Antibody and T Cell Responses to <em>Fusobacterium nucleatum</em> and <em>Treponema denticola</em> in Health and Chronic Periodontitis

<div><p>The characteristics of the T cell response to the members of oral flora are poorly understood. We characterized the antibody and T cell responses to FadA and Td92, adhesins from <em>Fusobacterium nucleatum</em>, an oral commensal, and <em>Treponema denticola</em>, a periodontal pathogen, respectively. Peripheral blood and saliva were obtained from healthy individuals and patients with untreated chronic periodontitis (CP, n = 11 paris) and after successful treatment of the disease (n = 9). The levels of antigen-specific antibody were measured by ELISA. In plasma, IgG1 was the most abundant isotype of Ab for both Ags, followed by IgA and then IgG4. The levels of FadA-specific salivary IgA (sIgA) were higher than Td92-specific sIgA and the FadA-specific IgA levels observed in plasma. However, the periodontal health status of the individuals did not affect the levels of FadA- or Td92-specific antibody. Even healthy individuals contained FadA- and Td92-specific CD4⁺ T cells, as determined by the detection of intracytoplasmic CD154 after short-term <em>in vitro</em> stimulation of peripheral blood mononuclear cells (PBMCs) with the antigens. Patients with CP tended to possess increased numbers of FadA- and Td92-specific CD4⁺ T cells but reduced numbers of Td92-specific Foxp3⁺CD4⁺ Tregs than the healthy subjects. Both FadA and Td92 induced the production of IFNγ and IL-10 but inhibited the secretion of IL-4 by PBMCs. In conclusion, <em>F. nucleatum</em> induced Th3 (sIgA)- and Th1 (IFNγ and IgG1)-dominant immune responses, whereas <em>T. denticola</em> induced a Th1 (IFNγ and IgG1)-dominant response. This IFNγ-dominant cytokine response was impaired in CP patients, and the Td92-induced IFNγ levels were negatively associated with periodontal destruction in patients. These findings may provide new insights into the homeostatic interaction between the immune system and oral bacteria and the pathogenesis of periodontitis.</p>