FIGURE 2.

OspC-expressing strains bind significantly more plasmin and plasminogen than OspC-deficient strains. Cultures of the OspC-expressing strains B31–A3 (B31) and ospC-comp (Comp) each retain over 4 × 10¹⁰ molecules of active plasmin (A) or plasminogen (B) per culture containing 10⁸ spirochetes, whereas the OspC-deficient strain, ospC-ko (KO) was associated with fewer than 0.25 × 10¹⁰ plasmin molecules per culture containing 10⁸ spirochetes. Preincubating strain B31–A3 with polyclonal anti-OspC antibodies (B31 + 1°Ab) inhibits the spirochetes from binding active plasmin or plasminogen. The lysine analog ϵ-ACA inhibits binding of plasmin and plasminogen to the OspC-expressing spirochetes (B31+ϵ-ACA) confirming the role of lysine molecules in this interaction. Active plasmin bound to the spirochetes is protected from inhibition by α₂-antiplasmin (α₂-AP), a potent serum-derived inactivator of unbound plasmin. The number of active plasmin molecules that remained in spirochete cultures after incubation and extensive washing was estimated by comparing plasmin activity levels associated with each culture to experimentally prepared standards. Plasmin activity data were normalized by the number of spirochetes remaining in each experimental culture after all washing steps.