Detection of Lewis a antigen in sera of ovarian carcinoma patients by MOv2-MOv8 double-determinant radioimmunoassay

The monoclonal antibodies MOv2 and MOv8, raised against ovarian carcinoma, were found to be directed against two non-crossreacting epitopes expressed on the same molecule. Immunochemical analysis of the MOv8 recognized epitope showed that the Le^a oligosaccharide, or commercial anti-Le^a MAb, but not the anti-Le^b MAb, prevented MOv8 binding to the reference target cell line (SW626), indicating that it is carried by the Le^a antigen. Since we previously reported that MOv2 also recognises the Le^a antigen, these data suggest that Mov8 and Mov2 were directed against different epitopes on the same oligosaccharide chain. Bearing in mind the knowledge of the biochemical nature of the monoclonal antibody recognized epitopes (CaMOv2 and CaMOv8), the presence of the circulating molecules recognized by them was analyzed by double determinant immunoradiometric assay (DDIRMA) in 103 sera from ovarian carcinoma patients. Patients with clinical evidence of the disease (ED) with MOv2 and MOv8 reactive and negative tumors had sera reactivity in 67% and 19% respectively. Also, 26% of the patients with no clinical evidence of disease (NED) had positive sera. When we investigated the relationship between MOv2-MOv8 DDIRMA sera positivity and red blood cells (RBC) Lewis phenotype, a strong correlation was found between the Le^a+ phenotype and DDIRMA sera reactivity in healthy donors (6/6) and in ovarian carcinoma patients (9/10) whatever their clinical condition. No Le^a healthy donors gave evidence of MOv2-MOv8 reactive sera. In contrast, 33% and 57% of the sera from ED carcinoma patients with respectively Le^{a b +} and Le^{a b} phenotype were positive.

In conclusion, the percentage of DDIRMA positive cases previously reported in healthy donors and found here in ovarian carcinoma patients with MOv2 and MOv8 negative tumors or NED, was in agreement with Le^{a +} phenotype frequency in the normal Caucasian population. However the Lewis^a could represent a tumor-associated antigen in ED patients with Le^a phenotype and MOv2-MOv8 DDIRMA might be useful for monitoring the disease