Immunochemical relations between pneumococcal group 19 and Klebsiella capsular polysaccharides

Pneumococcal type 19F polysaccharide, contained in the current U.S. licensed pneumococcal vaccine, is a poor immunogen in young children. Capsular polysaccharides of Klebsiella, reported to be cross-reactive with the pneumococcal type 19F, were isolated. Immunochemical relations between pneumococcal group 19 and Klebsiella capsular polysaccharides were studied by immunodiffusion with hyperimmune typing antisera, immune passive hemolysis, and plaque-forming cell response in mice following injection of polysaccharides. Klebsiella K2, K9, K32, K47, and K48 polysaccharides were reported to cross-react with horse pneumococcal type 19 antiserum by direct precipitation. However, these Klebsiella polysaccharides did not react in immunodiffusion with horse or with rabbit pneumococcal 19F and 19A typing antisera. Furthermore, pneumococcal type 19F and 19A polysaccharides did not precipitate with rabbit K2, K9, K32, K47, and K49 typing antisera. The cross-reactivity of the Klebsiella and pneumococcal polysaccharides was studied by 2 other methods. Rabbit pneumococcal 19F antiserum induced by low passive hemolysis of sheep erythrocytes coated with Klebsiella polysaccharides. In contrast, Klebsiella typing antisera exhibited higher hemolytic activity with erythrocytes coated with type 19F polysaccharides. The cross-reactivity of K2 and pneumococcal 19F polysaccharides was also exhibited by the direct IgM plaque-forming cell response. The cross-reactivity, determined by precipitin reactions using animal antisera, may not predict the cross-immunogenicity of purified polysaccharides in humans.