Clustered ribosomes RER

Class II molecules mainly present Peptides derived from proteins taken up by an antigen-presenting cell ft <0

Pinocytotic vesicle

Exogenous antigen-processing compartment

Exogenous antigen-processing compartment

Antigen-processing endosome with class II MHC molecules

Figure 20.3 Antigen processing and presentation in association with MHC molecules. Left: Antigen presentation of proteins produced within cells is mainly the property of class I MHC molecules. A proportion of proteins (black ribbons) produced within a cell on ribosomes (yellow) are broken down to peptides (black fragments) within a cytoplasmic molecular complex known as a proteasome. The resulting peptides are actively transported by TAP proteins (blue) through the wall of specialized endoplasmic reticulum (ER) that has MHC class I molecules blue in its wall. The peptides and P2-microglobulin associate with class I molecules and are then expressed on the cell surface (detailed in Figure 20.2). Right: Antigen presentation in association with class II MHC

which express increased amounts of MHC class II and constitut-ively express other molecules associated with T-cell activation such as CD40, CD80 (B7.1) and CD86 (B7.2). They also have markedly reduced capacity for pinocytosis. In the T zones of secondary lymphoid organs CD4+, and to a lesser extent CD8+, recirculating T cells are constantly migrating to the surface of IDCs, which they appear to scrutinize for the presence of an MHC-peptide complex to which they can bind with their T cell receptor. In this way, the T cells continually screen for the presence of peptides derived from both extra- and intracellular antigens (see section on immune responses).

The T-cell receptor (TCR) (Figure 20.4) has certain similarities to immunoglobulin in that it is made up of two non-identical polypeptide chains that have constant and variable regions. In addition, as described in the next section, the genes that encode for TCR and immunoglobulin are remarkably similar. The TCR has only one antigen binding site per molecule, as opposed to the two antigen binding sites of immunoglobulin.

molecules involves pinocytosis of antigen (black ribbon) and fusion with an antigen-processing endosome, which has class II MHC molecules (magenta and blue) inserted in its wall. The antigen-presenting grooves of the class II MHC molecules are kept empty by the association with invariant chain (CD74). Fusion of the pinosome with the endosome heralds the activation of proteolytic enzymes; the invariant chain and the ingested proteins are broken down to peptides. The resulting peptides (black fragments) are assembled into the antigen-presenting groove of class II MHC molecules that are held in the endosomal wall. The HLA class II molecules with bound peptides are then carried to the cell surface (detailed in Figure 20.2).

There are two types of TCR. The most common is composed of a heterodimer of an a-chain and a P-chain. The minority population of y8 TCRs is less well characterized, but they appear to recognize antigen in a different way from aP TCR. A number of different molecules are associated with the two TCR polypeptide chains in order to generate the TCR complex. This transmembrane signalling complex of molecules is collectively known as CD3 (detailed in Figure 20.4) and is linked to 'second messenger' signalling molecules, whose expression varies between different T-cell subsets and at different stages of T-cell differentiation. CD4 or CD8 molecules are closely associated with the CD3 complex and determine the class of MHC molecule to which the TCR can bind.

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