THE PROPERDIN SYSTEM AND IMMUNITY V. THE BACTERICIDAL ACTIVI~ O~ Tm~ PROPERDIN SYSTEM BY ALASTAIR

Properdin, complement, and magnesium ions constitute the recognized components of the properdin system (1, 2). This system, which is present in the normal serum of man and other mammals, has the ability in vitro to lyse certain cells, destroy bacteria, and inactivate viruses, and appears to be involved in the resistance or susceptibility of experimental animals to infection, irradiation, and shock (3-9). Preliminary reports have described briefly the action of the properdin system in bactericidal phenomena (1, 2, 5). The purpose of this paper is to report in detail experimental evidence which shows that the properdin system is the mechanism by which certain bacteria are destroyed by fresh, normal human serum. The factors required for the bactericidal action; and the variety of bacteria affected by the system are also considered. Properdin was originally recognized by its ability to combine with zymosan, the insoluble cell wall residue of yeast, to form a complex that inactivated the third component of complement (C'3). Two distinct reaction stages were demonstrated: First, the combination of zymosan with properdin, which required the presence of complement and magnesium and second, the destruction of C'3 by the properdin-zymosan complex (PZ) (1, 2). These interactions had a resemblance to the heat-labile viral neutralizing activities of serum described by Ginsberg and Horsfall (10). When it was evident, as a result of preliminary studies (1, 6), that the properdin system had viral inactivating properties, the analogy between zymosan and the bacterial cell wall suggested that the properdin system might also be bactericidal. The experiments, that confirmed this hypothesis, are described below.