BETting on BRD4 inhibition to combat adaptive resistance to CAR T cell therapy in glioblastoma
Authors: Caitlin R. Hopkins, Joseph A. Fraietta
Journal: Molecular Therapy (2021)

In this issue of Molecular Therapy, Xia et al.1Xia L. Liu J.-Y. Zheng Z.-Z. Chen Y.-J. Ding J.-C. Hu Y.-H. Hu G.-S. Xia N.-S. Liu W. BRD4 inhibition boosts the therapeutic effects of epidermal growth factor receptor-targeted chimeric antigen receptor T cells in glioblastoma.Mol. Ther. 2021; 29: 3011-3026Google Scholar demonstrate that chimeric antigen receptor (CAR) T cells engineered to recognize the epidermal growth factor receptor (EGFR) control glioblastoma multiforme (GBM) for a short time but then paradoxically potentiate adaptive resistance in tumors, whereby a battery of genes encoding various immunosuppressive molecules (i.e., programmed death-ligand 1 [PD-L1], programmed death ligand 2 [PD-L2], herpes virus entry mediator [HVEM], galectin-9 [Gal-9], indoleamine 2,3-dioxygenase 1 [IDO1], etc.) are upregulated during long-term therapy. Through mechanistic analyses, they demonstrate that the bromodomain and extraterminal (BET) protein, BRD4, a chromatin modifier functioning at both promoter and enhancer regions, was required for the activation of these immunosuppressive genes. Through an elegant sequence of experiments, the authors demonstrated that combining CAR T cells with the BRD4 inhibitor, JQ1, increased the efficacy of glioblastoma-directed cellular therapy (Figure 1). Malignant brain tumors account for more deaths than renal cancer or melanoma and are currently the leading cause of cancer-related mortality in children and young adults.2Curtin S.C. Minino A.M. Anderson R.N. Declines in cancer death rates among children and adolescents in the United States, 1999-2014.NCHS Data Brief. 2016; : 1-8Google Scholar,3Siegel R.L. Miller K.D. Fuchs H.E. Jemal A. Cancer statistics, 2021.CA Cancer J. Clin. 2021; 71: 7-33Google Scholar Current therapeutic approaches for GBM, the most common and malignant form of these tumors, are non-specific and limited by toxicity to systemic tissues and structures of the surrounding eloquent brain.4Nam J.Y. de Groot J.F. Treatment of glioblastoma.J. Oncol. Pract. 2017; 13: 629-638Google Scholar,5Proescholdt M.A. Schödel P. Doenitz C. Pukrop T. Höhne J. Schmidt N.O. Schebesch K.M. The management of brain metastases-systematic review of neurosurgical aspects.Cancers (Basel). 2021; 13: 1616Google Scholar Despite aggressive approaches, such as surgical resection followed by radiation and chemotherapy, GBM inexorably progresses and remains universally fatal. For these reasons, GBM is a major unresolved health problem and a therapeutic challenge. Thus, it is important to develop novel alternative therapies for this deadly disease. Brain tumors are known to express specific antigens that make them ideal targets for CAR T cell immunotherapy. CAR T cells have shown remarkable efficacy against relapsed and refractory hematological malignancies but have not yet proven nearly as effective in treating solid tumors, such as GBM. In clinical trials of systemically administered GBM-directed CAR T cells, engraftment in the peripheral blood, trafficking to the brain, and/or elicitation of antigen-directed activity have been observed.6O'Rourke D.M. Nasrallah M.P. Desai A. Melenhorst J.J. Mansfield K. Morrissette J.J.D. Martinez-Lage M. Brem S. Maloney E. Shen A. et al.A single dose of peripherally infused EGFRvIII-directed CAR T cells mediates antigen loss and induces adaptive resistance in patients with recurrent glioblastoma.Sci. Transl. Med. 2017; 9: eaaa0984Google Scholar, 7Goff S.L. Morgan R.A. Yang J.C. Sherry R.M. Robbins P.F. Restifo N.P. Feldman S.A. Lu Y.C. Lu L. Zheng Z. et al.Pilot trial of adoptive transfer of chimeric antigen receptor-transduced T cells targeting EGFRvIII in patients with glioblastoma.J. Immunother. 2019; 42: 126-135Google Scholar, 8Ahmed N. Brawley V. Hegde M. Bielamowicz K. Kalra M. Landi D. Robertson C. Gray T.L. Diouf O. Wakefield A. et al.HER2-specific chimeric antigen receptor-modified virus-specific T cells for progressive glioblastoma: a phase 1 dose-escalation trial.JAMA Oncol. 2017; 3: 1094-1101Google Scholar However, multiple challenges unique to GBM tumors likely contribute to the lack of therapeutic efficacy, including poor persistence and function in the toxic tumor microenvironment (TME), which becomes even more inhibitory after CAR T cell infusion.6O'Rourke D.M. Nasrallah M.P. Desai A. Melenhorst J.J. Mansfield K. Morrissette J.J.D. Martinez-Lage M. Brem S. Maloney E. Shen A. et al.A single dose of peripherally infused EGFRvIII-directed CAR T cells mediates antigen loss and induces adaptive resistance in patients with recurrent glioblastoma.Sci. Transl. Med. 2017; 9: eaaa0984Google Scholar CAR-T-cell-based therapeutic approaches for GBM therefore need to address how to increase T cell persistence and effector function in a TME that gradually upregulates immunosuppressive barriers due to adaptive resistance following adoptive transfer. BET polypeptides (e.g., BRD2, BRD3, and BRD4) contain tandem bromodomain motifs that interact with acetylated lysine residues in histones, thereby linking changes in chromatin structure with gene transcription. BET proteins therefore work as "readers" of the histone code and regulate chromatin signaling through the recruitment of transcriptional co-activators and repressors.9Fujisawa T. Filippakopoulos P. Functions of bromodomain-containing proteins and their roles in homeostasis and cancer.Nat. Rev. Mol. Cell Biol. 2017; 18: 246-262Google Scholar Due to the large number of genes and enhancers that are induced during long-term CAR T cell therapy, particularly the aforementioned immunosuppressive genes, it stands to reason that inhibiting a BRD4-modulated transcriptional program within the TME might improve the antitumor activity of EGFR CAR T cells. Accordingly, the authors show that JQ1-mediated displacement of BRD4 at active enhancers as well as enhancer-associated immunosuppressive genes augmented the potency of EGFR CAR T cells in in vitro co-culture experiments. The BRD4-specific effects of the compound were confirmed through a parallel BRD4 genetic knockdown strategy tested in the same experimental system. In in vivo models involving a GBM cell line and patient-derived xenografts, the combination of EGFR CAR T cells with JQ1 resulted in significantly increased control of tumor growth and metastasis compared to CAR T cells or JQ1 alone. Prolongation of overall survival was also observed in tumor-engrafted mice treated with a combination of CAR T cells and JQ1, relative to animals that received JQ1 or CAR T cell monotherapy.1Xia L. Liu J.-Y. Zheng Z.-Z. Chen Y.-J. Ding J.-C. Hu Y.-H. Hu G.-S. Xia N.-S. Liu W. BRD4 inhibition boosts the therapeutic effects of epidermal growth factor receptor-targeted chimeric antigen receptor T cells in glioblastoma.Mol. Ther. 2021; 29: 3011-3026Google Scholar Several approaches have been taken to improve the survival and function of CAR T cell responses, including combinations with checkpoint inhibitors and genetic engineering of the T cells to express cytokine-signaling systems (e.g., interleukin 2 [IL-2], IL-7, IL-12, IL-15, IL-18, and IL-21) or additional co-stimulatory molecules.10Rafiq S. Hackett C.S. Brentjens R.J. Engineering strategies to overcome the current roadblocks in CAR T cell therapy.Nat. Rev. Clin. Oncol. 2020; 17: 147-167Google Scholar Thus, the epigenetic modification strategy described here to attenuate CAR T cell extrinsic inhibition mediated by the immunosuppressive TME would likely need to be combined with other strategies to overcome T cell intrinsic resistance mechanisms. It is intriguing to speculate that, in addition to mitigating TME-associated adaptive resistance, JQ1 treatment may have directly boosted CAR T cell intrinsic fitness by preventing terminal differentiation or mitigating exhaustion, as has been previously reported.11Kagoya Y. Nakatsugawa M. Yamashita Y. Ochi T. Guo T. Anczurowski M. Saso K. Butler M.O. Arrowsmith C.H. Hirano N. BET bromodomain inhibition enhances T cell persistence and function in adoptive immunotherapy models.J. Clin. Invest. 2016; 126: 3479-3494Google Scholar,12Kong W. Dimitri A. Wang W. Jung I.Y. Ott C.J. Fasolino M. Wang Y. Kulikovskaya I. Gupta M. Yoder T. et al.BET bromodomain protein inhibition reverses chimeric antigen receptor extinction and reinvigorates exhausted T cells in chronic lymphocytic leukemia.J. Clin. Invest. 2021; 131: 145459Google Scholar In summary, the work performed by Xia and colleagues1Xia L. Liu J.-Y. Zheng Z.-Z. Chen Y.-J. Ding J.-C. Hu Y.-H. Hu G.-S. Xia N.-S. Liu W. BRD4 inhibition boosts the therapeutic effects of epidermal growth factor receptor-targeted chimeric antigen receptor T cells in glioblastoma.Mol. Ther. 2021; 29: 3011-3026Google Scholar identifies a novel epigenetic mechanism and an actionable pharmacological intervention to potentially overcome adaptive resistance to CAR T cell therapy in GBM. As the major limitations to the efficacy of CAR T cells for the treatment of solid tumors are multifactorial, this approach of administering engineered cells and an epigenetic modulator to render the TME less immunosuppressive will likely need to be combined with other modalities to further enhance the function and persistence of cellular therapy products. The National Cancer Institute (U54CA244711) and the Alliance for Cancer Gene Therapy (Investigator Award in Cell and Gene Therapy for Cancer) supported this work. Figure 1 was created using BioRender. BRD4 inhibition boosts the therapeutic effects of epidermal growth factor receptor-targeted chimeric antigen receptor T cells in glioblastomaXia et al.Molecular TherapyMay 28, 2021In BriefGlioblastoma is very difficult to treat, and a cure is often not possible. Scientists discovered that combination therapy with a small-molecule inhibitor and a way of immunotherapy can effectively slow down the growth and metastasis of glioblastoma in mouse models, which has great potential to be translated into the clinic. Full-Text PDF Open Access