PRF1 and infection: As an antigen-delivery system, attenuated L. monocytogenes establishes a self-limited, natural infection within the host leading to the secretion and endogenous presentation of encoded antigens, with induction of MHC Class I-restricted CD8 T cell responses18 and can be safely administered to interferon-gamma (IFN-γ) or perforin-deficient mice19, while maintaining the ability to induce CD8 T cell responses.