**6. Each FcγR leads to unique cellular responses**

The signaling pathways activated by immune complexes binding to Fcγ receptors stimulate different neutrophil responses including phagocytosis, respiratory burst, cytokine and chemokine production, and antibody-dependent cellular cytotoxicity (ADCC) [7, 8, 33]. However, our understanding of what particular function is activated in a cell responding to an individual type of FcγR is still very limited. This lack of knowledge is due, in part, to the fact that each cell expresses several types of FcγRs and all receptors can bind to 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. Traditionally, it

has been thought that each cell is set to activate a particular cell function after FcγR cross-linking. More recently, however, another interpretation has been considered: each FcγR activates a particular signaling pathway leading to a unique cell response. In the traditional view, each cell is already programmed to perform a particular cell function after FcγR cross-linking, independently of the receptor used. This idea is not really supported by experimental evidence. As indicated above, different IgG subclasses bind particular Fcγ receptors with different affinity, leading to unique cell functions *in vivo* [42]*.* In the most recent view, each FcγR activates a distinctive signaling pathway leading to an individual cell function. This view is supported by recent reports, where individual FcγRs on human neutrophils initiate particular cell responses [77, 78, 84–86].

The idea that particular Fcγ receptors could activate unique cell functions was initially published more than 20 years ago. It was found that the neutrophil FcγRIIIb induced actin polymerization in a Ca2+-dependent manner, while FcγRIIa did not [87]. This initial report was not followed by similar reports and the idea of one receptor one response was forgotten. However, with time, other reports have provided new evidence that supports this idea. Some years later, it was reported that FcγRIIa, but not FcγRIIIb caused shedding of L-selectin expression [88] (**Figure 6**). Consequently, it was proposed that binding of antibodies to FcγRIIIb could induce a proadhesive phenotype of neutrophils [88]. More recently, new evidence supporting this idea was found. When each receptor was selectively activated with specific monoclonal antibodies, FcγRIIIb but not FcγRIIa, was able to activate β1 integrins [77] (**Figure 7**). This activation resulted from an increase in binding affinity to fibronectin [77]. Thus, after neutrophils leave the circulation, engagement of FcγRIIIb could lead to activation β1 integrins, allowing the cells to adhere to extracellular matrix proteins and migrate into tissues [89] (**Figure 1**). In contrast, for antibody-mediated phagocytosis [17], FcγRIIa was the main Fcγ receptor mediating this response, while FcγRIIIb contribution to phagocytosis was minimal [86]. Therefore, at least in human neutrophils, each Fcγ receptor initiates particular cell functions. FcγRIIa induces phagocytosis (**Figure 6**), while FcγRIIIb promotes an adhesive phenotype via activation of β1 integrins (**Figure 7**).

In addition, it was also reported that FcγRIIIb signals to the neutrophil nucleus more efficiently than FcγRIIa. FcγRIIIb, but not FcγRIIa, induced a large increase

#### **Figure 6.**

*Neutrophil functions activated by FcγRIIa. In human neutrophils, FcγRIIa signaling induces L-selectin shedding from the cell membrane, and also activates efficient phagocytosis. The oval represents IgG-opsonized bacteria.*

**33**

**Figure 8.**

*for DNA.*

**Figure 7.**

*Neutrophil Activation by Antibody Receptors DOI: http://dx.doi.org/10.5772/intechopen.80666*

in phosphorylated ERK in the nucleus, and also efficient phosphorylation of the nuclear factor Elk-1 [84] (**Figure 7**). Interestingly, FcγRIIa also induced phosphorylation of ERK in the cytosol [84, 90], but this active ERK seems to function mainly

*Neutrophil functions activated by FcγRIIIb. Cross-linking of the human FcγRIIIb stimulates activation of β1 integrins promoting in this way a proadhesive neutrophil phenotype. FcγRIIIb also induces activation of the* 

A recently discovered antimicrobial function of neutrophils is the formation of neutrophil extracellular traps (NETs) [92, 93]. NETs are induced by several pathogens, including virus, bacteria, fungi, and parasites [94]. Also, pro-inflammatory stimuli such as IL-8, TNF-α, and phorbol-12-myristate-13-acetate (PMA) are efficient inducers of NETs [95]. Because, antigen-antibody complexes are also capable of inducing NET formation [96]; it was clear that FcγRs were involved in NET formation. Recently, it was found that FcγRIIIb, but not FcγRIIa, is the recep-

Together, all these reports strongly reinforce the modern view that each FcγR induces a particular signaling pathway that activates a single cellular function. Elucidating the conditions that engage a single type of FcγR to activate a particular cellular response would be very helpful in the future for controlling some of cellular

*Neutrophil FcγRIIIb, but not FcγRIIa, induces neutrophil extracellular traps (NETs) formation. Human neutrophils were stimulated by cross-linking FcγRIIa with the specific monoclonal antibody IV.3, or by crosslinking FcγRIIIb with the specific monoclonal antibody 3G8. After 4 hours, neutrophils were fixed and stained* 

in enhancing phagocytosis and not in nuclear signaling [91] (**Figure 4**).

tor responsible for NET formation [78–80] (**Figure 8**).

*nuclear factor Elk-1 and formation of neutrophil extracellular traps (NETs).*

*Neutrophil Activation by Antibody Receptors DOI: http://dx.doi.org/10.5772/intechopen.80666*

#### **Figure 7.**

*Neutrophils*

responses [77, 78, 84–86].

activation of β1 integrins (**Figure 7**).

has been thought that each cell is set to activate a particular cell function after FcγR cross-linking. More recently, however, another interpretation has been considered: each FcγR activates a particular signaling pathway leading to a unique cell response. In the traditional view, each cell is already programmed to perform a particular cell function after FcγR cross-linking, independently of the receptor used. This idea is not really supported by experimental evidence. As indicated above, different IgG subclasses bind particular Fcγ receptors with different affinity, leading to unique cell functions *in vivo* [42]*.* In the most recent view, each FcγR activates a distinctive signaling pathway leading to an individual cell function. This view is supported by recent reports, where individual FcγRs on human neutrophils initiate particular cell

The idea that particular Fcγ receptors could activate unique cell functions was initially published more than 20 years ago. It was found that the neutrophil FcγRIIIb induced actin polymerization in a Ca2+-dependent manner, while FcγRIIa did not [87]. This initial report was not followed by similar reports and the idea of one receptor one response was forgotten. However, with time, other reports have provided new evidence that supports this idea. Some years later, it was reported that FcγRIIa, but not FcγRIIIb caused shedding of L-selectin expression [88] (**Figure 6**). Consequently, it was proposed that binding of antibodies to FcγRIIIb could induce a proadhesive phenotype of neutrophils [88]. More recently, new evidence supporting this idea was found. When each receptor was selectively activated with specific monoclonal antibodies, FcγRIIIb but not FcγRIIa, was able to activate β1 integrins [77] (**Figure 7**). This activation resulted from an increase in binding affinity to fibronectin [77]. Thus, after neutrophils leave the circulation, engagement of FcγRIIIb could lead to activation β1 integrins, allowing the cells to adhere to extracellular matrix proteins and migrate into tissues [89] (**Figure 1**). In contrast, for antibody-mediated phagocytosis [17], FcγRIIa was the main Fcγ receptor mediating this response, while FcγRIIIb contribution to phagocytosis was minimal [86]. Therefore, at least in human neutrophils, each Fcγ receptor initiates particular cell functions. FcγRIIa induces phagocytosis (**Figure 6**), while FcγRIIIb promotes an adhesive phenotype via

In addition, it was also reported that FcγRIIIb signals to the neutrophil nucleus more efficiently than FcγRIIa. FcγRIIIb, but not FcγRIIa, induced a large increase

*Neutrophil functions activated by FcγRIIa. In human neutrophils, FcγRIIa signaling induces L-selectin shedding from the cell membrane, and also activates efficient phagocytosis. The oval represents IgG-opsonized bacteria.*

**32**

**Figure 6.**

*Neutrophil functions activated by FcγRIIIb. Cross-linking of the human FcγRIIIb stimulates activation of β1 integrins promoting in this way a proadhesive neutrophil phenotype. FcγRIIIb also induces activation of the nuclear factor Elk-1 and formation of neutrophil extracellular traps (NETs).*

in phosphorylated ERK in the nucleus, and also efficient phosphorylation of the nuclear factor Elk-1 [84] (**Figure 7**). Interestingly, FcγRIIa also induced phosphorylation of ERK in the cytosol [84, 90], but this active ERK seems to function mainly in enhancing phagocytosis and not in nuclear signaling [91] (**Figure 4**).

A recently discovered antimicrobial function of neutrophils is the formation of neutrophil extracellular traps (NETs) [92, 93]. NETs are induced by several pathogens, including virus, bacteria, fungi, and parasites [94]. Also, pro-inflammatory stimuli such as IL-8, TNF-α, and phorbol-12-myristate-13-acetate (PMA) are efficient inducers of NETs [95]. Because, antigen-antibody complexes are also capable of inducing NET formation [96]; it was clear that FcγRs were involved in NET formation. Recently, it was found that FcγRIIIb, but not FcγRIIa, is the receptor responsible for NET formation [78–80] (**Figure 8**).

Together, all these reports strongly reinforce the modern view that each FcγR induces a particular signaling pathway that activates a single cellular function. Elucidating the conditions that engage a single type of FcγR to activate a particular cellular response would be very helpful in the future for controlling some of cellular

#### **Figure 8.**

*Neutrophil FcγRIIIb, but not FcγRIIa, induces neutrophil extracellular traps (NETs) formation. Human neutrophils were stimulated by cross-linking FcγRIIa with the specific monoclonal antibody IV.3, or by crosslinking FcγRIIIb with the specific monoclonal antibody 3G8. After 4 hours, neutrophils were fixed and stained for DNA.*

functions in clinical settings. For example, in intense infections, it may be important to activate phagocytosis. Because IgG2 binds better to FcγRIIa than to FcγRIIIb [33, 35], it is likely that IgG2 antibodies would activate phagocytosis by neutrophils much better than other IgG subclass antibodies. In consequence, promoting IgG2 antibodies against certain pathogens would result in better phagocytosis against them.
