As we age, the immune system starts overreacting to ordinary triggers, like an orchestra where some instruments play louder than they should. Rapamycin is a drug that works like a tuner: it doesn't change the orchestra's lineup, but it quiets the instruments that are too loud. Scientists tested this in old mice that were fed a small dose of rapamycin for five months.
The main immune cells in the blood and organs stayed at their previous numbers. Only one rare group changed: gamma-delta T cells, which with age begin producing more of the inflammatory signal interleukin-17. Rapamycin brought this measure back to the level seen in young animals. When the mice were given a bacterial component that triggers inflammation, those receiving rapamycin responded less strongly: fewer inflammatory signals in the blood and less activity of inflammation genes in the brain.
Previously it was unclear whether such treatment might weaken immunity too much. Now it's clear that rapamycin doesn't touch the quiet instruments, it only quiets the ones that sound too loud when there's alarm. This supports the hope that rapamycin can slow aging safely.
Rapamycin was discovered in a soil sample from Easter Island and named after the island's local name, Rapa Nui. It was first used as an antifungal, and its immunosuppressive properties were discovered later.
The idea of a drug that slows aging without breaking the body echoes the motif of a 'soft' geroprotector from Bruce Sterling's novel Schismatrix, where life extension is achieved through fine-tuning biology rather than blunt intervention.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.