MONOCLONAL-ANTIBODY X18A4 IDENTIFIES AN ONCOFETAL FIBRONECTIN EPITOPEDISTINCT FROM THE FDC-6 BINDING-SITE

OBJECTIVE: Oncofetal fibronectin reactive with antibody FDC-6 has been associated with trophoblastic implantation and chorion structural sta bility. Abnormal release of this fibronectin into cervical and vaginal secretions has identified patients at risk for preterm labor and deli very. The aim of this study was to determine whether trophoblast-deriv ed oncofetal fibronectin contains other novel epitopes distinct from t he FDC-6 binding site. STUDY DESIGN: Antitrophoblast fibronectin hybri domas were generated and screened by comparative immunoassays. One spe cific monoclonal antibody, X18A4, was identified and compared with ant ibody FDC-6 by immunocytochemical and immunoblot analyses. Both antibo dies were also evaluated in ''sandwich''-type double monoclonal immuno sorbent assays. RESULTS: X18A4 and FDC-6 bind avidly and noncompetitiv ely to distinct epitopes within oncofetal fibronectin. They exhibit si milar immunohistochemical staining of the extracellular matrix within placental tissue, ovarian epithelial tumors, and cultured trophoblasts . However, in contrast to FDC-6, X18A4 has no detectable binding activ ity to human plasma fibronectin, and its binding to oncofetal fibronec tin was unaffected by enzymatic deglycosylation. Immunoblot analyses o f oncofetal fibronectin proteolytic digests suggest that X18A4 binds n ear or within the alternatively spliced type III connecting segment do main. CONCLUSIONS: X18A4 identifies and binds with high affinity to a new epitope within oncofetal fibronectin, distinct from the FDC-6 bind ing site. Because X18A4 displays no detectable binding to plasma fibro nectin, it could be used as an important adjunctive antibody for enhan cing the specificity of clinically based oncofetal fibronectin diagnos tic assays.