Characterisation of Mucosal Secretory IgA Responses to Salmonella Typhimurium Infection

Summary 

Non-typhoidal Salmonellae are a group of bacteria that typically cause diarrhoeal disease (dNTS). This group of bacteria are particularly harmful to individuals who are HIV-positive, malaria-infected, or malnourished in sub-Saharan Africa. In these individuals, the bacteria can spread throughout the body and cause a more severe illness known as invasive non-typhoidal Salmonella (iNTS). There is a growing interest in developing vaccines to prevent iNTS, particularly with the rise of antibiotic-resistant infections, and several vaccines are in early-stage development.

To develop effective vaccines, we need to better understand the way the immune system protects against NTS infection. Secretory IgA (sIgA) is a type of common mucosal antibody known to provide protection against some intestinal (gut) bacteria. Compared with other bacteria, the role of sIgA in preventing NTS infection in humans has been relatively understudied. 

Our research project aims to develop and validate a way to measure sIgA targeted at Salmonella Typhimurium (STm) in stool and saliva samples. We aim to do this by standardised enzyme-linked immunosorbent assay (ELISA) to measure sIgA against STm. We then aim to use this to measure the strength, durability and targets of the sIgA response in healthy volunteers challenged with STm and individuals exposed to STm in a high-burden setting. These will be linked with antibody measures in blood to see how closely they are related.  We propose to use these methods in future vaccine studies to better understand immunity to STm and NTS.

Project Outcomes

Non-typhoidal Salmonella (NTS) bacteria often cause diarrheal diseases but can lead to a severe condition called invasive non-typhoidal Salmonella (iNTS). This condition disproportionately affects vulnerable populations, such as individuals with HIV, malaria, or malnutrition, particularly in sub-Saharan Africa. With rising antibiotic resistance, Salmonella has been identified as a priority pathogen by the WHO. There is currently significant interest and momentum in developing vaccines to protect against this infection. This study focuses on a type of antibody in the gut called secretory IgA (sIgA), which may help prevent the Salmonella bacteria from crossing the gut barrier.

Our research project analysed sIgA levels in saliva and stool samples from a human challenge study in the UK, where healthy volunteers were deliberately exposed to two strains of Salmonella Typhimurium (STm): 4/74 and D23580. We developed a method to measure sIgA using an in-house ELISA test, which allowed us to measured IgA that targeted specific molecules on the surface of the bacteria. We went on to measure levels before infection and at several timepoints afterward (14, 28, and 90 days). This allowed us to measure how the immune response develops over time. We measured these antibodies in salvia and stool samples, and compared how well these correlated with one another.Our key findings were:• sIgA Levels over time: Salmonella specific IgA levels in stool varied significantly between individuals. The levels started off low and generally peaked 2–4 weeks after infection before returning to baseline by three months. IgA levels in saliva followed a similar pattern, with levels increasing significantly between 14 and 28 days and returning to baseline by three months.• Strain-specific responses: One of the Salmonella strains, D23580, triggered lower IgA responses to a key bacterial protein, OmpD, compared with the 4/74 strain. This may reflect differences in how well these strains survive in the gut.• Impact of pre-existing immunity: Participants with higher baseline IgA levels showed smaller increases in antibody levels post-infection, suggesting that prior exposure to Salmonella – or related bacteria – influences the immune response.• Antibodies in saliva correlate with antibodies in blood: IgA in saliva – but not stool – correlated strongly with Salmonella antibody levels in blood.

Overall, the study highlights the variability in immune responses among individuals and emphasizes the importance of factors such as the infecting Salmonella strain and pre-existing immunity. Our ongoing studies will help us to determine whether the presence of strong antibody response correlates with protection from disease. We are currently measuring the same antibodies in patients from Malawi with a history of iNTS infection. This will help us to better understand the relationship between natural infection in a high-burden country and healthy UK volunteers. Together, these findings will help us to understand how natural immunity develops, which in turn could inform the design of vaccines for Salmonella.