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Peptide Repertoire Changes Caused by Defects in Antigen ProcessingByKristin Camfield LindDoctor of Philosophy in Molecular and Cell BiologyUniversity of California, BerkeleyProfessor Nilabh Shastri, ChairMHC I molecules display peptides derived from intracellular proteins on the cell surface for recognition by CD8+ T cells. Presentation of these peptide:MHC I complexes (pMHC I) is important for elimination of viruses and transformed cells. In order to generate an effective CD8 T cell response, a high-quality set of pMHC I must be presented. However, the editing mechanisms that select these optimal pMHC I are not clear. In these studies, we used biochemical and immunological methods to examine the role of two ER-resident pMHC I editors, tapasin and ERAAP in shaping the pMHC I repertoire. Analysis of tapasin-deficient mice revealed a qualitative loss as well as gain of some pMHC I. However, these changes did not overlap with the ERAAP-unedited pMHC I repertoire, indicating that tapasin edits peptides in a distinct way. Sequencing of peptides unique to tapasin-deficient cells revealed altered C-termini, suggesting that tapasin determines the peptide C-terminus while ERAAP shapes the peptide N-terminus. We separately examined the allele-specific effects of ERAAP-deficiency on the H-2d pMHC I repertoire. In addition to Kd and Dd, these mice express Ld, an MHC I molecule which is particularly affected by loss of ERAAP. We found that ERAAP-deficiency has distinct effects on the peptide repertoire presented by each MHC I molecule. Analysis of the peptide sequences revealed unique peptides bound to all three MHC I, although long, structurally unique peptides were only found on Dd and Kd. Furthermore, using a T cell hybridoma specific for loss of ERAAP function, we identified and characterized an ERAAP-unedited peptide presented by Dd. Together, these findings highlight the importance of both tapasin and ERAAP in determining an optimal pMHC I repertoire.