Preferentially expressed antigen in melanoma (PRAME) is a cancer-testis antigen that is expressed in many cancers and leukemias; in healthy tissue, PRAME is limited to the testes and ovaries, making it a highly attractive cancer target. PRAME is an intracellular protein that cannot currently be drugged. After proteasomal processing, the PRAME300-309 peptide (ALY) is presented in the context of human lymphocyte antigen HLA-A*02:01 molecules, for recognition by the T cell receptor (TCR) of cytotoxic T cells. We describe Pr20, a TCR mimic (TCRm) human IgG1 antibody discovered through phage-display technology that recognizes the cell-surface ALY peptide/HLA-A2 complex; Pr20 is an immunological tool and potential therapeutic agent. Pr20 bound to ALY peptide-pulsed cells and PRAME+/HLA-A2+ cancers. An afucosylated Fc form (Pr20M) directed antibody-dependent cellular cytotoxicity against PRAME+/HLA-A2+ leukemia cells and was therapeutically active in human leukemia models in vivo as a monotherapy. Interestingly, in some tumors, Pr20 binding markedly increased upon IFN? treatment, mediated by induction of the immunoproteasome catalytic subunit ?5i. The immunoproteasome reduced internal destructive cleavages within the ALY epitope compared to the constitutive proteasome. The ALY/HLA-A2 epitope expression is far lower than traditional mAb targets and therefore we explored strategies to enhance potency of Pr20M. We demonstrate that combining Pr20M and CD47 ‘do not eat me’ signal blockade (using a SIRP?-variant peptide called CV1) led to enhanced antibody dependent cellular phagocytosis (ADCP) and dramatic therapeutic effects in leukemia xenograft models. We also engineer a bispecific T cell engager (BiTE) and chimeric antigen receptor (CAR) T cell using the Pr20 scFv. Pr20-BiTE and Pr20-CAR were both capable of potent redirected T cell lysis against PRAME+/HLA-A2+ leukemia in vitro, however these constructs also had off-target activities against HLA-A2+ cells despite undetectable binding of Pr20. Finally, we confirm the MAPK pathway as an important regulator of HLA-I expression through a high-throughput small molecule inhibitor screen and describe mTOR as a potential regulator of HLA-I. We also discover that a 5-lipoxygenase inhibitor BW-B70c can enhance HLA-I expression through transcriptional regulation but through a mechanism independent of 5-lipoxygenase. BW-B70c treatment increases tumor-antigen presentation on HLA-I and sensitizes cells to TCRm-mediated ADCC. We also explore the use of the cytomegalovirus immunoevasin US6 to modulate antigen presentation on HLA-I with the goal of identifying strategies to protect engineered cellular therapeutics harboring foreign or synthetic transgenes from immunological attack. The data provide rationale for developing TCRm antibodies as therapeutic agents for cancer, offer mechanistic insight on proteasomal regulation of tumor-associated peptide/HLA antigens, yield possible therapeutic solutions to these ultra-low surface presentation targets, and explore a strategy to protect genetically engineered cells harboring foreign genes from immunological attack.