TRAFFIC OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II MOLECULES IN RABBIT LACRIMAL GLAND ACINAR-CELLS

Purpose. It has been suggested that lacrimal gland acinar cells, which have been induced to express major histocompatibility complex class I I (MHC II) molecules, might initiate local autoimmunity by using mecha nisms similar to those operating in the specialized antigen-presenting cells to process and present autoantigens. Surface-labeling experimen ts indicate that constituents of the acinar cell plasma membrane parti cipate in a rapid recycling traffic. The authors have surveyed the sub cellular distribution of MHC II molecules and have evaluated their par ticipation in the traffic between plasma membranes and intracellular c ompartments. Methods. Acinar cells were isolated from rabbit lacrimal glands and maintained for two nights in a serum-free, hormone-suppleme nted culture medium containing 10 mu M carbachol. MHC II molecules wer e detected with a monoclonal antibody (MAB 2C4), biotinylated goat-ant imouse IgG (BGAM), and avidin-ferritin (AvFe) or streptavidin-gold (SA vAu) conjugates. Results. Postembedding labeling with MAB 2C4, BGAM, a nd AvFe revealed MHC II molecules at the surface membranes, in cytopla smic vesicles, and in secretory vesicles. When cells were chilled to 4 degrees C and subjected to preembedding labeling with MAB 2C4, BGAM, and AvFe, surface MHC II molecules were specifically labeled. Labeled complexes were rapidly internalized upon warming to 37 degrees C. Post embedding labeling with MAB 2C4, BGAM, and SAvAu revealed additional i ntracellular MHC II molecules, with a distribution overlapping that of the MHC II molecules labeled during the preembedding procedure. When cells were cultured overnight in the presence of MAB 2C4 and subjected to postembedding labeling with BGAM and AvFe, label was detectable in small vesicles and in secretory vesicles. However, the extent of labe ling appeared less than obtained with postembedding labeling with MAB 2C4, BGAM, and AvFe. Preembedding labeling of cells that had been incu bated overnight with MAB 2C4 indicated that the cells continued to exp ress MHC II molecules at their surface membranes, and rapid internaliz ation of label upon warming to 37 degrees C confirmed that MHC II mole cule traffic continued in the presence of MAB 2C4. Postembedding label ing with MAB 2C4, BGAM, and SAvAu indicated the continued presence of a large intracellular pool of MHC II molecules. Conclusions. MHC II mo lecules in lacrimal acinar cells are present in large intracellular an d small surface pools. They move rapidly between these two pools, but further work will be required to determine whether the MHC II molecule traffic represents recycling or turnover and whether recycling pools and sequestered pools coexist. The presently available data make it re asonable to propose that the traffic of MHC II molecules to plasma mem branes provides a mechanism by which acinar cells display intracellula rly generated autoantigens to potentially reactive helper T lymphocyte s.