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Background Breast cancer is the most common cause of cancer-related death among women, and prognosis is especially poor for patients with triple-negative breast cancer (TNBC); therefore, there is an urgent need for new effective therapies. Recent studies have demonstrated that the uracil auxotroph  Toxoplasma gondii  vaccine displays anti-tumor effects. Here, we examined the immunotherapy effects of an attenuated uracil auxotroph strain of  T. gondii  against 4T1 murine breast cancer. Methods We constructed a uracil auxotroph  T. gondii  RH strain via orotidine 5′-monophosphate decarboxylase gene deletion (RH-Δ ompdc ) with CRISPR/Cas9 technology. The strain’s virulence in the  T. gondii -infected mice was determined in vitro and in vivo by parasite replication assay, plaque assay, parasite burden detection in mice peritoneal fluids and survival analysis. The immunomodulation ability of the strain was evaluated by cytokine detection. Its anti-tumor effect was evaluated after its in situ inoculation into 4T1 tumors in a mouse model; the tumor volume was measured, and the 4T1 lung metastasis was detected by hematoxylin and eosin and Ki67 antibody staining, and the cytokine levels were measured by an enzyme-linked immunosorbent assay. Results The RH-Δ ompdc  strain proliferated normally when supplemented with uracil, but it was unable to propagate without the addition of uracil and in vivo, which suggested that it was avirulent to the hosts. This mutant showed vaccine characteristics that could induce intense immune responses both in vitro and in vivo by significantly boosting the expression of inflammatory cytokines. Inoculation of RH-Δ ompdc  in situ into the 4T1 tumor inhibited tumor growth, reduced lung metastasis, promoted the survival of the tumor-bearing mice and increased the secretion of Th1 cytokines, including interleukin-12 (IL-12) and interferon-γ (INF-δ), in both the serum and tumor microenvironment (TME). Conclusion Inoculation of the uracil auxotroph RH-Δ ompdc  directly into the 4T1 tumor stimulated anti-infection and anti-tumor immunity in mice, and resulted in inhibition of tumor growth and metastasis, promotion of the survival of the tumor-bearing mice and increased secretion of IL-12 and IFN-γ in both the serum and TME. Our findings suggest that the immunomodulation caused by RH-Δ ompdc  could be a potential anti-tumor strategy. Graphical abstract

A uracil auxotroph Toxoplasma gondii exerting immunomodulation to inhibit breast cancer growth and metastasis