CD40 Negatively Regulates ATP-TLR4-Activated Inflammasome in Microglia

Abstract

During acute brain injury and/or sterile inflammation, release of danger-associated molecular patterns (DAMPs) activates pattern recognition receptors (PRRs). Microglial toll-like receptor (TLR)-4 activated by DAMPs potentiates neuroinflammation through inflammasome-induced IL-1β and pathogenic Th17 polarization which critically influences brain injury. TLR4 activation accompanies increased CD40, a cognate costimulatory molecule, involved in microglia-mediated immune responses in the brain. During brain injury, excessive release of extracellular ATP (DAMPs) is involved in promoting the damage. However, the regulatory role of CD40 in microglia during ATP-TLR4-mediated inflammasome activation has never been explored. We report that CD40, in the absence of ATP, synergizes TLR4-induced proinflammatory cytokines but not IL-1β, suggesting that the response is independent of inflammasome. The presence of ATP during TLR4 activation leads to NLRP3 inflammasome activation and caspase-1-mediated IL-1β secretion which was inhibited during CD40 activation, accompanied with inhibition of ERK1/2 and reactive oxygen species (ROS), and elevation in p38 MAPK phosphorylation. Experiments using selective inhibitors prove indispensability of ERK 1/2 and ROS for inflammasome activation. The ATP-TLR4-primed macrophages polarize the immune response toward pathogenic Th17 cells, whereas CD40 activation mediates Th1 response. Exogenous supplementation of IFN-γ (a Th1 cytokine and CD40 inducer) results in decreased IL-1β, suggesting possible feedback loop mechanism of inflammasome inhibition, whereby IFN-γ-mediated increase in CD40 expression and activation suppress neurotoxic inflammasome activation required for Th17 response. Collectively, the findings indicate that CD40 is a novel negative regulator of ATP-TLR4-mediated inflammasome activation in microglia, thus providing a checkpoint to regulate excessive inflammasome activation and Th17 response during DAMP-mediated brain injury.

Microglia are less pro‐inflammatory than myeloid infiltrates in the hippocampus of mice exposed to status epilepticus

“…Our data suggest that astrocytes but not microglia are the main cells of the central nervous system (CNS) responsible for the pro-inflammatory milieu occurring during the early phase of epileptogenesis, corroborating previous published data (Ravizza and Vezzani, 2006;Ravizza et al, 2008). Moreover, a recent study demonstrated that expression of CD40 by microglia is able to decrease IL-1b secretion and consequently negatively regulates neurotoxic inflammasome activation during sterile inflammation (Gaikwad et al, 2016).…”

“…This is in line with data showing that microglia express IL-1b in the first 4 h after SE but not at later time-points where astrocytes drive inflammation during early epileptogenesis (Ravizza et al, 2008). Moreover, a recent study demonstrated that expression of CD40 by microglia is able to decrease IL-1b secretion and consequently negatively regulates neurotoxic inflammasome activation during sterile inflammation (Gaikwad et al, 2016). IL-1b is thought to contribute to neuronal hyperexcitability and excitotoxicity through potentiation of glutamatergic neurotransmission (Hu et al, 2000;Lai et al, 2006;Viviani et al, 2003;Ye and Sontheimer, 1996).…”

Negative regulators and their mechanisms in NLRP3 inflammasome activation and signaling

“…An important adaptive immune cytokine, IFN‐γ, exerts a regulatory effect on IL‐1 production and immunopathological responses through generation of nitric oxide, which inhibits the NLRP3 inflammasome assembly via nitrosylation of the NLRP3 protein 34 . CD40, a costimulatory molecule, exhibits an inhibitory function in ATP‐Toll‐like receptor 4‐mediated inflammasome activation in microglia 35 . IFN‐γ‐mediated CD40 expression leads to decreased production of IL‐1β and suppresses the neurotoxic inflammasome activation required for pathogenic Th17 polarization 35 .…”

“…CD40, a costimulatory molecule, exhibits an inhibitory function in ATP‐Toll‐like receptor 4‐mediated inflammasome activation in microglia 35 . IFN‐γ‐mediated CD40 expression leads to decreased production of IL‐1β and suppresses the neurotoxic inflammasome activation required for pathogenic Th17 polarization 35 . These findings corroborate results from another study, which found that increased nitric oxide in TXNIP‐deficient mice exerted a negative effect on excessive inflammatory responses and inflammasome activation during endotoxic shock through induction of S ‐nitrosylation of NLRP3 36 .…”

Toll-like receptor 4 is activated by platinum and contributes to cisplatin-induced ototoxicity

“…It is also widely accepted that TLR4 is activated by other agonists including damageassociated molecular patterns, viral proteins and transition metals (4)(5)(6)(7)(8). TLR4 was found to mediate immune hypersensitivity reactions to the Group 9/10 transition metals nickel, cobalt and palladium (9)(10)(11).…”