Antigen processing and presentation are crucial steps for recognition of protein antigens by T cells. The T-cell receptor (TCR) will not bind a protein directly, as do immunoglobulins, but only as peptides derived from the original protein (processing step) and bound to class I or class II molecules of the major histocompatibility complex, or MHC (presentation step). This applies to helper as well as to effector cytotoxic T lymphocytes and therefore condition severy aspect of the T-dependent immune response. These functions strictly condition immunogenicity and correlate well with the dichotomy between carrier and hapten that had indicated long ago that the hapten part of the complex was recognized as such by the B cell, but was unable to initiate an immune response unless conjugated to a carrier protein (see Antigen). Later it was shown that carrier recognition involved the T helper cell but was also strictly dependent on MHC molecules (MHC restriction). Antigen processing and presentation take place in specialized cells and involve a number of molecules, most of which are encoded by genes of the MHC.
The antigen-presenting cells that are most efficient and most extensively studied are the dendritic cells and the epidermal Langerhans cells. They originate from CD34+ bone marrow precursors and migrate to different tissues. They express FcR e I and FcRII, as well as class I and class II molecules of MHC (in humans, HLA-DR). They internalize the antigen by the endosomal pathway and, after processing, reexpress at the cell surface antigen-derived peptides associated with class I molecules. Alternatively, they may present peptides associated with class I MHC after macropinocytosis. They may become interdigitated cells and migrate to secondary lymphoid organs, where they present peptides specifically to CD4 T cells. B cells also behave as antigen-presenting cells, and this is of major importance for the antibody response induced by a T-dependent antigen, because it conditions the necessary direct interaction between T and B cells. Finally, macrophages and monocytes also can present antigens, in addition to their other functions.
There are two main pathways for antigen processing and presentation. One involves association with MHC class I molecules and presentation to CD8 T lymphocytes, while the other involves MHC class II molecules and interaction with CD4 lymphocytes.
1. Class I Pathway
MHC Class I molecules are composed of one a polypeptide chain of 43 kDa that forms a heterodimer with the b2 microglobulin. A peptide binding site, which accommodates nonapeptides, is shared by the first two domains of the a chain. There is a huge degeneracy in peptide recognition by one such site, as expected from the low number of different MHC molecules expressed in any individual. Class I molecules are specialized in the presentation of endogenous proteins, synthesized in situ by a virus or any intracellular bacteria or parasite. Processing starts as a fraction of these cytosolic proteins are hydrolyzed in the proteasome, a proteinase-rich complex involved in protein degradation. The resulting peptides will be transported by the specialized TAP gene products to class I molecules in the endoplasmic reticulum. The class I peptide complex will pass through the Golgi apparatus, where the a chain becomes glycosylated before being included in a secretory vesicle and finally expressed at the plasma membrane. Interaction with the CD8 T cell involves the TCR, which makes contact with amino acid residues of both the peptide and the class I molecule, whereas CD8 binds solely to the MHC molecule. In addition to this specific interaction, many other cell adhesion molecules contribute to this "immunological synapse," as immunologists sometimes call it.
2. Class II Pathway
MHC class II molecules are composed of two chains, a and b, each containing two domains. The peptide binding site is shared by the a1 and b1 domains. The ab heterodimer associates with the Ii (CD74) invariant chain that prevents any association of an endogenous peptide with the class II molecule. After glycosylation during passage through the Golgi, the complex is packed in the class II vesicles that fuse with endosomes, where the invariant Ii chain is hydrolyzed, leaving only the CLIP peptide still associated with the ab heterodimer. The last step, controlled by HLA-DM molecules, involves an exchange between CLIP and a peptide derived from an exogenous protein antigen. The complex of peptide and class II molecules may now be expressed at the cell surface. A fraction of the class II molecules is reinternalized and may bind new peptides, before being reexpressed at the cell surface. The size of peptides that bind to the class II molecule may vary from between 10 and 30 amino acid residues. Interaction with T cells is somewhat similar to the case previously described, except that it involves essentially CD4 T cells.
CD4 T cells are helper cells that will either (a) interact with the T effector compartment and promote the emergence of cytotoxic T cells or (b) interact with B cells that will ultimately produce antibodies. The latter interaction is of particular interest, because the B cell will act as both a presenting cell and an antibody-producing cell. The T-cell-B-cell interaction is initiated because the B cell presents peptides derived from the specific antigen that had been internalized after binding to the surface immunoglobulin. The antigenic peptide-class II complex will trigger the interacting TCR and activate the T-cell CD3 signaling module. Critical molecules are then produced by the T cell, amongst which are soluble cytokines and one membrane ligand, CD40L, which will bind to its receptor, the CD40 molecule, which is constitutively expressed on the B cell. This is the major signal that will ultimately activate the final phase of differentiation of the B cell and provide the key to clonal expansion and antibody production.
