Background/Purpose: Systemic lupus erythematosus (SLE) is a systemic autoimmune disease that affects multiple organs including kidney and skin. While its etiology and pathogenesis are still largely unknown, recent studies suggest significant involvement of neutrophils. Dysregulation of neutrophil functions has been proposed to trigger and perpetuate autoimmunity. Specifically, evidence suggests a role for neutrophil extracellular traps (NETs; the extrusion of nucleic acids and various antibacterial components by the neutrophils as a process of program cell death) in autoimmunity. Neutrophils also synthesize various cytokines including type I interferons (IFNs), B-cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL), which may be involved in the induction of proinflammatory responses in SLE.

The number and phenotype of neutrophils in circulation displays circadian oscillations. A recent study showed that this is influenced by one of the clock genes, brain and muscle aryl hydrocarbon receptor nuclear translocator-like 1 (Bmal1). Neutrophils that lack Bmal1 are defective in aging and more responsive to sterile tissue inflammation. Therefore, in this study, we hypothesized that Bmal1 plays a protective role in SLE and investigated its association with SLE using TLR7-induced murine lupus model and human peripheral blood neutrophils from lupus patients.

Methods: Myeloid conditional Bmal1 knockout mice (KO) and C57BL/6J mice (WT) were treated with imiquimod cream (IMQ) for 6 weeks to induce a lupus phenotype. Upon euthanasia, systemic inflammatory reactions, serum anti-dsDNA antibody, histological findings in each organ, immune complex (IC) deposition in kidney, proteinuria, and type I IFN responses were evaluated. Mouse neutrophils were obtained from bone marrow using negative selection beads and their NET-forming capacity and gene expression were evaluated. Human peripheral neutrophils were purified from SLE patients in the morning (7am – 11am) to assess gene responses.

Results: KO and WT treated with IMQ showed comparable levels of splenomegaly, anemia, thrombocytopenia, and local skin inflammation. Histological changes in glomeruli were minimal and prominent proteinuria was not observed in either group. However, serum anti-dsDNA antibody levels were higher, and more IC deposition in kidney was observed in KO mice. The expression levels of IFN-stimulated genes in kidney were higher in KO. More neutrophils infiltrated into skin lesions in KO. There was no difference in NET-forming capacity of bone marrow neutrophils and serum cell-free DNA level between the two groups of mice. NET formation in tissue was not different either. The expression levels of APRIL in bone marrow neutrophils and the percentage of B cells in spleen were significantly higher in KO. Bmal1 expression level in human peripheral blood neutrophils from SLE patients correlated with serum C3 level and inversely correlated with serum anti-dsDNA antibody level.

Conclusion: Bmal1 negatively regulates autoantibody production and immune complex deposition in TLR7-induced murine lupus model. These results indicate that perturbation in the circadian rhythm of neutrophils can have pathogenic consequences in SLE.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-circadian-clock-gene-bmal1-negatively-regulates-autoantibody-production-in-sle/