A new mouse study published in Nature reports that an experimental mRNA treatment can temporarily rejuvenate aging T cells, key immune cells that decline in number and function as the body ages. Researchers identified three proteins that diminish in older mice and delivered mRNA instructions for those proteins using lipid nanoparticles, which accumulated in the liver and exposed circulating T cells to the therapy. Treated mice produced more T cells and showed stronger responses to vaccines and cancer immunotherapy than untreated mice. However, the benefits faded quickly once injections stopped, reflecting the short-lived nature of mRNA, and the authors emphasize that repeated dosing would be required and that long-term safety in older individuals still needs careful study.