Listeriolysin O: A phagosome-specific cytolysin revisited

Listeriolysin O (LLO) is an essential determinant of Listeria monocytogenes pathogenesis that mediates the escape of L. monocytogenes from host cell vacuoles, thereby allowing replication in the cytosol without causing appreciable cell death. As a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming toxins, LLO is unique in that it is secreted by a facultative intracellular pathogen, whereas all other CDCs are produced by pathogens that are largely extracellular. Replacement of LLO with other CDCs results in strains that are extremely cytotoxic and 10,000-fold less virulent in mice. LLO has structural and regulatory features that allow it to function intracellularly without causing cell death, most of which map to a unique N-terminal region of LLO referred to as the proline, glutamic acid, serine, threonine (PEST)-like sequence. Yet, while LLO has unique properties required for its intracellular site of action, extracellular LLO, like other CDCs, affects cells in a myriad of ways. Because all CDCs form pores in cholesterol-containing membranes that lead to rapid Ca²⁺ influx and K⁺ efflux, they consequently trigger a wide range of host cell responses, including mitogen-activated protein kinase activation, histone modification, and caspase-1 activation. There is no debate that extracellular LLO, like all other CDCs, can stimulate multiple cellular activities, but the primary question we wish to address in this perspective is whether these activities contribute to L. monocytogenes pathogenesis.