Proteomic analysis of melanoma‐derived exosomes by two‐dimensional polyacrylamide gel electrophoresis and mass spectrometry

Exosomes are 40–100 nm vesicles released by numerous cell types and are thought to have a variety of roles depending on their origin. Exosomes derived from antigen presenting cells have been shown to be capable of initiating immune responses in vivo and eradicating established tumours in murine models. Tumour‐derived exosomes can be utilised as a source of tumour antigen for cross‐priming to T‐cells and are thus of interest for use in anti‐tumour immunotherapy. Further exploration into the protein composition of exosomes may increase our understanding of their potential roles in vivo and this study has examined the proteome of exosomes purified from cell supernatants of the melanoma cell lines MeWo and SK‐MEL‐28. The vesicular nature and size (30–100 nm) of the purified exosomes was confirmed by electron microscopy and sucrose density gradient centrifugation. Western blotting demonstrated the absence of calnexin and cytochrome c, verifying the purity of the exosome preparations, as well as enrichment of MHC class I and the tumour‐associated antigens Mart‐1 and Mel‐CAM. The two‐dimensional polyarcylamide gel electrophoresis (2‐D PAGE) protein profiles of exosomes from the two cell lines were highly comparable and strikingly different from the profiles of the total cell lysates. Mass spectrometric sequencing identified proteins present in 49 protein spots in the exosome lysates. Several of these have been identified previously in exosomes but some are novel, including p120 catenin, radixin, and immunoglobulin superfamily member 8 (PGRL). Proteins present in whole‐cell lysates that were significantly reduced or excluded from exosomes were also identified and included several mitochondrial and lysosomal proteins, again confirming the proposed endosomal origin of exosomes. This study presents a starting point for future more in‐depth protein studies of tumour‐derived exosomes which will aid the understanding of their biogenesis and targeting for use in anti‐tumour immunotherapy protocols.