Antigen storage compartments in mature dendritic cells facilitate prolonged cytotoxic T lymphocyte cross-priming capacity

Abstract

Dendritic cells (DCs) are crucial for priming of naive CD8 ؉ T lymphocytes to exogenous antigens, so-called ''cross-priming.'' We report that exogenous protein antigen can be conserved for several days in mature DCs, coinciding with strong cytotoxic T lymphocyte crosspriming potency in vivo. After MHC class I peptide elution, protein antigen-derived peptide presentation is efficiently restored, indicating the presence of an intracellular antigen depot. We characterized this depot as a lysosome-like organelle, distinct from MHC class II compartments and recently described early endosomal compartments that allow acute antigen presentation in MHC class I. The storage compartments we report here facilitate continuous supply of MHC class I ligands. This mechanism ensures sustained crosspresentation by DCs, despite the short-lived expression of MHC class I-peptide complexes at the cell surface.antigen processing ͉ cross-presentation ͉ endocytosis ͉ Fc receptor ͉ MHC class I D endritic cells (DCs) are crucial in the initiation and orchestration of the T cell immune response (1-3). DCs operate as sentinels of an infection in the periphery and subsequently as conductors of the T cell response in the lymph nodes. To exert these functions, DCs are specialized in the ingestion, processing, and presentation of antigens acquired by receptor-independent pinocytosis or by receptor-mediated endocytosis (4-6). To this end, they are equipped with a diverse set of receptors for uptake of antigen, such as scavenger receptors, lectin receptors, or IgG (Fc␥) receptors (7).Immature DCs have the capacity to efficiently acquire antigen, but a poor capacity to migrate and to stimulate T cells. Proper T cell response initiation requires maturation of the DCs, a process that is triggered by contact with infectious or inflammatory signals. Mature DCs characteristically show enhanced migratory capacity, up-regulation of the MHC class I processing machinery, and enhanced expression of MHC I and II and costimulatory molecules.DCs present antigenic peptides in either MHC class I to induce CD8 T cell responses and in MHC class II to induce CD4 T cell responses. Whereas MHC class I ligands are commonly derived from breakdown products of endogenous proteins that are degraded by the proteasome (8), DCs also have the unique capacity to present peptides derived from exogenous antigens in MHC class I to CD8 ϩ T cells, a process called ''crosspresentation.'' This process is crucial for induction of effective cytotoxic T lymphocyte (CTL) immunity against tumors, which lack direct priming capacity themselves, but also against microorganisms including viruses (1).DCs have dedicated organelles to facilitate efficient loading of antigenic peptides in MHC class II molecules. These MHC class II compartments (MIIC) are multivesicular endosomes that express high levels of MHC class II and invariant chain and are abundantly present in immature DCs (9). On maturation of the DC, rapid reorganization of the MIIC takes place that facilitates transport of MHC class II...

Fcγ receptor antigen targeting potentiates cross-presentation by human blood and lymphoid tissue BDCA-3+ dendritic cells

“…31 In ovalbumin cross-presentation by mouse DC systems, data collectively suggests that processing may occur in either the endosomal or cytosolic/proteasomal pathway, depending on the endocytic route taken (ie, choice of binding to endocytic receptors) and configuration of the antigen (ie, soluble or particulate). 4,15,28,32,33 Our data in human MoDCs now shows that for HCMV pp65, Fc␥R-mediated uptake potentiates crosspresentation in a manner that requires processing both in the endosomal pathway and by the proteasome. Earlier flow cytometrybased work suggested that Fc␥R are not expressed on BDCA-3 ϩ DCs found in PBMCs.…”

“…28 For immune complexes, the requirements for endosomal processing and proteasome-mediated peptide generation are not fully clear, particularly for human myeloid DCs. 3,29 We therefore assessed in human MoDCs the role of antigen processing in the endosomal pathway and by the proteasome.…”

Differential use of autophagy by primary dendritic cells specialized in cross-presentation

“…Prolonged antigen storage favors MHC I cross-presentation as it facilitates antigen transfer to the cytosol, rather than its rapid degradation in the lysosomes. 44,45 Indeed, LAP, the process by which LC3 is recruited to phagosomes, has recently been implicated in promoting antigen stability in DC 46 unlike in other cell types where it is proposed to enhance antigen degradation. 21,22,47 This autophagy-dependent mechanism could be of more importance for soluble antigen that is likely to be more rapidly degraded than cell-associated antigen.…”

Serum Antibodies Critically Affect Virus-Specific CD4+/CD8+ T Cell Balance during Respiratory Syncytial Virus Infections

“…Therefore, it can be envisaged that for an effective in vivo expansion of RSV-specific CD4 + and CD8 + T cells, indirect routes of Ag presentation are most effective for efficient induction of T cell responses whereby DC acquire noninfectious material like opsonized virus particles or necrotic infected cells. Cross presentation is facilitated by stimulation of innate immune receptors expressed by DCs (56)(57)(58)(59)(60). The efficacy of the cross presentation route via MHC class I molecules and the relative contribution of MHC class II presentation for in vivo RSV-specific T cell activation might depend on several factors.…”