6.6 Special endosomal compartments
Different cell types may have specialised endosomal compartments related to their specific functions. In this section we are going to look at just one example of this, the pathway of antigen processing and presentation that occurs in a number of different types of leukocyte, including B cells, the lymphocytes that make antibodies.
In order to synthesise antibodies, B cells interact with another class of lymphocyte, a T helper cell. To do this the B cell has to degrade foreign material that it has internalised and present antigen fragments to the T cell. These steps, called antigen processing and antigen presentation, play a central role in the induction of the immune response. As you read the following paragraphs, refer to Figure 40.
The T cell recognises small peptide fragments that are non-covalently bound to molecules encoded within the major histocompatibility complex (MHC class II molecules).
Now let us look at the steps in antigen processing and presentation that occur in the B cell, and specifically at the intracellular trafficking pathways. B cells have antibody on their surface, which acts as their receptor for antigen. Antigens that become bound to the surface antibody are endocytosed and passed through the early and late endosomal compartments, and thence to a lysosomal compartment, where they are partly degraded. Partial proteolysis generates polypeptide fragments, which are diverted to an acidic endosomal compartment called the MIIC compartment. Figure 41 shows the multilamellar appearance of an MIIC compartment in a B cell.
Meanwhile, in the ER, the B cell synthesises MHC class II molecules, with the associated invariant chain. These are directed to the trans Golgi network where they are sorted for delivery to endosomes by a signal sequence that is located in the cytoplasmic tail of the invariant chain. While in transit from the Golgi to the endocytic pathway, the invariant chain is degraded by endosomal proteases. From the trans Golgi network, the MHC class II molecules move to the MIIC compartment, where they intersect the pathway taken by polypeptide fragments coming from the lysosomes. Cleavage of the invariant chain causes it to dissociate from the MHC class II molecule, thus exposing its peptide binding site. A type of chaperone (DM in Figure 40) located in the compartment now loads the binding site with peptide fragments coming from the lysosomes. The MHC–peptide complex is sent to the cell surface, for potential recognition by a passing T cell.