Lyme disease is a significant and re-emerging disease that impacts the public health, particularly in endemic regions. The causative agent of Lyme disease, Borrelia burgdorferi, is transmitted to humans by Ixodes ticks. During tick feeding, B. burgdorferi is injected into the skin, and quickly adapts to mammalian environment by differentially expressing a number of genes, many of which encode lipoproteins and contribute to borrelial infectivity. Identification and characterization of these host specific proteins should provide insight into important steps in regard to B. burgdorferi colonization as well as borrelial pathogenesis. In the first part of this study, we identified a previously uncharacterized borrelial lipoprotein designated BBA33. We showed that a bba33 mutant failed to establish infection in mice and was cleared early in the infectious process, in a manner that could be rescued by genetic complementation with intact bba33. In terms of the protein function, we found that BBA33 specifically bound to human type IV and VI collagen in vitro. Our findings demonstrated that BBA33 functions as an essential adhesin in experimental infection via its interaction in collagen-enriched tissues. Previous observation showed that B. burgdorferi resists the classical complement-dependent killing; however, the mechanisms of resistance were not described. In the second part of this study, we asked how surface exposed lipoproteins might promote resistance to the classical complement pathway. Through the use of far-Western immunoblots, we found that the previously identified fibronectin binding protein BBK32 mediates binding to the classical complement C1 complex. Subsequent surface plasmon resonance assays showed that the BBK32 C-terminal domain bound to both C1 and C1r with high affinity. This binding was found to inhibit the activation of C1r. Furthermore, the production and surface localization of BBK32 in a non-virulent, serum-sensitive B. burgdorferi isolate increased the bacterial binding to C1 and C1r, and partially promoted bacterial resistance to the classical pathway-dependent killing. This study demonstrated a novel mechanism utilized by B. burgdorferi to evade complement system activation and survive in the mammalian host. In addition, we identified a new function of B. burgdorferi lipoprotein BBK32 in blocking the classical complement pathway. Given that several pathologies are associated with uncontrolled activation of C1, it is possible that inhibition of this activity by BBK32 derivatives/mimetics may provide therapeutic potential. Taken together, this study deciphered novel functions of borrelial surface lipoproteins. It also provided insight into the mechanisms of how different surface proteins of B. burgdorferi mediate the colonization of host tissues and the evasion of complement-dependent killing. As such, these studies provide additional insight into the role of borrelial surface proteins in the infectivity and pathogenesis of B. burgdorferi during experimental infection.
