The immune system must distinguish viable cells from cells damaged by physical and infective processes. The damaged cell-recognition molecule Clec9A is expressed on the surface of the mouse and human dendritic cell subsets specialized for the uptake and processing of material from dead cells. Clec9A recognizes a conserved component within nucleated and nonnucleated cells, exposed when cell membranes are damaged. We have identified this Clec9A ligand as a filamentous form of actin in association with particular actin-binding domains of cytoskeletal proteins. We have determined the crystal structure of the human CLEC9A C-type lectin domain and propose a functional dimeric structure with conserved tryptophans in the ligand recognition site. Mutation of these residues ablated CLEC9A binding to damaged cells and to the isolated ligand complexes. We propose that Clec9A provides targeted recruitment of the adaptive immune system during infection and can also be utilized to enhance immune responses generated by vaccines.

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DC-based vaccines that initiate T cell responses are well tolerated and have demonstrated efficacy for tumor immunotherapy, with the potential to be combined with other therapies. Targeting vaccine antigens (Ag) directly to the DCs in vivo is more effective than cell-based therapies in mouse models and is therefore a promising strategy to translate to humans. The human CD141 DCs are considered the most clinically relevant for initiating CD8 T cell responses critical for killing tumors or infected cells, and they specifically express the C-type lectin-like receptor CLEC9A that facilitates presentation of Ag by these DCs. We have therefore developed a human chimeric Ab that specifically targets CLEC9A on CD141 DCs in vitro and in vivo. These human chimeric Abs are highly effective at delivering Ag to DCs for recognition by both CD4 and CD8 T cells. Given the importance of these cellular responses for antitumor or antiviral immunity, and the superior specificity of anti-CLEC9A Abs for this DC subset, this approach warrants further development for vaccines.

could be achieved, the immunogenicity of TNE-encapsulated antigen was not explored. Here we show unexpectedly, given the lack of DC activation by anti-Clec9A-antigen conjugates, that in the absence of adjuvant, cross-presenting DCs targeted with antigenClec9A-TNE stimulate DC activation, antigen-specific CTLs, and highly effective tumor-specific immunity, dependent on the presence of CD4 helper epitopes and CD40 signaling.

Highlights d The Plasmodium ribosomal protein RPL6 is expressed during liver-stage infection d RPL6 can be targeted by specific liver T RM cells for efficient parasite elimination d Prime-and-trap vaccination targeting RPL6 induces effective protection against malaria d RPL6 is highly conserved across global P. falciparum clinical isolates

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Cross-presentation is the mechanism by which exogenous Ag is processed for recognition by CD8 + T cells. Murine CD8α + DCs are specialized at cross-presenting soluble and cellular Ag, but in humans this process is poorly characterized. In this study, we examined uptake and cross-presentation of soluble and cellular Ag by human blood CD141 + DCs, the human equivalent of mouse CD8α + DCs, and compared them with human monocyte-derived DCs (MoDCs) and blood CD1c + DC subsets. MoDCs were superior in their capacity to internalize and cross-present soluble protein whereas CD141 + DCs were more efficient at ingesting and cross-presenting cellular Ag. Whilst cross-presentation by CD1c + DCs and CD141 + DCs was dependent on the proteasome, and hence cytosolic translocation, cross-presentation by MoDCs was not. Inhibition of endosomal acidification enhanced cross-presentation by CD1c + DCs and MoDCs but not by CD141 + DCs. These data demonstrate that CD1c + DCs, CD141 + DCs, and MoDCs are capable of crosspresentation; however, they do so via different mechanisms. Moreover, they demonstrate that human CD141 + DCs, like their murine CD8α + DC counterparts, are specialized at cross-presenting cellular Ag, most likely mediated by an enhanced capacity to ingest cellular Ag combined with subtle changes in lysosomal pH during Ag processing and use of the cytosolic pathway.Keywords: Antigen processing r Cross-presentation r Human dendritic cells Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionDendritic cells (DCs) are professional APCs that are uniquely able to process and present antigen (Ag) to prime naïve T-cell Correspondence: Dr. Kristen J. Radford e-mail: kristen.radford@mater.uq.edu.au responses. DCs in human and mouse can be classified into a number of subsets that vary in location, phenotype, and specialized function [1]. These include (i) inflammatory monocyte-derived DCs (MoDCs) that develop from monocytes and are rapidly * These authors contributed equally to this work.C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 330 Meng-Chieh Chiang et al. Eur. J. Immunol. 2016. 46: 329-339 recruited to sites of inflammation, (ii) plasmacytoid DCs, which are key producers of type I IFN, and (iii) "classical" or "conventional" DCs (cDCs), which can be further categorized based on location into "lymphoid-resident" and "migratory" DCs [1]. The lymphoidresident DCs capture Ag directly in situ, whereas migratory DCs reside in the peripheral organs (e.g. lung, skin, and gut) where they capture Ag then migrate to lymphoid tissues to share their Ag with other lymphoid-resident DCs, or present Ag directly to T cells. cDCs can be further segregated into two main subsets: (i) the mouse CD11b + cDC subset and human CD1c + DC equivalent; and (ii) the mouse CD8α + lymphoid-resident DC, related mouse CD103 + tissue resident cDCs, and the human equivalent CD141 + DC that can now be collectively defined by coexpression of the C-type lectin-like re...

Liver resident-memory CD8+ T cells (TRM cells) can kill liver-stage Plasmodium-infected cells and prevent malaria, but simple vaccines for generating this important immune population are lacking. Here, we report the development of a fully synthetic self-adjuvanting glycolipid-peptide conjugate vaccine designed to efficiently induce liver TRM cells. Upon cleavage in vivo, the glycolipid-peptide conjugate vaccine releases an MHC I–restricted peptide epitope (to stimulate Plasmodium-specific CD8+ T cells) and an adjuvant component, the NKT cell agonist α-galactosylceramide (α-GalCer). A single dose of this vaccine in mice induced substantial numbers of intrahepatic malaria-specific CD8+ T cells expressing canonical markers of liver TRM cells (CD69, CXCR6, and CD101), and these cells could be further increased in number upon vaccine boosting. We show that modifications to the peptide, such as addition of proteasomal-cleavage sequences or epitope-flanking sequences, or the use of alternative conjugation methods to link the peptide to the glycolipid improved liver TRM cell generation and led to the development of a vaccine able to induce sterile protection in C57BL/6 mice against Plasmodium berghei sporozoite challenge after a single dose. Furthermore, this vaccine induced endogenous liver TRM cells that were long-lived (half-life of ~425 days) and were able to maintain >90% sterile protection to day 200. Our findings describe an ideal synthetic vaccine platform for generating large numbers of liver TRM cells for effective control of liver-stage malaria and, potentially, a variety of other hepatotropic infections.