bution. It was found that this complex of self pep- 
tide and self MHC makes up one-eighth of all com- 
plexes on cells involved in CD4 T cell tolerance or 
activation. How^ever, this complex is present at very 
low levels on thymic epithelial cells, the cells that 
provide the MHC ligands involved in positive selec- 
tion. In mice having either thymic epithelial cells or 
bone marrow-derived cells expressing this self pep- 
tide-.self MHC complex, one finds tolerance only 
when the self peptide is expressed on bone 
marrow-derived cells. 
Moreover, there appears to be excess positive se- 
lection by the low level of this peptide found on 
thymic cortical epithelium in two different systems. 
This suggests that positive selection involves inter- 
actions of the TCR with ligands at low density, while 
negative selection involves contact with antigen- 
presenting cells expressing higher levels of a lim- 
ited repertoire of self peptides. This hypothesis is in 
keeping with the low density of TCRs on thymocytes 
prior to positive selection and with the low den- 
sity of MHC class I molecules on thymic cortical 
epithelium. 
Orientation of the TCR Relative 
to Its MHC Ligand 
There has been much speculation about the na- 
ture of the crucial interaction in clonal selection of 
CD4 T cells — the binding of the peptide:MHC class 
II complex by the TCR. Dr. Janeway's laboratory has 
generated chimeric TCRs by genetic engineering. 
These studies have shown that the centrally placed 
hypervariable regions are required for all responses 
of a cloned T cell line. However, variable amino 
acids that map to the first and second complemen- 
tarity-determining regions of the TCR a chain are 
also involved in the fine specificity of MHC recogni- 
tion by this cloned T cell line. 
Exchanging these regions between two TCRs gen- 
erates novel responses not observed with either pa- 
rental line alone. The critical interaction is between 
the helical region of the a chain of the MHC class II 
molecule and the Va-encoded first and second hy- 
pervariable regions of the TCR. Resolution of this 
interaction by site-directed mutagenesis of the TCR 
and the MHC class II molecule is nearly complete 
and should provide a detailed picture of the orienta- 
tion of these two structures during antigen recogni- 
tion. To confirm this, the structure of the relevant 
molecules must be determined. In collaboration 
with Dr. Alfred Bothwell, soluble receptors have 
been prepared from this cloned T cell line for struc- 
tural analysis. A major goal of the coming year will 
be the analysis of these soluble molecules. (This 
project was supported in part by a grant from 
the National Institute of Allergy and Infectious 
Diseases.) 
Co-stimulation Is Also Required 
to Activate CD4 T Cells 
Activated CD4 T cells can respond directly to li- 
gand alone by producing cytokines that mediate ef- 
fector functions. However, the initiation of an im- 
mune response requires clonal expansion of naive 
CD4 T cells through the production of the T cell 
growth factor interleukin-2 (IL-2). To produce IL-2, 
a naive T cell needs to receive co-stimulatory signals 
in addition to receptor ligation. Dr. Janeway's labo- 
ratory has shown that one cell must present both 
ligand and co-stimulator in order to induce clonal 
expansion of normal CD4 T cells. At least two co- 
stimulatory molecules that act in concert have been 
defined, the B cell activation antigen B7/BB1 and 
the heat-stable antigen. Many distinct microbial con- 
stituents will induce expression of co-stimulatory 
activity on antigen-presenting cells. Dr. Janeway has 
proposed that recognition of invariant microbial 
components by primitive immune recognition sys- 
tems triggers the expression of co-stimulatory activ- 
ity, allowing the adaptive immune response to dis- 
tinguish infectious non-self from non-infectious 
self. The definition of these recognition systems is a 
major future goal of the laboratory. (This project 
was supported in part by a grant from the National 
Institute of Allergy and Infectious Diseases.) 
Homing of Activated Effector Cells 
to Tissues 
When naive CD4 T lymphocytes encounter anti- 
gen in lymph nodes or spleen, they must first prolif- 
erate and then differentiate into active effector 
cells. These cells must leave the lymphoid organs 
and home to sites of infection in order to mediate 
their functions. They leave the lymphoid organs by 
efferent lymphatics and enter the blood, where they 
circulate to all parts of the body. However, they are 
only useful if they can enter the site of infection. Dr. 
Janeway's laboratory has asked what molecules are 
involved in the homing of effector cells to potential 
infection sites. 
The iSj-integrin VlA-4 appears to be crucial for 
this step, binding to VCAM- 1 on endothelial cells. In 
the system studied, activated T cells specific for a 
brain antigen would enter normal brain only if they 
express VLA-4, and entry was VlA-4 and VCAM- 1 de- 
pendent. Dr. Janeway believes that VLA-4 and other 
adhesion molecules that are up-regulated on effec- 
tor T cells allow the cells to leave the vessels and 
enter tissues. If antigen or infection is present, then 
the cells respond and recruit other cells to mediate 
338 
