Cd9 and cd63 are used as biomarkers in evs but they can change from cell line to cell line
The amount of CD9 and CD63 (tetraspanins) could change from cell line to cell line[@hartjes2020EVQuant; high-throughput quantification and characterization of extracellular vesicle (sub)populations]. TR-FIA is a bulk measurement that does not allow to distinguish sub-populations, therefore single-particle measurements are an advantage. This is what quantifying the concentration of EVs in urine or plasma through confocal microscopy allows at a single-particle level.
More epitopes on the surface of an EV will increase the TR-FIA signal, but more epitopes at a single-particle level will not result in higher EV count. - [@hartjes2020EVQuant; high-throughput quantification and characterization of extracellular vesicle (sub)populations]
Importantly, although CD9 and CD63 are often used as general EV markers, they are only detected on a fraction of EV's.
Thorough analysis of the relative abundance of CD9, CD63, and CD81 in different populations of EV's show the very high variation[@kugeratski2021Quantitative proteomics identifies the core proteome of exosomes with syntenin-1 as the highest abundant protein and a putative universal biomarker]
See also: Syntenin-1 is the most abundant protein in EVs.
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