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 lesions using immunohistochemical methods. During the acute phase, both IL-1 alpha- and IL-1 beta-expressing cells were observed adjacent to many osteoclasts that contained large cytoplasms, and consequently the periapical lesions tended to expand. However, the number of IL-1 beta-expressing cells was about twice that of IL-1 alpha-expressing cells during the same period. On the other hand, there were significantly less IL-1 beta-expressing cells during the chronic phase and consequently a significant expansion of the periapical lesions was not seen. Furthermore, IL-1 alpha- and IL-1 beta-expressing cells were located far from the bone surfaces, whereas a fibroblastic cell layer lay between the IL-1-expressing cells and the bone surfaces. Both IL-1 alpha and IL-1 beta were mainly expressed by macrophages. In addition, after the percolated application of formocresol to experimentally induced periapical lesions, numerous macrophages expressing IL-1 beta became conspicuous within the vicinity of the extensive bone resorption sites. These results suggest that macrophages expressing IL-1 beta may play an important role in the activation and recrudescence of osteoclastic bone resorption, and this may be due to their localization close to the bone resorption sites noted within experimental periapical lesions.