Classical and alternative complement pathway activation by pneumococci

Abstract

Streptococcus pneumoniae strains (62) were studied for their abilities to consume selected components of classical and alternative complement [c] pathways in human sera. The classical pathway was blocked by chelating Ca with ethyleneglycol-bis(.beta.-aminoethyl ether)-N,N-tetraacetic acid and by removing C4. The alternative pathway was blocked by removing factor B. Each strain''s activation of the 2 pathways was compared with its nonimmune reactivity with the Fc region of immunoglobulin (Ig)G. Activation of the classical complement pathway appeared to be independent of Fc reactivity. Highly Fc-reactive strains activated the alternative pathway more effectively than did less Fc-reactive strains. Since pneumococcal activation of the alternative pathway requires non-immunospecific IgG, nonimmune binding of IgG on the pneumococcal surface may endow it with complement-activating properties.