Yersinia pestis biovar Microtus strain 201, an avirulent strain to humans, provides protection against bubonic plague in rhesus macaques

Abstract

Yersinia pestis biovar Microtus is considered to be a virulent to larger mammals, including guinea pigs, rabbits and humans. It may be used as live attenuated plague vaccine candidates in terms of its low virulence. However, the Microtus strain's protection against plague has yet to be demonstrated in larger mammals. In this study, we evaluated the protective efficacy of the Microtus strain 201 as a live attenuated plague vaccine candidate. Our results show that this strain is highly attenuated by subcutaneous route, elicits an F1-specific antibody titer similar to the EV and provides a protective efficacy similar to the EV against bubonic plague in Chinese-origin rhesus macaques. The Microtus strain 201 could induce elevated secretion of both Th1-associated cytokines (IFN-γ, IL-2 and TNF-α) and Th2-associated cytokines (IL-4, IL-5, and IL-6), as well as chemokines MCP-1 and IL-8. However, the protected animals developed skin ulcer at challenge site with different severity in most of the immunized and some of the EV-immunized monkeys. Generally, the Microtus strain 201 represented a good plague vaccine candidate based on its ability to generate strong humoral and cell-mediated immune responses as well as its good protection against high dose of subcutaneous virulent Y. pestis challenge.

Keywords: Rhesus macaques; Yersinia pestis; live attenuated vaccine; plague; protection.

Figure 1. Development of IgG titers to F1 in Chinese-origin rhesus macaques immunized with the Y. pestis strain 201 and the EV vaccine on week 2, 4, 6, and 8 post immunization.

Figure 2. Production of cytokines IFN-γ, TNF-α, IL-2, IL-4, IL-5, and IL-6 and chemokines IL-8, MCP-1 and RANTES in Chinese-origin rhesus macaques immunized with the Y. pestis strain 201 and the EV on week 2, 4, 6, and 8 post immunization. Cytokine or chemokine production was calculated by subtracting the cytokine or chemokine concentration in control animals from the concentration measured in the Y. pesti 201- or the EV-immunized animals.

Figure 3. The F1 antigen of Y. pestis was identified by Immunohistochemitry staining. Bacteria were visualized as those expressing F1 antigen (brown and yellow stain). Numerous bacteria were observed in the tissues of lungs (a), lymph node (b), liver (c), spleen (d), kidney (e), and heart (f) of one dead animal of the Y. pestis 201 group (C) and control animals (D).

Figure 4. Skin ulcerations at the site of the injection in the immunized animals after challenge with Y. pestis 141. (A) Photo taken from the animal of the Y. pestis 201 group 8 d after challenge with virulent Y. pestis 141. (B) Photo taken from the animal of the Y. pestis 201 group 8 d after challenge with virulent Y. pestis 141. (C) Photo taken from the animals of the EV group 8 d after challenge with virulent Y. pestis 141.

Figure 5. Histopathology of the tissues from the immunized animals and the control animals. Tissue sections were stained with hematoxylin and eosin for pathological examination after infection with Y. pestis. (A) Tissue sections from the animals immunized with the EV and then infected with virulent Y. pestis strain 141. (B) Tissue sections from the animal immunized with the Y. pestis 201 and then infected with virulent Y. pestis strain 141. (C) Tissue sections from one dead animal of the Y. pestis 201 group after challenge with virulent Y. pestis strain 141. (D) Tissue sections from the control animals after infection with virulent Y. pestis strain 141.