Biological Roles and Expression 
of Complement Receptors 
V. Michael Holers, M.D. — Assistant Investigator 
Dr. Holers is also Assistant Professor of Medicine and Pathology at the Washington University School 
of Medicine and Assistant Physician at Barnes Hospital, St. Louis. He received his undergraduate degree 
from Purdue University and his M.D. degree from Washington University. He did postdoctoral research 
at the University of Colorado, Denver, and then at Washington University. 
THE complement system was initially de- 
scribed as an activity found in serum that 
could mediate the killing of foreign infectious 
organisms such as bacteria or viruses. This sys- 
tem, in conjunction with specific immunity gen- 
erated by vaccination, was found to be critical to 
preventing and fighting infections. Later it was 
realized that complement also facilitates the in- 
teraction of antigen-antibody complexes with 
cells of the immune system, which greatly en- 
hances the specific immune response. Therefore 
complement not only helps to initiate an immune 
response but also plays an important role in the 
ability to clear infections and foreign antigens 
from the body. 
The complement system consists of at least 20 
serum proteins that are activated in a cascade fash- 
ion. As part of the activation process, protein 
fragments are released that attract inflammatory 
cells. In addition, antigen-antibody complexes 
are coated with specific complement fragments 
that covalently attach to this target. One of these 
fragments, complement component C3, is able to 
be cleaved proteolytically after attachment to a 
target. This cleavage reaction results in a number 
of different conformations of C3, which allow it 
to interact with at least three unique cell surface 
receptors. After binding to cells, specific trans- 
membrane signals are sent, and the complement 
receptors then mediate ingestion and processing 
or killing of the targets. Also as part of this pro- 
cess, C3 fragments may bind to self tissues, rather 
than to the antibody-bound target, thereby attack- 
ing at inappropriate sites. Other cell membrane 
C3-binding proteins are able to inactivate this C3 
and prevent inappropriate damage to self tissues. 
We are interested in the interaction of C3 with 
its specific receptors and regulatory proteins, par- 
ticularly the biological aspects of complement 
receptor 2 (CR2). In addition, we are studying 
mouse homologues of CR2 and other proteins of 
this type. Human CR2 serves as the receptor for 
the Epstein-Barr virus (EBV), which is responsi- 
ble for most cases of infectious mononucleosis 
and is causally associated with a number of hu- 
man tumors of B lymphocytes and epithelial 
cells. Patients who have forms of congenital or 
acquired immunodeficiency (such as AIDS or 
after organ transplantation) are particularly sus- 
ceptible to tumors associated with EBV. 
In the past few years we have cloned and ana- 
lyzed the structure and activities of human CR2, 
its mouse homologues, and a unique mouse pro- 
tein called Crry/p65. We have shown that ex- 
pression of recombinant forms of these proteins 
in other cells is sufficient to mediate the binding 
of ligands or to control inappropriate comple- 
ment deposition. By using other recombinant 
techniques and creating mutations within these 
proteins, we have shown that specific amino 
acids in small domains are important for ligand 
interactions. In addition, we have synthesized 
peptides that have the ability to block binding of 
some ligands, in particular, EBV binding to CR2. 
These studies should allow us to devise strategies 
to alter the function of this receptor in vivo. For 
instance, one type of reagent might block EBV 
binding to CR2 but not normal binding of C3. 
This could be useful in some illnesses associated 
with EBV. 
We have analyzed the murine homologues of 
these proteins to understand further the biologi- 
cal role of human CR2, in addition to other com- 
plement receptors and regulatory proteins. Once 
the activities of these proteins are understood, we 
should be able to utilize murine models of the 
normal immune response, as well as autoimmune 
diseases, to elucidate the in vivo roles of these 
proteins. 
Another aspect of CR2 expression is also under 
analysis. Expression of CR2 varies during human 
B lymphocyte development: it is expressed only 
on late pre-B cells and mature B cells and not on 
very early pre-B lymphocytes or on late immuno- 
globulin-secreting cells. The molecular mecha- 
nisms that underlie this phenotype, which is also 
found among other B cell-specific markers, are 
likely fundamental to the overall processes by 
which B cells mature and are activated. We are 
analyzing these mechanisms. As part of these stud- 
ies we have defined a promoter for CR2 and other 
sites within the gene that are likely to be impor- 
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