Anti-CmεX monoclonal antibodies potentially applicable for targeting mIgE-expressing B cells (140.20)

Abstract On B lymphocytes, membrane-bound IgE (mIgE) is part of BCR and essential for generating isotype-specific IgE response. On mIgE+ B cells in man, the membrane-bound ε chain (mε) exists predominantly in the long form (mεL), containing 52-a.a. CεmX domain between CH4 and the C-terminal membrane-anchoring segment; the conventional form of mε (mεS) exists in minor proportions. CεmX thus provides an attractive site for immunological targeting of mIgE+ cells. Here we have shown that 9 newly prepared CεmX-specific monoclonal antibodies (mAbs), as well as previously reported a20, can bind to mIgE.FcL-expressing CHO cells, while only 4B12 and 26H2 can bind to mIgE.FcL-expressing B cell line Ramos cells. In epitope mapping studies using synthetic peptides representing different parts of CεmX, 4B12 was shown to bind to the N-terminal part, 26H2 the middle part, and all others the C-terminal part of CεmX. These results suggest that the C-terminal part of CεmX is sterically hindered for mAb binding by associated components in the BCR. The chimerized mAbs c4B12 and c26H2, when cross-linked by secondary antibodies, could lyse mIgE.FcL-expressing Ramos cells by apoptosis through a BCR-dependent caspase pathway. Using PBMCs as the source of effector cells, c4B12 and c26H2 could cause ADCC against mIgE.FcL-expressing Ramos cells in a does-dependent fashion. These results indicate that anti-CεmX mAbs, 4B12 and 26H2, can bind to native mIgE on B cells and are potentially useful for targeting mIgE+ B cells for controlling IgE production.