FcgammaRIIIA Gene Research Articles

A triallelic Fc gamma receptor type IIIA polymorphism influences the binding of human IgG by NK cell Fc gamma RIIIa.

A donor-dependent difference in electrophoretic mobility of deglycosylated released Fc gamma RIIIa derived from NK cells and macrophages was observed. We investigated whether this was based on a polymorphism of the Fc gamma RIIIA gene. Cloning and sequencing of Fc gamma RIIIa-encoding cDNA derived from an apparently heterozygous donor showed one single nucleotide substitution at position 230 (T-->G), which was responsible for a leucine (L)-->arginine (R) substitution at position 48 in the first extracellular Ig-like domain (EC1) of Fc-gamma RIIIa and caused a higher electrophoretic mobility of Fc gamma RIIIa. An allele-specific primer annealing PCR assay was developed to amplify specifically an Fc gamma RIIIA gene-derived fragment, which was digested with AciI (recognizing G230) or MnlI (recognizing T230). MnlI restriction analysis revealed the presence of a third Fc gamma RIIIa allele with a T230-->A substitution, which predicts a change of 48-leucine into 48-histidine (H). A gene frequency of 86% for the T230 (48-L) allele, 6% for G230 (48-R), and 8% for A230 (48-H) was found. A significantly different genotype distribution was found among 12 unrelated Caucasian Fc gamma RIIIB gene-deficient donors. Fc gamma RIIIa-48R and Fc gamma RIIIa-48H showed a higher binding capacity of human (h)IgG1, hIgG3, and hIgG4 compared with Fc gamma RIIIa-48L. Finally, the CD16 mAbs 1D3 and MEM154 bound more strongly and Leu11c (B73.1) bound less to the newly identified Fc gamma RIIIa isoforms.

Fc gamma RIIA alleles are heritable risk factors for lupus nephritis in African Americans.

Allelic variants of Fc gamma R confer distinct phagocytic capacities providing a mechanism for heritable susceptibility to immune complex disease. Human Fc gamma RIIa has two codominantly expressed alleles, R131 and H131, which differ substantially in their ability to ligate human IgG2. The Fc gamma RIIa-H131 is the only human Fc gamma R which recognizes IgG2 efficiently and optimal IgG2 handling occurs only in the homozygous state. Therefore, since immune complex clearance is essential in SLE, we hypothesized that Fc gamma RIIA genes are important disease susceptibility factors for SLE, particularly lupus nephritis. In a two-stage cross-sectional study, we compared the distribution of Fc gamma RIIA alleles in African Americans with SLE to that in African American non-SLE controls. A pilot study of 43 SLE patients and 39 controls demonstrated a skewed distribution of Fc gamma RIIA alleles, with only 9% of SLE patients homozygous for Fc gamma RIIa-H131 compared with 36% of controls (odds ratio, 0.18; 95% CI, 0.05-0.69, P = 0.009). This was confirmed with a multicenter study of 214 SLE patients and 100 non-SLE controls. The altered distribution of Fc gamma RIIA alleles was most striking in lupus nephritis. Trend analysis of the genotype distribution showed a highly significant decrease in Fc gamma RIIA-H131 as the likelihood for lupus nephritis increased (P = 0.0004) consistent with a protective effect of the Fc gamma RIIA-H131 gene. The skewing in the distribution of Fc gamma RIIA alleles identifies this gene as a risk factor with pathophysiologic importance for the SLE diathesis in African Americans.

Reconstitution of human Fc gamma RIII cell type specificity in transgenic mice.

The human low affinity receptors for the Fc domain of immunoglobulin G, Fc gamma RIII, are encoded by two genes (IIIA and IIIB) which share >95% sequence identity in both coding and flanking sequences. Despite this extraordinary sequence conservation, IIIA is expressed in natural killer (NK) cells and macrophages and is absent in neutrophils, whereas IIIB is expressed only in neutrophils. To determine the molecular basis for this differential expression, we have generated transgenic mice using the genomic sequences of IIIA and IIIB. IIIA and IIIB transgenic mice show faithful reconstitution of this human pattern of cell type specificity. To determine the cis acting sequence elements that confer this specificity, we constructed chimeric genes in which 5.8 kb of 5' sequences of the IIIB gene has been replaced with a homologous region from the IIIA gene, and conversely, IIIA 5' sequences have been substituted for the analogous region of the IIIB gene. Promoter swap transgenic mice that carry IIIA 5' flanking sequences express Fc gamma RIII in macrophages and NK cells. In contrast, promoter swap transgenic mice that contain IIIB 5' sequences express Fc gamma RIII in neutrophils only. These studies define the elements conferring the cell type-specific expression of the human Fc gamma RIII genes within the 5' flanking sequences and first intron of the human Fc gamma RIIIA and Fc gamma RIIIB genes.

The Human Low Affinity Immunoglobulin G Fc Receptor III-A and III-B Genes: MOLECULAR CHARACTERIZATION OF THE PROMOTER REGIONS

The human Fc receptor with low affinity for IgG (Fc gamma RIII, CD16) is encoded by two nearly identical genes, Fc gamma RIII-A and Fc gamma RIII-B, resulting in tissue-specific expression of alternative membrane-anchored isoforms. The transmembrane CD16 receptor forms a heteromeric structure with the Fc epsilon RI (gamma) and/or CD3 (zeta) subunits on the surface of activated monocytes/macrophages, NK cells, and a subset of T cells. The expression of the glycosylphosphatidylinositol-anchored CD16 isoform encoded by the Fc gamma RIII-B gene is restricted to polymorphonuclear leukocytes and can be induced by Me2SO differentiation of HL60 cells. We have isolated and sequenced genomic clones of the human Fc gamma RIII-A and Fc gamma RIII-B genes, located their transcription initiation sites, identified a different organization of their 5' regions, and demonstrated four distinct classes of Fc gamma RIII-A transcripts (a1-a4) compared with a single class of Fc gamma RIII-Bb1 transcripts. Both CD16 promoters (positions -198 to -10) lack the classical "TATA" positioning consensus sequence but confer transcriptional activity when coupled to the human lysozyme enhancer. Both promoters also display different tissue-specific transcriptional activities reflecting the expected gene expression of Fc gamma RIII-A and Fc gamma RIII-B in NK cells versus polymorphonuclear leukocytes. Within the -198/-10 fragments, the sequences of the two CD16 genes have been identified to differ in 10 positions. It is suggested that these nucleotide differences might contribute to cell type-specific transcription of Fc gamma RIII genes.

Clustering of the platelet Fc gamma receptor induces noncovalent association with the tyrosine kinase p72syk.

The Fc receptor for IgG in platelets was identified as the integral membrane isoform encoded by the Fc gamma RIIA gene. Functional analysis of this molecule determined that activated Fc gamma RIIA is tyrosine phosphorylated and that activation induced the physical association with the protein tyrosine kinase p72syk. Other tyrosine-phosphorylated molecules also co-immunoadsorbed with the activated receptor. Tyrosine kinase activity co-immunoadsorbing with the platelet Fc gamma R was enhanced upon activation and specifically induced the phosphorylation, on tyrosine residues, of a physically associated 72-kDa protein. These data support a model of Fc gamma receptor-mediated platelet activation where signal is transduced through inducible association of the tyrosine kinase p72syk with the low affinity Fc gamma receptor. Thrombin, a potent platelet agonist, has been shown to up-regulate the activity of the tyrosine kinase p72syk in platelets. Consequently, our findings identify a second pathway by which p72syk is activated in platelets.

Intrinsic human glomerular mesangial cells can express receptors for IgG complexes (hFc gamma RIII-A) and the associated Fc epsilon RI gamma-chain.

Immune complexes localize to the glomerular mesangium in many forms of human glomerulonephritis. In this investigation, we determined whether primary cultured human glomerular mesangial cells (HMC) express mRNA and functional protein of FcR for IgG. Performing reverse transcription-PCR with subsequent control Southern blots, we showed that in contrast to human monocytes or granulocytes, mRNA for Fc gamma RI and Fc gamma RII is not detectable in either growth-arrested (48 h serum free) or unstimulated proliferating (medium with 10% FCS) HMC. However, IFN-gamma in combination with LPSE.coli (LPS), but not LPS alone elicited a significant transcription of hFc gamma RIII mRNA in resting HMC, whereas cytokines, such as IL-1 beta or TGF-beta, failed to induce any of the three Fc gamma Rs in resting HMC. Proliferating HMC showed a basal transcription of Fc gamma RIII mRNA, which was enhanced by IFN-gamma in combination with LPS. Slot-blot analysis indicated that only the Fc gamma RIII-A gene encoding the transmembrane isoform of Fc gamma RIII was expressed by HMC. For the first time, transcripts for the gamma-chain of Fc epsilon RI were found in HMC. Fc gamma RIII protein expression was detected after LPS/IFN-gamma stimulation both by specific staining of paraformaldehyde-fixed HMC and immunoprecipitation of Fc gamma RIII protein from HMC membranes. Fc gamma RIII-A receptors are functionally active, as HMC IL-6 mRNA synthesis was stimulated by heat-aggregated IgG or F(ab')2 fragments of CLBGran1 mAb only after induction of Fc gamma RIII-A. These in vitro data suggest that HMC that are basically negative for Fc gamma RIII can be stimulated to express low affinity Fc gamma Rs, and thus may participate actively in human glomerulonephritis involving immune complexes.

The major peripheral myelin protein zero gene: structure and localization in the cluster of Fc gamma receptor genes on human chromosome 1q21.3-q23.

We have characterized the human gene encoding the major peripheral myelin protein zero (P0) and assigned it, by in situ hybridization, to the q21.3-q23 region of human chromosome 1. This region is known to contain a cluster of interspersed genes coding for the related human leukocyte receptors of the Fc portion of the immunoglobulin G (Fc gamma RI, II, III). This colocalization was refined by the finding of a yeast artificial chromosome (YAC) of the Centre d'Etude du Polymorphisme Humain (CEPH) library, hybridizing to the P0 and Fc gamma RIIA genes, demonstrating their physical linkage. These data may have important implications in demyelinating diseases studies like Charcot-Marie-Tooth disease type 1B (CMT1B).

Response of human platelets to activating monoclonal antibodies: importance of Fc gamma RII (CD32) phenotype and level of expression

Certain monoclonal antibodies (MoAbs) specific for platelet membrane glycoproteins are known to be capable of activating platelets, and it is generally thought that platelets from normal subjects are equally susceptible to stimulation by such MoAbs. We found that platelets from 20 normal donors varied significantly in their sensitivity to three IgG1 murine MoAbs specific for membrane glycoproteins CD9, GPIV (CD36), and the GPIIb/IIIa complex (CD41), respectively. The response of platelets to these MoAbs was blocked by prior addition of MoAb IV.3 specific for the Fc gamma RII receptor, indicating that activation was Fc receptor mediated. Platelets that responded poorly to these MoAbs failed to bind the MoAb 41H.16, specific for the “responder” form of Fc gamma RII, but platelets that responded well reacted with this MoAb. The average number of Fc gamma RII receptors on platelets from “responders” and “non-responders” was approximately the same. However, the number of Fc gamma RII receptors expressed influenced sensitivity of a subgroup of “responder” platelets to the anti-CD41 MoAb. These platelets were judged on the basis of MoAb binding studies to be heterozygous for the two alleles of Fc gamma RIIA. In contrast to their varying sensitivity to IgG1 MoAbs, members of the platelet panel responded equally well to 50H.19, an IgG2a MoAb specific for CD9, and these responses could not be blocked by MoAb IV.3 in the presence of plasma. This appears to be because of dual actions of 50H.19 on platelets: one FcR-dependent and the other complement-dependent. Our findings confirm previous reports that certain IgG1 MoAbs activate platelets through binding of their Fc domains to Fc gamma RII receptors and demonstrate that this response is influenced both by Fc gamma RII phenotype and (in the case of the anti-CD41 MoAb) by the number of Fc gamma RII receptors expressed. The failure of nonresponding platelets to bind detectable amounts of MoAb 41H.16, which is thought to recognize all Fc gamma RII receptors except for one allele of the Fc gamma RIIA gene, is consistent with the possibility that Fc gamma RIIA gene products, but not Fc gamma RIIB or Fc gamma RIIC gene products, are expressed on platelets.

Response of Human Platelets to Activating Monoclonal Antibodies: Importance of FCγRII (CD32) Phenotype and Level of Expression

Certain monoclonal antibodies (MoAbs) specific for platelet membrane glycoproteins are known to be capable of activating platelets, and it is generally thought that platelets from normal subjects are equally susceptible to stimulation by such MoAbs. We found that platelets from 20 normal donors varied significantly in their sensitivity to three IgG1 murine MoAbs specific for membrane glycoproteins CD9, GPIV (CD36), and the GPIIb/IIIa complex (CD41), respectively. The response of platelets to these MoAbs was blocked by prior addition of MoAb IV.3 specific for the Fc gamma RII receptor, indicating that activation was Fc receptor mediated. Platelets that responded poorly to these MoAbs failed to bind the MoAb 41H.16, specific for the "responder" form of Fc gamma RII, but platelets that responded well reacted with this MoAb. The average number of Fc gamma RII receptors on platelets from "responders" and "non-responders" was approximately the same. However, the number of Fc gamma RII receptors expressed influenced sensitivity of a subgroup of "responder" platelets to the anti-CD41 MoAb. These platelets were judged on the basis of MoAb binding studies to be heterozygous for the two alleles of Fc gamma RIIA. In contrast to their varying sensitivity to IgG1 MoAbs, members of the platelet panel responded equally well to 50H.19, an IgG2a MoAb specific for CD9, and these responses could not be blocked by MoAb IV.3 in the presence of plasma. This appears to be because of dual actions of 50H.19 on platelets: one FcR-dependent and the other complement-dependent. Our findings confirm previous reports that certain IgG1 MoAbs activate platelets through binding of their Fc domains to Fc gamma RII receptors and demonstrate that this response is influenced both by Fc gamma RII phenotype and (in the case of the anti-CD41 MoAb) by the number of Fc gamma RII receptors expressed. The failure of nonresponding platelets to bind detectable amounts of MoAb 41H.16, which is thought to recognize all Fc gamma RII receptors except for one allele of the Fc gamma RIIA gene, is consistent with the possibility that Fc gamma RIIA gene products, but not Fc gamma RIIB or Fc gamma RIIC gene products, are expressed on platelets.

Genomic organization of mouse Fc gamma receptor genes.

We have isolated and characterized the gene coding for the mouse Fc receptor that is termed Fc gamma RIIa. The gene contains five exons and spans approximately 9 kilobases. Unlike most members of the immunoglobulin gene superfamily, this gene utilizes multiple exons to encode its leader peptide. The first exon encodes the hydrophobic region of the signal sequence; the second exon, which contains only 21 base pairs, encodes a segment of the signal peptidase recognition site; and the beginning of the third exon encodes the predicted site of peptidase cleavage. The third and fourth exons each code for immunoglobulin-like extracellular domains. The fifth exon encodes the hydrophobic transmembrane domain and the cytoplasmic tail. Partial characterization of the Fc gamma RIIb gene indicates that it also contains multiple leader exons, including a 21-base-pair exon and two exons coding for homologous immunoglobulin-like extracellular domains. However, the Fc gamma RIIb gene uses four exons to encode its intracytoplasmic region. Analysis using contour-clamped homogeneous electric field (CHEF) gels indicates that the Fc gamma RIIa and Fc gamma RIIb genes are linked within 160 kilobases on mouse chromosome 1.