Brief Definitive Report INTERACTION OF SOLUBLE FIBROBLAST SURFACE ANTIGEN WITH FIBRINOGEN AND FIBRIN

A cell-type-specific membrane glycoprotein (SFA) of chicken (1) and human (2) fibroblasts has been recently described from our laboratory . Malignantly transformed fibroblasts lack SFA on their surface (3) . The same antigen is present in serum and its molecular properties are similar to those of the cellular antigen (4). The surface labeled protein present on normal but not on transformed hamster fibroblasts (5, 6) seems to be the hamster counterpart of SFA.' The serum concentration of SFA has been estimated to be in the order of 100 ug/ml and it was originally thought to represent a previously unknown serum protein. Here we show that SFA interacts with fibrin and appears in the serum cryoglobulin fraction associated with cryofibrinogen . SFA has properties similar to those of "cold insoluble globulin" (CIG) (7-9) and the two proteins seem to be identical. Materials and Methods Reagents . Bovine fibrinogen (80% clottable) and fibrin powder were kindly provided by Dr . Gunnar Myllyla (The Finnish Red Cross Blood Transfusion Center, Helsinki). Human fibrinogen (90% clottable) was obtained from Kabi (Sweden) andwas further purified as described by Finlayson and Mosesson (10) . The purified preparation did not reveal any proteins other than fibrinogen when tested in immunodiffusion at the concentration of 1 mg/ml against antinormal human serum (Behringwerke, Marburg) . No SFA, a major contaminant of the original preparation, was detectable in the purified fibrinogen . Antisera . Production of antisera to SFA has been described (2). Human fibroblasts (MRC-5) cultured in 10% fetal calf serum were washed with buffer and treated with insoluble papain for 15 min. The material released was dialyzed, lyophilized, and used for immunization of rabbits or sheep. Such antisera were absorbed with calf serum and insolubilized papain and reacted with a single component in fibroblast extracts and human serum or plasma in immunodiffusion. Matrix-Bound Proteins . Fibrinogen was linked to Sepharose beads (Pharmacia Fine Chemicals, Inc., Sweden) according to Porath et al . (11) . Purification of CIG. Outdated human plasma taken to citrate was used as starting material and CIG was purified by cryoprecipitation followed by glycine precipitation and DEAE cellulose chromatography as described by Mosesson and Umfleet (9). Affinity Chromatography on Sepharose-Conjugated Purified HumanFibrinogen . The Sepharose