When activated by proinflammatory stimuli, conventional DCs downregulate MARCH-1, terminating MHCII protein turnover; in contrast, activated plasmacytoid DCs maintain this process (14). by disease-associated structural polymorphism. Differences in MHCII turnover were observed between NOD colonies with high and low T1D incidence, but fast turnover was dispensable Rabbit Polyclonal to CKLF3 for autoimmunity. Moreover, NOD mice with gene knockouts of peptide loading cofactors do not develop T1D. Thus, fast turnover does not appear pathogenic, and conventional antigen presentation is critical for autoimmunity in NOD mice. However, shared D-erythro-Sphingosine environmental factors may underpin colony differences in MHCII protein turnover, immune regulation, and pathogenesis. Keywords:autoimmune pathogenesis, type 1 diabetes mellitus, major histocompatibility complex class II, antigen-presenting cell, protein turnover, mass spectrometry == Introduction == Major histocompatibility complex class II (MHCII) molecules present peptides at cell surfaces for recognition by D-erythro-Sphingosine CD4+T lymphocytes. They shape a functional, self-tolerant T-cell repertoireviapositive and negative thymocyte selection and support peripheral CD4+T-cell homeostasis (1). Presentation of pathogen-derived peptides activates specific CD4 T cells, initiating adaptive immunity (2). Extensive polymorphism in the peptide-binding groove, shaping the presented peptide repertoire, is a key characteristic of MHCII proteins (3). These polymorphisms also control susceptibility to autoimmune diseases. The regulation of MHCII protein expression is critical to these functions. Here, we discuss MHCII protein turnover, a critical but poorly understood determinant of steady-state expression. We describe recent methodological advances in measuring MHCII protein turnoverin vivo, which enabled us to examine the postulated role of MHCII protein turnover in autoimmune pathogenesis in a mouse model. == Functional Significance of MHCII Protein Turnover == Major histocompatibility complex class II proteins are expressed constitutively by antigen-presenting cells [APCs: B cells, dendritic cells (DCs), macrophages, thymic epithelial cells] and inducibly by other D-erythro-Sphingosine cell types. MHCII gene transcription is controlled by the master regulator, class II transactivator (CIITA) (4). Post-translational regulation fine-tunes MHCII expression levels and determines antigen persistence. For example, in DCs, CIITA, and MHCII gene transcription are shut down following activation by bacterial lipopolysaccharide, yet MHCII surface levels rise, because MHCII protein degradation is also shut down (5). As a result, long-lived MHCII/peptide complexes are able to survive during DC migration to lymph nodes (6), where they provide persistent stimuli for CD4+T-cell priming (7). In other APC types, cytokine regulation of MHCII turnover also affects expression (8,9), and stimuli can increase turnover, rather than shutting it down. == Determinants of MHCII Protein Turnover == Major histocompatibility complex class II degradation in non-activated DCs begins with internalization from the cell surface, which is triggered by ubiquitination of the cytoplasmic tail of the MHCII chain (10) by the ubiquitin E3 ligase, membrane-associated RING-CH 1 (MARCH-1) (11,12). The chain is also ubiquitinated to a small extent (13). When activated by proinflammatory stimuli, conventional DCs downregulate MARCH-1, terminating MHCII protein turnover; in contrast, activated plasmacytoid DCs maintain this process (14). MARCH-1 also mediates cytokine regulation of MHCII turnover in monocytes and B cells (8,9,15). Other MARCH family members may be involved, as well (16). The internalized MHCII molecules are targeted for lysosomal degradation by unknown proteases. Studies using inhibitors suggest a role for cysteine proteases, although the effects are small (17) and MHCII molecules resist proteolysis by many cysteine cathepsinsin vitro(18). The serine protease, Cathepsin G, cleaves MHCII molecules at a specific membrane-proximal sitein vitro, but this does not detectably contribute to MHCII degradation in APCs (18). Major histocompatibility complex class II protein degradation is also influenced by the preceding assembly and maturation steps, which are reviewed elsewhere in this series (Figure1) (19). Briefly, heterodimers assemble with invariant chain (Ii) in the endoplasmic reticulum (ER) and travel to endosomes. There, Ii is cleaved, leaving class II-associated Ii peptides (CLIP) in the peptide-binding groove, which must be released to enable loading with endosomal peptides. A co-factor, DM (HLA-DM in humans, H2-DM/H2-M.
