Complement-dependent cytotoxicity induced by therapeutic antibodies in B-cell acute lymphoblastic leukemia is dictated by target antigen expression levels and augmented by loss of membrane-bound complement inhibitors.

To optimally utilize therapeutic monoclonal antibodies in the treatment of B-cell acute lymphoblastic leukemia (B-ALL) understanding their mechanisms of action and the factors influencing these mechanisms is required. We show strong correlations between target antigen expression levels and sensitivity to complement-dependent cytotoxicity (CDC) induced by rituximab, ofatumumab, or alemtuzumab in a panel of cell lines derived from primary B-ALL cells and in primary B-ALL samples. Simultaneous loss of expression of membrane-bound complement regulatory proteins (mCRP) CD55 and CD59 due to glycophosphatidylinositol-anchor deficiency, significantly increased sensitivity to CDC. Accordingly, induced increase in CD55 or CD59 expression protected cells against CDC. The extent of protection co-depended on antigen expression and antibody concentration. In contrast, natural variation in mCRP expression could not be used as a single factor to predict sensitivity to CDC. In conclusion, sensitivity of B-ALL cells to CDC was predominantly determined by antibody concentration and target antigen expression.