Inhibition of Fcγ Receptor-Mediated Phagocytosis by a Nonphagocytic Fcγ Receptor

The Fc receptor for IgG expressed in the villus endothelium of human placenta is Fc gamma RIIb2

To evaluate the potential role of human placental endothelial cells in the transport of IgG from maternal to fetal circulation, we studied Fc gamma receptor (Fc gamma R) expression by immunohistology and immunoblotting. Several pan-Fc gamma RII Abs that label the placental endothelium displayed a distribution pattern that correlated well with transport functions, being intense in the terminal villus and nil in the cord. In contrast, the MHC class 1-like IgG transporter, FcRn, and the classical Fc gamma RIIa were not expressed in transport-related endothelium of the placenta. Our inference, that Fc gamma RIIb was the likely receptor, we confirmed by analyzing purified placental villi, enriched in endothelium, by immunoblotting with a new Ab specific for the cytoplasmic tail of Fc gamma RIIb. These experiments showed that the Fc gamma RII expressed in villus endothelium was the b2 isoform whose cytoplasmic tail is known to include a phosphotyrosyl-based motif that inhibits a variety of immune responses. We suggest that this receptor is perfectly positioned to transport IgG although as well it may scavenge immune complexes.

Variant Genotypes of the Low-Affinity Fcγ Receptors in Two Control Populations and a Review of Low-Affinity Fcγ Receptor Polymorphisms in Control and Disease Populations

Fcγ-receptors (FcγR) provide a critical link between humoral and cellular immunity. The genes of the low-affinity receptors for IgG and their isoforms, namely, FcγRIIa, FcγRIIb, FcγRIIIa, FcγRIIIb, and SH-FcγRIIIb, are located in close proximity on chromosome 1q22. Variant alleles may differ in biologic activity and a number of studies have reported the frequencies of variant FcγR alleles in both disease and control populations. No large study has evaluated the possibility of a nonrandom distribution of variant genotypes. We analyzed 395 normal individuals (172 African Americans [AA] and 223 Caucasians [CA]) at the following loci: FcγRIIa, FcγRIIIa, and FcγRIIIb, including the SH-FcγRIIIb. The genotypic distributions of FcγRIIa, FcγRIIIa, and FcγRIIIb conform to the Hardy-Weinberg law in each group. There was no strong evidence that combinations of 2-locus genotypes of the 3 loci deviated from random distributions in these healthy control populations. The distribution of SH-FcγRIIIb is underrepresented in CA compared with AA (P < .0001) controls. A previously reported variant FcγRIIb was not detected in 70 normal individuals, indicating that this allele, if it exists, is very rare (<1%). In conclusion, we present data that should serve as the foundation for the interpretation of association studies involving multiple variant alleles of the low-affinity FcγR.

IgG-Mediated Enhancement of Antibody Responses Is Low in Fc Receptor γ Chain-Deficient Mice and Increased in FcγRII-Deficient Mice

Immunization with IgG/Ag or IgE/Ag complexes leads to a higher production of specific Abs than immunization with Ag alone. The enhancing effect of IgE is exclusively dependent upon the low-affinity receptor for IgE, FcεRII, whereas the mechanism behind IgG-mediated enhancement is unknown. We have investigated whether receptors for the Fc part of IgG are required for responses to IgG/Ag. Mice lacking the γ subunit of Fc receptors (FcRs) (FcRγ−/−), FcγRII (FcγRII−/−), or FcγRIII (FcγRIII−/−) were immunized with BSA-2,4,6-trinitrophenyl (TNP) alone or BSA-TNP complexed to monoclonal TNP-specific IgG1, IgG2a, or IgG2b. As expected, all subclasses enhanced the Ab-response to BSA in wild-type mice. Enhancement was in the same order of magnitude in FcγRIII−/− mice (≤177-fold of controls administered Ag alone), whereas it was abrogated in FcRγ−/− mice and augmented in FcγRII−/− mice (≤5147-fold of controls). The response to IgE/Ag complexes in FcRγ−/− and FcγRII−/− mice was similar to that seen for wild-type mice, demonstrating that non-FcγR-dependent responses were normal. Our observations suggest that IgG/Ag complexes enhance Ab responses via FcγRs. Moreover, they reveal a strong negative regulation of Ab responses to IgG/Ag exerted by FcγRII.