The Janus Face of α-Toxin: A Potent Mediator of Cytoprotection in Staphylococci-Infected Macrophages

After phagocytosis by macrophages, Staphylococcus aureus evades killing in an a-toxin-dependent manner, and then prevents apoptosis of infected cells by upregulating expression of antiapoptotic genes like MCL-1 (myeloid cell leukemia-1). Here, using purified a-toxin and a set of hla-deficient strains, we show that a-toxin is critical for the induction of MCL-1 expression and the cytoprotection of infected macrophages. Extracellular or intracellular treatment of macrophages with a-toxin alone did not induce cytoprotection conferred by increased Mcl-1, suggesting that the process is dependent on the production of a-toxin by intracellular bacteria. The increased expression of MCL-1 in infected cells was associated with enhanced NFκB activation, and subsequent IL-6 secretion. This effect was only partially inhibited by blocking TLR2, which suggests the participation of intracellular receptors in the specific recognition of S. aureus strains secreting a-toxin. Thus, S. aureus recognition by intracellular receptors and/or activation of downstream pathways leading to Mcl-1 expression is facilitated by a-toxin released by intracellular bacteria which permeabilize phagosomes, ensuring pathogen access to the cytoplasmatic compartment. Given that the intracellular survival of S. aureus depends on a-toxin, we propose a novel role for this agent in the protection of the intracellular niche, and further dissemination of staphylococci by infected macrophages.