Immuno-phage synergy driven by fitness costs: turning bacterial phage resistance into therapeutic gain through evolutionary traps.

The host immune system typically recognizes administered bacteriophages as foreign entities, potentially acting as a rate-limiting factor in phage therapy via neutralizing antibody production and immune-mediated clearance. Furthermore, the emergence of bacterial phage resistance during treatment remains a major hurdle for its clinical application. Although employing phage cocktails is the standard strategy to suppress phage resistance, clinical trials occasionally report the emergence of phage-resistant variants despite cocktail treatments. Therefore, a more proactive strategic approach that goes beyond merely preventing resistance and instead actively steers the trajectory of bacterial evolution must be applied. In this context, immuno-phage synergy, in which phages and host immunity act in an orchestrated manner rather than in competition, has recently gained significant traction. To evade phage infection, pathogenic bacteria often mutate or shed crucial surface structures (e.g., capsules, lipopolysaccharides, and pili), thereby incurring fitness costs as evolutionary trade-offs. This mini-review summarizes recent insights into how these trade-offs drastically enhance host immune clearance, including increased susceptibility to neutrophil-mediated opsonophagocytosis, complement-dependent killing, and host chemical barriers. By understanding and exploiting the dynamics of phage-induced selective pressure and immunological trade-offs, the evolutionary arms race could be altered to intentionally drive pathogens into evolutionary traps. Concurrently, we discuss emerging clinical challenges, such as unexpected immune evasion (trade-up) resulting from certain mutations, and the critical disconnect between in vitro resistance assays and in vivo evolution. Ultimately, we highlight how the formidable barrier of phage resistance can be converted into a therapeutic gain, thereby offering a robust framework for sustainable phage therapy.