Effective clearance of rituximab-resistant tumor cells by breaking the mirror-symmetry of immunoglobulin G and simultaneous binding to CD55 and CD20.

Complement-dependent cytotoxicity (CDC), which eliminates aberrant target cells through the assembly and complex formation of serum complement molecules, is one of the major effector functions of anticancer therapeutic antibodies. In this study, we discovered that breaking the symmetry of natural immunoglobulin G (IgG) antibodies significantly increased the CDC activity of anti-CD20 antibodies. In addition, the expression of CD55 (a checkpoint inhibitor in the CDC cascade) was significantly increased in a rituximab-resistant cell line generated in-house, suggesting that CD55 overexpression might be a mechanism by which cancer cells acquire rituximab resistance. Based on these findings, we developed an asymmetric bispecific antibody (SBU-CD55 × CD20) that simultaneously targets both CD55 and CD20 to effectively eliminate rituximab-resistant cancer cells. In various cancer cell lines, including rituximab-resistant lymphoma cells, the SBU-CD55 × CD20 antibody showed significantly higher CDC activity than either anti-CD20 IgG antibody alone or a combination of anti-CD20 IgG antibody and anti-CD55 IgG antibody. Furthermore, the asymmetric bispecific antibody (SBU-CD55 × CD20) exhibited significantly higher CDC activity against rituximab-resistant cancer cells compared to other bispecific antibodies with symmetric features. These results demonstrate that enhancing CDC with an asymmetric CD55-binding bispecific antibody could be a new strategy for developing therapeutics to treat patients with relapsed or refractory cancers.

Optimal combination of beneficial mutations for improved ADCC effector function of aglycosylated antibodies.

The Fc region of IgG antibodies is crucial for binding to Fc receptors expressed on the surfaces of various immune leukocytes and eliciting therapeutic effector functions such as clearance of antibody-opsonized tumor cells. Despite abrogated Fc gamma receptor (FcγR) binding and therapeutic effector function in the absence of N-linked glycosylation at Asn297, the aglycosylated Fc region of IgG antibodies has bioprocessing advantages such as the absence of glycan heterogeneity and simple bacterial antibody production. Therefore, these antibodies have been comprehensively engineered as effector functional units for human therapy. In this work, we constructed a huge library of Fc variants with combinations of 25 beneficial mutations that were previously identified to improve binding of glycosylated or aglycosylated Fc regions to human FcγRs in previous studies. High-throughput screening of the resulting library led to the identification of an aglycosylated Fc variant that exhibited almost double the antibody-dependent cell-mediated cytotoxicity than wild-type glycosylated Fc. All mutations in this aglycosylated Fc variant were derived from previously identified beneficial mutations for engineered aglycosylated Fc variants as opposed to glycosylated variants, suggesting that significantly different sets of beneficial mutations are necessary to improve the effector function of aglycosylated Fc.

Engineered aglycosylated full-length IgG Fc variants exhibiting improved FcγRIIIa binding and tumor cell clearance.

FcγRIIIa, which is predominantly expressed on the surface of natural killer cells, plays a key role in antibody-dependent cell-mediated cytotoxicity (ADCC), a major effector function of therapeutic IgG antibodies that results in the death of aberrant cells. Despite the potential uses of aglycosylated IgG antibodies, which can be easily produced in bacteria and do not have complicated glycan heterogeneity issues, they show negligible binding to FcγRIIIa and abolish the activation of immune leukocytes for tumor cell clearance, in sharp contrast to most glycosylated IgG antibodies used in the clinical setting. For directed evolution of aglycosylated Fc variants that bind to FcγRIIIa and, in turn, exert potent ADCC effector function, we randomized the aglycosylated Fc region of full-length IgG expressed on the inner membrane of Escherichia coli. Multiple rounds of high-throughput screening using flow cytometry facilitated the isolation of aglycosylated IgG Fc variants that exhibited higher binding affinity to FcγRIIIa-158V and FcγRIIIa-158F compared with clinical-grade trastuzumab (Herceptin®). The resulting aglycosylated trastuzumab IgG antibody Fc variants could elicit strong peripheral blood mononuclear cell-mediated ADCC without glycosylation in the Fc region.