Tumor‐shed antigen CA125 blocks complement‐mediated killing via suppression of C1q‐antibody binding

Abstract

C1q-engagement with IgG and IgM type antibodies is the initiating step of classical complement-mediated immunity. The tumor shed antigen CA125 has been reported to have immunosuppressive effects on host tumor responses as well as commercially approved and experimental monoclonal antibody (mAb)-based therapeutic agents. To better understand this effect, molecular and cellular studies were carried out testing the ability of CA125 to perturb the classical complement pathway. Here, we show that patient-derived CA125 inhibits IgG1, IgG3, and IgM-mediated complement-dependent cytotoxicity (CDC) by perturbing antibody-Fc interaction with the C1q complement-initiating protein only in those mAbs that are directly bound by CA125. This mechanism was found to impact naturally generated IgM antibodies as well as experimental and clinically approved mAbs, such as farletuzumab and rituximab, respectively. These data support a role for CA125 in humoral immune suppression and as a potential mechanism by which tumors may possibly avoid host immune responses.

Block‑Removed Immunoglobulin Technology to enhance rituximab effector function by counteracting CA125‑mediated immunosuppression

“…CA125 also inhibited the RTX-dependent activation of the Fc-gamma receptor FCGR3A/CD16a (‘Fc receptor’; Fig. 1B ), confirming the mechanism of action previously reported for the reduced ADCC activity of farletuzumab, whereby the binding of CA125 to this antibody reduces its engagement with both Fc receptor as well as C1q ( 13 , 15 , 19 ). Furthermore, CA125 did not decrease RTX binding to CD20, thus ruling out reduced binding to the target cells as a mechanism of decreased ADCC and CDC ( Fig.…”

“…3C ), RTX N109D showed CD20-specific binding and affinity comparable to parent RTX. Since CA125 inhibits C1q binding to RTX, leading to reduced CDC activity ( 19 ), we investigated whether this CA125 effect was attenuated when using RTX N109D. Our previous analysis already indicated that C1q binding to RTX N109D was in fact less suppressed by CA125 compared with RTX ( Fig.…”

“…We have previously reported that CA125 interacts with RTX and reduces its interaction with the complement factor C1q, leading to reduced CDC activity ( 19 ). Because CA125 has been reported to also inhibit ADCC activity ( 13 , 15 ), we hypothesized that CA125 could also affect RTX-mediated ADCC and thus investigated this hypothesis.…”

“…In a previous study, we demonstrated that CA125 binds to RTX, reducing its C1q binding and consequently its CDC activity ( 19 ). The present study built upon that initial observation and performed additional cellular and molecular assays to study RTX and CA125 interaction, as well as generating a library of RTX variants using a proprietary technology called Block-Removed Immunoglobulin Technology (BRITE) that combines randomized amino acid substitutions and high-throughput functional screenings to identify CA125-refractory RTX variants.…”

“…Similar to RTX, the modes of action of farletuzumab and amatuximab include ADCC and CDC ( 16 – 18 ). In addition, CA125 inhibits CDC by blocking C1q from binding to farletuzumab ( 19 ). Key evidence showing that ADCC plays an important role in the clinical effect of RTX was provided by the analysis of Fc gamma RIIIa (CD16a) polymorphisms, where patients carrying CD16a 158V/V genotype (associated with high affinity Fc gamma receptor) showed significantly improved objective response rates compared with FL patients carrying at least one 158F allele (associated with low affinity Fc gamma receptor) ( 20 ).…”