Surfactant Protein A Directly Interacts with TLR4 and MD-2 and Regulates Inflammatory Cellular Response

Abstract

The purpose of the current study was to examine the binding of pulmonary surfactant protein A (SP-A) to TLR4 and MD-2, which are critical signaling receptors for lipopolysaccharides ( In innate immune systems, toll-like receptors (TLRs) 2 are implicated in recognition and signaling of pathogen-associated molecular patterns (1). Stimulation of different TLRs induces distinct patterns of gene expression, which leads to the activation of innate immunity and instructs the development of antigen-specific acquired immunity (2). Among the TLR family, TLR4 plays a critical role in recognition and signaling of bacterial lipopolysaccharide (LPS) (3). TLR4 requires accessory protein MD-2 for an efficient response to LPS (4). We have recently demonstrated the direct interaction between MD-2 and extracellular TLR4 domain (5, 6). MD-2 binds LPS (7), but LPS has been demonstrated to be cross-linked with TLR4 and MD-2 only when coexpressed with CD14 (8), suggesting that LPS is in close proximity to the receptor complex.The lung is constantly challenged by inhaled pathogens, pollutants, and particles that are present in the environment. Pulmonary surfactant, a mixture of lipids and proteins that serves to reduce the surface tension of the alveoli, is involved in the innate immune system of the lung. Recent studies demonstrate that the most abundant component of surfactant protein, surfactant protein A (SP-A), plays important roles in pathogen clearance and inflammatory responses (9 -12). SP-A belongs to the collectin subgroup of the C-type lectin superfamily along with surfactant protein D (SP-D) and mannose-binding lectin. The primary structure of SP-A subunits are composed of a short amino-terminal segment, a collagen-like sequence characterized by Gly-X-Y repeats with an interruption near the midpoint of the domain, a neck domain, and a carbohydrate recognition domain (CRD) (13). Trimeric association occurs by the folding of collagenous domains into triple helices (14) and coiled-coil bundling of ␣-helices in the neck (15). Fully assembled SP-A is a bouquet-like octadecamer consisting of six trimeric subunits that are stabilized by the amino-terminal sequences and disulfide bonds (16).Recent studies from this and other laboratories have demonstrated that SP-A modulates inflammation by interacting with cell surface receptors including CD14 (17), TLR2 (18, 19), signal-inhibitory regulatory protein ␣, and calreticulin/CD91 (20). Although it has been suggested that SP-A activates cellular responses dependent on TLR4 (21), the interactions of SP-A * This work was supported in part by a grant-in-aid for scientific research from the Ministry of Education, Science, Sports and Culture, Japan and by Akiyama Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C.

Pulmonary Surfactant Protein A Regulates TLR Expression and Activity in Human Macrophages

“…SP-A binds to one or more receptors on macrophages (22) which triggers signaling events that alter the biology of these cells (28,46,47). Previous work from our laboratory has shown that SP-A increases MR expression on human MDMs, while another laboratory found that SP-A increases SR-A expression on rat macrophages (4, 5); both receptors are known PRRs.…”

Surfactant Protein A (SP-A)-mediated Clearance of Staphylococcus aureus Involves Binding of SP-A to the Staphylococcal Adhesin Eap and the Macrophage Receptors SP-A Receptor 210 and Scavenger Receptor Class A

“…In addition, SR-A is involved in combinatorial feedback inhibition of proinflammatory signaling mediated by Toll-like receptors (91). In this regard, anti-inflammatory activities of SP-A that were previously attributed to direct binding of SP-A to Toll-like receptors (92,93) may also involve indirect regulation of SR-A via SP-R210. It is noteworthy that TLR2 (Toll-like receptor 2) contributes to intracellular survival of S. aureus in macrophages (94).…”

Pulmonary Collectins Protect Macrophages against Pore-forming Activity of Legionella pneumophila and Suppress Its Intracellular Growth

“…Collectins-The 1.13-kb cDNA for human SP-A1 and the 1.181-kb cDNA for human SP-D were inserted into pEE14 plasmid vectors, and recombinant human SP-A and SP-D were expressed in CHO-K1 cells using the glutamine synthetase gene amplification system (20), as described previously (10,21). CHO-K1 cells expressing SP-A and SP-D were grown in glutamate-free Glasgow minimum essential medium (Sigma) containing 10% dialyzed fetal calf serum and methionine sulfoximine (50 M for SP-A and 25 M for SP-D) for gene amplification.…”