1 MHC II expression is tightly controlled at several levels. Transcriptional regulation confines constitutive MHC II expression to professional

APCs and thymic epithelial cells and allows up-regulation on other cell types after exposure to inflammatory cytokines.2 Post-translational events also regulate cellular localization of MHC II, thereby influencing MHC II half-life. In immature dendritic cells (DCs), MHC II molecules are efficiently targeted to lysosomes by the clathrin adaptor protein complex 2 (AP-2) and/or by the E3 ubiquitin ligase, membrane-associated RING-CH protein 1 (MARCH-I) and are degraded within a few hours; surface expression remains relatively low. DC activation stimulates a transient burst of MHC II synthesis, this website turn-off of MARCH-I and deposition of peptide/MHC II complexes at the plasma membrane, where they are long-lived (> 100 hr). Data from B-cell lines, melanoma lines and human monocytes see more implicate similar pathways in control of MHC II levels in these cell types.3–6 Expression levels of MHC II are also influenced by interaction with accessory molecules that regulate MHC II peptide loading: MHC II-associated invariant chain (Ii) and HLA-DM

(DM). Nascent MHC II molecules assemble in the endoplasmic reticulum with Ii; in cells from animals lacking Ii, surface levels of most MHC II alleles are substantially reduced because of inefficient assembly and egress.7–9 After assembly, MHC II/Ii complexes travel to endocytic compartments, directed by sequences in the Ii cytoplasmic tail; there, Ii is sequentially degraded by cathepsins.10 Groove-bound Ii remnants, the class TCL II-associated Ii peptides (CLIPs), are exchanged for antigenic peptides with the assistance of the peptide exchange factor DM.11 Chaperoning effects of DM provide further regulation of MHC II preservation/degradation1,2 (C. Rinderknecht and S. Roh, unpublished data). DM editing of peptides in favour of strong binders is also a factor, as the quality of peptide cargo is thought to influence

MHC II half-life.12–14 Despite active regulation of expression at the level of proteolysis, MHC II molecules must be relatively resistant to proteolytic attack. MHC II molecules traverse acidic, proteolytic endosomal compartments, where peptide loading occurs, for several hours en route to the plasma membrane.15–17 Moreover, in inflammatory settings, myeloid and stromal cells may release proteases into the extracellular fluid, yet MHC II molecules are abundantly expressed in such settings and must remain functional to allow local antigen presentation. The paradox of regulated turnover in the face of inherent proteolytic resistance is only beginning to be addressed. Only limited information exists regarding the proteases involved in constitutive or regulated MHC II turnover, or the factors that render MHC II molecules at least partially resistant to proteolytic attack.