Immune responses contribute to the success of radiation therapy of solid tumors; however, the mechanism of triggering CD8+ T cell responses is poorly understood. Antigen cross-presentation from tumor cells by dendritic cells (DC) is a likely dominant mechanism to achieve CD8+ T cell stimulation. We established a cross-presentation model in which DC present a naturally expressed oncofoetal tumor antigen (5T4) from irradiated DU145 prostate cancer cells to 5T4-specific T cells. The aim was to establish which immunogenic signals are important in radiation-induced cross-presentation. Radiation (12 Gy) caused G2/M cell cycle arrest and cell death, increased cellular 5T4 levels, high-mobility protein group-B1 (HMGB1) release and surface calreticulin and heat shock protein-70 (Hsp70) expression in DU145 cells. DC phagocytosed irradiated tumor cells efficiently, followed by upregulation of CD86 on phagocytic DC. CD8+ 5T4-specific T cells, stimulated with these DC, proliferated and produced IFNγ. Inhibition of HMGB1 or the TRIF/MyD88 pathway only had a partial effect on T cell stimulation. Unlike previous reports, we found no evidence that DC carrying Asp299Gly toll-like receptor-4 (TLR4) single nucleotide polymorphism had impaired ability to cross-present tumor antigen. However, pre-treatment of tumor cells with Hsp70 inhibitors resulted in a highly significant and robust prevention of antigen cross-presentation and CD86 upregulation on DC co-cultured with irradiated tumor cells. Blocking the Hsp70 receptor CD91 also abolished cross-presentation. Together, our study demonstrates that radiation induces immunologically relevant changes in tumor cells, which can trigger CD8+ T cell responses via a predominantly Hsp70-dependent antigen cross-presentation process.
- Received April 24, 2014.
- Revision received February 5, 2015.
- Accepted February 5, 2015.
- Copyright © 2015, American Association for Cancer Research.