In vivo modulation of rat complement activities by infusion of anti-H antibodies.

Regulation of the activity of the alternative pathway of complement occurs by the plasma proteins I and H. H is able not only to prevent formation of the amplification convertase C3bBb but also to cause the decay-dissociation of Bb from C3bBb. The present studies have investigated the role of H in vivo. Affinity-purified goat (Fab')2-anti H was infused intravenously in rats, and its effect on C4, C3 and CH50 activities was assessed. In addition, H, C3, C4 and B were quantitated by immunochemical methods. H depletion was dependent on the dose of anti-H, and maximal consumption in vivo occurred between 5 and 15 min. A maximum of 51% and 77% hemolytic C3-consumption was seen after 15 min with 3.8 and 8.4 mg anti-H, respectively. The injection of 6.3 mg affinity-purified goat IgG anti-rat H caused 96.8 +/- 0.7% and 85.0 +/- 1.4% consumption of CH50 and C3 hemolytic activity, respectively. Using this dose of IgG anti-rat H, it was found that the clearance in rats of rat erythrocytes sensitized with 8000 molecules of guinea pig IgG2 per E was impaired as compared to the clearance of these intermediates in control rats injected with a comparable dose of normal goat IgG. The results indicate that H functions as a potent regulator of C in vivo and that infusion of anti-H can be used as a method to achieve depletion of circulating complement components.