Key Points Antibodies causing FNAIT have decreased Fc fucosylation, unlike in refractory thrombocytopenia. Decreased Fc fucose increases affinity to FcγRIIIa/b, enhances platelet phagocytosis, and correlates with increased disease severity.

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Human IgG3 displays the strongest effector functions of all IgG subclasses but has a short half-life for unresolved reasons. Here we show that IgG3 binds to IgG-salvage receptor (FcRn), but that FcRn-mediated transport and rescue of IgG3 is inhibited in the presence of IgG1 due to intracellular competition between IgG1 and IgG3. We reveal that this occurs because of a single amino acid difference at position 435, where IgG3 has an arginine instead of the histidine found in all other IgG subclasses. While the presence of R435 in IgG increases binding to FcRn at neutral pH, it decreases binding at acidic pH, affecting the rescue efficiency—but only in the presence of H435–IgG. Importantly, we show that in humans the half-life of the H435-containing IgG3 allotype is comparable to IgG1. H435–IgG3 also gave enhanced protection against a pneumococcal challenge in mice, demonstrating H435–IgG3 to be a candidate for monoclonal antibody therapies.

Gene copy number variation (CNV) and single nucleotide polymorphisms (SNPs) count as important sources for interindividual differences, including differential responsiveness to infection or predisposition to autoimmune disease as a result of unbalanced immunity. By developing an FCGR-specific multiplex ligationdependent probe amplification assay, we were able to study a notoriously complex and highly homologous region in the hu-

We analyzed a genetic polymorphism of Fcγ receptor IIIa (CD16) that is present on position 158 (Phe or Val) in the membrane-proximal, IgG-binding domain. With a polymerase chain reaction–based allele-specific restriction analysis assay we genotyped 87 donors and found gene frequencies of 0.57 and 0.43 for FcγRIIIA-158F and −158V, respectively. A clear linkage was observed between the FcγRIIIA-158F and −48L genotypes on the one hand and the FcγRIIIA-158V and −48H or −48R genotypes on the other hand (χ2 test; P < .001). To determine the functional consequences of this FcγRIIIa-158V/F polymorphism, we performed IgG binding experiments with natural killer (NK) cells from genotyped donors. All donors were also typed for the recently described triallelic FcγRIIIa-48L/R/H polymorphism. NK cells were treated with lactic acid to remove cell-associated IgG. FcγRIIIaNK158F bound significantly less IgG1, IgG3, and IgG4 than did FcγRIIIaNK-158V, irrespective of the FcγRIIIa-48 phenotype. Moreover, freshly isolated NK cells from FcγRIIIa-158VV individuals carried significantly more cytophilic IgG than did NK cells from FcγRIIIa-158FF individuals. In addition, CD16 monoclonal antibody (MoAb) MEM154 bound more strongly to FcγRIIIa-158V, compared with -158F, again independently of the FcγRIIIa-48 phenotype. The binding of MoAb B73.1 was not influenced by the FcγRIIIa-158V/F polymorphism, but proved to depend solely on the amino acid present at position 48 of FcγRIIIa. In conclusion, the previously reported differences in IgG binding among the three FcγRIIIa-48L/R/H isoforms are a consequence of the linked, biallelic FcγRIIIa-158V/F polymorphism at amino-acid position 158.

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We analyzed a genetic polymorphism of Fcγ receptor IIIa (CD16) that is present on position 158 (Phe or Val) in the membrane-proximal, IgG-binding domain. With a polymerase chain reaction–based allele-specific restriction analysis assay we genotyped 87 donors and found gene frequencies of 0.57 and 0.43 for FcγRIIIA-158F and −158V, respectively. A clear linkage was observed between the FcγRIIIA-158F and −48L genotypes on the one hand and the FcγRIIIA-158V and −48H or −48R genotypes on the other hand (χ2 test; P < .001). To determine the functional consequences of this FcγRIIIa-158V/F polymorphism, we performed IgG binding experiments with natural killer (NK) cells from genotyped donors. All donors were also typed for the recently described triallelic FcγRIIIa-48L/R/H polymorphism. NK cells were treated with lactic acid to remove cell-associated IgG. FcγRIIIaNK158F bound significantly less IgG1, IgG3, and IgG4 than did FcγRIIIaNK-158V, irrespective of the FcγRIIIa-48 phenotype. Moreover, freshly isolated NK cells from FcγRIIIa-158VV individuals carried significantly more cytophilic IgG than did NK cells from FcγRIIIa-158FF individuals. In addition, CD16 monoclonal antibody (MoAb) MEM154 bound more strongly to FcγRIIIa-158V, compared with -158F, again independently of the FcγRIIIa-48 phenotype. The binding of MoAb B73.1 was not influenced by the FcγRIIIa-158V/F polymorphism, but proved to depend solely on the amino acid present at position 48 of FcγRIIIa. In conclusion, the previously reported differences in IgG binding among the three FcγRIIIa-48L/R/H isoforms are a consequence of the linked, biallelic FcγRIIIa-158V/F polymorphism at amino-acid position 158.

Haemolytic Disease of the Fetus and Newborn (HDFN) is caused by maternal alloimmunization against red blood cell antigens. In severe cases, HDFN may lead to fetal anaemia with a risk for fetal death and to severe forms of neonatal hyperbilirubinaemia with a risk for kernicterus. Most severe cases are caused by anti-D, despite the introduction of antental and postnatal anti-D immunoglobulin prophylaxis. In general, red blood cell antibody screening programmes are aimed to detect maternal alloimmunization early in pregnancy to facilitate the identification of high-risk cases to timely start antenatal and postnatal treatment. In this review, an overview of the clinical relevance of red cell alloantibodies in relation to occurrence of HDFN and recent views on prevention, screening and treatment options of HDFN are provided.