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 k>is B.Sc. 
and Ph.D. degrees in biochemistry from the University of Hull, England, and performed postdoctoral work 
in Amsterdam and Zurich. Before accepting his current position, Dr. Flavell was first Assistant Professor 
at the University of Amsterdam, then Head of the Laboratory of Gene Structure and Expression at the 
National Institute for Medical Research, Mill Hill, London, and subsequently President and Chief Scientific 
Officer of Bio gen Research Corporation, Cambridge, Massachusetts. Dr. Flavell is a Fellow of the Royal 
Society and a member of other distinguished societies. 
MY laboratory has concerned itself for many 
years with the regulation and function in 
the immune system of the genes of the murine 
MHC (major histocompatibility complex). In the 
mouse these genes are encoded on chromosome 
17, and prior work has shown that there are a 
large number of linked class 1-related genes and 
a handful of class II genes. Class I genes encode a 
protein of approximately 45,000 molecular 
weight that is found in association with a small 
subunit, i82"r"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 /?, 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 intra- 
cellular pathogens that are presented, this time to 
helper or inflammatory T cells that carry the CD4 
co-receptor. Both types of T cells secrete hor- 
mone-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. 
Class I and class II genes are both regulated in 
vivo by various lymphokines. Of these, the inter- 
ferons are important and are currently utilized in 
the therapy of several human diseases. For exam- 
ple, interferon-7 secreted by activated T cells 
stimulates the synthesis of MHC class 1 and II mol- 
ecules and, as a result, presumably renders a cell 
better able to present antigen and thus to poten- 
tiate an immune response. We have taken a ge- 
netic approach to attempt to understand how in- 
terferon-7 activates the synthesis of these MHC 
molecules. A new strategy was used to isolate a 
series of mutant cell lines that are not capable of 
responding to interferon-7. These mutants ap- 
pear to have a series of different defects. In some 
of these cell lines, the mutations result in a com- 
plete loss of the ability of all genes to respond to 
interferon-7 and even, surprisingly, interferon-a 
and which were previously believed to use 
distinct mechanisms. Other cell lines are defec- 
tive only in the response of the class II and related 
genes. This genetic approach should help us un- 
derstand how these important molecules regulate 
gene expression by elucidating the molecular 
steps that the cell utilizes to activate genes 
through interferons. 
One important issue in the functioning of the 
immune system is how the body discriminates its 
own tissues (self) from foreign components such 
as pathogens. The process that protects the indi- 
vidual against the destruction of self tissues is 
known as immune tolerance. Tolerance is gener- 
ally believed to be established during the produc- 
tion of new T cells in the thymus by a process 
called negative selection, which is mediated by 
clonal deletion; that is, self-reactive T cells are 
destroyed at the site of synthesis. The failure of 
self-tolerance leads to the autoimmunity that 
characterizes human diseases such as autoim- 
mune diabetes (insulin-dependent diabetes mel- 
litus [IDDM]) and rheumatoid arthritis. 
In the past few years we have been interested in 
determining the mechanisms of tolerance to 
those components of the body that are never 
found in the thymus and therefore pose a prob- 
lem for tolerance mechanisms operating in the 
thymus. Transgenic mice can be used to study 
this process, 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 
signals that function in that specific tissue. We 
have previously performed such experiments by 
directing the synthesis of MHC class II proteins to 
the pancreatic jS-cells of transgenic mice. In these 
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