Phage-specific immune responses impair efficacy of phage therapy targeting VRE in a murine model

Antimicrobial resistant bacteria are an emerging and prevalent global threat with an urgent need for alternative therapies. Bacteriophage (phage) therapy is a promising approach to address these infections that has gained renewed interest in recent years. Despite this, questions remain regarding the therapeutic use of phages, including the impact that the immune response may have on phage therapy, particularly when this treatment is administered long-term or when reusing a specific phage treatment in a single individual. To investigate this, we developed a mouse model to assess phage treatment using a cocktail of five phages from the Myoviridae and Siphoviridae families that target vancomycin-resistant enterococcus (VRE) gut colonization. Phage cocktail treatment significantly reduced the intestinal bacterial burden of VRE in mice. We characterized innate and adaptive immune responses elicited against the phage cocktail after one and multiple exposures, respectively. While minimal innate responses were observed after phage administration, two courses of phage therapy induced phage-specific neutralizing antibodies and appeared to accelerate phage clearance from tissues. Interestingly, the myophages in our cocktail induced a more robust neutralizing antibody response than the siphophages. Proteins targeted by phage-specific antibodies were also identified from each phage family of the cocktail. Importantly, we show that this anti-phage immunity reduced the effectiveness of the phage cocktail in our murine model, leading to significantly higher fecal bacterial burden following repeat treatment. Collectively, this study shows the immune system has the potential to impede effectiveness of phage therapy and that the phage-specific immune responses can differ significantly between phages. These findings can help inform decisions about inclusion of specific phages in cocktails for future studies.