Safety and colonization of two novel VirG(IcsA)-based live Shigella sonnei vaccine strains in rhesus macaques (Macaca mulatta)

Abstract

Shigella are gram-negative bacterium that cause bacillary dysentery (shigellosis). Symptoms include diarrhea and discharge of bloody mucoid stools, accompanied by severe abdominal pain, nausea, vomiting, malaise, and fever. Persons traveling to regions with poor sanitation and crowded conditions become particularly susceptible to shigellosis. Currently a vaccine for Shigella has not been licensed in the United States, and the organism quickly becomes resistant to medications. During the past 10 y, several live attenuated oral Shigella vaccines, including the strain WRSS1, have been tested in humans with considerable success. These Phase I vaccines lack the gene for the protein VirG also known as IcsA, which enables the organism to disseminate in the host target tissue. However, 5% to 20% of the vaccinated volunteers developed mild fever and brief diarrhea, and the removal of additional virulence-associated genes from the vaccine strain may reduce or eliminate these side effects. We administered 2 Shigella sonnei vaccines, WRSs2 and WRSs3, along with WRSS1 to compare their rates of colonization and clinical safety in groups of 5 rhesus macaques. The primate model provides the most physiologically relevant animal system to test the validity and efficacy of vaccine candidates. In this pilot study using a gastrointestinal model of infection, the vaccine candidates WRSs2 and WRSs3, which have additional deletions in the enterotoxin and LPS modification genes, provided better safety and comparable immunogenicity to those of WRSS1.

Figure 1.

Mean number of days with Shigella-positive fecal samples after vaccination. Positive culture indicates excretion of viable Shigella bacteria; positive PCR indicates continued colonization of the lower gastrointestinal tract.

Figure 2.

Mean peripheral blood leukocyte count change from baseline for each vaccine group. Any change from baseline identifies an immune-system–stimulating event.

Figure 3.

Mean peripheral blood neutrophil counts for each vaccine group before vaccination and in subsequent weeks. The normal range for the in-house clinical pathology laboratory is 2.2–to 5.6 × 10 cells/μl.

Figure 4.

Mean number of days of abnormal clinical signs. The only abnormal clinical sign seen for the WRSs3 group was stool consistency.

Figure 5.

The number of animals in each group with abnormal signs during the first 5 d after vaccination. No abnormal signs were seen in any group after day 5.

Figure 6.

Mean body temperature of sedated animals; the day 0 value is before vaccination. The in-house normal range for body temperature in sedated macaques is 36.1 to 39.5 °C.

Figure 7.

Mean percentage change in body weight compared to baseline weight. Baseline weight taken 4 days prior to vaccination