Microglia derived IL-6 suppresses neurosphere generation from adult human retinal cell suspensions

Following retinal degeneration or inflammation that disrupts tissue architecture, there is limited evidence of tissue regeneration, despite evidence of cells with progenitor properties in the adult human retina at all ages. With the prospect of tissue/cell transplantation, redressing homeostasis whilst overcoming glial barrier or gliosis remains key to successful graft versus host integration and functional recovery. Activated human retinal microglia (MG) secrete cytokines, including IL-6, which may suppress neurogenesis or cellular (photoreceptor) replacement. To investigate this hypothesis, adult human retinal explants were cultured in cytokine-conditioned media (TNFalpha, TGFbeta, LPS/IFNgamma) to activate microglia in situ. Following culture of retinal explants for 4 days, supernatant conditioned by resulting migrated microglia was collected after a further 3 days and fed to retinal cell suspensions (RCS). Neurosphere (NS) generation and survival analysis was performed after 7 and 14 days in culture, with or without addition of conditioned media and with or without concomitant IL-6 neutralisation. Neurosphere phenotype was analysed by immunohistochemistry and cell morphology. Migratory MG from retinal explants were activated (iNOS-positive) and expressed CD45, CD11b, and CD11c. LPS/IFNgamma-activated MG conditioned media (MG-CM) contained significant levels of IL-6 (1265 +/- 143) pg/ml, which inhibited neurosphere generation within RCS in the presence of optimal neurosphere generating N2-FGF2 culture medium. Neutralising IL-6 activity reinstated NS generation and the differentiation capacity was maintained in the spheres that formed. Even in the presence of high levels of IL-6, those few NS that did form demonstrated a capacity to differentiate. The data supports activated MG-derived IL-6 influence retinal cell turnover.