Destruction of li by proteolysis is required for MHC class II molecules to bind antigenic peptides, and for transport of the resulting complexes to the cell surface. The cysteine protease cathepsin S is highly expressed in spleen, lymphocytes, monocytes, and other class II-positive cells, and is inducible with interferon-gamma. Specific inhibition of cathepsin S in B lymphoblastoid cells prevented complete proteolysis of li, resulting in accumulation of a class II-associated 13 kDa li fragment in vivo. Consequently, the formation of SDS-stable complexes was markedly reduced. Purified cathepsin S, but not cathepsin B, H, or D, specifically digested li from alpha beta li trimers, generating alpha beta-CLIP complexes capable of binding exogenously added peptide in vitro. Thus, cathepsin S is essential in B cells for effective li proteolysis necessary to render class II molecules competent for binding peptides.

We have generated mice lacking H2-M complexes, critical facilitators of peptide loading onto major histo-compatibility complex class II molecules. Ab molecules in these mice matured into stable complexes and were efficiently expressed at the cell surface. Most carried a single peptide derived from the class II-associated invariant chain; the diverse array of peptides normally displayed by class II molecules was absent. Cells from mutant mice presented both whole proteins and short peptides very poorly. Surprisingly, positive selection of CD4+ T cells was quite efficient, yielding a large and broad repertoire. Peripheral T cells reacted strongly to splenocytes from syngeneic wild-type mice, no doubt reflecting the unique peptide complement carried by class II molecules in mutant animals.

The antigen-specific receptors of T lymphocytes rely on products of the major histocompatibility complex (MHC) to recognize and engage antigen. MHC molecules display antigen on the cell surface in the form of small peptides, generated intracellularly by fragmentation of the intact protein antigen. They acquire these peptides at distinct intracellular locations: In the endoplasmic reticulum (ER), class I molecules bind peptides derived from cytosolic proteins, whereas class II molecules acquire their peptide cargo in an endocytic compartment. Sequestration of class II molecules from the constitutive secretory pathway is mediated by their interaction with an additional polypeptide, the invariant chain (Ii). The Ii contains sorting signals in its cytoplasmic tail that target class II molecules to the endocytic pathway where they encounter peptides generated from protein antigens that have also accessed this route.