Separate groups demonstrate that IL-23 stimulation of cytokine by TH17 cells drives encephalitogenicity.

Two research teams have separately implicated the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) as playing a central role in the IL-23-driven neuroinflammatory process that occurs in autoimmune diseases such as multiple sclerosis. The separate groups, led by scientists at Thomas Jefferson University in Philadelphia and the University of Zurich in Switzerland, have found that IL-23 induces IL-17-producing T helper cells (TH17) to express GM-CSF, and this cytokine plays a key role in their encephalitogenicity. The Thomas Jefferson team notes that although the role of GM-CSF in the pathology of multiple sclerosis is unknown, these latest studies in animal models suggest that blocking GM-CSF activity may represent a feasible therapeutic strategy for multiple sclerosis.

Both research teams report their findings in Nature Immunology. The paper by Thomas Jefferson Medical College chairman and professor Abdolmohamad Rostami, M.D., and colleagues is titled “The encephalitogenicity of TH17 cells is dependent on IL-1- and IL-23-induced production of the cytokine GM-CSF.” The University of Zurich’s Laura Codarri, Ph.D., and team describe their findings in a paper titled “RORγt drives production of the cytokine GM-CSF in helper T cells, which is essential for the effector phase of autoimmune neuroinflammation.”

TH17 cells are widely believed to play a key role in autoimmune diseases of the CNS, with stimulation by IL-23 driving their encephalitogenicity, the researchers explain. However, to date, the mechanism behind this pathogenicity has remained unknown.

Dr. Rostami’s team differentiated naive CD4+ T cells into TH17 cells by stimulating the cells with TGF-β and IL-6 (first stimulation), and then reactivated the cells in the presence of various cytokines (second stimulation). During the first stimulation, a frac­tion of IL-17A+ T cells expressed GM-CSF, and small amounts of GM-CSF were present in culture supernatants. In the second stimulation, treatment with IL-23 resulted in a higher frequency of GM-CSF+ TH17 cells than in cultures treated with TGF-β and IL-6 without added cytokines. IL-23 also resulted in significant augmentation of GM-CSF secre­tion. When they then injected the resulting IL-23-treated TH17 cells into sublethally irradiated recip­ient mice, the animals all devel­oped severe experimental autoimmune encephalomyelitis (EAE) and died within seven days. In contrast, of the five mice injected with TH17 cells that had only been treated with TGF-β and IL-6, just two developed mild disease. They also found that adding IL-1β to IL-23 stimulation of the TH17 cells boosted production of both GM-CSF and IL-17A.

Significantly, when mice treated with an anti-GM-CSF antibody were injected with the IL-23-stimulated TH17 cells, they developed significantly milder disease with delayed onset rela­tive to that of control mice, and all survived. Notably, the researchers add, IL-23-deficient mice developed EAE when exogenous IL-23 was delivered into the CNS, which indicates that IL-23 can act on already-developed TH17 cells.

Experimental data has previously shown that GM-CSF induces IL-23 in APCs, which in turn induces GM-CSF expres­sion by TH17 cells, resulting in amplification of the inflammatory response, the authors note. TH1 cells also produced GM-CSF and responded to IL-1β by increasing GM-CSF production, which means they are another possible source of GM-CSF in EAE. However, the authors continue, it has also been shown that most CNS-infiltrating TH1 cells actually originate from TH17 cells.  “A plausible model can be proposed in which TH17 cells and their TH1 progeny (ex-TH17 cells) are essential in EAE, which includes the bulk of GM-CSF production, whereas classical TH1 cells have a marginal role,” Dr. Rostami’s team claims. “We propose a positive feedback loop whereby IL-23 produced by APCs induces GM-CSF production by TH17 cells, which in turn stimulates IL-23 production in APCs. Greater and/or prolonged production of IL-23 results in stronger and longer lasting TH17 responses, causing more profound inflammation.”

Dr. Codarri’s team has similarly implicated IL-23, along with the transcription factor RORγt, as driving expression of GM-CSF in helper T cells. Their research in addition showed that IL-12, interferon-γ (IFN-γ), and IL-27 acted as negative regulators.

They found that mice injected with T helper triggered to produce GM-CSF had a significantly earlier onset of EAE and greater disease severity than did recipients of cells polarized to secrete IFN-γ or IL-17. Also concurring with the Jefferson team’s findings, the Zurich team found that treating mice with an anti-GM-CSF antibody significantly reduced the severity of TH17 cell- or IFN-γ-secreting TH1 cell-mediated EAE. “This indicates that the secretion of GM-CSF is a critical feature for the encephalitogenicity of helper T cells in vivo regardless of whether they are polarized in vitro toward a TH17 or TH1 phenotype,” the team states.

Significantly, T cells lacking GM-CSF completely failed to induce EAE, whereas the loss of IFN-γ or IL-17A only minimally impaired T cell pathogenicity. And in mice injected with T cells that only produced GM-CSF or IL-17A or IFN-γ, it was the animals receiving the GM-CSF-expressing cells that developed the most severe EAE. This importance of GM-CSF over IL-17A or IFN-γ was upheld in studies of T cells taken from mice which were doubly deficient in both IFN-γ and IL-17A. When these T cells were reactivated and transferred into wild-type recipient mice, the resulting disease profile in treated animals was the same as in mice administered with wild-type T cells.

Further tests in mice showed that GM-CSF acts at least in part to stimulate the infiltration of myeloid cells into the CNS. “Conceptually, the finding that of all known T cell cytokines, GM-CSF seems to be the only one absolutely essential for endowing T cells with pathogenic properties links invasion of the CNS by T cells with the activation and maturation of cells belonging to the myeloid lineage,” the researchers concluded.

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