Fc gamma receptor IIb on target B cells promotes rituximab internalization and reduces clinical efficacy

The anti-CD20 mAb rituximab is central to the treatment of B-cell malignancies, but resistance remains a significant problem. We recently reported that resistance could be explained, in part, by internalization of rituximab (type I anti-CD20) from the surface of certain B-cell malignancies, thus limiting engagement of natural effectors and increasing mAb consumption. Internalization of rituximab was most evident in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL), but the extent of internalization was heterogeneous within each disease. Here, we show that the inhibitory Fc gamma RIIb on target B cells promotes this process and is largely responsible for the observed heterogeneity across a range of B-cell malignancies. Internalization correlated strongly with Fc gamma RIIb expression on normal and malignant B cells, and resulted in reduced macrophage phagocytosis of mAb-coated targets. Furthermore, transfection of Fc gamma RIIb into Fc gamma RIIb negative Ramos cells increased internalization of rituximab in a dose-dependent manner. Target-cell Fc gamma RIIb promoted rituximab internalization in a cis fashion and was independent of Fc gamma RIIb on neighboring cells. It became phosphorylated and internalized along with CD20:anti-CD20 complexes before lysosomal degradation. In MCL patients, high Fc gamma RIIb expression predicted less durable responses after rituximab-containing regimens. Therefore, target-cell Fc gamma RIIb provides a potential biomarker of response to type I anti-CD20 mAb. (Blood. 2011;118(9):2530-2540)