Traveling backward

Anew study of antigen-presenting cells has revealed a previously unseen pathway for recovering proteins from lysosomes, normally a dead-end for proteins. Amy Chow, Ira Mellman, and colleagues (Yale University, New Haven, CT) have visualized a selective retrograde transport pathway to the plasma membrane.

The pathway, which has also been seen by Marianne Boes, Hidde Ploegh (Harvard Medical School, Boston, MA), and colleagues, starts at lysosomes, which in most cells are highly degradative. But in immature dendritic cells (DCs), lysosomes house internalized antigens and MHC class II molecules until the cells mature. After maturation, MHC II molecules reside on the plasma membrane, where they activate T cells. Previous microscopy of maturing DCs has given glimpses of tubulating lysosomes and nonlysosomal vesicles that contain MHC II molecules, hinting at the presence...