Biology Reference
In-Depth Information
F i g u r e 6.1. Positive selection of T-cells in the thymus.
that they express the CD45RO isoform and lack the need for costimulatory
molecules, can remain in circulation at low levels for years (McMichael et
al., 2000). The CD3
CD4
cells typically di¨erentiate into e¨ector T-helper
(Th)1 and Th2 cells based on the type of cytokine they secrete. Th1 cells secrete
IFN-g, IL-2, and tumor necrosis factor ( TNF )-b, whereas Th2 cells secrete IL-
10, transforming growth factor (TGF )-b, IL-4, and IL-5, although this is an
oversimpli®cation, and clones with a variety of cytokine secretion patterns have
been reported. CD3
CD8
cells typically di¨erentiate into e¨ector cytotoxic
cells (CTL), which also secrete cytokines ( Tc1 and Tc2), subject to the same
conditions as CD4
T cells. The most e¨ective APC include dendritic cells (DC),
which are capable of picking up the pathogen (or proteins) and processing its
protein to produce MHC-restricted, speci®c antigenic determinants (epitopes)
that are recognized by the T cells. Thus, the presence of speci®c epitopes are
essential for the development of both e¨ector and memory T cells. Figure 6.2
shows a typical interaction between MHC-restricted, peptide-carrying APC and
a T cell via the T-cell receptor. This interaction leads to the development of
memory and e¨ector T cells.
Most of the focus on CTL is based on the ability of these memory T cells to
di¨erentiate into e¨ector cells on secondary encounters with the same antigens.
This is the basis of most of our current vaccine strategies where pathogen-
