What is ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity)?

Antibody-dependent cell-mediated cytotoxicity (ADCC) is an essential immune response where antibodies, primarily of the IgG1 class, engage immune cells to target and destroy cancer cells. By recognizing tumor cell antigens, monoclonal antibodies (mAbs) bridge immune effector cells, such as natural killer (NK) cells, with target cells to mediate cytotoxicity. Recent research highlights ADCC as a pivotal component in therapeutic antibody efficacy, suggesting strategies to improve ADCC could significantly advance cancer immunotherapy outcomes.  

ADCC Mechanisms in Monoclonal Antibody Therapy

ADCC is primarily facilitated by the interaction between an antibody’s Fc region and Fc receptors (FcRs) on immune effector cells. In antibody-based therapies, the mAb binds to a target antigen on the tumor cell via the Fab region, while the Fc region interacts with Fcγ receptors on effector cells like NK cells, macrophages, and other myeloid cells. FcγRIII (CD16A), expressed on NK cells, is particularly important for mediating ADCC. 

Once FcγR binds the antibody’s Fc region, the immune cell becomes activated, releasing cytotoxic molecules such as perforin and granzyme B, which induce apoptosis in the target cell. Perforin forms pores in the target cell’s membrane, allowing granzyme B to enter and trigger cell death through DNA fragmentation. Additionally, activated NK cells may express Fas ligand, which binds to the Fas receptor on tumor cells, initiating apoptosis through another pathway. These cytotoxic activities underscore the therapeutic potential of ADCC in mAb-based cancer therapies, where the presence and effectiveness of ADCC are linked to patient outcomes.

Related: Antibody Effector Functions

Clinical Relevance of ADCC in Cancer Immunotherapy

Therapeutic mAbs in oncology, such as trastuzumab, rituximab, and cetuximab, use ADCC as part of their antitumor mechanisms. Studies have shown that patients with certain FcγR polymorphisms, specifically high-affinity variants like FcγRIII-158V/V, tend to exhibit improved clinical outcomes in mAb therapy. These FcγR genotypes enhance ADCC, leading to better responses in cancers like breast cancer, lymphoma, and neuroblastoma. 

For instance, trastuzumab’s efficacy in HER2-positive breast cancer has been linked to enhanced ADCC activity in patients with favorable FcγR variants. Similarly, rituximab’s effectiveness in non-Hodgkin lymphoma correlates with the presence of FcγRIIIA-158V/V and FcγRIIA-131H/H alleles, which increase the binding affinity between IgG1 antibodies and activating FcγRs on immune effector cells. This genetic predisposition to more robust ADCC has opened avenues to further optimize mAbs for patients lacking these high-affinity FcγR variants.¹

Methods for Measuring ADCC

Accurate measurement of ADCC is critical in developing and assessing mAb therapeutics. Traditionally, the chromium release assay was used, where target cells were labeled with radioactive chromium (51Cr), and cytotoxicity was determined by measuring the release of 51Cr upon cell lysis. However, due to safety and accuracy concerns, newer, non-radioactive assays have largely replaced this method. 

Current ADCC quantification techniques include flow cytometry-based assays, where target cells are labeled with fluorescent dyes to monitor cytotoxicity. The VITAL assay, for instance, can measure multiple cell populations in vitro and in vivo, providing a more detailed analysis of cytotoxicity mechanisms. Further, lactate dehydrogenase release assays allow for cytotoxicity assessment without the need for labeling by detecting released enzymes upon cell death. 

Impedance-based assays offer another advantage by enabling real-time monitoring of cell lysis over extended periods. High-throughput flow cytometry combined with single-cell imaging has also shown promise in improving accuracy, which is essential for comparing ADCC properties across different mAbs and effector cell populations. 

Advances in mAb Engineering to Optimize ADCC

Innovations in antibody engineering are opening new possibilities for enhancing ADCC. Strategies primarily focus on modifying the Fc region to improve binding to activating FcγR, especially FcγRIIIA (CD16A) expressed on NK cells. For example, site-directed mutagenesis can optimize specific amino acid residues to increase FcγR binding affinity. In addition, glycoengineering—altering Fc glycosylation patterns—has been employed to boost ADCC efficacy. Modifications like afucosylation, where fucose is removed from the Fc region, increase binding to FcγRIII, enhancing the antibody’s ADCC activity.

Studies indicate that afucosylated versions of therapeutic mAbs display up to 50-fold higher binding affinities and improved cytotoxicity in preclinical cancer models. These modifications are now incorporated into clinical trials for afucosylated mAbs, testing efficacy across various cancers. Additionally, Fc asymmetry through engineering heterodimers has yielded mAbs with increased structural stability and improved ADCC properties, allowing for longer circulation and potentially lower dosing requirements.

Related: Afucosylated Antibody Expression

References: 

1. Zahavi, D., AlDeghaither, D., O’Connell, A., & Weiner, L. M. (2018). Enhancing antibody-dependent cell-mediated cytotoxicity: a strategy for improving antibody-based immunotherapy. Antibody Therapeutics, 1(1), 7–12. https://doi.org/10.1093/abt/tby002