When bacteria such as Listeria and Salmonella are taken up into a phagocytic cell of the immune system, they are engulfed into a phagocytic vacuole in the interior of the cell. Here they may be destroyed and fragments of antigens they carry will eventually egress to the cell surface to activate CD4 immune cells, which are important in assisting in the immune response. Listeria, unlike other bacteria, has evolved to break out of the vacuole and survive inside the immune cell. This way antigens that Listeria carries are targeted to a pathway in the cytoplasm where they are broken into peptides and taken to the cell surface for recognition by killer T cells. These killer T cells seek out and destroy tumor cells displaying tumor-specific antigens.

Credit: Yvonne Paterson, PhD, University of Pennsylvania School of Medicine