Comparative immunogenicity of group 6 pneumococcal type 6A(6) and type 6B(26) capsular polysaccharides

Abstract

The comparative immunogenicity of the 2 cross-reacting group 6 pneumococcal capsular polysaccharides, type 6A(6) and type 6B(26), was studied with hyperimmune rabbit typing antisera and with sera from adult volunteers injected with polyvalent pneumococcal vaccines containing either 50 μg of type 6A (U.S. designation, type 6) or 50 μg each of type 6A and type 6B (U.S. designation, type 26) polysaccharides. Both group 6 polysaccharides were linear copolymers composed of 1 mol each of D-galactose, D-glucose, L-rhamnose, and D-ribitol phosphate. They differed only in that type 6A had a rhamnopyranosyl-(1→3)-D-ribitol → 3)-D-ribitol bond and the type 6B had a rhamnopyranosyl-(1 → 4)-D-ribitol bond. Quantitative precipitation and absorption analysis with rabbit hyperimmune antisera induced by simultaneous injection with type 6A and type 6B organisms revealed extensive cross-reactions between the 2 group 6 polysaccharides. There was less, although still quite extensive, cross-reactivity between the 2 group 6 polysaccharides examined with antisera from rabbits injected with only 1 of the group pneumococci. In a radioimmunoassay, using 14C internally labeled type 6A or type 6B polysaccharide antigens, there was no difference in the serum antibody level to either type of volunteer injected with polyvalent pneumococcal vaccines containing type 6A or both type 6A and type 6B polysaccharides. These studies indicate that the structural similarity of the pneumococcal group 6 polysaccharides confers extensive cross-reactivity with hyperimmune typing antisera prepared with whole organisms or after injection of purified polysaccharides in adult volunteers. With the authors' current polysaccharides, it appears that a polyvalent pneumococcal vaccine formulation that contains only type 6A will serve to induce the maximum amount of serum antibodies to both group 6 organisms.