Noncovalent association with stress protein facilitates cross-priming of CD8+ T cells to tumor cell antigens by dendritic cells

A viral oncogene carrying well-defined K-b/D-b-restricted epitopes was expressed in a heat shock protein (hsp)-associated or nonassociated form in the murine tumor cells P815 and Meth-A. Wild-type SV40 large T-Ag (wtT-Ag) is expressed without stable hsp association; mutant (cytoplasmic cT-Ag) or chimeric (cT272-green fluorescent fusion protein) T-Ag is expressed in stable association with the constitutively expressed, cytosolic hsp73 (hsc70) protein. In vitro, remnants from apoptotic wtT-Ag- or cT-Ag-expressing tumor cells are taken up and processed by immature dendritic cells (DC), and the K-b/D-b-binding epitopes T1, T2/3, and T4 of the T-Ag are cross-presented to CTL in a TAP-independent way. DC pulsed with remnants of transfected, apoptotic tumor cells cross-presented the three T-Ag epitopes more efficiently when they processed ATP-sensitive hsp73/cT-Ag complexes than when they processed hsp-nonassociated (native) T-Ag. In vivo, more IFN-gamma -producing CD8(+) T cells were elicited by a DNA vaccine that encoded hsp73-binding mutant T-Ag than by a DNA vaccine that encoded native, non-lisp-binding T-Ag. Three- to 5-fold higher numbers of T-Ag (T1-, T2/3-, or T4-) specific, D-b/K-b-restricted IFN-gamma -producing CD8(+) T cells were primed during the growth of transfected H-2(d) Meth-A/cT tumors than during the growth of transfected Meth-A/T tumors in F-1(b x d) hosts. Hence. the association of an oncogene with constitutively expressed, cytosolic hsp73 facilitates cross-priming in vitro and in vivo of CTL by DC that process material from apoptotic cells.