Natural and vaccine mediated correlates of protection against enteric fever

Enteric fever is a febrile illness caused by systemic infection with Salmonella Typhi or Paratyphi. It causes approximately 136,000 deaths a year. Enteric fever has largely been eradicated in developed countries but there remains a high incidence in lower-middle income countries. Emergence of antimicrobial resistant strains poses a significant health problem. Vaccine intervention can help reduce the burden of disease and is recommended in regions of enteric fever endemicity. However, current enteric fever vaccines are not effective against paratyphoid fever and recent data show that in some regions the incidence and proportion of disease attributable to S. Paratyphi infection is increasing. Comprehensive control of enteric fever requires effective vaccines against both etiological agents. Currently there are no licensed vaccines against paratyphoid fever. Licensure of novel vaccines is partially hindered by our limited understanding of protective immunity and lack of immunological correlates of protection. Development and application of immunoassays to characterise the immune response to S. Paratyphi infection or vaccination can further our understanding of protective immunity and help support licensure of new vaccines to help control enteric fever. I have developed multiple robust immunoassays to reliably measure the functional humoral immune responses against S. Typhi and S. Paratyphi A. Using these standardised methods I have measured serum bactericidal activity (SBA), antibody dependent monocyte phagocytosis (ADMP), antibody dependent neutrophil phagocytosis (ADNP) responses to enteric fever vaccines and experimental challenge in a series of clinical trials. I measured pre-existing, and post-challenge functional immunity in two oral S. Paratyphi A experimental challenge studies. Although there was variability levels of pre-exisiting immunity at baseline, none of the features significantly correlated with protection from acute paratyphoid fever after challenge. Ninety days after challenge there were significant increases in SBA, ADNP, ADMP. For the SBA, challenge induced increases were dependant on systemic infection whereas increases in ADMP and ADNP occurred regardless of systemic infection. A subset of S. Paratyphi A challenged individuals undertook a second challenge. Significant waning of functional immunity occurred by 17 months, at the point of rechallenge and no features measured at the rechallenge baseline correlated with diagnostic outcome of the second challenge. In another study, healthy volunteers received either two doses two weeks apart of oral paratyphoid vaccine, CVD 1902, or sodium bicarbonate placebo control approximately one month before oral S. Paratyphi A challenge. SBA and ADMP titres were significantly increased 42 days after receiving vaccine or placebo. However, ADNP titres were significantly elevated after vaccine/placebo in those who were protected from infection. A similar trend was observed for ADMP, but it did not reach statistical significance. Analysis of this data while the study is still blinded may impact these findings and further analysis should be conducted once the study is unblinded. Integrative analysis approaches were unable to identify any immune signatures that correlated with protection in any of the three challenge studies. Functional immunity against S. Paratyphi A was also measured in participants in a phase 1 clinical trial evaluating the safety and immunogenicity of a novel S. Typhi – Paratyphi A bivalent conjugate vaccine, Sii-PTCV. A single dose of Sii-PTCV elicited significant increases in SBA, ADMP, ADNP suggesting polyfunctional immune stimulation. In conclusion, I have developed robust immunoassays that can reliably measure antibody effector functions. In paratyphoid naïve adults or adults who have experience a single exposure to S. Paratyphi A there are no baseline immune functions that correlate with protection in an experimental challenge model. I have used these assays to demonstrate that functional immunity can be stimulated by oral S. Paratyphi A challenge, a single dose of Sii-PTCV, and potentially by CVD1902. However further work is required to determine whether vaccine induced responses correlate with protection from paratyphoid fever. While no correlates of protection could be identified in these studies, this work adds to our growing understanding of host immune responses to typhoidal Salmonella. Furthermore, this work serves a ‘proof of principal’ demonstrating that these assays can be used, in addition to other assays, to assess immunogenicity and correlates of protection against paratyphoid fever future studies.