A landscape of checkpoint blockade resistance in cancer: underlying mechanisms and current strategies to overcome resistance

Abstract

The discovery of immune checkpoints and the development of immune checkpoint inhibitors (ICI) have achieved a durable response in advanced-stage cancer patients. However, there is still a high proportion of patients who do not benefit from ICI therapy due to a lack of response when first treated (primary resistance) or detection of disease progression months after objective response is observed (acquired resistance). Here, we review the current FDA-approved ICI for the treatment of certain solid malignancies, evaluate the contrasting responses to checkpoint blockade in different cancer types, explore the known mechanisms associated with checkpoint blockade resistance (CBR), and assess current strategies in the field that seek to overcome these mechanisms. In order to improve current therapies and develop new ones, the immunotherapy field still has an unmet need in identifying other molecules that act as immune checkpoints, and uncovering other mechanisms that promote CBR.

Keywords: Immune checkpoint inhibitors (ICI); checkpoint blockade resistance; checkpoint blockade resistance mechanisms; immune checkpoint blockade; immunotherapy resistance.

Figure 1.

Mechanism associated to ICI resistance. The TME represents a complex interaction between tumor cells, immune cells, and inhibitory cytokines leading to a plethora of mechanisms associated to ICI resistance. (a) Defects in T cells effector function and their exclusion from the TME can occur as a result of dysregulation of MAPK/ERK cell signaling pathways in tumor cells inducing the production of VEGF and IL-8, having a net inhibitory effect on the recruitment of T cells and defects in T-cell activation and differentiation. Due to the crosstalk between the MAPK and the PI3K signaling pathways, an oncogenic mutation in MAPK pathway or a loss of PTEN expression can cause the enhancement of PI3K-AKT signaling as well. Similarly, a mutation or silencing of $\mathit{\beta }$-catenin or tumor suppressor wnt protein, results in dysregulation of the WNT/$\mathit{\beta }$-catenin pathway, promoting aberrant signaling of b -catenin contributing to the absence of T cell expression signature. Mutations in the IFN-g receptor and defects on the IRF1 or JAK1/2 genes can contribute to T cell desensitization and consequently promote acquired resistance to ICIs. Specifically, mutations of PD-L1, PD-L2, and JAK2 genes resulting from the amplification of the locus that contains these genes promote the formation of the PD-1/TCR inhibitory microcluster, which results in the inhibition of T cell activation. (b) Another resistance mechanism is developed after defects in the APM specifically due to mutations of B2M and TAP proteins. These mutations make the antigen unable to reach the tumor cell’s surface and be recognized and cleared out by CD8⁺ T cells. (c) the interaction between T cells, tumor cells, APCs, and immunosuppressive cells (MDSCs) throughout immune checkpoints such as VISTA, LAG-3, TIGIT, and TIM-3, triggers and inhibitory signal causing the exhaustion of T cells, blockage of TCR signaling, and decreasing T cell activation and TCR expression. (d) Immunosuppressive signaling also contributes to primary and/or acquired resistance in cancer. Tregs can restrain the effector function of immunocompetent cells by inducing checkpoint-mediated suppression (CTLA-4, PD-1, TIGIT, TIM-3, and LAG-3), competing for IL-2 binding, or by producing anti-inflammatory cytokines. As such, TGF-b is a pleiotropic cytokine involved in tumor evasion and immunotherapy resistance. The upregulation of the metabolic modulator IDO within the TME allows the depletion of tryptophan, resulting in the decrease of T cells. Another metabolic modulator associated with T cell function suppression and ICI resistance is adenosine. Several ectonucleotidases (CD39 and CD73) catalyze the conversion of ADP or AMP to adenosine or NAD⁺ to AMP (CD38) causing the upregulation of adenosine in the tumor milieu. TME: tumor microenvironment; MAPK/ERK: mitogen-activated protein kinase/extracellular signal-regulated kinase; VEGF: vascular endothelial growth factor; IL-8: interleukin-8; PTEN: phosphatase and tensin homolog deleted on chromosome 10; PI3K-AKT: phosphoinositide-3-kinase – protein kinase B/Akt; WNT/$\mathit{\beta }$-catenin: wingless-related integration site/ b-catenin; IRF1: interferon regulatory factor 1; JAK1/2: Janus kinase 1/2; PD-L1/L2: program cell-death ligand 1/ligand 2; PD-1/TCR inhibitory microcluster: program cell-death 1/T cell receptor inhibitory microcluster; APM: antigen presenting machinery; $\mathit{\beta }$2 M; $\mathit{\beta }$2-microglobulin; TAP:transporter associated with antigen presentation; APC: antigen presenting cells; MDSCs: myeloid-derived suppressor cells; VISTA: V-domain ig suppressor of T cell activation; LAG-3: lymphocyte activation gene 3; TIGIT: T cell ImmunoGlobulin and ImmunoTyrosine inhibitory motif (ITIM) domain; TIM-3: T cell immunoglobulin and mucin-3; IDO: indoleamine 2,3-dioxygenase 1; CTLA-4:cytotoxic T lymphocyte antigen-4; TGF-$\mathit{\beta }$: transforming growth factor beta ; ADP: adenosine triphosphate; AMP: adenosine monophosphate; NAD⁺: nicotinamide adenine dinucleotide.

Figure 2.

Strategies to overcome associated mechanisms contributing to immunotherapy resistance. (a) Chemotherapy, radiotherapy, and small molecule inhibitors are among the strategies used to overcome deficiencies in T-cell priming. These agents can induce ICD in tumor cells causing the release of antigens and DAMPs, allowing the recruitment and maturation of APCs and the presentation of targetable antigens to effector T cells. Other interventions are used to increase the number of tumor-reactive T cells interacting in the TME, such as (b) ACT using TCR T cell or CAR T cell. TCR T directed against specific cancer antigens (testis antigen, NY-ESO-1) can recognize the antigen through MCH molecules and CAR T cells act in an MHC-independent manner targeting cell surface antigens. (c) the use of cancer vaccines is also used to expand tumor-specific T cell populations, broaden the T cell repertoire, and promote the transport of T cells to tumor lesions. Additionally, the use of (d) immunostimulatory cytokines such as NHS-IL-12 or N803 can promote the enhancement of effector cell recruitment and boost CD8⁺ T cell and NK cell cytolytic functions. (e) physical barriers also prevent the infiltration of effector T cells in the TME, and tumor vasculature represents one of these barriers. Therefore, VEGF targeting using small molecule inhibitors or mAbs represents an effective strategy to disrupt angiogenic pathways that are fostering the aberrant vasculature. (f) another physical barrier is the extracellular matrix that is composed mainly of collagen. Collagen is produced by TAMs, CAFs, and tumor cells and can impair immune activity through interactions with LAIR-1. Blockade of LAIR-1 combined with PD-1, and TGF-$\mathit{\beta }$ blockade can increase M1 TAMs population, CD8⁺ T cell infiltration, while decreasing TGF-$\mathit{\beta }$ and collagen. Figure adapted from J Clin Invest. 2022;132: e155148. https://doi.Org/10.1172/JCI155148. lastly, repolarization of the immunosuppressive microenvironment as an alternative strategy to overcome certain resistant mechanisms can be achieved by (g) the use of mAbs targeting CD33 and TRAIL-R2 receptors expressed in tumor cells were demonstrated to eliminate MDSCs. The use of anti-CFR-1 were demonstrated to eliminate M2 macrophages. Likewise, the depletion of tregs using anti-CD4, anti-CD25 or anti-GITR in combination with ICI have shown to improve CD8+ T cell activity. Another approach uses to decrease the immunosuppressive population that abrogates the effect of ICIs, is the use of small molecule inhibitors targeting CXCR1/2 which are receptors to chemokines essential for the recruitment of MDSCs and TANs. Growing evidence have shown that blocking TGF-b and PD-L1 simultaneously can decrease immunosuppressive population in the TME. (h) the use of inhibitors against IDO enzyme, halt the conversion of tryptophan to kynurenine allowing the increase of NK cells and CD8⁺ T cell and the decrease of Tregs.