Mammalian cell expression of dimeric small immune proteins (SIP).

We have designed and expressed bivalent small immune proteins (SIP) based on scFv fragments connected through a short linker of four amino acids to the CH3 domain of the human immunoglobulin gamma 1 H-chain. Three different versions have been designed and expressed in mammalian cells. In one construct a cysteine residue was included in the last amino acid of the flexible 15-amino acid long linker connecting the V(L) and V(H) domains, thus creating a disulphide bond stabilized molecule. A version with a shorter (five amino acids) V(L)/V(H) linker was also produced and shown to be efficiently assembled and secreted. All three SIPs form dimers retaining their antigenic specificity in Western blotting and having a comparable functional affinity (avidity) as determined by ELISA.