REGULATION OF CD59 EXPRESSION ON K562 CELLS - EFFECTS OF PHORBOL-MYRISTATE ACETATE, CROSS-LINKING ANTIBODY AND NONLETHAL COMPLEMENT ATTACK

CD59 is the major membrane attack complex of complement (MAC) inhibiti ng protein on human cells. Its regulation is therefore an important fa ctor in determining the fate of cells at sites of complement activatio n. We have chosen the K562 erythroleukaemia cell line as a model for s tudies of the regulation of CD59 expression, because it has previously been reported that phorbol 12-myristate 13-acetate (PMA) caused a 15- fold up-regulation of CD59 mRNA in these cells, implying a substantial capacity for CD59 synthesis. However, no assessment of CD59 protein e xpression was made in these studies. We show here that surface express ion of CD59, as assessed by flow cytometry, was increased four-fold ov er a 16-hr incubation with PMA, whereas surface expression of decay-ac celerating factor (DAF) (CD55) and membrane cofactor protein (MCP) (CD 46) was not altered. The newly expressed CD59 was functionally active and anchored through glycosyl-phosphatidylinositol (GPI). Increased ex pression was dependent upon de novo protein synthesis. CD59 released i nto cell supernatant was also increased seven-fold by PMA, this 'secre ted' CD59 retained its GPI anchor. Non-lethal complement attack did no t alter CD59 expression but antibody cross-linking of CD59 caused a ra pid loss of the CD59-antibody complexes. However, CD59 was quickly res tored to pre-attack levels. This rapid restoration was not dependent u pon protein synthesis, suggesting release from preformed stores.