Vaccinology is the most cost effective and efficacious of medical interventions for the eradication or management of many infectious diseases. The generation of a cellular immune response is key to survival against a variety of viral and intracellular bacterial pathogens and cancer. Therefore, the development of safe vaccines capable of inducing strong cellular immunity continues to be a pressing challenge for medicine. We have explored the use of an attenuated strain of Listeria monocytogenes as a novel live vaccine vector for this purpose. L. monocytogenes is a gram-positive intracellular bacterial pathogen whose genome has been fully sequenced (22), greatly facilitating detailed studies of its biochemistry, genetics, and physiology and contributing to the existing wealth of knowledge of its adjuvant properties and immunogenicity.Since the early work of Mackaness in the 1960s (30), L. monocytogenes has been studied extensively as a paradigm for understanding innate and cell-mediated immunity. The organism can enter phagocytic cells through Fc receptors or type I macrophage scavenger receptors (12, 48) and can invade nonphagocytic cells by the interaction of the bacterial surface proteins InlA (16) and InlB (18) with their cognate host cell receptors. Following internalization by phagocytic cells, the majority of engulfed organisms are degraded in the phagosome (11). However, listeriolysin O (LLO), a pore-forming cytolysin, and PI-PLC, a phosphatidylinositol-specific phospholipase C, mediate permeabilization of the phagosomal membrane, enabling a fraction of the organisms to enter the host cell cytosol (9,31,38,45). Subsequently, proteins secreted by the organism can be delivered directly into the major histocompatibility complex (MHC) class I pathway of antigen processing and presentation (2, 5). Mutants of L. monocytogenes unable to enter the cytosol are avirulent. These bacteria fail to stimulate protective immunity in mice, and cells infected by such mutants are not recognized by L. monocytogenes-immune CD8 ϩ T cells (7, 34). Conversely, mice infected with sublethal doses of wildtype bacteria develop long-lasting immunity, mediated predominantly by CD8 ϩ T cells (13,29). This unique property of L. monocytogenes makes it attractive as a potential live vaccine vector, and recombinant strains expressing foreign antigens have successfully protected mice against lymphocytic choriomeningitis (23, 44), papillomavirus (26) and influenza virus (25) infections, and tumor challenge (24,36,37).Because of the potential use of this organism as a vaccine vector for protection against infectious diseases and cancer, the safety of L. monocytogenes is a critical issue. While infections by L. monocytogenes are fairly rare and can readily be controlled by a number of antibiotics, the organism can cause meningitis and death, particularly in immunocompromised or pregnant patients. A vaccine strain of L. monocytogenes should therefore be avirulent but immunogenic. We previously sought to generate an attenuated strain that could e...