Why ADCC

Antibody-dependent cellular cytotoxicity (ADCC) is one of the main mechanisms of action of therapeutic monoclonal antibodies, which recognize and kill target cells expressing specific antigens through effector cells expressing Fc receptors, and require antibodies with Fc and Fab regions to participate.

Effector Cells

ADCC can be mediated by a variety of effector cells, which share three common features: leukocytes, granule-containing, and FcR-expressing. Cells that meet these conditions are: monocytes (NK cells, macrophages, γδ T cells) and polymorphonuclear leukocytes (neutrophils, basophages, eosinophils), among which NK cells are the most common research objects in terms of ADCC effect.

Receptors

There are three types of FcR mediating IGg-dependent ADCC effects: FcγRI (CD64), FcγRII (CD32), and FcγRIIIA (CD16), among which FcγRIIIA (CD16) is mainly expressed by NK cells, so most experiments focus on FcγRIIIA (CD16).

Table 1. Effector cells involved in ADCC and major Fc receptors in peripheral blood. (Creative Biolabs)

ADCC Development Process

• Antibody binds to the target cell through the Fab region

A prerequisite for effector cells to be activated is that the antibody Fab region binds to the target cell. After the antibody binds to the target cells, the conformation of its Fc region changes, resulting in an increase in the FcR affinity of the antibody for effector cells.

• Recognition and binding of effector cell Fc receptors to antibody Fc

The affinity of the antibody to FcR is affected by glycosylation or amino acids, for example, when fucose on the antibody is heavily glycosylated, the affinity of the antibody for FcγRIIIA (CD16a) will be reduced, and if fucose is removed, the affinity of the antibody for CD16a and the ability to mediate the ADCC effect will be enhanced; In rituximab-based experiments, there is a trend that the expression of amino acids in the 158-position polymorphism of CD16a as valine (V) or phenylalanine (F) affects the affinity of the antibody to effector cells, and the trend is roughly V/V>V/F>F/F.

Fig.1 The affinity of mAb for the effector cell receptor FcγRIIIA (CD16) is altered by amino acid substitution. ¹

• Phosphorylation

Cellular src kinases phosphorylate immune receptor tyrosine-based activation motifs (ITAMs) in effector cells. Upon cross-linking of the antibody to CD16, the src kinase is activated and the ITAM on the CD16γ subunit is phosphorylated.

• Polarization of cytotoxic particles

The phosphorylated ITAM recruits Syk and activates its associated signaling pathways, such as PLC-γ, PI-3K, and Vav/Rho-family G-protein, to mediate degranulation in effector cells.

Fig.2 NK cells mediate the three main signaling pathways involved in ADCC. ²

• Perforin/granzyme kills target cells

Activation of the relevant signaling pathways triggers an increase in intracellular calcium ions, the involvement of microtubule tissue centers (MTOCs), and the polarization and release of perforin/granzyme-containing organelles.

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References

1. Narvekar, Aditya.; et al. "ADCC enhancement: A conundrum or a boon to mAb therapy?." Biologicals: Journal of the International Association of Biological Standardization. (2022): 10-18.
2. Mir, Manzoor. Immunoglobulins, magic bullets and therapeutic antibodies. (2020).