The Leptospiral Outer Membrane Proteome & Immunity

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The overall goal of this proposal is to define the leptospiral surface proteome and the relevance of post-translational modifications to immunity. We have identified a number of surface-exposed lipoproteins that are expressed during infection of the mammalian host. However, many leptospiral surface lipoproteins remain to be identified and those that are known appear to undergo extensive post-translational modifications that likely affect recognition by the host immune system. Lipoproteins are dominant leptospiral surface antigens. The genome of Leptospira interrogans serovar Copenhageni encodes approximately 168 lipoproteins. We have described a number of these lipoproteins, localized them to either the inner or outer membrane, and determined whether they are surface exposed. L. interrogans has genes encoding two possible lipoprotein export pathways: The LOL pathway and Type II secretion. Methods for leptospiral genetic manipulation are now available to determine the signals required to target lipoproteins to the outer membrane and leptospiral surface, as has recently been achieved for the lipoproteins of Borrelia burgdorferi by Wolfram Z¿ckert, who is an export on spirochetal surface lipoprotein export pathways and a co-investigator on this proposal. Recent proteomic studies, including those performed in collaboration with co-investigator Caroline Cameron, reveal that many leptospiral surface proteins undergo post-translational modification, particularly by methylases. We now have evidence that the major outer membrane lipoprotein, LipL32, undergoes extensive differential methylation during infection. This would explain why recombinant LipL32 produced in E. coli is ineffective as a vaccine, even though it is an abundant surface lipoprotein. Understanding the nature of surface lipoprotein methylation provides an opportunity to create effective methylated peptide vaccines that target lipoprotein surface epitopes expressed during infection. The Research Plan has the following three Specific Aims: #1. What is the leptospiral surface lipoprotein export pathway? Our hypothesis is that, as in B. burgdorferi, leptospiral lipoproteins are exported to the leptospiral surface via the LOL export pathway. We will test this hypothesis by transforming L. interrogans with genes encoding lipoprotein-GFP fusions and test their susceptibility to surface proteolysis. We will determine the length of the tether needed for targeting lipoproteins to the surface and the role of negative-charged amino acids in preventing surface localization. #2. How does in vivo LipL32 methylation alter its surface epitopes? Our hypothesis is that increased methylation during infection alters the antigenic character of LipL32. We will isolate organisms from infected tissues and further define LipL32 sites that become methylated during infection. Those sites that are predicted to be surface-exposed based on the LipL32 crystal structure will be tested for recognition by infection-derived antibodies and T-cells. #3. Which methylated peptides are most effective at inducing protective immunity? Methylated peptides that are highly recognized by infection-derived antibodies and T-cells will be examined as immunoprotective antigens in the hamster model of leptospirosis. In vitro assays examining adherence inhibition, growth inhibition, bactericidal activity, and opsonophagocytosis will be performed to determine mechanisms of protective immunity.