Small antibodies block toxin injection by capping Pseudomonas aeruginosa's needle

Kyoto Prefectural University of Medicine and Institute of Science Tokyo develop next-generation mRNA therapy with potential against multidrug-resistant bacteria

A joint research group including Assistant Professor Mao Kinoshita of the Graduate School of Medical Science at Kyoto Prefectural University of Medicine, Hospital Director Teiji Sawa of the University Hospital at the same university, and Professor Satoshi Uchida of the Medical Research Laboratory, Institute of Integrated Research at Institute of Science Tokyo, has developed a next-generation antibody therapy that uses messenger RNA (mRNA) to produce antibodies against Pseudomonas aeruginosa. The team designed mRNA to produce small antibodies that cap "T3SS," the needle-like structure of P. aeruginosa. By administering this antibody, they confirmed excellent therapeutic effects in immunocompromised mice inoculated with patient-derived multidrug-resistant P. aeruginosa. This is expected to be an effective measure against multidrug-resistant P. aeruginosa. The results were published in Nature Communications on April 9.

Multidrug-resistant (AMR) bacteria, which are resistant to many antibiotics, are increasing worldwide. In 2019, direct deaths reached 1.27 million, and it is estimated that by 2050, this figure will exceed the number of cancer deaths. Six types of bacteria, including P. aeruginosa, account for 80% of these deaths.

P. aeruginosa typically does not infect healthy individuals but targets those with weakened immune systems, causing pneumonia and urinary tract infections. It is known to inject toxins into cells via its needle-like Type III Secretion System (T3SS), leading to cell destruction.

The research group investigated a treatment method that involves designing mRNA for an antibody targeting the T3SS, allowing the body to produce the antibody itself. This antibody physically caps the needle to inhibit toxin injection.

First, they designed an mRNA encoding "scFv-m166" (a small antibody), which consists only of the antigen-binding site at the tip of a Y-shaped IgG antibody. In addition, mRNAs for antibodies containing Fc regions other than the tip and antibodies with only antigen binding sites unrelated to P. aeruginosa (control) were designed. Each mRNA was experimentally encapsulated in lipid nanoparticles and intravenously administered to mice.

AlphaFold 3 (a protein structure prediction model) predicted that the small antibody would bind to the tip of the P. aeruginosa needle. It suggested that the binding of two antibody molecules could powerfully inhibit toxin injection.

In an experiment where mRNA was administered before inoculation with P. aeruginosa (prevention model), the control group (untreated) was completely wiped out within one to two days. In contrast, approximately 80% of the group treated with the small antibody survived, showing high therapeutic efficacy. The group treated with the Fc-region antibody showed the next highest survival rate.

In an experiment where mRNA was administered after inoculation (treatment model), the control group was similarly wiped out. The survival rate for the small antibody group remained high at approximately 80%, while the survival rate for the Fc-region antibody group was approximately 50%.

In clinical settings, the infection of immunocompromised patients with multidrug-resistant P. aeruginosa is a problem. The researchers conducted an experiment to mimic this situation. When immunocompromised mice were inoculated with patient-derived multidrug-resistant P. aeruginosa, the survival rate was higher for those treated with protein antibodies (not mRNA) than for those treated with an existing drug (colistin). Furthermore, when the small antibody and the Fc-region antibody were administered, both showed higher survival rates than the protein antibody, with the small antibody showing the highest survival rate.

Consequently, the team conducted experiments using existing drugs in combination with small antibodies and with the Fc region, respectively. The combination of the small antibody and the existing drug achieved a 100% survival rate. The survival rate for the combination of the Fc-region antibody and the existing drug remained at approximately 50%. The researchers confirmed that the administered "scFv-m166" caused antibody production in the liver and, due to its miniaturized size, reached deep into the lungs, the site of infection, more effectively than the antibody with the Fc region.

The low delivery rate of standard-sized antibodies to the lungs has been a long-standing challenge. When the small antibody mRNA is administered intravenously using lipid nanoparticles, blood levels rise within a few hours, and antibody production continues in the liver for several days. By miniaturizing the antibody to only the binding site, inflammatory reactions caused by immune activation were reduced, and it appears that the P. aeruginosa was eliminated by existing immunity.

While antibody drugs for infectious diseases are considered effective, they are often deemed impractical due to high costs. In contrast, this mRNA-based therapy is expected to reduce costs and be widely applicable.

Sawa stated: "Compared to conventional protein antibody therapy, this method using mRNA holds the potential to solve the challenges of cost and speed. We believe the technology we have developed is a new medical technology that could evolve into a new immunotherapy for infectious diseases, including those caused by bacteria and viruses."

Journal Information
Publication: Nature Communications
Title: Fc-free single-chain antibody mRNA therapy for airway infection of multidrug-resistant Pseudomonas aeruginosa
DOI: 10.1038/s41467-026-71040-8