The tailless complex polypeptide-1 ring complex of the heat shock protein 60 family facilitates cross-priming of CD8 responses specific for chaperoned peptides.

The tailless complex polypeptide-1 ring complex (TRiC) is a eukaryotic heat shock protein 60 (hsp60) molecule that has been shown to bind N-terminally extended precursors of OVA-derived SIINFEKL in vivo. Binding of peptides to TRiC was shown to be essential for their presentation on MHC class I. We demonstrate in this study that purified TRiC binds antigenic peptides in vitro as well; however, such binding is not restricted to N-terminally extended peptides, suggesting that the results obtained in vivo reflect the availability of peptides in vivo rather than structural constraints of TRiC-peptide binding. Immunization of mice with noncovalent complexes of peptides (derived from OVA or β-galactosidase) and TRiC results in cross-priming of CD8(+) T lymphocytes specific for K(b)/SIINFEKL or L(d)/TPHPARIGL. Mechanistic dissection of this phenomenon shows that TRiC binds APC, and TRiC-chaperoned peptides are processed within the APC and presented on their MHC class I. Immunogenicity of TRiC purified from OVA- or β-galactosidase-expressing cells, that is, of endogenously generated TRiC-peptide complexes, was investigated, and such preparations were observed not to be immunogenic. Consistent with this observation, SIINFEKL or its precursors were not observed to be associated with TRiC purified from cells expressing a fusion GFP-OVA protein. In contrast, immunization with TRiC purified from a tumor elicited specific protection against a challenge with that tumor. These results are interpreted with respect to the cell biological properties of TRiC and suggest that in vivo, TRiC binds a limited proportion of peptides derived from a limited set of intracellular proteins.