Lyme disease, caused by the spirochete Borrelia burgdorferi, is the most common vector-borne disease in U.S. B. burgdorferi can infect the skin at the site of the tick bite and spread via blood to diverse tissues, indicating that the bacterium can withstand bloodstream defenses. The complement system is a bloodstream defense that is activated via three different pathways. A critical step of the activation of this system is the formation of C3 convertases, C4b2a or C3bBb, which catalyze events resulting in inflammation, opsonization and pathogen lysis. Potential host damage following complement activation necessitates tight regulation by serum complement regulators that bind to complement components and promote their degradation. For example, the complement regulator factor H (FH) and C4BP modulates formation of C3bBb and C4b2a, respectively. We recently showed that B. burgdorferi produces DbpA, a surface protein that promotes tissue colonization by binding to dermatan sulfate and decorin, also fosters bacteremia by binding to C4BP. A B. burgdorferi strain producing DbpA-I156A, a point mutant deficient in C4BP binding, showed a delay in bacteremia and joint colonization. In addition, B. burgdorferi also produces CRASPs (Complement Regulator Acquiring Surface Proteins) that bind to complement regulators. The paralogs CspA, CspZ bind to FH and FH-like protein (FHL), and the paralogs ErpP, ErpC, and ErpA bind to FH and complement factor H-related protein (CFHR). Although several CRASPs have been demonstrated to contribute to serum resistance in vitro, none have been definitely shown to contribute to mammalian infection. Recently, we found that a B. burgdorferi strain harboring an erpA::Tn insertion displayed a defect in tissue colonization at early stages of infection. These results suggest that DbpA and ErpA modulate C4b2a and C3bBb, respectively, to promote bacteremia and tissue colonization. The ability of DbpA and ErpA to inhibit the formation of both classes of C3 convertases may signify a coordinated attack on host bloodstream defenses by B. burgdorferi. To test this hypothesis, the following aims will be pursued. (1) Test the role of FH-binding in bacteremia and colonization. We will test if a targeted, non-polar erpA deletion mutant display defects in FH/CFHR binding, complement activation in vitro, and bacteremia and tissue colonization in the murine model. If the mutant displays defects, we will test if FH binding by ErpA is essential for these functions, and whether other CRASPs can functionally complement the defects. (2). Test if C4BP- and FH-binding by B. burgdorferi provide non-redundant roles to promote bacteremia and colonization. To determine if a dramatic dissemination defect requires loss of both C4BP- and FH-binding activities, we will mutate erpA in the strain background of the B. burgdorferi DbpA-I156A mutant. These double mutant strains will be tested for the severe bacteremia and colonization defect in the murine model.