Introduction of FcαR
Fc receptor (FcR) belongs to the immune receptor family, and its function is closely related to antigen recognition. FcR exists on many cells, and FcR can act as a hub between the body fluids and cell branches of the immune system. The definition of FcR is based on their specificity for the Fc fragment of the immunoglobulin isotype, and the IgA receptor is called FcαR.
Molecular Characteristics of FcαRI
FcαRI (CD89) is a transmembrane receptor belonging to the Ig receptor gene family. It consists of an α chain that binds a ligand, which contains two extracellular Ig-like domains, a transmembrane region, and a short cytoplasmic tail. Generally speaking, polymerized IgA (pIgA) and IgA complexes bind closely to FcRI. However, in the case of monomeric IgA, it may interact with FcαRI only transiently. Unlike FcR, the ligand binding site of IgA is located in the first extracellular domain of FcαRI, while other white blood cell FcRs bind to the ligand in the second extracellular Ig-like domain.
Fig.1 The leukocyte IgA receptor FcαRI. (Van Egmond, 2001)
Biological Function
- FcαRI in immunity
- FcαRI signaling
FcαRI plays an important role in many aspects of host defense. The FcαRI linkage of the IgA immune complex induces a variety of biological processes, including antibody-dependent cytotoxicity, phagocytosis, superoxide production, the release of cytokines and inflammatory mediators, and antigen presentation.
The inhibition of ITAMi signaling by monomeric IgA through FcαRI may have an effect on maintaining steady-state conditions. And the interaction between IgA immune complex and FcαRI is closely related to the inflammatory response.
FcαRI as Therapeutic Target in Cancer
In recent years, antibodies against specific tumor-associated antigens, such as human epidermal growth factor 2 (HER2) and epidermal growth factor receptor (EGFR), have played an increasingly important role in the therapy of solid tumors. These therapeutic antibodies are usually similar to IgG and have the advantage of a long half-life. In addition, these antibodies can activate the complement system and recruit macrophages as cytotoxic effector cells. Based on these facts, studies have found that bispecific antibodies (BsAbs) targeting tumor antigens and FcαRI can recruit neutrophils in vitro. Benefiting from the antibody-dependent cytotoxicity induced by FcαRI, neutrophils can effectively destroy tumor cells. BsAb may treat cancer by enhancing the activation of immune cells expressing FcαRI, which provides a new direction for cancer therapy.
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Reference
- Van Egmond, M.; et al. IgA and the IgA Fc receptor. Trends in immunology. 2001, 22(4): 205-211.
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