Fractionated radiation severely reduces the number of CD8+ T cells and mature antigen presenting cells within lung tumors.

Abstract

PURPOSE\nThe combination of standard-of-care radiotherapy (RT) with immunotherapy is moving to the mainstream of non-small cell lung cancer (NSCLC) treatment. Multiple preclinical studies reported on the CD8+ T cell stimulating properties of RT, resulting in abscopal therapeutic effects. A literature search demonstrates that most preclinical lung cancer studies applied subcutaneous lung tumor models. Hence in-depth immunological evaluation of clinically relevant RT in orthotopic lung cancer models is lacking.\n\n\nMETHODS\nHere, we studied the therapeutic and immunological effects of low-dose fractionated RT on lungs from C57BL/6 mice, challenged two weeks before with firefly luciferase expressing Lewis Lung Carcinoma cells via the tail vein. Low-dose fractionation was represented by 4 consecutive daily fractions of image-guided RT at 3.2 Gy.\n\n\nRESULTS\nWe showed reduced lung tumor growth upon irradiation using in vivo bioluminescence imaging and immunohistochemistry. Moreover, significant immunological RT-induced changes were observed in irradiated lungs as well as in the periphery (spleen and blood). First, a significant decrease in the number of CD8+ T cells and trends towards more CD4+ and regulatory T cells were seen upon RT in all evaluated tissues. Notably, only in the periphery the remaining CD8+ T cells showed a more activated phenotype. In addition, a significant expansion of neutrophils and monocytes was observed upon RT locally and systemically. Locally, RT increased the influx of tumor-associated macrophages and conventional type 2 dendritic cells (DCs), while the alveolar macrophages and conventional type 1 DCs dramatically dropped. Functionally, these antigen presenting cells severely reduced their CD86 expression, suggestive for a reduced capacity to induce potent immunity.\n\n\nCONCLUSION\nOur results imply that low-dose fractionated RT of tumor-bearing lung tissue shifts the immune cell balance towards an immature myeloid cell dominating profile. These data argue for myeloid cell repolarizing strategies to enhance the abscopal effects of fractionated RT-treated NSCLC patients.