Peptide and protein antigens require distinct antigen-presenting cell subsets for the priming of CD4+ T cells.

Priming of naive CD4+ T cells to Ag requires an antigen-presenting cell (APC) that can take up the Ag and present peptide bound to MHC class II molecules. We have used both in vivo and in vitro approaches to demonstrate that the APC used to prime naive CD4+ T cells depends on the initial form in which an Ag is administered. Although Ag delivered as a peptide was presented most efficiently to CD4+ T cells by DC, these APC were poor at priming to a protein form of the same Ag. In contrast, the presence of B cells was a requisite for priming to protein Ag.