OX40 receptor (tumor necrosis factor receptor superfamily, member 4; CD134) is a T-cell co-stimulatory molecule that plays an important role in T-cell activation and survival. OX40 receptor is activated by its ligand, OX4OL; and modulation of the OX40-OX4OL interaction is a promising target for the treatment of autoimmune diseases and cancers. Here, we generated a high-affinity anti-OX40 single-chain variable fragment carrying a C-terminal cysteine residue (scFvC). Physicochemical and functional analyses revealed that the scFvC bound to OX40-expressing cells and was internalized via OX40-mediated endocytosis without inducing phosphorylation of I kappa B alpha. (nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha), an important complex in the classical NF kappa B (nuclear factor kappa light-chain-enhancer of activated B cells) signaling pathway. In addition, mutation of the 36th cysteine residue in variable region of light chain enabled site-specific chemical modification to carboxy terminal cysteine and improved the thermal stability of the scFvC. These results suggest that this novel high affinity anti-OX40 scFvC may be useful as a transporter for targeted delivery of small compounds, proteins, peptides, liposomes, and nanoparticles, into OX40-expressing cells for the treatment of autoimmune diseases and cancers. (C) 2018 Elsevier Inc. All rights reserved.