Background/Purpose: Neuropsychiatric systemic lupus erythematosus (NPSLE) is one of the most severe organ manifestations in SLE and is associated with poor quality of life. We have shown that abnormal activation of microglia plays an essential role in the pathogenesis of NPSLE through phagocytosis and the over-production of proinflammatory cytokines¹. The aim of this study was to clarify the involvement of microglia in the pathogenesis of NPSLE and to identify potential novel therapeutic targets.

Methods: Microglia were isolated from female MRL/lpr lupus-prone mice and MRL/MpJ mice using a magnetic-activated cell sorting column and anti-CD11b-coated magnetic microbeads. RNA sequencing was performed to explore genes involved in microglial activation. To identify a candidate gene with potential as a therapeutic target for NPSLE, we assessed its function in microglia by evaluating effects of an inhibitor of a candidate gene on MRL/lpr and the conditional knockout mice (cKO) using Cx3cr1^CreERT2/+ mice. The expression of inflammatory cytokines were analyzed by real-time PCR, and phagocytosis assay was performed using fluorescent bioparticles in cultured microglia from both inhibitor-treated MRL/lpr and cKO mice. Real-time PCR and phagocytosis assay were performed after stimulation by lipopolysaccharide (LPS) or cytokines (IFNα, IFNγ, TNFα and IL-6). To evaluate the effect of intracerebroventricular administration of the inhibitor on behavioral abnormalities and microglial activation, behavioral tests and immunohistochemical evaluation of the brain were performed in MRL/lpr mice. In cKO mice, generated from C57BL/6, behavioral abnormalities were induced by imiquimod applied to the right ear three times a week for 14 weeks before the behavioral tests.

Results: Among all the sequenced genes, 1,022 differentially expressed genes (887 up-regulated, 135 down-regulated) were detected in microglia of MRL/lpr compared with MRL/MpJ mice. In up-regulated genes, we focused on phosphodiesterase 1b (Pde1b), one of the top-fold changed genes. Consistent with the result from RNA sequencing, real-time PCR revealed that Pde1b expression was significantly high in MRL/lpr microglia. A PDE1B inhibitor significantly suppressed gene expression of cytokines (Tnf, Il6, and Il1b), and decreased the percentage of cells containing phagocytosed particles in cytokine-stimulated MRL/lpr microglia. Gene expression of cytokines (Tnf, Il6, and Il1b) and phagocytosis were also significantly suppressed in LPS- or cytokine-stimulated cKO (Cx3cr1^CreERT2/+-Pde1b^fl/fl) mice microglia compared with control mice (Cx3cr1^+/+-Pde1b^fl/fl) microglia. Intracerebroventricular administration of a PDE1B inhibitor ameliorated behavioral abnormalities and microglial activation in MRL/lpr. Furthermore, Pde1b cKO mice showed reduction in imiquimod-induced behavioral abnormalities.

Conclusion: Inhibition or microglia-specific cKO of Pde1b reduced microglial activation and ameliorated abnormal behavior. Our data suggested that PDE1B promoted microglial activation and was involved in the pathogenesis of NPSLE. PDE1B could be a novel therapeutic target for NPSLE. 

References: 1. Karino K, et al. Arthritis Rheumatol 2023;75:411-23.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/phosphodiesterase-1b-contributes-to-neuropsychiatric-manifestations-in-lupus-prone-mice-through-microglial-activation/