Preparation of single chain variable fragment of MG7 mAb by phage display technology

Development of an oral DNA vaccine against MG7-Ag of gastric cancer using attenuated salmonella typhimurium as carrier

AIM: To develop an oral DNA vaccine against gastric cancer and evaluate its efficacy in mice.

METHODS: The genes of the MG7-Ag mimotope and a universal Th epitope (Pan-DR epitope, PADRE) were included in the PCR primers. By PCR, the fusion gene of the two epitopes was amplified. The fusion gene was confirmed by sequencing and was then cloned into pcDNA3.1 (+) plasmid. The pcDNA3.1 (+)-MG7/PADRE was used to transfect an attenuated Salmonella typhimurium. C57BL/6 mice were orally immunized with 1 × 10⁸ cfu Salmonella transfectants. Salmonella harboring the empty pcDNA3.1 (+) plasmid and phosphate buffer saline (PBS) were used as negative controls. At the 6th week, serum titer of MG7-Ag specific antibody was detected by ELISA. At the 8th week cellular immunity was detected by an unprimed proliferation test of the spleenocytes by using a [³H]-thymidine incorporation assay. Ehrlich ascites carcinoma cells expressing MG7-Ag were used as a model in tumor challenge assay to evaluate the protective effect of the vaccine.

RESULTS: Serum titer of antibody against MG7-Ag was significantly higher in mice immunized with the vaccine than that in control groups (0.841 vs 0.347, P < 0.01; 0.841 vs 0.298, P < 0.01), while in vitro unprimed proliferation assay of the spleenocytes showed no statistical difference between those three groups. Two weeks after tumor challenge, 2 in 7 immunized mice were tumor free, while all the mice in the control groups showed tumor formation.

CONCLUSION: Oral DNA vaccine against the MG7-Ag momitope of gastric cancer is immunogenic. It can induce significant humoral immunity against tumor in mice, and the vaccine has partially protective effects.

Construction, expression and tumor targeting of a single-chain Fv against human colorectal carcinoma

AIM: A single-chain antibody fragment, ND-1scFv, against human colorectal carcinoma was constructed and expressed in E.coli, and its biodistribution and pharmacokinetic properties were studied in mice bearing tumor.

METHODS: V_H and V_L genes were amplified from hybridoma cell IC-2, secreting monoclonal antibody ND-1, by RT-PCR, and connected by linker (Gly₄Ser)₃ to form scFv gene, which was cloned into expression vector pET 28a(+) and finally expressed in E.coli. The expressed product ND-1scFv was purified by metal affinity chromatography using Ni-NTA, its purity and biological activity were determined using SDS-PAGE and ELISA. ND-1scFv was labeled with ^99mTc, and then injected into mice bearing colorectal carcinoma xenograft for phamacokinetic study in vivo.

RESULTS: SDS-PAGE analysis showed that the relative molecular weight of recombinant protein was 30 kDa with purity of 94%. ELIAS assay revealed that ND-1scFv retained the immunoactivity of parent mAb, being capable of binding specifically to human colorectal carcinoma cell line expressing associated antigen. Radiolabeled ND-1scFv exhibited rapid tumor targeting, with specific distribution in mice bearing colorectal carcinoma xenograft observed as early as 1 h following injection. In vivo pharmacokinetic studies also demonstrated that ND-1scFv had very rapid plasma clearance (T_1/2α of 5.7 min, T_1/2β of 2.6 h).

CONCLUSION: ND-1scFv shows significant immunoactivity, and better pharmacokinetic and biodistribution characteristics compared with intact mAbs, demonstrating the possibility as a carrier for tumor-imaging.

Expression and bioactivity identification of soluble MG7 scFv

AIM

To examine the molecular mass and identify the bioactivity of MG7 scFv for its application as a targeting mediator in gene therapy of gastric cancer.

METHODS

Two strongly positive recombinant phage clones screened from MG7 recombinant phage antibody library were separately transfected into E.coli TG1. Plasmid was isolated from the transfected E.coli TG1 and digested by EcoR I and Hind III to examine the length of exogenous scFv gene. Then, the positive recombinant phage clones were individually transfected into E.coli HB2151. The transfectant was cultured and induced by IPTG. Perplasmic extracts was prepared from the induced transfectant by osmotic shock. ELISA was used to examine the antigen-binding affinity of the soluble MG7 scFv. Immunodotting assay was adopted to evaluate the yield of soluble MG7 scFv produced by transfected E.coli HB2151. Western blot was used to examine the molecular mass of MG7 scFv. Finally, the nucleotide sequence of MG7 scFv was examined by DNA sequencing.

RESULTS

Two positive recombinant phage clones were found to contain the exogenous scFv gene. ELISA showed that MG7 scFv had strong antigen-binding affinity. Immuodotting assay showed that transfected E.coli HB2151 could successfully produce the soluble MG7 scFv with high yield via induction by IPTG. The molecular mass of MG7 scFv was 30 kDa by western blot. DNA sequencing demonstrated that the VH and VL genes of MG7 scFv were 363 bp and 321 bp,respectively.

CONCLUSION

We have successfully developed the soluble MG7 scFv which possessed strong antigen-binding affinity.