Protease-modified erythrocytes: CD55 and CD59 deficient PNH-like cells.

The increased susceptibility to homologous complement in paroxysmal nocturnal haemoglobinuria (PNH) is known to be associated with the deficiency of the membrane complement inhibitors CD59 and CD55. Proteases have been used in this study to modify normal human RBC to complement sensitive PNH-like cells. To investigate the protective role of CD59 and CD55, the relationship between the content of CD59 and CD55 and the complement susceptibility of the PNH-like cells has been determined. The differential resistance of the enzyme-treated RBC to complement-mediated injury was measured by acidified serum lysis. Pronase-treated erythrocytes lacked both CD59 and CD55 and were very susceptible to complement-mediated lysis. Papain treatment of RBC reduced the CD55 content but did not affect CD59 and induced slight susceptibility to complement-mediated lysis. Trypsin treatment of RBC destroyed 80% of CD59, had little effect on CD55 (unless incubation was extended) and slightly increased susceptibility to lysis. Thus, partial CD55 and CD59 activity was sufficient to protect cells from complement-mediated lysis. In the reactive lysis assay, anti-CD55 and anti-CD59 induced haemolysis, anti-CD59 having the more pronounced effect. Lysis was enhanced when RBC were treated by both antibodies simultaneously.