Radiotherapy-Mediated Immunomodulation and Anti-Tumor Abscopal Effect Combining Immune Checkpoint Blockade

SIMPLE SUMMARY: Combination of radiotherapy and immunotherapy to antagonize tumors is one of hotspot issues currently. Immunotherapy can effectively alleviate the non-lasting effect of radiotherapy and tumor recurrence. Local radiotherapy can mobilize immune cells into tumor microenvironment and make tumors unable to escape from immune supervision. Therefore, the radio-immunotherapy achieves better curative effects on cancer patients. We have reviewed the regulation of multiple immune cells with different functions by radiotherapy, as well as the mechanism research and clinical outcomes combined with immune checkpoint inhibitors for cancer treatment, which provides new insights in the treatment strategies for certain types of cancer patients in clinic. ABSTRACT: Radiotherapy (RT) is a conventional method for clinical treatment of local tumors, which can induce tumor-specific immune response and cause the shrinkage of primary tumor and distal metastases via mediating tumor infiltration of CD8+ T cells. Ionizing radiation (IR) induced tumor regression outside the radiation field is termed as abscopal effect. However, due to the mobilization of immunosuppressive signals by IR, the activated CD8+T cells are not sufficient to maintain a long-term positive feedback to make the tumors regress completely. Eventually, the “hot” tumors gradually turn to “cold”. With the advent of emerging immunotherapy, the combination of immune checkpoint blockade (ICB) and local RT has produced welcome changes in stubborn metastases, especially anti-PD-1/PD-L1 and anti-CTLA-4 which have been approved in clinical cancer treatment. However, the detailed mechanism of the abscopal effect induced by combined therapy is still unclear. Therefore, how to formulate a therapeutic schedule to maximize the efficacy should be took into consideration according to specific circumstance. This paper reviewed the recent research progresses in immunomodulatory effects of local radiotherapy on the tumor microenvironment, as well as the unique advantage for abscopal effect when combined with ICB, with a view to exploring the potential application value of radioimmunotherapy in clinic.