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Rapid constitutive generation of a specific peptide-MHC class II complex from intact exogenous protein in immature
Lisa Colledge1, Clare L Bennett, Philip A Reay
1Institute for Stem Cell Research, Institute for Cell, Animal and Population Biology, University of Edinburgh, Edinburgh, GB.
MHC class II molecules present peptides, derived largely from exogenous antigens, to CD4+ T cells. Complex-generation occurs mainly in the specialized late endosomal MHC class II-rich compartment (MIIC) vesicles of antigen-presenting cells (APC). Dendritic cells (DC) have been reported to store intact antigen in MIIC until the receipt of an activation signal, when they process it into peptide-MHC class II complexes. However, constitutive migration of DC from the periphery to secondary lymphoid organs has been observed, and antigen presentation by nonactivated DC is proposed to play a role in the induction of tolerance to peripheral antigens. Thus, constitutive peptide-MHC class II complex generation must also occur in DC in immunologically quiescent situations. We have used a monoclonal antibody that detects a specific peptide-MHC class II complex to directly demonstrate constitutive complex generation in immature murine DC. Protein-derived peptide-MHC class II complexes were detected by flow cytometry at the DC surface within 1 h of antigen exposure in the absence of an exogenous activation signal, and could be detected by confocal microscopy in the MIIC within 5 min of antigen exposure. This processing activity was endotoxin independent. These data provide evidence for constitutive peptide-MHC class II complex generation in immature DC, and thus support a role for this activity in the induction of peripheral tolerance.
MHC class II molecules present peptides, derived largely from exogenous antigens, to CD4+ T cells. Complex-generation occurs mainly in the specialized late endosomal MHC class II-rich compartment (MIIC) vesicles of antigen-presenting cells (APC). Dendritic cells (DC) have been reported to store intact antigen in MIIC until the receipt of an activation signal, when they process it into peptide-MHC class II complexes. However, constitutive migration of DC from the periphery to secondary lymphoid organs has been observed, and antigen presentation by nonactivated DC is proposed to play a role in the induction of tolerance to peripheral antigens. Thus, constitutive peptide-MHC class II complex generation must also occur in DC in immunologically quiescent situations. We have used a monoclonal antibody that detects a specific peptide-MHC class II complex to directly demonstrate constitutive complex generation in immature murine DC. Protein-derived peptide-MHC class II complexes were detected by flow cytometry at the DC surface within 1 h of antigen exposure in the absence of an exogenous activation signal, and could be detected by confocal microscopy in the MIIC within 5 min of antigen exposure. This processing activity was endotoxin independent. These data provide evidence for constitutive peptide-MHC class II complex generation in immature DC, and thus support a role for this activity in the induction of peripheral tolerance.