An ESR1 neoepitope vaccine targeting resistance mutations induces cross-protective anti-tumor immune responses against ER+ Breast Cancer

Estrogen receptor a (ER) positivity defines ~75% of breast cancers (BCs), treated with a variety of estrogen-targeting therapies. However, ~25% of patients with localized disease and nearly all metastatic disease patients develop resistance to such therapies. One major mechanism of resistance occurs through gain-of-function mutations in the gene encoding ER (ESR1), which occurs after prolonged endocrine therapy. These gain-of-function mutations render ESR1 constitutively active and represent critical neoepitopes of ER+ BC. We here confirm that these mutations permit ligand-independent estrogen signaling, as well as stimulate alternate pathways to enhance BC oncogenicity in vivo. As these oncogenic ESR1-mutated genes (ESR1mut) may be a significant, predictable contributor to the development of acquired resistance to endocrine therapy, we explored the potential of vaccination targeting these neoepitopes. In our study, we confirmed that these peptides are presented by MHC complexes and capable of stimulating CD8+ T-cell responses. Using recombinant adenoviral vectors encoding ESR1mut, we also demonstrate the induction of ESR1-specific immunity to generate anti-tumor responses against ESR1mut-expressing BC. Finally, we improved the safety profile, ESR1-specific immunity and anti-tumor efficacy of our vaccines using c-terminal subunit ESR1mut targeting, which has the potential to prevent the outgrowth of resistant ER+ BCs and improve patient outcomes. Overall design: MCF7 cells were transfected with WT or mutant ESR1 and cultured with or without estrogen. Cells were harvested and replicates sequenced for gene expression changes.