Fig. 6

Proposed therapeutic mechanism for sargramostim. During PD progression, native alpha-synuclein (α-syn) becomes modified and misfolded. Modification results in formation of oligomers that aggregate into fibrils due to dysfunctional protein clearance and breakdown. Fibrils coalesce into intra- and extraneuronal inclusion bodies (Lewy bodies) resulting in dopaminergic neuronal cell death. Dead and dying neurons release Lewy bodies and aggregated α-syn into the extracellular environment that is taken up by resident microglia and infiltrating macrophages, causing the initiation of an pro-inflammatory signaling and reactive phenotype. Reactive microglia/macrophages secrete neurotoxic mediators in response to misfolded protein, resulting in additional neuronal death. The imbalance of inflammatory monocytes and T effector cells (Teff) with anti-inflammatory regulatory T cells (Treg) contributes to the peripheral inflammatory milieu associated with disease. To suppress this response, peripheral administration of sargramostim (GM-CSF, Leukine) results in proliferation of myeloid progenitor cells within the bone marrow that mobilize to the bloodstream following maturation into anti-inflammatory monocytes, granulocytes, and tolerogenic dendritic cells. Transcriptomic and proteomic evaluations of circulating monocytes after treatment with sargramostim revealed a monocyte phenotype with increased expression of CD93, CD163, ATG7, and GABARAPL2, and decreased expression of LRRK2, HMOX1, TLR2, TLR8, and RELA, indicating increased antioxidant, anti-inflammatory, and autophagic functions. Additionally, sargramostim treatment results in induction of immunosuppressive Tregs. Resulting tolerogenic dendritic cell-induced Treg populations show elevated FOXP3, CTLA-4, ITGB7, CD45RO, and CD31, which support a stable immunosuppressive phenotype with enhanced migratory functions. Within the brain, infiltrating monocytes and microglia become polarized into an anti-inflammatory phenotype with enhanced phagocytosis, autophagy, and macroautophagy. This leads to increased protein clearance, proper oligomer breakdown, decreased Lewy body formation, restoration of a homeostatic microenvironment, and ultimately, decreased neuroinflammation and neurodegeneration. Additionally, infiltration of induced immunosuppressive Treg to the sites of inflammation enhances an anti-inflammatory microglial phenotype and control of neural homeostasis, which further contributes to a disease-modifying neuroprotective environment