4T1 breast carcinoma is a highly malignant and poorly immunogenic murine tumor model that resembles advanced breast cancer in humans, and is refractory to most immune stimulation-based treatments. We hypothesize that the ineffectiveness of immune stimulatory treatment is mediated by the suppressive effects of CD4(+)CD25(+) regulatory T (Treg) cells, which can be attenuated by engaging the glucocorticoid-induced tumor necrosis factor receptor family-related protein with its natural ligand (GITRL); further, combination treatment with existing immune stimulation regimens will augment anti-tumor immunity and eradicate metastatic 4T1 tumors in mice.A soluble homodimeric form of mouse GITRL (mIg-mGITRLs) was molecularly constructed and used to treat orthotopic 4T1 tumors established in immune-competent, syngeneic Balb/c mice. When applied in combination with adenovirus-mediated intratumoral murine granulocyte macrophage colony stimulating factor (GM-CSF) and interleukin-12 (IL-12) gene delivery plus systemic 4-1BB activation, mIg-mGITRLs attenuated the immune-suppressive function of splenic Treg cells, which led to elevated interferon-gamma (IFN-gamma) production, tumor-specific cytolytic T-cell activities, tumor rejection and long-term survival in 65% of the animals without apparent toxicities. The results demonstrate that addition of mIg-mGITRLs to an immune-stimulatory treatment regimen significantly improved long-term survival without apparent toxicity, and could potentially be clinically translated into an effective and safe treatment modality for metastatic breast cancer in patients.