Exosomes bearing HLA-DR1 molecules need dendritic cells to efficiently stimulate specific T cells

Exosomes are small vesicles (60-100 nm) secreted by various cell types upon the fusion of endosomal compartments with the plasma membrane. Exosomes from antigen-presenting cells (APC), such as B lymphocytes and dendritic cells (DC), bear MHC class II molecules. In addition, the injection of DC-derived exosomes was reported to elicit potent T cell responses in vivo. Here, we analyzed the activation of specific T cells by MHC class II-bearing exosomes in vitro. The rat mast cell line, RBL-2H3, was engineered to express human class II molecules uniformly loaded with an antigenic peptide [HLA-DR1-hemagglutinin (HA)]. These cells secreted exosomes bearing DR1 class II molecules upon stimulation by a calcium ionophore or IgE receptor cross-linking. Exosomes bearing DR1-HA(306-318) complexes activated HA/DR1-specific T cells only weakly, whereas the cross-linking of such exosomes to latex beads increased stimulation of specific T cells. By contrast, the incubation of free exosomes with DC resulted in the highly efficient stimulation of specific T cells. Thus, exosomes bearing MHC class II complexes must be taken up by professional APC for efficient T cell activation.     

The Hermansky-Pudlak syndrome (HPS) protein is part of a high molecular weight complex involved in biogenesis of early melanosomes

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder in which oculocutaneous albinism, bleeding tendency and a ceroid-lipofuscin lysosomal storage disease result from defects of multiple cytoplasmic organelles: melanosomes, platelet dense granules and lysosomes. The HPS polypeptide, a 700 amino acid protein which is unrelated to any known proteins, is likely to be involved in the biogenesis of these different organelles. Here, we show that HPS is a non-glycosylated, non-membrane protein which is a component of two distinct high molecular weight complexes. In non-melanotic cells the HPS protein is contained almost entirely in an approximately 200 kDa complex that is widely distributed throughout the cytosol. In melanotic cells the HPS protein is partitioned between this cytosolic complex and a >500 kDa complex that appears to consist of the approximately 200 kDa complex in association with membranous components. Subcellular fractionation, immunofluorescence and immunoelectron microscopy studies indicate that the membrane-associated HPS complex of melanotic cells is associated with tubulovesicular structures, small non-coated vesicles, and nascent and early-stage melanosomes. These findings suggest that the HPS complex is involved in the biogenesis of early melanosomes.     

Melanosomes and MHC class II antigen-processing compartments

Melanosomes are specialized intracellular compartments within melanocytes and retinal pigment epithelial cells that function in the synthesis, storage, and secretion of melanins, which are the major pigments made by mammals. The mechanisms that regulate the formation of melanosomes, and the pathways by which constituent proteins are targeted to them, are related to those involved in the biogenesis of major histocompatibility complex (MHC) class II antigen-processing compartments. Consequently, diseases that affect pigmentation may also affect antigen presentation to T cells. Moreover, many of the tissue-specific proteins that localize to melanosomes and participate in melanin formation double as tumor-associated antigens that are targets for T cells in patients with melanoma. Our studies on melanosome biogenesis are providing new ways of thinking about antigen-processing compartments and the mechanisms regulating presentation of tumor-associated antigens.