Migration and function of Th17 cells.

T cells play central roles in regulation of the immune system in mammals. T cell receptor alphabeta T cells that can produce the cytokine IL-17 are called Th17 cells and form a lineage of effector T cells that are distinct from Th1, Th2 and FoxP3(+) T cells. The primary function of Th17 cells is to fight infection by bacterial and fungal pathogens. Autoreactive Th17 cells are implicated in mediating inflammation in the central nerves system, joints and other tissues. Th17 cells express a number of chemokine receptors including a secondary lymphoid tissue homing receptor (CCR7), the B follicle homing receptor (CXCR5), and non-lymphoid tissue homing receptors (CCR4, CCR5, and CXCR6). While these receptors are heterogeneously expressed by Th17 cells and have the potential to guide the migration of Th17 cell subsets into various tissues, CCR6 is uniformly expressed by most Th17 cells regardless of their tissue tropism. CCR6 plays an important role in migration of Th17 cells to inflamed tissues. Another function of CCR6 is to localize Th17 cells close to CCL20 expressing cells in the intestine, which would be important for the homeostasis of Th17 cells. Thus, chemokine receptors appear to play complex roles in the biology of Th17 cells.