Leptospirosis, caused by pathogenic strains of the spirochete Leptospira spp, is the most common and geographically widespread zoonotic disease. The disease has been considered an important re-emerging infectious disease. Humans are accidental hosts who get infected through direct contact with body fluids of carrier animals or with contaminated water and soil. Human leptospirosis is a major problem in countries of tropical and, less often, subtropical climates. Numerous outbreaks have been reported in association with rainy seasons, floods, and recreational and sports activities. Globally, leptospirosis is estimated to occur at a frequency of approximately 1.03 million cases per year with a mortality of 5.7%. The disease also has a great economic impact in the agricultural industry since it affects livestock by inducing abortions, stillbirths, infertility, reduced milk production and death.
Due to the broad spectrum of symptoms, such as fever, chills, headache, and myalgias, and similarity with other tropical illness, leptospirosis often remains largely underdiagnosed. For diagnosis, serological tests are commonly used, and the diagnostic performance of a serological test is mainly influenced by the antigen used. An ideal serological test should cover all serovars of pathogenic leptospires with high sensitivity and specificity and use reagents that are relatively inexpensive to produce and can be used in tropical climates.
For prevention, commercially available vaccines, including heat or chemically inactivated leptospires, named bacterins, provide serovar-specific protection against infection. The lack of serovar cross-protection in addition to the need for annual revaccination imposes a major limitation of whole-cell Leptospira vaccines.
LPS are complex, amphipathic biomolecules that constitute the major surface component of Gram-negative bacteria Leptospire. The variations in the carbohydrate composition of LPS reflect the antigenic diversity among pathogenic leptospires. The protective immunity conferred by LPS as an immunogen is generally serovar specific. LPS are involved in host-adaption and also the target for antibody and vaccine development.
OmpL1 is a transmembrane protein while LipL41 is an outer membrane lipoprotein. Both proteins are surface-exposed. OmpL1 and LipL41 act synergistically to induce immunoprotection in the hamster model of leptospirosis, while neither of the individual proteins induces protective immunity. Although the mechanism of this synergistic protection remains to be solved, a steric hindrance of OmpL1 by LipL41 has been proposed.
As one of the most abundant proteins, LipL32 is an outer membrane lipoprotein, which is conserved both genetically and immunologically in the various pathogenic leptospires. Also called haemolysis associated protein-1 (Hap-1), LipL32 antigen induces antibodies in patients with leptospirosis. A recombinant LipL32 antigen has good sensitivity and specificity when used in an ELISA for human leptospirosis IgG.
Lig proteins are surface exposed outer membrane proteins punctuated by tandem repeats of about 90 amino acids of bacterial Ig like domains. LigA consists only of Ig-like domains, whereas LigB has an additional unique domain at the C terminus. Sera from human patients infected with different leptospiral serogroup strains reacted with Lig proteins. Studies suggest that Lig proteins could induce protective immunity against the challenge with strains of various leptospiral serovars.
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