Effect of the tumor-shed antigen CA125 on humoral immune responses of IgG1, IgG3, and IgM-type antibodies.

Abstract

15 Background: Human cancers employ a number of mechanisms to evade host immune responses. Here we report the effects of CA125 on suppressing humoral immune responses of naturally occurring and therapeutic antibodies (Abs). These data stem from prespecified subgroup analysis of a Ph3 trial testing farletuzumab, a monoclonal Ab (mAb) to folate receptor alpha, plus standard-of-care carboplatin-taxane (CT) chemotherapy in patients with recurrent platinum-sensitive ovarian cancer. Patients with low CA125 serum levels treated with farletuzumab plus CT demonstrated improvements in PFS (HR 0.49, p = 0.0028) and OS (HR 0.44, p = 0.0108) compared to placebo plus CT. Farletuzumab utilizes Ab-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) with the aim to kill target bound tumor cells. These functions were analyzed in patient samples to determine if CA125 negatively impacts mAb-mediated humoral responses. Methods: Molecular and cell based assays tested the effects of CA125 on humoral immune activity mediated by mAbs on ovarian cancer patient serum samples. Results: Here we show that CA125 inhibits ADCC and CDC by directly binding to a subset of mAbs and perturbing engagement with Fc-γ activating receptors CD16a and CD32a and the C1q complement initiating protein. The effects occur via CA125 binding to the mAb variable domain, which alters the structure of the CH2 motif leading to suppressed binding by CD16a/CD32a and C1q. The lack of inhibition by the high affinity CD64a Fc-γ receptor as well as lack of FcRn inhibition suggests a conformational change occurs within the CH2 motif that perturbs the binding of low affinity CD16a, CD32a and C1q proteins. The effect appears to involve a subset of Abs composed of IgG1, IgG3 and IgM isotypes. Conclusions: CA125 has an immunosuppressive effect on Ab-mediated humor immunity in a subset of tested Abs of varied isotypes including IgG1, IgG3, and IgM. The effects have implications in monitoring therapeutic mAbs that may be negatively affected by CA125 binding as well as potential implications for identifying patients who may be at risk for developing certain types of cancer, including ovarian.