Targeting the junction of CεmX and ε-migis for the specific depletion of mIgE-expressing B cells

Monoclonal antibodies targeting the extracellular region of the human IgE heavy chain membranetethering domain have been proposed for treating allergies caused by hyperproliferative monoclonal expansion of IgE-producing B cells. Antibodies against this target are expected to deplete membrane IgE (mIgE) displaying B cells and leave B cells of other immunoglobulin isotypes intact. Because of alternative splicing, the mIgE heavy chain has two isoforms that differ in their membrane-proximal segment. In the long isoform, the CH4 domain is followed by a 67-amino acid-long extracellular portion. Out of these 67 amino acids, the first 52 amino acids following the CH4 domain constitute the C epsilon mX segment while the rest of the 15 amino acids immediately adjacent to the membrane constitute the epsilon-migis. In the short isoform the C epsilon mX segment is absent and the CH4 domain is followed only by the 15-amino acid-long epsilon-migis segment. Using antibodies derived from a phage display library, we investigated: (1) epsilon-migis and (2) the junction of C epsilon mX and epsilon-migis (C epsilon mX.migis), as potential therapeutic antibody targets. Our results indicate that antibodies obtained from our phage library that target epsilon-migis bind to a variety of human cells irrespective of mIgE expression, possibly due to homology between epsilon-migis and a region of phosphoinositide-binding protein (ARAP3). In contrast, antibodies specific for the C epsilon mX.migis junctional region, bound specifically to transfected and primary B cells expressing human mIgE and elicited antibody-dependent cellular cytotoxicity and reduction in IgE production. These antibodies did not bind secreted IgE or the mIgE isoform in which CemX is absent. These results suggest that CemX.migis junctional region is a promising antibody target and the human antibodies we describe warrant further evaluation. (C) 2012 Elsevier Ltd. All rights reserved.