Objective Macrophage Activation Syndrome (MAS) is a devastating cytokine storm syndrome complicating many inflammatory diseases and characterized by fever, pancytopenia, and systemic inflammation. It is clinically similar to Hemophagocytic Lymphohistiocytosis (HLH), which is caused by viral infection of a host with impaired cellular cytotoxicity. Murine models of MAS and HLH illustrate Interferon-γ (IFN-γ) as the driving stimulus for hemophagocytosis and immunopathology. We sought to understand the inflammatory contributors to a murine model of Toll-like Receptor 9 (TLR9)-induced fulminant MAS. Methods Wild-type (WT), transgenic, and cytokine-inhibited mice were treated with an IL-10 receptor blocking antibody and TLR9 agonist, and parameters of MAS were evaluated. Results Fulminant MAS was characterized by dramatic elevations in IFN-γ, IL-12, and IL-6. Serum IFN-γ correlated with enhanced IFN-γ production within some hepatic populations, but fewer IFN-γ+ cells. Surprisingly, IFN-γKO mice developed immunopathology and hemophagocytosis comparably to WT mice. However, IFN-γKO mice did not become anemic and had greater numbers of splenic erythroid precursors. IL-12 neutralization phenocopied disease in IFN-γKO mice. Interestingly, Type I interferons contributed to the severity of hypercytokinemia and weight loss, but their absence did not otherwise affect MAS manifestations. Conclusion These data demonstrate that both fulminant MAS and hemophagocytosis can arise independently of IFN-γ, IL-12, or Type I interferons. They also suggest that IFN-γ-mediated dyserythropoiesis, not hemophagocytosis, is the dominant cause of anemia in fulminant TLR9-MAS. Thus, our data establish a novel mechanism for the acute anemia of inflammation, but suggest that a variety of triggers can result in hemophagocytic disease.

Dendritic cells (DCs) have been suggested to direct a type of Th differentiation through their cytokine profile, e.g., high IL-12/IL-23 for Th1 (named DC1/immunogenic DCs) and IL-10 for Th2 (DC2/tolerogenic DCs). Suppressor of cytokine signaling (SOCS)-3 is a potent inhibitor of Stat3 and Stat4 transduction pathways for IL-23 and IL-12, respectively. We thus hypothesize that an enhanced SOCS-3 expression in DCs may block the autocrine response of IL-12/IL-23 in these cells, causing them to become a DC2-type phenotype that will subsequently promote Th2 polarization of naive T cells. Indeed, in the present study we found that bone marrow-derived DCs transduced with SOCS-3 significantly inhibited IL-12-induced activation of Stat4 and IL-23-induced activation of Stat3. These SOCS-3-transduced DCs expressed a low level of MHC class II and CD86 on their surface, produced a high level of IL-10 but low levels of IL-12 and IFN-γ, and expressed a low level of IL-23 p19 mRNA. Functionally, SOCS-3-transduced DCs drove naive myelin oligodendrocyte glycoprotein-specific T cells to a strong Th2 differentiation in vitro and in vivo. Injection of SOCS-3-transduced DCs significantly suppressed experimental autoimmune encephalomyelitis, a Th1 cell-mediated autoimmune disorder of the CNS and an animal model of multiple sclerosis. These results indicate that transduction of SOCS-3 in DCs is an effective approach to generating tolerogenic/DC2 cells that then skew immune response toward Th2, thus possessing therapeutic potential in Th1-dominant autoimmune disorders such as multiple sclerosis.

IL-12 has long been considered important in the pathogenesis of multiple sclerosis. However, evidence from recent studies strongly supports the critical role of IL-12-related proinflammatory cytokine IL-23, but not IL-12, in the development of experimental autoimmune encephalomyelitis (EAE), an animal model of this disease. The role of IL-23 in the CNS immunity of multiple sclerosis patients has not been elucidated; nor is it known whether human microglia produce this cytokine. In this study we investigated the expression of IL-23p19 and p40, with its key subunit p19 as the focus, in histologically characterized CNS specimens from multiple sclerosis and control cases using in situ hybridization and immunohistochemistry. A significant increase in mRNA expression and protein production of both subunits of IL-23 was found in lesion tissues compared with non-lesion tissues. Double staining showed that activated macrophages/microglia were an important source of IL-23p19 in active and chronic active multiple sclerosis lesions. We also detected IL-23p19 expression in mature dendritic cells which were preferentially located in the perivascular cuff of active lesions. The finding that human microglia produce IL-23 was further confirmed by the inducible production of IL-23p19 and p40 in cultured human microglia in vitro upon different Toll-like receptor stimulations. Taken together, these findings on the expression of IL-23p19 in multiple sclerosis lesions may lead to a better understanding of the events culminating in human multiple sclerosis.

Key Points• Blockade of ST2 improves morbidity and mortality in murine FHL.• Danger signals such as may be required to amplify antigen-specific immune responses above the threshold for FHL disease in mice.

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