Distinct types of T-cell help for the induction of a humoral immune response to Streptococcus pneumoniae

Studies have indicated that purified soluble polysaccharide antigens can el icit T cell-independent Ig responses in vivo, although these responses can be modulated by T cells in a noncognate manner. Relatively little is known, however, concerning the parameters that regulate polysaccharide-specific. as well as protein-specific, Ig isotype responses to an intact extracellula r bacterium. Using the murine in vivo humoral response to intact Streptococ cus pneumoniae as a model it can be shown that CD4(+) T-cell receptor alpha beta (+) T cells deliver help for both polysaccharide- and protein-specifi c Ig responses. However, these responses differ fundamentally in their mech anism of action.