Composition of exosomes derived from Atlantic salmon (Salmo salar) head kidney leukocytes

Exosomes are secreted nanosize vesicles (30−100 nm) derived from multivesicular endosomes. Exosomes are released by different immune cell types, including Tand B-lymphocytes, mast cells and antigen-presenting cells (APCs). The composition of exosomes including protein and RNA content reflects their endosomal origin and the type of cells that produce them. Mammalian APCs produce large amounts of exosomes loaded with MHC class I and II molecules with important immunomodulatory properties. In the current study, exosomes were isolated from salmon head kidney leukocytes stimulated with CpG oligonucleotides and yeast beta-glucan (BG) and their protein composition was studied using Western blotting and LC-MS/MS analysis. Major exosome markers were detected including flotillin, CD63, CD81, CD9 and MHC-I and II molecules. Ontology analysis indicated that the exosome samples were enriched in ribosomal, membrane-associated as well as nuclear proteins the last of which reflects the abundance of histones within exosome preparations. Interestingly, label-free MS quantitation suggests that the exosomes might be derived from cross-presenting compartments as they are also enriched in proteins involved in trafficking and loading of MHC-I with exogenous antigens