γδ T cells: an important source of IL-17

IL-17 is a cytokine that plays an important role in orchestrating innate immune function. In addition, IL-17 has been shown to exacerbate autoimmune diseases. CD4+ αβ T cells, γδ T cells, and NK cells all produce IL-17. Th17 cells are a newly defined αβ+ T cell lineage characterized by IL-17 production. However, γδ T cells are often the major source of this cytokine. Their response can be very rapid during bacterial infections and has been shown to be protective, but IL-17-producing γδ T cells have also been found to exacerbate collagen-induced arthritis. Interestingly, some γδ T cells produce IL-17 in response to IL-23 alone, even in naïve animals, suggesting they are already differentiated and may develop differently than CD4+ αβ Th17 cells.