Modulation of immune-associated surface markers and cytokine production by murine retinal glial cells

Murine retinal glia are normally negative for major histocompatibility complex (MHC) Class II antigens and express low levels of MHC Class I and intercellular adhesion molecule-1 (ICAM-1) as detected by avidin-biotin-peroxidase immunohistochemistry. These surface molecules associated with immune function were either induced (Class II) or upregulated (Class I and ICAM-1) on cultured retinal glial cells in a dose- and time-dependent manner following exposure to recombinant interferon-γ (rlFN-γ). MHC Class I and II expression by passaged and primary cells was maximal (> 90% positive) after incubation with 100 U/ml of rIFN-γ for 48 h. ICAM-1 expression by primary and passaged cells tripled between 48 and 72 h after exposure to 25 or 50 U/ml of rIFN-γ. By 72 h after exposure to 100 U/ml of rIFN-γ, 62% of the retinal glia were positive for ICAM-1, whereas under normal culture conditions these molecules were detected on <3% of the retinal glia. Bacterial lipopolysaccharide (LPS), a known stimulator of central nervous system (CNS) astrocytes, increased ICAM-1 expression only 3-fold to 9% of cells staining positively, but neither MHC Class I nor Class II expression was altered from baseline levels. Surface expression of ICAM-I, MHC Class I, and MHC Class II was unaffected by exposure to either rTNF-α (1000 U/ml) or rIL-6 (100 U/ml) for 24 h. Under normal culture conditions, intracellular interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) were detected immunohistochemically. Exposure to either rIFN-γ or LPS induced more intense staining which correlated with increased secreted levels of both cytokines in culture supernatants. Levels of secreted TNF-α increased 6-fold after stimulation with LPS for 24 h, while secreted IL-6 increased over 9-fold. These results support the hypothesis that retinal glia may participate in intraretinal immune processes following stimulation during inflammatory and infectious processes via either cell surface- or soluble mediator-dependent mechanisms or a combination of both.