Supplementary Material for: The Janus Face of a-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.