Immune response to co-infection of periodontopathogens in a mouse model

Objectives: To determine the immune responses to a co-infection of periodontopathic bacteria using a mouse abscess model. Methods: Non-immunised BALB/c mice were divided into 7 groups of 3-4 mice, challenged subcutaneously with Fusobacterium nucleatum (Fn), Tannerella forsythensis (Tf) and Porphyromonas gingivalis (Pg) as follows: Group 1 mice received Pg and Fn; Group 2 mice, Tf and Fn; Group 3 mice, Pg and Tf; Group 4 mice, Pg only; Group 5 mice, Fn;Group 6 mice, Tf; and Group 7 mice, saline. On days 1, 4, 7, and 14 mice were weighed, euthanized, lesion size measured, excised and blood collected. Sections from the lesions were stained by an avidin-biotin immunoperoxidase method for CD4+ T-cells, CD8+ T-cells, CD19+ B-cells and Mac3+ macrophages. Levels of serum antibodies from each group specific for each bacterium were determined using an ELISA. Results: Lesions developed at the injection site in all groups except the control group. Lesion sizes peaked at day 4 with Pg/Tf and Pg; at day 7 with mice injected with Pg/Fn, Fn/Tf, Fn and Tf. All groups except those challenged with Pg/Tf and Tf/Fn had healed or were healing at day 14. All mice lost weight at 12 and 36hrs, gained weight on day 7 and lost weight unexpectedly on day 14. Pg/Fn mice showed an increased infiltration of CD4+ T-cells and Mac3+ cells into the lesion when compared to Pg or Fn mice. Pg/Tf mice also showed lesions with an increased infiltration of CD4+ T-cells compared with Pg or Tf mice. Mice challenged with combinations of bacteria typically had higher specific antibody levels than those challenged with individual bacteria. Conclusions: Co-infection resulted in a complex response to bacterial synergism not seen with individual bacteria. This may result from increased expression of bacterial virulence factors with the subsequent immune response.