Lyme disease (LD) is the most prevalent vector-borne disease in the United States, with up to 300,000 cases a year. LD is caused by several species of the spirochete bacteria Borrelia burgdorferi sensu lato (the Lyme borreliae), which are transmitted by ticks from rodent reservoirs to human hosts. While a short course of antibiotics is usually effective in eliminating the bacteria a sizeable number of LD patients continue to suffer long- term, debilitating sequelae, including pain, fatigue, cognitive dysfunction and other symptoms known as post- treatment Lyme disease (PTLD). As many as 1.9 million people in the US suffer from PTLD. There is currently no vaccine that can prevent LD or PTLD. We are developing an immunoprophylactic for LD and other tick-borne diseases (TBD) based on an understanding of the virulence mechanisms that the causal pathogens use to evade innate immunity. These pathogens protect themselves from elimination by the human complement system by binding to the human complement inhibitor Factor H (FH), a protein abundant in blood. FH bound to bacterial surfaces blocks the activation of the alternative complement pathway that would otherwise destroy the bacteria. We have produced recombinant proteins that are fusions of the FH domains that bind to Lyme borreliae with the constant region of human IgG3 (Fc3), using a plant expression system. In Phase I we showed that the fusion SCR(6-7)/Fc3 binds to B. burgdorferi (Bb) and B. afzelii (Ba) and, in the presence of human complement, kills the bacteria. We have also demonstrated that SCR(6-7)/Fc3 prevents Bb and Ba infection in mice by blocking bacterial survival in fed ticks and tick-to-host transmission at a dose as low as 2 mg/kg. Our overall goal is to develop a pre-exposure prophylactic (PrEP) to prevent LD and TBDs. In this Phase II STTR project we will optimize the in vivo efficacy and pharmacokinetics of SCR(6-7)/Fc3. We will conduct process development and carry out preliminary preclinical safety testing. The project is a collaboration of two research groups that are uniquely qualified to bring it to a successful conclusion. Planet Biotechnology Inc (the small business concern) will produce novel variants of SCR(6,7)/Fc3. Yi-Pin Lin at the New York State Department of Health will evaluate the ability of the proteins to mediate complement-dependent killing of Lyme and relapsing fever spirochetes by membrane attack complex and opsonophagocytosis by human macrophages and their ability to block infection when mice are bitten by ticks carrying a virulent Lyme borreliae strain. We will select a lead FH-Fc pre-immune prophylaxis drug candidate, scale up production, conduct toxicology studies in two animal species and analyze pharmacokinetics and pharmacodynamics in humanized mice and non-human primates.