INTERNALIZATION OF ISCOM-BORNE ANTIGENS AND PRESENTATION UNDER MHC CLASS-I OR CLASS-II RESTRICTION

Exogenous nonreplicating antigens (Ag) incorporated into into immunost imulating complexes (iscoms) induce CTE responses under MHC class I re striction. A requirement for inducing CTL responses is that the Ag is delivered to the cytosol of antigen-presenting cells (APC), a route re stricted to endogenously produced Ag. To investigate the mechanisms by which iscoms elicit MHC class I-restricted responses, the intracellul ar distribution of influenza virus envelope proteins incorporated in i scoms (flu-iscoms) or in micelles (flu-micelles) was studied in vitro using murine peritoneal cells (PEC). Ultrathin sections of cells pulse d with biotinylated flu-iscoms or flu-micelles were analyzed by electr on microscopy after detection of the biotin label by reaction with str eptavidin-gold. PEC pulsed with flu-iscoms showed a pattern of scatter ed gold particles distributed in clear and dense vesicles as well as i n the intracellular space but not associated with. organelles. In cell s pulsed with flu-micelles, Ag was also detected in most cellular comp artments but at a considerably lower concentration. The intracellular distribution of particulate Ag in iscom or micelle form was confirmed by lysis and differential centrifugation of Ag-pulsed APC. Furthermore , P815 cells pulsed with flu-iscoms were lysed by specific immune effe cters showing that time iscom-AE; was processed and presented by class I-expressing APC. flu-iscoms were internalized about 50-fold more eff iciently than ovalbumin iscoms (ova-iscoms) suggesting that the nature of the protein and/or the presence of cellular receptors are importan t factors influencing the capacity of APC to take up iscom-borne prote ins. PEC accounted for the most active internalization of iscom-borne Ag, although splenic dendritic cells and B cells also took up flu-isco ms with remarkable efficiency. (C) 1998 Academic Press.