Expression of LFA-1 identifies different prognostic subgroups in patients with advanced follicle center lymphoma (FCL)

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.

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.

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

Leukotoxin (LtxA) (trade name, Leukothera) is a protein secreted by the oral bacterium Aggregatibacter actinomycetemcomitans A. actinomycetemcomitans is an oral pathogen strongly associated with development of localized aggressive periodontitis. LtxA acts as a virulence factor for A. actinomycetemcomitans by binding to the β₂ integrin lymphocyte function-associated antigen-1 (LFA-1; CD11a/CD18) on white blood cells (WBCs) and causing cell death. In addition, because of its specificity for malignant and activated WBCs, LtxA is being investigated as a therapeutic agent for treatment of hematological malignancies and autoimmune diseases. Here, we report the successful generation and characterization of Jurkat T lymphocytes with deletions in CD18, CD11a, and Fas that were engineered using CRISPR/Cas9 gene editing. Using these clones, we demonstrate the specificity of LtxA for cells expressing LFA-1. We also demonstrate the requirement of the cell death receptor Fas for LtxA-mediated cell death in T lymphocytes. We show that LFA-1 and Fas are early events in the LtxA-mediated cell death cascade as caspase activation and mitochondrial perturbation do not occur in the absence of either receptor. To our knowledge, LtxA is the first molecule, other than FasL, known to require the Fas death receptor to initiate cell death. Knowledge of the mechanism of cell death induced by LtxA will facilitate the understanding of LtxA as a bacterial virulence factor and development of it as a potential therapeutic agent.

Aggregatibacter actinomycetemcomitans leukotoxin (LtxA; Leukothera) induces cofilin dephosphorylation and actin depolymerization during killing of malignant monocytes

Leukotoxin (LtxA; Leukothera), a protein toxin secreted by the oral bacterium Aggregatibacter actinomycetemcomitans, specifically kills white blood cells (WBCs). LtxA binds to the receptor known as lymphocyte function associated antigen-1 (LFA-1), a β2 integrin expressed only on the surface of WBCs. LtxA is being studied as a virulence factor that helps A. actinomycetemcomitans evade host defences and as a potential therapeutic agent for the treatment of WBC diseases. LtxA-mediated cell death in monocytes involves both caspases and lysosomes; however, the signalling proteins that regulate and mediate cell death remain largely unknown. We used a 2D-gel proteomics approach to analyse the global protein expression changes that occur in response to LtxA. This approach identified the protein cofilin, which underwent dephosphorylation upon LtxA treatment. Cofilin is a ubiquitous actin-binding protein known to regulate actin dynamics and is regulated by LIM kinase (LIMK)-mediated phosphorylation. LtxA-mediated cofilin dephosphorylation was dependent on LFA-1 and cofilin dephosphorylation did not occur when LFA-1 bound to its natural ligand, ICAM-1. Treatment of cells with an inhibitor of LIMK (LIMKi) also led to cofilin dephosphorylation and enhanced killing by LtxA. This enhanced sensitivity to LtxA coincided with an increase in lysosomal disruption, and an increase in LFA-1 surface expression and clustering. Both LIMKi and LtxA treatment also induced actin depolymerization, which could play a role in trafficking and surface distribution of LFA-1. We propose a model in which LtxA-mediated cofilin dephosphorylation leads to actin depolymerization, LFA-1 overexpression/clustering, and enhanced lysosomal-mediated cell death.

Physiology and pathophysiology of selectins, integrins, and IgSF cell adhesion molecules focusing on inflammation. A paradigm model on infectious endocarditis

The development of adhesion bonds, either among cells or among cells and components of the extracellular matrix, is a crucial process. These interactions are mediated by some molecules collectively known as adhesion molecules (CAMs). CAMs are ubiquitously expressed proteins playing a central role in controlling cell migration, proliferation, survival, and apoptosis. Besides their key function in physiological maintenance of tissue integrity, CAMs play an eminent role in various pathological processes such as cardiovascular disorders, atherogenesis, atherosclerotic plaque progression and regulation of the inflammatory response. CAMs such as selectins, integrins, and immunoglobulin superfamily take part in interactions between leukocyte and vascular endothelium (leukocyte rolling, arrest, firm adhesion, migration). Experimental data and pathologic observations support the assumption that pathogenic microorganisms attach to vascular endothelial cells or sites of vascular injury initiating intravascular infections. In this review a paradigm focusing on cell adhesion molecules pathophysiology and infective endocarditis development is given.

In vitro synergism between LFA-1 targeting leukotoxin (Leukothera™) and standard chemotherapeutic agents in leukemia cells

Leukotoxin (Leukothera™; LtxA) is a bacterial protein and experimental therapeutic that binds leukocyte function antigen (LFA-1) on white blood cells (WBCs) and induces cell death via apoptosis or necrosis. We previously found that LtxA preferentially targets WBCs with high levels of activated LFA-1, which is characteristic of many leukemias and lymphomas, and showed that LtxA exhibits significant anti-leukemia activity in vivo using the humanized SCID mouse model. In this report, we demonstrate that LtxA induces very rapid (1h) apoptosis in acute monocytic leukemia THP-1 cells characterized by binding of annexin V to cells, loss of mitochondrial membrane potential, depletion of cellular ATP, and fragmentation of chromosomal DNA. We tested the activity of LtxA in combination with the standard chemotherapeutic agents, etoposide, mitoxantrone, daunorubicin, busulfan, and imatinib against several leukemia cell lines, including THP-1, GDM-1, HL-60, and KU-812 cells. LtxA exhibited synergism with all the drugs, and the levels of synergy were dependent on the doses used and cell lines examined. In general, the greatest level of synergy was observed with LtxA and etoposide or imatinib. Combination index (CI) values were less than 0.1 for many of the combinations, indicating very strong synergism. In addition, LtxA alone was cytotoxic to primary cells from newly diagnosed, relapsed, and refractory patients with different hematological malignancies. Thus, LtxA is highly effective at inducing rapid apoptosis both as a single agent and in combination with approved leukemia therapies.

Aggregatibacter actinomycetemcomitans leukotoxin: from threat to therapy

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium that colonizes the human oral cavity and is the causative agent for localized aggressive periodontitis (LAP), an aggressive form of periodontal disease that occurs in adolescents. A. actinomycetemcomitans secretes a protein toxin, leukotoxin (LtxA), which helps the bacterium evade the host immune response during infection. LtxA is a membrane-active toxin that specifically targets white blood cells (WBCs). In this review, we discuss recent developments in this field, including the identification and characterization of genes and proteins involved in secretion, regulation of LtxA, biosynthesis, newly described activities of LtxA, and how LtxA may be used as a therapy for the treatment of diseases.

Anti-leukemia activity of a bacterial toxin with natural specificity for LFA-1 on white blood cells

The oral bacterium, Aggregatibacter actinomycetemcomitans, produces a leukotoxin (LtxA) that is specific for white blood cells (WBCs) from humans and Old World primates by interacting with lymphocyte function antigen-1 (LFA-1) on susceptible cells. To determine if LtxA could be used as a therapeutic agent for the treatment of WBC diseases, we tested the in vitro and in vivo anti-leukemia activity of the toxin. LtxA kills human malignant WBC lines and primary leukemia cells from acute myeloid leukemia patients, but healthy peripheral blood mononuclear cells (PBMCs) are relatively resistant to LtxA-mediated cytotoxicity. Levels of LFA-1 on cell lines correlated with killing by LtxA and the toxin preferentially killed cells expressing the activated form of LFA-1. In a SCID mouse model for human leukemia, LtxA had potent therapeutic value resulting in long-term survival in LtxA-treated mice. Intravenous infusion of LtxA into a rhesus macaque resulted in a drop in WBC counts at early times post-infusion; however, red blood cells, platelets, hemoglobin and blood chemistry values remained unaffected. Thus, LtxA may be an effective and safe novel therapeutic agent for the treatment of hematologic malignancies.

Clinical significance of co-expression of CD21 and LFA-1 in B-cell lymphoma

We previously reported that the prognosis of CD21-positive diffuse large B-cell lymphoma (DLBCL) is significantly favorable to that of CD21-negative DLBCL (Otsuka et al. in Br J Haematol 127:416-424, 2004). In this study, we attempted to clarify the biological significance of CD21 expression in B-cell lymphoma (BCL) by performing in vitro experiments using CD21 transfection into a CD21-negative lymphoma cell line and analyzing clinical data from lymphoma samples. Established clones of CD21 transfectants showed homotypic aggregation in suspension culture. Analysis of integrin expression revealed that LFA-1 appeared to be expressed on CD21 transfectants, and the cell aggregation was abrogated by anti-LFA-1 antibody. The CD21 transfectants could adhere to plastic plates coated with ICAM-1. Moreover, flow cytometry and/or immunohistochemical analyses of clinical BCL samples (n = 29) revealed positive for CD21 in all cases; LFA-1 was also expressed without exception. All BCL cells isolated from cavity fluids (n = 10) failed to express both CD21 and LFA-1. These data suggest that CD21 is tightly related to LFA-1 expression in BCL and the absence of CD21/LFA-1 expression is associated with pleural/peritoneal fluid involvement by BCL, a potential indicator of disease progression of BCL.

Characterization of a radiolabeled small molecule targeting leukocyte function-associated antigen-1 expression in lymphoma and leukemia

OBJECTIVE

Leukocyte function-associated antigen-1 (LFA-1) is constitutively expressed on leukocytes, including overexpression on lymphomas and leukemias. We have developed a derivative of BIRT 377, an allosteric inhibitor of LFA-1, which may be chemically tagged without affecting binding. In this study, we modified this derivative, (R)-1-(4-aminobutyl)-5-(4-bromobenzyl)-3-(3,5-dichlorophenyl)-5-methylimidazolidine- 2,4-dione (butylamino-NorBIRT), and demonstrated its potential as a noninvasive imaging agent.

METHODS

Specific binding of fluorescein-labeled butylamino-NorBIRT to both human and murine cells was demonstrated using equilibrium binding and dissociation techniques. A radiometal, lutetium-177 (Lu-177), was incorporated into the butylamino-NorBIRT through 1,4,7,10-tetraazacyclododecane-N,N',N",N'''- tetraacetic acid (DOTA) as a chelator.

RESULTS

Equilibrium-binding experiments demonstrated that fluorescein- labeled butylamino-NorBIRT specifically binds human and murine LFA-1 with affinity constants of 135 and 186 nM, respectively. Dissociation kinetic experiments demonstrated an off-rate of 0.168/second(1) on murine cells, consistent with the observed affinity constant. Lutetium-177 was used for labeling, with > or =99.99% radiochemical purity and incorporation yield. This radiolabeled derivative exhibited high stability in fetal bovine serum (FBS) at 37 degrees C over 72 hours. (177)Lu-DOTA-butylamino-NorBIRT showed a binding affinity of 235 nM to human LFA-1 for equilibrium binding and competitive binding experiments.

CONCLUSIONS

The radiolabeled DOTA-butylamino-NorBIRT may have potential as a noninvasive imaging or therapeutic agent in both human and mouse models.

2D NMR and conformational analysis of a prototype anti-tumour steroidal ester

The synthetic 3beta-hydroxy-17alpha-aza-d-homo-5-androsten-7,17-dione-p-N-N-bis(2-chloroethyl)aminophenylacetate (SOT-19, I) was found to be a very potent anti-leukaemic agent candidate. Its high biological activity and low toxicity rationalize the study of its conformational properties. It can also serve as a prototype and therefore as a template for a series of congener compounds possessing a variety of toxicity and anti-leukaemic activity in subsequent 3D-QSAR studies. Its low energy conformers were identified through a combination of conformational search methods and 2D NOESY NMR spectroscopy. The low energy conformers were mainly compact, with the alkylating aromatic group orienting either to the alpha- or beta-surface of the steroidal plane. The preference in the orientation of the alkyl chain may be steroid dependent and related to the mechanism by which they produce their anti-leukaemic action. This hypothesis is supported by the fact that small chemical modifications of the conformation on the steroidal skeleton produce significant alterations on the anti-leukemic activity.

The allylic 7-ketone at the steroidal skeleton is crucial for the antileukemic potency of chlorambucil’s active metabolite steroidal esters

We have investigated the role of the allylic 7-ketone in oxidized Delta5-steroids on antileukemic activity. We synthesized and studied a series of oxidized and non-oxidized steroidal esters of p-N,N-bis(2-chloroethyl)aminophenylacetic acid (PHE), chlorambucil's active metabolite. In a comparative study of these 7-keto derivatives, on a molecular basis, regarding their ability to induce sister chromatid exchanges (SCEs) and to inhibit cell proliferation in normal human lymphocytes in vitro, the results with these 7-keto derivatives, on a molecular basis, correlated well with their antileukemic potency against leukemia P388- and L1210-bearing mice, which proved to be significantly increased compared to that of the non-oxidized derivatives. Our results indicate that the role of the steroidal skeleton it is not only for the transportation of the alkylating agent into the cell, but also contributes directly to the mechanism of antileukemic action, by an as-yet unknown way. The main conclusion from this study is that the existence of the allylic 7-keto group in the skeleton of the Delta5-steroidal esters impressively enhances their antileukemic activity, while the toxicity remains at clinically acceptable levels, suggesting that this structural modification should be further investigated.