Interleukin‐17 family cytokines in protective immunity against infections: role of hematopoietic cell‐derived and non‐hematopoietic cell‐derived interleukin‐17s

Interleukin-17 family cytokines, consisting of six members, participate in immune response in infections and autoimmune and inflammatory diseases. The prototype cytokine of the family, IL-17A, was originally identified from CD4+ T cells which are now termed Th17 cells. Later, IL-17A-producing cells were expanded to include various hematopoietic cells, namely CD8+ T cells (Tc17), invariant NKT cells, γδ T cells, non-T non-B lymphocytes (termed type 3 innate lymphoid cells) and neutrophils. Some IL-17 family cytokines other than IL-17A are also expressed by CD4+ T cells: IL-17E by Th2 cells and IL-17F by Th17 cells. IL-17A and IL-17F induce expression of pro-inflammatory cytokines to induce inflammation and anti-microbial peptides to kill pathogens, whereas IL-17E induces allergic inflammation. However, the functions of other IL-17 family cytokines have been unclear. Recent studies have shown that IL-17B and IL-17C are expressed by epithelial rather than hematopoietic cells. Interestingly, expression of IL-17E and IL-17F by epithelial cells has also been reported and epithelial cell-derived IL-17 family cytokines shown to play important roles in immune responses to infections at epithelial sites. In this review, we summarize current information on hematopoietic cell-derived IL-17A and non-hematopoietic cell-derived IL-17B, IL-17C, IL-17D, IL-17E and IL-17F in infections and propose functional differences between these two categories of IL-17 family cytokines.