Exploring the impact of SBRT on dendritic cells in the tumor microenvironment of triple-negative breast cancer: Insights from combination with thymosin α1.

e13128 Background: Triple-negative breast cancer (TNBC) lacks estrogen receptor (ER), progesterone receptor (PR), and HER2 expression, which limits treatment options and results in a poor prognosis. Stereotactic body radiotherapy (SBRT) is a critical treatment modality that induces the release of tumor-associated antigens (TAAs) and enhances immune responses. However, radiotherapy may impair dendritic cell (DC) function, which is essential for antigen presentation and T-cell activation. Thymosin α1 (Tα1), an immunomodulatory agent, has demonstrated potential in enhancing T-cell responses and counteracting immune suppression induced by chemotherapy. This study investigates whether combining Tα1 with SBRT can improve radiotherapy efficacy in triple-negative breast cancer (TNBC) by modulating the immune microenvironment. Methods: A mouse model of triple-negative breast cancer (TNBC) was treated with different methods. Tumor volume and weight were measured to evaluate therapeutic outcomes. Changes in the immune microenvironment within tumors and draining lymph nodes were assessed using flow cytometry, immunofluorescence, ELISA, Western blot, and transcriptome sequencing. Key parameters included the expression of antigen-presenting and co-stimulatory molecules on dendritic cells, the proportion of mature dendritic cells, and T-cell infiltration and functionality. Statistical analysis was performed using ANOVA or t-tests, with significance defined as p < 0.05. Results: SBRT alone reduced the expression of MHC class I/II molecules in tumor tissues, impairing dendritic cell (DC)-mediated antigen presentation. Radiotherapy affected DC function, leading to an increased proportion of CD8 + and CD4 + double-positive T cells within the local tumor microenvironment. However, the combination of Tα1 and SBRT inhibited tumor growth, restored dendritic cell antigen presentation, and enhanced T-cell infiltration. Specifically, combination therapy elevated the expression of MHC class II, CD80, and CD86, while promoting the development of CD4 + T-cell subsets and increasing the proportion of IFN-ɣ + CD4 + T cells. Conclusions: Tα1 enhances SBRT efficacy by improving dendritic cell-mediated antigen presentation and promoting robust T-cell responses. These findings suggest that the Tα1+SBRT combination could be a promising immunotherapeutic strategy for TNBC.