Immunotherapy targeting inhibitory Fc? receptor IIB (CD32b) in the mouse is limited by monoclonal antibody consumption and receptor internalization

Genetic deficiency of the inhibitory Fc receptor, Fc?RIIB (CD32b), has been shown to augment the activity of activatory Fc?R and promote mAb immunotherapy. To investigate whether mAbs capable of blocking Fc?RIIB have similar capacity, we recently generated a panel of specific anti-mouse Fc?RIIB mAbs that do not cross-react with other FcRs, allowing us to study the potential of Fc?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?RIIB mAbs demonstrated limited therapeutic activity despite optimized treatment regimens. Unexpectedly, we observed that the anti-Fc?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?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?RIIB mAb immunotherapy was effective when used against Fc?RIIB+ tumors in Fc?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