Brief Report: Interferon-γ-Mediated Immunopathology Potentiated by Toll-Like Receptor 9 Activation in a Murine Model of Macrophage Activation Syndrome

Objective Macrophage activation syndrome (MAS) is a life-threatening cytokine storm syndrome that occurs in patients with underlying rheumatic diseases. Preclinical and clinical data suggest that interferon-gamma (IFN gamma) is pathogenic in MAS, but how IFN gamma may be linked to disease pathogenesis remains unknown. This study was undertaken to determine whether IFN gamma signals synergize with systemic innate immune responses to drive the cytokine storm in a murine model of MAS. Methods IFN gamma-deficient mice were treated with 5 doses of the Toll-like receptor 9 (TLR-9) agonist CpG 1826, IFN gamma, or a combination of the 2 stimuli over the course of 10 days. Immunopathologic features of MAS, including cytopenias, hepatitis, hepatosplenomegaly, and induction of inflammatory myelopoiesis, were assessed. Mixed bone marrow chimeras were created to determine whether TLR-9- and IFN gamma receptor 1 (IFN gamma R1)-dependent signals induce enhanced myelopoiesis in a cell-intrinsic or cell-extrinsic manner. Results IFN gamma-deficient mice did not develop features of MAS when treated with repeated doses of either the TLR-9 agonist or IFN gamma alone. In contrast, IFN gamma-deficient mice treated with both the TLR-9 agonist and IFN gamma developed cytopenias, hepatitis, and hepatosplenomegaly, reproducing major clinical features of MAS. TLR-9- and IFN gamma R1-dependent signals synergized to enhance myeloid progenitor cell function and induce myelopoiesis in vivo, which occurred through cell-extrinsic mechanisms and correlated with the induction of disease. Conclusion These findings demonstrate that TLR-9-driven signals potentiate the effects of IFN gamma to initiate murine MAS, and provide evidence that induction of inflammatory myelopoiesis is a common TLR-9- and IFN gamma-dependent pathway that may contribute to the pathogenesis of MAS.