Data from Enhanced Antitumor Immunity via Endocrine Therapy Prevents Mammary Tumor Relapse and Increases Immune Checkpoint Blockade Sensitivity

Abstract

<div>Abstract<p>The role of active antitumor immunity in hormone receptor–positive (HR<sup>+</sup>) breast cancer has been historically underlooked. The aim of this study was to determine the contribution of the immune system to antiprogestin-induced tumor growth inhibition using a hormone-dependent breast cancer model. BALB/c-GFP<sup>+</sup> bone marrow (BM) cells were transplanted into immunodeficient NSG mice to generate an immunocompetent NSG/BM-GFP<sup>+</sup> (NSG-R) mouse model. Treatment with the antiprogestin mifepristone (MFP) inhibited growth of 59–2-HI tumors with similar kinetics in both animal models. Interestingly, MFP treatment reshaped the tumor microenvironment, enhancing the production of proinflammatory cytokines and chemokines. Tumors in MFP-treated immunocompetent mice showed increased infiltration of F4/80<sup>+</sup> macrophages, natural killer, and CD8 T cells, displaying a central memory phenotype. Mechanistically, MFP induced immunogenic cell death (ICD) <i>in vivo</i> and <i>in vitro</i>, as depicted by the expression and subcellular localization of the alarmins calreticulin and HMGB-1 and the induction of an ICD gene program. Moreover, MFP-treated tumor cells efficiently activated immature dendritic cells, evidenced by enhanced expression of MHC-II and CD86, and induced a memory T-cell response, attenuating tumor onset and growth after re-challenge. Finally, MFP treatment increased the sensitivity of HR<sup>+</sup> 59–2-HI tumor to PD-L1 blockade, suggesting that antiprogestins may improve immunotherapy response rates. These results contribute to a better understanding of the mechanisms underlying the antitumor effect of hormonal treatment and the rational design of therapeutic combinations based on endocrine and immunomodulatory agents in HR<sup>+</sup> breast cancer.</p>Significance:<p>Antiprogestin therapy induces immunogenic tumor cell death in PRA-overexpressing tumors, eliciting an adaptive immune memory response that protects mice from future tumor recurrence and increases sensitivity to PD-L1 blockade.</p></div>