Abstract 2982: Somatic TP53 mutations alter the immune microenvironmentafter chemotherapy in breast cancer
Authors: Nixon Mellissa J. , Estrada V. Monica , Opalenki Susan R. , Hicks Donna , Korrer Michael , Sanders Melinda E. , Salgado Roberto , Kim Young , Cook Rebecca , Arteaga Carlos L. , Balko Justin M. 
Published: August 2018

Background: Neoadjuvant chemotherapy (NAC), followed by surgery, is the standard of care for triple-negative breast cancer (TNBC) patients. Unfortunately, only 30% achieve a pathologic complete response (pathCR, no evidence of invasive disease in the breast after NAC). In patients who do not achieve a pathCR, tumor-infiltrating lymphocytes (TILS) correlate with improved survival. This suggests there is an immune response to NAC in a subset of patients, which may be augmented by immune checkpoint inhibitors (ICI). Clinical trials are under way to examine the efficacy of ICI in breast cancer; therefore it is vital to identify biomarkers of response to better identify patients who may benefit from these modalities. Because many TNBC patients harbor somatic TP53 mutations and TP53 has been shown to be involved in immunity, we investigated the role of TP53 mutations on the immune microenvironment of breast cancer patients with residual disease.

Methods: We examined 34 matched pre- and post-NAC primary breast cancer. TILs were scored by a pathologist. RNA and DNA were extracted and analyzed using nanoString Pan-Cancer Immune panel (&gt;700 genes) and ImmunoSeq T-cell receptor (TCR) sequencing, respectively. CRISPR technology was used in mouse tumor cell lines to generate Trp53 mutations commonly found in TNBC. Cells were treated with chemotherapy in vitro to determine tumor-specific changes in cytokines and PD-L1 expression. To determine alterations to the immune microenvironment, isogenic cell lines were injected into syngeneic mice and expression of immune recruiting cytokines and immune checkpoint molecules by the tumor as well as immune phenotyping of TILs after NAC was performed.

Results: TILs in the residual disease of TNBC patients correlated with improved survival after surgery, as expected. A third of patients had increased immune gene signatures after NAC, which correlated with increased TCR clonality, increased overall survival, and increased T cell-recruiting chemokine expression (CXCL9/10, etc.). In isogenic murine breast cancer cells, Trp53 mutant cells had higher levels of immune-recruiting chemokines after chemotherapy treatment in vitro, but demonstrated loss of Cdkn1a, a canonical p53 target gene. Trp53 mutant cells also exhibited increased PD-L1 expression after NAC compared to Trp53 wild-type cells. Mouse studies examining the immune infiltrate are ongoing.

Conclusions: Our work suggests that NAC induces immune gene signatures in a subset of TNBC that is correlated with a better prognosis. TP53 mutations in the tumor may contribute to an increased immune infiltrate after chemotherapy through upregulation of chemokine expression and promote T cell exhaustion through upregulation of the immune checkpoint PD-L1. Ongoing in vivo studies with chemotherapy and ICI will indicate if p53-mutant tumors have a better response to ICI after NAC.

Citation Format: Mellissa J. Nixon, V. Monica Estrada, Susan R. Opalenki, Donna Hicks, Michael Korrer, Melinda E. Sanders, Roberto Salgado, Young Kim, Rebecca Cook, Carlos L. Arteaga, Justin M. Balko. Somatic TP53 mutations alter the immune microenvironmentafter chemotherapy in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2982.