Role of Lysosomal Proteinases in Regulation of MHC Class II Presentation to CD4 T Cells

Major histocompatibility complex (MHC) class II molecules sample peptides generated in the endocytic compartment of antigen-presenting cells (APCs) in the process of degradation of foreign and self-protein antigens and present these peptides to CD4 T cells. The majority of peptides presented by MHC class II molecules are derived from self-proteins, and these complexes are involved in shaping CD4 T cell repertoire in the thymus and the periphery.

MHC class II molecules assemble in the endoplasmic reticulum with the assistance of invariant chain (Ii). Binding of Ii prevents ‘‘premature’’ loading of peptides onto MHC class II molecules early in transport, preserves the peptide-binding capacity, and facilitates sorting of MHC class II molecules into endosomes. Upon arrival into endosomes, Ii is degraded by lysosomal proteinases, leaving a small Ii fragment, termed CLIP, bound in the MHC class II peptide–binding groove. These proteinases are also involved in the generation of peptides from various foreign and self-antigens entering the endocytic pathway. An MHC class II–like accessory molecule DM catalyzes the dissociation of CLIP from and loading of antigenic peptides into the peptide-binding groove. We previously used tandem mass spectrometry to characterize repertoires of peptides displayed by MHC class II molecules and analyzed the role of Ii and DM in shaping these repertoires.

Our recent studies showed that two lysosomal cysteine proteinases—cathepsins L and S—are critically involved in MHC class II processing and presentation: i.e., they mediate late stages of invariant chain (Ii) degradation. A surprising finding made in the course of these studies was differential functional expression of these two enzymes in distinct types of APCs in vivo. Thymic cortical epithelial cells involved in positive selection of T cells utilize cathepsin L, while bone marrow–derived cells (B cells, dendritic cells) that mediate deletion or negative selection of autoreactive T cells during thymic development utilize cathepsin S. More recently, we have found that in B lymphocytes cathepsin S is also involved in turnover of interferon-g–inducible thioreductase (another important player in MHC class II antigen processing) and cathepsin L, in addition to cathepsin S's role in Ii degradation. These studies suggest that the proteolytic environment in MHC class II–positive endosomal compartments in different APC types is distinct, and therefore repertoires of peptides displayed by MHC class II molecules may be distinct. This notion was supported by a direct comparison of MHC class II–bound peptide repertoires in an immortalized APC line isolated from the knockout animals reconstituted with cathepsin S or L. The tandem mass-spectrometric analysis of MHC class II–bound peptides revealed quantitative and qualitative effects of cathepsin S versus L expression on distinct subsets of peptides displayed by MHC class II molecules in these cells. Furthermore, our recent data implicate cathepsin L in generation of peptides involved in positive selection of CD4 T cells in thymic cortical epithelial cells.