Evaluation of an intragastric challenge model for Shigella dysenteriae 1 in rhesus monkeys (Macaca mulatta) for the pre-clinical assessment of Shigella vaccine formulations

Abstract

Shigellosis is a worldwide disease, characterized by abdominal pain, fever, vomiting, and the passage of blood- and mucus-streaked stools. Rhesus monkeys and other primates are the only animals that are naturally susceptible to shigellosis. A suitable animal model is required for the pre-clinical evaluation of vaccines candidates. In this study, the minimal dose of Shigella dysenteriae1 1617 strain required to produce dysentery in four of five (80% attack rate) monkeys using an escalating dose range for three groups [2 × 10(8) , 2 × 10(9) and 2 × 10(10) colony forming unit (CFU)] was determined. In addition, the monkeys were re-infected. The identified optimal challenge dose was 2 × 10(9) CFU; this dose elicited 60% protection in monkeys when they were re-challenged with a one log higher dose (2 × 10(10) CFU). The challenge dose, 2 × 10(10) CFU, produced severe dysentery in all monkeys, with one monkey dying within 24 h, elicited 100% protection when re-challenged with the same dose. All monkeys exhibited immune responses. This study concludes that the rhesus monkey model closely mimics the disease and immune response seen in humans and is a suitable animal model for the pre-clinical evaluation of Shigella vaccine candidates. Prior infection with the 1617 strain can protect monkeys against subsequent re-challenges with homologous strains.

Keywords: Shigellosis; challenge model; rhesus monkey (Macaca mulatta).

Figure 1

One monkey died within 24 h of challenge with a dose of 1.76 × 10¹⁰ CFU. (A) Inflamed intestine with only blood and mucus in feces, (B) bloody fluid in ileum.

Figure 2

Histology of colonic biopsies collected before challenge (A) and 2 days post-challenge, (B) H&E-stained post-challenge biopsy shows acute neutrophilic colitis, with crypt abscess, increased numbers of neutrophils, decreased numbers of goblet cells, and an increase in mitotic figures.

Figure 3

Moderate eosinophilic inflammation in the lamina propria.

Figure 4

IgA, IgG, and IgM antibody titers against SD1-LPS in group 1 monkeys (cross symbol), group 2 (open square symbol), and group 3 (closed round symbol), expressed as geometric mean titer (GMT) with GMT±SE after challenge on day 00 and re-challenge on day 31 with SD11617 strain.

Figure 5

Fecal s-IgA antibody titers to SD1-LPS in group1 (cross symbol), group 2 (open square symbol), and group 3 (closed round symbol) monkeys, expressed as MN with MN ± SE, after challenge on day 00 and re-challenge on day 31 with SD11617 strain.

Figure 6

Cytokine IL-8 levels (pg/ml), in plasma (A) and in fecal extract samples, (B) in monkeys from group1 (cross symbol), group 2 (open square symbol), and group 3 (closed round symbol) after challenge on day 00 and re-challenge on day 31 with SD11617 strain.