Molecular Regulation of Lymphoid Cell Growth and Development 
coordinate production of a variety of lympho- 
kines. Activation of this pathway involves CD28, 
a specific receptor on a T cell's surface. The mole- 
cule that normally stimulates this activation path- 
way is expressed on B cells, macrophages, and 
thymic epithelial cells. Thus this receptor-ligand 
interaction serves as a means for different cell 
types involved in the immune response to com- 
municate with each other. 
Within the T cell, the CD28 activation pathway 
appears to control the rate at which the mRNAs 
for these genes are degraded, rather than the rate 
at which the mRNAs are produced. Stimulation of 
this pathway after mitogenic activation of cells 
produces exponential growth of helper T cells. 
As a result, we have been able to keep helper T 
cells in exponential growth for over six months. 
This is exciting, because for the first time we are 
able to grow normal human helper T cells in cul- 
ture for long periods of time. In theory, we can 
now generate an infinite number of helper cells. 
Cells grown in this way are being tested for their 
ability to enhance an individual's immune re- 
sponse to cancer. In addition, we may be able to 
develop therapies to enhance the production of 
helper T cells within the body. Presently, how- 
ever, we do not know whether the activation 
pathway directly leads to the proliferation of 
helper T cells or whether a particular lympho- 
kine or combination of lymphokines produced as 
a result of this stimulation is responsible. Studies 
to clarify this issue are under way. 
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