The Evolution and Biological Roles 
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, which was discov- 
ered nearly 100 years ago, was initially de- 
scribed as an activity found in serum that medi- 
ates the lysis of erythrocytes or the killing of 
foreign organisms also treated with specific im- 
mune antibodies. Later it was realized that com- 
plement also facilitates the interaction of anti- 
gen-antibody complexes with cells of the 
immune system. It is now known that comple- 
ment can bind some foreign organisms or cells 
without the need for antibody. 
The complement system consists of at least 20 
serum proteins that are activated in a cascade fash- 
ion: initial activation of a small number of early 
components leads to the enzymatic generation of 
a large number of biologically active later compo- 
nents. As part of this process, protein fragments 
are released that attract inflammatory cells, and 
antigen-antibody complexes are coated with 
specific complement fragments that covalently 
attach to this target. One of these fragments, com- 
plement component C3, is able to be proteolyti- 
cally cleaved after attachment to targets. These 
cleavage reactions result in a number of different 
conformations; this allows C3 to interact with at 
least three unique cell surface receptors, the 
complement receptors. As part of this process, C3 
fragments may also 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). CR2 interacts with the C3d,g 
fragment, which is located near the site at which 
C3 covalently binds to its target. The C3d,g frag- 
ment remains attached to targets after the rest 
of C3 is trimmed away during processing of the 
antigen-antibody-C3 complex. 
CR2 also serves as the receptor for the Epstein- 
Barr virus (EBV) , which is responsible for most 
cases of infectious mononucleosis and is causally 
associated with a number of human tumors of B 
lymphocytes and epithelial cells. Patients who 
have forms of congenital or acquired immunode- 
ficiency (such as AIDS or after organ transplanta- 
tion) are particularly susceptible to tumors asso- 
ciated with EBV. 
In the past few years we have cloned and ana- 
lyzed the structure of the human CR2 gene. Ex- 
pression of the recombinant receptor in other 
cells is sufficient to mediate the binding of EBV 
and C3d,g. By using other recombinant tech- 
niques and creating mutations within the recep- 
tor, we have shown that specific amino acids in a 
small binding domain at the amino terminus of 
the receptor are important for ligand interac- 
tions. In addition, we have synthesized peptides 
that have the ability to block binding of EBV to 
the receptor. These studies should allow us to 
devise strategies to alter the function of this re- 
ceptor in vivo. For instance, one type of reagent 
might block viral binding but not normal binding 
of C3. This could be useful in some illnesses asso- 
ciated with EBV. 
To increase our understanding of the biologi- 
cal role of complement receptors and regulatory 
proteins, we have cloned and expressed mouse 
homologues for CR2 and complement receptor 
1, another C3 receptor. In addition, we have 
cloned and expressed related genes whose com- 
plement-binding and regulatory activities are not 
fully understood. Once we understand the activi- 
ties of these proteins, we will be able to utilize 
many murine models of the normal immune re- 
sponse, as well as autoimmune diseases, to exam- 
ine the roles that these proteins play in vivo. 
Another aspect of CR2 expression is also under 
analysis in my laboratory. Expression of CR2 var- 
ies during B lymphocyte development: it is ex- 
pressed only on late pre-B cells and mature B 
cells and not on very early pre-B lymphocytes or 
on late immunoglobulin-secreting cells. The mo- 
lecular mechanisms that underlie this pheno- 
type, 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 studies we have defined a promoter 
for CR2 and other sites within the gene that are 
likely to be important in gene regulation. In addi- 
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