From the editors

T helper 1 (T_H1) and T_H2 cells as we know them today — which is as CD4⁺ T-cell subsets producing distinct cytokines (interferon-γ and interleukin-4 (IL-4), respectively) — were first described in 1986 in The Journal of Immunology by Mosmann et al. Since then, our knowledge of T-cell biology has increased enormously, and many more distinct CD4⁺ T-cell subsets have been identified, including T_H3 cells, which are characterized by their production of transforming growth factor-β (TGFβ), and CD4⁺CD25⁺ regulatory T cells, which can produce IL-10 and TGFβ (although whether these cytokines are required for their suppressive function remains controversial). Recent studies have reported the existence of a population of CD4⁺ T cells that produces the pro-inflammatory cytokine IL-17. In the Opinion article on page 329, Chen Dong discusses the evidence supporting the suggestion that IL-17-producing CD4⁺ T cells are a unique T_H-cell lineage that is distinct from the T_H1- and T_H2-cell lineages in terms of the cytokines that are produced by these cells and the transcription factors that mediate their development.

One of the reasons for the huge interest in IL-17-producing T_H cells is that IL-17 has been shown to have a pro-inflammatory function in several animal models of autoimmunity, and its expression has been associated with several inflammatory diseases in humans, including rheumatoid arthritis. However, before IL-17-producing T_H cells can be considered as potential therapeutic targets, we need to learn more about their biology. And, as indicated in a Research Highlight on page 260 — which describes a recent paper in Immunity that reports a role for TGFβ in regulating the development of IL-17-producing T_H cells — we might be in for some surprises!