Covert cross reactants in a two-site immunoassay studied with monoclonal antibodies

The influence of naturally occurring heterophilic anti-immunoglobulin antibodies on direct measurement of serum proteins using sandwich ELISAs

Sandwich ELISAs have become a widely used method for the quantitative detection of serum proteins. However, they can be biased by a variety of interfering substances. As reported recently, we observed false-positive levels of interferon (IFN)-alpha and -beta in up to 27% of sera from healthy blood donors using commercial ELISAs. We now demonstrate that two different groups of naturally occurring heterophilic antibodies (IgG-type) are responsible for these titers. Group I (representing 85% of positive samples) binds to the Fab region of IgG from goat, mouse, rat, horse, and bovidae (but not rabbit). Group II (15%) recognizes an epitope in the Fc region of mouse, horse, bovine, and rabbit (but not goat or rat) immunoglobulins. The antibodies did not crossreact with human IgG subclasses but contributed to false-positive IgG rheumatoid factor levels obtained using a commercially available ELISA. To investigate the susceptibility of assays to these artifacts, various combinations of capture and detection antibodies have been tested. On this basis, we defined the relative risks that standard ELISAs might be influenced by heterophilic anti-immunoglobulin antibodies. In general, assays that use monoclonal antibodies for both capture and detection are less susceptible than others which include at least one polyclonal antiserum. However, only systems utilizing rabbit F(ab')(2) fragments have been found to be immune to this interference.

Specificity of two-site immunoassays

When cross-reaction in two-site immunoassays was investigated both theoretically and experimentally it was found that such systems do not always result in enhanced specificity. Computer simulation studies indicated that substances which display negligible cross-reaction in a radioimmunoassay could produce an assay response identical to that of the analyte in a two-site immunoassay using excess antibody. Cross-reactivity in two differing two-site immunoassays was compared to that obtained in radioimmunoassays using the same monoclonal antibodies for human chorionic gonadotrophin. In addition to the effects of excess antibody, cross-reactivity was observed in one of the two-site immunoassays which could not have been predicted from the specificity of the antibodies or cross-reactivity in the radioimmunoassays. The unexpected cross-reaction of the beta subunit of human chorionic gonadotrophin in the assay resulted from an apparent alteration in the specificity of one of the antibodies following binding of the beta subunit to the second antibody. These studies emphasise the complexity of binding reactions in two-site immunoassays.

Evidence for free beta-subunit secretion in so-called human chorionic gonadotropin-positive seminoma

In 349 patients with pure seminoma the authors determined human chorionic gonadotropin (HCG) plus the free beta-subunit (HCG-beta) by a polyclonal radioimmunoassay (RIA) and found elevated serum levels in 42 patients (12%). In addition, each HCG and HCG-beta were specifically measured by monoclonal immunoradiometric assays (IRMA) in 19 of these patients. Overall 12 patients were HCG-beta positive and 12 HCG positive. Elevated serum levels of both molecules were found in five patients, whereas seven patients showed elevation only in HCG and another seven patients only in HCG-beta. In serum samples of patients with high HCG-beta and minimally elevated HCG levels the free beta-subunit could be detected by gel filtration and immunoradiometric assays, whereas both HCG-beta and holo-HCG were found in ultrafiltered and concentrated urinary samples. The authors conclude that HCG-beta can be present as the major form in serum even if both HCG-beta and HCG are detectable in urine. When measuring hormone concentration as a tumor marker in seminoma patients, methods which determine both activities, HCG plus HCG-beta, seem to be the most reliable. Whereas 16 patients with Stage I-IIc had HCG-beta levels between 0.1 and 2.8 IU/l and currently have no evidence of disease, two patients with Stage IV had HCG-beta levels of 18 and 37 IU/l and died. Current investigations have to clarify the prognostic significance of beta-subunit secretion.