The first antibodies produced by the adaptive immune response belong to the immunoglobulin M (IgM) class. These antibodies present low affinity for pathogen antigens. However, as the adaptive immune response progresses, antibodies produced are mainly of the IgG class. These antibodies present higher affinity and greater specificity for their particular antigen. Thus, IgG antibodies are involved in protection from all types of pathogens, including viruses, bacteria, fungi, and protozoa (1).

Although, IgG molecules are key for controlling infections, these antibodies usually do not directly damage the microorganisms they recognize. Nowadays, it is well known that leukocytes of the innate immune system are responsible for the protective effects of these antibodies. Some antibodies can directly neutralize toxins or viruses, and activate complement. By binding to a toxin, antibodies prevent the toxin from reaching its receptor on a cell and thus protect the cell. Similarly, by binding to a virus, antibodies inhibit uncoating of the virus and prevent a productive viral infection (2). Antibodies can also activate complement, which is then deposited on pathogens to promote phagocytosis via complement receptors (3, 4), or to induce bacterial lysis via the formation of the membrane attack complex (5).

IgG antibodies recognize their associated antigen via their two Fab (fragment antigen-binding) portions and are in turn recognized though their Fc (fragment crystallizable) portion by specific Fcγ receptors (FcγRs) on the membrane of immune cells (6, 7). Crosslinking of FcγR on the surface of cells activates several effector functions. These effector functions are aimed toward the destruction of pathogens and the induction of an inflammatory response that is beneficial during infections (8). Depending on the cell type, and also on the Fcγ receptor type, these effector functions include phagocytosis, activation of the oxidative burst, cell degranulation, antibody-dependent cell-mediated cytotoxicity (ADCC), and activation of genes for production of cytokines and chemokines (8, 9).

Because FcγR-mediated cell effector functions vary considerably among different leukocytes and types of IgG, it is then of great interest to understand how a certain FcγR is activated to induce a particular cellular function. This knowledge would help us in the future to augment an effective anti-microbial response for example during infections, or to inhibit an exacerbated inflammatory or autoimmune response (10, 11). In addition, it will help us to develop new therapeutic antibodies capable of interacting with certain Fc receptors to induce particular effector cell functions (12). The first level of control is clearly the binding of IgG molecules to FcγRs. In the past few years, the binding of IgG molecules to FcγRs has been examined more carefully, and new evidences on the manner some factors modulate the IgG–FcγR interaction have been described. These factors include the particular IgG subclass (13, 14) and the glycosylation pattern of the antibody (15–17).

Despite these advances on how IgG molecules and FcγRs interact, our knowledge of what particular effector function is activated in a certain cell and in response to a specific type of FcγR remains very limited today. The traditional view has been that each immune cell could be programmed to perform a particular cell function after FcγR crosslinking. Another more recent view is that each FcγR activates a particular signaling pathway leading to a unique cell response. In this review, I describe the main types of FcγRs, and the recent evidence that supports the idea that a specific FcγR induces a unique cell response.

Fcγ receptors are a family of glycoproteins expressed on the membrane of immune cells, and capable of binding the Fc portion of IgG antibody molecules (9, 14). These receptors can bind to the various IgG subclasses with different affinities (8), and when crosslinked by multivalent antigen-antibody complexes, can induce different cellular responses. In mice, there are three exclusive IgG receptors (mFcγRI, mFcRn, and mTRIM21), and three receptors that can bind both IgG and IgE (mFcγRIIb, mFcγRIII, and mFcγRIV) (18) (Figure 1). The latter dual-specific receptors prefer binding to IgG (affinity is around 2 log higher) that they are usually described as IgG receptors (18). However, interacting with IgE can also induce biological responses (19). All these receptors bind IgG on the membrane of the cells expressing them, except the neonatal FcR (mFcRn) (20, 21) and the cytosolic tripartite motif-containing protein 21 (TRIM21) (22, 23) that bind antibody molecules once internalized. In addition, polymorphisms for mouse Fc receptors have been described. Ly17.1 and Ly17.2 are alleles for mFcγRIIb, and V, T, H are alleles for mFcγRIII (Figure 1). These receptors can also be divided into activating (mFcγRI, mFcγRIII, and FcγRIV) and one inhibitory (mFcγRIIb) receptors (14, 24).

In humans, also several activating receptors (FcγRI/CD64, FcγRIIa/CD32a, FcγRIIc/CD32c, and FcγRIIIa/CD16a), one inhibitory receptor (FcγRIIb/CD32b), and one glycosylphosphatidylinositol (GPI)-linked receptor, lacking a cytoplasmic tail (FcγRIIIb/CD16b) have been identified (Figure 2) (14, 24–26). These are also described as classical IgG receptors. In addition, non-classical receptors for IgG include two FcR-like receptors, FcRL4/CD307d and FcRL5/CD307e that are homologous to FcγRI, and the receptors hFcRn and hTRIM21. All these receptors, with the exception of FcRL4 (that binds both IgA and IgG) are truly IgG receptors since they do not bind any other class of immunoglobulin (27) (Figure 2).

FcRL4 and FcRL5 are inhibitory receptors that are expressed exclusively on B cells and downregulate B-cell receptor responses (28, 29). FcRL4 is restricted to a subset of memory B cells (30). The hFcRn is a transport receptor that allows IgG recycling. Expression of hFcRn on vascular endothelial cells and on intestinal epithelial cells permits bidirectional IgG transport, from the circulation into tissues and vice versa. Also, on placental syncytiotrophoblasts, this receptor allows the transport of maternal IgG into the fetus (20). The hFcRn seems also capable of transporting IgG-bound antigens in dendritic cells (31), macrophages (32), and neutrophils (33), thus promoting antigen presentation and modulating immune responses (21). Similarly to the mouse, the cytosolic receptor hTRIM21 is also ubiquitously expressed (23).

FcγRI is a high-affinity receptor, having three Ig-like extracellular domains. It binds mainly monomeric IgG (34). By contrast, FcγRII and FcγRIII are low-affinity receptors, having two Ig-like extracellular domains. They bind only multimeric immune complexes (34, 35).

Activating receptors are associated with a dimer of the common FcRγ chain, which contains an immunoreceptor tyrosine-based activation motif (ITAM) sequence (Figure 2). An ITAM is a conserved signaling motif with the consensus sequence YxxI/Lx_(6–12)YxxI/L, where x represents any amino acid (36). Exceptions to this rule are the human FcγRIIa and FcγRIIc, which contain their own ITAM within their cytoplasmic tail. By contrast, the inhibitory receptor FcγRIIb contains an immunoreceptor tyrosine-based inhibition motif (ITIM) within its cytoplasmic tail (Figure 2). An ITIM has the consensus sequence I/V/L/SxYxxL/V (37). FcγRIIb negatively regulates various cell functions including antibody production by the B cell (38), proliferation, degranulation, and phagocytosis in other leukocytes when it is crosslinked with activating FcγRs (37, 39). Most immune cells express both activating and inhibitory FcγRs, hence simultaneous crosslinking establishes a threshold for cell activation (40, 41) that maintains a balanced immune response (42, 43). The GPI-linked receptor FcγRIIIb is expressed mainly on neutrophils and on a subset of basophils (44). It is classified as an activating receptor, although it is not associated with the common FcRγ chain (34). In fact, no other subunits are known to associate with it, and its signaling mechanism remains unknown (4, 14). The human FcγRIIa and FcγRIIIb are exclusive receptors that are not found in other species (24, 45).

As described above, there is one high-affinity Fcγ receptor, FcγRI (CD64), and two groups of low-affinity FcγRs, FcγRII and FcγRIII. The FcγRII group includes FcγRIIa, FcγRIIc, and FcγRIIb (CD32a, CD32c, and CD32b), while the FcγRIII group includes FcγRIIIa and FcγRIIIb (CD16a and CD16b). This means that a single IgG molecule cannot bind to most FcγRs. On the contrary, antigen-antibody (immune) complexes promote many low-affinity interactions between FcγR and IgG. In consequence, only immune complexes are able to induce the crosslinking of FcγR on the membrane of immune cells leading to the various antibody-mediated cell functions (Tables 2 and 3).

All activating FcγR containing ITAM sequences seem to signal in a similar way at least at the first signaling step. After crosslinking of activating FcγRs, Src family kinases, such as Fyn, Lck, or Lyn, get activated followed by activation of Syk (spleen tyrosine kinase) family kinases. These kinases phosphorylate tyrosines within the ITAM. Phosphorylated tyrosines then become docking sites for Syk, which in turn phosphorylates multiple substrates leading to different cell responses (4, 14, 139). The Ras pathway can be activated through phosphorylation of Sos. This leads to activation of Ras, which in turn phosphorylates Raf, leading to activation of MAPK/ERK kinase (MEK) and extracellular signal-regulated kinase (ERK). This pathway is associated with activation of transcription factors such as AP-1, nuclear factor of activated T cells (NFAT), and NF-κB that control cytokine production and expression of cell survival proteins (Figure 3). Syk can also induce activation of phosphatidylinositol-3 kinase, which produces phosphatidylinositol 3,4,5-trisphosphate (PIP3). This phospholipid recruits pleckstrin homology domain-expressing proteins such as Bruton’s tyrosine kinase and other Tec family kinases involved in activation of small GTPases, such as Rho and Rac that are required for cytoskeleton remodeling. These small GTPases also impinge in activation of MEK and c-Jun N-terminal kinases, leading to nuclear factor activation (Figure 3). PIP3 also recruits phospholipase Cγ, which in turn generates diacylglycerol (DAG) and inositol triphosphate (IP₃). DAG activates PKC (protein kinase C), an important serine/threonine kinase that can lead to activation of the MAP kinases ERK and p38 (Figure 3). IP₃ induces release of intracellular calcium from the endoplasmic reticulum. Calcium regulates several proteins such as calmodulin and calcineurin, which are important for activation of some nuclear factors like NFAT (Figure 3). Activation of different nuclear factors induces expression of cytokines important for inflammation and immune regulation, such as IL-2, IL-6, IL-8, IL-10, tumor necrosis factor α (TNF-α), and IFN-γ (140–142) (Figure 3).

The signal transduction pathways activated by FcγRs binding to high avidity immune complexes, induce multiple cell responses including phagocytosis, respiratory burst, cytokine and chemokine production, and antibody-dependent cell-mediated cytotoxicity (ADCC) (14, 18, 41). The particular signaling molecules activated to initiate each cell response are not clearly defined in part because every cell has more than one type of FcγR and all receptors can bind more than one type of IgG. Thus, it is not clear whether each receptor leads to a particular response or the average signaling from various receptors activates a predetermined cell response. As discussed later in more detail, recent research is beginning to shade light into this issue.

As discussed above, it is now clear that different IgG subclasses engage different FcγRs to induce particular cellular responses in vivo. However, the data published so far does not explain what cell function is activated in response to a particular type of Fcγ receptor. We can think of at least two mechanisms to generate this IgG–FcγR response selectivity: in one, each immune cell is already programmed to perform a particular cell function after FcγR crosslinking, independently of the receptor used. This does not seem likely because as mentioned before, each type of immune cell can give different responses depending on the class of IgG and also on the conditions the cell encounters (such as inflammation, etc.) (42, 72). In the second mechanism, each FcγR activates a particular signaling pathway leading to a unique cell response. This mechanism is supported by recent reports where individual FcγR were crosslinked on human neutrophils (143–147).

Human neutrophils express only two FcγRs, FcγRIIa and FcγRIIIb (45). These receptors are different in the way they are anchored to the cell membrane. FcγRIIa has a typical transmembrane and cytoplasmic tail containing an ITAM for signaling. By contrast, FcγRIIIb is a GPI-linked receptor, lacking a cytoplasmic tail, and its signaling mechanism remains unknown. The first report suggesting that these receptors could initiate distinct cellular responses came out over 20 years ago. It was reported that both FcγRs were capable of signaling, but while FcγRIIIb induced actin polymerization in a Ca²⁺-dependent manner, FcγRIIa did not (148). This pioneer work did not manage to maintain the idea of one receptor one response. However with time other reports have provided new evidence that supports this idea. For example, it was later reported that FcγRIIa, but not FcγRIIIb could induce an increase in L-selectin expression (149). Based on this, it was suggested that FcγRIIIb-mediated activation of circulating neutrophils could lead to a proadhesive phenotype (149). Supporting this view, it was also found that after selective engagement of each receptor with specific monoclonal antibodies, FcγRIIIb, but not FcγRIIa, was able to activate β1 integrins (143). This activation was not due to an increase in integrin expression but rather to an increase in binding affinity for integrin ligands such as fibronectin (143). By contrast, when the major cell response of neutrophils, arguably phagocytosis (150, 151), was examined with receptor specific opsonized beads, FcγRIIa was the predominant FcγR mediating this response. FcγRIIIb contribution to phagocytosis was minimal (145). Thus, at least in human neutrophils each Fcγ receptor is used to activate unique cell responses. FcγRIIa induces mainly phagocytosis, while FcγRIIIb promotes an adhesive phenotype via activation of β1 integrins (Figure 4).

In addition, it was recently found that FcγRIIIb signaling to the neutrophil nucleus was much more efficient than FcγRIIa signaling. FcγRIIIb, but not FcγRIIa, promoted a robust increase in phosphorylated ERK in the nucleus, and also efficient phosphorylation of the nuclear factor Elk-1 (144) (Figure 5). Interestingly, FcγRIIa also induced phosphorylation of ERK in the cytosol (144, 152), but this active ERK seems to function mainly in enhancing phagocytosis and not in nuclear signaling (Figure 5). An important point still unresolved is the actual FcγRIIIb signaling pathway. For FcγRIIa, the FcγR signaling pathway resembles the classical ITAM-mediated pathway (4, 14, 139, 153) (Figure 3), while for FcγRIIIb, the signaling pathway remains a mystery and further research is needed in this area (Figure 5).

Another important cellular function of neutrophils to kill microbes is the formation of neutrophil extracellular traps (NETs) (154, 155). These structures are induced by several pathogens, including virus, bacteria, fungi, and parasites (156). Also, pro-inflammatory stimuli such as lipopolysaccharide, TNF-α, and PMA are efficient inducers of NETs (157). Because, antigen-antibody complexes are also capable of inducing NET formation (158, 159), it was clear that FcγRs were involved in NET formation. Recently, it was reported that FcγRIIIb is the receptor responsible for NET formation in response to immobilized immune complexes (160). In this study, NET formation induced by immobilized immune complexes was blocked by antibodies against FcγRIIIb, but not by antibodies against FcγRIIa (160), indicating that solely FcγRIIIb mediates NET release. Moreover, by direct crosslinking of each type of FcγR with specific monoclonal antibodies it was also confirmed that only FcγRIIIb is capable of inducing NET formation (147, 161). Although, the initial signaling mechanism for FcγRIIIb remains unknown, the signaling pathway for this cell response has been shown to involve the Syk and TAK1 kinases, as well as the MEK/ERK cascade (Figure 6) (161). Because FcγRIIIb is a GPI-linked receptor it is not clear how it can connect to the ERK pathway. However, it is known that GPI-linked proteins concentrate in lipid rafts on the cell membrane. In these rafts many signaling molecules such as Src family tyrosine kinases concentrate, and it is possible that FcγRIIIb upon ligand binding can connect somehow with these kinases and activate Syk. A possible mechanism is the binding of the receptor, within the lipid rafts, to a putative ITAM-containing molecule (151). After Syk activation, a signaling molecular complex can be organized leading to activation of other kinases such as TAK1 (Figure 6). Many steps are still unknown and future research will help in elucidate this signaling pathway.

Taken together, these reports strongly support the hypothesis that each FcγR is capable of initiating particular signaling pathways that lead to unique cell responses. This information would certainly be very helpful in the future for controlling some of the cellular responses in clinical settings. For example, during a strong infection efficient phagocytosis may be desirable. Considering that IgG2 displays a stronger binding to FcγRIIa than to FcγRIIIb (35) (Table 2), one could predict that antibodies of the IgG2 subclass would be much better at inducing phagocytosis by neutrophils. Thus, inducing the production of IgG2 antibodies against certain pathogens, would very likely improve the phagocytosis response against them. Following the same idea, new monoclonal antibodies against tumors have been developed for recognition of malignant cells. Because on NK cells the only activating Fcγ receptor is FcγRIIIa, finding antibodies with better binding (higher affinity) to FcγRIIIa should improve the activation of ADCC. Indeed, this has been shown to be the case for several anti-tumor antibodies (12, 162, 163). This means that, when we know what is the particular cellular response initiated by each FcγR on an immune cell, we could find ways to improve the IgG binding interaction and enhance the response, or vice versa to block the IgG binding interaction and in consequence inhibit the response.

Fcγ receptors expressed in many immune cells are capable of activating different cellular responses important not only for controlling microbial infections but also for regulating immunity. Different subclasses of IgG antibodies bind the various FcγRs with different affinities. These FcγRs are expressed on a wide variety of leukocytes and are capable of activating when crosslinked with immune complexes, different cellular responses of great importance for host defense and for immune regulation. Recent evidence suggests that a specific Fcγ receptor activates particular cell responses. At least for the human neutrophil it is clear that FcγRIIa activates efficient phagocytosis, while FcγRIIIb signals to the nucleus for nuclear factor activation and NETs formation. Therefore, in principle, a particular cell response could be induced or inhibited by engaging or blocking the corresponding FcγR. For example, using IgG2 antibodies a better phagocytosis response should be generated in neutrophils. Because, FcγRs are responsible not only of initiating anti-microbial responses, but also of controlling the intensity of the immune response, there is growing interest in revealing what specific Fcγ receptor activates a particular cell response. Information similar to the one described for neutrophil FcγRs on other immune cells, such as monocytes or dendritic cells, is not available. We will certainly see in the near future much more research in this area.

CR conceived the issues which formed the content of the manuscript and wrote the manuscript.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.