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Introduction
Candidate prophylactic vaccines against Listeria monocytogenes (Listeria) include live attenuated pathogens [1-4] , live vector-based approaches [5-7] and subunit vaccines [8,9] . However, the use of live vaccines in at-risk immunocompromised individuals including pregnant women poses major safety risks, making a non-living subunit vaccine a more preferable approach. Unfortunately, subunit vaccines by themselves are generally ineffective due to overall poor immunogenicity and inability, in particular, to generate the T-cell immunity required for protection against intracellular organisms such as Listeria.
Dendritic cell (DC) vaccines are produced by isolation of DC from a subject, in vitro loading of the DC with the relevant vaccine antigen, and then intravenous or subcutaneous injection of the live DC back into the subject. Based on their improved ability to induce T-cell immunity, DC vaccines have been proposed as solutions for induction of T-cell protection against intracellular infections, such as HIV [10] . Our group previously reported a DC vaccine loaded with a peptide from the Listeria protein, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), induced generation of GAPDH-specific CD8+ and CD4+ T cells and conferred protection against Listeria[11] , consistent with the importance of T-cell responses against GAPDH in Listeria protection [12-14] . Other Listeria antigens that contain CD4+ and CD8+ T-cell epitopes and that might therefore enhance the efficacy of a DC-based Listeria vaccine include listeriolysin (LLO), ActA, and p60 [11,15,16] . To date, only T-cell immunity against LLO has been reported in humans recovering from listeriosis [17] , suggesting a potential role of in protection. We therefore wished to test whether a T-cell vaccine targeting LLO could protect against listeriosis. In particular, we sought to test whether a nanoparticle-conjugated LLO peptide could increase DC vaccine efficacy [10,18] and also whether a novel T-cell adjuvant could enhance LLO vaccine immunogenicity [19] . Gold glyconanoparticles (GNPs) constitute a nanoscale metallic core to which self-assembled monolayers of carbohydrate ligands are covalently linked by means of thiol chemistry [20] . Due to their water dispersibility, biocompatibility, resistance to enzymatic degradation, ease of preparation, and ability to incorporate different ligands, GNPs are highly versatile [21,22] . In this study, therefore, we sought to test the potential of a glucose-labelled GNP presenting the LLO91-99 peptide (GNP-LLO) to protect against listeriosis in...