Killed but Metabolically Active Recombinant Listeria monocytogenes as an Antigen Delivery and Activation Platform for Human Dendritic Cell-Based Cancer Immunotherapy.

Dendritic cells (DCs) are bone marrow-derived professional antigen presenting cells which play a pivotal role in initiating immune responses. To achieve optimal potency for priming of naⁱve T cells, DCs must not only present high levels of MHC peptide complexes, but must also undergo maturation. An ideal DC-targeting vaccine formulation, therefore, should simultaneously deliver multiple antigens and activate DCs to increase expression of surface MHC-peptide complexes, costimulatory molecules, and production of immuno-stimulatory cytokines. Despite the development of a variety of ex vivo methods, no standard efficient and cost effective methods combining antigen loading and activation/maturation of DCs for clinical use currently exist. Here we report the development of a novel DC antigen-loading platform based on killed, but metabolically active (KBMA) recombinant Listeria monocytogenes (Lm). While recombinant Lm has a relatively long history as a cancer vaccine platform, its utility as a DC activation and loading platform has not been fully explored. KBMA Lm were 8-logs more sensitive to photochemical inactivation by the combined treatment of S-59 psoralen (amotosalen HCl) and long-wave ultraviolet light (S-59/UVA), by virtue of abrogation of nucleotide excision repair (NER) capacity, through engineered deletion of the bacterial UvrAB genes. Due to random distribution of infrequent psoralen adducts, KBMA Lm cannot propagate, but are metabolically active and can express their genetic repertoire, and program presentation of expressed heterologous antigens via the MHC class I and class II pathways. Infection of human monocyte-derived or blood myeloid DCs with KBMA Listeria induced their maturation and production of pro-inflammatory and pro- Th1 cytokines. The DCs achieved a fully mature phenotype, comparable to that induced by the standard maturation stimulus MCM-mimic. To determine whether KBMA Lm could program presentation of encoded antigens by the MHC class I pathway, we constructed a recombinant Lm vaccine expressing CD8+ T cell epitopes of influenza matrix protein (MP_58–66) and MelanA/Mart-1 protein (Mart1_26–35). DCs infected with the recombinant KBMA Lm vaccine presented the recombinant class I epitopes and induced IFN-γ production by MP_58–66- or Mart1_26–35-specific human CD8+ T cell clones. These studies demonstrate that KBMA Listeria is a novel and powerful ex vivo DC antigen loading platform combining a potent DC-maturation stimulus with the cytosolic delivery of recombinant antigen for presentation.