The causative agent of Lyme disease, Borrelia burgdorferi, can persistently infect humans and other mammalian hosts for great lengths of time. Understanding the mechanisms by which Lyme disease spirochetes establish and maintain chronic infections will be crucial for developing improved treatments for long-term human infections. Lyme disease spirochetes are generally resistant to their hosts'alternative pathway of complement activation. Studies with cultured bacteria found that either of two borrelial outer surface proteins facilitates resistance to host complement. These proteins, CRASP-1 and CRASP-2 (complement regulator-acquiring surface proteins 1 and 2), both bind the human complement regulatory proteins factor H and FHL-1 (factor H-like protein 1), plus other, unidentified serum components. However, we found production of CRASP-1 is repressed within the first few days of mammalian infection, while CRASP-2 production is greatly induced during mammalian infection. Thus, CRASP-2 is the only borrelial protein demonstrated to facilitate resistance to host complement that is produced during established mammalian infection. Our studies lead us to hypothesize that production of CRASP-2 during mammalian infection permits the bacterium to avoid killing by its host's alternative pathway of complement activation, and thereby enables the spirochete to persistently infect humans. Experiments are outlined in this proposal to test our hypothesis by (1) evaluating the essential nature of CRASP-2 production during mammalian infection and the necessity for precise control of CRASP-2 synthesis, and (2) characterizing mechanisms by which CRASP-2 expression is regulated. PUBLIC HEALTH RELEVANCE: The causative agent of Lyme disease can infect humans and other hosts for years, possibly even for the lifetime of the individual. Our recent studies demonstrated that a borrelial protein named CRASP-2 helps protect Borrelia burgdorferi from killing by the human innate immune system, and that CRASP-2 is produced at high levels during human infection. We propose to determine the mechanisms by which Lyme disease spirochetes control CRASP-2 synthesis, with the long-term goal of developing therapies that will disrupt CRASP-2 expression during human infection and thereby render the bacteria sensitive to elimination from the body.