NATURE IMMUNOLOGY VOLUME 11 NUMBER 4 APRIL 2010 313 Cross-presentation permits specialized dendritic cells (DCs) to present extracellular antigens to cytotoxic CD8 + T cells (cytotoxic T lymphocytes (CTLs)) 1–3 . Cross-presentation of autoantigens or innocuous circulating antigens usually results in cross-tolerance 4,5 . Immunogenic cross-presentation, also known as cross-priming, requires the presence of pathogen-associated molecular patterns, such as Toll-like receptor (TLR) ligands, and/or of specific CD4 + helper T cells 6,7 that ‘license’ the cross-presenting DC for cross- priming 1,8 . The underlying molecular mechanisms that result from such licensing are under intense investigation and include enhanced costimulatory signals and diminished proapoptotic signals by the DCs 9,10 . The necessity of DC licensing in cross-priming indicates that sev- eral rare cells of the immune response, that is, antigen-specific CTLs, specific helper T cells and cross-presenting DCs, must physically inter- act. Such interactions are usually governed by chemokines and their receptors 11 , which suggests that these molecules might be involved in cross-priming. In support of this idea, it has been shown that the interaction of helper T cells with TLR ligand–stimulated DCs causes the production of ligands for the chemokine receptor CCR5, which attract naive CTLs and increase the likelihood of their encounter with licensed DCs 12,13 . Activated CTLs themselves can then produce CCR5 ligands that attract further CTLs to DCs that have successfully cross-primed 14 . Also, natural killer T cells (NKT cells) can activate DCs for cross-priming 15–17 , but the underlying mechanisms are unclear. As NKT cell–dependent cross-priming by DCs also requires the encounter of three immune cell types, a role for chemokines is likely but has not yet been studied. NKT cells are thought to link innate and adaptive immunity of both the T helper type 1 (T H 1) and T H 2 type and have been associated with antimicrobial, autoimmune and antitumor immunity 18–20 . Most NKT cells express a semi-invariant T cell antigen receptor repertoire through which they recognize glycolipid antigens presented by the nonpolymorphic major histocompatibility complex (MHC) class I–like protein CD1d 21 . Bacterial ligands include α- galactosyldiacylglycerol (α-GalDAG) from Borrelia burgdorferi and α-glycuronosylceramides such as GSL-1′ from Sphingomonas species 22–24 . Isoglobotrihexosylceramide (iGb3) is an endogenous ligand that has been linked to NKT cell activation after infection with Gram-negative bacteria 23 . The glycosphingolipid α-galactosylceramide (α-GC) has been widely used as model NKT cell antigen 15,18,19 . OCH is a synthetic α-GC derivative with a truncated sphingosine chain that induces T H 2-like responses 25 . Mutagenesis studies indicate that CD1d binds α-GC and other ligands, including iGb3 and α-GalDAG, in a similar manner 26,27 . NKT cell activation causes rapid production of T H 1, T H 2 and T H 17 cytokines and various chemokines and chemokine 1 Institutes of Molecular Medicine and Experimental Immunology and 2 Life and Medical Sciences Institute, Friedrich-Wilhelms-Universität, Bonn, Germany. 3 III Medizinische Klinik, Universitätsklinikum Hamburg-Eppendorf, Hamburg, Germany. 4 Department of Biochemistry and Molecular Biology and 5 Department of Chemistry, Monash University, Clayton, Australia. 6 Department of Microbiology and Immunology, University of Melbourne, Parkville, Australia. 7 Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah, USA. 8 Molecular Immunology, Institut für Umweltmedizinische Forschung an der Heinrich- Heine-Universität Düsseldorf, Düsseldorf, Germany. 9 Present address: Dana-Farber Cancer Institute, Boston, Massachusetts, USA. 10 These authors contributed equally to this work. Correspondence should be addressed to I.F. (irmgard.foerster@uni-duesseldorf.de) or C.K. (ckurts@web.de). Received 22 October 2009; accepted 2 February 2010; published online 28 February 2010; doi:10.1038/ni.1848 Alternative cross-priming through CCL17-CCR4- mediated attraction of CTLs toward NKT cell–licensed DCs Verena Semmling 1,10 , Veronika Lukacs-Kornek 1,9,10 , Christoph A Thaiss 1 , Thomas Quast 2 , Katharina Hochheiser 1 , Ulf Panzer 3 , Jamie Rossjohn 4 , Patrick Perlmutter 5 , Jia Cao 5 , Dale I Godfrey 6 , Paul B Savage 7 , Percy A Knolle 1 , Waldemar Kolanus 2 , Irmgard Förster 8 & Christian Kurts 1 Cross-priming allows dendritic cells (DCs) to induce cytotoxic T cell (CTL) responses to extracellular antigens. DCs require cognate ‘licensing’ for cross-priming, classically by helper T cells. Here we demonstrate an alternative mechanism for cognate licensing by natural killer T (NKT) cells recognizing microbial or synthetic glycolipid antigens. Such licensing caused cross-priming CD8 + DCs to produce the chemokine CCL17, which attracted naive CTLs expressing the chemokine receptor CCR4. In contrast, DCs licensed by helper T cells recruited CTLs using CCR5 ligands. Thus, depending on the type of antigen they encounter, DCs can be licensed for cross-priming by NKT cells or helper T cells and use at least two independent chemokine pathways to attract naive CTLs. Because these chemokines acted synergistically, this can potentially be exploited to improve vaccinations. ARTICLES © 2010 Nature America, Inc. All rights reserved.