Purpose: Recent studies have revealed that fucosylated therapeutic IgG1s need high concentrations to compensate for FcγRIIIa-competitive inhibition of antibody-dependent cellular cytotoxicity (ADCC) by endogenous human plasma IgG. Here, we investigated whether ADCC of nonfucosylated therapeutic IgG1 is also influenced by plasma IgG in the same way as fucosylated IgG1s.

Experimental Design: Ex vivo ADCC upon CD20⁺ human B cells was induced by incubation of human whole blood with nonfucosylated and/or fucosylated anti-CD20 IgG1s rituximab, and quantified by measuring the remaining CD19⁺ human B cells using flow cytometry.

Results: Nonfucosylated anti-CD20 showed markedly higher (over 100-fold based on EC₅₀) ex vivo B-cell depletion activity than its fucosylated counterpart in the presence of plasma IgG. The efficacy of fucosylated anti-CD20 was greatly diminished in plasma, resulting in the need for a high concentration (over 1.0 μg/mL) to achieve saturated efficacy. In contrast, nonfucosylated anti-CD20 reached saturated ADCC at lower concentrations (0.01-0.1 μg/mL) with much higher efficacy than fucosylated anti-CD20 in all nine donors through improved FcγRIIIa binding. Noteworthy, the high efficacy of nonfucosylated anti-CD20 was inhibited by addition of fucosylated anti-CD20. Thus, the efficacy of a 1:9 mixture (10 μg/mL) of nonfucosylated and fucosylated anti-CD20s was inferior to that of a 1,000-fold dilution (0.01 μg/mL) of nonfucosylated anti-CD20 alone.

Conclusions: Our data showed that nonfucosylated IgG1, not including fucosylated counterparts, can evade the inhibitory effect of plasma IgG on ADCC through its high FcγRIIIa binding. Hence, nonfucosylated IgG1 exhibits strong therapeutic potential through dramatically enhanced ADCC at low doses in humans in vivo.