Anti-inflammatory effects of low-dose radiotherapy. Indications, dose, and radiobiological mechanisms involved.

Low-dose radiotherapy (LD-RT) has been used for several benign diseases, including arthrodegenerative and inflammatory pathologies. Despite its effectiveness in clinical practice, little is known about the mechanisms through which LD-RT modulates the various phases of the inflammatory response and about the optimal dose fractionation. The objective of this review is to deepen knowledge about the most effective LD-RT treatment schedule and radiobiological mechanisms underlying the anti-inflammatory effects of LD-RT in various in vitro experiments, in vivo studies, and clinical studies.

Effects of irradiation on brain vasculature using an in situ tumor model.

PURPOSE: Damage to normal tissue is a limiting factor in clinical radiotherapy (RT). We tested the hypothesis that the presence of tumor alters the response of normal tissues to irradiation using a rat in situ brain tumor model. METHODS AND MATERIALS: Intravital microscopy was used with a rat cranial window to assess the in situ effect of rat C6 glioma on peritumoral tissue with and without RT. The RT regimen included 40 Gy at 8 Gy/day starting Day 5 after tumor implant. Endpoints included blood-brain barrier permeability, clearance index, leukocyte-endothelial interactions and staining for vascular endothelial growth factor (VEGF) glial fibrillary acidic protein, and apoptosis. To characterize the system response to RT, animal survival and tumor surface area and volume were measured. Sham experiments were performed on similar animals implanted with basement membrane matrix absent of tumor cells. RESULTS: The presence of tumor alone increases permeability but has little effect on leukocyte-endothelial interactions and astrogliosis. Radiation alone increases tissue permeability, leukocyte-endothelial interactions, and astrogliosis. The highest levels of permeability and cell adhesion were seen in the model that combined tumor and irradiation; however, the presence of tumor appeared to reduce the volume of rolling leukocytes. Unirradiated tumor and peritumoral tissue had poor clearance. Irradiated tumor and peritumoral tissue had a similar clearance index to irradiated and unirradiated sham-implanted animals. Radiation reduces the presence of VEGF in peritumoral normal tissues but did not affect the amount of apoptosis in the normal tissue. Apoptosis was identified in the tumor tissue with and without radiation. CONCLUSIONS: We developed a novel approach to demonstrate that the presence of the tumor in a rat intracranial model alters the response of normal tissues to irradiation.

Time course of anti-inflammatory effect of low-dose radiotherapy: correlation with TGF-beta(1) expression.

BACKGROUND AND PURPOSE: Low-dose radiotherapy (LD-RT) has a potent anti-inflammatory effect, and transforming growth factor (TGF)-beta(1) is a potential mediator of this effect. The objectives of this study were to characterize the in vivo effects of LD-RT on leukocyte recruitment over time, and its relationship with TGF-beta(1) production. MATERIALS AND METHODS: Mice were submitted to abdominal irradiation with a dose of 0.3 Gy, or to sham radiation and studied 5, 24, 48 or 72 h after irradiation. Four hours before the study a proinflammatory stimulus consisting of LPS or placebo was administered. Leukocyte-endothelial cell interactions in intestinal venules were assessed using intravital microscopy. Circulating levels and intestinal tissue production of TGF-beta(1) were determined. RESULTS: Compared to non-irradiated LPS-challenged group, the number of adherent leukocytes was significantly reduced 5, 24 and 48 h, but not 72 h after irradiation in LPS-challenged mice. Rolling leukocytes were significantly decreased at all time points analyzed. Plasma TGF-beta(1) levels were increased 5 and 24h after irradiation. Increased intestinal production during this period was corroborated by in vitro culture experiments. CONCLUSIONS: LD-RT has a sustained inhibitory effect on leukocyte recruitment for 48 h, which is initially associated with an increase in TGF-beta(1) intestinal production.

Radiation-induced permeability and leukocyte adhesion in the rat blood-brain barrier: modulation with anti-ICAM-1 antibodies.

We assessed the acute effects of radiation on the rat blood-brain barrier. A cranial window model and intravital microscopy were used to measure changes in permeability and leukocyte adhesion in pial vessels after a localized, single dose of 20 Gy. Permeability was assessed using five sizes of fluorescein isothiocyanate (FITC)-dextran molecules (4.4-, 10-, 38.2-, 70-, and 150-kDa) with measurements performed before and 2, 24, 48, 72 and 96 h after irradiation for the 4.4 and 38.2-kDa molecules and before and 24 h after irradiation for the other three molecules. To demonstrate the nature of blood-brain barrier permeability, we concurrently studied the permeability of microvessels in the cremaster muscle. In both tissues, permeability to FITC-dextran was significantly greater 24 h after irradiation than before (P<0.05). The exception was that radiation did not affect the permeability of pial vessels to the 150-kDa molecule. The particle-size dependence of the permeability changes in the brain were indicative of altered integrity of endothelial tight junctions and occurred concomitantly with an increase in cell adhesion which was determined by fluorescent labeling of leukocytes with rhodamine 6G. An early inflammatory response to irradiation was apparent in the brain 2 h after irradiation. The numbers of rolling and adherent leukocytes increased significantly and peaked at 24 h. Injection with the anti-ICAM-1 mAb significantly reduced leukocyte adhesion and permeability thereby linking the two processes. These findings provide a target to reduce radiation-related permeability and cell adhesion and potentially the side effects of radiation in the CNS.