Aggregatibacter actinomycetemcomitans leukotoxin causes activation of lymphocyte function-associated antigen 1

The oral microbiome and oral and upper gastrointestinal diseases

Background

Microbiomes are essential components of the human body, and their populations are substantial. Under normal circumstances, microbiomes coexist harmoniously with the human body, but disturbances in this equilibrium can lead to various diseases. The oral microbiome is involved in the occurrence and development of many oral and gastrointestinal diseases. This review focuses on the relationship between oral microbiomes and oral and upper gastrointestinal diseases, and therapeutic strategies aiming to provide valuable insights for clinical prevention and treatment.

Methods

To identify relevant studies, we conducted searches in PubMed, Google Scholar, and Web of Science using keywords such as "oral microbiome," "oral flora, " "gastrointestinal disease, " without any date restrictions. Subsequently, the retrieved publications were subject to a narrative review.

Results

In this review, we found that oral microbiomes are closely related to oral and gastrointestinal diseases such as periodontitis, dental caries, reflux esophagitis, gastritis, and upper gastrointestinal tumors (mainly the malignant ones). Oral samples like saliva and buccal mucosa are not only easy to collect, but also display superior sample stability compared to gastrointestinal tissues. Consequently, analysis of the oral microbiome could potentially serve as an efficient preliminary screening method for high-risk groups before undergoing endoscopic examination. Besides, treatments based on the oral microbiomes could aid early diagnosis and treatment of these diseases.

Conclusions

Oral microbiomes are essential to oral and gastrointestinal diseases. Therapies centered on the oral microbiomes could facilitate the early detection and management of these conditions.

Kingella kingae RtxA Cytotoxin in the Context of Other RTX Toxins

The Gram-negative bacterium Kingella kingae is part of the commensal oropharyngeal flora of young children. As detection methods have improved, K. kingae has been increasingly recognized as an emerging invasive pathogen that frequently causes skeletal system infections, bacteremia, and severe forms of infective endocarditis. K. kingae secretes an RtxA cytotoxin, which is involved in the development of clinical infection and belongs to an ever-growing family of cytolytic RTX (Repeats in ToXin) toxins secreted by Gram-negative pathogens. All RTX cytolysins share several characteristic structural features: (i) a hydrophobic pore-forming domain in the N-terminal part of the molecule; (ii) an acylated segment where the activation of the inactive protoxin to the toxin occurs by a co-expressed toxin-activating acyltransferase; (iii) a typical calcium-binding RTX domain in the C-terminal portion of the molecule with the characteristic glycine- and aspartate-rich nonapeptide repeats; and (iv) a C-proximal secretion signal recognized by the type I secretion system. RTX toxins, including RtxA from K. kingae, have been shown to act as highly efficient ‘contact weapons’ that penetrate and permeabilize host cell membranes and thus contribute to the pathogenesis of bacterial infections. RtxA was discovered relatively recently and the knowledge of its biological role remains limited. This review describes the structure and function of RtxA in the context of the most studied RTX toxins, the knowledge of which may contribute to a better understanding of the action of RtxA in the pathogenesis of K. kingae infections.

Aggregatibacter actinomycetemcomitans Leukotoxin Activates the NLRP3 Inflammasome and Cell-to-Cell Communication

Carriers of highly leukotoxic genotypes of Aggregatibacter actinomycetemcomitans are at high risk for rapid degradation of tooth-supporting tissues. The leukotoxin (LtxA) expressed by this bacterium induces a rapid pro-inflammatory response in leukocytes that results in cell death. The aim of the present study was to increase the understanding of LtxA-induced leukocyte activation mechanisms and of possible associated osteoclast differentiation. The effect of LtxA on activation of the inflammasome complex was studied in THP-1 wild type and in NLRP3- and ASC knockout cells. Cell-to-cell communication was assessed by fluorescent parachute assays, and THP-1 differentiation into osteoclast-like cells was investigated microscopically. The results showed that LtxA induced inflammatory cell death, which involved activation of the NLRP3 inflammasome and gap junction cell-to-cell communication. THP-1 cells treated with lipopolysaccharide (LPS) and LtxA together differentiated into an osteoclast-like phenotype. Here, LPS prevented LtxA-mediated cell death but failed to induce osteoclast differentiation on its own. However, pit formation was not significantly enhanced by LtxA. We conclude that A. actinomycetemcomitans leukotoxicity mediates activation of the NLRP3 inflammasome and cell-to-cell communication in the induced pro-inflammatory cell death. In addition, LtxA stimulated differentiation towards osteoclasts-like cells in LPS-treated THP-1 cells.

Aggregatibacter actinomycetemcomitans as the Aetiological Cause of Rheumatoid Arthritis: What Are the Unsolved Puzzles?

Leukotoxin A (LtxA) is the major virulence factor of an oral bacterium known as Aggregatibacter actinomycetemcomitans (Aa). LtxA is associated with elevated levels of anti-citrullinated protein antibodies (ACPA) in rheumatoid arthritis (RA) patients. LtxA targets leukocytes and triggers an influx of extracellular calcium into cytosol. The current proposed model of LtxA-mediated hypercitrullination involves the dysregulated activation of peptidylarginine deiminase (PAD) enzymes to citrullinate proteins, the release of hypercitrullinated proteins through cell death, and the production of autoantigens recognized by ACPA. Although model-based evidence is yet to be established, its interaction with the host’s immune system sparked interest in the role of LtxA in RA. The first part of this review summarizes the current knowledge of Aa and LtxA. The next part highlights the findings of previous studies on the association of Aa or LtxA with RA aetiology. Finally, we discuss the unresolved aspects of the proposed link between LtxA of Aa and RA.

Etiology of aggressive periodontitis in individuals of African descent

Aggressive periodontitis (AgP) is a form of periodontitis that affects adolescents and has a significantly higher prevalence in individuals of African descent. AgP typically shows familial aggregation, suggesting a genetic predisposition. Young age, good health status, rapid attachment loss, and familial aggregation are the primary features of this disease. AgP has been closely linked to specific bacterial strains of Aggregatibacter actinomycetemcomitans. A. actinomycetemcomitans strains isolated from patients with AgP produce leukotoxin (LtxA), which specifically affects polymorphonuclear leukocytes in primates, especially humans. High-throughput 16S rRNA gene sequencing and bioinformatics analyses revealed differences in the subgingival microbiota between patients with AgP and those with chronic periodontitis (ChP). The genera Atopobium and Prevotella show increased prevalences in AgP than in ChP. According to AgP susceptibility, several single nucleotide polymorphisms have been detected in different genes in individuals of African descent. Interleukin (IL)-1α and IL-1β genetic polymorphisms may be associated with the severity of both ChP and AgP. An elevated serum level of IL-17 produced by Th17 cells may be a characteristic of AgP. Analyses of the relationships among bacteria, host defenses, genetic predisposition, and numerous other factors are required to understand the progression of this disease.

Epigallocatechin gallate alters leukotoxin secretion and Aggregatibacter actinomycetemcomitans virulence

OBJECTIVES

We and others have previously shown that epigallocatechin gallate (EGCg) inhibits the activity of an important virulence factor, leukotoxin (LtxA), produced by the oral bacterium Aggregatibacter actinomycetemcomitans, suggesting the potential use of this molecule as an anti-virulence strategy to treat periodontal infections. Here, we sought to better understand the effects of EGCg on toxin secretion and A. actinomycetemcomitans pathogenicity in a co-culture model.

METHODS

We used a quantitative immunoblot assay to determine the concentrations of LtxA in the bacterial supernatant and on the bacterial cell surface. Using a co-culture model, consisting of A. actinomycetemcomitans and THP-1 cells, we studied the impact of EGCg-mediated changes in LtxA secretion on the toxicity of A. actinomycetemcomitans.

KEY FINDINGS

EGCg increased production of LtxA and changed the localization of secreted LtxA from the supernatant to the surface of the bacterial cells. In the co-culture model, a single low dose of EGCg did not protect host THP-1 cells from A. actinomycetemcomitans-mediated cytotoxicity, but a multiple dosing strategy had improved effects.

CONCLUSIONS

Together, these results demonstrate that EGCg has important, but complicated, effects on toxin secretion and activity; new dosing strategies and comprehensive model systems may be required to properly develop these anti-virulence activities.

Aggregatibacter actinomycetemcomitans leukotoxin: From mechanism to targeted anti-toxin therapeutics

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium associated with localized aggressive periodontitis, as well as other systemic diseases. This organism produces a number of virulence factors, all of which provide some advantage to the bacterium. Several studies have demonstrated that clinical isolates from diseased patients, particularly those of African descent, frequently belong to specific clones of A. actinomycetemcomitans that produce significantly higher amounts of a protein exotoxin belonging to the repeats-in-toxin (RTX) family, leukotoxin (LtxA), whereas isolates from healthy patients harbor minimally leukotoxic strains. This finding suggests that LtxA might play a key role in A. actinomycetemcomitans pathogenicity. Because of this correlation, much work over the past 30 years has been focused on understanding the mechanisms by which LtxA interacts with and kills host cells. In this article, we review those findings, highlight the remaining open questions, and demonstrate how knowledge of these mechanisms, particularly the toxin's interactions with lymphocyte function-associated antigen-1 (LFA-1) and cholesterol, enables the design of targeted anti-LtxA strategies to prevent/treat disease.

Aggregatibacter actinomycetemcomitans LtxA Hijacks Endocytic Trafficking Pathways in Human Lymphocytes

Leukotoxin (LtxA), from oral pathogen Aggregatibacter actinomycetemcomitans, is a secreted membrane-damaging protein. LtxA is internalized by β2 integrin LFA-1 (CD11a/CD18)-expressing leukocytes and ultimately causes cell death; however, toxin localization in the host cell is poorly understood and these studies fill this void. We investigated LtxA trafficking using multi-fluor confocal imaging, flow cytometry and Rab5a knockdown in human T lymphocyte Jurkat cells. Planar lipid bilayers were used to characterize LtxA pore-forming activity at different pHs. Our results demonstrate that the LtxA/LFA-1 complex gains access to the cytosol of Jurkat cells without evidence of plasma membrane damage, utilizing dynamin-dependent and presumably clathrin-independent mechanisms. Upon internalization, LtxA follows the LFA-1 endocytic trafficking pathways, as identified by co-localization experiments with endosomal and lysosomal markers (Rab5, Rab11A, Rab7, and Lamp1) and CD11a. Knockdown of Rab5a resulted in the loss of susceptibility of Jurkat cells to LtxA cytotoxicity, suggesting that late events of LtxA endocytic trafficking are required for toxicity. Toxin trafficking via the degradative endocytic pathway may culminate in the delivery of the protein to lysosomes or its accumulation in Rab11A-dependent recycling endosomes. The ability of LtxA to form pores at acidic pH may result in permeabilization of the endosomal and lysosomal membranes.

RTX Toxins Ambush Immunity's First Cellular Responders

The repeats-in-toxin (RTX) family represents a unique class of bacterial exoproteins. The first family members described were toxins from Gram-negative bacterial pathogens; however, additional members included exoproteins with diverse functions. Our review focuses on well-characterized RTX family toxins from Aggregatibacteractinomycetemcomitans (LtxA), Mannheimiahaemolytica (LktA), Bordetella pertussis (CyaA), uropathogenic Escherichia coli (HlyA), and Actinobacillus pleuropneumoniae (ApxIIIA), as well as the studies that have honed in on a single host cell receptor for RTX toxin interactions, the β₂ integrins. The β₂ integrin family is composed of heterodimeric members with four unique alpha subunits and a single beta subunit. β₂ integrins are only found on leukocytes, including neutrophils and monocytes, the first responders to inflammation following bacterial infection. The LtxA, LktA, HlyA, and ApxIIIA toxins target the shared beta subunit, thereby targeting all types of leukocytes. Specific β₂ integrin family domains are required for the RTX toxin’s cytotoxic activity and are summarized here. Research examining the domains of the RTX toxins required for cytotoxic and hemolytic activity is also summarized. RTX toxins attack and kill phagocytic immune cells expressing a single integrin family, providing an obvious advantage to the pathogen. The critical question that remains, can the specificity of the RTX-β₂ integrin interaction be therapeutically targeted?

Aggregatibacter actinomycetemcomitans Leukotoxin (LtxA; Leukothera®): Mechanisms of Action and Therapeutic Applications

Aggregatibacter actinomycetemcomitans is an oral pathogen that produces the RTX toxin, leukotoxin (LtxA; Leukothera^®). A. actinomycetemcomitans is strongly associated with the development of localized aggressive periodontitis. LtxA acts as a virulence factor for A. actinomycetemcomitans to subvert the host immune response by binding to the β₂ integrin lymphocyte function-associated antigen-1 (LFA-1; CD11a/CD18) on white blood cells (WBCs), causing cell death. In this paper, we reviewed the state of knowledge on LtxA interaction with WBCs and the subsequent mechanisms of induced cell death. Finally, we touched on the potential therapeutic applications of LtxA (trade name Leukothera^®) toxin therapy for the treatment of hematological malignancies and immune-mediated diseases.

The Extracellular Domain of the β2 Integrin β Subunit (CD18) Is Sufficient for Escherichia coli Hemolysin and Aggregatibacter actinomycetemcomitans Leukotoxin Cytotoxic Activity

Urinary tract infections are one of the most common bacterial infections worldwide. Uropathogenic Escherichia coli strains are responsible for more than 80% of community-acquired urinary tract infections. Although we have known for nearly a century that severe infections stemming from urinary tract infections, including kidney or bloodstream infections are associated with expression of a toxin, hemolysin, from uropathogenic Escherichia coli, how hemolysin functions to enhance virulence is unknown. Our research defines the interaction of hemolysin with the β₂ integrin, a human white cell adhesion molecule, as a potential therapeutic target during urinary tract infections. The E. coli hemolysin is the prototype for a toxin family (RTX family) produced by a wide array of human and animal pathogens. Our work extends to the identification and characterization of the receptor for an additional member of the RTX family, suggesting that this interaction may be broadly conserved throughout the RTX toxin family.

The Escherichia coli hemolysin (HlyA) is a pore-forming exotoxin associated with severe complications of human urinary tract infections. HlyA is the prototype of the repeats-in-toxin (RTX) family, which includes LtxA from Aggregatibacter actinomycetemcomitans, a periodontal pathogen. The existence and requirement for a host cell receptor for these toxins are controversial. We performed an unbiased forward genetic selection in a mutant library of human monocytic cells, U-937, for host factors involved in HlyA cytotoxicity. The top candidate was the β₂ integrin β subunit. Δβ₂ cell lines are approximately 100-fold more resistant than wild-type U-937 cells to HlyA, but remain sensitive to HlyA at high concentrations. Similarly, Δβ₂ cells are more resistant than wild-type U-937 cells to LtxA, as Δβ₂ cells remain LtxA resistant even at >1,000-fold-higher concentrations of the toxin. Loss of any single β₂ integrin α subunit, or even all four α subunits together, does not confer resistance to HlyA. HlyA and LtxA bind to the β₂ subunit, but not to α_L, α_M, or α_X in far-Western blots. Genetic complementation of Δβ₂ cells with either β₂ or β₂ with a cytoplasmic tail deletion restores HlyA and LtxA sensitivity, suggesting that β₂ integrin signaling is not required for cytotoxicity. Finally, β₂ mutations do not alter sensitivity to unrelated pore-forming toxins, as wild-type or Δβ₂ cells are equally sensitive to Staphylococcus aureus α-toxin and Proteus mirabilis HpmA. Our studies show two RTX toxins use the β₂ integrin β subunit alone to facilitate cytotoxicity, but downstream integrin signaling is dispensable.