Lack of lipoprotein-dependent effects on the cytotoxic interactions of Actinobacillus actinomycetemcomitans leukotoxin with human neutrophils

Virulence and Pathogenicity Properties of Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is a periodontal pathogen colonizing the oral cavity of a large proportion of the human population. It is equipped with several potent virulence factors that can cause cell death and induce or evade inflammation. Because of the large genetic diversity within the species, both harmless and highly virulent genotypes of the bacterium have emerged. The oral condition and age, as well as the geographic origin of the individual, influence the risk to be colonized by a virulent genotype of the bacterium. In the present review, the virulence and pathogenicity properties of A. actinomycetemcomitans will be addressed.

Effects of Aggregatibacter actinomycetemcomitans leukotoxin on neutrophil migration and extracellular trap formation

Background

Aggressive periodontitis is associated with the presence of Aggregatibacter actinomycetemcomitans, a leukotoxin (Ltx)-producing periodontal pathogen. Ltx has the ability to lyse white blood cells including neutrophils.

Objectives

This study was aimed at investigating the interactions between neutrophils and Ltx with regard to the chemotactic properties of Ltx and the release of neutrophil extracellular traps (NETs).

Methods

Neutrophils from healthy blood donors were isolated and incubated for 30 min and 3 h with increasing concentrations of Ltx (1, 10, and 100 ng/mL) as well as with A. actinomycetemcomitans strains (NCTC 9710 and HK 1651) producing different levels of Ltx. Formation of NETs and cell lysis were assessed by microscopy, fluorescence-based assays, and measurement of released lactate dehydrogenase. Neutrophil migration in response to different Ltx gradients was monitored by real-time video microscopy, and image analysis was performed using ImageJ software.

Results

Although Ltx (10 and 100 ng/mL) and the leukotoxic A. actinomycetemcomitans strain HK 1651 lysed some neutrophils, other cells were still capable of performing NETosis in a concentration-dependent manner. Low doses of Ltx and the weakly leukotoxic strain NCTC 9710 did not lead to neutrophil lysis, but did induce some NETosis. Furthermore, all three concentrations of Ltx enhanced random neutrophil movement; however, low directional accuracy was observed compared with the positive control (fMLP).

Conclusions

The results indicate that Ltx acts both as a neutrophil activator and also causes cell death. In addition, Ltx directly induces NETosis in neutrophils prior to cell lysis. In future studies, the underlying pathways involved in Ltx-meditated neutrophil activation and NETosis need to be investigated further.

Aggregatibacter actinomycetemcomitans: virulence of its leukotoxin and association with aggressive periodontitis

Periodontitis is an infection-induced inflammatory disease that causes loss of the tooth supporting tissues. Much focus has been put on comparison of the microbial biofilm in the healthy periodontium with the diseased one. The information arising from such studies is limited due to difficulties to compare the microbial composition in these two completely different ecological niches. A few longitudinal studies have contributed with information that makes it possible to predict which individuals who might have an increased risk of developing aggressive forms of periodontitis, and the predictors are either microbial or/and host-derived factors. The most conspicuous condition that is associated with disease risk is the presence of Aggregatibacter actinomycetemcomitans at the individual level. This Gram-negative bacterium has a great genetic variation with a number of virulence factors. In this review we focus in particular on the leukotoxin that, based on resent knowledge, might be one of the most important virulence factors of A. actinomycetemcomitans.

Actinobacillus actinomycetemcomitans suppresses rat natural killer cell activity in vivo

OBJECTIVE

To examine the immune suppressive effect of Actinobacillus actinomycetemcomitans (Aa) on rat natural killer (NK) cell activity in vivo.

MATERIAL AND METHODS

Sprague Dawley rats were given Aa in 2 different manners: (i) by mixing Aa with food at a dose of 10(8) cells/rat/day for 3 months; or (ii) by a single i.m. injection of live Aa at doses of 10(6) and 10(7) cells/rat/day. NK cell activity was measured by means of a 51Cr-release assay using YAC-1 tumor cells as targets.

RESULTS

Rats that had been infected by Aa mixed with food experienced significant suppression of their NK cell activity; this reached approximately =50% of control values at 2 and 3 months post-Aa infection. The suppression in NK cell activity was related to decreases in the extent of conjugate formation between effectors and YAC-1 target tumor cells (51.7%) and the extent of lysis of target cells (75.7%). The results also showed that addition of an admixture of Aa-treated NK cells to the control NK cells caused 75% and 53% decreases in activity at effector:target ratios of 25:1 and 50:1, respectively. In addition, a significant increase in the extent of T-suppressor cells (154.8% of control) was detected at 3 months post-Aa infection. In contrast, the single injection of live bacteria resulted in a remarkable, dose-dependent inhibition of NK cell activity (55% and 71% at doses of 10(6) and 10(7) cells/rat/day, respectively) as early as 2 days post-treatment. This also reflected significant suppression in the effector:target conjugate formation ratio (52% of control). The data also revealed a 150-188% increase in the number of splenic lymphocytes post-Aa injection. These effects were transient and normal levels were re-established by the fifth day

CONCLUSION

Aa treatment causes suppression of NK cell activity and the mode of action may be due to induction of T-suppressor cells or dilution of NK cells with other lymphoid cell populations. The degree of suppression is affected by the way in which Aa is introduced to the host. These results may contribute to the understanding of how Aa evades host defense.