recognize phospholipids, phospholipid-binding proteins, or both has been associated with a thrombophilic disorder, the antiphospholipid syndrome (APS) (1, 2). This syndrome is characterized by recurrent vascular thromboses involving the venous, arterial, and placental circulation and may occur alone (primary APS) or in conjunction with other autoimmune disease (secondary APS) (1–3). Several mechanisms have been proposed for explaining the procoagulant state of APS (2). According to one hypothesis (4), antiphospholipid antibody (aPL) binding to protein–phospholipid complexes on platelets, endothelial cells, or other cells may result in their activation via crosslinking of Fcγ receptors of type IIa (FcγRIIa) (5, 6). This activation may induce a prothrombotic phenotype (4). If this hypothesis is true, FcγRIIA function may regulate APS pathogenesis.
FcγRIIa is widely expressed on hematopoietic cells, including neutrophils and mononuclear phagocytes. This isoform is the only FcγR expressed on platelets and endothelial cells (7). A common functional polymorphism of the FcγRIIA gene plays a particular role in the expression of IgG2-mediated antibody responses. The 2 allelic forms of FcγRIIA differ by a single amino acid at residue 131 (histidine or arginine). The H131 allele is essential for handling IgG2 immune complexes (8). This polymorphism has been proposed to be influential in a variety of autoimmune diseases (7). The low-binding R131 allele imparts a significant risk for systemic lupus erythematosus (SLE) (9). Binding to the H131 variant can initiate more pronounced monocyte, platelet, and endothelial cell activation in the context of an IgG2 immune response, inducing a prothrombotic phenotype. Since autoimmune disease–associated aCL show IgG2 predominance (10, 11), one might expect an enrichment of the H131 allele in APS patients (11). However, the data are not yet conclusive (12, 13). Besides, in patients with APS secondary to SLE, this anticipated selection of the H131 allele may not be evident, given the overrepresentation of the RR genotype in SLE (9). Thus, the role of the FcγRIIA-R/H131 polymorphism in APS is still unclear.
APS is relatively uncommon, and isolated studies regarding its genetic background are unlikely to be conclusive. Previous analyses for HLA-conferred APS susceptibility have required the combination of data from various study groups (14). The aim of the present study was to investigate the importance of FcγRIIA alleles for APS susceptibility in the context of an international collaborative meta-analysis. Such an approach enhances the power to detect modest, but clinically important, differences between groups and helps to avoid spurious findings due to inconsistencies of the data from different research teams.
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This international meta-analysis suggests a complex genetic background underlying the relationship between the FcγRIIA-R/H131 polymorphism and APS. A significant increase in RR homozygosity was documented in the whole group of APS patients. This selection was most striking in patients with APS secondary to SLE. The R131 allele seemed to confer risk for APS under a recessive model, whereas the effect of R131 on susceptibility to SLE has been found to have a dose-response character (9). This difference may be explained by the fact that among lupus patients, those who have APS also have an overrepresentation of homozygosity for the high-binding H131 allele. This may be even more prominent in primary APS, but data on primary APS comparisons were too limited to be definitive. As a result of these composite genetic influences, when the whole group of APS patients was contrasted against disease-free controls, there was a selection of the HH genotype as compared with the RH genotype. Thus, the meta-analysis results suggest that the observed genetic profile may be a composite of 2 different and opposing influences with regard to APS susceptibility.
Recent evidence suggests an effect of FcγR as potential initiators of thrombosis. This complication seems to be a consequence of platelet activation initiated when platelet FcγRIIa are crosslinked by antibodies (5, 32). Since the interaction of IgG2-containing antibodies with FcγRIIa is allotype-dependent (8), it has been hypothesized that the high-binding H131 allele would be overrepresented among subjects with antibody-mediated thrombosis (11, 33). The hypothesis has also been investigated in heparin-induced thrombocytopenia, another syndrome with similar immune-mediated thrombosis, but no consistent relationship between the H131 allele and this syndrome was shown (34).
The results of this meta-analysis may help to explain discrepancies among findings of previous studies of FcγRIIA-R/H131 in APS (11–13). HH homozygosity increases the risk of APS relative to RH heterozygosity. However, the effect of HH homozygosity for susceptibility to APS is overwhelmed by the larger effect of RR homozygosity for susceptibility to SLE in general (9), especially among patients with secondary APS. The R131 allele may confer risk for SLE through deficient handling of IgG2-containing immune complexes by the mononuclear phagocyte system, leading to their tissue deposition and to accelerated organ damage (7).
It is not clear whether RR homozygosity may also confer an increased risk of primary APS per se, aside from SLE, through some common pathophysiologic link to SLE. We should caution that the classification criteria for APS and SLE are functional criteria and may not fully correspond to the subgrouping of APS and SLE based on genetic predisposition. APS is a remarkably heterogeneous syndrome with different prognostic profiles (35).
Alternative hypotheses could explain an independent effect of RR homozygosity on the risk of APS. For example, apoptotic cells are a major source of autoantigens, and an impairment of their physiologic clearance may promote the development of autoimmunity. Anionic phospholipids redistribute from the inner leaflet to the outer leaflet of cell membranes during apoptosis (36). This systemic exposure could enable the binding of phospholipid-binding proteins such as β2-glycoprotein I (β2GPI) to apoptotic cell membranes (37) and may also trigger the production of aPL antibodies (38, 39). Phospholipid–β2GPI complexes on the surface of membrane blebs are recognized by aPL antibodies (37, 38, 40, 41), which leads to opsonization of apoptotic cells that are then phagocytosed by FcR-positive macrophages (40, 41). Considering that aPL (10, 11), especially those with reactivity to β2GPI (42, 43), show IgG2-dominant distribution, such antibodies would be predicted to be poor opsonins in RR homozygous subjects. Defective clearance of aPL-opsonized apoptotic particles by macrophages may lead to inflammatory removal pathways (44, 45), favoring an autoimmune, rather than an antiinflammatory, response to apoptotic cells. Thus, antigen processing and presentation by antigen-presenting cells (44, 45) provide an antigenic stimulus for specific T and B clones, leading to further aPL antibody production that may exert procoagulant effects (2). Moreover, persistently circulating apoptotic cells could express procoagulant properties, thus supporting thrombotic events (46).
That the RR genotype is mostly enriched among lupus patients with APS is also consistent with this explanation, since SLE is characterized by an increased rate of activation-induced cell death (47). Increased apoptotic load with the augmented exposure of anionic phospholipids may amplify the consequences of the defective handling of apoptotic cells in lupus patients who are homozygous for the low-binding allele. Remarkably, aPL antibodies occur more frequently and earlier in SLE patients with the RR genotype (20, 48). Furthermore, macrophages from SLE patients with sufficient expression of receptors implicated in phagocyte recognition of cells undergoing apoptosis (CD14 and CD36) exhibit defective engulfment of apoptotic cell material in vitro (49).
Patients with either primary or secondary APS have similar clinical profiles as far as thrombotic manifestations are concerned (3, 50). Nevertheless, arthritis, low C4 levels, and hematologic abnormalities such as hemolytic anemia, thrombocytopenia, leukopenia, and neutropenia are more common among patients with APS secondary to SLE (3, 50). The selection of the RR homozygous state that was demonstrated only in patients with secondary APS relative to disease-free controls could also be related to these differences, even though other factors may also play a role in the pathogenesis of these manifestations. It is noteworthy that such clinical features and serologic findings seem to be overrepresented in lupus patients with low-binding FcγR alleles (20, 21). Moreover, a critical role for FcγR has been demonstrated in models of collagen-induced arthritis as well as in models of experimental cytopenias (51, 52).
The relevance of the FcγRIIA-R/H131 polymorphism for APS susceptibility should be viewed at the population level. Although the summary OR estimates suggest only a moderate genetic effect, the importance of this effect may be considerable at the population level, given the high frequency of the RR genotype (∼25%) in populations of European descent (7). Thus, modest ORs translate to a clinically meaningful proportion of APS cases that could be attributed to RR homozygosity (at least 10% in populations of European descent). Empirical evidence suggests that for multigenetic diseases, the magnitude of the ORs is generally modest. Among 55 genetic associations examined in different disciplines, none had an OR exceeding 2.0, and only 13 showed a significant OR exceeding 1.5 (53).
Some limitations of this study should be discussed. First, the number of APS patients with specific clinical manifestations was too small to reliably assess the effect of the FcγRIIA-R/H131 polymorphism on the risk of vascular thromboses or other APS-related features. Multiple comparisons and small subgroups would make such inferences impractical, even with the sample size of the meta-analysis. Second, bias is possible in a meta-analysis. However, bias diagnostics did not suggest the presence of such problems in this study. Three study teams did not use the preliminary criteria for the classification of APS; nevertheless, similar results were obtained when data from these study teams were excluded. Last, the association of the FcγRIIA-R/H131 polymorphism with APS could be explained by the existence of linkage disequilibrium between this gene and other candidate genes on chromosome 1 that may also be more directly relevant for the risk of specific disease manifestations (54). This would require the investigation of extended haplotypes (55, 56) in the future. Genetic variants in different genes might also contribute to the pathogenesis of APS (57, 58). Recognition of specific disease-associated genetic factors may expand our understanding of disease pathogenesis and may also be useful for identifying subjects at increased risk of developing APS.
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Other key investigators of the collaborating study teams involved in this project were as follows: Toru Fukazawa, MD, PhD (Juntendo University, Tokyo, Japan), Hiroshi Hashimoto, MD, PhD (Juntendo University, Tokyo, Japan), Elisabeth Tournier-Lasserve, MD (Faculté de Médecine Necker, Paris, France), Kwang-Taek Oh, MD (Hanyang University, Seoul, South Korea), Jan G. J. van de Winkel, PhD (Department of Immunology and Genmab, University Medical Center Utrecht, Utrecht, The Netherlands).