LYMPHOCYTE SURFACE ANTIGENS IN HEALTH AND DISEASE 
Bei^amin D. Schwartz, M.D., Ph.D., Investigator 
The major histocompatibility complex class II 
molecules are crucial to both normal immune re- 
sponses and abnormal immune responses leading 
to the development of autoimmune disease. These 
molecules are highly polymorphic, and it is this 
polymorphism that is ultimately responsible for 
conferring both specific immune responsiveness 
and disease predisposition. The class II molecules 
consist of an a-chain glycoprotein and a (i-chain gly- 
coprotein; both are encoded by genes contained 
within the major histocompatibility complex. The 
proposed structure of the class II molecule suggests 
that the al domain of the a-chain and the pi do- 
main of the P-chain form a structure that consists of 
eight p strands forming a P pleated sheet, on which 
rest two a-helices that form a groove or cleft, which 
binds the antigenic peptide. This complex of the 
antigenic peptide and the class II molecule is the 
ligand that is recognized by the antigen-specific re- 
ceptor on T lymphocytes. 
The class II molecules have a limited tissue distri- 
bution and are expressed predominantly on immu- 
nocompetent cells, including B cells, antigen-pre- 
senting cells, and (in humans) activated T cells. 
However, class II expression can also be induced 
on a variety of other cells on which expression is 
normally absent. Dr. Schwartz and his colleagues 
have examined the regulation of class II gene ex- 
pression, the binding of antigenic peptides by class 
II molecules, and the presentation of antigenic pep- 
tide to T cells. 
I. HLA Class II Gene Regulation. 
Last year Dr. Schwartz's laboratory reported the 
development of the Southwestern methodology to 
identify and clone the cDNAs that encode DNA- 
binding proteins. One such protein binds to a 48 
bp oligomer, the X-Y box. The X-Y box corresponds 
to a portion of the class II gene promoter region 
that spans two highly conserved sequence motifs, 
the X box and Y box, and the spacer region be- 
tween them, and has been shown to be essential 
for class II gene expression. The protein binding to 
the X-Y box, YB-1, was shown to be specific for the 
inverted CCAAT box sequence in the Y box. 
Analysis of class Il-positive and -negative cell 
lines indicated that although YB-1 is ubiquitous, 
YB-1 mRNA levels vary inversely with class II mRNA 
levels. This inverse correlation was supported by 
the observation that interferon-7-induced class II 
expression on class Il-negative U937 cells is accom- 
panied by a fall in YB-1 levels. These results suggest 
that YB-1 is a repressor factor. This laboratory has 
also cloned and partially sequenced a murine ho- 
mologue of YB-1 that is similar to human YB-1. 
Because of a report that some CCAAT box-bind- 
ing proteins are heterodimers, the possibility that 
YB-1 might be one subunit of a heterodimeric pro- 
tein was explored. A Xgtll cDNA library prepared 
from the DR5 B lymphoblastoid cell line Swei was 
probed with the Y box to which the YB-1 protein 
had been bound. A second cDNA was isolated and 
shown to encode a protein that binds to the YB- 
1-Y box complex. In the absence of YB-1, this pro- 
tein, YB-2, did not bind to the Y box itself but did 
bind weakly to the X-Y box. In the presence of 
YB-1, YB-2 bound strongly to both the X-Y box and 
the Y box. Levels of YB-2 mRNA also inversely cor- 
related with class II mRNA levels. Restriction map- 
ping. Southern analysis, and DNA sequencing con- 
firmed that YB-2 was distinct from YB-1. Gel 
retardation studies in which the interaction of YB-1 
and YB-2 was studied suggested that YB-2 interacts 
with YB-1 to prevent it from binding to the X-Y box. 
A study of the proteins within various nuclear ex- 
tracts that interact with the class II gene promoter 
regions has also been initiated. Gel retardation 
analysis of DNA-binding proteins within these nu- 
clear extracts suggests that the binding patterns ob- 
served are similar in class Il-negative and class II- 
positive cells. In addition, class Il-negative cells 
appear to have higher levels of DNA-binding pro- 
teins and/or higher affinity binding proteins than 
do class Il-positive cells. These results are reminis- 
cent of the higher levels of YB-1 and YB-2 mRNAs 
observed in class Il-negative cells compared with 
class Il-positive cells and support the notion that, 
in the class II system, high levels of certain transact- 
ing factors act to repress class II transcription. 
II. Interaction of Class II Molecules and Antigenic 
Peptides. 
In collaboration with Drs. Immanuel Luescher, 
Dan Crimmins, and Emil Unanue, Dr. Schwartz has 
been studying the interaction of class II molecules 
and antigenic peptides. In the mouse system, the 
I-A'' molecule is known to bind the hen egg white 
lysozyme peptide (HEL) 46-61. Conjugates of this 
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