Analysis of various human cell lines revealed that IL-1β-induced iNOS-dependent reduction of IDO1 mRNA expression occurred in brain cell lines (A172; glioblastoma, IMR-32; neuroblastoma, and T98G; glioblastoma) and liver cell lines (Huh7 and HepG2), but not in other cell lines.Moreover, co-culturing type II <i>T. gondii</i>-infected THP-1 human monocytes with the brain cell lines inhibited the IDO1-mediated anti-<i>T. gondii</i> response in a GRA15-dependent manner.
RT response in glioblastoma involves immune stimulation, reflected by increases in activated and cytotoxic T cells, which was balanced by immune checkpoint reactivation, reflected by an increase in IDO1 expression and regulatory T cells (Treg).
However, rather than abandoning immune checkpoint inhibitor treatment for glioblastoma, which has shown promise in other types of cancer, ongoing studies are currently evaluating this therapeutic class when combined with other agents.<b>Experimental Design:</b> Here, we investigated immunocompetent orthotopic mouse models of glioblastoma treated with the potent CNS-penetrating IDO1 enzyme inhibitor, BGB-5777, combined with anti-PD1 mAb, as well as radiotherapy, based on our recent observation that tumor-infiltrating T cells directly increase immunosuppressive IDO1 levels in human glioblastoma, the previously described reinvigoration of immune cell functions after PD-1 blockade, as well as the proinflammatory effects of radiation.<b>Results:</b> Our results demonstrate a durable survival benefit from this novel three-agent treatment, but not for any single- or dual-agent combination.