Inhibition of TLR Activation and Up-Regulation of IL-1R-Associated Kinase-M Expression by Exogenous Gangliosides

Abstract

Gangliosides, sialic acid-containing glycosphingolipids present in the outer leaflet of plasma membranes, are produced at high levels by some tumors, are actively shed into the tumor microenvironment, and can be detected in high concentrations in the serum of cancer patients. These tumor-shed molecules are known to be immunosuppressive, although mechanisms remain to be fully elucidated. In this study, we show that membrane enrichment of human monocytes with purified exogenous gangliosides potently inhibits ligand-induced activation and proinflammatory cytokine production induced by a broad range of TLRs, including TLR2, TLR3, TLR6, and TLR7/8, in addition to a previously identified inhibitory effect on TLR4 and TLR5. Inhibition of TLR activation is reversible, with complete restoration of TLR signaling within 6–24 h of washout of exogenous gangliosides, and is selective for certain gangliosides (GM1, GD1a, and GD1b), whereas others (GM3) are inactive. To characterize the inhibition, we assessed the expression of the TLR signaling pathway inhibitor, IL-1 receptor associated kinase-M (IRAK-M). In response to ganglioside enrichment alone, we observed striking up-regulation of IRAK-M in monocytes, but without concomitant proinflammatory cytokine production. This contrasts with endotoxin tolerance, in which IRAK-M up-regulation follows proinflammatory cytokine expression caused by LPS exposure. We hypothesize that ganglioside treatment induces a state of tolerance to TLR signaling, leading to blunted activation of innate immune responses. In the tumor microenvironment, shed tumor ganglioside enrichment of APC membranes may likewise cause these cells to bypass the normal TLR signaling response and progress directly to the inhibitory state.

Multi-system disorders of glycosphingolipid and ganglioside metabolism

“…Ex vivo, gangliosides activate microglia to produce the proinfl ammatory mediators nitric oxide and TNF ␣ ( 237 ). Alternatively, gangliosides also suppress toll-like receptor-induced proinfl ammatory cytokine expression ( 238 ). In Sandhoff disease mice and humans, macrophage/microglial activation and subsequent cytokine expression result in neuronal dysfunction ( 211 ).…”

Ganglioside-exposed dendritic cells inhibit T-cell effector function by promoting regulatory cell activity

“…Ganglioside treatment alone had no effect on cytokine production compared with untreated BMDC. As with GD1a, pre-treatment of BMDC with GM1 ganglioside also inhibited IL-12 production 25 (Fig. 1c).…”

“…1c). In these experiments a dose of 50 lM of ganglioside was used, which has been shown to be optimal in previous experiments, 25 and does not induce a decrease in the viability of treated DC compared with control DC, as determined by trypan blue exclusion (data not shown).…”

“…Ganglioside exposure of BMDC inhibits LPS-dependent up-regulation of co-stimulatory molecules and cytokine production Previous studies showed that pre-treatment with gangliosides inhibits LPS-mediated up-regulation of co-stimulatory molecules and cytokine production by human peripheral blood monocytes and blood-derived DC, 17,22,25 and tumour gangliosides have also been shown to inhibit DC generation and function. 10 In the current study, we extended these findings using an antigen-specific mouse model and murine BMDC.…”

The glycolipid sulfatide protects insulin‐producing cells against cytokine‐induced apoptosis, a possible role in diabetes

“…The signalling of Toll-like receptors TLR3-7/8 is disrupted by exogenous addition of βDGalp (1)(2)(3) [39]. The exogenous addition of ceramide inhibits protein kinase C-mediated activation of NF-κB [40].…”