After IL-7 and IL-7 receptor provide the survival signals to early pro-T-cells,
the major event in T-cell development occurs,
which is the rearrangement of the T-cell receptor genes.
The T-cell receptor characterizes T-cells, this is what's different between a T-cell
and the other lineages of immune cells, so the rearrangement of the T-cell
receptor genes, and the expression of the T-cell receptor,
is the key event that will take place in the thymus.
This is dependent on a key enzyme called RAG, recombination activating
gene, is the enzyme that orchestrates the rearrangement of the T-cell receptor
genes, so once again, children that have mutations in RAG will have another type
of SCID (severe combined immunodeficiency).
In this case it's called Omenn syndrome, after the person who identified it,
and it means the absence of both T- and B-cells.
In contrast with X-linked SCID described in the previous slide - where T-cells were
absent but B-cells were there - in this case, because RAG is not expressed,
and RAG is required for both B- and T-cell development, the child will be devoid
of both B- and T-cells.
This will result in an even more aggressive immunodeficiency.
In this case, once again, bone marrow transplantation will be required, so that
the child can develop normal B- and T-cells, once it gets healthy bone marrow
with healthy T- and B-cell progenitors that can express the RAG protein.
What does RAG do?