Heat shock protein 72 (HSP72), a hyperthermia-inducible immunogenic determinant on leukemic K562 and Ewing's Sarcoma cells

Following non-lethal heat stress (41.8°C) and a recovery period at 37°C, the inducible 72 kDa HSP (HSP72) is detectable selectively on the cell surface of human Ewing's Sarcoma (ES) and of leukemic K562 cells but not on EBV transformed B cells (B-LCL) which were generated from PBL of healthy human volunteers. The HSP72 expression was measured by flowcytometric analysis using a monoclonal antibody (moAb) that specifically recognizes HSP72, the inducible form of the HSP70 group. The major histocompatibility complex (MHC) class I expression, detected with the moAb W6/32 was not affected by non-lethal heat exposure and a recovery period at 37°C for 12h: ES cells express MHC class I molecules on about 80% of the cells; K562 cells exhibited no MHC class I expression neither before nor after heat shock. Inhibition of RNA- (actinomycin D) or protein-synthesis (cycloheximide) prior to heat treatment completely inhibits the expression of HSP72 on the cell surface of both tumour cells, thus indicating that de novo protein synthesis is required for HSP72 cell surface expression. Since, apart from HSP72, protein synthesis in general is down-modulated by heat shock we speculate that HSP72 molecules that are expressed on the cell surface of tumour cells might be recruited from newly synthesized proteins. The heat-inducible HSP72 cell surface expression on tumour cells could be correlated with an increased sensitivity of leukemic and sarcoma cells to lysis mediated by NK effector cells. The results of cold target inhibition assays revealed that histologically different tumour cells (sarcoma and leukemic cells) that were exposed to non-lethal temperatures have to share a similar if not identical HSP72 immunogenic determinant.