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Monitoring Stem Cell Research 
dependent on recognition by the T cell receptor of a three- 
dimensional complex on the surface of antigen-presenting cells 
(APC) comprised of the HLA molecule and its bound peptide. The 
peptides are produced by complex antigen processing machineries 
within the APC (i.e. proteolytic enzymes, peptide transporters and 
molecular chaperones) which generate a pre-selected peptide pool 
for association with the HLA molecules. The different types of T cells 
require different HLA molecules for antigen presentation, so-called 
"HLA restriction" phenomena. T cell receptors on CD8+ cytotoxic T 
cells (CTLs) bind peptides presented by HLA class I molecules, 
whereas CD4+ T helper cells (Th) recognize peptides bound to HLA 
class II molecules. Of the 8-13 amino acid residues of a bound 
peptide within a class I or II HLA molecule, only three to four amino 
acid side chains are accessible to the T cell receptor, and a similar 
number of amino acids are involved in binding to the HLA molecule. 
Thymic Education Of T Cells 
T cells mature in the thymus to appropriately respond to foreign 
pathogens without inadvertently attacking the host. Under the 
influence of various thymic resident cells and factors they elaborate, 
maturing T cells fall into two categories: those that are able to 
discriminate between self and non-self and can appropriately 
respond to foreign pathogens without inadvertently attacking the 
host, and those which are unable to appropriately discriminate 
between self and non-self. Dendritic cells have been implicated in 
the deletion, or inhibition, of T cells reactive to self-antigens, 
particularly in the thymus during T cell development or in peripheral 
lymphoid organs. The process of self/non-self discrimination by the 
maturing T cells is dependent on thymic dendritic cell (DC) 
presentation of self-antigens in the context of self-HLA molecules. 
When maturing thymic T cells are highly reactive with self- 
antigen/HLA complexes, they are deleted so that potentially 
autoreactive T cells will not be released into the periphery. If a 
particular foreign antigen can be presented in such a way in the 
thymus as to fool the maturing T cells into believing that the antigen 
is part of self tissue, then T cells capable of reacting with this 
antigen will also be eliminated. Indeed, it has been demonstrated 
that when mouse thymic DC present transgenically introduced 
foreign antigens to developing T cells, the mature peripheral T cell 
repertoire of the mouse lacks T cells capable of reacting with the 
specific foreign antigen, i.e. it is tolerant to the foreign antigen. This 
has raised the possibility that injection of dendritic cells into an 
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