Lyme borreliosis is the most widespread vector-borne disease in temperate zones of Europe and North America. Although the infection is treatable, the symptoms are often overlooked resulting in infection of the neuronal system. In this work we uncover the underlying molecular mechanism of borrelial translocation across the blood-brain barrier (BBB). We demonstrate that neuroinvasive strain of Borrelia readily crosses monolayer of brain-microvascular endothelial cells (BMECs) in vitro and BBB in vivo. Using protein-protein interaction assays we found that CD40 of BMECs and OspA of Borrelia are the primary molecules in transient tethering of Borrelia to endothelium. OspA of neuroinvasive Borrelia, but not of non-neuroinvasive strain, binds CD40. Furthermore, only the neuroinvasive Borrelia and its recombinant OspA activated CD40-dependent pathway in BMECs and induced expression of integrins essential for stationary adhesion. Demonstration of the CD40-ligand interactions may provide a new possible perspective on molecular mechanisms of borrelial BBB translocation process.

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