Evaluation of nonspecific (alternative pathway) opsonic activity by neutrophil chemiluminescence

The role of alternative complement pathway activation in protection against infection is not well understood. We have investigated nonspecific opsonic activity in human adult, term neonatal and premature serum using the technique of neutrophil chemiluminescence (cl) to measure particle uptake. Following phagocytosis, neutrophils become metabolically active, produce excited molecular oxygen and emit a burst of light which can be detected and quantitated in a liquid scintillation counter. In the present study, zymosan particles were preopsonized in human serum and added to control neutrophils. Particles opsonized in adult serum produced a marked peak in CL. Serial dilutions of the serum prior to opsonization yielded proportionally lower peaks in CL. Opsonic activity as measured by the CL procedure was completely blocked by the addition of chelating agents which remove calcium and magnesium ions and block both classic and alternative complement pathway activation. Addition of an excess of magnesium ions (needed for alternative pathway activation) to the reaction mixture partially restored the opsonic activity. Opsonic activity as measured by the CL procedure was significantly depressed in approximately two-thirds of term and premature infant sera tested. Deficient nonspecific opsonic activity was closely correlated with serum levels of C3PA. These studies suggest that the CL procedure may be of value in the investigation of nonspecific opsonins in human serum.