When EV Dose Is Not What It Seems: Quantitative Mismatch Between Particle Number- and Protein-Based Dosing

Extracellular vesicles (EVs) are widely studied as mediators of intercellular communication, yet their quantitative characterization remains inconsistent. In this study, we investigated how vesicle release rate, cellular proliferation, and tumour origin jointly shape EV-associated protein output. Using a panel of 19 human, murine, canine and primate cancer- and non-cancer cell lines cultured under standardized conditions, we isolated small extracellular vesicle-enriched fractions and quantified particle number, EV-associated protein content, cell number and proliferation rate and integrated these parameters into three complementary, readily computable metrics: protein content per particle (PP; pg/particle), Vesicle Release Characteristics (VRC; particles/cell/24 h), and Protein Release Characteristics (PRC; pg/cell/24 h). Cancer cells showed significantly higher estimated EV release per cell than non-cancer cells (∼3.6-fold higher VRC), while their EV-enriched isolates exhibited a substantially lower protein-to-particle ratio (∼4.3-fold lower PP). Lower PP was independently associated with both cancer origin and proliferation rate and was not explained by differences in vesicle size. Despite the elevated vesicle output, cancer-derived cells exported less total vesicular protein per cell and time unit (∼2.1-fold lower PRC), and the relationship between vesicle output and total protein output itself differed between non-cancer and cancer cells. These findings demonstrate that particle number and protein content describe distinct and non-equivalent aspects of EV output. We therefore recommend integrated, multidimensional reporting (VRC, PRC, and the protein/particle descriptor PP - or vesicles per pg protein) to enable biologically meaningful comparison and dosing of EV preparations.