Human CD18 Is the Functional Receptor forAggregatibacter actinomycetemcomitansLeukotoxin

Abstract

Aggregatibacter (Actinobacillus) actinomycetemcomitans is the causative organism of localized aggressive periodontitis, a rapidly progressing degenerative disease of the gingival and periodontal ligaments, and is also implicated in causing subacute infective endocarditis in humans. The bacterium produces a variety of virulence factors, including an exotoxic leukotoxin (LtxA) that is a member of the repeats-in-toxin (RTX) family of bacterial cytolysins. LtxA exhibits a unique specificity to macrophages and polymorphonuclear cells of humans and other primates. Human lymphocyte function-associated antigen 1 (LFA-1) has been implicated as the putative receptor for LtxA. Human LFA-1 comprises the CD11a and CD18 subunits. It is not clear, however, which of its subunits serves as the functional receptor that confers species-specific susceptibility to LtxA. Here we demonstrate that the human CD18 is the receptor for LtxA based on experiments performed with chimeric ␤2-integrins recombinantly expressed in a cell line that is resistant to LtxA effects. In addition, we show that the cysteine-rich tandem repeats encompassing integrin-epidermal growth factor-like domains 2, 3, and 4 of the extracellular region of human CD18 are critical for conferring susceptibility to LtxA-induced biological effects.

Virulence Mechanisms of Leukotoxin from Aggregatibacter actinomycetemcomitans

“…The secreted leukotoxin has been shown to be easily inactivated by environmental proteases and superoxide radicals (Johansson et al, 2000b(Johansson et al, , 2001Balashova et al, 2007). This degradation of the toxin molecule can be inhibited by the presence of superoxide dismutase (SOD) produced by A. actinomycetemcomitans and the naturally occurring protease inhibitors of human serum (Johansson et al, 2001;de Haar et al, 2006;Balashova et al, 2007). In 1981, McArthur and co-workers showed that the activity of leukotoxin in interaction with polymorphonuclear leukocytes (PMNs) was enhanced by the presence of human serum (McArthur et al, 1981).…”

“…This property of leukotoxin further supports the importance of electrostatic forces for its association to the bacterial outer membrane. The secreted leukotoxin has been shown to be easily inactivated by environmental proteases and superoxide radicals (Johansson et al, 2000b(Johansson et al, , 2001Balashova et al, 2007). This degradation of the toxin molecule can be inhibited by the presence of superoxide dismutase (SOD) produced by A. actinomycetemcomitans and the naturally occurring protease inhibitors of human serum (Johansson et al, 2001;de Haar et al, 2006;Balashova et al, 2007).…”

“…The residues 1-128 on human CD11a has been shown to determine the human specificity of leukotoxin-induced cell lysis (Kieba et al, 2007). In addition, the extracellular region of human CD18 (residues 500-600) has been shown critical for conferring susceptibility to leukotoxin-induced cell lysis (Dileepan et al, 2007). The most important ligand of LFA-1 is the intercellular adhesion molecule 1 (ICAM-1), but this interaction does not coincide with the residues identified for leukotoxin binding (Dileepan et al, 2007;Kieba et al, 2007, Dustin et al, 2004.…”

“…This restricted host cell specificity indicates that the species-specific effect of leukotoxin is mediated through a unique receptor on the target cells and that a precise region in the toxin recognizes and interacts with the receptor (Dileepan et al, 2007;Kieba et al, 2007). The principal feature of this species recognition region of leukotoxin is a series of 14 tandemly repeated amino acid sequences in the repeat region of the toxin (Stanley et al, 1994).…”

“…In addition, the extracellular region of human CD18 (residues 500-600) has been shown critical for conferring susceptibility to leukotoxin-induced cell lysis (Dileepan et al, 2007). The most important ligand of LFA-1 is the intercellular adhesion molecule 1 (ICAM-1), but this interaction does not coincide with the residues identified for leukotoxin binding (Dileepan et al, 2007;Kieba et al, 2007, Dustin et al, 2004. This finding indicates that the intracellular signaling mediated by the LFA-1 ligand binding is not activated by the leukotoxin binding.…”