Evaluation of the role of the pneumococcal Forssman antigen (F-polysaccharide) in the cross-serotype protection induced by pneumococcal subcellular preparations.

We tested the hypothesis that the capacity of subcellular preparations of rough pneumococci to give cross-serotype protection is due to the presence of the pneumococcal Forssman antigen (F-polysaccharide). We found by hemagglutination inhibition that the Forssman antigen is present in the subcellular extracts. However, we concluded that the Forssman antigen is not the protective immunogen in the pneumococcal subcellular preparation, since absorption with sheep erythrocytes failed to remove the protective capacity from antiserum raised against the vaccine. Other evidence mitigating against the pneumococcal Forssman antigen being the protective immunogen included the absence of a detectable hemolytic titer in protective antiserum raised against the subcellular preparation, the failure of high-titered sheep hemolysin to passively protect mice against pneumococcal infection, and the failure of purified F-polysaccharide to immunize mice against pneumococcal infection.