STRUCTURE AND FUNCTION OF THE T CELL ANTIGEN RECEPTOR 
Arthur Weiss, M.D., Ph.D., Associate Investigator 
Dr. Weiss's goal is to understand how T lympho- 
cyte surface molecules regulate cellular responses. 
The T cell antigen receptor (TCR) is among the 
many cell surface molecules that regulate T cell ac- 
tivation. The TCR is a remarkably complex seven- 
chain structure that consists of two subunits. 1) 
The ligand-binding subunit that is responsible for 
antigen recognition comprises the a/p-chain 
heterodimer (Ti). The a- and (3-chains are clonally 
distributed and are derived from rearranging genes. 
2) The CD3 complex, comprising five invariant 
chains, 8, e, 7, and t,^. CD3 is thought to play a role 
in signal transduction. 
The TCR activates two signal transduction path- 
ways: the phosphatidylinositol (PI) and tyrosine ki- 
nase pathways. In the case of the PI pathway, stimu- 
lation of the TCR results in increases in inositol 
phosphates and diacylglycerol as a consequence of 
phospholipase C-induced hydrolysis of phos- 
phatidylinositol 4,5-bisphosphate (PIP2)- PlP^-de- 
rived second messengers, inositol 1,4,5-trisphos- 
phate and diacylglycerol, are responsible for the 
observed mobili2ation in cytoplasmic free calcium 
([Ca^"^].) and activation of protein kinase C. Several 
late biological responses by T cells are associated 
with these intracellular changes, including tran- 
scription of lymphokine and lymphokine receptor 
genes, expression of lymphocyte activation anti- 
gens, and activation of the cytolytic mechanism. 
The precise role of the components of the TCR and 
the identity of the intracellular elements involved 
in activation of the PI pathway are not understood. 
The second signal transduction pathway reg- 
ulated by the TCR involves the activation of a 
tyrosine kinase. This is best manifested by the ap- 
pearance of several new tyrosine-containing phos- 
phoproteins after TCR stimulation. Most notable is 
the tyrosine phosphorylation of the ^-chain of the 
TCR. The kinase responsible for this activity is not 
intrinsic to the TCR itself and has not been identi- 
fied. The relationship between this pathway and 
the PI pathway, as well as its role in cell activation 
responses, is not known. 
I. Isolation of TCR Signal Transduction Mutants. 
The complex nature of the TCR structure is not 
readily amenable to a simple structural and func- 
tional analysis. To identify the functional domains 
of the TCR and other components of the TCR-regu- 
lated PI signal transduction pathway, a somatic cell 
genetic system was developed to isolate TCR signal 
transduction mutants. A protocol involving ligand- 
induced growth inhibition and fluorescence cell 
sorting for cells that fail to increase [Ca^""^]. was 
used to isolate a family of three signal transduction 
mutants (J.CaMl-3) from the T cell leukemic line 
Jurkat. All three cells express high levels of TCR on 
the cell surface but fail to mobilize [Ca^"*^]. or in- 
crease inositol phosphate metabolites in response 
to anti-Ti monoclonal antibodies. Two of the mu- 
tants, J.CaMl and J.CaM3, do respond partially to a 
subgroup of anti-CD3 monoclonal antibodies. The 
similar functional phenotypes of these two mutants 
suggest that the molecules that are defective in 
these cells are involved in a similar function within 
a multimolecular complex. Utilizing a novel 
heterokaryon complementation assay, the defects in 
these cells have been mapped to three distinct 
genes other than the Ti chains. Current efforts are 
aimed at examining the functional competency of 
the tyrosine kinase pathway and at developing a ge- 
netic reconstitution system to identify the defective 
components within these cells. 
Characterization of the responses of these mu- 
tants has also established that 1) ligand-induced 
TCR internalization is not dependent on TCR in- 
duction of the PI pathway; 2) all three mutants 
with distinct defects fail to produce interleukin-2 
(IL-2), supponing a role for the PI pathway in this 
cellular activation response; 3) sustained activation 
of the PI pathway is required for commitment to IL- 
2 production; and 4) there is a directional transfer 
of information regarding ligand occupancy from Ti 
to CD3. 
II. Function of the Human Muscarinic Receptor in 
T Cells. 
As a further means of characterizing these mu- 
tants and to assess the role of the inositol phos- 
pholipid pathway in regulating the activation of T 
cells, a heterologous receptor that activates the ino- 
sitol phospholipid pathway was expressed in Jurkat 
cells and the signal transduction mutants. The 
human muscarinic acetylcholine receptor subtype 1 
(HMl) is expressed in neuronal, cardiac, and 
smooth muscle cells and functionally activates the 
PI pathway in these cells. When transfected into 
Jurkat cells, the HMl receptor efficiently activates 
Continued 
453 
