GENETICS, STRUCTURE, AND FUNCTION OF HISTOCOMPATIBILITY ANTIGENS 
KiRSTEN FiscHER-LiNDAHL, PH.D., Investigator 
Histocompatibility (H) antigens are traditionally 
divided into major and minor antigens, depending 
on the strength of the immune response they pro- 
voke. The major H antigens are encoded in a single 
gene complex, the major histocompatibility com- 
plex (MHC), whereas minor H antigens are en- 
coded by every chromosome. Work in Dr. Fischer- 
Lindahl's laboratory on an unusual H antigen, 
maternally transmitted antigen (Mta), has led to a 
unified model of H antigens: they are all variations 
on the theme of an MHC class I or class II protein 
presenting a peptidic ligand. An allelic difference 
in the MHC protein can be a major H antigen, 
whereas an allelic difference in one of the peptidic 
ligands is a minor H antigen. 
I. Mitochondrial Minor H Antigen. 
A. Processing of maternally transmitted factor 
(MTF). Mta is a complex of MTF, an MHC class I 
molecule (Hmt), and P^'^'^i'oslobulin. Last year 
the 5' end of the mitochondrial NDl gene was 
identified as the M//" gene. A synthetic peptide cor- 
responding to the first 17 amino acids of NDl can 
be added to cells and will be recognized by 
cytotoxic T lymphocytes as if it were the mitochon- 
drial gene product. The antigenic difference is due 
to allelic variation in the sixth residue, which can 
be isoleucine, alanine, valine, or threonine. A maxi- 
mal response is obtained with the peptide at a con- 
centration of 100 nM or more, which is very similar 
to observations made with viral peptide antigens. 
Cells must be incubated with the peptide for at 
least 5 h for optimal presentation of the antigen. 
The uptake and presentation of the peptide can be 
accelerated by inducing pinocytosis. All suitable tar- 
get cells are capable of presenting the peptide, sug- 
gesting that no specialized antigen-processing 
mechanism is required. Cells treated with the NDl 
peptide compete fully with target cells with the nat- 
ural mitochondrial antigen, showing that only 1 of 
the 15-40 allelic amino acid differences in mito- 
chondrial proteins is recognized as a minor H anti- 
gen in the Mta system. 
B. Absence of the thymus leukemia antigen (TL) on 
mitochondria. The mitochondrial peptide MTF is 
presented by Hmt, which is encoded by a gene 
closely linked to Tla, another MHC class I gene en- 
coding the thymocyte cell surface antigen TL. With 
gold-tagged monoclonal antibodies and frozen sec- 
tions of intact cells, reports of association of TL 
with isolated mitochondria were reexamined and 
discarded as based on contamination of the mito- 
chondria with plasma membrane during the isola- 
tion. In the sections of intact cells, the plasma 
membrane was well labeled with anti-TL and the 
mitochondria with a specific marker, but no specific 
labeling of the mitochondria with anti-TL could be 
detected. The techniques that were developed for 
fixation of mitochondria with minimal damage of 
epitopes will be used to study the cellular distribu- 
tion of MTF. 
II. MHC Class I Genes in the Hmt Region. 
A. t haplotypes. The Hmt region on chromosome 
17 is limited by two recombinational breakpoints, 
R4-e and R4-1, thought to be <0.7 cM apart. Two 
other breakpoints have been useful in the subdivi- 
sion of the Hmt region: the end of the distal t inver- 
sion and the duplication. In recombinants be- 
tween two t haplotypes that differ for H-2, Hmt, 
and Tpx-1, Hmt segregates with H-2 rather than 
with Tpx-1, showing that it is included in the t in- 
version. Because the currently known MHC class I 
genes of the Hmt region all have characteristic, con- 
served restriction fragment length polymorphisms 
(RFLP) in t haplotypes, they must lie within the t in- 
version and therefore belong to the proximal end 
of the Hmt region, closest to Tla. 
B. Cloning new genes. The R4-1 haplotype carries 
the Hmt region from Mus musculus castaneus 
{Hmt^) on a chromosome 17 otherwise derived 
from the laboratory strains C3H and C3H.SW 
(Hmt^). Because of the genetic divergence of the 
mice involved, this haplotype is well suited for RFLP 
analysis. Digests of genomic DNA from the recombi- 
nant and parental strains were screened with a 
probe for the conserved exon 4 of MHC class I 
genes, and several fragments that must map to the 
Hmt region were identified and cloned from band 
libraries. The same panel of strains has also been 
used to map two previously cloned MHC class I 
genes, Thy 19-4 a.nd Mb 1, to the Hmt region. 
The extremely low level of expression of Thy 19-4, 
except in the thymus, and negative results from 
transfection experiments suggest that it is not Hmt. 
Similarly, the lack of evidence for expression oiMbl 
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