Cell surface expression of heat shock protein gp96 enhances cross-presentation of cellular antigens and the generation of tumor-specific T cell memory.

Gp96 is an endoplasmic reticular heat shock protein (HSP). We have shown previously that surface expression of gp96 (96tm) on tumor cells led to the activation of dendritic cells and increased anti-tumor immunity. In this report, we have found that protective immunity elicited by 96tm+ tumor cells was tumor-specific and long-lasting. Both CD4+ and CD8+ T cell memory were elicited. By immunizing with tumor cells loaded with the chicken ovalbumin (ova) model antigen, we demonstrated that the priming of adoptively transferred ova-specific CD8+ T cells could occur across MHC haplotypes. The efficiency of this cross priming can be significantly increased when mice were immunized with whole cells that express both ova and cell surface gp96 (ova+96tm+). Mere mixture of soluble ova with 96tm-expressing tumor cells (ova-96tm+) was insufficient, arguing for further processing of ova and perhaps the participation of 96tm-ova complexes in this process. We further compared the relative efficiency of two whole cell vaccines based on the manipulation of gp96 expression in one system: 96tm+ whole cells and cells that secrete the gp96-Ig fusion protein. We found that both vaccines are effective in a prophylactic model against tumors. Our study has reinforced the notion that the manipulation of the site of expression of HSPs may be an effective approach for cancer immunotherapy.