Molecular Mechanisms That Regulate 
B Cell Development 
Michel C. Nussenzweig, M.D., Ph.D. — Assistant Investigator 
Dr. Nussenzweig is also Assistant Professor and Head of Laboratory at the Rockefeller University. He 
received his undergraduate and M.D. degrees from New York University and his Ph.D. degree from 
Rockefeller. He completed his residency and clinical fellowship at Massachusetts General Hospital 
and conducted postdoctoral research at Harvard Medical School with Philip Leder. 
WE are interested in understanding the mo- 
lecular mechanisms that regulate B lym- 
phocyte activation and differentiation. Our ap- 
proach has been to focus on one important 
transition in B lymphocytes, allelic exclusion. 
The immune system is responsible for protect- 
ing vertebrates from both invasion by infectious 
organisms and deregulated growth of endoge- 
nous malignancies. To accomplish this task, the 
immune system must be able to distinguish self 
from non-self. Evolution has solved this problem 
in higher vertebrates by providing a network of 
cell types and humoral agents. It is the lympho- 
cytes — T cells and B cells — that direct the speci- 
ficity of immune responses. 
Although the mechanism of antigenic recogni- 
tion differs for these two cell types, the genera- 
tion of receptor diversity is achieved in a similar 
fashion. In both cases the business end of the re- 
ceptor is created in individual somatic cells by a 
series of genetic recombinations at a minimum of 
two loci. For example, immunoglobulin mole- 
cules that serve as the B cell receptor are com- 
posed of two sets of rearranging genes that en- 
code the heavy- and light-chain immunoglobulin 
proteins. Furthermore, the same genes that en- 
code the B lymphocyte receptor also direct the 
production of secreted antibodies that are an 
important component of humoral immune re- 
sponses. Thus the regulation of T cell and B cell 
receptor rearrangements is a central feature of 
the generation of immune responses. 
The joining events that bring together the im- 
munoglobulin segments occur in an ordered and 
regulated fashion. In B lymphocytes, rearrange- 
ments begin at the heavy-chain locus with the re- 
combination of D and J segments. This is followed 
by joining of DJ with one of 100-1 ,000 variable- 
region segments. After a functional immunoglob- 
ulin heavy-chain transcription unit is created, the 
light-chain genes undergo a similar set of 
rearrangements . 
One poorly understood aspect of these events 
is the ability of lymphocytes to limit themselves 
to the production of a single receptor. Since pro- 
ductive rearrangements could occur in two 
heavy-chain and four light-chain alleles, a single 
B lymphocyte could potentially make several 
types of receptors, including hybrid molecules. 
The mechanism that ensures that only one recep- 
tor is produced is referred to as allelic exclusion. 
It is an important safeguard for the immune sys- 
tem, since production of multiple receptors by a 
lymphocyte would dilute the specificity of any 
given immune response. 
Much of the early work in the area of allelic 
exclusion was based on examining the status of 
immunoglobulin genes in transformed B cells. 
The transformed cells are frozen in one stage of 
lymphocyte development and for this reason 
offer only a static picture of important regulatory 
events. Unfortunately there is no in vitro system 
that faithfully reproduces regulated immuno- 
globulin gene rearrangements. To study how im- 
munoglobulin genes can regulate allelic exclu- 
sion, we turned to transgenic mice. 
Our approach has been to introduce into the 
germline of mice human immunoglobulin genes 
that have been modified to direct the synthesis of 
either membrane-associated or secreted immuno- 
globulin heavy chains. We found that the expres- 
sion of human membrane-bound immunoglobu- 
lin M (IgM) results in the exclusion of most 
endogenous mouse immunoglobulins. The se- 
creted version of the same transgene has little 
effect. 
This initial observation raises two important 
questions. First, how does membrane immuno- 
globulin signal? Second, how is exclusion regu- 
lated at the genetic level? During the past year we 
have made significant progress toward resolving 
both questions. To understand signaling by the 
membrane-associated immunoglobulin mole- 
cule, we have developed a system for complete 
functional reconstitution of the immunoglobulin 
receptor from cloned components in heterolo- 
gous cells. Transport of IgM to the surface of T 
cells requires coexpression of the immunoglobu- 
lin heavy and light chains with B29, an immuno- 
globulin-associated polypeptide. In addition, the 
transfected receptor is fully active in the pres- 
ence of B29. MBI , a second IgM-associated poly- 
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