Nakagawa, T.Y., Von Grafenstein, H., Sears, J.E., 
Williams, J., Janeway, C.A., Jr., and Flavell, 
R.A. 1991 ■ The use of the polymerase chain reac- 
tion to map CD4^ T cell epitopes. Eur J Immunol 
21:2851-2855. 
Sears, J.E., Fikrig, E., Nakagawa, T.Y., Deponte, K., 
Marcantonio, N., Kantor, F.S., and Flavell, R.A. 
1991- Molecular mapping of Osp-A mediated im- 
munity against Borrelia burgdorferi, the agent of 
Lyme disensc. J Immunol 147:1995-2000. 
REGULATION OF NF-kB AND IkB DURING B CELL DEVELOPMENT 
Sankar Ghosh, Ph.D., Assistant Investigator 
The process of differentiation and development of 
B cells from pluripotent stem cells serves as a useful 
experimental model to study various aspects of 
mammalian ontogeny. This process is characterized 
by regulated expression of different genes at differ- 
ent stages. Such developmentally regulated genes 
include the immunoglobulin heavy and light 
chains, A5, VpreB, B29, and mb-1. The major inter- 
est in Dr. Ghosh's laboratory is to understand how 
the expression of one of these genes, the immuno- 
globulin K light-chain gene, is regulated. 
The expression of the k gene depends on the activ- 
ity of two enhancers, one located in the intron be- 
tween and exons and the other ~ 9 kb down- 
stream of the gene. The activity profiles of the 
two enhancers overlap to a great degree during B 
cell development, although plasma cell lines in 
which the intronic enhancer is inactive are still able 
to express large amounts of k mRNA, indicating the 
importance of the 3' enhancer in later stages of B cell 
development. However, it is the intronic enhancer 
whose activity correlates exactly with the transition 
from a pre-B to a mature B cell. 
The developmental and tissue-specific activity of 
this enhancer depends on a transcription factor, NF- 
kB, which belongs to the rel family of proteins. Chro- 
mosomal translocations involving members of this 
family have been linked to the development of hu- 
man B cell lymphomas. In addition to its central role 
in the expression of the immunoglobulin k gene, 
NF-kB is also an important element in the inducible 
expression of a wide variety of cellular genes and in 
the replication of various viruses, including human 
immunodeficiency virus (HIV). An understanding 
of the molecular mechanisms that regulate the func- 
tion of NF-kB should therefore help to illuminate 
fundamental principles in oncogenesis, as well as 
normal development of B cells. 
Toward that goal. Dr. Ghosh's laboratory is focus- 
ing on IkB, an inhibitory protein that regulates the 
activity of NF-/cB. In pre-B cells the ubiquitous NF- 
/(B is present in an inactive form, being bound to I/cB. 
However, in mature B and plasma cells, NF-kB is a 
constitutively active, nuclear protein. Therefore the 
change of NF-kB from an inducible cytosolic protein 
to an active, nuclear protein is linked intimately to 
the progression of B cell development. The recent 
cloning of the genes encoding the subunits of NF-/cB 
(p50 and p65) and kB finally allows a detailed anal- 
ysis of the regulation of NF-kB activity in B cells. 
An important question that remains to be an- 
swered is. What causes the constitutive activation of 
NF-/cB in mature B cells? Using Northern and West- 
ern analyses, Dr. Ghosh's laboratory has found that 
I/cB mRNA and protein continue to be made in ma- 
ture B and plasma cells, yet the protein in these cells 
cannot bind to NF-kB. One possible explanation is 
that I/cB in these cells is post-translationally modi- 
fied and inactivated. Alternatively, the IkB in mature 
B and plasma cells is sequestered by binding to a 
new protein whose synthesis is regulated during de- 
velopment. Experiments are now in progress to de- 
termine if either of these or yet some other mecha- 
nism is responsible for regulating IkB activity in 
later stages of B cell development. 
IkB, like NF-kB, is also a member of a family of 
proteins that share a common structural motif 
known as ankyrin repeats. At present three different 
IkBs have been cloned — I/cB-a, I/cB-7, and the proto- 
oncogene bcl-3; another form, I/(B-/3, is yet to be 
cloned. The major form, I/cB-a, is not regulated 
through transcription in B cells (see above), but the 
expression of UB-y (an alternatively spliced version 
of the p50 NF-kB precursor) is regulated at the level 
of mRNA. Both pro- and pre-B cells have significant 
amounts of the I/cB-7 message, while mature B and 
plasma cells have almost none. 
Unlike UB-a, which interacts specifically with 
the p50:p65 NF-kB heterodimer, IkB-7 appears to 
target the p50 dimer. It is not clear what role the 
p50 dimer, and by extension I/cB-7, play during B 
cell differentiation. Because I/cB-7 levels decrease 
upon differentiation, it is likely that IkB-7 is impor- 
tant to maintain an undifferentiated state during ear- 
lier stages of development. Therefore inhibition of 
IkB-7 expression in pre-B cells may lead to differen- 
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