The mechanism underlying T cell help for induction of an antigen-specific in vivo humoral immune response to intact Streptococcus pneumoniae is dependent on the type of antigen.

Little is known concerning the role of T cells in regulating an anti-polysaccharide Ig response to an intact pathogen. We previously reported that the in vivo Ig responses to Streptococcus pneumoniae (strain R36A), specific for pneumococcal surface protein A (PspA) and for the phosphorylcholine (PC) determinant of C-polysaccharide, were both dependent on TCR-alphabeta(+) T cells and B7-dependent costimulation, although only PspA-specific memory was generated. In this report, we show that the T cell help underlying these two Ag-specific Ig responses is distinct. Using H-Y-specific T cell transgenic mice made "nonleaky" by crossing with mice genetically deficient for TCR-alpha, we demonstrate that the T cell help for the anti-PC, in contrast to the anti-PspA, response is TCR-nonspecific and occurs normally in the absence of germinal center formation, although it is still dependent on B7-dependent costimulation. Consistent with these data, we demonstrate, using cathepsin S(-/-) mice, that although the anti-PC response is largely dependent on CD4(+) T cells, there is a reduced (or lack of) dependence, relative to the anti-PspA response, on the generation of new peptide-MHC class II complexes. In this regard, the T cell help for an optimal anti-PC response is delivered more rapidly than that required for an optimal anti-PspA response. Collectively, these data demonstrate a novel accelerated TCR-nonspecific B7-dependent form of T cell help for augmenting a polysaccharide-specific Ig response to an intact bacterium without the generation of memory.