Bovine apoB-100 is the predominant target of the antibody response in patients treated with FCS cultured cells. (A) FCS was immunoprecipitated by isotype-matched control IgG1, 3E8.1 mAb, serum samples from 2 healthy human volunteers, or 2 serum samples from ADA-SCID patients in a gene therapy trial. Samples were analyzed by SDS-PAGE. (B) BW cells were incubated with the FITC-conjugated IgG fraction of serum from patient 2 with (dotted light line) or without (thick solid line) 3E8.1 mAb. (C) FCS treated with ( + ) or without (−) neuraminidase was immunoprecipitated by 3E8.1 or by serum from patient 2 and analyzed by SDS-PAGE (lower panel) or by Western blotting with anti-Neu5Gc antibody (upper panel). (D) FCS immunoprecipitated by serum from a healthy human volunteer, by serum from patient 2, or by serum samples from 4 patients in the breast cancer vaccine trial, were analyzed by SDS-PAGE. The definitions of + and − are in “Materials and methods, Patient sera/plasma from clinical trials and healthy donor cells.”