Avirulent ME49Δgra5Protecting Hosts AgainstToxoplasma gondiiInfection and Breast Tumors

Toxoplasma gondii is the causative agent of toxoplasmosis, a zoonotic disease that poses a threat to human health and a considerable loss to livestock farming. At present, clinical therapeutic drugs mainly target T. gondii tachyzoites and fail to eradicate bradyzoites. Developing a safe and effective vaccine against toxoplasmosis is urgent and important. T. gondii possesses dense granule organelles that secrete immunogenic dense granule proteins (GRAs). GRA5 localizes to the parasitophorous vacuole membrane (PVM) in the tachyzoite stage and to the cyst wall in the bradyzoite stage. Here we report that gra5 knockout T. gondii strain is avirulent and fails to form cysts, but stimulates sero-conversion in mice. We next investigated the protective efficacy of ME49Δ gra5 vaccination against T. gondii infection. All the immunized mice survived the challenge infection with either wild-type RH, ME49, VEG tachyzoites or ME49 cysts. Remarkably, ME49Δ gra5 tachyzoites inoculation in situ attenuated the growth of murine breast tumor (4T1) in mice and prevented 4T1’s lung metastasis. ME49Δ gra5 inoculation might upregulate the levels of Th1 cytokines and tumor-infiltrating T cells in tumor microenvironment (TME) and trigger anti-tumor responses by increasing the number of NK, B, T cells, macrophages, and dendritic cells (DCs) in spleen. Collectively, the results support the potential of ME49Δ gra5 as attenuated live vaccine against T. gondii infection and breast cancer. Author summary The zoonotic toxoplasmosis poses a great disease burden to humans and big loss to livestock farming. At present, clinical therapeutic drugs mainly target T. gondii tachyzoites but have no effect on bradyzoites. A safe and effective vaccine against toxoplasmosis is urgent and important. Dense granule proteins (GRAs) are a group of immunoactive proteins secreted by T. gondii dense granule organelles. GRA5 is located on the parasitophorous vacuole membrane (PVM) at the tachyzoite stage and on the cyst wall at the bradyzoite stage. We discovered that the gra5 knockout T. gondii strain became avirulent and failed to form cysts in mice. ME49Δ gra5 immunized mice survived the challenge infection of wild-type RH, ME49, and VEG tachyzoites, and ME49 cysts. Meanwhile, ME49Δ gra5 tachyzoites inoculation in situ remarkably attenuated the growth of murine breast tumor (4T1) in mice including the injected and distant tumors, as well as prevented 4T1’s lung metastasis. This results support the potential of ME49Δ gra5 as attenuated live vaccine against T. gondii infection and breast cancer.