Immunology A Syk Response to Dead Cells

Elizabeth M. Adler

Science Signaling, AAAS, Washington, DC 20005, USA

Necrotic cells elicit innate inflammatory responses involved in wound healing and adaptive immune responses that may play a role in tumor and transplant rejection. Noting that CLEC9A, a C-type lectin, is selectively expressed in a class of dendritic cells that phagocytose necrotic cells and cross-prime CD8⁺ T cells (called CD8⁺ cells in mice), Sancho et al. explored its role in mediating the adaptive response. Necrotic cells stimulated signaling by chimeric receptors containing mouse or human CLEC9A extracellular domains fused to CD3 (CLEC9A-CD3) expressed in cocultured T cell lines. CLEC9A-CD3 signaling correlated with necrotic cell number, assessed by permeability to DNA stains, and was independent of necrotic cell type or the method used to elicit necrosis. Confocal imaging of permeabilized fibroblasts exposed to labeled tetramers of the CLEC9A C-type lectin domain revealed that the CLEC9A ligand was cytoplasmic. The ability of CD8⁺ dendritic cells lacking CLEC9A to phagocytose necrotic material was unimpaired; however, they were unable to stimulate expansion and differentiation of T cells with antigen ingested from necrotic cells, although they did so with antigen from other sources. CLEC9A has a cytoplasmic hemITAM motif (a motif in which phosphorylation of a tyrosine residue enables binding of the kinase Syk), and Syk recruitment was critical to CLEC9A signaling: Phosphorylated Syk accumulated at the interface between necrotic and dendritic cells; cells lacking Syk were unable to stimulate T cells with dead-cell–associated antigen; and wild-type CLEC9A, but not a mutant in which the tyrosine was substituted with phenylalanine, restored this ability in dendritic cells lacking CLEC9A. Cross-priming to dead-cell–associated antigens was decreased in transgenic mice lacking CLEC9A and in wild-type mice injected with an antibody directed against CLEC9A. Thus, CLEC9A appears to link the ability to recognize necrotic cells with activation of the adaptive immune response through a pathway that depends on its interaction with Syk.

D. Sancho, O. P. Joffre, A. M. Keller, N. C. Rogers, D. Martínez, P. Hernanz-Falcón, I. Rosewell, C. Reis e Sousa, Identification of a dendritic cell receptor that couples sensing of necrosis to immunity. Nature 458, 899–903 (2009). [PubMed]