CD8+ cytotoxic T lymphocytes (CTLs) mediate resistance to infectious agents and tumours. Classically, CTLs recognize antigens that are localized in the cytoplasm of target cells, processed and presented as peptide complexes with class I molecules of the major histocompatibility complex (MHC). However, there is evidence for an exogenous pathway whereby antigens that are not expected to gain access to the cytoplasm are presented on MHC class I molecules. The most dramatic example is the in vivo phenomenon of cross-priming: antigens from donor cells are acquired by bone-marrow-derived host antigen-presenting cells (APCs) and presented on MHC class I molecules. Two unanswered questions concern the identity of this bone-marrow-derived cell and how such antigens are acquired. Here we show that human dendritic cells, but not macrophages, efficiently present antigen derived from apoptotic cells, stimulating class I-restricted CD8+ CTLs. Our findings suggest a mechanism by which potent APCs acquire antigens from tumours, transplants, infected cells, or even self-tissue, for stimulation or tolerization of CTLs.

SummaryDendritic cells, but not macrophages, efficiently phagocytose apoptotic cells and cross-present viral, tumor, and self-antigens to CD8 ϩ T cells. This in vitro pathway corresponds to the in vivo phenomena of cross-priming and cross-tolerance. Here, we demonstrate that phagocytosis of apoptotic cells is restricted to the immature stage of dendritic cell (DC) development, and that this process is accompanied by the expression of a unique profile of receptors, in particular the ␣ v ␤ 5 integrin and CD36. Upon maturation, these receptors and, in turn, the phagocytic capacity of DCs, are downmodulated. Macrophages engulf apoptotic cells more efficiently than DCs, and although they express many receptors that mediate this uptake, they lack the ␣ v ␤ 5 integrin. Furthermore, in contrast to DCs, macrophages fail to cross-present antigenic material contained within the engulfed apoptotic cells. Thus, DCs use unique pathways for the phagocytosis, processing, and presentation of antigen derived from apoptotic cells on class I major histocompatibility complex. We suggest that the ␣ v ␤ 5 integrin plays a critical role in the trafficking of exogenous antigen by immature DCs in this cross-priming pathway.

Cell death by necrosis is typically associated with inflammation, in contrast to apoptosis. We have identified additional distinctions between the two types of death that occur at the level of dendritic cells (DCs) and which influence the induction of immunity. DCs must undergo changes termed maturation to act as potent antigen-presenting cells. Here, we investigated whether exposure to apoptotic or necrotic cells affected DC maturation. We found that immature DCs efficiently phagocytose a variety of apoptotic and necrotic tumor cells. However, only exposure to the latter induces maturation. The mature DCs express high levels of the DC-restricted markers CD83 and lysosome-associated membrane glycoprotein (DC-LAMP) and the costimulatory molecules CD40 and CD86. Furthermore, they develop into powerful stimulators of both CD4+ and CD8+ T cells. Cross-presentation of antigens to CD8+ T cells occurs after uptake of apoptotic cells. We demonstrate here that optimal cross-presentation of antigens from tumor cells requires two steps: phagocytosis of apoptotic cells by immature DCs, which provides antigenic peptides for major histocompatibility complex class I and class II presentation, and a maturation signal that is delivered by exposure to necrotic tumor cells, their supernatants, or standard maturation stimuli, e.g., monocyte-conditioned medium. Thus, DCs are able to distinguish two types of tumor cell death, with necrosis providing a control that is critical for the initiation of immunity.

TRANCE (tumor necrosis factor [TNF]–related activation-induced cytokine) is a new member of the TNF family that is induced upon T cell receptor engagement and activates c-Jun N-terminal kinase (JNK) after interaction with its putative receptor (TRANCE-R). In addition, TRANCE expression is restricted to lymphoid organs and T cells. Here, we show that high levels of TRANCE-R are detected on mature dendritic cells (DCs) but not on freshly isolated B cells, T cells, or macrophages. Signaling by TRANCE-R appears to be dependent on TNF receptor–associated factor 2 (TRAF2), since JNK induction is impaired in cells from transgenic mice overexpressing a dominant negative TRAF2 protein. TRANCE inhibits apoptosis of mouse bone marrow–derived DCs and human monocyte-derived DCs in vitro. The resulting increase in DC survival is accompanied by a proportional increase in DC-mediated T cell proliferation in a mixed leukocyte reaction. TRANCE upregulates Bcl-xL expression, suggesting a potential mechanism for enhanced DC survival. TRANCE does not induce the proliferation of or increase the survival of T or B cells. Therefore, TRANCE is a new DC-restricted survival factor that mediates T cell–DC communication and may provide a tool to selectively enhance DC activity.