Mechanisms of processing and presentation of the antigens of Listeria monocytogenes.

Antigen processing is the series of events through which protein antigens become degraded and are sent to the cell surface for recognition by T cells. These events within the cell have been studied extensively using the model system of infection with Listeria monocytogenes. This bacteria resides primarily intracellularly; immunity to it is mediated by cellular responses. Upon phagocytosis by a macrophage, engulfed Listeria express a hemolytic molecule, listeriolysin O (LLO), and can disrupt the membrane of the endosome and escape into the cellular cytoplasm. This allows the organism to escape the hostile environment of the endosome and also gives it access to the processing machinery of the cytoplasm. Therefore, Listeria monocytogenes can be processed within an endosome and be presented by class II MHC, or can escape into the cytoplasm and be processed there and presented by class I MHC. This LLO molecule is not only an important virulence factor, it is also a dominant antigen in the cellular immune response to Listeria. The outcome of antigen processing and presentation can be influenced by the expression of LLO, by the state of activation of the macrophage, and by the cytokines involved in the immune response.