Overexpression of protectin (CD59) down-modulates the susceptibility of human melanoma cells to homologous complement

The clinical efficacy of therapeutic complement (C)-activating monoclonal a ntibodies (mAb) to melanoma-associated antigens can be impaired by the leve ls of expression of C-inhibitory molecules on neoplastic cells. Protectin ( CD59) is a glycosylphosphatidylinositol (GPI)-anchored cell membrane glycop rotein, acting as terminal regulator of C cascade, which is heterogeneously expressed in melanomas and represents the main restriction factor of C-med iated lysis of melanoma cells. Thus, we investigated whether the overexpres sion of CD59 could influence the constitutive susceptibility of distinct me lanoma cells to homologous C. Infection of CD59-positive Mel 100 and 70-W m elanoma cells by a retroviral vector carrying the CD59 cDNA, significantly (P < 0.05) upregulated their constitutive expression of CD59, whereas it di d not affect that of additional C-regulatory molecules. Transduced CD59 was entirely CPI-anchored and showed a molecular weight identical to native CD 59. Additionally, higher amounts of soluble CD59 were detected in the condi tioned media of CD59-transduced melanoma cells compared with parental cells . CD59-transduced melanoma cells, sensitized by the anti-GD3 disialoganglio side mAb R24, were significantly (P < 0.05) less susceptible to homologous C-lysis than were parental cells; this effect was fully reverted by the mas king of CD59 with F(ab')(2) fragments of the anti-CD59 mAb YTH53.1. These r esults provide conclusive evidence demonstrating that absolute levels of CD 59 expression regulate the susceptibility to homologous C of specific melan oma cells, and suggest an additional explanation for the poor clinical resu lts obtained with C-activating mAb in the clinical setting. J. Cell. Physio l. 185:317-323, 2000. (C) 2000 Wiley-Liss, Inc.