Activation of the fourth component of complement (C4): assessment by rocket immunoelectrophoresis and correlation with the metabolism of C4.

The classical pathway of complement (C) is activated in several diseases, and this activation characteristically involves the activation of C4, the fourth component of C. Since activation of C4 ultimately produces the polypeptide fragment C4d, we have applied immunoelectrophoresis in gels containing antibodies that precipitate C4d and C4 (rocket immunoelectrophoresis) as a means of detecting and quantitating C4 activation. Plasma C4 produced a single precipitin line after rocket immunoelectrophoresis, whereas plasma containing C4d produced two precipitin lines corresponding to C4d and C4. The areas enclosed by the respective precipitin line(s) were quantitated by planimetry and approximated the amounts of C4d and C4 present in the specimen. The ratio of the areas of C4d/C4 as measured in this analysis correlated significantly with the in vivo metabolism of radiolabeled C4. Our method detected C4d in the plasmas of some patients with rheumatoid arthritis, hereditary angioedema, systemic lupus erythematosus, or chronic urticaria with hypocomplementemia. These studies indicated that determination of the C4d/C4 ratio is useful in the evaluation of in vivo C activation and also may be applicable to the study and management of diseases associated with C activation.