An alternative mechanism for the properdin system

Evidence is presented that phenomena ascribed to the properdin system may be explained in terms of classical antibody (Ab) in combination with three of the components of complement (C'), i.e., C'(1), C'(4), and C'(2) respectively. The results suggest that the complex of properdin (P) with zymosan (Z) represents a mixture of Z.Ab, Z.Ab.C'(1, 4), Z.Ab-C'(1, 4, 2), and Z.Ab.C'(1, 4, 2) in a decayed state. The purported preferential reactivity of Z with the third component of C' (C'(3)) is not supported by the present experiments in which Z was reacted with C' in both guinea pig and human sera. Approximately the same number of reactive units of C'(1, 4) and of C'(3) were inactivated by Z, as well as by D. pneumoniae and a washed specific precipitate of bovine albumin-anti-albumin. The latter evidence that small amounts of antigen-antibody complex fix significant amounts of C'(3) stands in contrast with the classical concept that C'(3) is not fixed in ordinary complement fixation reaction. The observed reactivity of C'(3) is explained on the basis of the present use of essentially undiluted serum as a source of C'(3) and of 37 degrees as the temperature for fixation. Ancillary data indicate that purified properdin contains Ab. In the presence of a chelating agent and at 0 degrees properdin agglutinated Z granules. Measurements of nitrogen (N) uptake by Z suggest that 0.25 microg. N were contributed by solutions stated to contain 1 unit of properdin. The broad spectrum of reactivity or cross-reactivity of the Ab in normal serum is likely due to the wide distribution in nature of closely related polysaccharides. Further immunochemical studies are necessary to establish definitively the origin and mode of action in "natural resistance" of antibodies reactive with these polysaccharides.

Accessory plasma factors involved in the bactericidal test for type-specific antibody to group A Streptococci. II. Human plasma cofactor (s) enhancing opsonization of encapsulated organisms

A study was made of the nature of the thermolabile plasma factors in human blood which promote the phagocytosis of Group A streptococci in vitro in the presence of optimal amounts of type-specific M antibody. The plasmas of individuals with strong opsonic activity (normal) were compared with those of some individuals whose opsonic activity was consistently weak (deficient). A general relationship was established between encapsulation of streptococci and the opsonic requirement for thermolabile plasma factor(s). Marked differences in phagocytosis of Group A organisms by human bloods were demonstrated with encapsulated strains only. Human bloods deficient in the cofactor required for opsonization of encapsulated streptococci (coopsonin) showed a normal rate of phagocytosis against all other organisms and particles studied. Furthermore, coopsonin-deficient bloods contained normal levels of four components of complement, of properdin, of lysozyme, and of direct bactericidal activity against several species of Gram-negative organisms and of E. coli bacteriophage. The independence of the streptococcal coopsonin from complement was also demonstrated by absorption of plasma with bentonite and with zymosan. Under appropriate conditions, the coopsonin was reduced without significant loss of complement. The data support the concept that the capsule of the streptococcus imposes an opsonic requirement for a plasma factor(s) which is present in varying amounts in human bloods and which appears to be independent of the complement system. The possibility that it is accessory to the latter components has not been excluded.