Summary
The studies reported here probe the existence of a receptor-mediated mode of fibrin-binding
by macrophages that is associated with the chemical change underlying the fibrinogen-fibrin
conversion (the release of fibrinopeptides from the amino-terminal domain) without
depending on fibrin-aggregation. The question is pursued by 1) characterization of
binding in relation to fibrinopeptide content of both the intact protein and the CNBr-fragment
comprising the amino-terminal domain known as the NDSK of the protein, 2) tests of
competition for binding sites, and 3) photo-affinity labeling of macrophage surface
proteins. The binding of intact monomers of types lacking either fibrinopeptide A
alone (α-fibrin) or both fibrinopeptides A and B (αβ-fibrin) by peritoneal macrophages
is characterized as proceeding through both a fibrin-specific low density/high affinity
(BMAX ≃ 200–800 molecules/cell, KD ≃ 10−12 M) interaction that is not duplicated with fibrinogen, and a non-specific high density/low
affinity (BMAX ≥ 105 molecules/cell, KD ≥ 10−6 M) interaction equivalent to the weak binding of fibrinogen. Similar binding characteristics
are displayed by monocyte/macrophage cell lines (J774A.1 and U937) as well as peritoneal
macrophages towards the NDSK preparations of these proteins, except for a slightly
weaker (KD ≃ 10−10 M) high-affinity binding. The high affinity binding of intact monomer is inhibitable
by fibrin-NDSK, but not fibrinogen-NDSK. This binding appears principally dependent
on release of fibrinopeptide-A, because a species of fibrin (β-fibrin) lacking fibrinopeptide-B
alone undergoes only weak binding similar to that of fibrinogen. Synthetic Gly-Pro-Arg
and Gly-His-Arg-Pro corresponding to the N-termini of to the α- and the β-chains of
fibrin both inhibit the high affinity binding of the fibrin-NDSKs, and the cell-adhesion
peptide Arg-Gly-Asp does not. Photoaffinity-labeling experiments indicate that polypeptides
with elec-trophoretically estimated masses of 124 and 187 kDa are the principal membrane
components associated with specifically bound fibrin-NDSK. The binding could not be
up-regulated with either phorbol myristyl acetate, interferon gamma or ADP, but was
abolished by EDTA and by lipopolysaccharide. Because of the low BMAX, it is suggested that the high-affinity mode of binding characterized here would
be too limited to function by itself in scavenging much fibrin, but may act cooperatively
with other, less limited modes of fibrin binding.
