[Th17 lymphocytes in bacterial infections]

Th17 cells are a relatively newly discovered subpopulation of helper CD4+ T lymphocytes. It has been shown that these cells may contribute to tissue damage during certain inflammatory and autoimmune diseases and also play an important role in antitumor and antimicrobial, particularly antibacterial, immunity. Bacteria stimulate the Th17 response through several Toll-like (TLR), NOD-like (NLR) and C-type lectin (CLR) receptors. When activated, Th17 lymphocytes produce several cytokines, mainly interleukin (IL)-17 and chemokines, that further attract and activate phagocytes to mediate bacterial clearance. Thus Th17 cells contribute to induction of host protective immunity, particularly against extracellular bacterial pathogens: Staphylococcus aureus, Streptococcus pneumoniae and Klebsiella pneumoniae. Furthermore, numerous studies indicate the importance of Th17 lymphocytes in immunity against intracellular bacteria such as Francisella tularensis and Chlamydia muridarum. In this case, the protective immune response is mediated mainly through stimulation of local dendritic cell (DC) function for establishing a Th1 immune response, indispensable for controlling intracellular infectious agents. However, deregulation of the Th17/IL17 response during bacterial infections may lead to profound pathologies. As a result, Th17 cells participate in chronic inflammatory diseases, leading to tissue destruction and favoring tumor development. This article summarizes current understanding of the bacteriainduced Th17 response in the context of the protective immune response and immunopathology.