An immunohistochemical study of the behavior of cells expressing interleukin-1 alpha and interleukin-1 beta within experimentally induced periapical lesions in rats

Cells expressing interleukin-1 (IL-1) were examined in rat periapical lesio ns using immunohistochemical methods. During the acute phase, both IL-1 alp ha- and IL-1 beta-expressing cells were observed adjacent to many osteoclas ts that contained large cytoplasms, and consequently the periapical lesions tended to expand. However, the number of IL-1 beta-expressing cells was ab out twice that of IL-1 alpha-expressing cells during the same period. On th e other hand, there were significantly less IL-1 beta-expressing cells duri ng the chronic phase and consequently a significant expansion of the periap ical lesions was not seen. Furthermore, IL-1 alpha- and IL-1 beta-expressin g cells were located far from the bone surfaces, whereas a fibroblastic cel l layer lay between the IL-1-expressing cells and the bone surfaces. Both I L-1 alpha and IL-1 beta were mainly expressed by macrophages. In addition, after the percolated application of formocresol to experimentally induced p eriapical lesions, numerous macrophages expressing IL-1 beta became conspic uous within the vicinity of the extensive bone resorption sites. These resu lts suggest that macrophages expressing IL-1 beta may play an important rol e in the activation and recrudescence of osteoclastic bone resorption, and this may be due to their localization close to the bone resorption sites no ted within experimental periapical lesions.