Background/Purpose: Hemophagocytic syndromes are caused by an uncontrolled systemic inflammatory response resulting in cytopenias, multisystem organ failure, and rapid death, often despite aggressive therapy. Macrophage activation syndrome (MAS) is one of the hemophagocytic syndromes known to complicate severe inflammatory diseases, and can be seen in patients with systemic rheumatologic conditions. Previous work describes a novel murine model utilizing repeated injection of CpG, a Toll-like Receptor 9 (TLR9) agonist, into wild-type mice to stimulate MAS. This model recapitulates the inflammatory, “cytokine storm” milieu and clinical features of MAS. TLR9, Interleukin (IL)-12 and interferon gamma are necessary for the development of MAS in this model. However, the initial cellular responses contributing to CpG-induced MAS remain elusive. 

Methods: Wild-type (WT) and TLR9^-/- bone marrow chimeras, transgenic clec4-DTR mice, and IL-12 reporter mice were treated with or without CpG, and parameters of MAS were measured. WT and TLR9^-/- mixed in vitro splenocyte cultures were treated with or without CpG, and IL-12 production was measured from supernatants by ELISA and intracellular IL-12 was assessed by flow cytometry.

Results: We show that TLR9 expression within the hematopoietic compartment is necessary and sufficient for CpG-induced systemic inflammation. In contrast, TLR9 expression in radioresistant cells is not sufficient to drive CpG-mediated MAS. Surprisingly, depletion of individual innate immune cell populations does not alter the course of CpG-induced systemic inflammation including the depletion of plasmacytoid dendritic cells, cells known for their potent proinflammatory response to CpG. Using IL-12 reporter mice, IL-12 production upon CpG stimulation is shown to be redundant, as multiple innate immune cell populations produce IL-12 in this model. Finally, in contrast to in vivo infection models, mixed in vitro cultures of TLR9-deficient and TLR9-sufficient cells demonstrate that IL-12 production is intrinsic to cells capable of sensing CpG.

Conclusion: These data demonstrate that multiple innate immune cells contribute to CpG-induced immunopathology by producing IL-12. This redundancy in IL-12 production explains the inability to cure disease by targeting depletion of a single innate immune cell population. Futhermore, the ability of a cell to produce IL-12 is dependent on its ability to sense CpG through TLR9 expression. Thus, our data identify the most proximal cellular events leading to CpG-induced immunopathology providing novel insights into disease mechanisms and robust targets for the development of future therapeutic interventions in MAS.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/innate-immune-cell-production-of-interleukin-12-drives-cpg-induced-macrophage-activation-syndrome/