The Role of C3 as an Opsonin in the Early Stages of Infection 1

Summary

In order to investigate the role of C3 in host defense in vivo, normal AKR/J mice, genetically deficient in C5, were depleted of serum C3 by the injection of purified cobra venom factor (CoVF). Concurrent with their C3 depletion, their serum opsonizing activity decreased to a level less than 20% of normal. When these mice were challenged with an intraperitoneal injection of pneumococci 2 hr after the CoVF treatment, the LD₅₀ was from 30 to 80 times lower than the LD₅₀ in saline-treated control animals. When the CoVF was given only 6 hr after the pneumococcal challenge, the LD₅₀ was the same as in the control mice. If the pneumococci were first preopsonized in vitro and then injected into CoVF-treated animals, the LD₅₀ was the same as that in control animals.

These experiments demonstrate that C3 plays a significant role in vivo in the host's defense against infection and that a major part of that role is through its action as an opsonin. Furthermore, these experiments demonstrate that the role of C3 is most significant during the early stages of bacterial invasion.

The authors are grateful to Andrea Swift for her expert technical assistance.