Abstract
We previously demonstrated that intact, inactivated Streptococcus pneumoniae (unencapsulated strain R36A) inhibits IgG responses to a number of coimmunized soluble antigens (Ags). In this study, we investigated the mechanism of this inhibition and whether other extracellular bacteria exhibited similar effects. No inhibition was observed if R36A was given 24 h before or after immunization with soluble chicken ovalbumin (cOVA), indicating that R36A acts transiently during the initiation of the immune response. Using transgenic cOVA-specific CD4(+) T cells, we observed that R36A had no significant effect on T-cell activation (24 h) or generation of regulatory T cells (day 7) and only a modest effect on T-cell proliferation (48 to 96 h) in response to cOVA. However, R36A mediated a significant reduction in the formation of Ag-specific splenic germinal center T follicular helper (GC Tfh) and GC B cells and antibody-secreting cells in the spleen and bone marrow in response to cOVA or cOVA conjugated to 4-hydroxy-3-nitrophenylacetyl hapten (NP-cOVA). Of note, the inhibitory effect of intact R36A on the IgG anti-cOVA response could be reproduced using R36A-derived cell walls. In contrast to R36A, neither inactivated, unencapsulated, intact Neisseria meningitidis nor Streptococcus agalactiae inhibited the OVA-specific IgG response. These results suggest a novel immunosuppressive property within the cell wall of Streptococcus pneumoniae.