Targeting ERAP enhances anti-tumor immunity by disrupting the NKG2A/HLA-E inhibitory checkpoint

New targets that enhance anti-tumor immunity must be identified to improve the efficacy of cancer immunotherapy. Here we show that loss of endoplasmic reticulum aminopeptidase (ERAP) family proteins improves anti-tumor immunity and synergizes with immune checkpoint blockade. Mechanistically, we show that loss of ERAP inactivates the HLA-E/NKG2A checkpoint, which normally restrains tumor killing by both CD8+ T cells and NK cells. The inhibitory activity of HLA-E is dependent on its presentation of a restricted set of invariant epitopes which form the binding surface for the NKG2A/CD94 receptor complex. Using genetic screening, in vivo models, cell-based assays, and immunopeptidomics, we show that loss of ERAP activity prevents the processing of these invariant peptides and alters the presented peptidome of both HLA-E and classical MHC-I. HLA-E neo-peptides presented after ERAP deletion are unable to bind the NKG2A/CD94 receptor, rendering tumor cells highly susceptible to killing by NKG2A+ cytotoxic T and NK cells. Thus, loss of ERAP phenocopies the loss or inhibition of the HLA-E/NKG2A pathway and represents an attractive therapeutic approach to inhibit this critical checkpoint. More broadly, this work identifies ERAP1/2 as druggable intracellular enzymes that could be targeted using small molecules to inactivate a cell-surface inhibitory pathway and represents a novel approach to therapeutic modulation of immune responses.