Inhibition of the Apelin/APJ Axis Modulates Type 1 Cytotoxic T Cell and Type 2 Cytotoxic T‐Related Immune Responses in an Animal Model of Metastatic Breast Cancer

Apelin, an adipose-derived peptide, is overexpressed in the microenvironment of various tumors, modulating anti-tumor immune responses. In the immune system, cytotoxic T cells perform a fundamental role in cancerous cell elimination. Therefore, inhibiting apelin/APJ can prevent apelin-induced immunosuppression. In this study, we investigated the impact of an apelin receptor antagonist, ML221, on modulating type 1 cytotoxic T (TC1) and TC2 lymphocyte-associated parameters in mice with breast cancer (BC). BC was generated in BALB/c mice by subcutaneous inoculation of 4T1 malignant cells. Phosphate-buffered saline (PBS) or ML221 was administered to the BC-bearing animals for 21 days, starting at 15 days following tumor cell injection. After animal euthanasia on day 37, serum IL-12/IL-4 concentrations and the frequency of splenic TC1/TC2 cells were measured by ELISA and flow cytometry techniques, respectively. BC metastasis to the liver was examined by hematoxylin and eosin staining. Treatment of BC animals with ML221 was more efficient in decreasing tumor growth (p < 0.002), avoiding liver metastases (p < 0.0001), and enhancing the rate of survival (p = 0.001) in comparison to the non-treated group. When compared to the non-treated group, treatment with ML221 substantially raised the frequency of TC1 cells and the TC1/TC2 ratio in the spleen (p < 0.01). Serum IL-12 concentrations and the IL-12/IL-4 ratio were likewise higher in BC mice treated with ML221 (p < 0.05). TC1-associated anti-tumor responses can be improved by apelin/APJ axis suppression, which can contribute to decreasing tumor growth and progression.