Characterization of receptor interactions with ALO and activation of signaling pathways

Bacillus anthacis, a Gram-positive bacterium responsible for causing anthrax, produces virulence factors that alter host cellular signaling and immune functions. Anthrolysin O (ALO) is a cholesterol-dependent cytolysin (CDC) produced by B. anthracis. This toxin has been shown to bind to cholesterol (1), bind to TLR4 (manuscript submitted) and signal through TLR4 (2). We wanted to further characterize ALO and the interaction between ALO and the membrane of macrophages. We know ALO binds to cholesterol [1]; however, low concentrations of ALO require the presence of Toll-like Receptor 4 (TLR4) to signal [2]. Therefore, we wanted to determine if ALO binds to TLR4 and to characterize this interaction. Previously, pneumolysin, a CDC, was shown to bind to TLR4 positive cells [3]. Additionally, intermedilysin [4] and vaginolysin [5] bind to CD59, adding to the list of CDCs that can bind receptors along with cholesterol. We hypothesized that ALO binds directly to TLR4. Our data show that rALO binds directly to TLR4, and stimulates cells through TLR4-dependent and - independent mechanisms. Next, we wanted to further characterize the signaling pathways that ALO activated in macrophages. We hypothesize that ALO activated cells through TLR4-dependent and -independent mechanisms. Our data support our hypothesis that ALO signals through TLR4-dependent and -independent mechanisms. We observed that ALO activates both MyD88-dependent and -independent pathways. The activation by ALO through TLR4-independent pathways is at least partially c-Src-dependent. These data indicate that TLR4 is not just a receptor for Gram-negative LPS; TLR4 can bind both Gram-negative and Gram-positive bacterial components. These data together suggest that ALO binds to TLR4 activating signaling pathways, and binds to cholesterol within cell membranes causing activation of additional signaling pathways independent of TLR4. Thus, ALO can activate cells through multiple mechanisms.