Research Article

Targeting a neoantigen derived from a common TP53 mutation

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Science  05 Mar 2021:
Vol. 371, Issue 6533, eabc8697
DOI: 10.1126/science.abc8697

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Turning a tumor suppressor into a target

Tumor-suppressor genes such as TP53 (tumor protein P53) play key roles in the pathogenesis of cancer but, unfortunately, they are difficult to target because they do not create an overactive protein that can be inhibited with a drug. Hsiue et al. discovered a way to target a cancer-associated mutant form of the p53 protein using the body's own immune system (see the Perspective by Weidanz). The authors identified a distinct fragment of this mutant protein, characterized the structural basis for its presentation to T cells, and designed a bispecific antibody to stimulate T cell killing of p53-mutant cancer cells.

Science, this issue p. eabc8697; see also p. 996

Structured Abstract

INTRODUCTION

TP53 (tumor protein P53), a tumor suppressor gene, is the most commonly mutated cancer-driver gene. Nevertheless, drugs that target mutant p53, the protein product, remain unavailable today, decades after the discovery of its critical role in cancer. Although drugs that inactivate proteins encoded by mutant oncogenes, such as epidermal growth factor receptor (EGFR) or BRAF, are available, proteins encoded by tumor suppressor genes are already inactivated through mutation. Reactivating such proteins by use of pharmacologic agents is very challenging. Thus, new approaches to target such inactivated proteins, including those encoded by TP53, are actively being sought.

RATIONALE

We attempted to develop an immunotherapeutic approach to target proteins encoded by a mutant TP53 gene. p53 is an intracellular protein, primarily located within the nucleus, thus out of reach for conventional antibody-based therapies. However, proteins are degraded into peptides by the proteasome, and a fraction of these peptides can be presented by the human leukocyte antigen (HLA) on the cell surface. This in principle makes it possible for suitably designed proteins to recognize peptide fragments of intracellular proteins when bound to HLA on the cell surface. The arginine-to-histidine substitution at codon 175 (R175H) is the most common TP53 mutation and is the most frequent mutation in any tumor suppressor gene. The peptide HMTEVVRHC (mutant amino acid underlined), derived from the p53R175H mutation, can bind to a particular HLA allele (HLA-A*02:01) and form a peptide-HLA complex on the cell surface. HLA-A*02:01 is the most frequent HLA-A type in the U.S. population. Thus, the p53R175H/HLA-A*02:01 complex is a particularly attractive therapeutic target, shared among many cancer patients. However, such neoantigen peptide-HLA complexes typically exist at low density on cell surfaces, and a potent therapeutic format will be required to achieve meaningful therapeutic effects. T cells can be activated in the presence of a very low number of antigens. We therefore attempted to generate a T cell–based therapy that links T cells to cancer cells through a newly developed antibody that specifically binds to the p53R175H peptide-HLA complex.

RESULTS

Using a large phage library that displays diverse antibody variable fragments, we identified H2, an antibody fragment that binds with high affinity to the p53R175H peptide-HLA complex but not to its wild-type counterpart. We converted H2 into a T cell–based immunotherapeutic agent—a bispecific single-chain diabody—by fusing it with an antibody fragment that binds to the T cell receptor–CD3 complex on T cells. This bispecific antibody binds to the p53R175H peptide-HLA complex with an affinity [dissociation constant (Kd) = 86 nM] higher than that typical for T cell receptors and redirects T cells to recognize cancer cells that express the complex. Despite a very low density of the peptide-HLA complex on the cell surface, as quantified with mass spectrometry, the bispecific antibody effectively activated T cells to secrete cytokines and kill target cancer cells. This killing was dependent on the expression of both the cognate HLA and particular TP53 mutation. The bispecific antibody also resulted in regression of human xenograft tumors in mice, both when treatment was initiated soon after tumor engraftment and when the tumors were already well established. Structure of the H2 antibody fragment with the p53R175H peptide-HLA complex showed that H2 formed a cage-like structure around the mutant amino acid (His175) and one adjacent amino acid (Arg174). The stability imparted by this cage provided the structural basis for the highly specific recognition of the mutant peptide-HLA complex by H2.

CONCLUSION

We developed an antibody-based therapeutic approach that targets a neoantigen derived from a common TP53 mutation in a highly specific fashion. It effectively activated T cells and lysed tumor cells both in vitro and in vivo despite the low antigen density on their surface. This approach could in theory be used to treat cancers containing other mutations that are difficult to target by conventional means.

Mechanism of action of bispecific antibody H2-scDb.

A mutant peptide containing the common TP53 mutation R175H can form a complex with HLA-A*02:01 on the tumor cell surface. H2-scDb, a bispecific antibody, binds to the p53R175H peptide-HLA complex with one arm and the T cell receptor complex with the other arm, activating T cells to kill tumor cells.

CREDIT: E. COOK

Abstract

TP53 (tumor protein p53) is the most commonly mutated cancer driver gene, but drugs that target mutant tumor suppressor genes, such as TP53, are not yet available. Here, we describe the identification of an antibody highly specific to the most common TP53 mutation (R175H, in which arginine at position 175 is replaced with histidine) in complex with a common human leukocyte antigen–A (HLA-A) allele on the cell surface. We describe the structural basis of this specificity and its conversion into an immunotherapeutic agent: a bispecific single-chain diabody. Despite the extremely low p53 peptide-HLA complex density on the cancer cell surface, the bispecific antibody effectively activated T cells to lyse cancer cells that presented the neoantigen in vitro and in mice. This approach could in theory be used to target cancers containing mutations that are difficult to target in conventional ways.

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