Immunodetection and Pathogenesis Mediated by Bacterial Membrane Vesicles

Animals have evolved sophisticated means to detect and defend themselves against colonization by microorganisms. Mammals differentiate self from nonself by the use of germline encoded pattern recognition receptors (PRRs) of the innate immune system in addition to pathogen-specific recognition by the adaptive immune system. Conserved microbial structures, termed microbe-associated molecular patterns (MAMPs) are recognized by the innate immune system. Pathogens succeed in subverting these immune mechanisms by the production of effector proteins and modification of MAMPs. While many PRRs and their cognate ligands have been extensively examined, most studies addressed the immune response toward the whole microorganism, isolated MAMPs, or effector proteins. Both Gram-negative and Gram-positive bacteria produce membrane vesicles that contain several MAMPs and bacterial effector proteins, and the study of the interplay of these with immune PRRs is a recently emerging field. Here we summarize key components of the innate and adaptive immune systems and discuss current knowledge regarding the immune recognition and immunomodulatory functions of bacterial membrane vesicles in mammalian hosts.