Journal of Experimental Medicine, Vol 180, 2259-2267, Copyright © 1994 by Rockefeller University Press
Further characterization of the thrombasthenia-related idiotype OG. Antiidiotype defines a novel epitope(s) shared by fibrinogen B beta chain, vitronectin, and von Willebrand factor and required for binding to beta 3
Y Gruel, E Brojer, DJ Nugent and TJ Kunicki
Roon Research Center for Arteriosclerosis and Thrombosis, Scripps Research Institute, La Jolla, California 92037.
A patient (OG) with Glanzmann thrombasthenia became refractory to platelet
transfusion after the production of an immunoglobulin G (IgG) isoantibody
(Ab1) specific for the integrin subunit beta 3. To determine the frequency
at which the OG idiotype is found in the general population and in
immune-mediated disease states, we developed a rabbit polyclonal antibody
(Ab2) specific for affinity-purified OG anti-beta 3 Fab. The binding of Ab2
to Ab1 is inhibited by purified alpha IIb beta 3. Ab2 als binds to IgG
specific for alpha IIb beta 3 obtained from one nonrelated Glanzmann
thrombasthenia patient ES who has developed isoantibodies of similar
specificity. On the other hand, Ab2 does not recognize alpha IIb beta
3-specific antibodies produced by two Glanzmann thrombasthenia patients, AF
and LUC, who have developed isoantibodies with specificities distinct from
that of the OG isoantibody. Moreover, Ab2 does not recognize alpha IIb beta
3-specific antibodies developed by three representative patients with
(autoimmune) thrombocytopenic purpura or six representative patients with
alloimmune thrombocytopenias, nor does it bind to IgG from any of 13
nonimmunized individuals. We have found that Ab2 also binds to selected
protein ligands of alpha IIb beta 3 namely, fibrinogen, vitronectin, and
von Willebrand factor, but not to other protein ligands or control
proteins, such a fibronectin, type I collagen, and albumin. The epitope(s)
recognized by Ab2 on each adhesive protein are either very similar or
identical since each protein can inhibit the binding of Ab2 to any of the
other proteins. The epitope on fibrinogen recognized by Ab2 resides in the
B beta chain, and is likely contained within the first 42 amino acids from
the NH2 terminus. Since OG IgG inhibits fibrinogen binding to alpha IIb
beta 3, the specificity of the OG idiotype defines a novel binding motif
for the integrin alpha IIb beta 3 that is shared by fibrinogen,
vitronectin, and von Willebrand factor, but distinct from previously
described RGD-containing sites on the fibrinogen, A alpha chain or the
fibrinogen gamma chain COOH-terminal decapeptide site. Our findings
reported here represent an excellent example of molecular mimicry in which
an antigen-selected, IgG inhibitor of alpha IIb beta 3 function shares a
novel recognition sequence common to three physiologic protein ligands of
that receptor.
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