LEVELS OF CELL-MEMBRANE CD59 REGULATE THE EXTENT OF COMPLEMENT-MEDIATED LYSIS OF HUMAN-MELANOMA CELLS

Normal and neoplastic cells are protected from autologous complement ( C) attack by different cell-surface C-regulatory proteins including CD 59 (protectin), CD46 (membrane cofactor protein) and CD55 (decay-accel erating factor). Indirect immunofluorescence (IIF) analysis showed a d ifferential expression of CD59, CD46, and CD55 in nine human melanoma cell lines and that the expression of CD59 was highly heterogeneous co mpared with that of CD46 and CD55. Levels of cell membrane CD59 were f ound to regulate the differential sensitivity of melanoma cells invest igated to homologous C-mediated lysis; in fact, an inverse correlation (r > 0.7, p < 0.05) was found between levels of cell membrane CD59, b ut not of CD46 and CD55, and extent of C-mediated lysis of melanoma ce lls sensitized with scalar concentrations of the anti-GD3 ganglioside mAb R24. Masking of CD59 by 2.5 mu g/ml of the anti-CD5S mAb YTH53.1 i nduced or enhanced C-mediated lysis of melanoma cells sensitized with 2.5 mu g/ml of mAb R24; the latter phenomenon was found to be directly correlated (r = 0.865, p < 0.01) with levels of cell membrane CD59. C D59 is bound to melanoma cells by a glycosylphosphatidylinositol ancho r: treatment of C-resistant melanoma cells Mel 97, by increasing doses of phosphatidylinositol-specific phospholipase C (PI-PLC), progressiv ely decreased cell-surface expression of CD59 and increased C-mediated lysis of cells sensitized with mAb R24. Staining of 38 benign and mal ignant lesions of melanocytic origin by mAb YTH53.1 demonstrated that CD59 is consistently expressed in vivo and confirmed the heterogeneous expression detected in vitro. Our data, altogether, demonstrate that CD59 is the main restriction factor of C-mediated lysis of melanoma ce lls and that levels of CD59 may account for their differential resista nce to C-mediated lysis. The analysis of the levels of CD59 could repr esent at; useful strategy in selecting melanoma patients who may benef it from immunotherapeutic treatment(s) that trigger C activation.