FcR in ADCC

FcR receptor

Fc receptors (FcRs) are located on the surface of immune cells and are able to bind to the Fc terminus of antibodies, playing a vital role in regulating the immune defense system. Fc receptors also affect the efficacy of therapeutic antibodies, so they are of great interest in drug development. In fact, the efficacy of a therapeutic antibody depends not only on its binding to the target, but also on how long it stabilizes in the patient's bloodstream and whether the antibody's interaction with the Fc receptor is effective in eliciting an appropriate immune response.

Therapeutic antibodies can be used to treat cancer, immune diseases, or viral infections. It mainly includes: cytokine or cytokine receptor neutralizing antibodies, as well as cytotoxic antibodies and antibody-drug complexes that bind to specific targets of cancer cells and directly kill tumor cells. Antibodies against immune targets are designed to enhance the immune response within the tumor microenvironment, such as PD-1/PD-L1 or CTLA4 antibodies.

The half-life of antibodies is regulated by the binding of neonatal Fc receptor (FcRn) to IgG, which plays an important regulatory role in IgG trafficking and homeostasis. Other Fc receptors trigger an immune response through an antibody-dependent cell-mediated cytotoxicity (ADCC) mechanism in which natural killer (NK) cells are activated by antibodies binding to tumor cells, followed by lysis of tumor cells. These Fc receptors can be classified according to the type of immunoglobulin (Ig) recognized: Fcγ receptor binding IgG, Fcα receptor binding IgA, and Fcε receptor binding IgE.

The binding of new therapeutic antibodies (primarily IgG classes) to FcRn or appropriate FcγR generally needs to be optimized to modulate their half-life and other biological effects associated with antibodies.

Antibody Checkpoint

The Fcγ receptor contains multiple extracellular immunoglobulin domains that bind to the Fc region of IgG. FcγRI (CD64), FcγRIIa (CD32a), FcγRIIb (CD32b), FcγRIIc (CD32c), FcγRIIIa (CD16a), and FcγRIIIb (CD16b) all belong to the FcγR family. They have different affinities for IgG due to differences in their molecular structure. In addition, five of the FcγR family are activators, while FcγRIIb is an inhibitory receptor.

Fig.1 Different therapeutic antibodies require different FcγR binding activity. ¹

The cytoplasmic region of the activated Fcγ receptor contains two immunoreceptor tyrosine activation motifs (ITAMs) with an amino acid sequence of YxxL/Ix(6-8)YxxL/I. FcγRI and FcγRIIIa do not contain ITAM but are signaled through another ITAM-containing membrane-anchored subunit.

When IgG binds to the activating receptor, the two tyrosine residues of ITAM are phosphorylated by the tyrosine kinases of the intracellular Src family and become binding sites for signaling proteins in the SH2 domain, initiating a signal transduction cascade to generate a biological response.

Fig.2 Schematic diagram of FcεRI, FcγRIII, and FcγRIIB signal transduction. ²

Inhibitory FcγRIIb is expressed in most B cells, and it can provide negative feedback regulation of B cell stimulation. This receptor contains an intracellular immune receptor tyrosine phosphatase such as SHP1, SHP2, or SHIP that binds to antagonistic phosphotyrosine signaling.

FcγR is involved in a variety of biological functions. Receptors present on NK cells can promote the lysis of infected cells or pathogens when they bind to antibodies attached to infected cells or invading pathogens. FcγR, presenting on phage cells, binds to antibodies on invading bacteria, triggering the process of phagocytosis, while FcγR, presenting on eosinophils, causes degranulation to occur.

FcγRIIIa and ADCC

Therapeutic antibodies can activate ADCC via FcγRIIIa. Cytology-based methods such as co-culture assays of stable cell lines expressing targets can be used to evaluate the optimization of antibodies during the optimization of therapeutic antibodies.

The typical ADCC process involves two steps: activating NK cells expressing FcγRIIIa (CD16a) and releasing cytotoxins to attack target cells. Human FcγRIIIa exhibits dimorphism at residue 158, with the Val-158 variant encoding a better affinity than the Phe-158 variant. FcγRIIIa-mediated ADCC enhances the efficacy of therapeutic antibodies used in the treatment of solid tumors and can be used as a direct therapeutic target for hematopoietic tumors.

References

1. Chen, Xin.; et al. "FcγR-binding is an important functional attribute for immune checkpoint antibodies in cancer immunotherapy." Frontiers in immunology. (2019) 10 292.
2. Pullen, N.A.; et al. "The Fyn-STAT5 pathway: A new frontier in IgE- and IgG-mediated mast cell signaling." Frontiers in immunology. (2012) 3 117.