The Role of Aggregation in Fusobacterium nucleatum-induced Immune Cell Death

Introduction: Fusobacterium nucleatum, an anaerobic oral bacterium, has been shown to be highly abundant in endodontic infections. Its role in these infections remains unclear. Previous studies have shown that F nucleatum could aggregate immune cells. We have demonstrated that F nucleatum can induce significant apoptosis in peripheral blood mononuclear cells (PBMCs). In this in vitro study, we sought to determine what role this aggregation phenomenon has on the induction of apoptosis in PBMCs. Methods: F nucleatum bacteria were treated as follows: formaldehyde-fixed, heat-treated, or sonicated before co-culturing with PBMCs. Cell aggregation and apoptosis of the PBMCs were assessed under light microscopy and analyzed by flow cytometry, respectively. PBMCs were then immobilized with a Matrigel matrix before treatment with F nucleatum. Aggregation and apoptosis were assessed as before. Surface staining of activation marker CD69 was assessed by flow cytometry. The apoptosis and CD69 data underwent one-way analysis of variance, followed by post hoc Bonferroni test and chi(2) test, respectively, to determine statistical significance. Results: Viable and formaldehyde-treated but not sonicated or heat-treated F nucleatum bacteria were able to cause severe aggregation and apoptosis of the immune cells. Disruption of F nucleatum mediated aggregation by immobilization of the cells with a Matrigel matrix resulted in a significant diminution of cell death but not cell activation when assessed by using surface expression of CD69 early activation antigen. Conclusions: F nucleatum's ability to induce cell death in immune cells appears to be mediated through the immune cells being aggregated, which might have important implications for its pathogenesis. (J Endod 2011;37:1531 1535)