PGE2-CXCL9 Turing System Explains the Spatial Selectivity of Aspirin's Anti-Tumor Effect in CRC

Two seemingly unrelated fields collide here. The first is a 70-year-old idea from mathematician Alan Turing — yes, the codebreaker — who showed mathematically that two chemicals diffusing at different speeds can spontaneously create repeating patterns, like stripes on a zebra or spots on a leopard. The second is a newer discovery in cancer biology: inside colorectal tumors, immune cells called CD8 T-cells (the ones that kill cancer) don't spread evenly. They cluster in some spots and are completely absent from others — so-called 'immune deserts' — and this patchy distribution predicts how well patients do. This hypothesis proposes that those immune patterns inside tumors are actually a Turing pattern — a biological stripe or spot system generated by the push-pull between two molecules. The activator is CXCL9, a chemical that attracts immune cells. The inhibitor is PGE2, an inflammatory molecule produced by an enzyme called COX-2 that diffuses much farther and faster, suppressing immune activity over a wide area. Together, they could generate the repetitive waves of immune presence and absence seen in real tumors. Aspirin and related drugs (like ibuprofen) block COX-2, cutting off PGE2 — and the hypothesis predicts this would collapse the Turing pattern, flooding tumors with immune cells more uniformly and making them more vulnerable. This is genuinely elegant because it reframes a well-known clinical observation — aspirin takers get less colorectal cancer — through a completely new lens. Instead of a vague 'anti-inflammatory' effect, it offers a precise geometric mechanism: aspirin disrupts the spatial mathematics that tumors use to hide from the immune system.

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