Antigenic specificity of two antibodies directed against the thymic hormone serum thymic factor (FTS)

Serum thymic factor (facteur thymique serique, FTS) induces in vitro differentiation of T-cell precursors into more mature cells with T-cell characteristics. As isolated from porcine serum, FTS is the nonapeptide GIp-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn. We have described two radioimmunoassays that detect FTS but not other thymic hormones [Ohga et al., (1982) J. Immun. Methods, in press]. One assay is based on a monoclonal antibody from a hybridoma derived by fusion of mouse myeloma cells and spleen cells from a mouse immunized with an FTS-mouse IgG conjugate. The second assay is based on the antiserum from a rabbit immunized with FTS bound to F(ab')2 fragments of rabbit IgG. The detailed antigenic specificity of these anti-FTS antibodies was determined by measuring the ability of FTS and 12 synthetic FTS peptide analogues to compete with a radioiodinated FTS analogue in these radioimmunoassays. The mouse monoclonal antibody and the rabbit antiserum showed similar structural requirements for binding of the FTS peptides. Since FTS had been attached to the carrier proteins through the ϵ-amino group of Lys-3, both antibodies were relatively insensitive to omission of 1 or 2 N-terminal residues, replacement of Glp-1 with either Ala or Tyr-Ala, or substitution of Lys-3 with Ala. In contrast, binding of FTS to the antibodies was substantially decreased by omission of 3 or 4 N-terminal residues, omission of 2 or 3 C-terminal residues, or replacement of Gly-7 or Asn-9 with Ala. Relative to the mouse monoclonal antibody, the rabbit antiserum was more sensitive to the omission of 4 N-terminal residues and much more sensitive to replacement of Gln-5, Gly-6, or Asn-9 by Ala. Both antibodies were relatively specific for molecules ending in -Xxx-Xxx-Gly-Gly-Ser-Asn-OH, which corresponds to the biologically active region of FTS.