Antibodies to phage fiber and nozzle proteins block Acinetobacter baumannii phage therapy by preventing infection and accelerating clearance

# What the study did Researchers used AbP20, a podovirus active against Acinetobacter baumannii, to test how antibodies raised against individual phage structural proteins affect phage therapy in mice. They induced phage-specific antibody responses by administering AbP20 once daily for seven days, then examined which antibody specificities impaired therapeutic efficacy and why.

# Main findings The study found that antibodies induced by two AbP20 proteins — the fiber and the nozzle — are the dominant causes of phage therapy failure. Antibodies targeting other structural proteins tested (portal, capsid, adaptor) did not drive the same therapeutic loss. The two problematic antibody types act through separate but complementary mechanisms:

• Nozzle-specific antibodies block genomic injection into the host bacterium, preventing phage replication even if adsorption occurs.

Both antibody classes also cause infected phage particles to form large aggregates and markedly increase macrophage-mediated phagocytosis. This phagocytosis enhancement occurs independently of Fc receptor engagement, indicating a different pathway for immune clearance of opsonized phage.

# What the evolved phage variant showed The authors tested an evolved AbP20 variant with improved capacity to escape neutralizing antibodies. That variant partially restored therapeutic activity in the presence of neutralizing antibodies, but it did not prevent accelerated phagocytosis and immune clearance driven by those antibodies. In other words, neutralization escape reduced the direct blocking of infection but did not fully overcome antibody-driven removal of phage by the host immune system.

# Why this matters

# Practical implications for therapy development and use

• Repeated or prolonged administration of the same phage may induce neutralizing antibodies that target critical structural proteins and reduce efficacy.
• Selecting or engineering phages to alter fiber and nozzle antigenicity could reduce neutralization, but escaping neutralization alone may not prevent antibody-accelerated clearance.
• Combination strategies should consider both neutralization (blocking infection) and antibody-driven immune removal when assessing phage persistence and dosing schedules.

# Bottom line Antibodies to AbP20 fiber and nozzle proteins disrupt phage therapy through two complementary mechanisms: direct neutralization of infection steps and promotion of phage aggregation with accelerated macrophage clearance. Neutralization-escape variants can mitigate one mechanism but not the antibody-driven immune clearance, indicating both mechanisms need attention when developing phage therapeutics.