Cell membrane associated free kappa light chains are found on a subset of tonsil and in vitro-derived plasmablasts.

The monoclonal antibody, MDX-1097, is currently progressing through clinical trials as a possible therapy for multiple myeloma. MDX-1097 targets a cell membrane bound form of free immunoglobulin kappa light chain (FκLC), termed kappa myeloma antigen (KMA), which is found on the surface of malignant plasma cells. The clinical potential of MDX-1097 highlights the need to characterise the expression of its cognate antigen, KMA, in normal tissue. In this study, we have analysed the expression of KMA on B cell subsets found in tonsils, peripheral blood and bone marrow. We found KMA expression on a small population of tonsillar and in vitro derived plasmablasts. In contrast, no KMA expression was observed on peripheral blood or bone marrow resident B cell subsets. This study yields important insights into the possible subsets of B cells that might be depleted as a result of an immunotherapy targeting KMA.