PROJECT SUMMARY Trypanosoma brucei is a causative agent of human and animal African trypanosomiasis, devastating diseases that endanger public health and present a major barrier to economic development in sub-Saharan Africa. The extracellular parasite manages to sustain an infection in the blood and tissues of its mammalian host by periodically “switching” its dense variant surface glycoprotein (VSG) coat. Drawing from a genomic repertoire of ~2000 VSG-encoding genes, the parasite changes its expressed VSG coat throughout infection. Before the host antibody response can clear a population expressing one VSG, the parasite switches expression to a new VSG, rendering it invisible to host antibody for a time. This process occurs continually and allows the parasite to maintain a chronic infection. The interaction between antibody and VSG, therefore, represents a critical interface in this infection. Despite its importance, the principles governing antibody recognition of VSG remain poorly understood. Here we propose to investigate the antibody-VSG interface during infection in high resolution using a high-throughput epitope mapping approach called phage immunoprecipitation sequencing (PhIP-seq). Using a phage display library containing peptides from every VSG ever annotated, we will measure antibody responses to VSG during chronic infections, while simultaneously using a high-throughput sequencing method developed by our lab to track the VSGs expressed during infection. In the first aim, we will characterize the dynamics and targets of anti-VSG antibody during chronic experimental infections. In the second aim, we will use PhIP-seq to map VSG epitopes in natural human infections. The proposed study will further our understanding of anti-VSG antibody responses while establishing T. brucei as a tractable model for the study of antibody responses to antigenically variable pathogens. Long term, this work could inform strategies for diagnosis or prevention of an important neglected tropical disease.