Age-Related Changes in Interleukin-1 Expression within Experimentally Induced Periapical Lesions in Rats

Objectives: This study investigated the effects of aging on the relationship between interleukin-1 (IL-1) expression and expansion of periapical lesions in young (5-week-old) and mature (1-year-old) rats.
Methods: Periapical lesions were induced in rats by opening pulp chambers of the mandibular first molars and filing the distal root canal with #25 K-files. Teeth were exposed to the oral environment during the experimental period. Rats were sacrificed 7, 14 and 28 days following treatment and examined histochemically.
Results: In young animals, numerous cells expressing IL-1α and IL-1β were observed adjacent to osteoclasts that contained large amounts of cytoplasms; consequently, the periapical lesions tended to expand. Both IL-1α and IL-1β were expressed mainly in macrophages. Young animals showed a significant increase in IL-1β expression, which was approximately one-tenth greater than that in mature animals during the acute phase. During the chronic phase, there were significantly fewer cells expressing IL-1β, and no significant expansion of the periapical lesion was seen. However, enlarged lesions were not repaired despite the presence of numerous osteoblasts on most resorbed bone surfaces around the lesions. Conversely, in mature animals, only a few cells expressed IL-1β within the lesions throughout the duration of the experiment. Although some small osteoclasts were detected, active bone resorption was not observed around the lesions. While cells expressing IL-1α were observed in the lesion, the size of the lesion had increased gradually. However, active bone remodeling was not observed around the lesion.
Conclusions: These results reveal differences in the expression of IL-1 subtypes within periapical lesions between young and mature animals. This leads to speculation that the types of cytokines produced in inflamed periapical tissues vary with age.