Research Article: Role of Toll-Like Receptors 2 and 4 in Pulmonary Inflammation and Injury Induced by Pneumolysin in Mice

Date Published: November 24, 2009

Publisher: Public Library of Science

Author(s): Mark C. Dessing, Robert A. Hirst, Alex F. de Vos, Tom van der Poll, Adam J. Ratner.

Abstract: Pneumolysin (PLN) is an intracellular toxin of Streptococcus pneumoniae that has been implicated as a major virulence factor in infections caused by this pathogen. Conserved bacterial motifs are recognized by the immune system by pattern recognition receptors among which the family of Toll-like receptors (TLRs) prominently features. The primary objective of the present study was to determine the role of TLR2 and TLR4 in lung inflammation induced by intrapulmonary delivery of PLN.

Partial Text: Streptococcus pneumoniae is the most frequently isolated pathogen in community acquired pneumonia [1], [2]. Several pneumococcal proteins and enzymes have been implicated in the virulence of this bacterium and the pathogenesis of pneumonia [3]. Pneumolysin (PLN) is a protein expressed by Streptococcus pneumoniae intracellularly and/or in the cell wall, that is present in virtually all clinical isolates [4], [5], [6]. PLN is considered to be an important virulence factor of the pneumococcus. Indeed, mice infected with a PLN-deficient strain of S. pneumoniae showed a reduced lethality and a diminished inflammatory response when compared to animals infected with PLN-producing S. pneumoniae[7], [8]. PLN remains within the pneumococcus during bacterial growth, but is released when the pathogen autodestructs by expressing autolysin [9] or after destruction by the host immune system or antibiotic treatment [10]. In addition, a recent report raised doubt as to whether PLN truly is an intracellular protein, showing that PLN primarily localized to the cell wall compartment during growth of pneumococci in the absence of detectable cell lysis [6]. At low doses, PLN activates the classical pathway of the complement system, induces cytokine production by macrophages and monocytes, inhibits the migration, respiratory burst and antibacterial activity of neutrophils and macrophages and affects ciliary beating of epithelial cells [11]–[15]. At high doses, PLN can induce cell death; PLN interacts with cholesterol in the host-cell membrane resulting in the formation of transmembrane pores and death of the host (immune) cell [16]. Our laboratory recently demonstrated that purified PLN induces neutrophil influx and the production of cytokines and chemokines in the lungs of mice [17]. In addition, PLN dose dependently induced vascular permeability and pulmonary edema in mice [18], [19]. Together these data suggest that PLN has a strong impact on the host response to invasion of the lower respiratory tract by S. pneumoniae.

The pneumococcal cell wall consists of several proteins and enzymes that contribute to the virulence of the pathogen and the pathogenesis of pneumonia [3]. PLN is a toxin of S. pneumonia, expressed intracellularly and/or in the cell wall compartment, that is present in all clinical isolates [4], [5], [6]. Several studies have demonstrated that PLN is recognized by the immune system through a specific interaction with TLR4 [21]–[25]. The primary objective of the present investigation was to determine the contribution of TLR2 and TLR4 in lung inflammation and injury induced by PLN in vivo. First we confirmed that our PLN preparation activated HEK cells via TLR4. We then revealed that intrapulmonary delivery of PLN induces an inflammatory response in the mouse lung that is dependent in part not only on TLR4, but also on TLR2. Our data expand a recent investigation showing that the induction of PAI-1 mRNA in the lungs of mice at least in part depends on TLR4 [26].



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