Date Published: July 13, 2017
Publisher: Public Library of Science
Author(s): Karen V. Evangelista, Kristel Lourdault, James Matsunaga, David A. Haake, Paulo Lee Ho.
Leptospirosis is the most widespread zoonosis and is considered a major public health problem worldwide. Currently, there is no widely available vaccine against leptospirosis for use in humans. A purified, recombinant subunit vaccine that includes the last six immunoglobulin-like (Ig-like) domains of the leptospiral protein LigA (LigA7’-13) protects against lethal infection but not renal colonization after challenge by Leptospira interrogans. In this study, we examined whether the addition of the first seven Ig-like domains of LigB (LigB0-7) to LigA7’-13, can enhance immune protection and confer sterilizing immunity in the Golden Syrian hamster model of acute leptospirosis. Hamsters were subcutaneously immunized with soluble, recombinant LigA7’-13, LigB0-7, or a combination of LigA7’-13 and LigB0-7 in Freund’s adjuvant. Immunization with Lig proteins generated a strong humoral immune response with high titers of IgG that recognized homologous protein, and cross-reacted with the heterologous protein as assessed by ELISA. LigA7’-13 alone, or in combination with LigB0-7, protected all hamsters from intraperitoneal challenge with a lethal dose of L. interrogans serovar Copenhageni strain Fiocruz L1-130. However, bacteria were recovered from the kidneys of all animals. Of eight animals immunized with LigB0-7, only three survived Leptospira challenge, one of which lacked renal colonization and had antibodies to native LigB by immunoblot. In addition, sera from two of the three LigB0-7 immunized survivors cross-reacted with LigA11-13, a region of LigA that is sufficient for protection. In summary, we confirmed that LigA7’-13 protects hamsters from death but not infection, and immunization with LigB0-7, either alone or in combination with LigA7’-13, did not confer sterilizing immunity.
Leptospirosis is the most widespread zoonotic disease affecting both humans and animals. Infection by pathogenic strains of Leptospira spp. commonly occurs through direct contact with an infected animal’s urine or indirectly through contaminated water. Almost all mammals can harbor Leptospira in the proximal renal tubules, from which spirochetes are excreted in urine. Rats serve as a major carrier in human leptospirosis particularly in urban settings. Infection in rats is mostly asymptomatic, with bacteria cleared from the bloodstream and organs except the renal tubules leading to urinary shedding of up to 107 leptospires per milliliter months after initial infection [1, 2]. Humans, on the other hand, serve as accidental hosts with whom infection is acute and occasionally fatal. Leptospirosis in humans can present as an acute, self-limited disease characterized by an abrupt onset of fever, headache, chills, nausea and vomiting, myalgia, and less commonly skin rashes. A severe disease form is Weil’s syndrome, characterized by multiorgan system complications including jaundice, acute hepatic and renal dysfunction, and hemorrhage [1–5]. An infected individual may also be asymptomatic and shed leptospires, similar to maintenance animal hosts. Asymptomatic renal colonization and urinary shedding were observed among a group of individuals in the Peruvian Amazon . However, the prevalence of persistently infected individuals and their relevance to disease transmission are unknown.
In an effort to develop a vaccine that confers protection across all pathogenic serovars and protects the kidneys from bacterial colonization, we tested the immunoprotective properties of LigA7’-13 in combination with LigB0-7. We confirmed results of previous studies that the immunization of hamsters with LigA7’-13 protected hamsters from death but not from infection, while animals that received LigB0-7 protein were mostly susceptible to leptospiral infection [18, 26]. We found that addition of LigB0-7 to LigA7’-13 neither conferred sterilizing immunity nor reduced the bacterial burden in kidneys.