Project Summary/Abstract Adoptive cell therapy (ACT) is a promising therapeutic approach for the treatment of cancer. However, the initial success of ACT has been limited to chimeric antigen receptor (CAR)-T cell therapies for hematological malignancies. Applying this cell therapy to solid tumors is challenged by the lack of targetable tumor antigens, the severe systemic toxicity and the suppressive tumor microenvironment. T cell receptor (TCR) gene therapy can overcome some of these challenges because it enables targeting of intracellular proteins presented as peptide antigens on the human leukocyte antigen (HLA) complex. However, the majority of naturally occurring TCRs are of low-affinity to their peptide-HLA targets. Engineering these TCRs via phage display or yeast display for higher affinity is complicated by the introduction of unwanted cross-reactivity and the poor association between affinity and function. This project seeks to tackle each of the major challenges of ACT in order to effectively reprogram the immune system to combat solid tumors. The F99 phase is focused on a TCR engineering platform for the creation, modification, and profiling of TCRs that can target tumor-associated self-proteins with minimal toxicity profiles. In this approach, I first raise T cells from the natural repertoire that recognize a related ‘foreign’ peptide that differs by one amino acid from the self-peptide. Then, I modulate the fine specificity of the TCR by directed evolution of the peptide binding region to switch its specificity towards the tumor self-antigen of interest. I demonstrate the value of this approach by the creation of libraries of viral-specific TCRs and the subsequent in vitro selection of TCRs that switched specificity to a closely related epitope. The engineered TCRs showed robust T-cell activation after ligand recognition and are of equal or higher efficiency than the parental receptor. Importantly, the engineered TCRs displayed no additional promiscui