Aggregatibacter actinomycetemcomitans leukotoxin: from threat to therapy

Abstract

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium that colonizes the human oral cavity and is the causative agent for localized aggressive periodontitis (LAP), an aggressive form of periodontal disease that occurs in adolescents. A. actinomycetemcomitans secretes a protein toxin, leukotoxin (LtxA), which helps the bacterium evade the host immune response during infection. LtxA is a membrane-active toxin that specifically targets white blood cells (WBCs). In this review, we discuss recent developments in this field, including the identification and characterization of genes and proteins involved in secretion, regulation of LtxA, biosynthesis, newly described activities of LtxA, and how LtxA may be used as a therapy for the treatment of diseases.

Figure 1.

Leukotoxin operon from the 652 strain harboring the full-length promoter region and JP2 strain with a 530-bp deletion at the 3′ end of the promoter region.

Figure 2.

Type I secretion system for export of LtxA from A. actinomycetemcomitans. The model is based on experimental evidence and data from other systems. Reproduced in modified form from Kuhnert and Christensen (2008) with permission.

Figure 3.

Hemolytic properties of A. actinomycetemcomitans on different growth media. The names on the left indicate different strains of A. actinomycetemcomitans, and the letters refer to the media. All media contained 5% sheep blood. Lane A, Columbia agar from Accumedia; lane B, Columbia agar from BBL-Becton Dickinson; lane C, Trypticase soy agar from BBL-Becton Dickinson; lane D, AAGM. Reproduced with permission from Balashova et al. (2006a).

Figure 4.

Regulation of LtxA biosynthesis. Activation of the PTS results in the production of cAMP, which has a positive effect on the expression of ltx genes. LtxA protein is modified with fatty acid residues by LtxC and then secreted through a channel consisting of LtxB, LtxD, and TdeA. The presence of iron blocks the secretion of LtxA. After being secreted into the environment, LtxA may be stabilized by SOD and protected from the ROS and RNS released by host cells.

Figure 5.

Host response to A. actinomycetemcomitans infection in the mouth. Interaction between bacterial cells and gingival tissue causes the release of cytokine signals by lymphocytes and other WBCs. Cytokine signaling results in the activation of circulating WBCs in the underlying vasculature. Activation results in the immunological priming of WBCs and a conformational change of LFA-1 from a low-affinity state to a high-affinity, exposed state. High-affinity, activated LFA-1 can bind ICAM-1 expressed by vascular endothelial cells. Interaction between ICAM-1 and LFA-1 results in the migration of WBCs into the infected tissue via the process of diapedesis. In the gingival tissue, WBCs are incapacitated by secreted LtxA binding to activated LFA-1, allowing A. actinomycetemcomitans to colonize, persist, and invade deeper into the periodontal pocket.

Figure 6.

A model for the mechanism of how LtxA interacts with host cells and ultimately causes cellular intoxication by apoptosis. See text for details.

Figure 7.

Efficacy of LtxA therapy in a humanized mouse model for leukemia. (A) SCID mice were injected with the human leukemia cell line, HL-60, in the peritoneal cavity and then remained untreated (top) or were treated with 3 doses of LtxA (bottom). (B) Kaplan-Meier survival plot shows that 7/8 leukemic mice that received LtxA were long-term survivors and remained disease-free, while the untreated leukemic mice all succumbed to disease. Reproduced with permission from Kachlany et al. (2009).