Pulmonary Surfactant Protein A Augments the Phagocytosis of Streptococcus pneumoniae by Alveolar Macrophages through a Casein Kinase 2-dependent Increase of Cell Surface Localization of Scavenger Receptor A

Koji Kuronuma,Hitomi Sano,Kazunori Kato,Kazumi Kudo,Naoki Hyakushima,Shin-Ichi Yokota,Hiroki Takahashi,Nobuhiro Fujii,Hiroshi Suzuki,Tatsuhiko Kodama,Shosaku Abe,Yoshio Kuroki

doi:10.1074/jbc.m312490200

Abstract

Pulmonary surfactant proteins A (SP-A) and D (SP-D), members of the collectin family, play important roles in the innate immune system of the lung. Here, we show that SP-A but not SP-D augmented phagocytosis of Streptococcus pneumoniae by alveolar macrophages, independent of its binding to the bacteria. Analysis of the SP-A/SP-D chimeras, in which progressively longer carboxyl-terminal regions of SP-A were replaced with the corresponding SP-D regions, has revealed that the SP-D region Gly(346)-Phe(355) can be substituted for the SP-A region Leu(219)-Phe(228) without altering the SP-A activity of enhancing the phagocytosis and that the SP-A region Cys(204)-Cys(218) is required for the SP-A-mediated phagocytosis. Acetylated low density lipoprotein significantly reduced the SP-A-stimulated uptake of the bacteria. SP-A failed to enhance the phagocytosis of S. pneumoniae by alveolar macrophages derived from scavenger receptor A (SR-A)-deficient mice, demonstrating that SP-A augments SRA-mediated phagocytosis. Preincubation of macrophages with SP-A at 37 degrees C but not at 4 degrees C stimulated the phagocytosis. The SP-A-mediated enhanced phagocytosis was not inhibited by the presence of cycloheximide. SP-A increased cell surface localization of SR-A that was inhibitable by apigenin, a casein kinase 2 (CK2) inhibitor. SP-A-treated macrophages exhibited significantly greater binding of acetylated low density lipoprotein than nontreated cells. The SP-A-stimulated phagocytosis was also abolished by apigenin. In addition, SP-A stimulated CK2 activity. These results demonstrate that SP-A enhances the phagocytosis of S. pneumoniae by alveolar macrophages through a CK2-dependent increase of cell surface SR-A localization. This study reveals a novel mechanism of bacterial clearance by alveolar macrophages.

Highlights

Pulmonary surfactant is a mixture of lipids and proteins that function to keep alveoli from collapsing during expiration [1]
This study reveals a novel mechanism of bacterial clearance by alveolar macrophages
The results indicate that the augmentation of the phagocytosis by Surfactant protein A (SP-A) is independent of the binding of SP-A to S. pneumoniae, since the addition of EDTA that inhibited SP-A binding to S. pneumoniae showed no effect on the uptake enhanced by SP-A

Summary

Introduction

Pulmonary surfactant is a mixture of lipids and proteins that function to keep alveoli from collapsing during expiration [1]. SP-A-deficient mice exhibit reduced bacterial clearance and elevated pulmonary inflammation in response to microbial challenge (4 – 6) Recent studies from this and other laboratories have revealed that SP-A and SP-D modulate lung inflammation by interacting with cell surface receptors on macrophages including CD14, Toll-like receptor 2, signal-inhibitory regulatory protein ␣, and calreticulin/CD91 [7,8,9,10]. Macrophage scavenger receptor A (SR-A) interacts with a number of ligands including the modified lipoproteins, lipopolysaccharides, lipoteichoic acid, and Gram-negative and -positive bacteria [15, 16] and functions as a pattern recognition receptor as a mannose receptor [17] These receptors are responsible for the phagocytosis of various bacteria and are essential components in the innate immune system. This study reveals a novel mechanism of bacterial clearance by alveolar macrophages

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