Stability-indicative and conformation-specific enzyme linked immunosorbent assay for analysis of recombinant human gamma interferon

Recombinant human interferon gamma (rhIFN-gamma) is a promising molecule for the treatment of several diseases. A pair of conformation-specific monoclonal antibodies (mAbs) against rhIFN-gamma was selected from generated hybridoma cell lines to design a sensitive, stability-indicative, sandwich-type ELISA. The main assay parameters were optimized by the checkerboard method for the highest signal-to-noise ratio: assay buffer composition, coating buffer pH and composition, coating temperature-incubation time parameters, and coating mAb concentration and conjugate dilution. Detection and quantification limits were estimated between 0.019 and 0.078 ng/mL, respectively, and recovery values were from 92.03% to 98.40%. The coefficient of variation of intra-assay precision parameters ranged from 2.32% to 9.21% while the inter-analyst variation was between 4.70% and 10.63%, supporting the method's repeatability. The ELISA was specific for correctly folded and non-aggregated molecular species, as compared to intrinsic Trp fluorescence (chemical denaturation) and optical density at 340 nm (thermal aggregation), respectively. However, the method was not sensitive to the small C-terminal degradation of full-length rhIFN-gamma 1-144 (losses of 6-12 amino acid residues) as compared to results with mass spectrometry and gel electrophoresis. ELISA showed good correlation with rhIFN-gamma antiviral biological activity. This method was applied to the stability evaluation of rhIFN-gamma in physiological buffer at low concentrations using polypropylene and glass vials also in the presence of adsorption protectant excipients. Furthermore, ELISA could be adapted to other applications such as quantification of IFN-gamma in serum samples, Mycobacterium tuberculosis diagnosis, etc.