Enhancement of clinical signs in C3H/HeJ mice vaccinated with a highly immunogenic Leptospira methyl-accepting chemotaxis protein following challenge
Leptospirosis is the most widespread zoonosis and a life-threating disease of humans and animals. Licensed killed whole-cell vaccines are available for animals; however, they do not offer heterologous protection, do not induce a long-term protection, or prevent renal colonization. In this study, we characterized an immunogenic Leptospira methyl-accepting chemotaxis protein (MCP) identified through a reverse vaccinology approach, predicted its structure, and tested the protective efficacy of a recombinant MCP fragment in the C3H/HeJ mice model. The predicted structure of the full-length MCP revealed an architecture typical for topology class I MCPs. A single dose of MCP vaccine elicited a significant IgG antibody response in immunized mice compared to controls (P < 0.0001), especially the IgG1 and IgG2a subclasses. The vaccination with MCP despite eliciting a robust immune response, did not protect mice from disease and renal colonization. However, survival curves were significantly different between groups, and the MCP vaccinated group developed clinical signs faster than the control group. There were differences in gross and histopathological changes between the MCP vaccinated and control groups. The factors leading to enhanced disease process in vaccinated animals needs further investigation. We speculate that anti-MCP antibodies may block the MCP signaling cascade and may limit chemotaxis, preventing Leptospira from reaching its destination, but facilitating its maintenance and replication in the blood stream. Such a phenomenon may exist in endemic areas where humans are highly exposed to Leptospira antigens, and the presence of antibodies might lead to disease enhancement. The role of this protein in Leptospira pathogenesis should be further evaluated to comprehend the lack of protection and potential exacerbation of the disease process. The absence of immune correlates of protection from Leptospira infection is still a major limitation of this field and efforts to gather this knowledge is needed. Author summaryLeptospirosis is one of the underrecognized and neglected diseases of humans and animals. The presence of numerous Leptospira species/serovars infecting a broad range of animal reservoirs, and the resulting environmental contamination, makes control and prevention a cumbersome task. The bacterin-based vaccines available for animals do not offer protection against disease or renal colonization. A broader cross-protective vaccine is essentially needed to prevent Leptospira infections in humans and animals. Here we rationally selected a protein target based on its capacity to be recognized by antibodies of naturally infected animals and designed a recombinant vaccine. Our MCP vaccine was not effective in protecting mice from acute and chronic disease, and likely led to exacerbation of clinical signs in these animals. The development of an effective vaccine would contribute to control Leptospira infection in humans and animals and is important especially in low-income regions where leptospirosis is more prevalent and interventions to control the disease are not currently available.