Synergy between Toxoplasma gondii type I Delta GRA17 immunotherapy and PD-L1 checkpoint inhibition triggers the regression of targeted and distal tumors

Background In this study, we hypothesize that the ability of the protozoan Toxoplasma gondii to modulate immune response within the tumor might improve the therapeutic effect of immune checkpoint blockade. We examined the synergetic therapeutic activity of attenuated T. gondii RH Delta GRA17 strain and programmed death ligand-1 (PD-L1) treatment on both targeted and distal tumors in mice. Methods The effects of administration of T. gondii RH Delta GRA17 strain on the tumor volume and survival rate of mice bearing flank B16-F10, MC38, or LLC tumors were studied. We characterized the effects of Delta GRA17 on tumor biomarkers' expression, PD-L1 expression, immune cells infiltrating the tumors, and expression of immune-related genes by using immunohistochemistry, immunofluorescence, flow cytometry, NanoString platform, and real-time quantitative PCR, respectively. The role of immune cells in the efficacy of Delta GRA17 plus PD-L1 blockade therapy was determined via depletion of immune cell subtypes. Results Treatment with T. gondii Delta GRA17 tachyzoites and anti-PD-L1 therapy significantly extended the survival of mice and suppressed tumor growth in preclinical mouse models of melanoma, Lewis lung carcinoma, and colon adenocarcinoma. Attenuation of the tumor growth was detected in the injected and distant tumors, which was associated with upregulation of innate and adaptive immune pathways. Complete regression of tumors was underpinned by late interferon-gamma-producing CD8(+) cytotoxic T cells. Conclusion The results from these models indicate that intratumoral injection of Delta GRA17 induced a systemic effect, improved mouse immune response, and sensitized immunologically 'cold' tumors and rendered them sensitive to immune checkpoint blockade therapy.