Immune checkpoint pathways in glioblastoma: a diverse and evolving landscape

Abstract

Immune checkpoint (IC) inhibition in glioblastoma (GBM) has not shown promising results in the last decade compared to other solid tumors. Several factors contributing to the lack of immunotherapy response include the profound immunosuppressive nature of GBM, highly redundant signaling pathways underlying immune checkpoints, and the negative immunogenic impact of current standard of care on the tumor microenvironment. In this review, we will discuss various ICs in the context of GBM, their interplay with the tumor immune microenvironment, relevant pre-clinical and clinical studies, and the impact of current treatment modalities on GBM IC blockade therapy. Understanding the molecular mechanisms that drive ICs, and how they contribute to an immunosuppressive tumor microenvironment is critical in advancing IC inhibition therapy in GBM. Furthermore, revisiting current treatment modalities and their impact on the immune landscape is instrumental in designing future combinatorial therapies that may overcome treatment resistance.

Keywords: glioblastoma; immune checkpoints; immune microenvironment; immunotherapy; tumor immunosuppression.

Figure 1

Immune microenvironment in glioblastoma. The GBM immune landscape is characterized by a (A) paucity of tumor infiltrating lymphocytes that express multiple immune checkpoints and exhaustion markers leading to an impaired effector cell function and decreased tumor cell killing. There is also enrichment of immunosuppressive cellular subsets including Tregs, MDSCs, and TAMs. (B) Tregs release immunosuppressive cytokines including IL-10, IL-35, and TGF-β that impair T cell activation. Furthermore, (C) MDSCs promote Treg proliferation and function, while impairing T cell signaling via amino acid depletion (L-arginine) leading to reduced T cell proliferation. (D) TAMS comprise most of the immune cell tumor bulk with a larger proportion polarized towards an immunosuppressive phenotype (classically designate “M2”). TAMs impair T cells through tryptophan depletion via IDO expression as well as the release of immunosuppressive cytokines. (E) GBM-derived EVs carry bioactive molecules that promote immunosuppressive TAMs and limit the pro-inflammatory subtype. EVs also carry immunosuppressive cargo such as PD-L1 that can disrupt T cell activation and proliferation. (F) The hypoxic environment in GBM can also induce gene expression of ICs on lymphocytes through HIF-1α, promote MDSC function via NO production leading to IL-2 signaling inhibition and maintenance of the GSC population. (G) GSCs support Treg proliferation via STAT3 signaling and support the TAM population. Tregs, Regulatory T cells; MDSCs, Myeloid-derived suppressor cells; TAM, Tumor-associated macrophages; IDO, indoleamine 2,3-dioxygenase 1; EVs, Extracellular vesicles; HIF-1α, Hypoxia-inducible factor 1 alpha; NO, Nitric oxide.

Figure 2

Immune checkpoint pathways in glioblastoma. (A) PD-1. PD-1/PD-L1 binding leads to the recruitment and activation of the SHP-2 phosphatase by the ITIM and ITSM domains, which de-phosphorylates ZAP70 and downregulates subsequent downstream proteins (i.e. PI3K, LAT, SLP76) resulting in lymphocyte activation and proliferation. ZAP70 phosphorylation by Lck is also inhibited by the ITSM domain of PD-1 impairing downstream TCR signaling. (B) CTLA-4. CTLA-4 is a homolog of CD28 and has a higher affinity to their common ligand CD80/86 thereby displacing CD28. Competitive binding of CTLA-4 to CD80/86 recruits SHP-2 to its YVKM motif leading to the downregulation of similar signaling pathways that lead to T cell activation. (C) TIM-3. Gal-9 binding to TIM-3 results in calcium influx that leads to lymphocyte apoptosis. Similarly, PtdSer from apoptotic cells can bind to TIM-3 expressed on APCs leading to phagocytosis. In contrast, binding of PtdSer to TIM-3 on lymphocytes inducing apoptotic signals instead. HMGB1 competitively binds TIM-3 displacing PAMPs from apoptotic tumor cells inhibiting the processing of these PAMPs and their expression via MHC-I leading to tumor surveillance escape. Ceacam1 and TIM-3 are co-expressed on lymphocytes and function to regulate co-stimulatory signals from the CD28-CD80/86 complex leading to immunosuppression. (D) TIGIT. TIGIT competitively binds CD155 displacing the stimulatory protein DNAM-1 leading to lymphocyte exhaustion and anergy. Binding of TIGIT expressed on other immune cells to CD155 on tumor cells can lead to reduced NK Cell cytotoxicity via downregulation of the NF-KB pathway and increased immunosuppressive capacity of TREGs. (E) LAG-3. LAG-3 has a higher affinity to the TCR/MHC-II complex than CD-4. Binding of LAG-3 to the TCR/MHC-II complex leads to lymphocyte impairment via the cytoplasmic KIEELE motif. In the absence of MHC-II binding, acidification within the LAG-3 immunological synapse via calcium influx results N the dissociation of Lck from the TCR complex, preventing its ability to phosphorylation downstream activator proteins. (F) B7-H3. The ligands of B7-H3 are currently unknown. Activation of B7-H3 leads to impairment of lymphocyte activation and proliferation. DNAM-1, DNAX accessory molecule-1; Gal-9, Galectin-9; TCR, T-cell receptor; Gal-9, Galectin-9; PtdSer, Phosphatidylserine; MHC-I TIM-3, T cell immunoglobulin and mucin-domain containing-3; TIGIT, T cell immunoreceptor with Ig and ITIM domains; Ceacam1, Carcinoembryonic antigen-related cell adhesion molecule 1; HMGB1, High mobility group box 1 protein.

Figure 3

Immunogenic effects of standard glioblastoma treatment modalities. TMZ and Dex have immunosuppressive effects on GBM TME. Dex impairs the effect of anti-PD-1 therapy and reduces T cell tumor infiltration. TMZ induces the expression of certain ICs including PD-1, increases tumor mutational burden contributing to tumor heterogeneity, and augments TREG immunosuppressive function. BEV has pro-inflammatory effects including enhancing DC maturation and impairing TREGs. (BEV) Bevacizumab. (DEX) Dexamethasone. (IC) Immune checkpoint. (ICB) Immune checkpoint blocker. (TMZ) Temozolomide.