AIM:To clone and express the antigen of monoclonalantibody(MAb)PD4 for further investigation of its function.METHODS:MGC803 cDNA expression library was constructedand screened with PD4 as probes to clone the antigen.Afterfailed in the library screening,immunoprecipitation and SDS-polyacrylamide gel electrophoresis were applied to purifythe antigen for sequence analysis.The antigen coming fromMycoplasma hyorhinis(M.hyorhinis)was further confirmedwith Western blot analysis by infecting M.hyorhinis-freeHeLa cells and eliminating the M,hyorhinis from MGC803cells.The full p37 gene was cloned by PCR and expressedsuccessfully in Escherichia coli after site-directed mutations.Immunofluorescence assay was used to demonstrate if p37protein could directly bind to gastric tumor cell AGS.RESULTS:The cDNA library constructed with MGC803 cellswas screened by MAb PD4 as probes.Unfortunately,thepositive clones identified with MAb PD4 were also reactedwith unrelated antibodies.Then,immunoprecipitation wasperformed and the purified antigen was identified to be amembrane protein of Mycoplasma hyorhinis(M.hyorhinis)by sequencing of N-terminal amino acid residues.Themembrane protein was intensively verified with Western blotby eliminating M.hyorhinisfrom MGC803 cells and by infectingM.hyorhinis-free HeLa cells.The full p37 gene was clonedand expressed successfully in Escherichia coli after site-directedmutations.Immunofluorescence demonstrated that p37protein could directly bind to gastric tumor cell AGS.CONCLUSION:The antigen recognized by MAb PD4 is fromM.hyorhinis,which suggests the actions involved in MAbPD4 is possibly mediated by p37 protein or M.hyorhinis.Asp37 protein can bind directly to tumor cells,the pathogenicrole of p37 involved in tumorigenesis justifies further investigation. AIM: To clone and express the antigen of monoclonalantibody (MAb) PD4 for further investigation of its function. METHODS: MGC803 cDNA expression library was constructed and screened with PD4 as probes to clone the antigen. AfterFailed in the library screening, immunoprecipitation and SDS-polyacrylamide gel electrophoresis were applied to purify the antigen for sequence analysis. The antigen coming from Mycoplasma hyorhinis (M. hyorhinis) was further confirmed by Western blot analysis by infecting M. hyorhinis-free HeLa cells and eliminating the M, hyorhinis from MGC803 cells. The full p37 gene was cloned by PCR and expressed successfully in Escherichia coli after site-directed mutations. Immunofluorescence assay was used to demonstrate if p37protein could directly bind to gastric tumor cells AGS. RESULTS: The cDNA library constructed with MGC803 cells was screened by MAb PD4 as probes. Unfortunately, the positive clones identified with MAb PD4 were also reacted with unrelated antibodies. Chen, immunoprecipitation w asperformed and the purified antigen was identified to be a membrane protein of Mycoplasma hyorhinis (M. hyorhinis) by sequencing of N-terminal amino acid residues. The protein was was verified by Western blotby removing M.hyorhinisfrom MGC803 cells and by infecting M.hyorhinis-free HeLa cells. The full p37 gene was clonedand expressed successfully in Escherichia coli after site-directedmutations. Immunofluorescence demonstrated that p37protein could directly bind to gastric tumor cells AGS. CONCLUSION: The antigen recognized by MAb PD4 is from M.hyorhinis, which suggests the actions involved in MAbPD4 is possibly mediated by p37 protein or M.hyorhinis. Asp37 protein can bind directly to tumor cells, the pathogenicrole of p37 involved in tumorigenesis justifies further investigation.