Introduction: Normalization cancer immunotherapy is a new strategy to treat breast cancer. Sialic acid binding Ig-like lectin 15 (Siglec-15) is a new potential target for normalization cancer immunotherapy. In this study, we evaluated the role of Siglec-15 in breast cancer and investigated the influence of Siglec-15 on the microenvironment of infiltrating immune cells in cancer. Methods We performed immunohistochemical staining to analyse Siglec-15 expression in primary invasive breast cancer tissue microarrays. The tissue specimens were from 90 patients. Furthermore, the relationship between Siglec-15 and clinicopathological features was analysed with logistic regression and the Wilcoxon signed-rank test. The association between clinical characteristics and overall survival in The Cancer Genome Atlas (TCGA) and Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) patients was assessed. Results Immunohistochemical staining of tissue microarrays showed that Siglec-15 had higher expression in breast cancer tissues than in adjacent normal tissues. Breast cancer tissues had higher Siglec-15 expression than normal tissues did. Kaplan–Meier survival analysis suggested that triple-negative breast cancer with high Siglec-15 expression had poorer survival than those with lower Siglec-15 expression (p = 0.042). Furthermore, the high Siglec-15 expression group had low activated dendritic cells, follicular helper T cells, and M1 macrophages. Conclusions Siglec-15 had a high expression in breast cancer tissues. High Siglec-15 expression is associated with low activated dendritic cell, follicular helper T cell, and M1 macrophage proportions in breast cancer tissue and predicts poor prognosis in triple-negative breast cancer. Siglec-15 expression may be a potential prognostic molecular marker of poor survival in breast cancer.
