Cross‐reactive adaptive immune response to oral commensal bacteria results in an induction of receptor activator of nuclear factor‐κB ligand (RANKL)‐dependent periodontal bone resorption in a mouse model

Introduction: The present study examined whether induction of an adaptive immune response to orally colonizing non‐pathogenic Pasteurella pneumotropica by immunization with the phylogenetically closely related bacterium, Actinobacillus actinomycetemcomitans, can result in periodontal bone loss in mice. Methods: BALB/c mice harboring P. pneumotropica (P. pneumotropica⁺ mice) in the oral cavity or control P. pneumotropica‐free mice were immunized with fixed A. actinomycetemcomitans. The animals were sacrificed on day 30, and the following measurements were carried out: (i) serum immunoglobulin G and gingival T‐cell responses to A. actinomycetemcomitans and P. pneumotropica; (ii) periodontal bone loss; and (iii) identification of receptor activator of nuclear factor‐κB ligand (RANKL) ‐positive T cells in gingival tissue. Results: Immunization with A. actinomycetemcomitans induced a significantly elevated serum immunoglobulin G response to the 29‐kDa A. actinomycetemcomitans outer membrane protein (Omp29), which showed strong cross‐reactivity with P. pneumotropica OmpA compared to results in the control non‐immunized mice. The A. actinomycetemcomitans‐immunized P. pneumotropica⁺ mice developed remarkable periodontal bone loss in a RANKL‐dependent manner, as determined by the abrogation of bone loss by treatment with osteoprotegerin‐Fc. The T cells isolated from the gingival tissue of A. actinomycetemcomitans‐immunized P. pneumotropica⁺ mice showed an in vitro proliferative response to both A. actinomycetemcomitans and P. pneumotropica antigen presentation, as well as production of soluble(s)RANKL in the culture supernatant. Double‐color confocal microscopy demonstrated that the frequency of RANKL⁺ T cells in the gingival tissue of A. actinomycetemcomitans‐immunized P. pneumotropica⁺ mice was remarkably elevated compared to control mice. Conclusion: The induction of an adaptive immune response to orally colonizing non‐pathogenic P. pneumotropica results in RANKL‐dependent periodontal bone loss in mice.