Senior Research Scientist/Assistant Professor Feinstein Institutes for Medical Research, Northwell Health Manhasset, New York, United States
Disclosure(s):
Ailing Lu, PhD: No financial relationships to disclose
Introduction/Rationale: Macrophage Activation Syndrome (MAS) is a severe systemic inflammatory disorder linked to autoimmune diseases like sJIA. It involves macrophage hyperactivation, causing tissue damage, multi-organ failure, and hematopoietic issues. Overexpression of Toll-like receptor 7 (TLR7) has been associated with MAS. Increased GM-CSF levels and IRF5 activation observed in TLR7.1 transgenic mice suggest the GM-CSF-IRF5 pathway’s role in TLR7-driven MAS.
Methods: WT, Irf5–/–, and GM-CSF–/–mice received topical R848 treatment to activate TLR7. Flow cytometry and imaging flow cytometry were used to quantify blood and splenic Ly6C^hi monocytes and inflammatory hemophagocytes (iHPCs). Red blood cell and platelet counts were measured with the HEMAVET analyzer. To assess disease severity, WT, TLR7.1, TLR7.1Irf5+/–, TLR7.1Irf5–/–, TLR7.1GM-CSF+/–, TLR7.1GM-CSF–/– mice were evaluated. Cytokine levels (GM-CSF, IL-12, IL-6, TNFα) were measured using LegendPlex assays across all groups.
Results: Topical R848 in WT mice mimicked the TLR7.1 transgenic phenotype, with significant expansion of Ly6C^hi monocytes and iHPCs, contributing to dysregulated hematopoiesis and anemia. These effects were attenuated in Irf5–/– and GM-CSF+/– mice. TLR7.1Irf5+/– and TLR7.1Irf5–/– mice showed reduced GM-CSF levels.GM-CSF deficiency partially protected against early MAS symptoms, with age-dependent effects. Irf5 deficiency resulted in a more profound rescue, markedly decreasing Ly6C^hi monocytes and iHPCs, and fully preventing anemia and thrombocytopenia, thereby restoring hematopoietic balance.
Conclusion: TLR7 overexpression drives excessive GM-CSF production, which in turn promotes IRF5 activation and expansion of Ly6C^hi monocytes. These cells differentiate into iHPCs, contributing to anemia and inflammatory pathology in MAS. Targeting the GM-CSF–IRF5 axis offers a promising therapeutic strategy for MAS and related autoimmune diseases.
