Therapeutic Applications of Aggregatibacter actinomycetemcomitans Leukotoxin

Abstract

Aggregatibacter actinomycetemcomitans is a Gram-negative oral bacterium that has been primarily studied for its role in causing periodontal disease. The bacterium has also been implicated in several systemic diseases such as endocarditis and soft tissue abscesses. Leukotoxin (LtxA) is perhaps the best studied protein virulence factor from A. actinomycetemcomitans. The protein can rapidly destroy white blood cells (WBCs), helping the bacterium to subvert the host immune system. The functional receptor for LtxA is lymphocyte function associated antigen-1 (LFA-1), which is expressed exclusively on the surfaces of WBCs. Bacterial expression and secretion of the protein are highly regulated and controlled by a number of genetic and environmental factors. The mechanism of LtxA action on WBCs varies depending on the type of cell that is being killed, and the protein has been shown to activate numerous cell death pathways in susceptible cells. In addition to serving as an important virulence factor for the bacterium, because of its exquisite specificity and rapid activity, LtxA is also being investigated as a therapeutic agent that may be used to treat diseases such as hematological malignancies and autoimmune/inflammatory diseases. It is our hope that this review will inspire an increased intensity of research related to LtxA and its effect on Aggressive Periodontitis, the disease that led to its initial discovery.

Keywords: Crohn’s disease; LFA-1; allergic asthma; autoimmune disease; drug development; inflammation; inflammatory bowel disease; leukemia; lymphoma; protein drug; translational research; ulcerative colitis.

Figure 1

LFA-1 on peripheral blood mononuclear cells (PBMCs) from AML patients. PBMC samples from patients with AML were stained with anti-CD11a antibodies and then analyzed via flow cytometry. The x-axis represents the levels of LFA-1, and the y-axis represents intensity of the peak. Cell viability was also measured in cells that had been separately treated with LtxA for 4 h. Viability was determined using the ATP-based Cell-Titer Glo Assay. Leukothera is a potential commercial name for LtxA.

Figure 2

LFA-1 levels in PBMCs derived from healthy controls and patients with IBD. PBMCs were isolated and analyzed using anti-LFA-1 antibody and analyzed via flow cytometry. HC = healthy control; IBD = patients with Crohn’s disease (CD) or ulcerative colitis (UC). ** indicates p ≤ 0.05.

Figure 3

Sensitivity of PBMCs derived from CD patients to LtxA. PBMCs from two CD patients were treated with LtxA, and then cells were stained with anti-LFA-1 antibody (to separate high and low populations) and Annexin-V (to detect cell death).

Figure 4

Immunohistochemical staining of LFA-1 on GI tissue. (Top) Intestinal tissue from CD patients or non-CD individuals were stained with anti-CD11a antibody and processed using immunohistochemistry. Shown are representative histological samples. (Bottom) The intensity of LFA-1 staining in the tissue samples was quantified as the percentage positive staining area compared to the total tissue area. Red bars represent CD samples, while green bars represent non-CD samples.

Figure 4

Immunohistochemical staining of LFA-1 on GI tissue. (Top) Intestinal tissue from CD patients or non-CD individuals were stained with anti-CD11a antibody and processed using immunohistochemistry. Shown are representative histological samples. (Bottom) The intensity of LFA-1 staining in the tissue samples was quantified as the percentage positive staining area compared to the total tissue area. Red bars represent CD samples, while green bars represent non-CD samples.