INTRODUCTION
Properties of T CellsT lymphocytes (T cells) playa central role in the immune response acting both to mediate specific immune functions and to modulate the function of all other aspects of the immune system. T cells mediate these effects both through direct cell-cell interactions and through the production of soluble mediators known as lymphokines.T cells recognize antigen through a specific receptor, which is a heterodimeric structure similar to immunoglobulin1. Most T cells (-95%) express a T cell receptor (TCR) composed of an a and a /3 chain; these chains associate to form the antigen recognition portion of the TCR. The selective recognition of antigens is determined by variable portions of the molecules that are located in their amino-terminal half. The TCR associates on the cell surface with the pentameric CD3 complex, which links the TCR to other cellular components needed to mediate transduction of signals from the receptor to the cell interior2,3. A much smaller fraction (3-5%) of T cells express a TCR composed of a yand a ~ chain; in humans the fraction of T cells expressing the y~ form of the TCR in neonates was similar to that in adults in one study4 and less in another4 a . Most y~ TCR expressing T cells do not express either the CD4 or CD8 molecule. In contrast to T cells bearing the all TCR, the function of T cells bearing the y~ form of the TCR is not yet clear, although it has been suggested that they may playa role in defense against mycobacteria, in removal of damaged cells expressing heat shock proteins and in autoimmune diseaseS,Sa. Virtually all T cells bearing the a/3 TCR also express on their surface either the CD4 (T4) or the CD8 (T8) molecule. These molecules act with the TCR to endow the T cell with the capacity to recognize specific antigen 1,6,7.T cells recognize antigen in a way that is fundamentally different from the manner in which B cells (and immunoglobulin, which is the B cell antigen receptor) recognize antigen. Immunoglobulin binds to free antigen, often recognizing secondary or tertiary structure. T cells generally recognize relatively short (-8-9 amino acids) pep tides, derived by the processing of larger proteins, and only do so when the peptide is bound to a major histocompatibility (MHC) molecule (known as HLA antigens in humans). The CD4 and CDS molecules determine whether a T cell will recognize antigen bound to class II MHC molecules (HLA-D in man) or bound to class I MHC molecules (HLA-A, -B, and -C in man), respectively6. The corecognition of antigen in association with self MHC molecules is central to the role of T cells in self/non-self discrimination.
Development of T Cells from Hematopoietic PrecursorsT cells are derived from hematopoietic precursors, which migrate to and colonize the human embryonic thymus at approximately S weeks of gestation8-10 . In the thymus, prothymocytes acquire the TCR and the CD3 complex and initially express on their surface both the CD4 and the CDS molecules; because they express both the CD4 and the CDS molecules these ...
