**5. Fcγ receptor signaling**

The human neutrophil expresses two unique activating Fc receptors: FcγRIIa and FcγRIIIb. FcγRIIa is a receptor containing ITAM sequences [36, 69], and it signals similarly to other typical immunoreceptors, such as the antigen receptor of T lymphocytes (TCR) and the antigen receptor of B lymphocytes (BCR) [70]. The initial signaling steps for all immunoreceptors are alike and involve first crosslinking of the receptors on the membrane of the cell, followed by the activation of Src family tyrosine kinases (**Figure 4**). These kinases lead to activation of spleen

tyrosine kinase (Syk), which in turn phosphorylates tyrosines within the ITAM sequence. Phosphorylated ITAM then becomes a binding site for Syk. After binding to the receptor, Syk phosphorylates multiple substrates leading to different cell responses [6, 31, 71] (**Figure 4**). Syk can phosphorylate and activate phospholipase Cγ (PLCγ), which in turn generates diacylglycerol (DAG) and inositol triphosphate (IP3). DAG also activates protein kinase C (PKC), an important serine/threonine kinase that can lead to the activation of MAP kinases extracellular signal-regulated kinase (ERK) and p38 (**Figure 4**). IP3 induces release of intracellular calcium from the endoplasmic reticulum. Calcium regulates several proteins such as calmodulin and calcineurin. Syk can also induce activation of phosphatidylinositol-3 kinase (PI3K), which produces phosphatidylinositol 3,4,5-trisphosphate (PIP3). This phospholipid is relevant to the activation of small GTPases, such as Rho and Rac, which are involved in cytoskeleton remodeling for phagocytosis. Rac also leads to activation of the MAPK/ERK kinase (MEK)—ERK pathway, and to activation of c-Jun N-terminal kinases (JNK). These kinases are important for activation of nuclear factors, such as Elk-1, AP-1, and nuclear factor of activated T cells (NFAT) (**Figure 4**). These nuclear factors induce the 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 [72–74] (**Figure 4**).

#### **Figure 4.**

*Signaling transduction pathway of the neutrophil FcγRIIa. Engagement of activating FcγRIIa by IgGantigen immune complexes induces receptor cross-linking and phosphorylation of tyrosine residues in the immunoreceptor tyrosine-based activation motif domains (green oval) by Src family kinases. Phosphorylated tyrosines then become docking sites for Syk, which in turn phosphorylates multiple substrates leading to different signaling pathways that ultimately activate various cell responses. See text for details. P represents a phosphate group; Syk, spleen tyrosine kinase; PI3K, phosphatidylinositol-3 kinase; PIP2, phosphatidylinositol 4,5-bisphosphate; PIP3, phosphatidylinositol 3,4,5-trisphosphate; JNK, c-Jun N-terminal kinase; NFAT, nuclear factor of activated T cells; PLC***γ***, phospholipase C***γ***; DAG, diacylglycerol; IP3, inositol triphosphate; IP3R, receptor for IP3; ER, endoplasmic reticulum; PKC, protein kinase C; MEK, MAPK/ERK kinase; ERK, extracellular signal-regulated kinase; p38, p38 MAP kinase; AP-1, activator protein 1; Elk-1, Ets LiKe gene1 (ETS) transcription factor; IL-2, interleukin-2; IL-6, interleukin-6; TNF-α, tumor necrosis factor α; and IFN-γ, interferon-γ.*

**31**

**Figure 5.**

*extracellular traps (NETs).*

elucidate this signaling pathway.

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

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

In contrast, the human FcγRIIIb is a GPI-linked receptor that lacks an intracellular portion. Thus, it is not clear how it can connect to intracellular signaling molecules. However, there is no doubt that FcγRIIIb is an activating receptor inducing several neutrophil responses such as increase in calcium concentration [75], activation of the respiratory burst [76], activation of integrins [77], and induction of NETosis [78, 79]. Despite, the initial signaling mechanism for FcγRIIIb remains unknown, the signaling pathway for this receptor engages Syk and then transforming growth factor-β-activated kinase 1 (TAK1), as well as the MEK/ERK cascade [80] (**Figure 5**). One possibility to connect FcγRIIIb with Syk is that the receptor could link with signaling molecules such as Src family tyrosine kinases on the plane of the cell membrane. Because GPI-linked proteins, like the FcγRIIIb, concentrate in lipid rafts on the cell membrane together with Src kinases [81, 82], we can imagine that after cross-linking FcγRIIIb, it associates somehow with these kinases and activates Syk. A possible connection is the binding of the receptor, within the lipid rafts, to a putative ITAM-containing molecule [83]. Many steps are still unknown and future research will help in completely

*Signaling transduction pathway of the neutrophil FcγRIIIb. Cross-linking of the human FcγRIIIb by IgGantigen immune complexes induces activation of spleen tyrosine kinase (Syk) by a mechanism not yet described. Syk then activates transforming growth factor-β-activated kinase 1 (TAK1). TAK1 is in turn required for activation of ERK kinase (MEK) and extracellular signal regulated kinase (ERK). Activated ERK signals to the nucleus and contributes to activation of NADPH oxidase, which together lead to formation of neutrophil* 

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

#### **Figure 5.**

*Neutrophils*

tyrosine kinase (Syk), which in turn phosphorylates tyrosines within the ITAM sequence. Phosphorylated ITAM then becomes a binding site for Syk. After binding to the receptor, Syk phosphorylates multiple substrates leading to different cell responses [6, 31, 71] (**Figure 4**). Syk can phosphorylate and activate phospholipase Cγ (PLCγ), which in turn generates diacylglycerol (DAG) and inositol triphosphate (IP3). DAG also activates protein kinase C (PKC), an important serine/threonine kinase that can lead to the activation of MAP kinases extracellular signal-regulated kinase (ERK) and p38 (**Figure 4**). IP3 induces release of intracellular calcium from the endoplasmic reticulum. Calcium regulates several proteins such as calmodulin and calcineurin. Syk can also induce activation of phosphatidylinositol-3 kinase (PI3K), which produces phosphatidylinositol 3,4,5-trisphosphate (PIP3). This phospholipid is relevant to the activation of small GTPases, such as Rho and Rac, which are involved in cytoskeleton remodeling for phagocytosis. Rac also leads to activation of the MAPK/ERK kinase (MEK)—ERK pathway, and to activation of c-Jun N-terminal kinases (JNK). These kinases are important for activation of nuclear factors, such as Elk-1, AP-1, and nuclear factor of activated T cells (NFAT) (**Figure 4**). These nuclear factors induce the 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 [72–74] (**Figure 4**).

*Signaling transduction pathway of the neutrophil FcγRIIa. Engagement of activating FcγRIIa by IgGantigen immune complexes induces receptor cross-linking and phosphorylation of tyrosine residues in the immunoreceptor tyrosine-based activation motif domains (green oval) by Src family kinases. Phosphorylated tyrosines then become docking sites for Syk, which in turn phosphorylates multiple substrates leading to different signaling pathways that ultimately activate various cell responses. See text for details. P represents a phosphate group; Syk, spleen tyrosine kinase; PI3K, phosphatidylinositol-3 kinase; PIP2, phosphatidylinositol 4,5-bisphosphate; PIP3, phosphatidylinositol 3,4,5-trisphosphate; JNK, c-Jun N-terminal kinase; NFAT, nuclear factor of activated T cells; PLC***γ***, phospholipase C***γ***; DAG, diacylglycerol; IP3, inositol triphosphate; IP3R, receptor for IP3; ER, endoplasmic reticulum; PKC, protein kinase C; MEK, MAPK/ERK kinase; ERK, extracellular signal-regulated kinase; p38, p38 MAP kinase; AP-1, activator protein 1; Elk-1, Ets LiKe gene1 (ETS) transcription factor; IL-2, interleukin-2; IL-6, interleukin-6; TNF-α, tumor necrosis factor α; and* 

**30**

*IFN-γ, interferon-γ.*

**Figure 4.**

*Signaling transduction pathway of the neutrophil FcγRIIIb. Cross-linking of the human FcγRIIIb by IgGantigen immune complexes induces activation of spleen tyrosine kinase (Syk) by a mechanism not yet described. Syk then activates transforming growth factor-β-activated kinase 1 (TAK1). TAK1 is in turn required for activation of ERK kinase (MEK) and extracellular signal regulated kinase (ERK). Activated ERK signals to the nucleus and contributes to activation of NADPH oxidase, which together lead to formation of neutrophil extracellular traps (NETs).*

In contrast, the human FcγRIIIb is a GPI-linked receptor that lacks an intracellular portion. Thus, it is not clear how it can connect to intracellular signaling molecules. However, there is no doubt that FcγRIIIb is an activating receptor inducing several neutrophil responses such as increase in calcium concentration [75], activation of the respiratory burst [76], activation of integrins [77], and induction of NETosis [78, 79]. Despite, the initial signaling mechanism for FcγRIIIb remains unknown, the signaling pathway for this receptor engages Syk and then transforming growth factor-β-activated kinase 1 (TAK1), as well as the MEK/ERK cascade [80] (**Figure 5**). One possibility to connect FcγRIIIb with Syk is that the receptor could link with signaling molecules such as Src family tyrosine kinases on the plane of the cell membrane. Because GPI-linked proteins, like the FcγRIIIb, concentrate in lipid rafts on the cell membrane together with Src kinases [81, 82], we can imagine that after cross-linking FcγRIIIb, it associates somehow with these kinases and activates Syk. A possible connection is the binding of the receptor, within the lipid rafts, to a putative ITAM-containing molecule [83]. Many steps are still unknown and future research will help in completely elucidate this signaling pathway.
