Background: In immune-mediated diseases of the central nervous system, astrocytes exposed to interferon-gamma (IFN-gamma) can express major histocompatibility complex (MHC) class II molecules and antigens on their surface. MHC class II molecules are thought to be delivered to the cell surface by membrane-bound vesicles. However, the characteristics and dynamics of this vesicular traffic are unclear, particularly in reactive astrocytes, which overexpress intermediate filament (IF) proteins that may affect trafficking. The aim of this study was to determine the mobility of MHC class II vesicles in wild-type (WT) astrocytes and in astrocytes devoid of IFs. Methods: The identity of MHC class II compartments in WT and IF-deficient astrocytes 48 h after IFN-gamma activation was determined immunocytochemically by using confocal microscopy. Time-lapse confocal imaging and Alexa Fluor(546)-dextran labeling of late endosomes/lysosomes in IFN-gamma treated cells was used to characterize the motion of MHC class II vesicles. The mobility of vesicles was analyzed using ParticleTR software. Results: Confocal imaging of primary cultures of WT and IF-deficient astrocytes revealed IFN-gamma induced MHC class II expression in late endosomes/lysosomes, which were specifically labeled with Alexa Fluor(546)-conjugated dextran. Live imaging revealed faster movement of dextran-positive vesicles in IFN-gamma-treated than in untreated astrocytes. Vesicle mobility was lower in IFN-gamma- treated IF-deficient astrocytes than in WT astrocytes. Thus, the IFN-gamma-induced increase in the mobility of MHC class II compartments is IF-dependent. Conclusions: Since reactivity of astrocytes is a hallmark of many CNS pathologies, it is likely that the up-regulation of IFs under such conditions allows a faster and therefore a more efficient delivery of MHC class II molecules to the cell surface. In vivo, such regulatory mechanisms may enable antigen-presenting reactive astrocytes to respond rapidly and in a controlled manner to CNS inflammation.
