Immunotherapy Targeting Inhibitory Fcγ Receptor IIB (CD32b) in the Mouse Is Limited by Monoclonal Antibody Consumption and Receptor Internalization

Genetic deficiency of the inhibitory Fc receptor, Fc gamma RIIB (CD32b), has been shown to augment the activity of activatory Fc gamma R and promote mAb immunotherapy. To investigate whether mAbs capable of blocking Fc gamma RIIB have similar capacity, we recently generated a panel of specific anti-mouse Fc gamma RIIB mAbs that do not cross-react with other FcRs, allowing us to study the potential of Fc gamma RIIB as a therapeutic target. Previous work revealed a number of these mAbs capable of eliciting programmed cell death of targets, and in the present study we demonstrated their ability to promote target cell phagocytosis. However, in a variety of murine tumor models, anti-Fc gamma RIIB mAbs demonstrated limited therapeutic activity despite optimized treatment regimens. Unexpectedly, we observed that the anti-Fc gamma RIIB mAbs are rapidly and extensively consumed in vivo, both by the tumor and host cells, including B cells, leading to a precipitous loss from the circulation. Closer analysis revealed that the anti-Fc gamma RIIB mAbs become extensively internalized from the cell surface within 24 h in vivo, likely explaining their suboptimal efficacy. Subsequent studies revealed that anti-Fc gamma RIIB mAb immunotherapy was effective when used against Fc gamma RIIB+ tumors in Fc gamma RIIB-/- recipients, indicating that consumption of the mAb by nontumor cells is the primary limitation of these reagents. Importantly, similar rates of internalization were not seen on human target cells, at least in vitro. These studies further highlight the need to determine the propensity of mAb therapeutics to internalize target receptors and also identify potential key differences between human and mouse cells in this respect.