Listeriolysin O-liposome-mediated cytosolic delivery of macromolecule antigen in vivo: enhancement of antigen-specific cytotoxic T lymphocyte frequency, activity, and tumor protection

Cytotoxic T lymphocytes (CTLs) are primed by peptide antigens that are endogenously processed in the cytosol and presented in the context of major histocompatibility complex I (MHC I) molecules of antigen-presenting cells (APCs). Exogenous soluble protein antigens do not gain efficient entry into the cytosol of APCs, and therefore requires a special cytosolic delivery method. We have developed such a delivery strategy adopting the well-elucidated cytosol-invading listerial endosomal escape mechanism, and report here an efficient delivery of exogenous whole protein antigen into the cytosol in a mouse model. Co-encapsulation of listeriolysin O (LLO) inside liposome (LLO-liposome) was required for delivery of ovalbumin (OVA) into the cytosol of APCs in primary cultures. LLO-liposome-mediated OVA immunization in mice engendered significantly higher OVA-specific CTL activity and increased antigenic peptide-specific CTL precursor (CTLp) frequency as compared to non-LLO-liposome or soluble OVA immunizations. Interferon-gamma (IFN-gamma) production upon specific stimulation by MHC I-restricted peptide was also significantly stronger by the inclusion of LLO in the liposomes. Rerouting of antigen into the cytosol by LLO-liposomes, however, did not reduce the extent of anti-OVA antibody responses. Moreover, LLO-liposome-antigen vaccination was robust in conferring protection to mice from lethal challenges with antigen-expressing tumor cells. Our study demonstrates a novel delivery system for efficient introduction of exogenous protein into the cytosol in vivo, priming cellular immune responses, which are protective in nature. (C) 2002 Elsevier Science B.V. All rights reserved.