Memory B and T cell responses induced by serotype 4 Streptococcus pneumoniae vaccines: longitudinal analysis comparing responses elicited by free polysaccharide, conjugate and carrier.

We have conducted a 1-year longitudinal study in mice vaccinated by free serotype 4 Streptococcus pneumoniae PS (PS4), the corresponding tetanus toxoid (TT)-conjugated vaccine, or the TT carrier alone. B and T cell immunity induced by these three types of antigen, were compared by monitoring the (i) long-term persistence of specific serum antibodies, (ii) frequency of memory B cell precursors in spleen, and (iii) T cell responses against the carrier. While PS4-specific antibody response appeared later than the anti-carrier response upon primary immunization, PS4-specific B memory and serum responses were quantitatively and qualitatively similar to the ones observed against TT upon immunization by either the free carrier or the conjugate. We also observed a parallel persistent carrier-specific T cell response in the spleen. These data indicate that the nature and long-term kinetics of the anti-PS4 antibody response induced by the conjugate vaccine are similar to "classical" T-dependent response elicited by conventional protein antigens.