The spirochete Borrelia burgdorferi is the causative agent of Lyme borreliosis (Lyme disease) and is transmitted to mammalian hosts by tick vectors. In humans, the bacteria induce a complex disease, which involves the skin, joints, heart, and nervous system. However, the pathogenic principles of this multisystem illness are far from being understood. To disseminate from the site of the tick bite and invade multiple organ sites, spirochetes have to penetrate normal tissue barriers, such as vascular basement membranes and other organized extracellular matrices. Substantial evidence from other invasive bacterial infections suggest that spirochetes may use endogenous or host-derived enzymes-in particular, proteinases-for this purpose. Here we show that B. burgdorferi binds human plasmin(ogen) -mainly via its outer cell surface lipoprotein A. Binding of plasminogen to spirochetal receptor leads to an accelerated formation of active plasmin in the presence of host-derived plasminogen activator. The cellsurface-associated plasmin cannot be regulated by the serum inhibitor ac2-antiplasmin and degrades high-molecular-weight glycoproteins, such as fibronectin. It is suggested that the acquisition of host-derived proteinase plasmin(ogen) contributes to the pathogenicity of B. burgdorferi.

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Borrelia burgdorferi is transmitted by infected ticks and causes Lyme disease. To infect distant organ sites, B. burgdorferi spirochetes must disseminate from the site of the tick bite. During dissemination from the dermal tissue, they breach tissue barriers, probably by proteolysis. The previous findings that spirochetes bind serum-derived plasminogen and that plasmin favors spirochetal invasiveness and infectivity suggested a role for plasmin in the pathogenicity of B. burgdorferi. Binding of plasminogen to spirochetes and activation into plasmin is favored in a microenvironment that is rich in plasminogen and plasminogen activators. Plasminogen is abundant in plasma and interstitial fluids, and it is increased in inflammatory exudates. Since B. burgdorferi does not express endogenous plasminogen activators, the conversion of spirochete-bound plasminogen depends on host-derived plasminogen activators. In this report, we show that both intact B. burgdorferi organisms and its recombinant outer surface lipoprotein A induce human monocytes to express and secrete urokinase-type plasminogen activator in its zymogen form (pro-uPA). Moreover, we demonstrate that the presence of B. burgdorferi accelerates the interaction between (pro-)uPA and plasmin(ogen), leading to spirochete-bound plasmin. In a pro-uPA-serum mixture, spirochete-bound plasmin activity is generated. Taken together, the data suggest that B. burgdorferi may induce pro-uPA in a monocyte-containing inflammatory site and that the spirochetal surface provides an appropriate milieu for subsequent interactions between (pro-)uPA and plasmin(ogen), which result in spirochete-bound plasmin even in the presence of inhibitors for plasminogen activators and plasmin.

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