Aggregatibacter actinomycetemcomitans Leukotoxin (LtxA) Requires Death Receptor Fas, in Addition to LFA-1, To Trigger Cell Death in T Lymphocytes

Periodontal pathogens and cancer development

Increasing evidence suggests a significant association between periodontal disease and the occurrence of various cancers. The carcinogenic potential of several periodontal pathogens has been substantiated in vitro and in vivo. This review provides a comprehensive overview of the diverse mechanisms employed by different periodontal pathogens in the development of cancer. These mechanisms induce chronic inflammation, inhibit the host's immune system, activate cell invasion and proliferation, possess anti-apoptotic activity, and produce carcinogenic substances. Elucidating these mechanisms might provide new insights for developing novel approaches for tumor prevention, therapeutic purposes, and survival improvement.

Therapeutic Applications of Aggregatibacter actinomycetemcomitans Leukotoxin

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.

Periodontal Disease Pathogens, Pathogenesis, and Therapeutics: The CRISPR-Cas Effect

Periodontal disease (PD) is an immune-inflammatory disease affecting the supporting structures of the teeth, which results in progressive destruction of the hard and soft tissues surrounding teeth, ultimately resulting in tooth loss. The primary etiological factor for this disease is the presence of pathogenic microorganisms. Pathogenic bacteria face antagonistic conditions and foreign DNA components during the infection stage and depend on defense mechanisms such as clustered regularly interspaced short palindromic repeats (CRISPR)-Cas to counter them. Virulence genes regulated by the CRISPR-Cas system are often expressed by bacteria as part of the stress response to the presence of stress conditions and foreign elements. There is ever-growing evidence regarding the role of CRISPR-Cas in virulence of periodontal pathogens. The same CRISPR-Cas system may also be targeted to reduce bacterial virulence and it may also be utilized to develop diagnostic and therapeutic strategies for prevention and control of PD progression. This review article describes the CRISPR-Cas systems in the periodontal dysbiotic microbial communities, their role in the virulence of periodontal pathogens, and their potential role in understanding the pathogenesis of periodontitis and treatment of PD.

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 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.

Impact of Posttranslational Modification in Pathogenesis of Rheumatoid Arthritis: Focusing on Citrullination, Carbamylation, and Acetylation

Rheumatoid arthritis (RA) is caused by prolonged periodic interactions between genetic, environmental, and immunologic factors. Posttranslational modifications (PTMs) such as citrullination, carbamylation, and acetylation are correlated with the pathogenesis of RA. PTM and cell death mechanisms such as apoptosis, autophagy, NETosis, leukotoxic hypercitrullination (LTH), and necrosis are related to each other and induce autoantigenicity. Certain microbial infections, such as those caused by Porphyromonasgingivalis, Aggregatibacter actinomycetemcomitans, and Prevotella copri, can induce autoantigens in RA. Anti-modified protein antibodies (AMPA) containing anti-citrullinated protein/peptide antibodies (ACPAs), anti-carbamylated protein (anti-CarP) antibodies, and anti-acetylated protein antibodies (AAPAs) play a role in pathogenesis as well as in prediction, diagnosis, and prognosis. Interestingly, smoking is correlated with both PTMs and AMPAs in the development of RA. However, there is lack of evidence that smoking induces the generation of AMPAs.

Leukotoxin (LtxA/Leukothera) induces ATP expulsion via pannexin-1 channels and subsequent cell death in malignant lymphocytes

Leukotoxin (LtxA) (Trade name, Leukothera) is a protein that is secreted from the oral bacterium Aggregatibacter actinomycetemcomitans, which targets and kills activated white blood cells (WBCs) by binding to lymphocyte function associated antigen-1 (LFA-1). Interaction between LtxA and Jurkat T-cells results in cell death and is characterized by increased intracellular Ca²⁺, activation of caspases, clustering of LtxA and LFA-1 within lipid rafts, and involvement of the Fas death receptor. Here, we show that LtxA can kill malignant lymphocytes via apoptotic and necrotic forms of cell death. We show that LtxA causes activation of caspases and PARP, cleavage of pannexin-1 (Panx1) channels, and expulsion of ATP, ultimately leading to cell death via apoptosis and necrosis. CRISPR-Cas9 mediated knockout (K/O) of Panx1 in Jurkat cells prevented ATP expulsion and resulted in resistance to LtxA for both apoptotic and necrotic forms of death. Resistance to necrosis could only be overcome when supplementing LtxA with endogenous ATP (bzATP). The combination of LtxA and bzATP promoted only necrosis, as no Panx1 K/O cells stained positive for phosphatidylserine (PS) exposure following the combined treatment. Inhibition of LtxA/bzATP-induced necrosis was possible when pretreating Jurkat cells with oATP, a P2X₇R antagonist. Similarly, blockage of P2X₇Rs with oATP prevented the intracellular mobilization of Ca²⁺, an important early step in LtxA induced cell death. We show that LtxA is able to kill malignant lymphocytes through an apoptotic death pathway which is potentially linked to a Panx1/P2X₇R mediated necrotic form of death. Thus, inhibition of ATP release appears to significantly delay the onset of LtxA induced apoptosis while completely disabling the necrotic death pathway in T-lymphocytes, demonstrating the crucial role of ATP release in LtxA-mediated cell death.

Bacterial RTX toxins and host immunity

PURPOSE OF REVIEW

RTX toxin action often defines the outcome of bacterial infections. Here, we discuss the progress in understanding the impacts of RTX toxin activities on host immunity.

RECENT FINDINGS

Bordetella pertussis CyaA activity paralyzes sentinel phagocytic cells by elevating cellular cAMP levels and blocks differentiation of infiltrating monocytes into bactericidal macrophages, promoting also de-differentiation of resident alveolar macrophages into monocyte-like cells. Vibrio cholerae multifunctional autoprocessing repeats-in-toxins (MARTX), through Rho inactivating and α/β-hydrolase (ABH) domain action blocks mitogen-activated protein kinase signaling in epithelial cells and dampens the inflammatory responses of intestinal epithelia by blocking immune cell recruitment. The action of actin crosslinking effector domain and Ras/Rap1-specific endopeptidase (RRSP) domains of MARTX compromises the phagocytic ability of macrophages. Aggregatibacter actinomycetemcomitans LtxA action triggers neutrophil elastase release into periodontal tissue, compromising the epithelial barrier and promoting bacterial spreads into deeper tissue.

SUMMARY

Action of RTX toxins enables bacterial pathogens to cope with the fierce host immune defenses. RTX toxins often block phagocytosis and bactericidal reactive oxygen species and NO production. Some RTX toxins can reprogram the macrophages to less bactericidal cell types. Autophagy is hijacked for example by the activity of the V. cholerae ABH effector domain of the MARTX protein. Subversion of immune functions by RTX toxins thus promotes bacterial survival and proliferation in the host.

The role of CRISPR-Cas in advancing precision periodontics

The significant advancement of molecular biology has revolutionized medicine and provided important technologies to further clinical research development. Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) are DNA sequences derived from bacteriophages which have previously infected the bacterial species. The CRISPR-Cas system plays a key role in bacterial defense by detecting and destroying DNA fragments during subsequent bacteriophage invasions. The Cas9 enzyme recognizes and cleaves new invading CRISPR-complementary DNA sequences. Researchers have taken advantage of this biological device to manipulate microbes' genes and develop novel therapeutics to tackle systemic disease. In this review, we discuss the potential of utilizing CRISPR-Cas systems in the periodontal field to develop personalized periodontal care. We summarize promising attempts to bring this technology to the clinical setting. Finally, we provide insights regarding future developments to best utilize the CRISPR-Cas systems to advance precision periodontics. Although further research is imperative to evaluate the safety and potential of using CRISPR-Cas to develop precision periodontics approaches, few studies showed promising data to support the investment into this important technology in the dental sector. CRISPR-Cas9 can be a useful tool to create knockouts in vitro and in vivo as a screening tool to identify cellular pathways involved in the pathogenesis of periodontitis. Alternative CRISPR systems such as CRISPRa, CRISPRi, and Cas13 can be used to modify the transcriptome and gene expression of genes involved in periodontitis progression. CRISPR systems such as Cas3 can be used to target the periodontal biofilm and to develop new strategies to reduce or eliminate periodontal pathogens. Currently, the utility of CRISPR-Cas applications in clinical settings is limited. Through this review, we hope to foster further discussion in the periodontal research and clinical communities with respect to the potential clinical application of novel, CRISPR-Cas based, therapeutics for periodontitis.

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 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.