β-endorphin primes Natural Killer cells and NK-derived Extracellular Vesicle to enhance anti-tumor cytotoxicity

Psychoneuroimmunology suggests that positive physiological states, including laughter, could affect anti-tumor immunity, but the underlying mechanisms remain unclear. Here, we investigated whether β-endorphin (BE), an endogenous opioid peptide associated with positive physiological stimuli, modulates Natural Killer (NK) cell cytotoxicity and the anti-tumor activity of NK-derived extracellular vesicles (EVs). Using NK-92 cells, we assessed cytotoxicity against JIMT1 breast cancer cells, CD107a mobilization, cytotoxic activity of conditioned medium (CM), and EV yield, cargo, and function. BE enhanced NK-92-mediated killing of JIMT1 cells without increasing CD107a mobilization, suggesting that improved cytotoxicity was not driven by classical degranulation. Consistently, CM from BE-treated NK cells retained contact-independent cytotoxicity. NK-EVs were enriched in granzyme B and perforin following BE treatment exhibiting enhanced cytotoxicity against JIMT1 and BW tumor cells. BE also increased the cytotoxic activity of primary human NK cells, and BE-conditioned NK-EVs primed naïve NK-92 cells for enhanced tumor killing. These findings indicate that BE enhances NK anti-tumor immunity by remodeling the cytotoxic secretome and generating EVs that act as both direct cytotoxic effectors and mediators of NK cell priming.