Low-Dose Radiation Therapy Promotes Radiation Pneumonitis by Activating NLRP3 Inflammasome

To determine the role of NLRP3 inflammasome activation in low-dose radiation-induced radiation pneumonitis (RP) and to assess whether inhibition or deletion of the NLRP3 inflammasome is critical for conferring protection against RP.

The human monocytic THP-1 cells were treated with increasing doses of radiation to assess the activation of NLRP3 by Western blot and enzyme-linked immunosorbent assay. Reactive oxygen species (ROS) production was measured by flow cytometry, with or without ROS inhibitor treatment. A mouse thoracic radiation model that received different doses of radiation was used, and the lung tissues of thoracic-irradiated nlrp3^-/- and wild-type C57BL/6 mice were examined by hematoxylin and eosin and immunofluorescence staining. The concentrations of cytokines in the bronchoalveolar lavage fluid were measured by enzyme-linked immunosorbent assay and Luminex multiplex assays. Lipopolysaccharide (LPS) was administered intranasally 28 days after thoracic irradiation, and NLRP3 inhibitor, MCC950 was administered intraperitoneally after irradiation at 2 different doses.

(1) The NLRP3 inflammasome was activated in 2 Gy irradiated THP-1 cells; NLRP3 and cleaved-caspase-1 levels were not associated with dose escalation of irradiation. (2) Activation of the NLRP3 inflammasome was mediated by ROS, and ROS inhibitor treatment decreased the production of IL-1β and IL-18 in vitro. (3) NLRP3 was activated in mouse lungs by irradiation at 2 Gy, 4 Gy, and 16 Gy, and NLRP3 activation was continuous for 8 weeks. (4) NLRP3 deletion protects against LPS-mediated monocyte infiltration in the mouse lung. (5) The administration of MCC950 decreased the inflammation score of the mice irradiated with 2 Gy or 16 Gy in vivo.

Our results indicate that the NLRP3 inflammasome is activated by low-dose irradiation both in vitro and in vivo. Inhibition or deletion of NLRP3 can specifically alleviate the mouse lung inflammation caused by radiation and LPS treatment. This study reveals the mechanism of low-dose radiation therapy-induced RP and offers a possible treatment strategy.