Inflammatory mediators are insufficient for full dendritic cell activation and promote expansion of CD4+ T cell populations lacking helper function.

Dendritic cells (DCs) can be activated directly by triggering of receptors for pathogens or, indirectly, by exposure to inflammatory signals. It remains unclear, however, whether the two pathways result in qualitatively similar DCs or lead to equivalent adaptive immune responses. Here we report that indirect activation by inflammatory mediators generated DCs that supported CD4(+) T cell clonal expansion but failed to direct T helper cell differentiation. In contrast, exposure to pathogen components resulted in fully activated DCs that promoted T helper responses. These results indicate that inflammation cannot substitute for contact with pathogen components in DC activation and suggest that the function of pattern recognition by DCs is to couple the quality of the adaptive immune response to the nature of the pathogen.