Development of a combined GMMA and glycoconjugate iNTS-TCV trivalent vaccine against invasive salmonellosis

In many parts of sub-Saharan Africa, a silent and deadly threat lurks in the water and food that children consume. It comes from a family of bacteria known as Salmonella. While some types of this bacteria cause the familiar stomach upset known as food poisoning, other types can invade the bloodstream, leading to a severe condition called invasive non-typhoidal salmonellosis. This disease strikes hardest at young children, often killing them before they can be properly diagnosed or treated, especially in regions where access to effective antibiotics is limited or where the bacteria have become resistant to common drugs. At the same time, a related but distinct version of the bacteria, which causes typhoid fever, continues to sicken millions of people across the globe. For decades, scientists have worked to create vaccines against these threats. While a vaccine for typhoid fever exists, there has been no licensed vaccine to stop the invasive bloodstream infections caused by the other Salmonella types. The challenge has been to find a way to protect against multiple dangerous strains at once without creating a vaccine that is too complex to make or too harsh for the human body to handle.

A team of researchers at the GSK Vaccines Institute for Global Health has taken a significant step toward solving this problem by developing a new, three-part vaccine candidate. Their work focuses on combining two different vaccine technologies into a single shot. The first technology involves tiny bubbles taken from the outer shell of the bacteria, which act as a natural delivery system for the immune system. The second is a proven method for creating a vaccine against typhoid fever, where a sugar coating from the bacteria is attached to a harmless protein to help the body recognize it. The goal was to see if these two very different approaches could work together in one mixture to protect against the two most common invasive Salmonella strains and the typhoid bacteria simultaneously.

The researchers began by creating the bacterial components needed for the vaccine. They engineered strains of Salmonella Typhimurium and Salmonella Enteritidis, the two types responsible for the vast majority of invasive infections, to release their outer membrane bubbles naturally. These bubbles, known as GMMA, carry the specific markers that the immune system needs to learn to fight the bacteria. In a move to make future production easier and more adaptable for developing nations, the team tested a new method of growing these bacteria in single-use bioreactors, which are flexible containers that do not require the heavy cleaning and validation of traditional steel tanks. They successfully produced large batches of these bubbles, confirming that the new manufacturing method worked just as well as the old one. They then mixed these bubbles with the typhoid vaccine component, which consists of a sugar molecule from the typhoid bacteria attached to a carrier protein. The mixture was carefully formulated so that the bacterial bubbles stuck to a mineral gel, while the typhoid component remained free-floating in the liquid, ensuring all parts of the vaccine remained stable and effective.

When the team tested this new combination in mice, they discovered something unexpected and promising. They had worried that mixing these different vaccine parts might cause them to interfere with each other, making the final product less effective than if the parts were given separately. Instead, the results showed the opposite. The mice that received the combined vaccine produced strong antibodies against all three bacterial targets. More importantly, the immune response to the typhoid component was actually stronger in the combined vaccine than when the typhoid vaccine was given alone. The mice produced higher levels of antibodies, and those antibodies were better at killing the bacteria in laboratory tests. This suggested that the bacterial bubbles were not just sitting passively in the mix; they were actively helping to boost the body's reaction to the typhoid part of the vaccine.

To understand why this boost happened, the researchers looked closely at how the immune cells reacted to the vaccine. They found that the bacterial bubbles triggered a strong inflammatory signal, releasing a specific set of chemical messengers that recruit and activate immune cells. This environment appeared to encourage the immune system to produce a more diverse and powerful type of antibody against the typhoid bacteria, one that is better suited for long-term protection. In contrast, the typhoid vaccine given on its own did not trigger this same level of activity. The study also confirmed that the new vaccine was safe. When tested in rabbits, the vaccine caused only mild, temporary reactions at the injection site, such as swelling, which is a normal sign that the immune system is working. The animals did not show signs of serious illness or systemic toxicity, and their body temperatures returned to normal quickly after the injections.

The researchers also checked the stability of the vaccine over time. They stored samples at standard refrigeration temperatures and tested them after eighteen months. The vaccine remained potent, meaning it still worked just as well as when it was first made, and its physical properties did not change. This stability is crucial for a vaccine intended for use in regions where the cold chain for storage might be difficult to maintain. The team successfully produced the vaccine using both small-scale laboratory methods and larger-scale manufacturing processes, proving that the recipe could be scaled up to make enough doses for clinical trials.

This work demonstrates that it is possible to merge two distinct vaccine technologies into a single, effective product. The combination did not just work; it enhanced the protection against typhoid fever while simultaneously providing protection against the invasive Salmonella strains that currently have no vaccine. The findings suggest that the bacterial bubbles act as a natural helper, or adjuvant, that strengthens the immune response without needing additional chemical boosters. While these results are from animal studies, they provide a strong foundation for moving forward. The vaccine candidate has already advanced to human trials to test its safety and effectiveness in people, offering a potential new tool to reduce the burden of deadly bacterial infections in the world's most vulnerable populations.