Novel Approaches to Improve the Efficacy of Immuno-Radiotherapy

Abstract

Radiotherapy (RT) has been applied for decades as a treatment modality in the management of various types of cancer. Ionizing radiation induces tumor cell death, which in turn can either elicit protective anti-tumor immune responses or immunosuppression in the tumor micromilieu that contributes to local tumor recurrence. Immunosuppression is frequently accompanied by the attraction of immunosuppressive cells such as myeloid-derived suppressor cells (MDSCs), M2 tumor-associated macrophages (TAMs), T regulatory cells (Tregs), N2 neutrophils, and by the release of immunosuppressive cytokines (TGF-β, IL-10) and chemokines. Immune checkpoint pathways, particularly of the PD-1/PD-L1 axis, have been determined as key regulators of cancer immune escape. While IFN-dependent upregulation of PD-L1 has been extensively investigated, up-to-date studies indicated the importance of DNA damage signaling in the regulation of PD-L1 expression following RT. DNA damage dependent PD-L1 expression is upregulated by ATM/ATR/Chk1 kinase activities and cGAS/STING-dependent pathway, proving the role of DNA damage signaling in PD-L1 induced expression. Checkpoint blockade immunotherapies (i.e., application of anti-PD-1 and anti-PD-L1 antibodies) combined with RT were shown to significantly improve the objective response rates in therapy of various primary and metastatic malignancies. Further improvements in the therapeutic potential of RT are based on combinations of RT with other immunotherapeutic approaches including vaccines, cytokines and cytokine inducers, and an adoptive immune cell transfer (DCs, NK cells, T cells). In the current review we provide immunological rationale for a combination of RT with various immunotherapies as well as analysis of the emerging preclinical evidences for these therapies.

Keywords: PD-1; PD-L1; immune checkpoint inhibitors; immunosuppression; radiotherapy.

Figures

Figure 1

Radiation-induced immunosuppressive effects in the tumor micromilieu. RT induces recruitment, proliferation and polarization of immunosuppressive cell subtypes including myeloid-derived suppressor cells (MDSCs), M2 tumor-associated macrophages, N2 neutrophils, and regulatory T cells (CD4+CD25+Foxp3+). RT induces increased levels of suppressive factors including nitric oxide synthase (NOS) and reactive nitrogen intermediates (RNI), reactive oxygen species (ROS), cytokines (IL-4, IL-10, TGF-β), matrix metalloproteinases (MMPs), arginase I, collagenase, lipoxygenase (LOX) which in turn leads to the suppression of the T cell activity.

Figure 2

Regulation of PD-L1 expression in response to DNA damage in cancer cells. As per the DNA damage response pathway, DNA damage induced by IR or chemotherapeutic regents activates ATM/ATR/Chk1 signals, followed by the STAT-IRF pathway. In this pathway, STAT1/3-IRF1 appears to play an important role in PD-L1 upregulation after DNA damage. Alternatively, PD-L1 expression is regulated by the neoantigen pathway in the context of DNA damage and repair in cancer cells. Levels of mutation burden are associated with MSI. Mutation burdens and MSI are augmented by the defect of mismatch repair, chromatin remodeling, or abnormal DNA replication. Neoantigens presented by MHC class I, which is recognized by T cell receptors, activate T cells, followed by the release of IFNγ. IFNγ stimulates the STAT-IRF pathway via the IFNγ receptor (IFNGR) and upregulates PD-L1 expression in cancer cells. Another novel pathway, the cGAS/STING pathway, may also be involved in the activation of the IFN-STAT/IRF-PD-L1 pathway. Cytosolic DNA fragments induced by DNA damage activate the cGAS/STING pathway. Activation of the cGAS/STING pathway induces IFN type I (IFNα and IFNβ), which is incorporated into cancer cells via the IFNα/β receptor (IFNAR). IFNα/β-dependent signaling also activates the STAT-IRF pathway, resulting in PD-L1 upregulation.

Figure 3

Repair pathways and signaling in response to DSB induction by IR. After DSB induction, the Ku70/80 heterodimer complex (Ku) rapidly binds to all DSB ends. Ku bound to DSB ends plays the following roles: 1) Ku70/80 complex promotes NHEJ and 2) Ku70/80 complex protects DNA ends from unscheduled digestion by DNA nucleases. In the NHEJ pathway, DSBs are rapidly rejoined by DNA-PKcs and XLF/XRCC4/LIG4 following Ku binding to DSB ends (81). On the other hand, DSB ends are digested in the 5′ to 3′ direction by EXO1 to direct repair pathway toward HR. The resected ssDNA is coated with RPA. BRCA2 promotes the protein switch from RPA to RAD51, facilitating strand invasion into the template strand for recombination-mediated repair. In terms of DNA damage signaling, ATM, which serves as a sensor of DSBs, is the major DNA damage response (DDR) kinase and is activated at unresected DSB ends. At DSB ends during HR, resection promotes a switch from ATM to ATR activation, followed by Chk1 activation. In the context of DNA damage-dependent PD-L1 expression, the activation of Chk1 is a critical step leading to STAT/IRF-mediated PD-L1 upregulation.