MELANOMA-CELLS CONSTITUTIVELY RELEASE AN ANCHOR-POSITIVE SOLUBLE FORMOF PROTECTIN (SCD59) THAT RETAINS FUNCTIONAL ACTIVITIES IN HOMOLOGOUSCOMPLEMENT-MEDIATED CYTOTOXICITY

Protectin (CD59), a glycosylphosphatidylinositol-anchored cell membran e glycoprotein, is differentially expressed on melanocytic cells and r epresents the main restriction factor of C-mediated lysis of melanoma cells, In this study, we report that CD59-positive melanoma cells cons titutively release a soluble form of CD59 (sCD59), and that its levels directly correlate (r = 0.926; P < 0.05) with the amount of membrane- bound CD59, SDS-PAGE analysis showed that the molecular components of sCD59 are similar to those of cellular CD59 expressed by melanoma cell s, Melanoma-released sCD59 is anchor positive since it inserts into ce ll membranes of homologous cells that transiently increase their expre ssion of CD59, Moreover, sCD59 is functional: it blocks the binding of the anti-CD59 mAb YTH53.1 to melanoma cells and reverses its effects on C-mediated lysis, In fact, preincubation of mAb YTH53.1 with scalar doses of conditioned media of CD59-positive but not of CD59-negative melanoma cells reduced significantly (P < 0.05), and in a dose-depende nt fashion, the enhancement of C-mediated lysis of anti-GD3-sensitized melanoma cells induced by the masking of cellular CD59 by mAb YTH53.1 . Altogether, these data demonstrate that CD59-positive human melanoma cells release a soluble form of CD59 that is structurally similar to cellular CD59, retains its anchoring ability, is functional, and may i mpair the effectiveness of clinical approaches to humoral immunotherap y for human melanoma.