Purpose: Cancer cells produce innate immune signals following radiation damage, with STING pathway signaling as a critical mediator. High linear energy transfer (LET) radiations create larger numbers of DNA double-strand breaks (DSBs) per unit dose than low-LET radiations and may therefore be more immunogenic. We studied the dose response characteristics of pro-immunogenic type-I interferon, interferon-beta (IFNβ), and its reported suppressor signal, three-prime repair exonuclease 1 (TREX1), in vitro with low-LET x-rays and high-LET fast neutrons.
Methods: Merkel cell carcinoma cells (MCC) were irradiated by graded doses of x-rays (1-24 Gy) or fast neutrons (1-8 Gy). IFNβ was measured as a function of dose via ELISA assay, and exonuclease TREX1 expression via immunofluorescence microscopy. The Monte Carlo damage simulation (MCDS) was used to model fast neutron relative biological effectiveness for DSB induction (RBEDSB) and compared to laboratory measurements of the RBE for IFNβ production (RBEIFNβ) and TREX1 upregulation (RBETREX1). RBEIFNβ models were also applied to radiation transport simulations to quantify the potential secretion of IFNβ in representative clinical beams.
Results: Peak IFNβ secretion occurred at 5.7 Gy for fast neutrons and at 14.0 Gy for x-rays, i.e., an effective RBEIFNβ of 2.5 ± 0.2. The amplitude (peak value) of secreted IFNβ signal did not significantly differ between x-rays and fast neutrons (P > 0.05). TREX1 signal increased linearly with absorbed dose, with a four-fold higher upregulation per unit dose for fast neutrons relative to x-rays (RBETREX1 of 4.0 ± 0.1). Monte Carlo modeling of IFNβ suggests Bragg peak-to-entrance ratios of IFNβ production of 40, 100, and 120 for proton, alpha, and carbon ion beams, respectively, a factor of 10-20-fold higher compared to their corresponding physical dose peak-to-entrance ratios. The spatial width of the Bragg peak for IFNβ production is also a factor of two smaller.
Conclusion: High-LET fast neutrons initiate a larger IFNβ response per unit absorbed dose than low-LET x-rays (i.e., RBEIFNβ value of 2.5). The RBE value for IFNβ is quite similar to data reported in the literature for DSB induction and cellular, post-irradiation micronucleation formation for neutrons and x-rays. The increased IFNβ release after high-LET radiation may be a contributing factor in stimulating a systemic anti-tumor, adaptive immune response (abscopal effect). However, our results indicate that TREX1 anti-inflammatory signaling in vitro for MCC cells is larger per unit dose for fast neutrons than for x-rays (RBETREX1 of 4.0). Given these competing effects, additional studies are needed to clarify whether or not high-LET radiations are therapeutically advantageous over low-LET radiation for pro-inflammatory immune signaling in other cell lines in vitro and for in vivo cancer models.
Erratum: “Effective use of x rays in diagnostic radiology: Guidance for the optimization of image quality and absorbed dose in the patient by use of a Monte Carlo computational model of the imaging chain” [Med. Phys. 23, 177 (1996)]