Immunoepidemiological studies from Africa support the concept that antibody acquisition with age likely provides protection against invasive non-typhoidal Salmonella disease. Induction of antibodies to key antigenic targets such as the O antigen of lipopolysaccharide has been a significant target for vaccine development. Whilst pre-clinical studies in animals have shown promise for such vaccines, in humans we a lack a true correlate of protection. It would be valuable to better understand antibody induction to non-typhoidal Salmonella antigens to help develop an effective vaccine for all individuals.
Most studies of invasive non-typhoidal Salmonella disease tend to revolve around young children who, along with HIV and malaria-infected individuals are amongst the most susceptible to infection. Measuring antibody titres in such individuals is usually performed using serum samples. Particularly in young children, obtaining serum through venepuncture can be stressful to both child and parent. As a result, patient retention or recruitment of healthy subjects can be difficult as parental consent is often withdrawn once their child is healthy or is not given in the first place.
We propose to explore the use of non-invasive methods of measuring antibody responses to non-typhoidal Salmonella to increase patient acceptability and seek to identify comparable signatures to serum-derived antibody responses. Oral-fluid antibody has been successfully measured as an alternative to serum for a number of bacterial and viral antigens and the two responses have often correlated strongly. This alternative method may also provide additional data to support new correlates of protection, susceptibility or carriage, given Salmonella is an orally-ingested pathogen and this is a measure of a relevant mucosal response. This study will compare antibody responses across different age groups in both Kenya and the UK.
This study sought to prove that oral fluid can be used as a non-invasive surrogate for serum in measuring antibody responses to non-typhoidal Salmonella (NTS). We have clearly shown that for IgG the responses in oral fluid strongly correlates with that in serum. Though the concentration of IgG in oral fluid is lower than in serum our assays gave sufficient sensitivity to detect IgG in most adults who were IgG serum positive. Infants up to 6 months show low levels of serum IgG which is assumed to be mostly derived from their mother. The observation that most of these infants are IgG negative in oral fluid supports this observation.
Though IgG correlated between oral fluid and serum, IgA did not. Whilst this means it is not a useful surrogate our observations suggest that measuring IgA in oral fluid is a relevant measurement of a different immune response to NTS than that seen in serum. In infants we observed very high IgA responses in contrast to low to often negative responses in serum. Low serum responses were expected as infants to not derive IgA from their mother and so any antibody would likely be self-derived following exposure to NTS. The high IgA in oral fluid in infants is likely derived from breast milk which has 10-100 times the concentration of total IgA of serum and is known to remain in the mouth after feeding. In adults we also saw no correlation in oral fluid and serum. In this population the explanation for this is that IgA also originates from alternative components of saliva, those that are linked to the serum source and others that are not.
Looking at the overall picture, by measuring both oral fluid and serum responses we gain a much bigger snapshot of the immune response to NTS at any given time. In combination particularly for infants in the first year of life we get a better idea of the age of first exposure from comparing the age at which IgG is first observed in oral fluid and IgA is observed in serum as neither is contradicted by antibody from the infant’s mother. In contrast serum IgG and oral fluid IgA can give us potential insights into the immune response of the mother which may be critical for understanding prevention of NTS disease in infants in their first year of life.
In studies of infants, children and adults oral fluid has one clear advantage over serum, the fact it is a non-invasive procedure. This improves acceptability in a population and decreases stress on participants. We believe it has also lead to quicker recruitment on the basis it can be done at participant’s homes rather than clinic and can be done by any trained individual rather than a designated phlebotomist. In future studies we believe that oral fluid collection can be a useful tool in the design of clinical studies’ replacing serum collection on some occasions or complementing it on others, depending on the questions being asked.