The long term objective of this proposal is to improve our understanding of the mechanisms of pathogenesis of ehrlichial diseases and related tick-borne rickettsial pathogens of humans and animals. These pathogens include ehrlichial infections of humans caused by Ehrlichia chaffeensis and the causative agent of human granulocytic ehrlichiosis as well as E. canis, E. equi, Cowdria ruminantium and Anaplasma marginale infections of animals. A common feature of infections caused by these rickettsial pathogens is their ability to persist in human and/or animal reservoir hosts and cause long-term infection. Long-term infections of animal reservoir hosts effectively maintain the pathogens in the environment and ensure onward transmission by the tick vector. It has been recognized recently that these rickettsiae also share several features at the molecular level. They synthesize similar major outer membrane proteins (MSP2 homologs) that are strongly recognized by sera from infected humans and animals and that can be encoded by multigene families. MSP2 homologs have been proposed as diagnostic antigens in several of these rickettsial species and are known to be at least partially protective antigens against challenge infection with live rickettsiae. However, sequence variation of MSP2 homologs may limit use of these antigens for diagnosis or vaccination and may be a factor in long term persistence of the rickettsiae. MSP2 was initially defined in A. marginale and infections of cattle with this rickettsia provide an excellent model for analyzing mechanisms of persistence and the influence of sequence variation in MSP2. In this proposal, the molecular mechanisms of sequence variation in gene transcripts encoding MSP2 (msp2) will be defined. The specific aims are: 1) Define the complete structure of an msp2 gene RNA transcript in Anaplasma marginale; 2) Determine variable and conserved regions of msp2 gene transcripts in acute and persistent infections with A. marginale; 3) Characterize protein expression from msp2 gene transcripts in acute and persistent infections with A. marginale; 4) Determine the genomic mechanisms of msp2 variation in A. marginale; 5) Compare msp2 RNA transcripts in A. marginale and in the agent of human granulocytic ehrlichiosis. This study will provide a basis for comparison of pathogenic mechanisms involving MSP2 variation in these rickettsiae, particularly in the closely related rickettsiae causing anaplasmosis and human granulocytic ehrlichiosis.