Immunogenicity of outer membrane derivatives ofHaemophilus influenzae type b

We prepared outer membrane derivatives ofHaemophilus influenzae type b to determine whether the residual capsular and noncapsular surface components are immunogenic and protective. These fragments consist primarily of six major proteins and lipopolysaccharide. By transmission electron microscopy, they appeared as small, membrane-like fragments or larger, cellshaped double-track ghosts. Rabbits immunized with ghosts responded with increases in serum anticapsular antibody and bactericidal activity. Antisera absorbed with capsular antigen to remove anticapsular antibody remained bactericidal and passively protected infant rats. These data suggest that antibodies to noncapsular surface antigens are protective, and that outer membrane derivatives retain some of the constituents responsible for stimulating immunity.