Immunology, Biology 328

TcR Continued

VARIABLE REGION GENE REARRANGEMENT
Similar mechanisms for rearrangement as for Immunoglobulin.

Obeys the heptamer- nonamer 12/23 rule
RAG 1 and RAG 2 are expressed in the pre T cell
Shows allelic exclusion [for the most part] - 1 functional type of TcR per cell
Allelic exclusion is stringent for the beta chain of the TcR but appears to be not as stringent for the alpha chain. It is rare, but occasionally you will find a single T cell expressing the same beta chain with two different alpha chains. However, it is believed that only one of the TcRs is self MHC restricted and therefore only one of the TcRs is functional.

MECHANISMS FOR GENERATING TCR DIVERSITY
Recombinatorial Diversity
generated by multiple V gene segments and J gene segments for the alpha and gamma chain and by multiple V, D, and J gene segments for the beta and delta chain. Diversity in D region Usage
Due to the arrangement of the heptamer 12 nonamer and heptamer 23 nonamer sequences in the beta and delta genes, differential D region usage has been observed. (see figure in text)
In beta chain gene rearrangement both direct V-J rearrangement and V-D-J rearrangement occurs. In delta chain gene rearrangement, V-J, V-D-J, and V-D-D-J rearrangement is observed.

Combinatorial Diversity
Due to the random combination of alpha and beta chains or gamma and delta chains.

Junctional Diversity
N- nucleotide addition Diversity
terminal deoxynucleotidyl transferase can add nucleotides at joining junctions of all TcR genes
No Somatic Mutation - so TcR do not show affinity maturation.

T CELL RECEPTOR COMPLEX
There are also other polypeptides closely associated with the TcR
CD3 complex
CD3 forms a complex with the receptor. A cell that is CD3+ is also TcR +. Appearance of CD3 is a very good marker for cells expressing the T cell receptor. The CD3 is actually a complex of 5 polypeptides. Gamma, delta, and epsilon occur as 2 dimers [gamma-epsilon and delta-epsilon] complexed with either a homodimer of two zeta chains or a heterodimer or zeta and eta chains. Zeta and eta are encoded by the same gene and only differ in their carboxyl terminus. The two forms are generated by differential RNA splicing of the primary transcript. About 90% of the CD3 complexes examined to date incorporate the homodimer.

SEE DIAGRAM OF TCR COMPLEX IN BOOK

Short length of the cytoplasmic tail of the alpha & beta chains suggests that they are unsuitable for signal transduction. CD3 functions in this regard.
The TcR exists on the membrane as a molecular complex with CD3. Mutation in either the CD3 or the TcR results in loss of the entire complex from the membrane.

The transmembrane domain of all CD3 polypeptides contains a - charged aspartic acid. Correspondingly, there are either 1 or 2 + charged amino acids in the transmembrane domain of each TcR chain. The alpha chain contains a lysine and an arginine while the beta chain contains a single lysine.

Once the TcR recognizes an antigen-MHC complex, the associated CD3 complex is thought to transmit a signal to the cell interior that contributes to cellular activation. Monoclonal antibody specific for CD3 bypasses the antigen-specific T cell receptor. Following activation, several of the CD3 polypeptides are phosphorylated at tyrosine or serine residues. Phosphorylation is thought to lead to the activation of second messengers involved in T cell activation.

CD4 and CD8 Accessory Molecules
T helper cells express CD4 but not CD8
T cytotoxic cells express CD8 but not CD4

CD4 and CD8 are associated in the membrane with the TcR. Both CD4 and CD8 function as adhesion molecules; CD4 binds to Class II MHC molecules and CD8 binds to Class I MHC molecules. Binding of the TcR to the peptide/MHC complex is greatly augmented if CD4 or CD8 are assisting. Their cytoplasmic domains may also allow for signal transduction to occur. The signal-transduction property of CD4 and CD8 is mediated through their cytoplasmic domains. Both CD4 and CD8 are noncovalently associated with the protein kinase Lck.

Other TcR accessory molecules
A variety of other membrane molecules play an important accessory role in antigen recognition and T cell activation.

In addition to CD4 and CD8, T cells possess several other accessory molecules including CD2, LFA-1, CD28, CD45R. CD-2 is present on most thymocytes, all peripheral T cells, and on large granular lymphocytes. The ligand for CD2 is LFA-3. The CD2/LFA-3 interaction facilitates the binding of the Th to the APC and cytotoxic T cells to their target cells.

LFA-1 is another adhesion molecule which strengthens the interaction of the T cell with an APC or target cell. LFA-1 is an integrin which binds to ICAM-1.

Of course CD28 (and CTLA-4) on Th cell binds to B7 on an APC.

Comments or questions should be directed to Cheryl.Davis@wku.edu
Last Modified: March 31, 1998
All contents copyright (C) 1998.
Western Kentucky University.