Analysis of the expression of NLRP3 and AIM2 in periapical lesions with apical periodontitis and microbial analysis outside the apical segment of teeth

Objective To detect the distribution and expression levels of the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) and the absent in Melanoma 2 (AIM2) inflammasomes in periapical lesions and to analyse the possible microbial stimuli outside of teeth. Design The distribution of NLRP3 and AIM2 inflammasomes in sixteen periapical lesions was investigated by immunohistochemistry. Meanwhile, the relative gene expression levels of NLRP3 and AIM2 in sixteen periapical lesions and three health periodontal tissue were quantified by real-time polymerase chain reaction (PCR). Moreover, forty-seven teeth without sinus tracts were obtained in the clinic and included in bacterial analysis using PCR. Then, the mRNA levels of apoptosis-associated speck-like protein (ASC), caspase-1, interleukin-1β (IL-1β), NLRP3 and AIM2 in THP-1-derived macrophages treated with lipopolysaccharides (LPS) of Porphyromonas were also quantified by real-time PCR, and the IL-1β secretion level was investigated using enzyme-linked immunosorbent assay (ELISA). Results NLRP3 and AIM2 were positively expressed in periapical lesions and were mainly distributed in inflammatory cells. Most of the samples that demonstrated up-regulation of NLRP3 mRNA also demonstrated up-regulation of caspase-1 mRNA. Microbial analysis revealed that Porphyromonas endodontalis was the most commonly detected species and was detected in 27 of 47 cases (57.4%), followed by Fusobacterium nucleatum (20/47, 42.6%), Porphyromonas gingivalis (19/47, 40.4%), Tannerella forsythia (19/47, 40.4%), Actinomyces sp. (17/47, 36.17%), Treponema denticola (10/47,21.28%), Actinomyces israelii (9/47,19.15%), Prevotella intermedia (6/47, 12.77%), Enterococcus faecalis (1/47,2.13%) and Enterococcus faecium (0/47,0). Furthermore, we found that LPS of P. gingivalis induced THP-1 cells to produce IL-1β and to activate NLRP3 and AIM2 inflammasomes. Conclusions Our results suggest that the NLRP3 and AIM2 proteins play a part in the pathogenesis of periapical periodontitis. Anaerobes, such as P. endodontalis, P. gingivalis, F. nucleatum and T. forsythia, were the main detected microbial stimuli that might activate inflammasomes in periapical tissues.