Aggregatibacter actinomycetemcomitans Leukotoxin (LtxA; Leukothera®): Mechanisms of Action and Therapeutic Applications

Abstract

Aggregatibacter actinomycetemcomitans is an oral pathogen that produces the RTX toxin, leukotoxin (LtxA; Leukothera^®). A. actinomycetemcomitans is strongly associated with the development of localized aggressive periodontitis. LtxA acts as a virulence factor for A. actinomycetemcomitans to subvert the host immune response by binding to the β₂ integrin lymphocyte function-associated antigen-1 (LFA-1; CD11a/CD18) on white blood cells (WBCs), causing cell death. In this paper, we reviewed the state of knowledge on LtxA interaction with WBCs and the subsequent mechanisms of induced cell death. Finally, we touched on the potential therapeutic applications of LtxA (trade name Leukothera^®) toxin therapy for the treatment of hematological malignancies and immune-mediated diseases.

Keywords: Aggregatibacter actinomycetemcomitans; RTX (repeats-in-toxin) toxin; cell death; leukotoxin (LtxA); lymphocyte function-associated antigen-1 (LFA-1); oral microbiology; toxin therapy; virulence factor; β2 integrins.

Figure 1

Model for LtxA association with host cell membranes. 1. LtxA (shown with fatty acyl chains) passively adsorbs onto cell membranes and anchors via the fatty acylation. This is believed to elevate intracellular calcium levels. 2. Association of LtxA with the target cell membranes results in lipid bilayer destabilization, demonstrated by the formation of cell surface depression. 3. The elevated calcium levels activate the calcium-dependent protease calpain. Calpain cleaves talin, a cytoskeletal tethering protein that holds LFA-1 in place in the cell membrane. 4. LFA-1 can cluster in the lipid raft compartment where LtxA interaction with LFA-1 is mediated by the fatty acyl chains at the membrane interface, allowing for the toxin to become partially embedded into the cell membrane. Downstream signaling results in cell death.

Figure 2

Model of LtxA-mediated cell death in monocytes. LtxA preferentially targets CD14⁺ monocytes expressing P2X₇R. Upon treatment with LtxA, monocytes rapidly release ATP into the extracellular milieu, activating P2X₇R. Activation of P2X₇R allows Ca²⁺ to be mobilized, resulting in the clustering of LFA-1 and other β₂ integrins into the lipid raft compartment. LtxA/LFA-1 interaction leads to the activation of caspases. LtxA is internalized in an LFA-1/ β₂ integrin-dependent manner where LtxA is shuttled to the lysosome. LtxA ruptures the lysosomal membrane, allowing leakage of lysosomal proteases, such as Cathepsin D, into the cytosol leading to rapid and irreversible cell death.

Figure 3

Model of LtxA-mediated cell death in lymphocytes. LtxA induces clustering of LFA-1 in the lipid rafts, where LFA-1 colocalizes strongly with Fas. After association with LFA-1, LtxA partially transverses the cell membrane and interacts with membrane proximal domains of LFA-1, interfering with its activation. The literature supports that LtxA only interacts with LFA-1 1, but it is possible that LtxA interacts with LFA-1 and Fas independently 2. or LtxA simultaneously interacts with both LFA-1 and Fas 3. Faulty activation of LFA-1 may allow the β₂ tail to either directly activate caspase-8 or to signal via the Fas-mediated cell death pathway. The activation of caspase-8 can result in perturbation of the mitochondrial membrane, releasing cytochrome c into the cytosol, or further activation of effector caspases. Both pathways culminate in cell death.