ID #534 Overcoming TGFβ-mediated immunosuppression to enhance B7-H3 CAR-T cell therapy in Group 3 Medulloblastoma

Abstract Group 3 medulloblastoma (G3 MB) is an aggressive poor prognosis paediatric brain tumour. Chimeric Antigen Receptor (CAR) T-cell therapy targeting B7H3, an immune checkpoint molecule overexpressed in medulloblastoma and minimally expressed in healthy brain tissues, represents a promising therapeutic. However, the immunosuppressive tumour microenvironment (TME), often mediated by transforming growth factor (TGFβ), limits CAR T-cell efficacy and persistence. We hypothesised that inhibition of TGFβ signalling through blockade of TGFβ receptor I (TGFBRI) could preserve CAR T-cell function and enhance anti-tumour activity in G3 MB. TGFβ pathway activation was assessed in both immunocompetent and immunodeficient MB mouse models, as well as in patient tumour samples using immunohistochemical detection of phosphorylated SMAD3 (pSMAD3). Anti-B7H3 CAR T-cells were generated from healthy donor PBMCs using gamma-retroviral transduction, either alone or co-expressing a dominant negative TGFBRII (DNR). CAR T-cells functionality was evaluated in in vitro co-culture assays with G3 MB cell lines in the presence or absence of exogenous TGFβ (10ng/ml). Pharmacological inhibition of TGFβ signalling was tested using the selective TGFBRI inhibitors Vactosertib and LY3200882. Robust TGFβ pathway activation was observed across preclinical models and patient samples, with significantly higher pSMAD3 levels in Group 3 tumours compared with other medulloblastoma subgroups (SHH n = 71, Group 3 n = 63, Group 4 n = 71, WNT n = 15). Exogenous TGFβ markedly impaired CAR T-cell proliferation, phenotype and cytokine release; these effects were reversed by DNR expression. Both TGFBRI inhibitors were well tolerated, preserved CAR T-cell expansion and phenotype, and restored effector function in TGFβ-rich conditions. Drug treatment protected CAR-T effector function (cytokine release and proliferation) upon repeated tumour and TGFβ challenge. In vivo studies are evaluating efficacy, safety, feasibility, and pharmacokinetic and pharmacodynamic profiles. These findings provide an initial preclinical rationale for combining TGFβ-pathway inhibition with anti-B7H3 CAR T-cell therapy in G3 medulloblastoma.