Summary
The binding of radiolabeled monoclonal antifibrin antibody 59D8 (specific for fibrin
but not fibrinogen) to a series of degraded fibrin clots showed that the availability
of the Bβ(15-21) epitope (against which 59D8 had been raised) was inversely proportional
to the extent of clot lysis. Examination of digest supernatants revealed that the
Bβ(15-21) epitope was released from clots as a high molecular weight degradation product
in the presence of calcium ions but that the generation of low molecular weight peptides
occurred in the absence of calcium ions. To address the question of epitope accessibility,
we compared levels of fibrin clot binding among four radioactively labeled antibodies:
antifibrin monoclonal antibody 59D8, two antifibrinogen monoclonal antibodies that
cross-reacted with fibrin, and an affinity-purified polyclonal antifibrinogen antibody.
We expected that the antifibrinogen antibodies would show enhanced binding to clots
in comparison with the antifibrin antibody. However, the epitope accessibility experiments
showed that all four antibody preparations bound fibrin clots at comparable levels.
Taken together, these studies demonstrated that one fibrin-specific epitope, Bβ(15-21),
remains available on clots as they undergo degradation by plasmin and, importantly,
that the epitope is not solubilized at a rate faster than the rate at which the clot
is itself solubilized. The availability of the Bβ(15-21) epitope during the course
of plasminolysis assures the potential utility of antifibrin antibodies such as 59D8
for detecting thrombi and targeting plasminogen activators.
