Complement-Dependent Activity of CD20-Specific IgG Correlates With Bivalent Antigen Binding and C1q Binding Strength.

Area of Science:

• Immunology
• Pharmacology
• Oncology

Background:

• Monoclonal antibodies targeting CD20 are crucial for B cell malignancy therapy, inducing B cell depletion through various effector functions.
• Rituximab (RTX), Ofatumumab (OFA), and Obinutuzumab (OBI) are key anti-CD20 IgG1 antibodies, but their therapeutic efficacy, particularly complement-dependent cytotoxicity (CDC), varies significantly.
• Discrepancies in reported RTX efficacy may stem from diverse in vitro experimental systems.

Purpose of the Study:

• To systematically compare C1q binding, complement activation, and CDC induced by RTX, OFA, and OBI.
• To investigate these effector functions across human B cell lines with varying CD20 and complement regulatory protein expression, and on primary human B cells.
• To elucidate the relationship between antibody binding kinetics, complement activation, and B cell depletion mechanisms.

Main Methods:

• Real-time interaction analysis to measure C1q binding to antibody-coated B cells.
• Kinetic analysis of C1q binding modes and affinities.
• Assessment of complement-dependent cell killing (CDCC) and C3b deposition on various B cell types.

Main Results:

• C1q binding strength correlated with antibody bivalency to CD20 and varied between antibodies and cell lines.
• OFA demonstrated superior and cell-independent CDCC, while RTX and OBI showed cell-line dependent killing.
• All antibodies initiated C3b deposition, indicating complement activation, but CDC efficiency varied, suggesting alternative depletion pathways like phagocytosis.

Conclusions:

• Antibody-dependent complement activation and cell depletion are complex, influenced by antibody characteristics, target cell properties, and binding kinetics.
• OFA's potent CDC is cell-independent, whereas RTX and OBI efficacy is context-dependent, highlighting the need for tailored therapeutic strategies.
• Complement activation, even without robust CDC, may contribute to B cell depletion via mechanisms like complement-dependent phagocytosis.