Genetic Approaches to Immune Function 
and Tolerance 
Richard A. Flavell, Ph.D. — Investigator 
Dr. Flavell is also Professor of Immunobiology at Yale University School of Medicine. He received his B.Sc. 
and Ph.D. degrees in biochemistry from the University of Hull, England, where he worked with I. Gwyn 
Jones. His postdoctoral fellowships included research with Piet Borst at the University of Amsterdam and 
with Charles Weissmann at the University of Zurich. Before accepting his current position. Dr. Flavell 
was first Head of the Laboratory of Gene Structure and E.xpression at the National Institute for Medical 
Research, Mill Hill, London, and then President and Chief Scientific Officer of Biogen Research Corpora- 
tion, Cambridge, Massachusetts. Dr. Flavell is a Fellow of the Royal Society and a member of several other 
distinguished societies. 
MY laboratory has concerned itself for many 
years with the expression of the genes of 
the murine MHC (major histocompatibility com- 
plex) . In the mouse these genes are encoded on 
chromosome 17, and prior work has shown that 
there are approximately 25 class 1-related genes 
and a handful of class II genes. Class I genes en- 
code a protein of approximately 45,000 molecu- 
lar weight that is found in association with a small 
subunit, ;82'"^icroglobulin. Together this com- 
plex forms a symmetrical molecule consisting of 
four extracellular globular domains anchored 
through the cell membrane with the transmem- 
brane segment and having a short stretch of 
amino acids that extend into the cytoplasm. Class 
II molecules achieve a similar symmetry, but 
with two polypeptide chains, a and /3, each of 
which has two extracellular domains and a trans- 
membrane and cytoplasmic segment. 
Both class I and class II gene products serve as 
recognition elements, which bind antigenic pro- 
tein fragments and present them to T cells. In the 
case of class I genes, the presentation is to T cells 
carrying the CDS co-receptor molecule. These 
cells are usually cytotoxic T cells, whose role is 
to destroy cells that are virally infected. In the 
case of class II molecules, it is commonly soluble 
antigen that is presented, this time to helper or 
inflammatory T cells that carry the CD4 co-recep- 
tor. Both types of T cells secrete hormone-like 
molecules called lymphokines, which in turn act 
on other cell types — for example, on B cells, 
which are stimulated to multiply and to make 
antibody. 
Much of our work since joining HHMI has cen- 
tered on the regulation of expression of the class 
II genes and the study of their biological role in 
transgenic animals. Class I and class II genes are 
both regulated in vivo by various lymphokines. 
For example, interferon-7 secreted by activated T 
cells stimulates the synthesis of MHC class I and II 
molecules and, as a result, presumably renders a 
cell better able to present antigen and thus to 
potentiate an immune response. 
In the past year we have taken a genetic ap- 
proach to attempt to understand how interferon- 
7 activates the synthesis of these MHC molecules. 
A new approach has been used to isolate a series 
of mutant cell lines that are not capable of re- 
sponding to interferon-7. These cell lines appear 
to have a series of different defects, which we are 
attempting to elucidate. This genetic approach 
should help us understand how this important 
hormone regulates gene expression in this situa- 
tion and elsewhere. 
In studying the expression of class II genes, we 
have also analyzed class II protein function in an 
animal setting, employing the strategies of molec- 
ular genetics to perturb that expression and then 
observing the efl'ect on immune function. A key 
issue in the functioning of the immune system is 
how the body discriminates its own tissues (self) 
from foreign components such as pathogens. An 
organism's inability to destroy self tissues is 
known as immune tolerance. Tolerance is gener- 
ally believed to occur during the production of 
new T cells in the thymus by a process of clonal 
deletion; that is, self-reactive T cells are de- 
stroyed at the site of synthesis. 
We have been interested in determining the 
mechanisms of tolerance to those components of 
the body that are never found in the thymus. 
Transgenic mice can be used to study this pro- 
cess, since the expression of a given gene — and 
hence the protein encoded by that gene — can be 
directed to the tissue of choice by linking the 
gene for the desired protein to the regulatory sig- 
nals that function in that specific tissue. We have 
previously performed such experiments by di- 
recting the synthesis of MHC class II proteins to 
the pancreatic ^ cells of transgenic mice. In these 
experiments the mice were indeed found to be 
tolerant to the MHC antigens, even though they 
appeared to be expressed specifically at these 
sites very distant from the thymus. Interestingly, 
it appears that T cells that would normally react 
with this MHC product are not eliminated, which 
is what would be found if the class II antigen is 
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