The neutralization of interferons by antibody. II. Neutralizing antibody unitage and its relationship to bioassay sensitivity: the tenfold reduction unit.

The importance of establishing a common method of reporting neutralizing antibody levels is emphasized by the fact that patients injected repeatedly with a human interferon (HuIFN) may develop such antibodies that can abrogate the beneficial effects of the treatment. The earlier experimental and theoretical constructs of Kawade led to certain recommendations by the World Health Organization (WHO) concerning the methodology of neutralization tests and how to report the resultant data. A WHO international collaborative study on two human sera with antibodies against HuIFN-alpha and HuIFN-beta provided the opportunity not only to test the theoretical concepts concerning the neutralization reaction with data obtained in different bioassay systems in different laboratories but also to obtain enough data points for statistical evaluation with bioassays having a great range of sensitivity to IFN. The analyses substantiate and extend the original conclusions of Kawade that the neutralization follows the reaction mode of low-affinity antibody, in accord with the constant proportion hypothesis by which antibody reduces IFN activity in a set ratio of added/residual biologically active IFN, a consequence of the low molar concentration of free IFN at the neutralization end point. The present results support the recommendation that the preferred way to state the index of neutralization of antibodies is a titer (t), calculated by the formula t = f(n - 1)/9, where f is the reciprocal of the antibody dilution achieving the end point, and n is the IFN concentration measured in that day's titration. The tenfold reduction unit (TRU) of neutralization is proposed for use in expressing the quantity, or unitage, of IFN neutralizing antibody. The utility of its application is explained. The use of the index of neutralization described and the proposed derivative term of antibody unitage, TRU, should help make the results from different laboratories employing different bioassay systems more readily comparable and interpretable, provided the bioassays are sufficiently sensitive to IFN.