Low‐dose radiotherapy (LD‐RT) and the modulation of iNOS expression in adjuvant‐induced arthritis in rats

Purpose: Low‐dose radiotherapy (LD‐RT) of arthritic joints applied during the peak of the acute inflammatory response improves the clinical and histomorphological development of adjuvant arthritis. The study was undertaken to investigate the cellular composition of the inflammatory infiltrate and the expression of the pro‐inflammatory and anti‐inflammatory enzymes, inducible nitric oxide synthase (iNOS), cyclo‐oxygenase 2 (COX‐2) and haem‐oxygenase 1 (HO‐1), in response to LD‐RT.Materials and methods: Adjuvant arthritis in female Lewis rats was induced by intradermal injection of heat‐inactivated mycobacterium tuberculosis on day 0. Both arthritic hind paws were sham irradiated (group 1) or X‐irradiated with either 5×1.0 Gy (group 2) or 5×0.5 Gy (group 3) from days 15 to 19 after induction (15 animals/group). On days 21 (n=12 joints/group) and 30 (n=18 joints/group), cryostat sections were analysed histologically and immunohistologically after specific staining for macrophages, iNOS, COX‐2 and HO‐1.Results: A total of 5×1.0 Gy or 5×0.5 Gy led to a significant reduction of clinical symptoms from days 21 to 29, and a highly significant reduction of cartilage and bone destruction on day 30. Macrophage‐positive areas could be detected continuously throughout the periarticular infiltrate, and were slightly reduced after LD‐RT on days 21 and 30. This reduction was more pronounced after 5×1.0 Gy. Following LD‐RT, the iNOS score was reduced by about 45–50% on days 21 (p<0.05) and 30 (p<0.001). In contrast, the HO‐1 score was increased by about 50% on days 21 (p=0.08) and 30 (p=0.03).Conclusions: The clinically and histologically observed prevention of the pro‐gression of adjuvant arthritis after LD‐RT given during the peak of the acute inflam‐matory response and the reduction of cartilage and bone destruction in the chronic phase appears to be related to the modulation of iNOS activity by low X‐ray doses.