Abstract Introduction The impact of pro-inflammatory cytokines on neuroinflammation and cognitive function after lipopolysaccharide (LPS) challenge remains elusive. Herein we provide evidence that there is a temporal correlation between high-mobility group box 1 (HMGB-1), microglial activation, and cognitive dysfunction. Disabling the interleukin (IL)-1 signaling pathway is sufficient to reduce inflammation and ameliorate the disability. Methods Endotoxemia was induced in wild-type and IL-1R-/- mice by intra peritoneal injection of E. Coli LPS (1 mg/kg). Markers of inflammation were assessed both peripherally and centrally, and correlated to behavioral outcome using trace fear conditioning. Results Increase in plasma tumor necrosis factor-α (TNFα) peaked at 30 minutes after LPS challenge. Up-regulation of IL-1β, IL-6 and HMGB-1 was more persistent, with detectable levels up to day three. A 15-fold increase in IL-6 and a 6.5-fold increase in IL-1β mRNA at 6 hours post intervention (P < 0.001 respectively) was found in the hippocampus. Reactive microgliosis was observed both at days one and three, and was associated with elevated HMGB-1 and impaired memory retention (P < 0.005). Preemptive administration of IL-1 receptor antagonist (IL-1Ra) significantly reduced plasma cytokines and hippocampal microgliosis and ameliorated cognitive dysfunction without affecting HMGB-1 levels. Similar results were observed in LPS-challenged mice lacking the IL-1 receptor to those seen in LPS-challenged wild type mice treated with IL-1Ra. Conclusions These data suggest that by blocking IL-1 signaling, the inflammatory cascade to LPS is attenuated, thereby reducing microglial activation and preventing the behavioral abnormality.

Surgery and critical illness often associate with cognitive decline. Surgical trauma or infection can lead independently to learning and memory impairments via similar, but not identical, cellular signaling of the innate immune system that promotes neuroinflammation. In this study we explored the putative synergism between aseptic orthopedic surgery and infection, the latter reproduced by postoperative lipopolysaccharide (LPS) administration. We observed that surgery and LPS augmented systemic inflammation up to postoperative d 3 and this was associated with further neuroinflammation (CD11b and CD68 immunoreactivity) in the hippocampus in mice compared with those receiving surgery or LPS alone. Administration of a selective α7 subtype nicotinic acetylcholine receptor (α7 nAChR) agonist 2 h after LPS significantly improved neuroinflammation and hippocampal-dependent memory dysfunction. Modulation of nuclear factor-kappa B (NF-κB) activation in monocytes and regulation of the oxidative stress response through nicotinamide adenine dinucleotide phosphate (NADPH) signaling appear to be key targets in modulating this response. Overall, these results suggest that it may be conceivable to limit and possibly prevent postoperative complications, including cognitive decline and/or infections, through stimulation of the cholinergic antiinflammatory pathway.

Rationale

Pulmonary arterial hypertension (PAH) is characterized by vascular cell proliferation and endothelial cell apoptosis. TLR3 (Toll-like receptor 3) is a receptor for double-stranded RNA and has been recently implicated in vascular protection.

Objectives

To study the expression and role of TLR3 in PAH and to determine whether a TLR3 agonist reduces pulmonary hypertension in preclinical models.

Methods

Lung tissue and endothelial cells from patients with PAH were investigated by polymerase chain reaction, immunofluorescence, and apoptosis assays. TLR3^-/- and TLR3^+/+ mice were exposed to chronic hypoxia and SU5416. Chronic hypoxia or chronic hypoxia/SU5416 rats were treated with the TLR3 agonist polyinosinic/polycytidylic acid (Poly[I:C]).

Measurements and main results

TLR3 expression was reduced in PAH patient lung tissue and endothelial cells, and TLR3^-/- mice exhibited more severe pulmonary hypertension following exposure to chronic hypoxia/SU5416. TLR3 knockdown promoted double-stranded RNA signaling via other intracellular RNA receptors in endothelial cells. This was associated with greater susceptibility to apoptosis, a known driver of pulmonary vascular remodeling. Poly(I:C) increased TLR3 expression via IL-10 in rat endothelial cells. In vivo, high-dose Poly(I:C) reduced pulmonary hypertension in both rat models in proof-of-principle experiments. In addition, Poly(I:C) also reduced right ventricular failure in established pulmonary hypertension.

Conclusions

Our work identifies a novel role for TLR3 in PAH based on the findings that reduced expression of TLR3 contributes to endothelial apoptosis and pulmonary vascular remodeling.