Calreticulin is an endoplasmic reticulum (ER) chaperone that displays
lectin activity and contributes to the folding pathways for nascent
glycoproteins.
Calreticulin also participates in the reactions yielding assembly of
peptides onto nascent
MHC class I molecules. By chemical and immunological criteria, we identify
calreticulin as a
peptide-
binding protein and provide data indicating that
calreticulin can elicit CTL responses to components of its bound
peptide pool. In an adoptive immunotherapy protocol, dendritic cells pulsed with
calreticulin isolated from B16/F10.9 murine
melanoma, E.G7-OVA, or EL4
thymoma tumors elicited a CTL response to as yet unknown
tumor-derived Ags or the known OVA Ag. To evaluate the relative efficacy of
calreticulin 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 responses was compared. In both the B16/F10.9 and E.G7-OVA models,
calreticulin 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 disulfide isomerase. The observed antigenic activity of
calreticulin was recapitulated in in vitro experiments, in which it was observed that pulsing of bone marrow dendritic cells with E.G7-OVA-derived
calreticulin elicited sensitivity to lysis by OVA-specific CD8+ T cells. These data identify
calreticulin as a
peptide-
binding protein and indicate that
calreticulin-bound
peptides can be re-presented on dendritic cell class I molecules for recognition by CD8+ T cells.