Enhancing the efficacy of immunotherapy using radiotherapy

Abstract

Recent clinical breakthroughs in cancer immunotherapy, especially with immune checkpoint blockade, offer great hope for cancer sufferers - and have greatly changed the landscape of cancer treatment. However, whilst many patients achieve clinical responses, others experience minimal benefit or do not respond to immune checkpoint blockade at all. Researchers are therefore exploring multimodal approaches by combining immune checkpoint blockade with conventional cancer therapies to enhance the efficacy of treatment. A growing body of evidence from both preclinical studies and clinical observations indicates that radiotherapy could be a powerful driver to augment the efficacy of immune checkpoint blockade, because of its ability to activate the antitumor immune response and potentially overcome resistance. In this review, we describe how radiotherapy induces DNA damage and apoptosis, generates immunogenic cell death and alters the characteristics of key immune cells in the tumor microenvironment. We also discuss recent preclinical work and clinical trials combining radiotherapy and immune checkpoint blockade in thoracic and other cancers. Finally, we discuss the scheduling of immune checkpoint blockade and radiotherapy, biomarkers predicting responses to combination therapy, and how these novel data may be translated into the clinic.

Keywords: apoptosis; immune checkpoint blockade; immunogenic cell death; radiotherapy.

Figure 1

DNA damage and the programmed cell death pathway (apoptosis) induced by radiotherapy. Double‐ and single‐strand breaks induced by radiotherapy are sensed by Mre11/Rad50/Nbs1 and Rad1/Rad9/Hus1 and Rad17/RFC complexes, respectively. Mre11/Rad50/Nbs1 then recruits ataxia telangiectasia‐mutated protein (ATM), which phosphorylates checkpoint protein kinase 2 (CHK2). Rad1/Rad9/Hus1 and Rad17/RFC recruit ataxia telangiectasia and rad 3‐related (ATR), which phosphorylates checkpoint protein kinase 1 (CHK1). Activated CHK2 and CHK1 arrest cell cycle progression for DNA repair through homologous recombination (HR), nonhomologous end‐joining (NHEJ), poly (ADP‐ribose) polymerase (PARP) and γ‐H2AX. If DNA is repaired, cells will resume their normal cycles. However, if the damage is substantial, CHK2/CHK1 will phosphorylate P53 by dissociating P53 from mouse double minute 2 (MDM2), leading to the accumulation of transcriptionally active P53. The P53 then triggers the expression of pro‐apoptotic genes, namely p53‐upregulated modulator of apoptosis (PUMA) and BCL2‐associated X protein (BAX). PUMA disassembles complex P53 and anti‐apoptotic protein (BCL‐XL) in the cytoplasm. Liberated P53 disrupts pro‐apoptotic BAX and anti‐apoptotic BCL2 complex. Released BAX permeabilises mitochondrial outer membrane releasing cytochrome C, which binds to apoptotic protease‐activating factor (Apaf) and adenosine triphosphate (ATP) to form apoptosome and activate caspase‐9 and finally effector caspase‐3 and caspase‐7 inducing intrinsic apoptosis. Moreover, P53 also transactivates the death receptor (Fas/CD95) and death ligand (FasL/CD178). The interaction of Fas and FasL leads to trimerisation of CD95 and clustering of intracellular death domain (DD). DD then recruits FAS cell‐surface death receptor‐associated death domain (FADD). The FADD activates procaspase‐8 (PCASP‐8) forming death‐inducing signalling complex (DISC), which will further produce effector caspase‐3 and caspase‐7 cleaving DNA repair proteins such as PARP, structural protein, inducing membrane blebbing and DNA fragmentation leading to extrinsic cell death.

Figure 2

Induction of immunogenic cell death after radiotherapy. Tumor cells are treated with radiation. The dying tumor cells then induce the translocation of damage‐associated molecular patterns (DAMPs) such as CRT to plasma membrane of the cells and release other DAMPs such as high mobility group box‐1 protein (HMGB‐1) and adenosine triphosphate (ATP) into the immune milieu. Resident dendritic cells (rDCs) interact with CRT via CD91 receptor, leading to the phagocytosis of tumor cells to generate peptide antigen. rDCs migrate to draining lymph nodes under the direction of chemokine – CCL‐21 – to present tumor antigen to naïve CD4⁺ T cells and cross‐present the antigen to CD8⁺ T cells through major histocompatibility classes (MHC)‐I and MHC‐II, respectively. Effector T cells, particularly CD8⁺ T cells precisely directed by chemokines – CXCL‐9 and CXCL‐10 – home in on the tumor, killing it by inducing apoptosis through granzyme/ perforin pathway and Fas/ Fas‐ligand interaction. HMGB‐1 and ATP also bind to TLR‐4 and P2RX7, respectively, on dendritic cells/ tissue macrophages, inducing the activation of these cells to release pro‐inflammatory cytokines such as interferon, interleukin (IL)‐1β and IL‐18 to inflame the environment for more robust antitumor immune responses.

Figure 3

Effects of low‐ and high‐dose radiotherapy on tumor immune microenvironment. Both low‐ and high‐dose radiotherapies drive the upregulation of immunostimulatory (e.g. CD8⁺ T cells, NK cells) and immunosuppressive immune cells (e.g. Treg, MDSC, M2 macrophage) but influence differing cell subsets.