Aggregatibacter actinomycetemcomitans leukotoxin: a powerful tool with capacity to cause imbalance in the host inflammatory response

Abstract

Aggregatibacter actinomycetemcomitans has been described as a member of the indigenous oral microbiota of humans, and is involved in the pathology of periodontitis and various non-oral infections. This bacterium selectively kills human leukocytes through expression of leukotoxin, a large pore-forming protein that belongs to the Repeat in Toxin (RTX) family. The specificity of the toxin is related to its prerequisite for a specific target cell receptor, LFA-1, which is solely expressed on leukocytes. The leukotoxin causes death of different leukocyte populations in a variety of ways. It activates a rapid release of lysosomal enzymes and MMPs from neutrophils and causes apoptosis in lymphocytes. In the monocytes/macrophages, the toxin activates caspase-1, a cysteine proteinase, which causes a proinflammatory response by the activation and secretion of IL-1β and IL-18. A specific clone (JP2) of A. actinomycetemcomitans with enhanced leukotoxin expression significantly correlates to disease onset in infected individuals. Taken together, the mechanisms by which this toxin kills leukocytes are closely related to the pathogenic mechanisms of inflammatory disorders, such as periodontitis. Therapeutic strategies targeting the cellular and molecular inflammatory host response in periodontal diseases might be a future treatment alternative.

Keywords: Aggregatibacter actinomycetemcomitans; leukotoxin; proinflammatory response; virulence mechanisms.

Figure 1

Organization of the A. actinomycetemcomitamns leukotoxin operon.

Figure 2

Surface extract of bacteria from a highly leukotoxic (JP2) strain of A. actinomycetemcomitans (HK1619) separated by SDS-PAGE and stained with Coomassie blue. The 116 kD LtxA is released from the bacterial surface in the presence of °300 mM NaCl and dominates the protein profile in these extracts [15].

Figure 3

Schematic illustration of the type I secretion system required for export of the expressed A. actinomycetemcomitamns leukotoxin to the bacterial outer membrane (OM). IM = Inner membrane; TdeA = a TolC like protein (from Kachlany [20] with permission).

Figure 4

Schematic illustration of the molecular structure of the interaction between leukotoxin and the target cell membrane (from Lally [9] with permission).

Figure 5

Polymorphonuclear leukocytes (PMNs) were exposed to live A. actinomycetemcomitans for 10 min in a ratio of 25 bacteria/PMN, in the presence of 10% human non-immune sera at 37 °C under anaerobic conditions with gentle agitation. The low leukotoxic bacteria were phogocytized and killed by the PMN (left), while the highly leukotoxic bacteria (JP2) resisted PMN phagocytosis and caused extracellular release of lysosomal components (right) [58].

Figure 6

Schematic illustration of cellular mechanisms involved in LtxA-induced monocyte/macrophage death. It is suggested that LtxA adheres to the target cell membrane through binding to the LFA-1 dimer and further anchorage of the molecule by insertion of the fatty acid into the membrane lipid bilayer. The hydrophobic domain of the toxin is thought to cause small pores in the target cell membrane. Through still undefined intracellular signaling pathways, this interaction with the target cell membrane causes activation of caspase-1 and IL-1β that are secreted in a bioactive form from the affected cell.