Activation of complement by hemodialysis membranes: polyacrylonitrile binds more C3a than cuprophan

Conventionally, complement activation by hemodialysis membranes has been determined by measuring fluid phase C3a. Based on such measurements, polyacrylonitrile (PAN) membranes have been classified as weak activators compared to cuprophan. Previous studies have demonstrated, however, that PAN adsorb fluid phase C3a. Based on that observation, we hypothesized that complement activation by PAN might be artifactually underestimated if relatively large amounts of C3a remained membrane bound. In the present study, a method that allows the simultaneous quantification of both fluid phase and membrane bound C3a was used to assess complement activation by PAN and cuprophan. Pieces of membrane were incubated with C3-depleted serum that had been repleted with radiolabeled C3. Subsequently, the supernates and membranes were subjected to SDS-PAGE, and complement activation was quantified by determining the radioactivity of the C3a bands in the gel. The results showed that while the serum exposed to cuprophan membranes contained almost five times more C3a than that exposed to PAN, approximately 80 times more C3a was bound to the PAN membranes. Consequently, the total amount of C3a generated in the presence of PAN was higher than that generated in the presence of cuprophan. We conclude that assessment of complement activation by hemodialysis membranes using fluid phase C3a measurements alone may be misleading.