Enantiomeric Agonists of the Type 2 Cannabinoid Receptor Reduce Retinal Damage during Proliferative Vitreoretinopathy and Inhibit Hyperactive Microglia In Vitro

Microglia are resident immune cells of the central nervous system (CNS) and propagate inflammation following damage to the CNS, including the retina. Proliferative vitreoretinopathy (PVR) is a condition that can emerge following retinal detachment and is characterized by severe inflammation and microglial proliferation. The type 2 Cannabinoid Receptor (CB₂) is an emerging pharmacological target to suppress microglial-mediated inflammation when the eyes or brain are damaged. CB₂-knockout mice have exacerbated inflammation and retinal pathology during experimental PVR. We aimed to assess the anti-inflammatory effects of CB₂ stimulation in the context of retinal damage and also explore the mechanistic roles of CB₂ in microglia function. To target CB₂, we used a highly selective agonist, HU-308, as well as its enantiomer, HU-433, which is a putative selective agonist. First, β-arrestin2 and Gα_i recruitment was measured to compare activation of human CB₂ in an in vitro heterologous expression system. Both agonists were then utilized in a mouse model of PVR, and the effects on retinal damage, inflammation, and cell death were assessed. Finally, we used an in vitro model of microglia to determine the effects of HU-308 and HU-433 on phagocytosis, cytokine release, migration, and intracellular signaling. We observed that HU-308 more strongly recruited both β-arrestin2 and Gα_i compared to HU-433. Stimulation of CB₂ with either drug effectively blunted LPS- and IFNγ-mediated signaling as well as NO and TNF release from microglia. Furthermore, both drugs reduced IL-6 accumulation, total Caspase-3 cleavage, and retinal pathology following the induction of PVR. Ultimately, this work supports that CB₂ is a valuable target for drugs to suppress inflammation and cell death associated with Infection or sterile retinopathy, although the magnitude of effector recruitment may not be predictive of anti-inflammatory capacity.