The standardization of NK cell assays for use in studies of biological response modifiers

This paper deals with the standardization of human natural killer (NK) cell assays for the sequential evaluation of patients with various disease states, or who are being treated with biological response modifiers. The method is described for the calculation of lytic units which results in numbers proportional to effector cell activity. It is shown that normal donors are relatively consistent in their cytotoxicity, making it possible to use a 'bank' of normal controls against which patients data can be normalized. Cryopreserved lymphocytes, as well as fresh lymphocytes, can be used as controls. Under the usual conditions for recovering cryopreserved lymphocytes, NK activity is markedly reduced; but by preincubation of the lymphocytes at 37 degrees C for 5 h or more the activity is recovered. A number of currently used methods for the selection of controls are described and discussed with respect to their practicality and validity. The principle problem with currently used methods is the necessity for a large number of controls so that a uniform distribution of NK activity about the 'true' normal is assured. The method which we advocate in this paper is based on the selection of normal control donors from a group of individuals who have been tested repeatedly over several months or years and whose NK activity relative to normal donors as a whole can be stated with reasonable confidence. These values are then used as correction factors which are applied to the relevant control donor's cytotoxicity every time that that person is used. In any particular experiment the mean of the corrected control lytic unit values is used as the denominator for the calculation of patient NK activity relative to normal. This method can be applied 'retrospectively' if a variety of donors have been used repeatedly. Two 'tests' of the methodology are described, with the following results: (1) the relative NK activity of a randomly selected group of normal women, whose data were handled as if they were a patient population, was approximately one, and (2) complete Freund's adjuvant, administered intradermally to patients with resected lung cancer, caused an increase in natural killer cell activity compared to pretreatment levels. The use of these methods should make it possible to derive meaningful NK results which are comparable from laboratory to laboratory.