## 2 Materials and Methods
### Central Nervous System Tumors  
Malignant CNS tumors are one of the most prevalent cancer types and the third most common cause of cancer-related death in AYA [131; 150]. This heterogenous group consists of a variety of tumors; among them the major categories are gliomas, intracranial germ cell tumors and, in younger patients, embryonal tumors typical for pediatric age, such as medulloblastoma (MB). The prognosis in CNS malignancies in AYA is poorer than in children and the survival rates decrease with age [150].  
The research on Hsps in malignant CNS tumors addressed mainly MB and high-grade gliomas. Various Hsps, including Hsp27, Hsp40, Hsp60, Hsp70, and Hsp90\(\alpha\) were found to be expressed in MB specimens [151; 152]. One study reported that expressions of particular Hsps significantly differed between MB histological subtypes [151]. In another, Hsp90\(\beta\) was one of the proteins detected in MB, but not in benign pilocytic astrocytoma [153].  
The studies on glioblastoma multiforme (GBM) confirmed the high expressions of Hsps, including Hsp27, Hsp40, Hsp60, Hsp70, Hsp72, and Hsp90\(\alpha\) in tumor specimens and GBM stem cells [154; 155; 156; 157; 158; 159]. These proteins were found to be involved in pathways responsible for glioma cell invasiveness and motility and their overexpression may be associated with treatment resistance [160]. The spontaneous and radiation- or heat-shock induced release of Hsp27 and Hsp70 from glioma cells was also reported [161; 87; 162; 163]. Some studies suggested the key role of extracellular Hsp90 in GBM metastasis formation [164; 165; 166]. These findings translated into in vitro and animal model studies on Hsps inhibitors and Hsp-peptide complex vaccines in GBM treatment [167; 168; 169; 170; 171; 172]. Some of them recently reached the I and II phases of clinical trials [173; 174; 175; 176].  
Several studies also analyzed the role of exosomes in both pediatric and adult CNS tumors with a focus on the microRNA role [177; 178] and the possible use of other exosomal proteins as biomarkers [179; 180]. However, the field of exosomal Hsps in CNS malignancies was not extensively studied.  
[PERSON] et al. were the first to report the exosomal pathway of Hsp27, Hsp60, Hsp70, and Hsp90 secretion from malignant CNS tumor cells (adult glioma and murine anaplastic astrocytoma) [87]. Later, various Hsps were detected in the proteomic analysis of extracellular vesicles released by eight glioma cell lines [136]. However, the role of exosomal release of these proteins is not fully understood. [PERSON] et al. confirmed that exosomal Hsp90\(\alpha\) is released from various malignant cell lines, including glioma cells [88]. They also reported that, additionally to previously known mechanisms of activating matrix metalloproteinase 2 (MMP2) [81] and human epidermal growth factor receptor 2 (HER2) [181], extracellular Hsp90\(\alpha\) increases motility of the tumor cells by also activating plasmin. Interestingly, Hsp90\(\alpha\)-containing exosomes enhanced the movement of the glioma cells more than Hsp90\(\alpha\) protein alone, which indicates the cooperation of Hsp90\(\alpha\) with other exosomal proteins in promoting GBM progression and metastasis formation. Another study, however, showed the antitumor immune-stimulating function of exosomes carrying Hsp70. The glioma-derived exosomes were incubated with dendritic cells, leading to stimulation of the glioma-specific cytotoxic CD8+ T-cell response [99]. Further research is needed to better understand these mechanisms and, possibly, allow one to target exosomal Hsps in experimental therapies.