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Milani, Valeria (2005): Effects of intracellular and extracellular heat shock proteins on anti-tumor immune responses: Implications for the treatment of solid tumors using hyperthermia and HSP-based vaccines. Dissertation, LMU München: Medizinische Fakultät



The goal of this study was to investigate the function of the heat shock protein 70 family members, expressed in tumors under physiological and stress conditions and to dissect their role in tumor immune recognition as a function of intra- versus extracellular location. Another goal was to investigate whether heat-treatment at clinically relevant thermal doses affects the immunophenotype of a given tumor, as defined by tumor cell sensitivity to immune effector cells. For these questions, the human melanoma system was selected because it is well characterized with regards to tumor-associated antigens, like tyrosinase and Melan-A/MART-1, their epitopes and restriction elements for MHC class I presentation. In the first part of the study the focus specifically was on the time-temperature dependent effects of heat exposure. Two different thermal doses (41,8°C/120 minutes and 45°C/22 minutes) were selected that mimic the heterogeneity of the achieved temperature distribution within the tumor and the time-temperature dependent changes were determined in: a) antigen expression (tyrosinase and Melan-A/MART-1) at the protein and mRNA level; b) expression of the inducible HSP70 and the constitutive HSC70; c) processing and presentation of tyrosinase and MART-1 via MHC class I; d) susceptibility of melanoma cell lines to cytotoxic T lymphocytes like CD8+ T cells, LAK and NK cells. It was demonstrated that HSP70 and antigen expression display distinct expression and kinetics that reflect the thermohistory of the cells, i.e. exposure to high or low thermal doses. Immunologically, a low thermal dose did not alter immune recognition of the cells despite the fact that intracellular HSP70 and tyrosinase protein were upregulated. High thermal dose induced a pleiotropy of effects, including stronger upregulation of HSP70 and tyrosinase protein but downregulation of tyrosinase at mRNA level. Concordant with reduced HLA-A2 surface expression and tyrosinase mRNA levels, immune recognition of the heat-treated cells was initially reduced, but pretreatment levels were restored after 72 hours of recovery. The observation that tumor cells treated with temperatures below the breakpoint temperature maintain an immunological homeostasis during the heat shock response is of critical importance for the clinical application of hyperthermia in the treatment of tumors. In the second part of the study, the ability of HSP70 to cross-present a naturally expressed human tumor antigen, tyrosinase, that is of low immunigenicity, a situation that more closely resembles the patient situation was investigated. It was demonstrated that HSP70-peptide complexes (HSP70-PC) purified from tyrosinase-positive (HSP70-PC/tyr+) but not from tyrosinase-negative (HSP70-PC/tyr-) melanoma cells deliver the tyrosinase antigen to immature DCs for MHC class I restricted T cell recognition. T cell stimulation by HSP70-PC/tyr+ incubated with immature DCs with was very efficient even without additional DC maturation signals (e.g. exogenous TNF-?) demonstrating the ability of tumor-derived HSP70-PC to act as a chaperone for peptides and a signal for DC maturation. HSP70-PC in exerting both functions on DCs, delivering antigens and maturing DCs, ensures that the peptides that are delivered to the DCs are presented in an immunogenic context optimal for T cell stimulation. In conlusion, induction of intracellular heat shock proteins (HSPs) by heat does not interfere with the tumor immune recognition and when HSPs are expressed extracellularly they acquire immunostimulatory properties. These observations open new perspectives for the application of hyperthermia in combination with HSP-based vaccine in the treatment of solid tumors.