INTERACTIONS OF SOLUBLE CD59 WITH THE TERMINAL COMPLEMENT COMPLEXES -CD59 AND C9 COMPETE FOR A NASCENT EPITOPE ON C8

Human CD59-Ag is a glycophosphoinositollipid-anchored inhibitor of the membrane attack complex of complement (MAC). We have examined the int eractions of CD59 with the terminal complement complexes using soluble CD59 purified from human urine (CD59U). CD59U bound to SC5b-8, SC5b-9 , and MAC complexes when present during their formation. When SC5b-8, SC5b-9, and MAC were allowed to preform, progressively less I-125-CD59 U bound to the complexes. Terminal SC5b-9 complexes isolated from acti vated sera no longer bound CD59U, indicating that the binding sites ha d become inaccessible in the fully assembled SC5b-9 complex. Unlike ac ylated myocardial CD59 (CD59H) neither CD59U nor PIPLC-treated CD59H b ecame incorporated into poly-C9 complexes, suggesting that the interac tion of CD59 with C9 requires the lipid anchor. Human C9 and heterolog ous C9 from guinea pig serum, as well as the YTH53.1 anti-CD59 mAb, in hibited the binding of CD59U to SC5b-8. On the other hand, soluble CD5 9U did not inhibit binding of C9 to SC5b-8, although CD59E has been sh own to limit the number of C9 molecules entering into MAC. This sugges ts that two interaction sites between C5b-8 and C9 exist: one conferri ng the initial binding of C9 into the C5b-8 complex, and a second dire cting the insertion of C9 into the lipid bilayer. The latter interacti on is the prerequisite for C9 polymerization and the target for interf erence by CD59.