BORRELIA-BURGDORFERI BINDS PLASMINOGEN, RESULTING IN ENHANCED PENETRATION OF ENDOTHELIAL MONOLAYERS

Several strains of Borrelia burgdorferi and Borrelia hermsii can bind human Lys-plasminogen specifically. Affinity blots using I-125-labeled plasminogen showed that numerous polypeptides of all the strains and species tested could bind via lysine residues to the plasminogen molec ule since binding could be completely inhibited by the lysine analog e psilon-aminocaproic acid. Binding analysis using I-125-labeled plasmin ogen on live intact organisms showed that the organisms possess two bi nding sites for plasminogen: a high-affinity site with a K-d of 24 +/- 12 pM and 106 +/- 14 binding sites per spirochete and a low-affinity site with a K-d of 20 +/- 4 nM and 2,683 +/- 36 binding sites per spir ochete. Indirect immunofluorescence and confocal microscopy showed a g eneralized but punctate pattern of plasminogen binding to the spiroche te surface. Exogenously provided urokinase-type plasminogen activator converted B. burgdorferi surface-bound plasminogen to enzymatically ac tive plasmin as demonstrated by the breakdown of the chromogenic plasm in substrate S2251. Plasmin-coated organisms showed an enhanced abilit y to penetrate endothelial cell monolayers grown on connective tissue substrates compared to untreated controls (P < 0.001). This functional assay demonstrated that enzymatically active plasmin on the surface o f spirochetes can lead to greater invasion of tissues.