Fab fragments retain the catalytic properties of anti-DNA antibodies. (a and b) Intact anti-DNA IgG3, and Fab and F(ab′)2 fragments, were mixed with 500 bp dsDNA (molar ratio=150:1) at 4°C overnight. The results showed that the Fab and F(ab′)2 fragments, but not the Fc fragment, catalyzed dsDNA (a). The Fab and F(ab′)2 fragments from the control IgG3 mAb had no effect on dsDNA (b). (c) The Fab fragment of PL9-11 IgG3 was mixed with ALW (molar ratio=1:2) at 37°C for 4 hours before MALDI-TOF mass spectrometry, showing cleavage peaks (including the 947 peak) similar to those induced by the parent mAb. Data were from three to five independent experiments. Representative images were shown.