Radiotherapy Both Promotes and Inhibits Myeloid-Derived Suppressor Cell Function: Novel Strategies for Preventing the Tumor-Protective Effects of Radiotherapy

Abstract

Cancer immunotherapies aimed at neutralizing the programmed death-1 (PD-1) immune suppressive pathway have yielded significant therapeutic efficacy in a subset of cancer patients. However, only a subset of patients responds to antibody therapy with either anti-PD-1 or anti-PD-L1 antibodies. These patients appear to have so-called "hot" tumors containing tumor-reactive T cells. Therefore, checkpoint blockade therapy may be effective in a larger percentage of cancer patients if combined with therapeutics that also activate tumor-reactive T cells. Radiotherapy (RT) is a prime candidate for combination therapy because it facilitates activation of both local antitumor immunity and antitumor immunity at non-radiated, distant sites (abscopal response). However, RT also promotes tumor cell expression of PD-L1 and facilitates the development of myeloid-derived suppressor cells (MDSC), a population of immune suppressive cells that also suppress through PD-L1. This article will review how RT induces MDSC, and then describe two novel therapeutics that are designed to simultaneously activate tumor-reactive T cells and neutralize PD-1-mediated immune suppression. One therapeutic, a CD3xPD-L1 bispecific T cell engager (BiTE), activates and targets cytotoxic T and NKT cells to kill PD-L1⁺ tumor cells, despite the presence of MDSC. The BiTE significantly extends the survival time of humanized NSG mice reconstituted with human PBMC and carrying established metastatic human melanoma tumors. The second therapeutic is a soluble form of the costimulatory molecule CD80 (sCD80). In addition to costimulating through CD28, sCD80 inhibits PD-1 suppression by binding to PD-L1 and sterically blocking PD-L1/PD-1 signaling. sCD80 increases tumor-infiltrating T cells and significantly extends survival time of mice carrying established, syngeneic tumors. sCD80 does not suppress T cell function via CTLA-4. These studies suggest that the CD3xPD-L1 BiTE and sCD80 may be efficacious therapeutics either as monotherapies or in combination with other therapies such as radiation therapy for the treatment of cancer.

Keywords: bi-specific T cell engager (BiTE); myeloid-derived suppressor cells (MDSC); programmed death ligand 1 (PD-L1); radiotherapy-induced immune suppression; solubilized CD80.

Figure 1

Conventional fractionated radiotherapy (CFRT) increases MDSC while ablative hypofractionated radiotherapy (ABHRT) decreases MDSC. CFRT increases the quantity of MDSC by (i) inducing the complement component C5a; (ii) causing DNA damage resulting in the up-regulation of CSF1; or (iii) signaling through STING to increase IFNβ which up-regulates CCL2, CCL7, and CCL12, chemoattractants for MDSC. MDSC up-regulated by CFRT facilitate tumor cell survival by their production of arginase 1 which decreases nitric oxide, a radiosensitizing molecule. ABHRT enhances antitumor immunity by reducing intratumoral hypoxia which decreases the quantity of MDSC and MDSC expression of PD-L1, resulting in increased levels of CD40L⁺CD4⁺ T cells and CD8⁺ DC which activate CD8⁺ TIL.

Figure 2

CD3xPDL1 BiTE blocks PD-L1 and induces T cell-mediated cytotoxic death. The CD3xPDL1 BiTE consists of the V_H and V_L regions of anti-CD3 and anti-PDL1 linked together to form a 55 kDa single chain structure. The CD3xPDL1 BiTE binds to PD-L1 on PD-L1⁺ tumor cells blocking interaction with PD-1 on T cells, thereby preventing PD-1 mediated T cell exhaustion. The BiTE simultaneously binds to CD3 on CD4⁺ T cells, CD8⁺ T cells, and NKT cells, activates the cells, and forms a cytotoxic synapse. The activated effector cells then kill the PD-L1⁺ tumor cells.

Figure 3

Soluble CD80 activates T cells, blocks PD-1 mediated immune suppression, and promotes anti-tumor immunity. sCD80 acts as a checkpoint inhibitor by blocking PD-L1 on tumor cells and antigen presenting cells while simultaneously binding and activating T cells through CD28. T cells activated by sCD80 have increased IFNγ and IL-2 production and upregulate TCR and CD28 signaling, resulting in an immune-reactive tumor microenvironment with T cell killing of target tumor cells.