Icariin Attenuates Neuroinflammation and Dopaminergic Degeneration in a Rodent Model of Parkinson's Disease by Promoting the Expansion of Regulatory T Cells

Background: Icariin, a bioactive flavonoid, exhibits significant neuroprotective properties and has emerged as a promising candidate for preventing neurodegenerative diseases, including Parkinson's disease (PD). However, the mechanism underlying its action is not fully understood. Enhancing the function and frequency of peripheral regulatory T cells (Tregs) may mitigate dopaminergic degeneration. This study investigates the role of Tregs in icariin's neuroprotective effects in a rodent model of PD.

Methods: PD was induced in mice via stereotactic injection of 6-hydroxydopamine (6-OHDA). Mice underwent pretreatment with saline, icariin, an Androgen Receptor inhibitor (ARI), or a combination of icariin and ARI. Motor function was assessed in each experimental group, and dopaminergic neuronal injury was evaluated using immunohistochemistry (IHC) staining for tyrosine-hydroxylase (TH) in the substantia nigra (SN) and striatum. IHC was used to quantify CD4⁺ T-cell infiltration in the SN. Neuroinflammation was assessed through mRNA levels of the pro-inflammatory M1 phenotype of microglia, the anti-inflammatory M2 phenotype of microglia, and the levels of indicated pro-inflammatory cytokines in the SN. The frequency of Tregs among peripheral blood mononuclear cells (PBMCs) was analyzed by flow cytometry, and with Tregs were depleted using PC61 monoclonal antibodies. In vitro Androgen Receptor (AR) knockdown using shRNA in naïve CD4⁺ T cells was performed to validate the AR-dependent mechanism.

Results: The results revealed that icariin significantly alleviated dopaminergic degeneration. Mechanistically, icariin promotes the expansion of peripheral neuroprotective and immunosuppressive Tregs, thereby restricting CD4⁺ T cell migration into the SN. This improvement in the inflammatory microenvironment reduced neuroinflammation and mitigated neurodegeneration. However, the neuroprotective and anti-inflammatory effects of icariin were lost when combined with ARI. Additionally, Treg depletion before 6-OHDA injection reversed the positive effects observed in the PD model.

Conclusions: Icariin protects against neurodegeneration and neuroinflammation by boosting Tregs expansion in an androgen receptor-dependent manner.