Genetics, Structure, and Function of 
Histocompatibility Antigens 
Kirsten Fischer Lindahl, Ph.D. — Investigator 
Dr. Fischer Lindahl is also Associate Professor of Microbiology and Biochemistry at the University of Texas 
Southwestern Medical Center at Dallas. She began the study of histocompatibility with Morten Simonsen 
in Copenhagen, Denmark, and received her Ph.D. degree in immunobiology from the University of Wis- 
consin-Madison. She was a postdoctoral fellow with Darcy Wilson at the University of Pennsylvania, 
Philadelphia, and Klaus Rajewsky at the Institute for Genetics in Cologne, West Germany. Before accepting 
her current position, she was a member of the Basel Institute of Immunology in Switzerland. 
HISTOCOMPATIBILITY antigens are cell sur- 
face molecules that, when foreign, lead to 
the rejection of grafted tissues and organs by the 
vertebrate immune system. Because they form a 
major obstacle to clinical transplantation, H anti- 
gens have been studied for over 50 years. They 
are complexes of a small peptide ligand and an 
MHC molecule (encoded by genes of the major 
histocompatibility complex). A given individual 
has MHC molecules of a few different kinds, each 
of which can present to the immune system a 
large variety of peptides on the surface of cells. 
These peptides might be derived from proteins 
produced by intracellular parasites, bacteria, or 
viruses or by the body's own cells, such as tumor- 
specific or minor H antigens. The amino acid side 
chains that line the peptide-binding groove of an 
MHC molecule determine which peptides that 
molecule can bind, and therefore what antigens 
can be presented to induce an immune response 
in the individual with this MHC. 
The immune system is capable of recognizing a 
difference in either of the H antigens' two parts. A 
difference in the MHC molecule itself will alter 
many complexes and induce a strong immune re- 
sponse, hence the term "major" H antigen. By 
contrast, a difference in a peptide alters only one 
of many kinds of complexes and induces a weaker 
response, hence the term "minor" H antigen. Un- 
like the major H antigens, human minor H anti- 
gens remain ill-defined. In the mouse, however, 
more than 50 genes that encode minor H antigens 
have been mapped, and almost every chromo- 
some, including the mitochondrial genome, car- 
ries at least one. 
The Maternally Transmitted Antigen 
The sixth amino acid in the mouse mitochon- 
drial protein NDl is polymorphic. When cells 
with one form are transplanted to a mouse with 
another form, the amino-terminal peptide of NDl 
will act as a transplantation antigen, called Mta. 
This peptide is presented on the cell surface by 
an MHC class I molecule called M3. Mta was the 
first minor H antigen to be analyzed in molecular 
detail. It has turned out to be an excellent model, 
although discovered by virtue of its unusual fea- 
tures of mitochondrial, hence maternal, inheri- 
tance and its presentation by a novel, highly con- 
served MHC molecule. 
M3 only binds the NDl peptide when the me- 
thionine at the end carries a formyl group, and 
M3 can also bind other peptides with a formyl- 
methionine. This is characteristic of the amino 
terminus of mitochondrial and bacterial proteins, 
and distinguishes them from proteins made in the 
cytoplasm of mammalian cells. It is of great inter- 
est to understand how the formyl-methionine 
peptides are bound by M3 and which amino acids 
in M3 are important for this specificity. 
We have now cloned the gene M3 from mice, 
and considerable effort has been spent during the 
past year on various systems for deriving M3 in 
amounts sufficient for a structural analysis. We 
can express the M3 gene in insect cells, which 
make milligrams of the protein in a single culture 
flask. However, the protein is not expressed on 
the surface of these cells, probably because it is 
folded incorrectly, and is detectable only with 
rabbit antisera against short fragments of it. The 
heavy chain of MHC class I molecules is known to 
be unstable in the properly folded conformation 
at body temperature unless binding the ^2'^^' 
croglobulin light chain as well as a peptide. We 
are now trying to achieve correct folding by mix- 
ing the M3 heavy chain from a cell extract with 
(82-microglobulin light chains and Mta. 
We can express M3 in mouse fibroblasts, where 
Mta can be detected by killer T lymphocytes. The 
M3 protein is therefore folded correctly, but in 
amounts too small for biochemical analysis. This 
system does allow us, however, to change single 
amino acids in the protein by introducing muta- 
tions at specific sites in the gene. By testing 
whether killer T cells still recognize the mutant 
protein, we can assay whether it presents Mta. 
From sequencing naturally occurring variants of 
the M3 gene from wild mice, we are learning 
more about which amino acids are essential for 
the ability to present the peptide. 
133 
