Nature of the cross-protective antigen in subcellular vaccines of Streptococcus pneumoniae

Studies have been carried out to investigate the nature of the antigen present in subcellular extracts of a rough strain of Streptococcus pneumoniae A662b which has been shown to confer protection in mice against challenge with smooth, virulent organisms of the homologous and heterologous serotypes. The finding that whole, heat-killed cells were also capable of immunizing mice against challenge with organisms of heterologous serotypes suggests that the immunogen is present on the surface of the rough pneumococcal cell. Ribosomes purified by sucrose gradient centrifugation were not protective, but material recovered in the pellet retained activity. Subcellular extracts prepared from spheroplasts with a partial absence of cell wall showed decreased protective capacity, and extracts prepared from wall-deficient protoplasts were not protective. Crude cell walls evidenced cross-serotype protection, but purified walls did not protect. These results are interpreted as suggesting that the active moiety in the subcellular vaccine is present on the surface of rough pneumococci and is either a wall antigen that must be part of a larger macromolecular complex to be immunogenic, or a substance associated with the cell wall that is present in crude, but not purified, cell wall fractions.