Follicular dendritic cells carry MHC class II-expressing microvesicles at their surface

Follicular dendritic cells (FDCs) present in lymphoid follicles play a crit ical 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 a lso 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 w as not clear. Here we show by cryoimmunogold electron microscopy that MHC c lass II molecules are not present at the plasma membrane of FDCs. In contra st, microvesicles attached to the FDC surface contain MHC class II and othe r surface proteins not expressed by FDCs themselves. The size and marker pr ofiles of these microvesicles resemble exosomes, Exosomes, which are secret ed internal vesicles from multivesicular endosomes, have been shown earlier to stimulate proliferation of specific T lymphocytes in vitro, but their t arget in vivo remained a matter of speculation. We demonstrate here that is olated exosomes in vitro bind specifically to FDCs and not to other cell ty pes, suggesting that FDCs might be a physiological target for exosomes.