Abstract PR03: Analysis of immune infiltrate identifies checkpoint blockade and TLR3 activation as efficient synergistic combination of immunotherapy in rhabdoid tumors
Authors: Leruste Amaury , Han Zhi-Yan , Tauziède-Espariat Arnault , Caudana Pamela , Waterfall Joshua J. , Andrianteranagna Mamy , Sedlik Christine , Ramos Rodrigo , Viel Sophie , Chauvin Céline , Delattre Olivier , Piaggio Eliane , Bourdeaut Franck 
Published: February 2019

Abstract
Rationale: Rhabdoid tumors (RTs) are highly aggressive cancers of infancy, arising from the central nervous system (atypical teratoid rhabdoid tumors, AT/RT) and other miscellaneous extracranial locations. RTs are characterized by the biallelic inactivation of the SMARCB1 tumor suppressor gene and an otherwise highly stable genome. Prospective trials based on intensive multimodal therapies have led to few survivals, and the side effects are a great matter of concern. In the prospect of new therapeutic strategies, we analyzed the immune contexture of both human and mouse RT. We subsequently explored in vivo targeting of the immune infiltrate in order to determine the best immunotherapy strategy adapted to the specific immune context.
Methods: We first analyzed the immune infiltrate of a cohort of human tumor samples, including both AT/RT and extracranial RT, by immunohistochemistry, expression profiling, and flow cytometry. In parallel, we performed the same analyses on tumors obtained from the genetically engineered mouse model (GEMM) of RT Smarcb1flox/flox;Rosa26-CreERT2 previously established in our lab, and syngeneic heterotopic engraftment models that we derived from the GEMM. After identifying new therapeutic targets, we validated them in preclinical models of RT.
Results: Despite a well-established low mutational burden and stable genome, we found that both human and mouse RT are highly infiltrated by immune cells of both lymphoid and myeloid lineages. Tumor-infiltrating lymphocytes consist in equal proportion of CD4+ and CD8+ T cells with few regulatory T cells. We observed that T cells coexpress PD-1 and TIM-3 activation/exhaustion markers, while tumor and myeloid cells express their respective ligands, i.e., PD-L1 and Galectin 9. Consistently, we found that blockade of the PD-1/PD-L1 pathway was able to impair tumor growth in syngeneic models of RT and induced memory against second engraftment. Furthermore, combination of anti-TIM-3 and anti-PD-1 shows synergistic activity. In the context of RT syngeneic engraftment, we observed the expansion of aberrant myelopoiesis corresponding to suppressive myeloid cells. Tumor-infiltrating myeloid transcriptomic signatures correlated with the expression of chemokines known to recruit such myeloid cells. We found that tumor-infiltrating myeloid cells were a particularly intense and constant feature both in human and mouse RT, consisting primarily of macrophages of a protumoral phenotype. Targeting this myeloid infiltrate by TLR3 activation with poly(I:C) induced a potent antitumor effect. Combination of poly(I:C) with PD-1 blockade had a synergistic effect, leading to complete tumor regression in around 90% of treated mice.
Conclusion: We show here for the first time that rhabdoid tumors are highly immune-infiltrated tumors susceptible to PD-1/PD-L1 and TIM-3 blockade, and to reprogramming of myeloid suppressive cells. Immunogenicity of rhabdoid tumor cells is a surprising observation in the field of pediatric tumors; we hypothesize that rhabdoid tumor recognition by T cells is initiated by a SMARCB1-dependent mechanism under investigation.
Citation Format: Amaury Leruste, Zhi-Yan Han, Arnault Tauziède-Espariat, Pamela Caudana, Joshua J. Waterfall, Mamy Andrianteranagna, Christine Sedlik, Rodrigo Ramos, Sophie Viel, Céline Chauvin, Olivier Delattre, Eliane Piaggio, Franck Bourdeaut. Analysis of immune infiltrate identifies checkpoint blockade and TLR3 activation as efficient synergistic combination of immunotherapy in rhabdoid tumors [abstract]. In: Proceedings of the AACR Special Conference: Pediatric Cancer Research: From Basic Science to the Clinic; 2017 Dec 3-6; Atlanta, Georgia. Philadelphia (PA): AACR; Cancer Res 2018;78(19 Suppl):Abstract nr PR03.