Surfactant Protein A Inhibits Peptidoglycan-induced Tumor Necrosis Factor-α Secretion in U937 Cells and Alveolar Macrophages by Direct Interaction with Toll-like Receptor 2

Seiji Murakami,Daisuke Iwaki,Hiroaki Mitsuzawa,Hitomi Sano,Hiroki Takahashi,Dennis R Voelker,Toyoaki Akino,Yoshio Kuroki

doi:10.1074/jbc.m106671200

Seiji Murakami, Daisuke Iwaki + Show 6 more

Open Access

https://doi.org/10.1074/jbc.m106671200

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Abstract

Pulmonary surfactant protein A (SP-A) plays an important role in modulation of the innate immune system of the lung. Peptidoglycan (PGN), a cell wall component of Gram-positive bacteria, is known to elicit excessive proinflammatory cytokine production from immune cells. In this study we investigated whether SP-A interacts with PGN and alters PGN-elicited cellular responses. Binding studies demonstrate that PGN is not a ligand for SP-A. However, SP-A significantly reduced PGN-elicited tumor necrosis factor alpha (TNF-alpha) secretion by U937 cells and rat alveolar macrophages. The inhibitory effect on TNF-alpha secretion was dependent upon SP-A concentrations in physiological range. Coincubation of SP-A and PGN with human embryonic kidney 293 cells that had been transiently transfected with the cDNA of Toll-like receptor 2 (TLR2), a cell signaling receptor for PGN, significantly attenuated PGN-induced nuclear factor-kappaB activity. SP-A directly bound to a soluble form of the recombinant extracellular TLR2 domain (sTLR2). Coincubation of sTLR2 with SP-A significantly reduced the binding of sTLR2 to PGN. These results indicate that the direct interaction of SP-A with TLR2 alters PGN-induced cell signaling. We propose that SP-A modulates inflammatory responses against the bacterial components by interactions with pattern-recognition receptors.

Highlights

Pulmonary surfactant is a mixture of lipids and proteins that functions to keep alveoli from collapsing at expiration [1]
We have previously shown that Surfactant protein A (SP-A) bound to rough LPS (Re 595) with high affinity but not to smooth LPS (O26:B6) [26]
SP-A Attenuates PGN-induced NF-␬B Activation in TLR2transfected HEK293 Cells—Because recent in vivo and in vitro studies [22,23,24,25] demonstrate that PGN-induced signaling is mediated by Toll-like receptor 2 (TLR2), we examined the effect of SP-A on PGNinduced NF-␬B activation in TLR2-transfected HEK293 cells

Summary

Introduction

Pulmonary surfactant is a mixture of lipids and proteins that functions to keep alveoli from collapsing at expiration [1]. SP-A is recognized as playing an important role in regulating innate immunity within the lung This protein enhances the phagocytosis of Staphylococcus aureus [4], herpes simplex virus type I [5], type A Hemophilus influenzae [6], Mycobacterium tuberculosis [7], and Klebsiella [8] by alveolar macrophages. PGN, like lipopolysaccharides (LPS) from Gram-negative bacteria, can elicit the excessive release of proinflammatory cytokines from immune cells, which contribute to many of the adverse clinical manifestations of bacterial infections (16 –18). Taken together with the in vitro observations of the inhibitory function of SP-A on smooth LPS-elicited TNF-␣ secretion [26, 28], there is growing evidence that SP-A promotes an anti-inflammatory response to some bacterial ligands. Many inhaled pathogens that reach the alveolus are thought to interact immediately with lung collectins because these proteins are highly enriched at this biological interface

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