The Diversity of Receptor Function 
olfactory sensory neurons. This channel traverses 
the membranes six times, but also contains a do- 
main within the cytoplasm of the cell that binds 
cyclic nucleotides. The interaction of odors with 
their specific receptors results in an elevation in 
cAMP, which in turn leads to the opening of this 
channel. 
The demonstration that the olfactory epithe- 
lium expresses a unique ion channel controlled 
by intracellular levels of cyclic nucleotides now 
provides a mechanism linking odorant-elicited in- 
creases in cyclic nucleotides with the generation 
of an electrical signal transmitted to the brain. 
The cloning of a highly diverse multigene family 
of odor receptors, along with an ion channel re- 
sponsive to intracellular changes elicited by the 
receptors, should provide further insight into the 
recognition mechanisms and neural processing 
events that permit the discrimination of a diverse 
array of odors. 
Diversity of Recognition in the Cellular 
Immune System 
The cellular immune system must recognize 
and respond to an even greater diversity of signal- 
ing molecules in the form of foreign antigens. 
This diversity is encoded by an extremely large 
family of T cell receptor genes. Although thymo- 
cytes express a T cell antigen receptor, T cells 
segregate into discrete functional classes. One 
set, T helper cells, initiate an immune response 
to foreign antigen by activating other T cells. The 
second subset, T killer cells, respond to foreign 
antigen by the elaboration of cytotoxic functions 
resulting in the death of cells expressing an 
antigen. 
Most T helper cells express an additional sur- 
face membrane receptor, the CD4 molecule; 
most T killer cells express a different but related 
receptor, the CDS molecule. We have cloned the 
genes encoding these two receptors and charac- 
terized their functions. Our studies and others 
indicate that the coordinate engagement of a T 
cell antigen receptor (TCR) and a CD4 receptor 
with molecules on the antigen-bearing target cell 
is required for efficient T cell help. The coordi- 
nate engagement of a TCR and CDS appears to be 
required for efficient T cell killing. 
We have further demonstrated that the T cell 
surface receptors, CD4 and CDS, are not only re- 
quired for the mediation of an efficient cellular 
immune response, but are essential for shaping 
the repertoire of T cells during development. We 
have introduced an exogenous CDS gene into 
mouse embryos to generate transgenic mice that 
express a CDS gene on all T cells. Analysis of the 
T lymphocytes within the thymus and periphery 
of these mice leads to a model of T cell develop- 
ment in which the interaction of either CD4 or 
CDS with appropriate molecules on the surface 
of cells within the thymic epithelium is a primary 
determinant of whether a cell will be either a 
helper or a killer cell. 
Thus our studies on the structure and function 
of receptors from the immune system, olfactory 
system, and brain provide examples of how the 
eukaryotic chromosome can generate the diver- 
sity required to recognize and respond to a vast 
array of signaling molecules in the environment. 
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