BACKGROUND: Exosomes are released from multiple cell types, contain protein and 
RNA species, and have been exploited as a novel reservoir for disease biomarker 
discovery. They can transfer information between cells and may cause pathology, 
for example, a role for exosomes has been proposed in the pathophysiology of 
Alzheimer's disease. Although studied in several biofluids, exosomes have not 
been extensively studied in the cerebrospinal fluid (CSF) from humans. The 
objective of this study was to determine: 1) whether human CSF contains exosomes 
and 2) the variability in exosomal protein content across individuals.
METHODS: CSF was collected from 5 study participants undergoing 
thoraco-abdominal aortic aneurysm repair (around 200 - 500 ml per participant) 
and low-density membrane vesicles were concentrated by ultracentrifugation. The 
presence of exosomes was determined by western blot for marker proteins, 
isopycnic centrifugation on a sucrose step gradient and transmission electron 
microscopy with immuno-labelling. Whole protein profiling was performed using 
Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR).
RESULTS: Flotillin 1 and tumor susceptibility gene 101 (TSG101), two exosomal 
marker proteins, were identified in the ultracentrifugation pellet using western 
blot. These markers localized to a density consistent with exosomes following 
isopycnic centrifugation. Transmission electron microscopy visualized structures 
consistent with exosomes in size and appearance that labelled positive for 
flotillin 1. Therefore, the pellet that resulted from ultracentrifugation of 
human CSF contained exosomes. FT-ICR profiling of this pellet was performed and 
84-161 ions were detected per study participant. Around one third of these ions 
were only present in a single study participant and one third were detected in 
all five. With regard to ion quantity, the median coefficient of variation was 
81% for ions detected in two or more samples.
CONCLUSIONS: Exosomes were identified in human CSF and their proteome is a 
potential new reservoir for biomarker discovery in neurological disorders such 
as Alzheimer's disease. However, techniques used to concentrate exosomes from 
CSF need refinement to reduce variability. In this study we used relatively 
large starting volumes of human CSF, future studies will focus on exosome 
isolation from smaller 'real life' clinical samples; a key challenge in the 
development of exosomes as translational tools.