Tehran University of Medical Sciences Iranian Journal of Allergy, Asthma and Immunology 1735-1502 21 3 2022 06 18 287 299 EN Davoud Rostamzadeh Shiraz Institute for Cancer Research, School of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran AND Department of Immunology, School of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran Mohammad Reza Haghshenas Shiraz Institute for Cancer Research, School of Medicine, Shiraz Universit of Medical Sciences, Shiraz, Iran Mahdi Samadi Shiraz Institute for Cancer Research, School of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran Zahra Mojtahedi Shiraz Institute for Cancer Research, School of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran Zohreh Babaloo Department of ImmunologDepartment of Immunology, School of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran AND Immunology Unit, Drug Applied Research Center, Tabriz University of Medical Sciences, Tabriz, Iran Abbas Ghaderi Shiraz Institute for Cancer Research, School of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran 2021 05 23 2022 02 27 To investigate the effects of everolimus, a mechanistic/mammalian target of rapamycin (mTOR) inhibitor, on tumor growth and immune response in a mouse model of breast cancer. Human hormone receptor-positive (HR+)/human epidermal growth receptor 2-negative (HER2-) MC4-L2 cell line was used to establish a mouse model of breast cancer. The inhibitory effects of high (10 mg/kg) and low (5 mg/kg) doses of everolimus were investigated on tumor growth. Additionally, the frequency of CD4+Foxp3+ regulatory T cells (Tregs), CD8+Foxp3+ Tregs, and CD4+ and CD8+ T cells expressing cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) was explored by flow cytometry in bone marrow, lymph nodes, and spleen. Our results showed that both 10 mg/kg and 5 mg/kg doses of everolimus efficiently inhibited tumor growth, resulting in reduced breast tumor volume. In addition, it was revealed that everolimus-treated mice induced a higher frequency of CD4+Foxp3+ Tregs, CD8+Foxp3+ Tregs, and CD4+Foxp3+CTLA-4+ Tregs as well as CD4+ and CD8+ T cells expressing CTLA-4 in their bone marrow, lymph nodes, and spleen compared with standard control (vehicle-treated) in a dose-dependent manner. Furthermore, we found that everolimus treatment with 10 mg/kg and 5 mg/kg increased the frequency of Helios+Foxp3+ Tregs in the bone marrow of treated mice compared with the control group. Our results indicate that treatment with everolimus not only inhibits tumor growth but also exerts an immunomodulatory effect by inducing Tregs in the lymphoid organs of breast cancer-bearing mice. The combination of therapy with other anti-cancer agents may negate immune suppression and improve the efficacy of mTOR-targeted breast cancer therapy. https://ijaai.tums.ac.ir/index.php/ijaai/article/view/3231 https://ijaai.tums.ac.ir/index.php/ijaai/article/download/3231/1820