In the development of multiple sclerosis (MS), (re)activation of infiltrating T cells by myelin-derived Ags is considered to be a crucial step. Previously, αB-crystallin has been shown to be an important myelin Ag to human T cells. Since αB-crystallin is an intracellular heat shock protein, the question arises at what stage, if any, during lesional development in MS this Ag becomes available for CD4+ T cells. In 3 of 10 active MS lesions, αB-crystallin could be detected inside phagocytic vesicles of perivascular macrophages, colocalizing with myelin basic protein and myelin oligodendrocyte glycoprotein (MOG). Although the detectability of MOG in phagosomes is considered as a marker for very recent demyelination, MOG was detected in more macrophages and in more lesions than αB-crystallin. The disappearance of αB-crystallin from macrophages even before MOG was confirmed by in vitro studies; within 6 h after myelin-uptake αB-crystallin disappears from the phagosomes, αB-Crystallin-containing macrophages colocalized with infiltrating T cells and they were characterized by expression of MHC class II, CD40, and CD80. To examine functional presentation of myelin Ags to T cells, purified macrophages were pulsed in vitro with whole myelin membranes. These macrophages activated both myelin- primed and αB-crystallin-primed T cells in terms of proliferation and IFN-γ secretion. In addition, αB-crystallin-pulsed macrophages activated myelin- primed T cells to the same extent as myelin-pulsed macrophages, whereas myelin basic protein-pulsed macrophages triggered no response at all. These data indicate that, in active MS lesions, αB-crystallin is available for functional presentation to T cells early during inflammatory demyelination. Chemicals/CAS: Antigens, CD40; Antigens, CD80; Autoantigens; Cathepsins, EC 3.4.-; Crystallins; Histocompatibility Antigens Class II; Myelin Proteins