DESIGN AND CONSTRUCTION OF A HYBRID IMMUNOGLOBULIN DOMAIN WITH PROPERTIES OF BOTH HEAVY AND LIGHT-CHAIN VARIABLE REGIONS

The complementarity-determining regions (CDRs) of a human kappa light chain were replaced with CDRs from a murine gamma-1 heavy chain and, b y use of molecular modeling, key heavy chain framework residues were i dentified and thus included to preserve the native conformation of the heavy chain CDRs, Go-expression of this hybrid human kappa chain (VHB CL) With a human kappa chain counterpart (VLCL, engineered to contain murine light chain CDRs) resulted in the secretion of high levels of a heterodimeric protein (VHBCL::VLCL) termed 'kappabody'. This protein also had equivalent affinity for antigen as the Fab' of the parent mur ine IgG(1). High-level secretion was also observed for the hybrid chai n as homodimers (VHBCL::VHBCL), which is not observed for chimeric cha ins consisting of a heavy chain variable region and light chain consta nt region, i.e. VHCL homodimers or single chains are not secreted, Thi s indicates that regions within the variable domain, required for secr etion of light chains, reside outside of the hypervariable regions (CD Rs) and that the heavy chain CDRs and supporting residues do not preve nt secretion, These results demonstrate the possibility of designing s mall, single-domain molecules possessing a given binding activity whic h may be secreted at high levels from mammalian cells.