Invariant chain structure and its functions
The two MHC class II molecule forms a peptide binding groove between the ?1 and ?1 domains, the ? chain contributing most of the specificity. When first synthesized, this binding is prevented by a protein called the invariant chain, which is progressively cleaved off and replaced newly produced peptides in the endosomes (Chain and Playfair, 2001, p.41).
Recently, it has been demonstrated that expression of the invariant chain (Ii) facilitates the presentation of various antigens by major histocompatibility complex (class II) molecules. Ii is a type II membrane protein with the C terminal domain expressed at the luminal side of the endoplasmic reticulum membrane. The following support for two roles of Ii was found. I) Ii carries a signal sequence on its cytoplastic domain that is responsible for sorting Ii and associated MHC class II molecules from the secretory to endocytic route. ii) Ii impedes loading of peptides to the MHC class II grooves (Freisewinkel et al, 1993).
The major histocompatibility complex (MHC) encodes highly polymorphic polypeptides that serve the immune system as peptide receptors. Early in biosynthesis invariant chain (Ii) acts as a chaperone by assisting initial folding and oligomerisation of the MHCII subunits (Anderson, Miller, 1992). Ii is a non-polymorphic type II membrane protein that has important functions in the MHCII processing pathway. Ii promotes MHCII antigen processing and presentation (Stokinger et al, 1989). In the absence of Ii, MHCII ? and ? chains bind to unfolded ER polypeptides and are largely contained in aggregates, including the immunoglobulin binding protein. Ii binds to the peptide binding cleft and to some other regions of ?? heterodimers and stabilizes the conformation of MHCII molecules (Germain and Rinker, 1993).
CLIP region and its function
The exchange of CLIP for an appropriate antigenic peptide occurs in a late esdosomal compartment known as MHC class II compartment. The ? DM dimer remains intracellular and is abundant in MHC vesicles. DM molecules appear to bind and stabilize classical class II dimmers and catalyze the removal of CLIP from the antigen binding groove to allow the binding of other antigenic peptides available in the compartment (Koopman, Moorland, p.599). MHC class II molecules associate with the invariant chain (Ii) in the endoplasmic reticulum (ER) of antigen-presenting cells (APCs), with the CLIP (class-II-associated Ii peptide) region of Ii occupying the peptide-binding groove of the class II a–b heterodimer. This association serves both to prevent peptide binding to the class II molecules within the ER and to direct class II molecules into the endosomal/lysosomal pathway, where antigen processing and assembly of peptide-loaded MHC class II molecules occur (Watts, 1997).
The endosomal/lysosomal system is rich in proteases, particularly cysteine proteases, and these enzymes are crucial both for the generation of peptides from antigenic proteins, and for the removal of Ii from class II molecules. CLIP is the final fragment of Ii and is generated by the protease cathepsin S (Riese, 1996). CLIP is not spontaneously released from class II molecules in most cases, and exchange of this fragment for antigenic peptides is mediated by the accessory protein DM [HLA-DM in humans, H2-DM in the mouse] (Morris et al, 1994).
DM was initially identified because cells that lacked this protein expressed high levels of CLIP–MHC-class-II complexes at their cell surface, and as a consequence had an altered reactivity with certain class II reactive monoclonal antibodies (Mellins et al, 1990). DM is mainly localized in the vesicles of the endosomal/lysosomal system of APCs, where it interacts with newly synthesized class II molecules, and catalyzes the release of CLIP from the peptide binding groove (Sloan et al, 1995).
Antigen presentation of MHC class II molecules
Constitutive expression of major histocompatibility complex (MHC) class II molecules is confined to professional antigen-presenting cells (APCs) of the immune system. Activated T cells from many species synthesize and express MHC class II molecules at their cell surface (Otto et al, 2006). Durable adaptive immunity is dependent upon CD4 T-cell recognition of MHC class II molecules that display peptides from exogenous and endogenous antigens. Endogenously expressed cytosolic and nuclear antigens access MHC class II by way of several intracellular autophagic routes. These pathways include macroautophagy, microautophagy and chaperone-mediated autophagy. Macroautophagy can deliver antigens into autophagosomes for processing by acidic
proteases before MHC class II presentation. However, other endogenous antigens are processed by cytoplasmic proteases, yielding fragments that translocate via chaperonemediated autophagy into the endosomal network to intersect MHC class II (.Strawbridge and Blum, 2007).
Class II MHC complexes with antigenic peptides are generated by fundamentally different intra cellular mechanism, since the antigen presenting cells which interact with T helper cells need to sample the antigen from both intracellular and extracelluar compartments. A trans-Golgi vesicle containing class II has to intersect with a late endosome containing exogenous protein antigen taken into the cell by an endocytic mechanism (Roitt, Delves, p.95). The concept of antigen processing and presentation of antigen derived peptides complexed with major MHC on antigen presenting cells, such as macro phages and dendritic cells to specific T cell receptors has also been confirmed by the elucidation of the crystal structure of MHC class II molecules. CD4+ T cell bind class II MHC molecules on antigen presenting cells and are principally concerned with handling exogenous antigens (Bousquet, 1999, p.20).
Major histocompatibility complex (MHC) class II molecules present products of lysosomal proteolysis to CD4+ T cells. Although extracellular antigen uptake is considered to be the main source of MHC class II ligands, a few intracellular antigens have been described to gain access to MHC class II loading after macroautophagy. However, the general relevance and efficacy of this pathway is unknown. Schmid et al (2007) demonstrated constitutive autophagosome formation in MHC class II-positive cells, including dendritic, B, and epithelial cells. The autophagosomes continuously fuse with multivesicular MHC class II-loading compartments. The study suggested that macroautophagy constitutively and efficiently delivers cytosolic proteins for MHC class II presentation and can be harnessed for improved helper T cell stimulation (Schmid et al, 2007).
T-cell antigen recognition results in cellular activation, cell surface expression of Class II major histocompatibility complex glycoproteins (MHCII) on T cells, and a limited phase of T-cell-mediated antigen presentation in association with the induction of anergy and apoptosis of responder T cells. Unlike professional APC, naive T cells express no MHCII unless activated with specific antigen. The frequency of T cells bearing MHCII increases upon immunization or during an immune response (Patel and Mannie, 2002).
Major histocompatibility complex (MHC) class II proteins are heterodimers of ? and ?chains that bind peptide antigens in their peptide binding groove for presentation to helper T cells. Genes of the MHC are highly polymorphic, resulting in a large number of allelic variants of molecules in the population. These polymorphic residues are located in pockets of the peptide binding site, and critically influence the specificity of peptide binding by imparting selectivity for certain side chains at defined peptide positions (Rosloniec et al, 2006).
Antigen presentation via MHC class II is a complex process. The early stage of this process involves the induction of the class II transactivator (CIITA), which is the ‘master regulator’ of the MHC class II expression. CIITA is known to respond to different pro-inflammatory stimuli and to induce the expression of the classical MHC class II molecules (RT1-B, RT1-D) as well as the accessory molecule invariant chain (CD74, also known as li-chain) (for review see (Landmann et al, 2004). After the induction, the assembly of class II molecules (RT1-B? and ?, RT1-D? and ?) with the CD74, a type II membrane protein, occurs followed by the stepwise processing of the CD74 into the CLIP (class II associated li-peptide) starting from the C-terminus. The invariant chain directs the MHC class II complex to the late endocytic compartment and prevents the premature loading of the antigen-binding groove. There are many different proteases involved in the processing of CD74. The most effective proteases involved in the last step of this process are cathepsins S and L. These enzymes release the CLIP from the lip10 (leupeptin induced polypeptide). Depending on the cell type, cathepsin L and cathepsin S are involved in this step-wise degradation of the invariant chain in thymic epithelial cells and in B-cells, macrophages and dendritic cells respectively (Nakagawa et al, 1998).
Although exogenous antigens can readily access MHC class II molecules by way of endocytic sorting, it is less evident how endogenous nuclear and cytoplasmic antigens are processed and reach MHC class II molecules. Sequence analysis of MHC class II ligands and studies of T-cell responses to viral antigens indicate that MHC class II molecules access endogenous antigen from the cytoplasm and from nuclear and mitochondrial compartments (Zhou and Blum, 2004). However, these antigens are generally not detected within the endocytic pathway which raises the questions as to how these endogenous peptides access MHC class II. These endogenous antigens are presumed to employ certain routes to access MHC class II, namely, antigen release and re-internalization via the endosomal network (where the antigen is no longer technically endogenous), antigen sequestration from the cytoplasm for processing and presentation, and cytoplasmic antigen processing to yield peptides for transport to MHC class II (Strawbridge and Blum, 2007).
Antigen capture and presentation onto MHC class II molecules by B lymphocytes is mediated by their surface antigen receptor, i.e., the B-cell receptor (BCR). The BCR must therefore coordinate the transport of MHC class II- and antigen containing vesicles for them to converge and ensure efficient processing (Vascotto et al, 2007). Mature resting B lymphocytes capture antigen (Ag) via their specific B-cell receptor (BCR), which comprises a membrane immunoglobulin (mIg) coupled to a signalling module formed by the Iga–Igb dimer (Niiro & Clark, 2002). In addition to Ag internalization, BCR stimulation triggers a complex cascade of signaling events that ultimately leads to the activation of B lymphocytes, which can then initiate the development of germinal centres (GCs) for production of high affinity antibodies. To complete GC formation, activated lymphocytes must process and present internalized Ag on MHC class II process referred to as T cell–B cell cooperation (Mitchison, 2004).
Coupling phagocytosis with signaling pathways downstream of Toll like receptors (TLRs) significantly impacts the presentation of phagocytosed antigens by MHC class II molecules (MHC-II). The intersection of endocytic pathways with MHC-containing subcellular compartments determines crucially the course and character of antigen presentation. Phagocytosis is a particularly effective route for delivering antigens into MHC-II-rich compartments (Jutras & Desjardins, 2005). Newly synthesized MHC-II associate with invariant chains (Ii) in the endoplasmic reticulum (ER) and, through a targeting sequence within the Ii cytoplasmic domain, are directed to the endocytic pathway, where MHC-II can encounter and bind to peptides derived from internalized antigens (Watts, 2004).
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