deciphering the interface between a cd40 receptor and borrelial ligand ospa neuroborreliosis is serious sequelae of lyme borreliosis neuroinvasion is largely relied on successful translocation of borrelia across the bloodbrain barrier adherence of borrelia to brain microvascular endothelial cell bmec seems to be critical for translocation here we unfold the interface between ospa and cd40 molecules major ligand and receptor that are involved in adhesion of borrelia to bmecs we found that a region between asn127 and asp205 of ospa forms the cd40receptor binding site this region encompasses human umbilical vein endothelial cell huvec binding domain and contains a potential ligandbinding pocket lined by three amino acid residues arg139 glu160 and lys189 disruption of this pocket by truncation of the huvec binding domain caused complete abrogation of its ability to bind cd40 to identify the amino acid residues within the huvec binding domain involved in the cd40 binding sitedirected mutagenesis and binding assays were performed results showed that asp149 phe165 ala172 val186 and leu192 might form interface with cd40 molecule other side of the interface was also identified with the help of a ligandbinding assay with ospa and truncated cd40 fragments results exposed that cysteine rich domain 2 crd2 of cd40 might be the site for ospa binding precise knowledge of the molecular basis of the ligandreceptor interactions is essential in order to understand mechanisms of pathogenesis and could help in the development of novel therapeutics and vaccines