The capsular polysaccharide ofHaemophilus influenzaetype B prevents killing by complement and antibodies against outer membrane proteina

Abstract

The ability of antibodies, raised in rabbits against purified outer membrane protein a (Mr 47 000) of Haemophilus influenzae type b, to promote complement‐dependent killing of these encapsulated organisms was investigated. Killing of encapsulated strains was not induced by these antibodies in conjunction with either human, mouse, rabbit or guinea‐pig complement. Acapsular mutants were effectively killed by complement in the presence of antibodies against protein a. Killing was dependent on the presence of the 47‐kDa protein a and was not influenced by the outer membrane protein subtype or lipopolysaccharide serotype of the strain. The killing‐promoting activity could be absorbed from the sera with cells of strains with the same protein a, purified protein a, but not by purified lipopolysaccharide and capsular polysaccharide. Binding experiments showed that the encapsulated strain and its acapsular mutant bound antibodies against protein a with the same rate and to the same extent, indicating that the capsule probably interferes with complement activation or insertion of the membrane attack complex into the bacterial cell.