Molecular Mechanisms of Lymphocyte 
Differentiation 
Stephen V. Desiderio, M.D., Ph.D. — Associate Investigator 
Dr. Desiderio is also Associate Professor of Molecular Biology and Genetics at the Johns Hopkins University 
School of Medicine. He received his B.A. degree in biology and in Russian from Haverford College and his 
M.D. and Ph.D. degrees from the Johns Hopkins University School of Medicine in biochemistry and cellular 
and molecular biology. His postdoctoral fellowship was done with David Baltimore at the Massachusetts 
Institute of Technology. 
ONE remarkable feature of the immune sys- 
tem is its ability to recognize and respond to 
an extraordinarily large variety of foreign mole- 
cules, or antigens. This exquisite specificity is 
mediated through protein receptors that bind 
tightly to specific antigens. Such receptors are 
found on the surfaces of two types of immune 
cells: B cells and T cells. 
Antibody molecules, or immunoglobulins, rep- 
resent one type of antigen receptor. The site on 
the antibody molecule that binds to a specific an- 
tigen is genetically encoded by multiple, short 
segments of DNA. At the onset of development of 
the immune system, these DNA segments are lo- 
cated at separate places in the genome; during 
the maturation of antibody-producing cells (B 
cells) , segments are joined to form intact immu- 
noglobulin genes. In addition to antibody, there 
is another class of antigen receptor that is found 
on the surfaces of cells that mediate cellular im- 
munity (T cells). The T cell receptor's antigen- 
binding site, like that of the antibody molecule, is 
encoded by multiple DNA segments that are 
brought together during T cell maturation. After 
their genes are assembled, these receptor mole- 
cules are expressed at the surfaces of B and T 
cells, where specific interactions between recep- 
tor and antigen trigger division and further matu- 
ration of B and T cells. 
The total number of immunoglobulin or T cell 
receptor gene segments is large, but when any 
particular immunoglobulin or T cell receptor 
gene is assembled, only a handful of segments are 
selected and joined. As a result, many different 
combinations of segments are possible. It is this 
shuffling of small bits of DNA that generates 
much of the diversity of the immune response. 
Antibody Gene Rearrangement 
The rearrangement of antibody genes does not 
occur haphazardly during development, but is a 
well-orchestrated process in which some seg- 
ments are joined first and others later. The avail- 
able evidence suggests that this orchestration is 
accomplished by mechanisms related to those 
that regulate gene expression. Antibodies are 
made of two kinds of protein chains — heavy and 
light. The genes that encode the heavy chains are 
split into three segments — V, D, and J — that are 
brought together by two rearrangements. First a D 
segment joins to a J segment; then a V segment is 
fused to the DJ element. 
It has been known for several years that incom- 
pletely assembled antibody genes, the DJ ele- 
ments, are transcribed into RNA. We have re- 
cently worked out some of the details of this 
process. Each D segment carries a DNA sequence 
that supports initiation of transcription, a pro- 
moter sequence. The promoter is inactive, how- 
ever, unless it is near a second regulatory ele- 
ment, an enhancer. When a D segment joins to a J 
segment, the D promoter is brought near the en- 
hancer region, and transcription is initiated. The 
promoters that drive expression of completely as- 
sembled antibody genes — the promoters that lie 
upstream of V segments — are activated in much 
the same way, but the V and D promoters are 
structurally quite dissimilar. Thus, at the level of 
DNA-protein interactions, the D promoters are 
likely to be regulated differently from the V pro- 
moters. An understanding of these differences 
will shed light on regulatory mechanisms that are 
in force during the earliest stages of B cell 
development. 
Although specific DNA rearrangements play a 
central role in the development of the immune 
system, we know little about the mechanics of 
this process. We have taken two approaches to 
the problem. One has been to analyze the prod- 
ucts of rearrangement and the DNA sequences 
that mediate the reaction. The second has been to 
determine how these DNA sequences are recog- 
nized by the machinery that carries out 
recombination. 
To facilitate our study of antibody gene rear- 
rangement, we have made artificial DNA mole- 
cules that contain unrecombined antibody gene 
segments. By simple tests, we can detect rear- 
rangements of these artificial substrates after they 
are introduced into immature B cells. By follow- 
ing the fates of these engineered molecules we 
have been able to infer some general features of 
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