Prevention Of Complement Activation

Fluid Phase Inhibition

Manipulation of the normal pathway of complement activation has proven to be extremely effective for overcoming HAR. The most effective agent to date has been cobra venom factor (CVF). Cobra venom contains, in addition to several toxins and enzymes, a C3b-like molecule, CVF, that is resistant to human and rat C3b inactivating factors. This C3b analogue combines with components of the alternative pathway of complement to form a highly stable enzyme complex that causes massive consumption of C3, factor B, and members of the MAC. The result is exhaustion of the complement cascade. Removal of phospholipase contaminants in cobra venom has greatly reduced its toxicity in rodents. To date, toxicity has not been a significant factor in primates. CVF has extended xenograft survival in several rodent models, and has prolonged pig-to-baboon cardiac xenograft survival. Clinical use may be limited by the potential toxicity of CVF as well as its immunogenicity. Repeated use may induce anti-CVF antibodies. A variety of complement-inhibiting agents have been evaluated in small and large animal models. Agents such as soluble complement receptor (sCR1) have been shown to significantly prolong porcine xenograft survival in primates.

Host-Specific Regulators

Several biologically important regulators of complement activation (RCA) are located on the membranes of most cells that are in contact with blood or other body fluids containing complement system proteins. Decay accelerating factor (DAF or CD55), membrane inhibitor of reactive lysis (CD59 or protectin), homologous restriction factor (HRF), and membrane cofactor protein (CD46) interfere with the complement reaction only at the membrane sites where they are located. DAF inhibits formation of the classical and alternative pathway C3 and C5 convertases; if such convertases have been formed it promotes the dissociation of these enzymatic complexes. CD59 and HRF interfere with formation of the MAC at the C8 and C9 binding steps. CD46 acts as a cofactor for the inactivation of C3b and C4b by factor I. DAF, CD59, and HRF are uniquely anchored to the cell membrane by a phosphatidylinositol tail, which affords these molecules great mobility in the plane of the cell membrane and allows purified forms of RCA to be inserted into cell membranes. Membrane-associated RCA display the phenomenon of homologous restriction, i.e., they are inhibitory to homologous complement but have little inhibitory capacity over xenogeneic complement. This phenomenon has led several investigators to explore whether the introduction of human RCA molecules into xenogeneic cells would afford protection against human complement-mediated damage. Dalmasso et al were the first to show that purified human DAF protected these cells from the cytotoxic effects of human complement. They obtained similar results with purified human CD59. However, the protection afforded by inserted human RCA molecules is relatively short-lived, as they undergo rapid turnover.

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