- Antigen Processing and
Presentation Both CD4+ and CD8+ T cells can recognize antigen ONLY when it has been processed and presented on the cell membrane of an Antigen Presenting Cell (APC) or Target Cell (respectively) in association with self MHC. This phenonmenon has been termed: Restriction to Self MHC. In this terminology, CD4+ T cells are Class II MHC restricted while CD8+ T cells are Class I MHC restricted. By convention, the cells which present antigen to CD4 Th cells are termed antigen presenting cells while the cells which present antigen to CD8 Tc cells are termed target cells. Processing= protein antigen is degraded into peptides Presentation= association of peptide with MHC and transportation of MHC/peptide complex to the cell membrane These peptides are derived from proteins that are synthesized within the cytoplasm of the cell, thus this pathway is also termed the cytosolic pathway. Examples: viral proteins, tumor-specfic antigens, and proteins synthesized by intracellular protozoa or bacteria. Protein levels are carefully regulated in the cell. Misfolded, aged or damaged proteins are naturally degraded within the cytoplasm of all nucleated cells. Proteins targeted for proteolysis often have a small protein known as UBIQUITIN associated with them. Such conjugates are known to be degraded within structures known as PROTEOSOMES (basically a cylinder composed of hydrolytic enzymes which functions as a large protease complex) Degradation is thought to occur within the hollow center of the cylinder. Two alternative subunits of the proteasome (termed LMP2 and LMP7) are encoded within the MHC complex and their synthesis is triggered by the Th cell cytokine, IFN-gamma. The proteolytic activities of these two subunits apparently generates peptides which preferentially bind to Class I MHC proteins. Peptides generated in the
cytoplasm are transported across the membrane of the RER by a transporter
protein known as the TAP protein. The TAP protein is a heterodimer
composed of two subunits known as TAP1 and Interestingly, the genes for TAP 1 and TAP 2 map to the Class II region of the MHC complex. TAP 1 and 2 are classified as ABC proteins (ATP-binding cassette proteins) because such proteins are involved in the ATP-dependent transport of ions, sugars, amino acids, peptides, etc. across membranes. Meanwhile..... Class I MHC
alpha chains are being synthesized by the cell at ribosomes associated
with the RER. An alpha chain must associate with a beta-2- microglobluin
molecule and a processed PEPTIDE before it is The "completed" Class I MHC protein + peptide can now be transported to the Golgi in membrane-bound vesicles for further modification, packaging and sorting to the plasma membrane. The presentation of Class I MHC/ peptide by a target cell to a CD8+ Tc cell results in the proliferation and subsequent differentiation of a Tc into a CTL (killer/effector cell). The CTL can then participate in TARGET CELL KILLING. Target cell killing occurs due to induction of APOPTOSIS.
Processing and Presentation of Exogenous Antigens by Antigen Presenting Cells Web resource - Animated degradation and transport of antigens that bind major histocompatibility complex (MHC) class II molecules (Flash movie) Process performed only by
specialized antigen presenting cells (or cells which have been induced
to function as APCs): Mature Dendritic cells are the most effective antigen presenting cells in that they constitutively produce a high level of Class II MHC protein and the co-stimulatory protein, B-7. B cells constitutively express the Class II MHC protein but must be activated to produce B-7. Macrophages must be induced (activated) by the process of phagocytosis before expressing class II MHC or B7. In addition, there is a category of cells known as: NON-Professional APCs that can act as APCs for short periods of time, particularly during periods of sustained inflammatory activity. Such cells must be induced to express both Class II MHC and B7. Macrophages and dendritic cells internalize particulate antigen by phagocytosis and soluble antigens by endocytosis. B lymphocytes internalize antigen by receptor-mediated endocytosis [membrane-bound Ig serves
as the receptor and antigen serves as the ligand] Within the endocytic pathway or the phagocytic pathway - protein antigens are processed by proteolytic enzymes into peptides [13-24 amino acids in length]. For example, B lymphocytes internalize antigen through receptor mediated endocytosis. In this process, surface Ig bound to antigen is internalized in clathrin-coated vesicles. As this vesicle becomes an early endosome, the pH drops to 6-6.5 and the clathrin can be recycled to the plasma membrane. The vesicle then becomes a late endosome with a pH of 5-6 and finally a lysosome with a pH of 4.5-5. The lysosome contains a battery of over 50 different hydrolytic enzymes including: proteases, nucleases, lipases, glycosidases, etc. At this stage, proteins associated with the antigen are degraded (hydrolyzed) by proteases into peptides. Meanwhile...... Class II
MHC molecules are translated at ribosomes associated with RER. The 2
polypeptide chains [designated alpha & beta] enter the RER where
they attach to each other and bind to a third protein known as Ii
- the invariant chain. Membrane-bound vesicles traffic the MHC Class II (Ii serves as a targeting protein in this process) to the endosomal pathway. Under acidic conditions within this compartment, Ii is first cleaved leaving a fragment known as CLIP. (class II associated invariant chain peptide). The CLIP must either dissociate or be displaced to allow peptides to bind to the peptide-binding cleft. HLA-DM (a non-classical class II-like protein) catalyzes the release of CLIP and the binding of other peptides to the Class II MHC. HLA-DM also catalyzes the release of unstably bound peptides in a process known as "peptide editing". The peptide-MHC complex must be stable on the cell surface. It may take a long time for an appropriate Th lymphocyte to encounter the APC. Once a stable peptide-MHC interaction occurs, the complex is released and the Class II MHC/ peptide complex is then transported to the Golgi and ultimately to the plasma membrane where it is inserted. The Class II MHC/peptide
complex is now presented to a CD4+ Th cell. The result of this interaction
is: As with Class I, Class II proteins can also bind and present self peptides (many derived from MHC proteins themselves). These presented "self peptides" would be ignored since self reactive Th cells are deleted during thymic education. Interestingly, there are some pathogens which actually reside within the endosomal or phagosomal pathway of antigen presenting cells. Some examples include Mycobacterium tuberculosis and Mycobacterium leprae (the bacteria which cause tuberculosis and leprosy respectively), and the protistan parasite, Leishmania sp. (the causative agent of Leishmaniasis). Peptides derived from these
pathogens can also be processed and presented in association with Class
II MHC proteins on the surface of the infected APC. In this way, pathogen-specific
Th cells can be activated. Cytokines (especially IFNgamma) produced
by these activated Th cells can in turn activate the APC to destroy
the internalized pathogen through the production of the highly toxic
nitrogen intermediate - nitric oxide. As mentioned earlier in the
semester, it is believed that Th lymphocytes must first respond to Class
II MHC + peptide presented by a dendritic cell. Once activated,
these Th cells undergo clonal expansion leading to greatly increased
numbers of Th cells with this same TcR specificity. These cells
can now interact wtih Class II MHC + peptide on macrophages and B lymphocytes.
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