Molecular mechanism of phosphopeptide neoantigen immunogenicity

Abstract

Altered protein phosphorylation in cancer cells often leads to surface presentation of phosphopeptide neoantigens. However, their role in cancer immunogenicity remains unclear. Here we describe a mechanism by which an HLA-B*0702-specific acute myeloid leukemia phosphoneoantigen pMLL _747-755 (EPR(pS)PSHSM) is recognized by cognate TCR27, which is a candidate for immunotherapy of AML. We show that the replacement of phosphoserine P ₄ with serine or phosphomimetics does not affect the pMHC conformation or peptide-MHC affinity but abrogates the TCR27-dependent T cell activation and weakens binding between TCR27 and pMHC. We determined the crystal structures for TCR27 and cognate pMHC, mapped the pMHC-TCR interface by TROSY-NMR, generated a ternary pMHC-TCR complex using information-driven protein docking, and identified key polar interactions between phosphate group at P ₄ and TCR27 that are crucial for ternary complex stability and TCR27 specificity. These data will support development of cancer immunotherapy through target expansion and TCR optimization. *The authors would like to note that Yury Patskovsky and Aswin Natarajan contributed equally.