Immunomodulatory Effects of Radiotherapy

Abstract

Radiation therapy (RT), an integral component of curative treatment for many malignancies, can be administered via an increasing array of techniques. In this review, we summarize the properties and application of different types of RT, specifically, conventional therapy with x-rays, stereotactic body RT, and proton and carbon particle therapies. We highlight how low-linear energy transfer (LET) radiation induces simple DNA lesions that are efficiently repaired by cells, whereas high-LET radiation causes complex DNA lesions that are difficult to repair and that ultimately enhance cancer cell killing. Additionally, we discuss the immunogenicity of radiation-induced tumor death, elucidate the molecular mechanisms by which radiation mounts innate and adaptive immune responses and explore strategies by which we can increase the efficacy of these mechanisms. Understanding the mechanisms by which RT modulates immune signaling and the key players involved in modulating the RT-mediated immune response will help to improve therapeutic efficacy and to identify novel immunomodulatory drugs that will benefit cancer patients undergoing targeted RT.

Keywords: FLASH-RT; abscopal effects; cancer vaccines; carbon ion therapy; charged particle therapy; clustered DNA damage; immune signaling; radiation therapy; tumor antigens.

Figure 1

Schematics of the double-strand break (DSB) repair pathways after low- and high-linear energy transfer (LET) radiation. Low-LET radiation induces simple DSBs, which are repaired by non-homologous end joining pathway (NHEJ). The NHEJ process is initiated by the binding of a protein complex, the Ku70/80 heterodimer, to both ends of the broken DNA molecule, which, together with the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), generates the DNA-PK complex. Non-ligatable DNA termini are processed by the MRN complex (Mre11/Rad50/Nbs1), and the DNA ends are later rejoined by ligase IV and XRCC4. High-LET radiation causes clustered DNA lesions (or multiply damaged sites). Clustered DNA damage is mostly repaired by homologous recombination (HR). HR is initiated by the resection of the DNA ends through the combined action of the MRN complex (MRE11, RAD50, and NBS1) and CtIP to generate single-stranded DNA overhangs. Subsequently, replication protein-A (RPA) binds to the newly created single-strand region before being exchanged with RAD51. The RAD51/ssDNA complex invades the homologous template DNA, creating a temporary triple-DNA structure in which strand exchange occurs, then DNA synthesis proceeds until the second end is captured. In subsequent steps, a Holliday junction is generated to prime DNA synthesis, which is further resolved by specific nucleases.

Figure 2

Schematic diagram of the comprehensive immunological effects of radiotherapy. DNA fragments generated by radiotherapy (RT) can trigger both adaptive and innate immune signaling pathways. RT causes double-strand DNA damage and destroys tumor cells. Tumor cell death exposes tumor antigens (neoantigens), which are specifically expressed on the tumor cell surface and activate antigen presenting cells (APC) like dendritic cells (DC). Primed DCs initiate adaptive immune response and kill tumor cells recognized by T cells, which play an important role in antitumor immunity. In addition, DNA damage releases fragmented DNA into the cytosol or generates micronuclei, which activates a cGAS-mediated innate immune response that destroys the tumor. RT primarily treats local tumors, but it can also inhibit distant tumors after local radiation, which is known as an abscopal effect. Thus, the systemic antitumor response to local treatment can target both local tumor cells and distant metastatic tumors.

Figure 3

Schematic representation of a working hypothesis on the immunomodulatory role of radiation. Radiation induces DNA damage and generates micronuclei and fragmented dsDNA. During phagocytosis of tumor cells by dendritic cells (DCs), the DNA fragments hidden in irradiated tumor cells are released from phagosomes into the cytoplasm, where they act as a “danger signal”. The DNA sensor cGAS recognizes this signal, becomes catalytically active, and forms cGAMP. cGAMP binds to STING, which in turn activates IRF3 to induce type I IFN production. Type I IFN signaling activates DCs and promotes CD8⁺ T cell priming, which leads to tumor control. Alternatively, RT induces dying tumors to express more tumor antigens on their surface and to release danger-associated molecular patterns (DAMP), such as ATP and HMGB1, that stimulate DC activation. DCs express tumor antigen on their surface, interact with the T cell receptors on CD4⁺/CD8⁺ T cells, and activate immune cells that mediate tumor-specific killing.