Although gamma delta (gammadelta) T cell receptors represent our third lineage of antigen receptors in adaptive immunity, along with antibodies and alphgamma T cell receptors, their structural biology, ligand specificity, and function is poorly understood. Yet, emerging functional studies point to important roles in defense against cancers and pathogens, and the maintenance of host homeostasis. Gammadelta TCR use DNA rearrangement to generate potentially greater combinatorial diversity than antibodies or aa TCR, but controversy persists about whether this diversity is used for specific antigen recognition, or whether the germline segments are sufficient for gammadelta TCR function (2). Also, unlike aa TCR, gammadelta TCR can recognize both MHC-like and non-MHC ligands in the absence of antigen processing, similar to antibody/antigen recognition. Do gammadelta TCR recognize their ligands in a convergent manner reminiscent of alphabeta TCR interactions with peptide-MHC, or with a diversity of recognition solutions as seen in antibody/antigen complexes? Currently there is no structural information on the interactions of gammadelta TCR with their ligands. The broad goals of this application are to begin to elucidate the molecular and structural principles of ligand recognition by the gammadelta TCR. We aim to express, biochemically characterize, and determine the x-ray crystal structures of gammadelta TCR complexes with protein ligands, in companion with collaborative functional studies to understand the immunological relevance of our findings. We are focusing our structural efforts on a functionally well-characterized gammadelta TCR system, the interaction of the G8 gammadelta TCR with the non-classical MHC T22 and T10. In collaboration with Prof. Y.-h. Chien, we will also express recombinant forms of new gammadelta TCR with defined specificities for classical and non-classical MHC in order to ask whether there is a universally convergent gammadelta recognition mode of MHC-like ligands. Finally, we will begin to investigate whether the structural basis of gammadelta TCR recognition of non-MHC protein antigens is fundamentally different from 78 TCR recognition of MHC-like antigens.