Calreticulin displays in vivo peptide-binding activity and can elicit CTL responses against bound peptides

Calreticulin is an endoplasmic reticulum (ER) chaperone that displays lecti n activity and contributes to the folding pathways for nascent glycoprotein s, Calreticulin also participates in the reactions yielding assembly of pep tides onto nascent MHC class I molecules. By chemical and immunological cri teria, we identify calreticulin as a peptide-binding protein and provide da ta indicating that calreticulin can elicit CTL responses to components of i ts bound peptide pool. In an adoptive immunotherapy protocol; dendritic cel ls pulsed with calreticulin isolated from B16/F10.9 murine melanoma, E.G7-O VA, or EL4 thymoma tumors elicited a CTL response to as yet unknown tumor-d erived Ags or the known OVA Ag, To evaluate the relative efficacy of calret iculin in eliciting CTL responses, the ER chaperones GRP94/gp96, BiP, ERp72 , and protein disulfide isomerase were purified in parallel from B16/F10.9, EL4, and E.G7-OVA tumors, and the capacity of the proteins to elicit CTL r esponses was compared, In both the B16/F10.9 and E.G7-OVA models, calreticu lin was as effective as or more effective than GRP94/gp96 in eliciting CTL responses. Little to no activity was observed for BiP, ERp72, and protein d isulfide isomerase. The observed antigenic activity of calreticulin was rec apitulated in in vitro experiments, in which it was observed that pulsing o f bone marrow dendritic cells with E.G7-OVA-derived calreticulin elicited s ensitivity to lysis by OVA-specific CD8(+) T cells. These data identify cal reticulin as a peptide-binding protein and indicate that calreticulin-bound peptides can be re-presented on dendritic cell class I molecules for recog nition, by CD8(+) T cells.