Detection of Genetic Recombination in Germ Cells 
Jan Geliebter, Ph.D. — Assistant Investigator 
Dr. Geliebter is also Assistant Professor and University Fellow at the Rockefeller University. He received his 
Ph.D. degree in microbiology and immunology from the State University of New York, Downstate Medical 
Center. He was a postdoctoral fellow and research associate in the laboratory of Stanley Nathenson at the 
Albert Einstein College of Medicine, Bronx, New York. 
THE immune system functions to rid the body 
of foreign objects such as bacteria, viruses, 
tumors, and transplants. The portion of such mat- 
ter that is recognized as foreign by the immune 
system is called an antigen. Antigens that are 
found on cells are "presented" to the immune 
system by cell surface molecules called histocom- 
patibility molecules (also called HLA molecules 
in humans and H-2 molecules in the mouse). His- 
tocompatibility molecules are able to bind anti- 
genic fragments of, for example, viruses, and 
stimulate white blood cells (lymphocytes) to at- 
tack the virus-infected cell, thereby limiting the 
spread of infection. Without these antigen- 
presenting molecules the host would be unable 
to mount an immune response against pathogens 
and would not survive. 
Different H-2 molecules can bind and present 
different types of antigens. Because inbred mice 
have about three different types of H-2 molecules 
on their cells, they can bind and present a large, 
but limited, number of antigens to the immune 
system. 
To ensure the survival of the species, it is bene- 
ficial that many varieties of H-2 molecules be 
present in the population. In this way there will 
always be some portion of the population that 
will mount an immune response to a given anti- 
gen. An extraordinary number of different histo- 
compatibility molecules have been found in al- 
most all species investigated. In humans, the 
large variety of HLA molecules ensures our sur- 
vival but is the major obstacle confounding tissue 
transplantation. Our research interest lies in the 
genetic mechanism that generates the different 
histocompatibility genes in mice and other 
species. 
The H-2 genes of the mouse are part of the 
larger major histocompatibility complex class I 
multigene family. This gene family also contains 
genes that are structurally similar to H-2 genes 
and have unknown functions. The genetic mecha- 
nism that generates variety in H-2 genes is the 
microrecombination process, which reassorts 
DNA among H-2 genes and other related class I 
genes. By substituting small segments of class I 
gene sequences into H2 genes, the microrecom- 
bination process can create new H-2 molecules 
that have different antigen-presenting capabili- 
ties, thereby expanding the immune responsive- 
ness of the population. 
Our interest is to understand better the mecha- 
nism underlying the microrecombination pro- 
cess. This process has previously been studied by 
identifying microrecombinant mice that differed 
from their otherwise identical siblings by altered 
H-2 genes. Since microrecombinant H-2 mole- 
cules elicit skin graft rejection, these studies 
were accomplished by testing thousands of mice 
with skin grafts. The rejection of a skin graft by a 
sibling mouse signaled an alteration in H-2 mole- 
cules. These labor-intensive studies found that, 
on the average, one microrecombinant mouse 
was detected for every 5,000 skin grafts 
performed. 
To gain further insight into the microrecom- 
bination process, we are using a novel approach 
to detect microrecombinant H-2 genes. Mice 
with microrecombinant H-2 genes arise from 
germ cells (sperm or egg) that contain microre- 
combinant H-2 genes formed in the gonads (ova- 
ries or testes) of normal mice. Therefore, if one 
looks in the sperm or eggs of normal mice, one 
should find microrecombinant H-2 genes. We are 
using this approach to investigate the microre- 
combination process. Since we can obtain thou- 
sands of eggs or millions of sperm from each 
mouse, we can analyze the equivalent of millions 
of mice for microrecombinant H-2 genes. We are 
using the polymerase chain reaction (PGR) to 
amplify specific H-2 genes and then clone the 
genes. Small DNA probes are being utilized to 
identify the clones containing microrecombinant 
H-2 genes. 
We are presently analyzing the microrecom- 
bination frequency in ovarian cells, as data from 
previous studies indicate that microrecombina- 
tions occur in female germ cells. We also hope to 
determine whether microrecombinations occur 
in sperm cells as well, and at what frequency. 
(Evidence from skin graft studies suggested that 
microrecombination may occur only in female 
mice.) Some strains of mice may undergo micro- 
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