Molecular Approaches to T Lymphocyte Recognition and Differentiation 
tion and indicate that TCR binding to peptide- 
MHC ligands is so weak energetically that other, 
antigen-independent receptor-Iigand systems must 
govern the initial stages of T cell contact with 
antigen-presenting or target cells. The best candi- 
dates for such regulators of T cell interaction are 
adhesion molecules, such as LFA-1 (lymphocyte 
function antigen 1) or CD2. This would give ad- 
hesion molecule-ligand interactions a major role 
in orchestrating which T cells interact with 
which antigen-presenting cells. This has impor- 
tant implications for autoimmunity: normally T 
cells would be expected to focus on appropriate 
antigen-presenting cells (such as B cells or macro- 
phages) and would be hindered in surveying 
most other cells or tissues (which would lack the 
appropriate ligand expression) . 
Generation of Memory T Cells in Vitro 
A hallmark of the vertebrate immune system is 
its ability to respond much more strongly and 
quickly to a second encounter with an antigen. 
Although some of this effect is due to the clonal 
expansion of B and T cells, a significant, and per- 
haps the major, contribution is thought to involve 
the induction of antigen-specific memory lym- 
phocytes. Evidence suggests that this differen- 
tiation step is crucial to mounting a successful 
immune response, both in appropriate and inap- 
propriate (e.g., autoimmune) circumstances. 
Studies of B or T lymphocyte memory have been 
hampered by the lack of well-defined systems in 
which uniform populations of primary and mem- 
ory cells can be studied. We have approached this 
problem in T lymphocytes by making use of TCR 
a/3 transgenic mice that have an essentially mono- 
clonal T cell immune system, thereby eliminating 
the clonal expansion component of T cell mem- 
ory and allowing us to focus on the physiological 
changes that characterize this transition. 
We find that primary T cells can only produce 
interleukin-2 (IL-2) in response to TCR stimula- 
tion with immobilized peptide-MHC complexes 
if PMA (phorbol 12-myristate 12-acetate) is in- 
cluded in the culture. In contrast, secondary, or 
memory, T cells, derived from the identical start- 
ing population, readily produce lymphokine on 
this substrate, irrespective of PMA addition. Both 
T cell populations are inhibited by protein kinase 
C (PKC) inhibitors H7 and staurosporine. Thus 
secondary T cells are functionally distinct from 
primary cells; the establishment of T cell mem- 
ory, which may include alterations in PKC signal- 
ing, followed by the addition of exogenous IL-2, 
is sufficient to convert primary T cells to second- 
ary cells in vitro. This provides an excellent op- 
portunity to characterize and manipulate this im- 
portant juncture in T cell differentiation. 
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