Humoral immunity to Streptococcus pneumoniae induced by a pneumococcal ribosomal protein fraction.

Isolation of a protective subfraction of ribosomes from Streptococcus pneumoniae has been achieved, and the immune response it induces has been investigated. Mice immunized with pneumococcal ribosomes or purified protein extracted from the ribosomal preparation (2-CE protein) exhibit similar survival rates upon challenge by virulent S. pneumoniae. In contrast, recipients of purified ribosomal ribonucleic acid were never protected against pneumococcal challenge. Serum from mice immunized with pneumococcal ribosomes or 2-CE protein passively immunized syngeneic recipients against pneumococcal challenge, whereas spleen cells from the same donors were unable to transfer immunity. Passive immunization with antiribosome serum could be abrogated by absorption with whole ribosomes, 2-CE protein, or various serotypes of S. pneumoniae (capsular types 2, 3, 6, and 14). Antiribosome serum significantly enhanced clearance of S. pneumoniae from mouse blood in vivo and in vitro. This required phagocytic cells, since antiribosome serum alone, with or without complement, supported growth of S. pneumoniae to an extent comparable to normal serum. The data suggest that the primary immunogen of pneumococcal ribosomes resides in the protein fraction. Further, the immunity induced by the protein fraction is mediated by antibody that appears to function as an opsonin for S. pneumoniae.