ROLE OF ANNEXINS IN ENDOCYTOSIS OF ANTIGENS IN IMMATURE HUMAN DENDRITIC CELLS

We have evaluated the uptake of a soluble protein antigen, denitrophen ylated human serum albumin (DNP-HSA), and two different intracellular bacteria; Chlamydia trachomatis serovar L2 and Mycobacterium tuberculo sis strain H37Ra, by immature human dendritic cells. These were genera ted by culturing progenitor cells from blood in the presence of cytoki nes (granulocyte-macrophage colony-stimulating factor and interleukin- 4). Dendritic cells play a crucial part in antigen presentation for th e induction of T-cell-dependent immune responses in various tissues. R ecently, macropinocytic and phagocytic activity has been shown for imm ature dendritic cells of mouse, rat and human origin. In the present s tudy, macropinocytosis characterized the uptake of the soluble protein -antigen DNP-HSA, whereas the C. trachomatis were ingested via recepto r-mediated endocytosis in coated pits, and opsonized M. tuberculosis v ia phagocytosis. To follow the intracellular routes of the antigens, t heir positions were compared with the localization of annexins, a fami ly of Ca2+-and phospholipid-binding proteins, involved in membrane fus ion, aggregation and transport of different vesicles. To elucidate fur ther the intracellular pathway of the antigens, two other proteins, ly sosome-associated membrane protein-1 (LAMP-1) and cathepsin D, were la belled. They are known to colocalize with major histocompatibility com plex class II compartments in the immature dendritic cells. We observe d a distinct translocation of annexin V to DNP-HSA containing endosome s, and annexin III to vesicles with C. trachomatis. Furthermore, annex in III, IV and V redistributed to phagosomes with M. tuberculosis. Bot h LAMP-1 and cathepsin D colocalized with DNP-HSA endosomes, and with phagosomes with M. tuberculosis. Thus, immature human dendritic cells have the capacity to phagocytose. Moreover, the handling of these anti gens by dendritic cells may represent three distinct intracellular pat hways, albeit some properties and compartments are shared.