Activated B Cells Contribute to Osteoclastogenesis During T. forsythia Infection Associated Alveolar Bone Loss

Objectives: The ‘red complex’ periodontal pathogen Tannerella forsythia (Tf) induces a TLR2 dependent Th2 response in mice which leads to alveolar bone loss. Since Th2 responses activate B cells and recent evidence showing that B cell stimulating cytokines APRIL and BLyS are elevated during periodontitis suggest that B cell factors might contribute to alveolar bone resorption. This led us to seek the identity and function of B cell factors involved in periodontal bone loss. In this study we sought to determine to what extent the osteoclastogenic factor RNAKL expression in B cells might contributes to Tf induced alveolar bone loss.
Methods: Wild-type BALB/cJ, Th2 deficient (STAT6 KO) and B-cell KO (Igh-J^tm1Dhu) mice were infected to assess the effect of Th2 on RANKL expression on B cells and bone loss. FACS, morphometric method and TRAP staining were used to quantify the B cells, bone loss and in vitro osteoclastogenesis respectively. Data were analyzed using a Student's t test (between two groups) or ANOVA (multiple group comparisons), as appropriate. Statistical significance was defined as p < 0.05.
Results: Tf infected wild-type mice showed higher net alveolar bone loss over STAT6 ^KO and Igh-J^tm1Dhu mice. RANKL⁺ B cells were significantly higher in Tf infected wild-type BALB/cJ mice than in Th2 abrogated mice. Activated B cells and their spent medium from Tf infected mice induced higher number of multi nucleated TRAP⁺ cells, which corroborates with bone loss levels.
Conclusions: Our study shows that T. forsythia infection induces TLR2 mediated Th2 immune response, which leads to increased RANKL⁺ B cells and driving alveolar bone resorption.