The clinical efficacy of therapeutic
complement (C)-activating
monoclonal antibodies (mAb) to
melanoma-associated
antigens can be impaired by the levels of expression of C-inhibitory molecules on neoplastic cells.
Protectin (CD59) is a
glycosylphosphatidylinositol (GPI)-anchored cell membrane
glycoprotein, acting as terminal regulator of C cascade, which is heterogeneously expressed in
melanomas and represents the main restriction factor of C-mediated lysis of
melanoma cells. Thus, we investigated whether the overexpression of CD59 could influence the constitutive susceptibility of distinct
melanoma cells to homologous C.
Infection of CD59-positive Mel 100 and 70-W
melanoma cells by a retroviral vector carrying the CD59
cDNA, significantly (P < 0.05) upregulated their constitutive expression of CD59, whereas it did not affect that of additional C-regulatory molecules. Transduced CD59 was entirely GPI-anchored and showed a molecular weight identical to native CD59. Additionally, higher amounts of soluble CD59 were detected in the
conditioned media of CD59-transduced
melanoma cells compared with parental cells. CD59-transduced
melanoma cells, sensitized by the anti-GD3 disialoganglioside mAb R24, were significantly (P < 0.05) less susceptible to homologous C-lysis than were parental cells; this effect was fully reverted by the masking of CD59 with F(ab')(2) fragments of the anti-CD59 mAb YTH53.1. These results provide conclusive evidence demonstrating that absolute levels of CD59 expression regulate the susceptibility to homologous C of specific
melanoma cells, and suggest an additional explanation for the poor clinical results obtained with C-activating mAb in the clinical setting.