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until file(s) become available
Effects of linear energy transfer and hypoxia on radiation-induced immunogenicity through STING
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
Purpose: Preclinical studies have demonstrated that cancer cells may produce innate immune
signals such as type-I interferons following radiation damage, which derives from activation
of the cGAS-STING pathway following detection of cytosolic dsDNA. Limited studies have
explored how these mechanisms vary from the conditions of the radiation exposure. High-
linear energy transfer (LET) radiation induces more DNA double-strand breaks (DSB) per
dose than low-LET radiation, thus is expected to be more immunogenic. However, DNA
damage in hypoxic cells is more probable to undergo chemical repair due to limitations in
oxygen fixation, thus is expected to be more immunosuppressive. Our goal is to study and
model the dose response characteristics of IFNβ and Trex1 in vitro following exposure of
radiations with varying LET and to develop techniques for further study in vivo.
Methods: Reference data from Vanpouille-Box (2017) on STING dose response was applied to develop empirical models of cytosolic dsDNA and Trex1 regulation as a function of dose and quantity of DNA DSB, the latter of which is dependent on particle LET and oxygenation and is calculated using Monte Carlo Damage Simulation (MCDS) software. These models were used as preliminary data to guide in vitro experiments using Merkel cell carcinoma cells. The dose response of pro-inflammatory IFNβ and exonuclease Trex1, an anti-inflammatory suppressor of cGAS-STING, was measured post-irradiation. MCDS was again used to model fast neutron relative biological effectiveness for DSB induction (RBEDSB) and compared to laboratory measurements of the RBE for IFNβ production (RBEIFNβ). RBEIFNβ models were applied to radiation transport simulations to quantify the potential secretion of IFNβ in representative clinical beams. To enable intra-tumor radiation targeting of tumor hypoxia, mice were seeded with syngeneic tumors and imaged longitudinally with PCT- spectroscopy to determine local variations hemoglobin concentration (Hb) and oxygen saturation (SaO2) over time. Hypoxia classification was based on SaO2 levels in voxels containing hemoglobin relative to a “hypoxia threshold” of SaO2 < 0.2.
Results: Based on analysis of published data, our preliminary models of cytosolic DNA and
Trex1 dose responses demonstrate dose enhancements from high-LET radiation, such as that
at the distal edge of a Bragg peak, and suppression from cellular hypoxia. This manifests as
an RBE-dependent ‘shift’ in STING response. Laboratory measurements in MCC13 cells
show peak IFNβ production at 6.1 Gy following fast neutron irradiation and 14.5 Gy
following x-rays (RBEIFNβ = 2.4). However, IFNβ signal amplitudes were not significantly
different between these radiation types. Trex1 signal increased linearly with dose, with
fourfold higher upregulation per dose for fast neutrons. Modeling of RBE in clinical beams
suggests that ion sources may induce spatially localized IFNβ near their end of range, which
is potentially advantageous for initiation of tumor-specific immune activity. Uncharged
sources stimulate IFNβ more uniformly with depth. Longitudinal PCT-S scanning is able to localize and distinguish chronic and acute hypoxia in vivo. Changes in the hypoxic
classification from tumor growth and following anti-angiogenic therapy are distinguishable.
Conclusion: Radiation-induced immunogenicity can be induced differentially based on
radiation quality and is expected to be affected by cellular oxygenation. High-LET radiation,
such as fast neutrons, drives greater IFNβ innate immune response per dose than low-LET
radiation, such as x-rays, which may enhance abscopal effects when used in combination
with immune-stimulating agents. However, anti-inflammatory signaling is greater per dose
for fast neutrons, and it remains unclear if high-LET radiations are therapeutically
advantageous over low-LET radiation for pro-inflammatory tumor signaling. High
resolution in vivo imaging of tumor hypoxia is feasible with photoacoustic techniques, which
can potentially be leveraged to study selective immunogenicity enhancement of the hypoxic
niche following radiation therapy.