PEPTIDE AND PROTEIN ANTIGENS REQUIRE DISTINCT ANTIGEN-PRESENTING CELLSUBSETS FOR THE PRIMING OF CD4(-CELLS() T)

Priming of naive CD4(+) T cells to Ag requires an antigen-presenting c ell (APC) that can take up the Ag and present peptide bound to MHC cla ss 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 delivere d as a peptide was presented most efficiently to CD4(+) T cells by DC, these APC were poor at priming to a protein form oi the same Ag. In c ontrast, the presence of B cells was a requisite for priming to protei n Ag.