Question

T cells have to work in a partnership with an Antigen Presenting Cell (APC). Before this can occur, the APC must modify the antigen. Please discuss how this process happens and the major protein that is involved. Be detailed

Answer 1

Answer -

The Antigen Presenting Cell (APC) undoergoes bascically two types of process depending upon the type of antigen before presenting antigen to particular T-cell. The following are two processes -

Processing of endogenous antigens via cytosolic pathway

In this process of presentation, the antigen is present endogenously i.e. inside the cell in cytosol. These antigens first cleaved by proteasome resulting small peptides. Then, cytosolic peptidase further cleaves small peptide fragments to smaller fragments. After this, the small antigenic petide fragment through an energy-dependent process using a pore term as transporters associated with antigen processing (TAP), transferred to ER where the antigenic peptide bound with Major Histocompatibility Complex-I protein. Then, the MHC I-peptide complex using endomembrane system is transported to the plasma membrane or cell surface where, the APC presents the processed antigen complex with MHC I to cytosolic T-cells.

Processing of exogenous antigens via endocytic pathway

In this process of presentation, the antigen is present exogenous i.e. outside the cell in the extracellular part of the system. These antigens are transported inside the APC via endocytosis and as the antigens passes through the endosomes of endocytosis pathway, the acidic pH cleaves off the antigen to smaller fragments. Simultaneously, in another vesicle, containg Major Histocompatibility complex class II molecule coupled with a chain peptide known as invariant chain (preventing MHC II protein clogging before its interaction with antigen), fuses with the vesicle carrying the exogenous antigen.

For interaction of MHC II with antigen the chain has to be removed, thus, in late endosome vesicles carrying only the MHC II, the invariant chain cleaved off and only a smaller fragment of invariant chain known as CLIP remains which is there just to prevent premature binding of MHC II with antigen peptide.

Later, the CLIP is removed by a protein term as HLA-DM and the MHC II-antigen peptide complex is formed in the fused endosomal-vesicle. The endosomal vesicle then transports that MHC-peptide complex to the cell surface where APC presents the processed antigen complex with MHC II to helper T-cells.