Follicular dendritic cells (FDCs) present in lymphoid follicles play a critical role in germinal center reactions. They trap native Ags in the form of immune complexes providing a source for continuous stimulation of specific B lymphocytes. FDCs have been reported to express MHC class II molecules, suggesting an additional role in the presentation of not only native, but also processed Ag in the form of peptide-loaded MHC class II. Adoptive bone marrow transfer experiments have shown that MHC class II molecules are only passively acquired. Up to now the origin of these MHC class II molecules was not clear. Here we show by cryoimmunogold electron microscopy that MHC class II molecules are not present at the plasma membrane of FDCs. In contrast, microvesicles attached to the FDC surface contain MHC class II and other surface proteins not expressed by FDCs themselves. The size and marker profiles of these microvesicles resemble exosomes. Exosomes, which are secreted internal vesicles from multivesicular endosomes, have been shown earlier to stimulate proliferation of specific T lymphocytes in vitro, but their target in vivo remained a matter of speculation. We demonstrate here that isolated exosomes in vitro bind specifically to FDCs and not to other cell types, suggesting that FDCs might be a physiological target for exosomes.

To investigate the participation of microvascular pericytes in the process of capillary sprouting, we examined whole-mount preparations of the rat mesentery by use of a double immunofluorescence approach. Angiogenesis was induced by intraperitoneal injections of either the mast cell-degranulating substance compound 48/80 or tumor cell-conditioned medium. Capillary sprouts were visualized by staining with rhodamine-conjugated phalloidin and pericytes were simultaneously stained by an antibody to the intermediate filament protein desmin. Developing pericytes were negative for the smooth-muscle isoform of alpha-actin, but were clearly reactive for desmin. Pericytes appear to be involved in the earliest stages of capillary sprouting. Pericytes were regularly found lying at and in front of the advancing tips of endothelial sprouts. At many sites pericytes were seen to bridge the gap between the leading edges of opposing endothelial sprouts, which were apparently preparing to merge, suggesting that pericytic processes may serve as guiding structures aiding outgrowth of endothelial cells.

Exosomes are small membrane vesicles that are secreted by a multitude of cell types as a consequence of fusion of multivesicular late endosomes/lysosomes with the plasma membrane. Depending on their origin, exosomes can play roles in different physiological processes. Maturing reticulocytes externalize obsolete membrane proteins such as the transferrin receptor by means of exosomes, whereas activated platelets release exosomes whose function is not yet known. Exosomes are also secreted by cytotoxic T cells, and these might ensure specific and efficient targeting of cytolytic substances to target cells. Antigen presenting cells, such as B lymphocytes and dendritic cells, secrete MHC class-I- and class-II-carrying exosomes that stimulate T cell proliferation in vitro. In addition, dendritic-cell-derived exosomes, when used as a cell-free vaccine, can eradicate established murine tumors. Although the precise physiological target(s) and functions of exosomes remain largely to be resolved, follicular dendritic cells (accessory cells in the germinal centers of secondary lymphoid organs) have recently been shown to bind B-lymphocyte-derived exosomes at their cell surface, which supports the notion that exosomes play an immunoregulatory role. Finally, since exosomes are derived from multivesicular bodies, their molecular composition might provide clues to the mechanism of protein and lipid sorting in endosomes.

The cytoplasmic tail of the human 46 kDa mannose 6-phosphate receptor (MPR 46) is necessary for rapid internalization of the receptor and sufficient to mediate internalization of a resident plasma membrane protein. To localize the internalization sequences within the 67 amino acids of the cytoplasmic tail, the tail was progressively shortened from its C-terminus, internal deletions of between four and eight amino acids were introduced into the tail, and individual residues were substituted by alanine, glycine or serine. Three sequences were identified that contribute to the internalization of MPR 46. The first is located within the 23 juxtamembrane cytoplasmic residues of the tail. It contains four essential residues within a heptapeptide and does not resemble known internalization signals. The second sequence contains as a critical residue Tyr-45. The third region is located within the C-terminal seven residues and contains a di-leucine pair as essential residues. The first and third sequences were shown to function as autonomous internalization sequences. Substitution of critically important residues within a single internalization sequence was tolerated, with no or only a moderate decrease in the internalization rate. When essential residues from two or all three internalization sequences were substituted, however, the internalization rate was decreased by more than 60% and 90% respectively. This indicates that the autonomous internalization signals in the cytoplasmic tail of MPR 46 function in an additive manner, but are partly redundant.