Purified populations of human monocytes (greater than 99% monocytic) contain a subpopulation of cells (8-20%) that will surround themselves with a radial array of needles within 15 s after exposure to plasma. The needles are composed of fibrin as demonstrated by gel electrophoresis of isolated needles and by their reaction with a monoclonal antibody, UC45, that is specific for the alpha chain of fibrin. The addition of UC45 antibody to plasma inhibits the formation of the fibrin needles by monocytes, suggesting that the recognized antigen is important for polymerization. In contrast, UC45 binds weakly to conventional plasma fibrin and does not prevent clotting of plasma although the process is prolonged. Thus, the UC45 antigen is more prominently represented or more accessible on monocyte fibrin. At least 1% of the monocytes were able to form fibers at the earliest time that they could be tested. A greater proportion developed this ability between 2 and 6 h in culture. Macrophages from lung, thymus, and breast milk, when incubated for similar periods of time, do not form fibrin needles. However, phagocytes from tonsils were extremely active in this respect. It is speculated that monocytes may be induced to express fibrin as part of their activities in inflammatory lesions.

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