Abstract
Sphingosine-1-phosphate (S1P) levels significantly increase in bronchoalveolar lavage (BAL) of asthmatic patients following segmental allergen challenge and this increase well correlates with pulmonary inflammation. Epidemiological, genetic, clinical, and experimental data indicate a potential for the toll-like receptor 4 (TLR4) to initiate and exacerbate allergic airway diseases. The aim of this study was to evaluate the contribution of TLR4 in S1P-dependent asthma-like disease in mice. BALB/c or TLR4 defective (C3H/HeJ) mice received S1P (10 ng/mouse), LPS (0.1 μg/mouse) or S1P + LPS. Furthermore, S1P-treated BALB/c mice were injected with the purified rabbit anti-TLR4 antibody (10 μg/mouse). S1P administration induced airway hyperreactivity and pulmonary inflammation associated to an increase in the percentage of dendritic cells (DCs) and macrophages into the lung of BALB/c mice. These effects were coupled to a reduction of DCs in the mediastinic lymph node. All these S1P-mediated effects were absent in TLR4 defective mice or reversed by treatment with a purified rabbit anti-TLR4 antibody. Confocal analysis of pulmonary sections showed a significant increase in TLR4+ cells and a similar presence of S1P1 and TLR4 following S1P challenge. Accordingly, the immunoprecipitation evidenced an increased S1P1/TLR4 interaction. In conclusion, our findings suggest that a functional interaction between S1P1 and TLR4 leads to an enhanced allergic inflammatory response. Thus, S1P pathway contributes to the sentinel role played by innate immunity providing new targets for prevention and treatment of allergic airway diseases.
Highlights
Sphingosine-1-phosphate (S1P) is generated from the cell membrane component sphingomyelin [1]
The exact mechanism by which major allergens are recognized by the host have not been completely clarified [39], but recently it has been demonstrated that toll-like receptor 4 (TLR4) plays a key role in the response to common allergens, i.e., house dust mite protein [26] and the metal nickel [28]
It has been hypothesized that TLR4 may be activated by endogenous molecules released by damaged cells or chemical substances generated during tissue injury in severe inflammatory processes [40]
Summary
Sphingosine-1-phosphate (S1P) is generated from the cell membrane component sphingomyelin [1]. S1P levels are regulated by the balance between its synthesis through sphingosine kinases and its degradation by the S1P lyasi [2,3,4]. S1P binds plasma membrane G-protein-coupled receptors or acts on intracellular targets [5, 6]. It has been widely demonstrated that the inhibition of S1P biosynthesis reduces allergen-induced asthma-like features in the mouse [7]. S1P administration exacerbates antigen-induced airway inflammation and hyperresponsiveness [8].
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