Phospholipids inhibit cytotoxic effects of Actinobacillus actinomycetemcomitans leukotoxin on human polymorphonuclear leukocytes

Structure/Function Aspects of Actinobacillus actinomycetemcomitans Leukotoxin

Actinobacillus actinomycetemcomitans has been implicated as a causative organism in early-onset periodontitis. The mechanisms by which A. actinomycetemcomitans is pathogenic are not known, but the organism produces several potential virulence factors, one of which is a leukotoxin. As a group, bacterial protein toxins are made up of structural domains which control various aspects of toxic activity, such as target cell recognition, membrane insertion, and killing. The purpose of this article is to review the structure of RTX, with special emphasis to its relation to toxin function. In addition, we will propose a model based upon other bacterial proteins whereby the water-soluble A. actinomycetemcomitans leukotoxin is able to achieve insertion into a biological membrane. J Periodontol 1996;67:298-308.

Virulence factors of Actinobacillus actinomycetemcomitans relevant to the pathogenesis of inflammatory periodontal diseases

There is strong evidence implicating Actinobacillus actinomycetemcomitans as the causative agent of localised juvenile periodontitis (LJP), a disease characterised by rapid destruction of the tooth-supporting tissues. This organism possesses a large number of virulence factors with a wide range of activities which enable it to colonise the oral cavity, invade periodontal tissues, evade host defences, initiate connective tissue destruction and interfere with tissue repair. Adhesion to epithelial and tooth surfaces is dependent on the presence of surface proteins and structures such as microvesicles and fimbriae. Invasion has been demonstrated in vivo and in vitro although the mechanisms involved are poorly understood. The organism has a number of means of evading host defences which include: (i) inhibiting poloymorphonuclear leukocyte (PMN) chemotaxis; (ii) killing PMNs and monocytes; (iii) producing immunosuppressive factors; (iv) secreting proteases capable of cleaving IgG; and (v) producing Fc-binding proteins. Surface components of A. actinomycetemcomitans are potent stimulators of bone resorption and can induce the release of a range of cytokines which can initiate tissue destruction. A number of surface components can also inhibit the proliferation of fibroblasts and their production of components of the extracellular matrix. Little is known, however, regarding the way in which these factors operate in vivo to produce the pathological features of the disease.

The role of antibody, complement and neutrophils in host defense against Actinobacillus actinomycetemcomitans

A. actinomycetemcomitans is a facultative Gram-negative coccobacillus which has been implicated in the etiology and pathogenesis of localized juvenile periodontitis and has also been recognized for its potential to cause serious extraoral infections, particularly endocarditis. The polymorphonuclear neutrophil has been suggested to play a key role in host resistance to periodontopathic organisms, as indicated by the association between defective production or function of these phagocytic cells and severe periodontal disease. This association has engendered interest in the study of the interaction between neutrophils and A. actinomycetemcomitans, as well as the role of immunoglobulin and complement in facilitating this interaction. The objective of this review is to summarize current knowledge of the nature and consequences of the interaction between A. actinomycetemcomitans and the host defense triad consisting of neutrophils, complement and immunoglobulin.

Immunosuppressive effects of Centipeda periodontii: selective cytotoxicity for lymphocytes and monocytes

We have examined soluble sonic extracts prepared from several strains of Centipeda periodontii for their ability to alter human lymphocyte function. These organisms were isolated from subgingival plaque of patients with periodontal disease. We found that sonicates from several, but not all, strains of C. periodontii caused a dose-dependent inhibition of lymphocyte responsiveness to concanavalin A, phytohemagglutinin, pokeweed mitogen, and formalinized Staphylococcus aureus. Inhibition was associated with a concomitant decrease in cell viability assessed by trypan blue exclusion, 51Cr release, and electron microscopy. The maximal number of dead cells was observed 20 to 24 h after exposure to the sonic extract. Susceptible cells include human lymphocytes (both B and T), monocytes, and erythrocytes, whereas polymorphonuclear cells, murine L-929 fibroblasts, and sheep erythrocytes were not affected. Preliminary characterization of the cytotoxic activity indicates that it is heat labile and trypsin sensitive and has an Mr of 60,000. It has been proposed that impaired host defense may play a pivotal role in the pathogenesis of periodontal diseases. The data presented in this paper suggest that immunosuppression (local or systemic or both) could be initiated by C. periodontii. This immunosuppression may alter the nature and consequences of host-parasite interactions, thereby enhancing the pathogenicity of C. periodontii itself or some other opportunistic organism.