Activation of complement by 405-nm light in serum from porphyria cutanea tarda

Sera from three patients with porphyria cutanea tarda (PCT) were examined for evidence of complement activation. The irradiation of sera in vitro with 405-nm light resulted in a dose-dependent diminution of total hemolytic complement activity and the hemolytic titers of C1, C4, C2, C3, and C5. Furthermore, such irradiated sera showed immunoelectrophoretical C3 conversion, chemotactic activity for rat polymorphonuclear leukocytes inhibited by incubation with anti-C5 antisera but not anti-C3 antisera, and C5a generation as measured by radioimmunoassay. Factor-B conversion did not occur in such irradiated sera. Using Sephadex G-75 chromatography, the irradiated sera showed a multiphasic elution profile of chemotactic activity similar to that of zymosan-activated serum. The generation of C5a even occurred in factor-B-depleted serum. These studies indicate that the irradiation of PCT serum with 405-nm light induces the activation of the complement system via the classical pathway, resulting in the development of a chemotactic factor.