Bone‐resorptive cytokine gene expression in periapical lesions in the rat

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Abstract
Periapical bone destruction is an important pathogenic sequela of pulpal infection. Recent findings from this laboratory have demonstrated that most bone‐resorbing activity in extracts of rat periapical lesions can be neutralized by an anti‐interleukin (IL‐1α antiserum. To further clarify pathogenic mechanisms, bone‐resorptive cytokine messenger RNA (mRNA) expression was analyzed in developing rat periapical lesions. The molar teeth of 20 Sprague‐Dawley rats were surgically exposed and leftopen to permit infection from the oral environment. Total cell RNA was isolated from periapical granuloma tissue obtained on days 3, 7, 15 and 30 after exposure. mRVA for IL‐1α, IL‐1β and tumor necrosis factor a (TNF‐α) was amplified by revene transcription polymerase chain reaction, and levels were approximated by comparison to the parallel amplification of the housekeeping gene glyceraldehyde phosphate dehydrogenase. IL‐1α and TNF‐α mRNA were both highly expressed begining on day 7, increased on day 15, and declined somewhat on day 30. In contras, IL‐1β mRNA was expressed at much lower levels, but with similar kinetics. The kinetics of steady state IL‐1α and TNF‐α mRNA levels were confirmed using the quantitative RNase protection assay, whereas IL‐1β mRNA could not be detected by this technique. IL‐1α mRNA‐expressing cells were identified using in situ hybridization and included infiltrating macrophages, as well as resident fibroblasts, endothelial cells and osteoclasts. These results demonstrate that the IL‐1α and TNF‐α genes are highly expressed in developing periapical lesions in the rat and confirm previous studies at the protein level in this model.