Despite significant advances in understanding the biology of Borrelia burgdorferi, the agent of Lyme disease, the pathogenesis of Lyme disease remains poorly understood. The overall objective of this project is to use a mouse model of Lyme disease to investigate spirochete population kinetics and differential expression of specific Borrelia burgdorferi genes in selected target tissues during the dissemination/disease evolution phase, disease resolution (immune) phase, and persistent phases of B. burgdorferi infection. During the past project period, gene products were defined that are associated with antibody-mediated protective, but not arthritis- or carditis-resolving immunity (DbpA), and gene products that are associated with arthritis-, but not carditis- resolving or protective immunity (Arp and P37-42). In the current project period, the search will continue for other arthritis- and carditis-resolving antigen targets, as well as to characterize antigens expressed during persistent infection. Highly sensitive assays (real time PCR) have been optimized for quantitative analysis of spirochete population kinetics and for examining prototype gene expression in tissues at different stages of infection. Having optimized these assays, Specific Aim 1 will continue the search for B. burgdorferi antigens associated with biologically relevant antibody responses by screening a B. burgdorferi genomic expression library with sera from infected mice. Recombinant proteins and antisera will be tested for biologic activity (protective, disease-resolving, and dissemination-preventing activity) in the model. Specific Aim 2 will examine prototype gene expression during different phases of infection, and examine the effects of immunity or immune tolerance to biologically relevant antigens on these events. Specific Aim 3 will investigate mechanisms of persistent infection and disease quiescence by examining expression of prototype genes during this phase of infection, and define antigens that are involved in maintaining the host-agent equilibrium.