Molecular cloning of mesothelin, a differentiation antigen present on mesothelium, mesotheliomas, and ovarian cancers

A Lewis(y) epitope mimicking peptide induces anti-Lewis(y) immune responses in rabbits and mice

Lewis(y) carbohydrate antigens are abundant on the surface of many carcinomas. Mab B3 directed against this carbohydrate antigen has been used to make an immunotoxin that is very cytotoxic to cancer cells expressing the Lewis(y) antigen. Mab B3 was also used to screen a phage-displayed peptide library and identified a peptide mimicking the Lewis(y) epitope. In this report, we demonstrate that the Lewis(y) epitope-mimicking peptide induces anti-Lewis(y)immune responses in both rabbits and mice. In addition, Lewis(y) antigens induce anti-peptide immune responses. These results indicate that carbohydrate-mimicking peptides provide a novel strategy to elicit immune responses for tumor-associated carbohydrates.

111Indium-labeled monoclonal antibody K1: biodistribution study in nude mice bearing a human carcinoma xenograft expressing mesothelin

The monoclonal antibody (MAb) K1 is a murine IgG1 that recognizes mesothelin, a differentiation antigen present on mesothelium which is highly expressed on cancers derived from mesothelium, including most ovarian cancers and epithelioid mesotheliomas. MAb K1 was conjugated to 2-(p-isothiocyanatobenzyl)-cyclohexyl- diethylenetriaminepentaacetic acid and labeled with 111In. The biodistribution of 111In-K1 was studied in athymic nude mice bearing 2 s.c. tumors, one expressing a stably transfected plasmid encoding mesothelin and one composed of the parental untransfected A431 epidermoid carcinoma cells which do not express mesothelin. Tumor-bearing mice were given an i.v. injection of 111In-K1 and killed at different time points to determine the uptake of radiolabeled antibody. Significantly higher uptake was seen in antigen-positive tumors at all time points, with peak values at 72 hr (52.9% vs. 8% of the injected dose/g tissue for antigen-positive and antigen-negative tumors, respectively). Uptake in antigen-positive tumors was higher than the blood level at all time points, and the tumors contained a high level of the radiolabeled MAb even at 7 days (28.6% of the injected dose/g tumor).

Improved stability and yield of a Fv-toxin fusion protein by computer design and protein engineering of the Fv

The conversion of the anti-mesothelin monoclonal antibody K1 to a single-chain Fv (scFv) that is fused to a truncated form of Pseudomonas exotoxin A (PE) results in a fusion protein (immunotoxin) that is unstable and refolds very inefficiently. We have devised a method that identifies candidate residues in the framework region of K1 Fv that, when mutated, improved the yield and stability of the protein. The method works by initially aligning the framework sequences of K1 VH and VL with those of other scFvs that are stable and give a good yield as immunotoxins. Then we assigned a character to each residue that indicates its state of exposure based on the known crystal structures of Fabs. This identifies residues that are not compatible with their environment in the folded state of the protein. Next we calculated the frequencies of different amino acids for each position of the Fvs based on the available sequence database. This identifies residues that are not commonly present in the conserved positions. If these residues are compatible with their exposure profile they are left unaltered. Otherwise, they are identified as candidate residues for mutation. We identified two such residues in the VH (T82 and A85) and two in the VL (H36 and V60) of K1 that did not seem appropriate for their respective positions. By mutating these residues in K1 into those that occur most commonly in the sequence database or in stable scFvs, we significantly improved the stability and yield of the K1 scFv immunotoxins. By making single and combined mutations we assessed the relative contribution of mutations at these four sites towards the stability and yield of K1 scFv immunotoxins. The method we devised is probably general and can be used to improve other scFvs.

Isolation of a high-affinity stable single-chain Fv specific for mesothelin from DNA-immunized mice by phage display and construction of a recombinant immunotoxin with anti-tumor activity

Mesothelin is a differentiation antigen present on the surface of ovarian cancers, mesotheliomas, and several other types of human cancers. Because among normal tissues, mesothelin is present only on mesothelial cells, it represents a good target for antibody-mediated delivery of cytotoxic agents. In the present study mice were immunized with an eukaryotic expression vector coding for mesothelin. When high serum antibody titers were obtained, a phage display library was made from the splenic mRNA of these mice. After three rounds of panning on recombinant mesothelin, a single-chain Fv (scFv)-displaying phage was selected that bound specifically to recombinant mesothelin and mesothelin-positive cells. The scFv was used to construct an immunotoxin by genetically fusing it with a truncated mutant of Pseudomonas exotoxin A. The purified immunotoxin binds mesothelin with high affinity (Kd 11 nm), is stable for over 40 hr at 37 degrees C and is very cytotoxic to cells expressing mesothelin. It also produces regressions of tumors expressing mesothelin. This combination of selective cytotoxicity, high activity, and stability makes the immunotoxin a good candidate for development as a therapeutic agent. This work also shows that DNA immunization can be used to isolate and clone antibodies against epitopes present on human proteins in their native conformation.

Cloning and expression of the recombinant FAb fragment of monoclonal antibody K1 that reacts with mesothelin present on mesotheliomas and ovarian cancers

The monoclonal antibody (MAb) K1 recognizes an approximate 40 kDa glycoprotein, mesothelin, that is present on the surface of human mesothelial cells, mesotheliomas and ovarian cancers. We have cloned the cDNAs encoding the variable regions of MAb K1 and constructed plasmids for expression of recombinant K1(FAb). Recombinant FAb was produced in Escherichia coli in inclusion bodies that were solubilized and refolded to active protein. Binding of K1 MAb and FAb was compared by radioactive binding and competition assays and by surface plasmon resonance (BIAcore). Recombinant K1(FAb) binds to cells expressing K1-antigen with a similar affinity as papain derived FAb from K1(IgG) and with a 4- to 10-fold reduced affinity compared with bivalent IgG. The cloned FAb can be used to make higher affinity antibodies and immunoconjugates that could be useful for various types of immunotherapies.