Leukotoxin (Leukothera®) targets active leukocyte function antigen-1 (LFA-1) protein and triggers a lysosomal mediated cell death pathway.

Leukotoxin (LtxA) is a protein toxin that is secreted from the oral bacterium, Aggregatibacter actinomycetemcomitans. LtxA targets specifically the ß(2) integrin, leukocyte function antigen-1 (LFA-1) on white blood cells (WBCs) and causes cell death. LtxA preferentially targets activated WBCs and is being developed as a therapeutic agent for the treatment of WBC diseases such as hematologic malignancies and autoimmune/inflammatory diseases. However, the mechanism by which interaction between LtxA and LFA-1 results in cell death is not well understood. Furthermore, how LtxA preferentially recognizes activated WBCs is not known. We show here that LtxA interacts specifically with LFA-1 in the active (exposed) conformation. In THP-1 monocytes, LtxA caused rapid activation of caspases, but LtxA could overcome the inhibition of caspases and still intoxicate. In contrast, inhibiting the vesicular trafficking pathway or cathepsin D release from the lysosome resulted in significant inhibition of LtxA-mediated cytotoxicity, indicating a more potent, lysosomal mediated cell death pathway. LtxA caused rapid disruption of the lysosomal membrane and release of lysosomal contents into the cytosol. Binding of LtxA to LFA-1 resulted in the internalization of both LtxA and LFA-1, with LtxA localizing specifically to the lysosomal compartment. To our knowledge, LtxA represents the first bacterial toxin shown to localize to the lysosome where it induces rapid cell death.

Expression and targeting of lymphocyte function-associated antigen 1 (LFA-1) on white blood cells for treatment of allergic asthma.

Allergic asthma is a chronic respiratory disease that results from an exaggerated inflammatory response in the airways. Environment stimuli, such as pollen and HDM, cause activation and migration of inflammatory WBCs into the respiratory tract, where they cause lung damage. Migration of these WBCs is dependent on the active configuration of the ß2 integrin LFA-1. The experimental therapeutic agent LtxA specifically targets active LFA-1 and causes cell death. We investigated the association between LFA-1 and allergic asthma and hypothesized that targeting LFA-1 with LtxA could be an attractive strategy for treatment of the condition. We examined LFA-1 (CD11a) levels on PBMCs from patients with allergic asthma compared with healthy controls. Patients exhibited a significantly higher percentage of PBMCs expressing LFA-1 than healthy controls. Furthermore, the level of LFA-1 expression on patient PBMCs was greater than on healthy PBMCs. We identified a unique cellular population in patients that consisted of CD4(-) CD11a(hi) cells. We also evaluated LtxA in a HDM extract-induced mouse model for allergic asthma. LtxA caused resolution of disease in mice, as demonstrated by a decrease in BALF WBCs, a reduction in pulmonary inflammation and tissue remodeling, and a decrease in proinflammatory cytokines IL-4, IL-5, IL-9, IL-17F, and IL-23α in lung tissue. LFA-1 may serve as an important marker in allergic asthma, and the elimination of activated WBCs by use of LtxA could be a viable therapeutic strategy for treating patients with this condition.

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.