Human monoclonal antibodies reactive with colon carcinoma: identification by a novel screening procedure with xenografts.

To identify tumor-associated antigens immunogenic to man, human monoclonal antibodies (MAbs) were generated by fusing regional lymph node lymphocytes (LNLs) of each of three colon adenocarcinoma patients with the M5 mouse myeloma cell line. Heterohybridomas in 19 wells secreted human immunoglobulin (Ig) for at least 60 days. To identify immunoreactive MAbs, we devised an immunohistochemical assay with xenografts of six colon tumors. Binding was visualized with a biotin-conjugated goat antihuman Ig antibody followed by an avidin-biotin-peroxidase complex. This assay detected binding with a sensitivity of 0.5 microgram/ml human Ig. Reactivity against tumor-associated antigens (TAAs) in frozen sections of the colon xenografts was detected in 14 of the 19 supernates. However, only nine (three IgGs, five IgMs and one IgA) were reactive against formalin-fixed sections, showing that formalin-fixation destroyed some but not all antigenicity. Four MAbs were cloned and tested against the entire panel of 19 xenografts. One IgM MAb reacted only against colon carcinomas, while the other three (one each of IgA, IgG and IgM) had broader reactivities. Three of the four human MAbs did not cross-react with melanomas. In summary, human regional lymph nodes contain a high proportion of B lymphocytes sensitized to the autochthonous tumor. Immunostaining of human colon tumor xenografts can readily identify human MAbs reactive with colon carcinomas.