Radioimmunoassay of serum thymic factor (FTS)

We established a radioimmunoassay (RIA) method which enables a quantitative estimation of serum thymic factor (FTS). This assay is based on the displacement of 125I-(Lys[Tyr] 3)-FTS bound to the anti-FTS antibodies by FTS. Formaldehyde-fixed Staphylococcus aureus Cowan I cells were used to precipitate the antigen-antibody complex. The anti-FTS antiserum was raised in rabbit by repeated injections of FTS-conjugated bovine serum albumin. The antiserum seemed to recognize the carboxy-terminal seven amino acid residues, Lys-Ser-Gln-Gly-Gly-Ser-Asn-OH, of the FTS molecule. As little as 0.5 to 1 pg FTS can be detected by this method. To measure the FTS content in the serum, a serum sample was pretreated with 10% trichloroacetic acid and then with ethanol for removal of serum proteins. The FTS content in pig serum was around 22 pg/ml, and that in BALB/c mouse serum was 1.3 pg/ml. The FTS contents in the sera of DBA/2, C57BL/6 and (C57BL/6 X DBA/2)F1 mice were also less than 5 pg/ml. Since more than 70% of FTS exogenously added to pig or mouse serum was recovered, the mouse serum probably contains only a small amount of FTS.