Monoclonal antibody X18A4 identifies an oncofetal fibronectin epitope distinct from the FDC-6 binding site.

OBJECTIVE Oncofetal fibronectin reactive with antibody FDC-6 has been associated with trophoblastic implantation and chorion structural stability. Abnormal release of this fibronectin into cervical and vaginal secretions has identified patients at risk for preterm labor and delivery. The aim of this study was to determine whether trophoblast-derived oncofetal fibronectin contains other novel epitopes distinct from the FDC-6 binding site. STUDY DESIGN Antitrophoblast fibronectin hybridomas were generated and screened by comparative immunoassays. One specific monoclonal antibody, X18A4, was identified and compared with antibody FDC-6 by immunocytochemical and immunoblot analyses. Both antibodies were also evaluated in "sandwich"-type double monoclonal immunosorbent assays. RESULTS X18A4 and FDC-6 bind avidly and noncompetitively to distinct epitopes within oncofetal fibronectin. They exhibit similar 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 activity to human plasma fibronectin, and its binding to oncofetal fibronectin was unaffected by enzymatic deglycosylation. Immunoblot analyses of oncofetal fibronectin proteolytic digests suggest that X18A4 binds near or within the alternatively spliced type III connecting segment domain. CONCLUSIONS X18A4 identifies and binds with high affinity to a new epitope within oncofetal fibronectin, distinct from the FDC-6 binding site. Because X18A4 displays no detectable binding to plasma fibronectin, it could be used as an important adjunctive antibody for enhancing the specificity of clinically based oncofetal fibronectin diagnostic assays.