PDCD1 and neoplasm: Even for well-studied axes, translation can bifurcate: butyrate may potentiate PD-1 therapy yet limit CTLA-4 efficacy; [89–92] TMAO shows tumor-promoting programs in some contexts but improves ICB response in others [34, 174–178], and can be depleted by broad-spectrum antibiotics that also impair CAR-T potency [179].