found this online seems to say t cells are formed in the thymus and in bone marrow, so far i havent come across anything saying they cannot be regenerated.
Building the T-cell Repertoire
T cells have receptors (TCRs) that bind to antigen fragments nestled in MHC molecules. But,
all cells express class I MHC molecules containing fragments derived from self proteins;
many cells express class II MHC molecules that also contain self peptides.
This presents a risk to the animal of the T cells recognizing these self-peptide/self-MHC complexes and mounting an autoimmune attack against them. Fortunately, this is usually avoided by a process of selection that goes on in the thymus (where all T cells develop).
The process works like this:
The precursors of T cells — like all blood cells — are formed in the bone marrow.
These cells then migrate to the cortex of the thymus. At this time they have neither a complete TCR nor either CD4 or CD8 (thus are called "double-negative" or DN cells).
In the cortex of the thymus, they
begin to form a TCR; Link to discussion of how TCRs are generated.
synthesize both CD4 and CD8 (so now they are "double-positive" or DP cells).
The cortical cells of the thymus express a wide variety of small molecules, usually a peptide of 6–8 amino acids derived from body proteins; that is, "self" proteins such as
proteins within the cytosol
serum proteins; i.e., proteins circulating in the blood and lymph
nestled in a histocompatibility molecule (encoded by the MHC). [View]
Most of the cells (~97%) will produce a TCR that does not bind to any of the peptide-MHC molecules present on the surface of the cortical cells. Unless they can try again with a new TCR, these cells die by "neglect" (by apoptosis, actually).
Those remaining cells whose TCR
has bound a peptide antigen presented in class II MHC molecule stop expressing CD8 and become CD4+ T cells. It is these cells that will go on to become
Th1 cells in cell-mediated immune responses;
Th1 helper cells for cytotoxic T lymphocytes (CTLs);
Th2 helper cells for B cells [View].
has bound a peptide antigen presented in class I MHC molecule stop expressing CD4 and become CD8+ T cells.
Both sets of cells are said to have undergone positive selection.
After positive selection, these cells migrate to the medulla of the thymus.
There those cells whose TCR binds very strongly to complexes of self-peptide and self-MHC are destroyed (again by apoptosis).
This process of negative selection is important as it eliminates cells that might otherwise mount an autoimmune attack. It is one of the ways in which tolerance to self antigens is achieved. [Link to discussion of T-cell tolerance.]
The cells whose TCRs bind antigen at an affinity below the threshold that triggers apoptosis are free to leave the thymus and migrate throughout the immune system (lymph nodes, spleen, etc.)
It is this population which we depend on to mount immune responses against foreign antigens. A TCR that binds self-peptide/self-MHC with low affinity may well bind a foreign-peptide in self MHC with high affinity.