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 an tibody levels is emphasized by the fact that patients injected repeatedly w ith a human interferon (HuIFN) may develop such antibodies that can abrogat e the beneficial effects of the treatment. The earlier experimental and the oretical constructs of Kawade led to certain recommendations by the World H ealth Organization (WHO) concerning the methodology of neutralization tests and how to report the resultant data. A WHO international collaborative st udy on two human sera with antibodies against HuIFN-alpha and HuIFN-beta pr ovided the opportunity not only to test the theoretical concepts concerning the neutralization reaction with data obtained in different bioassay syste ms in different laboratories but also to obtain enough data points for stat istical evaluation with bioassays having a great range of sensitivity to IF N. 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 reduce s IFN activity in a set ratio of added/residual biologically active IFN, a consequence of the low molar concentration of free IFN at the neutralizatio n end point. The present results support the recommendation that the prefer red 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 m easured in that day's titration. The tenfold reduction unit (TRU) of neutra lization is proposed for use in expressing the quantity, or unitage, of IFN neutralizing antibody. The utility of its application is explained. The us e of the index of neutralization described and the proposed derivative term of antibody unitage, TRU, should help make the results from different labo ratories employing different bioassay systems more readily comparable and i nterpretable, provided the bioassays are sufficiently sensitive to IFN.