Homologous and heterologous re-challenge with Salmonella Typhi and Salmonella Paratyphi A in a randomised controlled human infection model

Abstract

Enteric fever is a systemic infection caused by Salmonella Typhi or Paratyphi A. In many endemic areas, these serovars co-circulate and can cause multiple infection-episodes in childhood. Prior exposure is thought to confer partial, but incomplete, protection against subsequent attacks of enteric fever. Empirical data to support this hypothesis are limited, and there are few studies describing the occurrence of heterologous-protection between these closely related serovars. We performed a challenge-re-challenge study using a controlled human infection model (CHIM) to investigate the extent of infection-derived immunity to Salmonella Typhi or Paratyphi A infection. We recruited healthy volunteers into two groups: naïve volunteers with no prior exposure to Salmonella Typhi/Paratyphi A and volunteers previously-exposed to Salmonella Typhi or Paratyphi A in earlier CHIM studies. Within each group, participants were randomised 1:1 to oral challenge with either Salmonella Typhi (104 CFU) or Paratyphi A (103 CFU). The primary objective was to compare the attack rate between naïve and previously challenged individuals, defined as the proportion of participants per group meeting the diagnostic criteria of temperature of ≥38°C persisting for ≥12 hours and/or S. Typhi/Paratyphi bacteraemia up to day 14 post challenge. The attack-rate in participants who underwent homologous re-challenge with Salmonella Typhi was reduced compared with challenged naïve controls, although this reduction was not statistically significant (12/27[44%] vs. 12/19[63%]; Relative risk 0.70; 95% CI 0.41-1.21; p = 0.24). Homologous re-challenge with Salmonella Paratyphi A also resulted in a lower attack-rate than was seen in challenged naïve controls (3/12[25%] vs. 10/18[56%]; RR0.45; 95% CI 0.16-1.30; p = 0.14). Evidence of protection was supported by a post hoc analysis in which previous exposure was associated with an approximately 36% and 57% reduced risk of typhoid or paratyphoid disease respectively on re-challenge. Individuals who did not develop enteric fever on primary exposure were significantly more likely to be protected on re-challenge, compared with individuals who developed disease on primary exposure. Heterologous re-challenge with Salmonella Typhi or Salmonella Paratyphi A was not associated with a reduced attack rate following challenge. Within the context of the model, prior exposure was not associated with reduced disease severity, altered microbiological profile or boosting of humoral immune responses. We conclude that prior Salmonella Typhi and Paratyphi A exposure may confer partial but incomplete protection against subsequent infection, but with a comparable clinical and microbiological phenotype. There is no demonstrable cross-protection between these serovars, consistent with the co-circulation of Salmonella Typhi and Paratyphi A. Collectively, these data are consistent with surveillance and modelling studies that indicate multiple infections can occur in high transmission settings, supporting the need for vaccines to reduce the burden of disease in childhood and achieve disease control. Trial registration NCT02192008; clinicaltrials.gov.

Fig 1. Time to diagnosis after challenge with a) Salmonella Typhi and b) Salmonella Paratyphi A.

Kaplan-Meier survival cumulative incidence of participants meeting the composite diagnostic endpoint, measured from challenge agent ingestion to development of first fever ≥38°C or first positive blood culture sampling. Non-diagnosed participants censored at day 14 hours. P = log-rank test between three groups. ST = S. Typhi naïve challenge. ST-ST = Homologous Re-Challenge with S. Typhi. SPT = S. Paratyphi naïve challenge. SPT-SPT = Homologous Re-Challenge with S. Paratyphi. SPT-ST Re-Challenge = Heterologous re-challenge with S. Typhi following previous S. Paratyphi challenge. ST-SPT Re-Challenge = Heterologous re-challenge with S. Paratyphi following previous S. Typhi challenge.

Fig 2

Clinical profile following Salmonella Typhi and Salmonella Paratyphi A challenge and re-challenge: Kaplan-Meier curves indicating time to bacteraemia following Salmonella Typhi (a) and Salmonella Paratyphi (b) challenge, in naïve and re-challenge participants; Time first fever >38oC following Salmonella Typhi (c) and Salmonella Paratyphi (d) challenge, in naïve and re-challenge participants. P = log-rank test between three groups;. Comparison of cumulative symptom severity scores (Day 0 to 21) in all participants diagnosed with Salmonella Typhi (e) or Salmonella Paratyphi (f) (median, interquartile range, p = Mann-Whitney U test); Quantitative blood culture at time of typhoid (g) or paratyphoid (h) diagnosis. Box plots represent median, interquartile range. Samples with no colonies were assigned a value corresponding to half the lower limit of detection (0.05 CFU/ml). p = Mann-Whitney U test; Maximum symptom severity score in participants diagnosed with typhoid (i) or paratyphoid (j) fever. Percentage of participants reporting one or more events. Symptoms were recorded using an electronic diary from Day 0 up to Day 21 post challenge. Stacked columns display percentage of participants reporting maximum symptom severity graded as mild (present but no interference with daily activity), moderate (some limitation of daily activity) or severe (unable to perform normal daily activity).

Fig 3. Antibody response to Salmonella Typhi and Paratyphi A challenge/re-challenge.

Serum IgG response to O9:LPS following Salmonella Typhi challenge (a) and O2:LPS following Salmonella Paratyphi challenge (b) grouped according to outcome (ED = Met criteria for enteric fever diagnosis. nED = Did not meet criteria for enteric fever diagnosis); Comparison of serum IgG response to O9:LPS (c) and O2:LPS (d) according to challenge/re-challenge group. Coloured lines connect median values for each timepoint. Grey lines connect paired samples across timepoints; p = Wilcoxon signed rank test; Magnitude of anti-O9:LPS IgG following Salmonella Typhi challenge (e) and anti-O2:LPS IgG following Salmonella Paratyphi challenge (f) expressed as log10 fold change from baseline at Day 28 and Day 90, p = Mann-Whitney U test; ST = S. Typhi naïve; ST-ST = Homologous S. Typhi re-challenge. SPT-ST = Heterologous S. Typhi re-challenge. SPT = S. Paratyphi naïve; SPT-SPT = Homologous S. Paratyphi re-challenge. ST-SPT = Heterologous S. Paratyphi re-challenge.