Engineering the Fc region of monoclonal antibodies (mAb) to enhance effector functions is likely to be a promising approach for next-generation mAb therapy. 113F, a complement-dependent cytotoxicity-(CDC)-enhancing variant of rituximab is such an antibody (Cancer Res 2008;68:3863-72). The first of the three major aims of the present study was to identify tumor-associated factors influencing tumor susceptibility to 113F-induced CDC, especially focusing on complement regulatory proteins (CRPs). The second aim of the present study was to compare 113F-induced CDC against primary lymphoma cells with rituximab in vitro. A current crucial problem in the field of human immunotherapy research, including antibody therapy, is the lack of suitable small animal models for in vivo preclinical testing. With respect to antibody-dependent cellular cytotoxicity (ADCC), we have established a human tumor-bearing mouse model, using NOD/Shi-scid, IL-2Rγnull (NOG) mice as recipients, in which human immune cells are engrafted and mediate ADCC (Cancer Immunol Immunother 2009;58:1195-206, J Immunol. 2009;183:4782-91). On the other hand, there is no mouse model in which human CDC can be evaluated. Thus, the third and final aim of the present study was to establish a mouse model in which it is human complement that mediates CDC against human tumor cells.Using this model, we assessed the therapeutic potential of 113F in comparison with rituximab.
Rituximab- and 113F-induced human CDC was compared in vitro, and in vivo using NOG mice with human complement.
First, we determined that tumor-associated factors influencing tumor susceptibility to 113F-induced CDC included the quantity of CRPs such as CD55 and CD59 on the cell surface, as observed in rituximab-induced CDC. Second, we found that 113F mediated highly enhanced CDC against primary CD20-expressing lymphoma cells from patients, greater than rituximab. Finally, a novel human tumor-bearing mouse model has been developed in which human complement functions in CDC. NOG mouse serum is defective in its capacity to induce CDC against human cells, and endogenous immune cells from NOG mice are unable to mediate ADCC of therapeutic antibodies with an Fc region consisting of human IgG. Thus, we were able to evaluate purely human CDC without interference from endogenous mouse immune cells or complement-mediated mAb induced antitumor effects in this NOG mouse model. The present observation of significant therapeutic efficacy of rituximab together with pooled human serum (PHS) compared to rituximab with inactivated PHS indicated that human complement does function in rituximab-induced CDC in these mice in vivo. The finding of specific localization of human C1q on CD20-expressing tumor cell membranes indicated that human CDC indeed contributed to the antitumor effect in this model. In addition, enhanced therapeutic efficacy of 113F together with PHS compared to rituximab with PHS in this mouse model was observed in vivo. In the cell proliferation assay, viability of the target cell line was not affected by rituximab or 113F alone, and thus no significant difference between these mAbs was observed in vitro. Therefore, the present in vivo observation emphasizes the concept of this type of CDC-enhancing antibody. Furthermore, the more abundant, denser signals of C1q at the tumor cell membrane in these human serum-bearing mice receiving 113F compared with rituximab, are consistent with the Fc region of the former having a much higher C1q binding affinity than the latter. These findings are concordant with the present observation of the greater therapeutic efficacy of 113F compared to rituximab in vivo.
This animal model overcomes the limitations of preclinical in vivo investigations of CDC caused by species incompatibilities between humans and mice. This model also makes it possible to reconstitute the human complement system during mAb-based immunotherapy and to perform more appropriate preclinical evaluations of novel therapeutic mAb which mediate CDC. In the present study, highly enhanced human CDC mediated by this type of CDC-enhancing mAb was demonstrated both in vitro and in a humanized mouse model in vivo. In the near future, the efficacy of the type of CDC-enhancing antibody described here will be established in planned clinical trials in humans.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.