Natural antibodies recognize cytoplasmic myosin that is cleaved and transferred to the cell membrane during apoptosis (89.4)

Abstract

Abstract Natural antibodies (Abs) and Abs from patients with chronic lymphocytic leukemia (CLL) bind a subset of apoptotic cells that expose intracellular myosin on their surface. To test the mechanisms that produce myosin exposed apoptotic cells (MEACs), Jurkat T cells were induced to undergo intrinsic (spontaneous or camptothecin-induced) or extrinsic (Fas ligand- or anti-Fas-induced) apoptosis. After prolonged incubation by all methods, a high percentage of apoptotic cells became MEACs. Thus, both intrinsic and extrinsic apoptotic pathways lead to MEAC formation, suggesting that a common downstream mediator is involved. Consistent with this, a caspase-3 inhibitor reduced both apoptosis and MEAC formation, whereas a caspase-1 inhibitor had no effect. To test if caspase-3 cleavage resulted in membrane exposure of myosin, cytoplasmic and membrane protein extracts were examined for caspase-3 cleaved myosin fragments. MEAC extracts predominantly exhibited the expected 149 and 94 kDa myosin fragments in the membrane fraction. In contrast, live cell extracts exhibited predominantly the full-length myosin protein in the cytoplasmic fraction. These results indicate that MEAC formation occurs during intrinsic and extrinsic apoptosis when caspase-3 is activated. Caspase-3 cleavage of myosin produces fragments that reside in the cell membrane, where they become available to react with CLL and natural Abs.