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A leukotoxin from Actinobacillus actinomycetemcomitans was isolated by a procedure that includes polymyxin B extraction, ion-exchange chromatography, and gel filtration chromatography. The procedure resulted in the recovery of 48% of the toxin with a 99-fold increase in specific activity. The isolated toxin has a molecular mass of 180,000 daltons by gel filtration and 115,000 daltons by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It retains all the major biological characteristics previously documented for crude leukotoxin preparations, including susceptibility to heat and proteolytic enzymes and neutralization by sera from patients with juvenile periodontitis. The isolated leukotoxin destroys human but not rat or guinea pig polymorphonuclear leukocytes and has no apparent effect on human erythrocytes. The availability of the A. actinomycetemcomitans leukotoxin should facilitate studies on its chemistry and mode of action as well as its role in the pathogenesis of human periodontal disease.

Juvenile periodontitis is associated with a high incidence of infection by Actinobacillus actinomycetemcomitans (Aa). The data presented indicate that the ability of Aa to destroy human PMNs is altered during the course of infection. Leukotoxic strains of Aa are characteristically found in isolates obtained from younger patients (6-12 years of age) but not in older subjects (13-25 years old). This suggests that the leukotoxin may be more important during early as opposed to more advanced phases of juvenile periodontitis.

The plaque-derived gram-negative microorganism Y4 identified as a member of the genus Actinobacillus, was tested for a soluble cytotoxic factor(s). Sonication or incubation of viable Y4 microorganisms in distilled water or normal human serum resulted in liberation of a soluble material which was cytotoxic in vitro for human polymorphonuclear leukocytes (PMNs). The Y4 soluble sonic extract was also cytotoxic to human peripheral blood monocytes. However, human lymphocytes, platelets, and fibroblasts, as well as rabbit, rat, and mouse leukocytes and chicken embryo fibroblasts, were not killed by exposure to the Y4 sonic extract. No hemolytic activity was detected in the Y4 sonic extract. No hemolytic activity was detected in the Y4 sonic extract. Consequently, the factor(s) in the Y4 sonic extract was referred to as Y4 leukotoxin. The Y4 leukotoxin was inactive at 4 degrees C, heat sensitive (56 degrees C, 30 min), and inactivated by proteases. The cytotoxic effect of Y4 leukotoxin on PMNs was dose, time, and temperature dependent. The leukotoxin did not bind to viable PMNs at 4 degrees C but did bind to dead PMN membrane components at both 4 and 37 degrees C. The addition of bovine serum albumin (51 mg/ml) to PMN-Y4 leukotoxin cultures inhibited the release of lactate dehydrogenase from the PMNs, but did not prevent the death of the cells as indicated by electron microscopy. Lysosomal markers were released in parallel to the cytoplasmic enzyme lactate dehydrogenase from Y4 leukotoxin-treated PMNs. The addition of 0.02 M ethylenedinitrilotetraacetic acid to these cultures inhibited release of lysosomal markers but enhanced the release of lactate dehydrogenase. These results suggested that a soluble leukotoxin with specificity for only human PMNs and monocytes can be liberated from viable Y4. What role this leukotoxin plays in the pathogenicity of the Y4 microorganism is not yet known. However, this leukotoxin is one of the first materials from a plaque-derived microorganism with a potential role in the pathogenesis of juvenile periodontitis.

A relatively high incidence of infection by Actinobacillus actionomycetemcomitans can be shown in subgingival plaque samples obtained from patients with juvenile periodontitis. These organisms possess a potent leukotoxin(s) which rapidly destroys isolated human polymorphonuclear leukocytes (PMNs) and monocytes. If such leukotoxins operate in vivo, they could deprive the gingival crevice area of an essential antibacterial defense mechanism. We have found that sera from juvenile periodontitis patients consistently (greater than 90%) contain antibodies which neutralize Actinobacillus actinomycetemcomitans leukotoxin(s). On the other hand, sera from normal individuals or patients with other types of periodontal disease usually amplified rather than inhibited the leukotoxic reaction. Many patients with juvenile periodontitis have demonstrable defects in PMN or monocyte chemotaxis and this may place them at risk to gingival infection by Actinobacillus actinomycetemcomitans. The immune response against these organisms could be a crucial determinant in the course of juvenile periodontitis. While this disease is relatively rare, it does cause immeasurable emotional, physical and economic hardship for patients and their families. The identification of Actinobacillus actinomycetemcomitans as a potential pathogen in this disorder may eventually lead to specific forms of therapy to prevent and eliminate infection by this organism in these patients.

Isolated human peripheral blood neutrophils were exposed to sonic extracts of Actinobacillus actinomycetemcomitans. Such bacterial preparations contain a potent leukotoxin which rapidly kills the leukocytes as reflected by cellular uptake of trypan blue, extracellular release of lactate dehydrogenase, or discharge of 51Cr from pre-labeled cells. Exogenous phospholipids with a glycerol skeleton esterified by fatty acids or positively charged liposomes inhibited cytotoxic phenomena. The data suggest that cell damage may involve the interaction of leukotoxin with phospholipids in the neutrophil cell membrane and that exogenous lipids either compete for or sterically block binding of the leukotoxin to these moieties in the membrane.

Hybridoma cell lines which produce monoclonal antibodies to a leukotoxin from Actinobacillus actinomycetemcomitans were prepared. The monoclonal antibodies were selected for their ability to neutralize the cytotoxic activity of the leukotoxin and recognize the toxin on nitrocellulose blots. The antibodies belonged to either the immunoglobulin G1 (IgG1) or IgG2 subclass and differed in their ability to bind to the leukotoxin on nitrocellulose blots. However, only slight differences in neutralization titers were observed. Use of the monoclonal antibodies revealed that polymyxin B-extracted or osmotic shock-released leukotoxin could be separated into several high-molecular-weight polypeptides by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoblot analysis with the monoclonal antibodies also demonstrated that the leukotoxin was present in eight oral strains of A. actinomycetemcomitans that had been previously classified by a biological assay as leukotoxic. The availability of these monoclonal antibodies should facilitate and expand studies concerning the role of the leukotoxin in the pathogenicity of A. actinomycetemcomitans.

We have examined soluble sonic extracts of Actinobacillus actinomycetemcomitans for their ability to alter human and murine fibroblast proliferation. We found that extracts of all A. actinomycetemcomitans strains examined (both leukotoxic and nonleukotoxic) caused a dose-dependent inhibition of both murine and human fibroblast proliferation as assessed by DNA synthesis ([3H]thymidine incorporation). Addition of sonic extract simultaneously with [3H]thymidine had no effect on incorporation, indicating that suppression was not due to the presence of excessive amounts of cold thymidine. Inhibition of DNA synthesis was also paralleled by decreased RNA synthesis ([3H]uridine incorporation) and by a decrease in cell growth as assessed by direct cell counts; there was no effect on cell viability. The suppressive factor(s) is heat labile; preliminary purification and characterization studies indicate that it is a distinct and separate moiety from other A. actinomycetemcomitans mediators previously reported, including leukotoxin, immune suppressive factor, and endotoxin. Although it is not clear how A. actinomycetemcomitans acts to cause disease, we propose that one aspect of the pathogenicity of this organism rests in its ability to inhibit fibroblast growth, which in turn could contribute to the collagen loss associated with certain forms of periodontal disease, in particular juvenile periodontitis.

The suitability of cynomolgus monkeys (Macaca fascicularis) for studies concerned with the biologic properties of Actinobacillus actinomycetemcomitans is the subject of the present investigation. We found that normal monkeys harbored leukotoxic strains of A. actinomycetemcomitans in subgingival plaque samples. Monkey peripheral blood PMNs and monocytes were killed following in vitro exposure to sonic extracts of leukotoxic strains of A. actinomycetemcomitans. Monkey sera were capable of inhibiting the leukotoxic properties of A. actinomycetemcomitans sonic extracts due to the presence of IgG antibodies which neutralized the leukotoxin. Similarly, sera from patients with juvenile periodontitis (but not normal human sera) abolished leukotoxin-mediated killing of monkey PMNs. Monkey peripheral blood lymphoid cells were not killed by A. actinomycetemcomitans but demonstrated depressed responses to mitogens following pre-incubation with A. actinomycetemcomitans sonicates prepared from either leukotoxic or "non-leukotoxic" human strains. These studies suggest that cynomolgus monkeys may serve as a suitable in vitro and in vivo model for delineating more about host-A. actinomycetemcomitans interrelationships in the etiology of human periodontal disease. [ABSTRACT FROM AUTHOR]

A sonic extract of Actinobacillus actinomycetemcomitans Y4, a microorganism originally isolated from a patient with juvenile periodontitis, contains a leukotoxin (Aa-Ieukotoxin) which specifically kills human polymorphonuclear leukocytes (PMNs) and monocytes. In the presence of normal human sera, the toxicity of the leukotoxin is enhanced whereas sera from patients with juvenile periodontitis neutralize leukotoxic activity. In juvenile periodontitis serum immunoglobulin G was identified as the specific inhibitor of the Aaleukotoxin. The leukotoxin enhancing factor(s) in normal human serum appeared to be a large molecular weight protein which was not immunoglobulin. Pooled sera from rabbits immunized with A. actinomycetemcomitans Y4 sonic extract also neutralized Aa- Icukotoxic activity while normal rabbit serum enhanced toxicity. The presence of anti- leukotoxin antibody in sent from individuals with juvenile periodontitis suggests that these people have been immunized with A. actinomycetemcomitans microorganisms. Monitoring of this antibody may be a convenient tool in the study of the etiology of juvenile periodontitis. [ABSTRACT FROM AUTHOR]