Recombinant cobra venom factor.

Cobra venom factor (CVF) is the complement-activating protein from cobra venom. CVF is a three-chain protein that functionally resembles C3b, the activated form of complement component C3. Like C3b, CVF forms a C3/C5 convertase with factor B in the presence of factor D and Mg(2+). Although CVF exhibits functional activity of C3b, it structurally resembles the C3b degradation product C3c, which is not able to form a C3/C5 convertase. CVF has become an important research tool to decomplement laboratory animals in order to study the role of complement in host defense, immune response, and pathogenesis of disease. As the Asian cobras of the Naja species are on the list of endangered species, cobra venom as the source for CVF has become increasingly difficult to obtain. Methods have been developed to recombinantly produce active forms of CVF. This manuscript reviews the production of recombinant pro-CVF using both prokaryotic and eukaryotic expression systems. The recombinant production of pro-CVF in two insect cell expression systems (baculovirus-infected Sf9 Spodoptera frugiperda cells, stably transfected S2 Drosophila melanogaster cells) generates three forms of pro-CVF: single-chain pro-CVF resembling pro-C3, a two-chain form of pro-CVF resembling C3, and another two-chain form of pro-CVF resembling C3b. All three forms of pro-CVF exhibit functional activity of mature, natural CVF. Recombinant pro-CVF supports the activation of factor B in the presence of factor D and Mg(2+), forms a bimolecular convertase pro-CVF,Bb that exhibits cleaving activity for both C3 and C5, and depletes the serum complement activity. The activity of pro-CVF and the resulting C3/C5 convertase is indistinguishable from CVF and the CVF,Bb convertase. Recombinant production of functionally active forms of pro-CVF ensures the availability of an important research reagent for future research involving complement depletion. The experimental systems to recombinantly produce active forms of CVF will also be invaluable for studies to delineate the structure/function relationship of CVF and its differences from C3, and to generate human C3 derivatives with CVF-like function ("humanized CVF") for therapeutic complement depletion.