Recombinant Human Low affinity immunoglobulin gamma Fc region receptor II-c (FCGR2C)

Gene Structure and Variants

The FCGR2C gene exhibits notable polymorphism, with significant implications for protein expression and function. A critical single nucleotide polymorphism (SNP) in exon 3 of FCGR2C results in either expression of the activating FcγRIIc (FCGR2C-open reading frame [ORF]) or its absence due to a stop codon (FCGR2C-Stop) . This polymorphism is functionally important as it determines whether an individual expresses the activating receptor.

Additional complexity arises from "nonclassical" FCGR2C-ORF alleles, in which FcγRIIc expression is unexpectedly absent despite the presence of an open reading frame. This phenomenon has been traced to novel splice site mutations near exon 7 that introduce another stop codon . Furthermore, certain individuals with FCGR2C-Stop alleles have been found to express FcγRIIb on NK cells, which normally do not express this inhibitory receptor. This expression pattern has been linked to a deletion of FCGR2C-FCGR3B that extends into the promoter region of the adjacent FCGR2B gene, possibly removing a negative regulatory element in the FCGR2B promoter in NK cells .

Copy Number Variation

Unlike some related genes (FCGR2A and FCGR2B), which do not show copy number variation (CNV), the FCGR2C gene exhibits significant CNV in human populations. In a study of 100 healthy white controls, the distribution of FCGR2C gene copy numbers was found to be as follows :

This variation in gene copy number is directly linked to similar variation in the FCGR3B gene, demonstrating the complex genetic architecture of this region .

Expression Pattern and Cellular Distribution

The expression pattern of FCGR2C shows cell-type specificity that contributes to its functional significance in the immune system. mRNA expression studies have revealed that transcripts for FcγRIIc are found in neutrophils, monocytes, and notably, NK cells . This distribution is significant, as NK cells express FcγRIIc as their only FcγRII isoform.

The expression of FcγRIIc protein on NK cells correlates directly with the presence of an FCGR2C-ORF allele. Individuals with the FCGR2C ORF/stop genotype express FcγRII on their NK cells, whereas those with the FCGR2C stop/stop genotype show a complete absence of FcγRII on NK cells . This pattern confirms that the expression of functional FcγRIIc depends on the genetic status of the FCGR2C gene.

Functional Significance in Immunity

FCGR2C plays a critical role in the immune system by serving as an activating IgG receptor that mediates antibody-dependent cellular cytotoxicity (ADCC) by immune cells. This function is particularly important for NK cell-mediated cytotoxicity against antibody-coated target cells .

The balance between activating and inhibitory signals from different FcγR subtypes is crucial for maintaining homeostasis in immune complex-driven inflammatory responses . FCGR2C contributes to this balance as an activating receptor, with its expression potentially influencing the threshold for immune activation or inhibition.

In individuals where FcγRIIb is expressed on NK cells (due to the genetic variations described earlier), this inhibitory receptor effectively suppresses killing mediated by FcγRIIIa (CD16a) in antibody-dependent cytotoxicity assays . This finding highlights the functional importance of the relative expression of activating versus inhibitory Fc receptors on immune cells.

Association with Autoimmune Disorders

FCGR2C has been strongly linked to autoimmune conditions, particularly idiopathic thrombocytopenic purpura (ITP). The activating FCGR2C-ORF genotype is significantly overrepresented in ITP patients compared to healthy controls, with an odds ratio of 2.4 (1.3-4.5, p < 0.009) . This association suggests that the expression of functional FCGR2C may predispose individuals to ITP by altering the balance of activating and inhibitory FcγR on immune cells.

In one study, the FCGR2C-ORF allele was found in 18% of healthy individuals but was present in 34.4% of ITP patients, representing a significant enrichment . Researchers have proposed that this activating genotype contributes to unbalanced immunity, potentially leading to autoimmune manifestations.

Role in Sepsis Prognosis

More recent research has identified FCGR2C as an emerging immune gene for predicting sepsis outcomes. Studies suggest that FCGR2C could serve as an immune biomarker associated with prognosis in septic patients . This finding points to a potential new direction for immunotherapy approaches aimed at reducing sepsis mortality.

Correlation analyses between FCGR2C and clinical indicators in septic patients have shown associations with severity indices such as SOFA scores, APACHE II scores, and inflammatory markers like interleukins . These correlations reinforce the role of FCGR2C in modulating inflammatory responses during severe infections.

Recombinant FCGR2C: Production and Applications

Recombinant Human FCGR2C is produced through various expression systems, with E. coli being commonly used for commercial production. The protein is typically fused to tags such as GST to facilitate purification and downstream applications . The recombinant form of FCGR2C enables researchers to study its structural and functional properties in controlled laboratory settings.

Applications of recombinant FCGR2C include:

1. Functional studies of receptor-ligand interactions

2. Development of diagnostic tools for autoimmune conditions

3. Screening of therapeutic antibodies for Fc receptor binding profiles

4. Structural studies to inform drug design targeting FcγR interactions