Traffic of major histocompatibility complex class II molecules in rabbit lacrimal gland acinar cells

It has been suggested that lacrimal gland acinar cells, which have been induced to express major histocompatibility complex class II (MHC II) molecules, might initiate local autoimmunity by using mechanisms similar to those operating in the specialized antigen-presenting cells to process and present autoantigens. Surface-labeling experiments indicate that constituents of the acinar cell plasma membrane participate in a rapid recycling traffic. The authors have surveyed the subcellular distribution of MHC II molecules and have evaluated their participation in the traffic between plasma membranes and intracellular compartments.

Acinar cells were isolated from rabbit lacrimal glands and maintained for two nights in a serum-free, hormone-supplemented culture medium containing 10 microM carbachol. MHC II molecules were detected with a monoclonal antibody (MAB 2C4), biotinylated goat-antimouse IgG (BGAM), and avidin-ferritin (AvFe) or streptavidin-gold (SAvAu) conjugates.

Postembedding labeling with MAB 2C4, BGAM, and AvFe revealed MHC II molecules at the surface membranes, in cytoplasmic 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. Postembedding labeling with MAB 2C4, BGAM, and SAvAu revealed additional intracellular 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 labeling appeared less than obtained with postembedding labeling with MAB 2C4, BGAM, and AvFe. Preembedding labeling of cells that had been incubated overnight with MAB 2C4 indicated that the cells continued to express MHC II molecules at their surface membranes, and rapid internalization of label upon warming to 37 degrees C confirmed that MHC II molecule traffic continued in the presence of MAB 2C4. Postembedding labeling with MAB 2C4, BGAM, and SAvAu indicated the continued presence of a large intracellular pool of MHC II molecules.

MHC II molecules in lacrimal acinar cells are present in large intracellular and 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 reasonable to propose that the traffic of MHC II molecules to plasma membranes provides a mechanism by which acinar cells display intracellularly generated autoantigens to potentially reactive helper T lymphocytes.