T-cell Receptor Rearrangement- unravelled

Cells of the adaptive immune system, i.e. B- and T-cells are amazingly diverse. They have to be to deal with myriad antigens that the body is exposed to. This diversity is achieved by rearrangement of the receptors on B and T cells during development. While I was aware of this, there was one unanswered question that had always puzzled me...until today. I'll share this shortly.

It is common knowledge that B cell "receptors" are comprised of immunoglobulins, which consist of heavy and light chains. Both have invariant constant (C) regions and variable regions. It is obviously the latter that contribute to their diversity and specificity. In the heavy chains, the variable regions are further comprised of V, D, and J regions ( stand for variable, diversity and joining). The light chain variable chains lack a D region and only consist of V & J regions. Each V, D or J domain has numerous alleles- up to 60, 70 or even more. Thus there are a high number of combinations possible, remembering that a given Ig chain will contain only one each of V, D or J. (Figure 1). These regions are flanked by signal sequences that allow their recognition in order that recombination may occur. These recognition sites undergo further changes -mutations- on exposure to antigens, a process called somatic hypermutation, resulting in increased affinity of the antibody for its antigen- affinity maturation.

Now we come to the reason for this post. T cells, instead of having immunoglobulins on their cell surface, have dimeric receptor chains- either a combination of alpha and beta or a combination of gamma and delta. alpha-beta T cells make up ~95% of all T cells, while the gamma-delta T cells are in a minority. It was therefore puzzling for me to note that in haematological malignancies such as leukaemias and lymphomas, it was usual to test for rearrangement of receptor chains in gamma chains, which are present in only 5% of T cells, rather than in alpha or beta chains, which make up the other 95%.

Here's why. Using standard methods such as Southern blotting, or increasingly now, PCR, you need a much smaller number of probes if were looking for clonality in T-gamma chains, than if you employed T-alpha or T-beta chains. Firstly, T-gamma chains only have V & J region and lack a D region, unlike T-alpha or T-beta. More importantly, the T-gamma chain has far less recombination-capable segments than either T-alpha or T-beta chains. For example, T-alpha has 70 V segments and at least 61 J segments spread over a large area (thus increasing the difficulty of Southern blotting). On the other hand the T-gamma chain has only 14 V segments and 5 J segments, and is therefore much easier to probe.

BTW, the same enzymes- RAG 1 and RAG 2, are responsible for splicing and rejoining of the various V, D and J regions in both B and T cells, which is the basis of severe combined immunodeficiency when there is a problem with RAG. However, unlike B cells, somatic hypermutation and affinity maturation does not occur in T cells. Nevertheless, further diversity is introduced by an enzyme called Terminal deoxynucleotidyl transferase (TdT) which randomly makes changes in the junctions between V, D and J chains. As the enzyme TdT is only present in immature lymphocytes, its presence or absence can inform the stage in lymphocyte ontogeny at which a lymphoid neoplasm arose.

Figure 1. Germline organisation of the T cell receptor genes