Immunogenicity of tumor peptides: importance of peptide length and stability of peptide MHC class II complex

Nonameric P815AB, a cytotoxic-T-lymphocyte-defined minimal core peptide enc oded by the murine mastocytoma gene P1A, fails to initiate CD4(+) cell-depe ndent reactivity in vivo to class-I-restricted epitopes when mice are admin istered peptide-pulsed dendritic cells. Effective immunization requires T h elper effects, such as those mediated by coimmunization with class-II-restr icted (helper) peptides or by the use of recombinant interleukin-12 (rIL-12 ). Although P815AB does possess class-II-restricted epitopes, they are like ly suboptimal, resulting in poor affinity and/or stability of MHC/P815AB co mplexes and inadequate activation of the antigen-presenting cell function o f dendritic cells. The present study has examined a series of:longer, P815A B-centered peptides (11-14 amino acids in length, all P1A-encoded) for thei r ability to initiate CD4(+) and CD8(+) cell-mediated responses to the nona mer in vivo, their ability to bind class II MHC in vitro, and their ability to assemble class II molecules stably. By means of a class-I-restricted sk in test assay in mice receiving peptide-pulsed dendritic cells, we found th at a 12-mer and a 13-mer effectively immunized against the core P815AB pept ide, and that this correlated with IL-2 production in vitro by CD4+ cells i n response to the nonamer. In vitro studies, involving affinity-purified cl ass II molecules, showed that the capacity to assemble class II molecules s tably, more than the affinity for class II MHC, correlated with the ability of the different P815AB peptides to prime the host to the core peptide see n by the T cells.