Structurally Identical Capsular Polysaccharide Expressed by Intact Group B Streptococcus versus Streptococcus pneumoniae Elicits Distinct Murine Polysaccharide-Specific IgG Responses In Vivo

We previously reported distinct differences in the murine in vivo Ig polysaccharide (PS)-specific responses to intact Streptococcus pneumoniae compared with responses to Neisseria meningitidis and that in each case, the bacterial subcapsular domain markedly influences the Ig response to the associated PS. In light of potentially unique contributions of biochemically distinct capsular PS and/or their characteristic attachments to the underlying bacterium, it remains unresolved whether different bacterial subcapsular domains can exert differential effects on PS-specific Ig responses to distinct bacterial pathogens. In this report, we used a mutant strain of group B Streptococcus ( Streptococcus agalactiae ) type III (GBS-III) that expresses desialylated capsular polysaccharide of GBS-III, biochemically identical to capsular pneumococcal polysaccharide type 14 (PPS14) of Streptococcus pneumoniae (intact inactivated Streptococcus pneumoniae , capsular type 14, Pn14), directly to compare the in vivo PPS14-specific IgG responses to two distinct Gram-positive bacteria. Although both GBS-III and Pn14 elicited relatively rapid primary PPS14-specific IgG responses dependent on CD4 + T cells, B7-dependent costimulation, and CD40–CD40L interactions, only GBS-III induced a highly boosted ICOS-dependent PPS14-specific IgG response after secondary immunization. Of note, priming with Pn14 and boosting with GBS-III, although not isolated PPS14, elicited a similar boosted PPS14-specific IgG response that was dependent on CD4 + T cells during secondary immunization, indicating that Pn14 primes for memory but, unlike GBS-III, fails to elicit it. The inability of Pn14 to elicit a boosted PPS14-specific IgG response was overcome by coimmunization with unencapsulated GBS-III. Collectively, these data establish that structurally identical capsular PS expressed by two distinct Gram-positive extracellular bacteria can indeed elicit distinct PS-specific IgG responses in vivo.