Leptospiral outer membrane proteins OmpL1 and LipL41 exhibit synergistic immunoprotection.

New vaccine strategies are needed for prevention of leptospirosis, a widespread human and veterinary disease caused by invasive spirochetes belonging to the genus Leptospira. We have examined the immunoprotective capacity of the leptospiral porin OmpL1 and the leptospiral outer membrane lipoprotein LipL41 in the Golden Syrian hamster model of leptospirosis. Specialized expression plasmids were developed to facilitate expression of leptospiral proteins in Escherichia coli as the membrane-associated proteins OmpL1-M and LipL41-M. Although OmpL1-M expression is highly toxic in E. coli, this was accomplished by using plasmid pMMB66-OmpL1, which has undetectable background expression without induction. LipL41-M expression and processing were enhanced by altering its lipoprotein signal peptidase cleavage site to mimic that of the murein lipoprotein. Active immunization of hamsters with E. coli membrane fractions containing a combination of OmpL1-M and LipL41-M was found to provide significant protection against homologous challenge with Leptospira kirschneri serovar grippotyphosa. At 28 days after intraperitoneal inoculation, survival in animals vaccinated with both proteins was 71% (95% confidence interval [CI], 53 to 89%), compared with only 25% (95% CI, 8 to 42%) in the control group (P < 0.001). On the basis of serological, histological, and microbiological assays, no evidence of infection was found in the vaccinated survivors. The protective effects of immunization with OmpL1-M and LipL41-M were synergistic, since significant levels of protection were not observed in animals immunized with either OmpL1-M or LipL41-M alone. In contrast to immunization with the membrane-associated forms of leptospiral proteins, hamsters immunized with His(6)-OmpL1 and His(6)-LipL41 fusion proteins, either alone or in combination, were not protected. These data indicate that the manner in which OmpL1 and LipL41 associates with membranes is an important determinant of immunoprotection.

Characterization of leptospiral outer membrane lipoprotein LipL36: downregulation associated with late-log-phase growth and mammalian infection.

We report the cloning of the gene encoding a 36-kDa leptospiral outer membrane lipoprotein, designated LipL36. We obtained the N-terminal amino acid sequence of a staphylococcal V8 proteolytic-digest fragment in order to design an oligonucleotide probe. A Lambda-Zap II library containing EcoRI fragments of Leptospira kirschneri DNA was screened, and a 2.3-kb DNA fragment which contained the entire structural lipL36 gene was identified. Several lines of evidence indicate that LipL36 is lipid modified in a manner similar to that of LipL41, a leptospiral outer membrane lipoprotein we described in a previous study (E. S. Shang, T. A. Summers, and D. A. Haake, Infect. Immun. 64:2322-2330, 1996). The deduced amino acid sequence of LipL36 would constitute a 364-amino-acid polypeptide with a 20-amino-acid signal peptide, followed by an L-X-Y-C lipoprotein signal peptidase cleavage site. LipL36 is solubilized by Triton X-114 extraction of L. kirschneri; phase separation results in partitioning of LipL36 exclusively into the hydrophobic, detergent phase. LipL36 is intrinsically labeled during incubation of L. kirschneri in media containing [3H]palmitate. Processing of LipL36 is inhibited by globomycin, a selective inhibitor of lipoprotein signal peptidase. After processing, LipL36 is exported to the outer membrane along with LipL41 and lipopolysaccharide. Unlike LipL41, there appears to be differential expression of LipL36. In early-log-phase cultures, LipL36 is one of the most abundant L. kirschneri proteins. However, LipL36 levels drop considerably beginning in mid-log phase. LipL36 expression in vivo was evaluated by examining the humoral immune response to leptospiral antigens in the hamster model of leptospirosis. Hamsters surviving challenge with culture-adapted virulent L. kirschneri generate a strong antibody response to LipL36. In contrast, sera from hamsters surviving challenge with host-adapted L. kirschneri do not recognize LipL36. These findings suggest that LipL36 expression is downregulated during mammalian infection, providing a marker for studying the mechanisms by which pathogenic Leptospira species adapt to the host environment.