High prevalence of human anti-bovine IgG antibodies as the major cause of false positive reactions in two-site immunoassays based on monoclonal antibodies.

A sandwich ELISA for quantification of the endometrial protein PP14 revealed false positive reactions in 81% of male sera (n = 54). The PP14 ELISA was based on two monoclonal antibodies (Mabs) with different epitope specificities--a catcher and a biotinylated indicator. The monoclonal antibodies were purified by protein G affinity chromatography from culture supernatant containing 10% (v/v) fetal calf serum (FCS). Human anti-animal IgG (bovine, mouse, horse, and swine) antibodies and human anti-bovine serum albumin antibodies were measured using an ELISA design, with direct bridging of the solid phase and biotinylated antigens. The false positive reactions were abolished by addition of 1% (v/v) bovine serum to the dilution buffer (DB). Human anti-bovine IgG antibodies (HABIA) were detected in 99 out of 104 sera from blood donors (50 females; 54 males). HABIA levels in male sera (n = 54) were positively correlated to the false positive signals in the PP14 ELISA (r = 0.923; p < 0.0001). Antibodies to IgG from other mammalian species (mouse, horse, and swine) were also detected in the donor sera, but levels and frequencies were lower compared to that of HABIA. Furthermore, HABIA were positively correlated to human anti-bovine serum albumin antibodies in the donor sera (r = 0.639; p < 0.0001; n = 103). HABIA (prevalence 95%) cause false positive reactions due to crossbinding of contaminating bovine IgG and/or crossreaction with mouse IgG in two-site immunoassays. The apparent presence of human anti-mouse IgG antibodies (HAMA), described to create false positive results, may be due to a crossreacting fraction of the polyclonal circulating antibodies against bovine IgG.