(Fc-Gamma Receptors/Immunoglobulin-Gamma Fc Receptors) expressed on hematopoietic cells play a key role in immune defenses by linking humoral and cellular immunity.Fc-GammaR
display coordinate and opposing roles in immune responses depending on their cytoplasmic region and/or their associated chains (Ref.1). These receptors recognize the Fc (Crystalline Fragment) domains of IgGs (Immunoglobulin-G), and play a crucial in vivo role in both initiation and regulation of inflammatory and cytotoxic responses, and also in the process of Phagocytosis in phagocytic cells: Macrophages, Neutrophils, and Monocytes (Ref.2). These Fc-GammaR-mediated immune responses can be exploited to develop novel immunotherapies. There are two major classes of Fc-GammaRs: Activation Receptors/Activating Receptors (receptors that activate effector functions) and Inhibitory Receptors (receptors that inhibit these functions). The Activating receptors, (Fc-GammaRI, Fc-GammaRIIA and Fc-GammaRIIIA) are characterized by the presence of cytoplasmic ITAM (Immunoreceptor Tyrosine-based Activation Motif) sequences associated with the receptor, and the Inhibitory Receptor (Fc-GammaRIIB) is characterized by the presence of an ITIM (Immunoreceptor Tyrosine-Based Inhibitory Motif) sequence (Ref.3). Fc-GammaRI has high affinity for monomeric IgG, whereas Fc-GammaRII and Fc-GammaRIII exhibit low affinity for monomeric IgG but avidly bind IgG-containing ICs (Immune Complexes). Activating Fc-GammaRs are found on most effector cells of the Immune System, notably Monocytes, Macrophages, NK (Natural Killer) cells, Mast cells, Eosinophils, Neutrophils, and Platelets, while absent from Lymphoid cells. In general, Activating and Inhibitory Fc-GammaRs are coexpressed on the same cell, a physiologically important means of setting thresholds for activating stimuli, because the IgG ligands coengage both types of receptors. The ratio of expression of these two opposing signaling systems will determine the cellular response (Ref.1).
Upon interaction with IgG (or IgG containing ICs), Fc-GammaRs initiate a plethora of signaling cascades involving receptor signaling motifs, and PTKs (Protein Tyrosine Kinases) and Phosphatases. These cascades ultimately culminate in activation or deactivation of effector cells, resulting in initiation or down-modulation of cellular processes. The interaction between Activation Fc-GammaRs and the Fc region of IgGs trigger rapid phosphorylation of specific tyrosine residues in the receptor within the ITAM motifs. Receptor cross-linking stimulates Src Family PTKs to phosphorylate Tyrosine (Y) residues within the ITAM domain of Fc-GammaRIIA whereas, the relevant ITAMs for Fc-GammaRI and Fc-GammaRIIIA are contained within a separate molecule, the dimmeric Gammaƒn-subunit, which associates with the receptor (Ref.1 & 3). The initial Tyrosine phosphorylation event is carried out by members of the Src Family: HCK (Hemopoietic Cell Kinase) and Lyn (V-YES-1 Yamaguchi Sarcoma Viral Related Oncogene Homolog) which phosphorylate the receptor (or associated Gamma-subunit). The Tyrosine kinase SYK (Spleen Tyrosine Kinase) is then recruited to the phosphorylated ITAM domain, where it is activated. SYK is also capable of autoactivation. SYK, in turn, transmits downstream signals leading to Actin polymerisation followed by particle internalization, and cell proliferation (Ref.4). The Src-Family PTKs are further activated by the phosphorylated ITAMs of the Activating Fc-GammaR . The Src-family PTKs then activate PYK2 (Proline-Rich Tyrosine Kinase-2) and Vav , which facilitate Actin reorganization and recruitment of cytoskeletal components to the nascent Phagosome during the phagocytic process. Among downstream effectors of SYK are PI3K (Phosphatidylinositde-3 Kinase), PLC-Gamma (Phospholipase-C-Gamma) and BTK (Brutons Tyrosine Kinase). Ultimately, a cascade of signaling pathways, including the Ras , BLNK (B-Cell Linker Protein) and Calcium Signaling leads to transcriptional activation of target genes and contribute to the differentiation and proliferation of the Immune cells, an essential component of the early immune response (Ref.1). BTK phosphorylates its downstream substrate BLNK, which results in the activation of Rac and JNK (C-Jun Kinase). p38 and other MAPKs (Mitogen Activated Protein Kinases) are also activated by SYK which result in cell proliferation.
The elimination of the invading pathogens and other harmful foreign particles is an important component of the Immune Response. This is brought about by the process of Phagocytosis which involves polymerization of Actin and formation of Phagosomes. PI3K , the Rho family of GTPases, and PKC-Alpha (Protein Kinase-C-Alpha) play important role in the process. PI3K catalyzes phosphorylation at the D-3 position of the Inositol ring of PI (Phosphatidylinositol), PIP (Phosphatidylinositol Phosphate) and PIP2 (Phosphatidylinositol 4,5-Bisphosphate) leading to subsequent production of PIP3 (Phosphatidylinositol 3,4,5-Trisphosphate). Cross-linking of Fc-GammaRI and Fc-GammaRII increases PI3K activity. PIP3 activates various members of the Rho Family of GTPases, which act hierarchically during Phagocytosis: CDC42 (Cell Division Cycle-42) partipates in the formation of filopodia and in the activation of Rac ; Rac stimulates membrane ruffling and activates Rho , and Rho
stimulates the formation of focal adhesions and stress fibers (Ref.4 & 5). PLC-Gamma activation by SYK leads to generation of IP3 (Inositol 1,4,5-Trisphosphate), DAG (Diacylglycerol) and sustained Ca2+ (Calcium) mobilization. DAG triggers localization of PKC-Alpha (Protein Kinase-C-Alpha) to nascent Phagosomes. All these events culminate in Actin Nucleation, Reorganization and the subsequent Membrane Extension. This dramatic, Actin-dependent extension of the plasma membrane around the particle is followed by secondary activity, such as the production of Superoxide and the release of inflammatory cytokines from the Phagocyte, an important component of the immune response (Ref.6).
Ca2+ mobilization from the intracellular stores (Endoplasmic Reticulum) through the activation of IP3R (IP3 Receptor) and Ca2+ influx through the Capacitance-Coupled Ca2+ Channel (CaCn) results in sustained Calcium Signaling, which gives rise to Calcium-dependent processes such as Degranulation, Phagocytosis, ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity), Cytokine Release and Proinflammatory Activation (Ref.6). These phenotypes are indicative of the central role of these receptors in mediating inflammatory responses to cytotoxic IgGs or IgG ICs. The Inhibitory Fc-GammaR (Fc-GammaRIIB) is also coexpressed on the immune effector cells (Ref.3). The inhibitory activity of Fc-GammaRIIB depends on the presence of an ITIM within its cytoplasmic tail, which counteracts the signal transduction triggered by the ITAM-containing Fc-GammaRs . Phosphorylation of the tyrosine of the ITIM motif occurs upon coligation to an Activation partner containing ITAM and is required for its inhibitory activity. This modification generates an SH2 recognition domain that is the binding site for the inhibitory signaling molecule SHIP (SH2-Containing Inositol Phosphatase) that leads to the abrogation of ITAM activation signaling by inhibiting the Src-Family PTKs, and by hydrolyzing the membrane Inositol Phosphate PIP3 thus interrupting the further downstream signaling by the Activating Fc-GammaRs (Ref.7).
Fc-GammaRs trigger the inflammatory, cytotoxic and hypersensitivity functions of immune effector cells. Activation or deactivation of effector cells via Fc-GammaRs can be exploited to develop novel therapies for Cancer, infectious diseases and autoimmune disorders. Antibodies directed against neoplastic cells provide new therapeutic approaches against various malignancies, including Lymphoma, Leukemia, Melanoma, and breast and colorectal Carcinoma. The Fc portion of the anti-tumor IgG is a major component of their therapeutic activity, along with other mechanisms such as activation of apoptosis, blockade of signaling pathways, or masking of tumor antigens. Thus, by binding to Activating Fc-GammaRs expressed by immune effector cells, such as Macrophages, Monocytes, Neutrophils, or NK cells, Tumor-specific antibodies trigger the destruction of malignant cells via ADCC or Phagocytosis. In contrast, inhibitory Fc-GammaR decreases the in vivo efficacy of antibodies against Tumors (Ref.8).