This proposal is centered on the molecular basis of host-adaptation by Borrelia burgdorferi in mice and on the phenotypic changes of host-adapted Borrelia (HAB) that relate to pathogenesis and immunity. The small numbers of B. burgdorferi (Bb) present in infected mice has been a hindrance to determination of their surface antigenic structure. The signals in the host environment that initiate the process of host-adaptation by Bb have not been reported. Several recent findings in our laboratory form the basis for this proposal. We have learned that the magnitude of spirochetemia in scid mice is orders of magnitude greater than previously appreciated, making possible direct study of HAB spirochetes. These HAB circulating in the blood of scid mice are bound to blood cells, and have an antigenic composition distinct from that of in vitro cultivated Bb. Further, we learned that contact of in vitro cultivated Bb with blood cells results in expression of proteins otherwise poorly expressed in vitro, but upregulated during infection. Three specific aims are proposed. The first is "proteomic analysis of Bb host-adaptation in the mouse model of Lyme disease." We have developed novel methods for efficient extraction of the surface proteins expressed by HAB in mouse skin, blood, heart, and joint. Using the tools of proteomics we will catalogue the full set of HAB surface proteins expressed in different tissues and the relative amounts in which they are expressed. The second specific aim is "molecular basis of host-adaptation." Contact with host cells in vitro triggers Bb to upregulate the expression of certain surface antigens. These events will be related to the findings of our proteomic analysis of host adaptation in the mouse. The Bb receptor(s) and host cell ligand(s) that mediate this process will be defined. The third specific aim is "relationship of surface antigenic structure of host-adapted Bb to protective immunity." HAB bound to circulating blood cells will be used to assess the relative representation of specific molecules on their surface. Novel HAB surface proteins will be tested as protective immunogens.