Recombinant light chain of human monoclonal antibody HB4C5 as a potentially useful lung cancer-targeting vehicle

Recombinant λ light chain of lung cancer-reacting human monoclonal antibody HB4C5 was expressed in Escherichia coli. Expression in bacteria ensured the generation of homogeneous light chain species devoid of activity-hampering N-linked glycosylation usually found in the light chain CDR-1 of HB4C5. Molecular engineering was also employed to eliminate the C-terminal two amino acid residues, i.e., Cys and Ser, to prevent the formation of λ light chain dimers which are less reactive than the monomeric form. The λ light chain was overexpressed in E. coli as inclusion bodies, which were solubilized, refolded, and treated with Aeromonas proteolytica aminopeptidase to remove the N-terminal Met with subsequent natural cyclization of the penultimate Gln residue to pyroglutamate, the same N-terminal end as that of naturally occurring λ light chain in HB4C5. Monomeric recombinant λ light chains, both before and after removal of the N-terminal Met residue, were 40 times more immunoreactive than the parent HB4C5. The immunostaining of lung cancer tissue sections with the recombinant λ light chain indicated cancer-specific reactions to all specimens of adenocarcinoma, squamous cell carcinoma and large cell carcinoma histologies, but did not react with small cell carcinoma. Tumor radioimmunoimaging experiments in LC6 (lung squamous cell carcinoma line) – xenografted nude mice by the i.p. injection of ¹²⁵I-labeled recombinant λ light chain and ¹²⁵I-labeled human λ light chain control gave tumor-specific and recombinantλ light chain-dependent images on day 5 postinjection, and images were also detectable on day 3. Biodistribution studies with ¹²⁵I-labeled recombinant λ light chain demonstrated that the λ light chain could penetrate better into the tumor sites, both at the necrotic and solid parts of the xenograft, as compared to our previous results with ¹²⁵I-labeled HB4C5 which could localize to the necrotic part only. These results suggest that the recombinant λ light chain is potentially useful as a lung cancer-targeting vehicle, for such as radioimmunoimaging and radioimmunotherapy, with least possible adverse immunogenic effects.