Interleukin-17 in Chronic Inflammatory Neurological Diseases

Interleukin 17 (IL-17) has been linked to autoimmune diseases, although its regulation and function have remained unclear. Here we have evaluated in vitro and in vivo the requirements for the differentiation of naive CD4 T cells into effector T helper cells that produce IL-17. This process required the costimulatory molecules CD28 and ICOS but was independent of the cytokines and transcription factors required for T helper type 1 or type 2 differentiation. Furthermore, both IL-4 and interferon-gamma negatively regulated T helper cell production of IL-17 in the effector phase. In vivo, antibody to IL-17 inhibited chemokine expression in the brain during experimental autoimmune encephalomyelitis, whereas overexpression of IL-17 in lung epithelium caused chemokine production and leukocyte infiltration. Thus, IL-17 expression characterizes a unique T helper lineage that regulates tissue inflammation.

We describe de novo generation of IL-17-producing T cells from naive CD4 T cells, induced in cocultures of naive CD4 T cells and naturally occurring CD4+ CD25+ T cells (Treg) in the presence of TLR3, TLR4, or TLR9 stimuli. Treg can be substituted by TGFbeta1, which, together with the proinflammatory cytokine IL-6, supports the differentiation of IL-17-producing T cells, a process that is amplified by IL-1beta and TNFalpha. We could not detect a role for IL-23 in the differentiation of IL-17-producing T cells but confirmed its importance for their survival and expansion. Transcription factors GATA-3 and T-bet, as well as its target Hlx, are absent in IL-17-producing T cells, and they do not express the negative regulator for TGFbeta signaling, Smad7. Our data indicate that, in the presence of IL-6, TGFbeta1 subverts Th1 and Th2 differentiation for the generation of IL-17-producing T cells.

Interleukin-12 (IL-12) is a heterodimeric molecule composed of p35 and p40 subunits. Analyses in vitro have defined IL-12 as an important factor for the differentiation of naive T cells into T-helper type 1 CD4+ lymphocytes secreting interferon-gamma (refs 1, 2). Similarly, numerous studies have concluded that IL-12 is essential for T-cell-dependent immune and inflammatory responses in vivo, primarily through the use of IL-12 p40 gene-targeted mice and neutralizing antibodies against p40. The cytokine IL-23, which comprises the p40 subunit of IL-12 but a different p19 subunit, is produced predominantly by macrophages and dendritic cells, and shows activity on memory T cells. Evidence from studies of IL-23 receptor expression and IL-23 overexpression in transgenic mice suggest, however, that IL-23 may also affect macrophage function directly. Here we show, by using gene-targeted mice lacking only IL-23 and cytokine replacement studies, that the perceived central role for IL-12 in autoimmune inflammation, specifically in the brain, has been misinterpreted and that IL-23, and not IL-12, is the critical factor in this response. In addition, we show that IL-23, unlike IL-12, acts more broadly as an end-stage effector cytokine through direct actions on macrophages.