Long-Peptide Cross-Presentation by Human Dendritic Cells Occurs in Vacuoles by Peptide Exchange on Nascent MHC Class I Molecules

Abstract

Cross-presentation enables dendritic cells to present on their MHC class I molecules antigenic peptides derived from exogenous material, through a mechanism that remains partly unclear. It is particularly efficient with long peptides, which are used in cancer vaccines. We studied the mechanism of long-peptide cross-presentation using human dendritic cells and specific CTL clones against melanoma Ags gp100 and Melan-A/MART1. We found that cross-presentation of those long peptides does not depend on the proteasome or the transporter associated with Ag processing, and therefore follows a vacuolar pathway. We also observed that it makes use of newly synthesized MHC class I molecules, through peptide exchange in vesicles distinct from the endoplasmic reticulum and classical secretory pathway, in an SEC22b- and CD74-independent manner. Our results indicate a nonclassical secretion pathway followed by nascent HLA-I molecules that are used for cross-presentation of those long melanoma peptides in the vacuolar pathway. Our results may have implications for the development of vaccines based on long peptides.

The Biology and Underlying Mechanisms of Cross-Presentation of Exogenous Antigens on MHC-I Molecules

“…In this pathway the presentation of exogenous antigens does not require proteasomes or TAP (33, 83, 84, 85). Instead, the presented peptides are generated by protein catabolism within the endocytic compartments and then loaded on MHC I molecules in these vesicles (33, 86–88) (Figure 3, Step 2).…”

“…This may be particularly important for the P2C pathway because in many systems P2C XPT is inhibited by BFA, indicating that the export of some newly synthesized proteins, most likely MHC I molecules, out of the ER/golgi is needed for this pathway (76, 77). Direct transport of MHC I from the ER to endocytic compartments may also be important to vacuolar XTP because, in at least one system, it required newly synthesized MHC I molecules and was not blocked by an inhibitor of endosomal recycling, primaquine (83). …”

“…CD74 is well known to bind and target MHC II molecules to these vesicles and it turns out that it can do the same thing for MHC I molecules (125). However, the role of CD74 in XPT is controversial, with one group finding that loss of CD74 impairs XPT (126) while other groups found that XPT was normal in CD74-deficient dendritic cells and mice (33, 83). Therefore, there may be additional as yet undefined mechanisms that traffic MHC I from the ER to endocytic compartments.…”

Blood monocytes sample MelanA/MART1 antigen for long‐lasting cross‐presentation to CD8+T cells after differentiation into dendritic cells

“…The experimental protocol is shown in Figure . Two antigens, the long (26 aa) MelanA peptide, which requires processing to be presented on HLA‐A2, and the short (10aa) 26–35 epitopic peptide, which directly binds the HLA‐A2 groove, were compared at equimolar concentrations for their ability to stimulate the LT12 MelanA‐specific HLA A2‐restricted CTL clone. Monocytes were pulsed for 3 hr, extensively washed before co‐culture with T lymphocytes either immediately (Fig.…”

Perpetual complexity: predicting human CD8+ T‐cell responses to pathogenic peptides

“…Cross-presentation has been shown to be facilitated by a TAP-dependent cytosolic pathway of phagocytosis, 71 and also by TAP independent pathways. 72 Recently, Ma et al 73 demonstrated a novel vacuolar pathway of cross-presentation, in which long peptides derived from melanoma tumor associated antigens (TAAs) are cross presented by DCs to CD8 + T cells in a proteasome and TAP-independent fashion. The direct contribution of cross-presentation to immunodominance has been best demonstrated in the context of influenza.…”