Direct construction of an open-sandwich enzyme immunoassay for one-step noncompetitive detection of thyroid hormone T4

To establish a sensitive noncompetitive immunoassay for thyroxine (T4), we attempted to isolate anti-T4 antibodies from a phage display library based on a phagemid pDongl (Dong et al. Anal. Biochem. 2009, 36, 386), which was designed to enable open-sandwich enzyme-linked immunosorbent assay (OS-ELISA) after selection on immobilized antigen. After the Fab-displaying phage library made from the splenocytes of T4-KLH immunized mice was subjected to biopanning on T4-BSA, two T4-specific clones were obtained. When they were assayed by indirect competitive ELISA, both clones showed low IC₅₀ (5-13 ng/mL), indicating their high affinity to T4. When they were used for OS-ELISA that detects antigen-dependency of the interaction between variable domains V _H and V_L, a clone successfully detected 1 ng/mL of T4 with a working range superior to that of competitive IA OS-ELISA was also performed with maltose binding protein (MBP)-fused V_H/V_L of this clone, which showed a detection limit less than 0.1 ng/mL T4. Moreover, the assay showed crossreactivity with T3 similar to that of competitive ELISA, and also gave a reasonable total serum T4 concentration (90 ng/mL) from ethanol-extracted sample serum using the recombinant proteins. This is the first direct construction of an OS-ELISA system bypassing hybridoma, which will be applicable to the detection of many other small molecule antigens.