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CD32

CD32, also known as Fcγ receptor II (FcγRII), is a family of low-affinity receptors for the Fc portion of immunoglobulin G (IgG) antibodies, which are integral membrane glycoproteins belonging to the immunoglobulin gene superfamily and expressed primarily on hematopoietic cells such as monocytes, macrophages, neutrophils, B cells, and platelets.^[1] These receptors play a pivotal role in bridging adaptive humoral immunity and innate cellular responses by binding immune complexes and modulating effector functions like phagocytosis, antibody-dependent cellular cytotoxicity (ADCC), cytokine release, and antigen presentation.^[1] Unlike high-affinity FcγRI (CD64), CD32 exhibits lower binding affinity for monomeric IgG but efficiently recognizes IgG aggregates during late-stage immune responses.^[2] The CD32 family is encoded by three closely related genes—FCGR2A, FCGR2B, and FCGR2C—resulting in distinct isoforms with approximately 85% amino acid sequence identity in their extracellular domains, each comprising two Ig-like domains, a transmembrane region, and a cytoplasmic tail; FCGR2C encodes the activating isoform FcγRIIC in approximately 20% of individuals due to a common stop codon polymorphism in the remainder.^[1] The activating isoforms, FcγRIIA and FcγRIIC, contain an immunoreceptor tyrosine-based activation motif (ITAM) in their cytoplasmic tails, promoting pro-inflammatory responses, while the inhibitory isoform FcγRIIB features an immunoreceptor tyrosine-based inhibition motif (ITIM) that dampens immune activation and serves as a key regulatory checkpoint.^[1] FcγRIIA is the most widely expressed and polymorphic, with variants like H131 and R131 influencing IgG subclass binding affinities and functional outcomes.^[3] Expression levels of CD32 isoforms are dynamically regulated by cytokines, such as IFN-γ upregulating FcγRIIA on myeloid cells and IL-10 enhancing FcγRIIB on B cells.^[1] In health, CD32 receptors maintain immune homeostasis by facilitating the clearance of immune complexes and preventing excessive inflammation, with FcγRIIB acting as an essential negative feedback mechanism in B cell signaling and antibody production.^[1] Dysregulation or genetic polymorphisms in CD32 are implicated in various diseases; for instance, FcγRIIA-H131 variants confer enhanced protection against bacterial infections like Staphylococcus aureus, while reduced FcγRIIB function or certain polymorphisms increase susceptibility to autoimmune disorders such as systemic lupus erythematosus (SLE) and rheumatoid arthritis.^[1] In cancer immunotherapy, CD32 influences the efficacy of monoclonal antibodies like rituximab, where FcγRIIA promotes ADCC-mediated tumor clearance, but FcγRIIB expression on tumors or immune cells can attenuate therapeutic responses.^[1] Ongoing research explores CD32 as a therapeutic target; recent studies (as of 2025) have identified FcγRIIB as a tumor immune checkpoint, with inhibitory antibodies enhancing anti-PD-1 efficacy and promoting T-cell antitumor activity.^[1]^[4]^[5]

Overview and Nomenclature

Definition and Role

CD32, also known as FcγRII, is a low-affinity Fc gamma receptor (FcγR) that functions as a cell surface glycoprotein, binding to the Fc region of immunoglobulin G (IgG) antibodies with micromolar affinity for monomeric IgG.^[1] As part of the broader Fc receptor family, CD32 plays a crucial role in bridging humoral and cellular immunity by recognizing IgG-containing immune complexes, thereby facilitating immune effector functions without engaging soluble monomeric IgG under normal conditions.^[6] This family of receptors is evolutionarily conserved across mammalian species, with orthologs identified in humans, mice, cows, pigs, and other vertebrates, underscoring its fundamental importance in host defense.^[7] In contrast to the high-affinity FcγRI (CD64), which binds monomeric IgG with nanomolar affinity (K_d \approx 10^{-9} M), CD32 exhibits lower binding affinity for monomeric IgG (K_d \approx 10^{-6} to $10^{-7} M) but achieves effective interaction through multivalent binding to IgG aggregates or opsonized targets, enabling physiological activation only in the context of immune complexes.^[8] The primary immunological roles of CD32 include mediating antibody-dependent cellular cytotoxicity (ADCC), where it triggers the release of cytotoxic granules from effector cells like macrophages and neutrophils; promoting phagocytosis of IgG-opsonized pathogens or debris by myeloid cells; and facilitating the clearance of circulating immune complexes to prevent tissue deposition and inflammation.^[9] These functions are essential for controlling infections, regulating inflammatory responses, and maintaining immune homeostasis.^[10] The human CD32 family comprises three main isoforms—FcγRIIA (activating), FcγRIIB (inhibitory), and FcγRIIC (activating, often pseudogene-derived)—each contributing to balanced immune regulation through differential signaling.^[1]

Historical Discovery

The low-affinity receptor for the Fc portion of IgG, known as FcγRII, was first identified in the early 1980s as a 40-kDa glycoprotein expressed on the surface of human B cells and monocytes. This discovery came through the use of monoclonal antibodies that recognized the protein's ability to bind immune complexes, distinguishing it from the high-affinity FcγRI receptor. Early studies highlighted its role in mediating antibody-dependent cellular functions on these immune cells. The molecular cloning of FcγRII isoforms marked significant progress in the late 1980s and early 1990s. In 1988, Hibbs and colleagues isolated cDNA clones encoding the activating isoform FcγRIIA (now CD32A), demonstrating its expression as a transmembrane protein capable of triggering cellular responses upon IgG binding. This was followed in 1990 by the Ravetch group, who cloned the inhibitory isoform FcγRIIB (CD32B) and identified FcγRIIC (CD32C) as a closely related gene that often functions as a pseudogene in humans due to mutations, though functional alleles exist in some individuals. These findings revealed the genetic organization of the FCGR2 locus on chromosome 1q23, arising from gene duplication events.^[11]^[12] The nomenclature for FcγRII shifted to CD32 during the Fifth International Workshop on Human Leukocyte Differentiation Antigens in 1990, with formal adoption in 1991 under the Cluster of Differentiation (CD) system to standardize leukocyte surface marker identification. This change facilitated comparative immunological research across species and cell types.^[13] In the 1990s, studies on genetic polymorphisms in CD32, particularly in the FCGR2A gene, established its linkage to autoimmune diseases. For instance, the H131R polymorphism in FcγRIIA was associated with increased risk of lupus nephritis in African American patients, highlighting how allelic variations influence immune complex clearance and disease susceptibility. Similar associations were noted with systemic lupus erythematosus and other conditions, underscoring CD32's impact on immune regulation.^[14]

Molecular Structure

General Architecture

CD32, also known as Fcγ receptor II (FcγRII), is a type I transmembrane glycoprotein characterized by an extracellular domain, a single transmembrane α-helix, and a cytoplasmic tail. This overall topology enables its role as a low-affinity receptor for the Fc region of immunoglobulin G (IgG) on the surface of immune cells. The mature protein has a molecular weight of approximately 40 kDa, though this can vary due to post-translational modifications such as N-linked glycosylation at multiple sites in the extracellular region, which contributes to structural heterogeneity and influences ligand interactions.^[15] The extracellular region consists of two immunoglobulin-like domains: the membrane-distal D1 domain (N-terminal) and the membrane-proximal D2 domain (C-terminal), both adopting a β-sandwich fold typical of the Ig superfamily. These domains are connected by a flexible linker, forming a compact, heart-shaped structure with an interdomain angle of approximately 70°. The D1 and D2 domains collectively mediate IgG binding, with the interface primarily involving exposed loops on the D2 domain that interact with the CH2 domain of the IgG Fc region.^[16]^[15] Crystal structures of the soluble extracellular domain of FcγRIIb, determined at 1.7 Å resolution, have provided key insights into this architecture, revealing conserved disulfide bridges stabilizing each Ig-like domain and surface-accessible glycosylation sites (e.g., at Asn61, Asn135, and Asn142) that play a minor role in direct IgG binding but affect overall receptor stability and specificity. The transmembrane domain spans the lipid bilayer as a single hydrophobic α-helix, facilitating proper orientation of the extracellular ligand-binding region toward the extracellular space. Variations in the cytoplasmic tail across isoforms include activating (ITAM-containing) or inhibitory (ITIM-containing) motifs, which dictate downstream signaling without altering the core domain architecture.^[16]^[1]

Isoform-Specific Features

The CD32 receptor, also known as FcγRII, exists in three main isoforms in humans: CD32A (encoded by FCGR2A), CD32B (encoded by FCGR2B), and CD32C (encoded by FCGR2C), each exhibiting distinct structural features that differentiate their signaling potential and ligand interactions.^[1] These isoforms share a common transmembrane architecture but diverge primarily in their cytoplasmic tails and select extracellular elements, influencing their roles without altering the overall two Ig-like extracellular domain framework.^[17] CD32A is the activating isoform, characterized by a cytoplasmic tail containing an immunoreceptor tyrosine-based activation motif (ITAM) with a non-canonical sequence, including the YxxL motif flanked by specific residues such as YMTL NPRAPTDDDKNI YLTL and three aspartic acid residues.^[1] A key allelic variant in CD32A is the histidine-131 (H131) polymorphism (H131R), where the H131 allele enhances binding affinity for IgG2 through altered interactions in the extracellular domain, while the R131 variant reduces this affinity.^[17] This polymorphism arises from a single nucleotide change (rs1801274) and is located in the second Ig-like domain, impacting ligand specificity without affecting the core domain structure.^[18] In contrast, CD32B is the inhibitory isoform, featuring a cytoplasmic tail with an immunoreceptor tyrosine-based inhibition motif (ITIM) defined by the S/IxYxxL sequence, which enables negative signaling upon phosphorylation.^[1] CD32B also produces two major splice variants: CD32B1, which includes a 19-amino-acid insertion in the cytoplasmic tail for membrane anchoring, and CD32B2, which lacks this insert, resulting in a shorter tail that facilitates different trafficking patterns.^[1] Additional polymorphisms, such as I232T in the transmembrane region, can influence membrane mobility but do not alter the primary ITIM structure.^[17] CD32C represents a non-functional isoform in most humans due to a premature stop codon (Gln13Stop polymorphism, present in approximately 80% of individuals), which truncates the protein early in the leader sequence, preventing full-length expression and rendering it incapable of surface presentation.^[1] Where expressed (in individuals homozygous for the Gln13 allele), CD32C displays an activating ITAM in its cytoplasmic tail identical to that of CD32A, derived from a gene duplication event involving recombination between FCGR2A and FCGR2B, with its extracellular domains mirroring those of CD32B and the transmembrane/intracellular regions akin to CD32A.^[18] In cases of expression, CD32C may appear as a soluble form due to alternative splicing or truncation, though full transmembrane functionality is rare in humans and more common in certain primates.^[1] Across the isoforms, sequence homology is high in the extracellular regions, with approximately 85% amino acid identity, reflecting their shared evolutionary origin from gene duplications on chromosome 1q23; however, the transmembrane and intracellular domains show greater divergence, particularly in the signaling motifs (ITAM vs. ITIM).^[1] These structural variations, including the H131R allelic variant in CD32A, underscore the isoform-specific adaptations that fine-tune IgG interactions while maintaining the conserved Ig-like binding scaffold.^[17]

Signaling Mechanisms

Activating Pathways

The activating pathways of CD32, particularly the CD32A isoform (FcγRIIA), are initiated by the binding of multimeric IgG immune complexes, which induce receptor clustering essential for signal transduction. This clustering is a critical threshold for activation, as monomeric IgG fails to trigger signaling, whereas multimeric complexes promote lateral aggregation of multiple CD32A molecules on the cell surface, leading to conformational changes that expose the cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM).^[19]^[20] Upon clustering, Src family kinases, such as Lyn, phosphorylate the tyrosine residues within the ITAM of the CD32A cytoplasmic tail, generating diphosphorylated ITAMs that serve as docking sites for downstream effectors. This phosphorylation step is indispensable for propagating the activation signal and is rapidly induced following ligand engagement.^[21]^[22]^[23] The diphosphorylated ITAM recruits and activates spleen tyrosine kinase (Syk), which in turn phosphorylates phospholipase Cγ (PLCγ), resulting in the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). This cascade mobilizes intracellular calcium stores, promoting calcium influx and activating downstream effectors like protein kinase C, which facilitate cellular responses.^[24]^[25]^[26] Activated signaling through CD32A culminates in pro-inflammatory outcomes, including the release of cytokines such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), as well as cytoskeletal rearrangements that drive phagocytosis of opsonized targets. These effects are mediated by Syk-dependent activation of transcription factors like NF-κB and NFAT, which upregulate cytokine gene expression, while actin polymerization pathways enable particle engulfment.^[27]^[28]^[29]

Inhibitory Pathways

CD32B, the inhibitory isoform of the CD32 family, mediates suppressive signaling primarily through its immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the cytoplasmic tail, which become phosphorylated upon receptor crosslinking with immune complexes or co-ligation with activating receptors. This phosphorylation is initiated by the Src-family kinase Lyn, which targets the tyrosine residues within the ITIMs, creating docking sites for downstream effectors.^[27]^[30] The primary effector recruited to the phosphorylated ITIM is the SH2 domain-containing inositol 5-phosphatase 1 (SHIP-1), which binds via its SH2 domain to the phosphotyrosine at position Y282. SHIP-1 then hydrolyzes phosphatidylinositol 3,4,5-trisphosphate (PIP3) to phosphatidylinositol 3,4-bisphosphate (PIP2), thereby depleting PIP3 levels and inhibiting the PI3K-Akt signaling pathway that drives cell activation and survival. This mechanism reduces B cell proliferation and antibody production as well as mast cell degranulation, providing a counterbalance to activating signals from ITAM-bearing receptors. Additionally, the ITIM at Y300 can recruit the protein tyrosine phosphatase SHP-1, further amplifying inhibition through dephosphorylation of key signaling molecules.^[27]^[30] In the context of B cell receptor (BCR) or T cell receptor (TCR) crosstalk, CD32B co-engagement leads to recruitment of SHIP-1 to the immune synapse, suppressing calcium mobilization, MAPK activation, and ultimately promoting B cell apoptosis or anergy, which raises the activation threshold for immune responses. This regulatory function is critical for preventing excessive immune activation and autoimmunity, as evidenced by autoimmune phenotypes in FcγRIIB-deficient models. The distinct ITIM motifs (Y282 for SHIP-1 and Y300 for SHP-1) thus serve as structural hubs that ensure precise inhibitory control, distinguishing CD32B's suppressive role from the stimulatory pathways of other CD32 isoforms.^[31]^[27]^[30]

Expression Patterns

In Immune Cells

CD32, also known as FcγRII, exhibits high expression on various hematopoietic cells, particularly myeloid lineages, where it plays a key role in immune responses. On monocytes and macrophages, all three isoforms—FcγRIIA (activating), FcγRIIB (inhibitory), and FcγRIIC (activating, though less common)—are expressed, with FcγRIIA being predominant.^[1] Neutrophils primarily express the activating isoform FcγRIIA, which supports functions such as phagocytosis of immune complexes.^[1] Quantitative assessments indicate approximately 10^4 to 10^5 CD32 receptors per monocyte, reflecting substantial surface density for ligand binding.^[32] In B cells, CD32 expression is dominated by the inhibitory isoform FcγRIIB, which provides negative feedback to modulate B cell activation and antibody production.^[1] Platelets express primarily FcγRIIA, contributing to platelet activation in immune contexts.^[18] Dendritic cells display a mix of isoforms, including both activating (FcγRIIA) and inhibitory (FcγRIIB) forms, which influence antigen presentation and T cell priming.^[28] Expression of CD32 on these immune cells is dynamically regulated. Interferon-gamma (IFN-γ) upregulates FcγRIIA on monocytes, macrophages, and dendritic cells, enhancing responsiveness to IgG-opsonized targets.^[28] Conversely, in chronic inflammatory conditions such as systemic lupus erythematosus, FcγRIIB expression is often downregulated, potentially exacerbating immune dysregulation.^[1] This regulated expression pattern supports CD32's role in phagocytosis and immune complex clearance across myeloid and lymphoid cells.^[18]

In Non-Immune Tissues

CD32 expression in non-immune tissues is dominated by the CD32B isoform (FcγRIIB), the sole inhibitory member of the low-affinity Fcγ receptor family, with minimal or absent CD32A in most non-hematopoietic cells, though low CD32A levels occur in certain vascular endothelial populations.^[1] FcγRIIB is broadly distributed across endothelial, epithelial, hepatic, and stromal cells, where it binds immune-complexed IgG to modulate local responses without triggering strong activation.^[33] In endothelial cells, CD32B is constitutively expressed and contributes to inflammatory regulation by binding IgG-containing immune complexes.^[34] Activation of endothelial FcγRIIB by hyposialylated IgG or immune complexes promotes vascular permeability, as seen in models of metabolic inflammation where it induces endothelial dysfunction and barrier disruption.^[34] This process facilitates leukocyte extravasation during acute inflammation while providing inhibitory feedback to limit excessive leakage.^[33] Epithelial tissues, particularly in the lung and kidney, express CD32B on alveolar and tubular epithelial cells, respectively, supporting IgG handling across barriers.^[1] In these sites, FcγRIIB participates in IgG transcytosis, aiding the directional transport of maternal or circulating IgG to maintain mucosal and renal immune homeostasis, though primary transcytosis is mediated by the neonatal Fc receptor (FcRn).^[35] For instance, in renal podocytes and tubular epithelia, CD32B expression correlates with IgG internalization and clearance from the glomerular basement membrane.^[36] Hepatocytes express CD32B, which binds IgG and promotes lipid droplet formation in contexts like nonalcoholic fatty liver disease, but also contributes to systemic immune complex clearance by facilitating hepatic uptake and degradation.^[37] These patterns ensure protective IgG distribution while adapting to inflammatory challenges.

Biological Functions

In Immune Regulation

CD32B, the inhibitory isoform of CD32 (FcγRIIB), plays a critical role in suppressing humoral immune responses by coligating with the B cell receptor (BCR) on B cells, thereby dampening BCR signaling and reducing B cell activation and proliferation. This mechanism involves recruitment of the SH2 domain-containing inositol polyphosphate 5-phosphatase (SHIP) to the immunoreceptor tyrosine-based inhibitory motif (ITIM) in FcγRIIB's cytoplasmic tail, which dephosphorylates key signaling molecules and inhibits calcium mobilization essential for B cell responses. As a result, this interaction limits autoantibody production and helps prevent excessive humoral immunity, particularly in contexts of self-antigen recognition.^[38]^[39]^[40] The activating isoform CD32A (FcγRIIA) on antigen-presenting cells (APCs) such as dendritic cells and macrophages enhances antigen uptake and presentation via immune complexes, promoting T cell priming and adaptive immunity, while its balance with the inhibitory CD32B prevents pathological overactivation. FcγRIIA signals through an immunoreceptor tyrosine-based activation motif (ITAM), facilitating cross-presentation of antigens to CD8+ T cells, but co-expression with FcγRIIB ensures that inhibitory signals counteract excessive inflammatory responses, maintaining immune homeostasis. This equilibrium is evident in myeloid cells where FcγRIIA-driven activation is modulated by FcγRIIB to avoid chronic inflammation.^[27]^[41]^[40] CD32 contributes to immune tolerance by modulating activation thresholds in B cells, leading to anergy or deletion of autoreactive clones, and indirectly supporting regulatory mechanisms in T cell responses. On immature B cells, reduced FcγRIIB expression heightens BCR signaling sensitivity, promoting anergy in self-reactive cells to enforce peripheral tolerance, while on APCs, it fine-tunes T cell activation to favor regulatory T cell (Treg) suppression over effector responses. Additionally, FcγRIIB interacts with Toll-like receptors (TLRs) through co-signaling, attenuating TLR4- and TLR9-mediated NF-κB activation in B cells and macrophages to prevent unchecked inflammation and promote balanced responses.^[42]^[43]^[44]^[45] Evidence from FcγRIIB-deficient mouse models underscores its essential regulatory function, as these animals exhibit hyperactive B cell responses, elevated autoantibody levels, and spontaneous development of systemic autoimmunity resembling lupus, highlighting CD32B's role in restraining innate and adaptive immunity to avert self-reactivity.^[46]^[47]^[48]

In Pathogen Defense

CD32, particularly its activating isoform CD32A (FcγRIIA), plays a key role in antibody-dependent cellular cytotoxicity (ADCC) by enabling natural killer (NK) cells and macrophages to lyse IgG-opsonized target cells, including those infected by pathogens.^[1] On macrophages, CD32A engagement triggers phagocytosis and release of reactive oxygen species (ROS) and other cytotoxic mediators, contributing to the elimination of virus-infected or bacteria-coated cells, while subsets of NK cells expressing functional CD32 isoforms contribute to this process through ITAM-mediated signaling.^[49] This mechanism enhances host defense against intracellular pathogens by bridging humoral and cellular immunity without requiring T-cell involvement.^[8] In phagocytosis, CD32A on neutrophils and monocytes facilitates the uptake and destruction of IgG-opsonized bacteria and viruses, serving as a primary effector for clearing extracellular threats.^[8] Neutrophils, for instance, use CD32A to engulf opsonized particles, leading to lysosomal degradation and reactive oxygen species production that amplify microbial killing.^[50] Monocytes similarly internalize coated pathogens, transitioning into macrophages that sustain prolonged phagocytic activity in tissues.^[51] This process is crucial for controlling bacterial dissemination, as demonstrated in defenses against Streptococcus pneumoniae, where CD32A polymorphisms (e.g., H131 variant) enhance opsonophagocytosis efficiency and correlate with reduced infection severity.^[1] The inhibitory isoform CD32B (FcγRIIB) on Kupffer cells in the liver contributes to pathogen defense by clearing immune complexes formed during infection, thereby preventing excessive inflammation while maintaining immune homeostasis.^[52] Kupffer cells express high levels of CD32B, which binds small IgG-containing complexes derived from opsonized pathogens, facilitating their rapid disposal via endocytosis without triggering proinflammatory responses.^[53] This disposal mechanism limits the recirculation of pathogen-antibody complexes, reducing the risk of vascular damage or secondary infections.^[54] CD32 synergizes with complement opsonization to enhance phagocytosis of pathogens by immune cells, where dual opsonization with IgG and complement fragments (e.g., C3b) on bacteria amplifies uptake by neutrophils expressing CD32A, as the receptors cross-link to generate stronger activation signals.^[55] This interplay is evident in defenses against encapsulated bacteria like Streptococcus, where complement augments IgG-mediated targeting for more effective clearance.^[56]

Clinical and Therapeutic Relevance

Genetic Variants

The CD32 family, encoded by the FCGR2A, FCGR2B, and FCGR2C genes, exhibits several key polymorphisms that influence receptor function and expression. A prominent variant in FCGR2A is the H131R polymorphism (rs1801274), which substitutes histidine for arginine at position 131 in the second immunoglobulin-like domain. This change alters the binding affinity for IgG subclasses, with the H131 allele exhibiting higher affinity for IgG2 compared to the R131 allele, which shows reduced binding to this subclass.^[57]^[58] The frequency of the R131 allele (minor allele frequency) varies by ethnicity, ranging from approximately 13-20% in European populations to 30-47% in certain African groups, such as the Dogon (34%) and Fulani (47%).^[59]^[60] In FCGR2B, the I232T polymorphism (rs1050501) in exon 5 results in an isoleucine-to-threonine substitution that disrupts proper membrane localization by causing endoplasmic reticulum retention and reduced surface expression of the inhibitory FcγRIIb receptor.^[61] This variant has been associated with increased susceptibility to systemic lupus erythematosus (SLE), particularly in homozygous T/T individuals, as observed in Japanese and Chinese cohorts.^[62]^[63] The FCGR2C gene, which encodes an activating receptor, contains an inactivating stop codon polymorphism (rs10917661) in exon 3 that renders it a non-expressed pseudogene in the majority of humans.^[64] However, the open reading frame (ORF) haplotype, which allows functional expression, occurs at higher frequencies in African populations (approximately 20%) compared to Europeans (10-15%) and is nearly absent in Asians.^[65]^[66] Haplotypes across the FCGR2 locus, combining variants like H131R in FCGR2A and the ORF/stop in FCGR2C, can modulate overall FcγR function by altering ligand binding, signaling balance between activating and inhibitory receptors, and immune cell responses.^[65]^[67] For instance, certain FCGR2A-FCGR2C haplotypes influence neutrophil-mediated phagocytosis and cytokine release.^[68] Detection of these variants typically involves PCR-based genotyping methods, such as PCR-restriction fragment length polymorphism (PCR-RFLP), real-time PCR with hybridization probes, or TaqMan assays for allele-specific discrimination.^[69]^[70] Functional assays, including flow cytometry for surface expression and IgG binding studies on transfected cells, complement genotyping to assess impacts on receptor activity.^[71]^[72]

Applications in Therapy

CD32, particularly its isoform CD32B (FcγRIIB), has been targeted by monoclonal antibodies designed to deplete or modulate B cells in autoimmune diseases. For instance, BI-1206, an anti-CD32B monoclonal antibody, has been evaluated in phase I/II clinical trials for B-cell lymphomas but shows promise for autoimmune conditions due to its ability to induce B-cell apoptosis and enhance rituximab-mediated cytotoxicity without significant off-target effects.^[73] As of 2025, early phase 2a data from ongoing trials combining BI-1206 with rituximab and acalabrutinib in relapsed/refractory non-Hodgkin lymphoma have demonstrated safety and promising clinical activity.^[74] Similarly, PRV-3279, a bispecific antibody targeting CD32B and CD79B on B cells, has demonstrated proof-of-mechanism in phase Ib trials for systemic lupus erythematosus (SLE), reducing autoreactive B-cell responses and inflammatory markers without B-cell depletion; however, the phase IIa trial was terminated for strategic reasons in 2025.^[75] These approaches leverage CD32B's inhibitory role to suppress aberrant humoral immunity, with preclinical data indicating reduced autoantibody production in lupus models.^[76] In cancer immunotherapy, engineering IgG1 antibodies to enhance engagement with CD32A (FcγRIIA) improves antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis by myeloid cells. Variants such as those with S239D/I332E mutations in the Fc domain increase binding affinity to CD32A, boosting ADCC against tumor cells expressing targets like CD20, as seen in optimized rituximab formulations that show superior tumor clearance in preclinical models and patient-derived xenografts.^[77] This engineering enhances rituximab's efficacy in non-Hodgkin lymphoma by promoting macrophage-mediated tumor killing, with clinical correlations to improved progression-free survival in responsive patients.^[78] Such modifications prioritize CD32A's role in bridging IgG-opsonized targets to activating Fc receptors on effectors, amplifying antitumor responses without altering antibody specificity.^[79] Blocking CD32A with inhibitory antibodies represents an emerging strategy to mitigate excessive immune activation in conditions like sepsis and transplant rejection. The monoclonal antibody IV.3, specific to CD32A, has been used in preclinical models to prevent FcγRIIA-mediated platelet activation and cytokine storms in sepsis, reducing inflammation and organ damage by interrupting immune complex signaling.^[80] In transplant settings, CD32A blockade attenuates antibody-mediated rejection by limiting neutrophil and macrophage infiltration into allografts, as demonstrated in rodent models where anti-CD32A treatment prolonged graft survival. These inhibitors target CD32A's activating function to dampen hyperinflammation while preserving baseline immunity. Genotyping of CD32A polymorphisms serves as a biomarker for personalizing therapy responses, particularly in immune thrombocytopenia (ITP). The H131R polymorphism in FCGR2A influences rituximab efficacy, with the H131 allele associated with stronger ADCC and higher remission rates in ITP patients, enabling tailored dosing or combination therapies.^[81] Similarly, in SLE and other autoimmunities, CD32A genotyping predicts outcomes for anti-CD20 therapies, guiding patient stratification in clinical trials.^[1] Recent advances post-2020 include bispecific antibodies that engage Fcγ receptors on macrophages to enhance phagocytosis. For example, macrophage-engaging bispecifics such as HER2-VEGFA antibodies promote tumor cell engulfment in solid tumors, showing superior antitumor activity in preclinical models compared to monospecific antibodies.^[82] These constructs exploit CD32's expression on macrophages to redirect innate immunity.

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Coexpression of activating and inhibitory FcγRs by DCs reveals a homeostatic checkpoint for inducing tolerance or immunity by immune complexes.<|control11|><|separator|>
[26]
Immune complexes bind preferentially to Fc gamma RIIA (CD32) on ...
Immune complexes bind preferentially to Fc gamma RIIA (CD32) on apoptotic neutrophils, leading to augmented phagocytosis by macrophages and release of ...Missing: FcγRIIA signaling
[27]
Cell-specific, activation-dependent regulation of neutrophil CD32A ...
FcγRII (CD32), a low-affinity receptor for monomeric IgG, is a 40-kd protein ... or unactivated neutrophils, suggesting that CD16B and CD32A cannot bind monomeric ...
[28]
Regulation of hematopoietic cell function by inhibitory ... - NIH
SHIP-1 has been implicated as the mediator of many inhibitory functions within FcγRIIB-containing signaling complexes. It is required for inhibition of antigen ...
[29]
A 13-amino-acid motif in the cytoplasmic domain of FcγRIIB ... - Nature
Mar 3, 1994 · The 13-residue motif in FcyRIIB controls lymphocyte activation by inhibiting a Ca 2+ sig-nalling pathway triggered through ARH-1 motifs.
[30]
A Comprehensive Quantitation of Fc Gamma Receptor Numbers on ...
Feb 5, 2020 · In this study we provide a comprehensive data set assessing quantitatively how many individual human and mouse FcγRs are expressed on B cells, NK cells, ...
[31]
FcγRIIB in autoimmunity and infection: evolutionary and therapeutic ...
All low-affinity FcγRs have two extracellular immunoglobulinlike domains termed D1 and D2; D2 interacts with the CH2 domains of the IgG Fc region. The ...Fcγriib And Autoimmunity · Fcγriib And Infection · Fcγriib And Ivig
[32]
Dermal microvascular endothelial cells express CD32 receptors in ...
We asked whether dermal microvascular endothelial cells (DMEC) express Fc gamma Rs. In cryostat sections of normal human skin, mAb IV.3 or AT10, both ...
[33]
Hyposialylated IgG activates endothelial IgG receptor FcγRIIB to ...
Nov 27, 2017 · We have shown that activation of the IgG receptor FcγRIIB in endothelium by hyposialylated IgG plays an important role in obesity-induced insulin resistance.Missing: lung | Show results with:lung
[34]
International Journal of Molecular Medicine - Spandidos Publications
Mar 17, 2015 · These findings suggest that FCGR2B2 compartments participate in the transcytosis of maternal IgG across the human placental endothelium.
[35]
Presence and possible impact of Fcγ receptors on resident kidney ...
Jun 30, 2022 · FcγRIIb is expressed on the largest spectrum of immune cell types (i.e. monocytes, macrophages, neutrophils, platelets, memory CD8+ T cells, B ...
[36]
Fc Gamma Receptor IIb Expressed in Hepatocytes Promotes Lipid ...
Sep 26, 2018 · This study reveals a novel role for FcγRIIb in regulating lipid metabolism and glycometabolism, and provides a new therapeutic target to improve NAFLD.
[37]
FcγRIIB regulates autoantibody responses by limiting marginal zone ...
Jul 12, 2022 · When FcγRIIB is crosslinked to the B cell receptor (BCR), downstream BCR signaling and B cell activation are inhibited (1, 2). There are alleles ...
[38]
FcγRIIb-BCR coligation inhibits B-cell receptor signaling in chronic ...
In normal B cells, BCR signaling can be inhibited by the low affinity receptor FcγRIIb (CD32b). This receptor exerts its negative effect when coligated with the ...
[39]
Immunoregulatory functions of the inhibitory Fc gamma receptor ...
Feb 6, 2018 · Review of the canonical and non-canonical functions of FcγRIIB and its potential as a therapeutic target for immunotherapy of cancer and ...
[40]
Physiological and Pathological Inflammation Induced by Antibodies ...
FcγRIIa is a low affinity receptor that signals via its own cytoplasmic tail, which carries an ITAM. FcγRIIb is generally considered to be an “inhibitory ...
[41]
FcγRIIb differentially regulates pre-immune and germinal center B ...
Apr 29, 2019 · Here we show that reduced expression of FcγRIIb enhances the deletion and anergy of autoreactive immature B cells, but in contrast promotes autoreactive B cell ...
[42]
Inhibitory Fcγ receptor is required for the maintenance of tolerance ...
In previous studies, FcγRIIB has been shown to play an important role in modulating antibody-medicated effector functions by setting thresholds for immune ...Inhibitory Fcγ Receptor Is... · Materials And Methods · Ot-I T Cell Expansion
[43]
FcγRIIb attenuates TLR4‑mediated NF‑κB signaling in B cells
The present study demonstrated that FcγRIIb ligation by the immune complex (IC) attenuated the TLR4‑triggered nuclear factor (NF)‑κΒ activation, and decreased ...
[44]
Immune complex negatively regulates Toll‐like receptor 9‐mediated ...
Dec 30, 2014 · The major finding of this study is that IC negatively regulates TLR9 activation of B cells through its interaction with FcγRIIb. It has been ...
[45]
FcγRIIb inhibits immune complex-induced VEGF-A production and ...
Mice deficient in FcγRIIb demonstrate hyperactive immune responses and are susceptible to antibody-mediated autoimmune diseases (2). In humans, a single ...
[46]
Systemic lupus erythematosus-associated defects in the inhibitory ...
Mice deficient in FcγRIIb demonstrate hyperactive immune responses (3) and are prone to autoimmunity (2), particularly systematic lupus erythematosus (SLE) ...
[47]
The Complex Association of FcγRIIb With Autoimmune Susceptibility
FcγRIIb on renal mesangial cells might protect against IgG-IC induced inflammation in the kidney (89). Both mechanisms might protect against the ...
[48]
Expression of Functional CD32 Molecules on Human NK Cells Is ...
These data show that certain individuals express high levels of functional FcγRIIc isoforms on their NK cells.
[49]
ADCC & ADCP | Cellular assay - InvivoGen
FcγRIIA (CD32A) is expressed on myeloid cells, including monocytes, macrophages, and dendritic cells (DCs); FcγRIIIa (CD16A) is expressed on macrophages and ...
[50]
Crucial Role of FcγRIIa (CD32) in Assessment of Functional Anti ...
Immunoglobulin G (IgG)—mediated phagocytosis by polymorphonuclear leukocytes (PMNL) constitutes the main defense against Streptococcus pneumoniae. Two leukocyte ...
[51]
Fc Receptor - an overview | ScienceDirect Topics
The FcγRs, by triggering phagocytosis, ADCC, and release of mediators, represent the primary effector molecules involved in the removal of IgG-coated pathogens ...
[52]
Dengue Virus Neutralization in Cells Expressing Fc Gamma Receptors
Activating Fc gamma receptors (FcγRs) in hematopoietic cells serve to remove antibody-opsonized antigens, including dengue virus (DENV), from systemic ...
[53]
Tissue-specific expression of IgG receptors by human macrophages ...
Oct 15, 2019 · Kupffer cells in the liver were the only tissue-resident macrophages that expressed the inhibitory IgG receptor, FcγRIIB. This inhibitory ...
[54]
Understanding Fc Receptor Involvement in Inflammatory Diseases
Following their crosslinking by immune complexes, FcRs play various roles such as modulation of the immune response by released cytokines or of phagocytosis.Fc Receptor Modes Of Action · Fc Receptors And Diseases · Gene Alleles
[55]
There Is Strength in Numbers: Quantitation of Fc Gamma Receptors ...
The central functions of FcγRs on tissue macrophages include the clearance of opsonized pathogens, the removal of small immune complexes from the circulation ...2. Results · 2.3. Fcγr Quantification On... · 2.4. Fcγr Quantification On...
[56]
The role of complement in the acquired immune response - PMC - NIH
... FcγRII (CD32), with CR2 playing a synergistic role. Likewise, IC ligation to either CR2 or FcγRII can activate another costimulatory molecule, lymphocyte ...
[57]
Antibody opsonization enhances MAIT cell responsiveness to ...
Jan 29, 2019 · We hypothesized that opsonization with IgG and complement impacts the interaction of bacteria with APCs through respective Fcγ-receptors (FcγR) ...<|control11|><|separator|>
[58]
Neutrophils dominate in opsonic phagocytosis of P. falciparum ...
Aug 19, 2021 · Some studies have suggested FcγRIIIB may act synergistically with activating receptors such the ITAM-bearing single-chain FcγRIIA for signaling, ...
[59]
Association of a coding polymorphism in Fc gamma receptor 2A and ...
Position 131 is polymorphic for binding of human IgG2 but not of human IgG1, as the allelic form of FCGR2A R/R131 has comparably lower binding affinity to human ...
[60]
FCGR2A - an overview | ScienceDirect Topics
FCGR2A is a cell surface receptor (formerly CD32) that mediates phagocytosis and inflammatory mediator release, and plays a role in immune complex clearance.
[61]
Fc gamma Receptor IIa-H131R Polymorphism and Malaria ...
As expected, allele frequencies at the rs1801274-R131H SNP were different between the ethnic groups (H frequency: Dogon 0.341, Fulani 0.474, P < 0.0001).
[62]
Single-nucleotide polymorphisms and copy number variations ... - NIH
Discussion. In this study, the MAF for rs1801274 (A>G, H131R) of FCGR2A was 0.155 and only 1 subject (0.9%) had the G/G genotype. A previous study indicated ...Missing: IgG2 | Show results with:IgG2
[63]
Entry - *604590 - Fc FRAGMENT OF IgG RECEPTOR IIb; FCGR2B
The surface expression of FCGR2B receptors was significantly reduced in activated B cells from -343C/C SLE patients. Blank et al. (2005) suggested that ...
[64]
Transmembrane domain dependent inhibitory function of FcγRIIB
In 2002, Kyogoku first demonstrated the correlation of FcγRIIB-I232T with the susceptibility to SLE in Japanese (Kyogoku et al., 2002a), while the epidemiology ...
[65]
FcγRIIB-I232T polymorphic change allosterically suppresses ligand ...
In this study, we evaluate the susceptibility of FCGR2B rs1050501 to SLE in an enlarged Chinese SLE patient cohort with complete clinical documents. Therefore, ...
[66]
Genetic Variation in Low-To-Medium-Affinity Fcγ Receptors - PMC
FcγRIIC (CD32c) has long been considered not to be expressed at all, as its gene (FCGR2C) was thought to be a pseudogene (14, 15), and therefore relatively ...
[67]
Different Genetic Associations Revealed in Kawasaki Disease - PMC
Mar 21, 2019 · The FCGR2C-ORF haplotype and rs1801274 (FCGR2A-H131R) were in weak LD (r2 = 0.08) in Europeans. We evaluated the importance of ethnic variation ...
[68]
Nonclassical FCGR2C haplotype is associated with protection from ...
Nov 9, 2017 · Because of a premature stop codon, the FCGR2C gene is a nonexpressed pseudogene in FCGR2C-stop individuals. A SNP in exon 3 can lead to an ...<|separator|>
[69]
Haplotypes of FcγRIIa and FcγRIIIb polymorphic variants influence ...
Mar 15, 2014 · We have set out to systematically analyze in a large cohort of healthy FCGR2/3-genotyped volunteers how the different haplotypes of neutrophil FcγRs ...
[70]
Association of FCGR2A and FCGR2A-FCGR3A haplotypes with ...
The increased association observed with a FCGR2A-FCGR3A haplotype suggests the presence of additional genetic polymorphisms in linkage disequilibrium with this ...Missing: FCGR2 | Show results with:FCGR2
[71]
Combined FCGR2A (131H/R) and FCGR3A (158F/V) genotypes ...
Oct 14, 2025 · The FcγRIIa-131R/H genotyping was performed using the PCR-restriction fragment length polymorphism (PCR-RFLP) method. The primers for the FCGR2A ...Missing: FCGR2 | Show results with:FCGR2
[72]
Development of a real-time PCR detection method for a FCGR2A ...
Design and methods: We designed a novel asymmetric real-time PCR method in the LightCycler that uses two hybridization probes and is followed by melting curve ...Missing: FCGR2 assays
[73]
Genotyping Single Nucleotide Polymorphisms (SNPs) and ... - NIH
Here we provide new detailed methodology that allows for the specific amplification of these FCGRs in a single PCR reaction, allowing for genotyping of both the ...
[74]
Comprehensive genetic and functional analyses of Fc gamma ... - NIH
Nov 11, 2022 · FCGR2C QSV assay and functional interpretation of FCGR2C genotyping. To supplement the MLPA panels, we developed a FCGR2C QSV assay to more ...
[75]
https://www.clinicaltrials.gov/study/NCT05087628
[76]
Study Details | NCT05087628 | PRV-3279-2a Trial in Systemic Lupus
This is a randomized, double-blind, placebo-controlled study in adult patients with active SLE. Approximately 100 eligible patients will be randomized at a ...
[77]
Provention Announces Positive Data from Phase 1b PREVAIL Study ...
Mar 12, 2020 · Provention has prioritized lupus as our lead autoimmune indication for its unmet need and since proof-of-mechanism for CD32B agonism has been ...<|control11|><|separator|>
[78]
Engineered antibody Fc variants with enhanced effector function
The S239D/I332E variant clearly shows increased potency relative to WT rituximab, consistent with its enhanced receptor affinity and ADCC in vitro, and with the ...Missing: IgG1 CD32A immunotherapy
[79]
Fcγ Receptor Polymorphisms and Clinical Efficacy of Rituximab in ...
Rituximab, an IgG1 MoAb, binds to CD20 on the surface of tumor cells and then bridges effector cells via the FcγRs. The effector cells then become activated and ...
[80]
Engineering Anti-Tumor Monoclonal Antibodies and Fc Receptors to ...
In this review, we detail the functions of Fc receptors in human NK cells and expand upon current research illustrating how engineering monoclonal antibodies ...
[81]
InVivoMAb anti-human CD32 (FcγRIIA) | Bio X Cell
$$172.00CD32 is expressed on monocytes/macrophages, granulocytes, platelets and B cells. CD32 enables interaction between Fc γ RII-expressing cells and opsonized ...
[82]
The impact of Fc gamma receptor IIa and IIIa gene polymorphisms ...
Aug 31, 2017 · The response to rituximab in ITP is variable; the effectiveness of rituximab is influenced by the process of activation of effector fragment C ...
[83]
Co-phagocytosis of VEGFA with HER2-overexpressing cancer cells ...
Sep 4, 2025 · Bispecific antibodies (BsAbs) represent a new class of therapeutic antibodies that bind to and exert therapeutic activities against 2 different ...
[84]
Bispecific, T-Cell-Recruiting Antibodies in B-Cell Malignancies
Bispecific antibodies (BsAbs) are designed to recognize and bind to two different antigens or epitopes. In the last few decades, BsAbs have been developed ...Missing: CD32 | Show results with:CD32