Interleukin-17 (IL-17A) is a pro-inflammatory
cytokine that is primarily secreted from T lymphocytes, mediators of adaptive immunity. Recently,
IL-17 was shown to be the defining
cytokine of a new T helper subset termed "Th17." Discovery of the Th17 population was a groundbreaking discovery that has triggered major revisions of the prevailing paradigms in T cell biology. Although produced by T cells,
IL-17 promotes expansion and recruitment of innate immune cells such as neutrophils, and also cooperates with TLR
ligands,
IL-1 beta, and
TNF alpha to enhance inflammatory reactions and stimulate production of
beta-defensins and other
antimicrobial peptides. Its receptor, IL-17RA, is ubiquitously expressed and shares many features with classical innate immune receptors such as shared intracellular tail motifs and convergence on common inflammatory transcription pathways. The role of
IL-17 in
periodontal disease is still uncertain, since
IL-17 has been shown to promote bone destruction in
arthritis, but is nonetheless essential to protect the host from pathogens, including periodontopathic organisms. Recent evidence has shown that Th17 cells are more osteoclastogenic than other T helper subsets such as Th1 or Th2. Ablation of
IL-17 signaling prior to onset of
infection with Porphyromonas gingivalis increases susceptibility to
periodontal bone loss, but this finding does not rule out the efficacy of therapeutic inhibition of
IL-17 after onset of severe disease.
IL-17 sits at the center of many complex diseases that integrate innate and adaptive immune mechanisms and requires careful study to maximize host protective effects and minimize host deleterious effects.