Anti-interferon-γ autoantibodies in patients with indeterminate QuantiFERON®-TB Gold Plus Assay results

Abstract

BackgroundThe QuantiFERON®-TB Gold Plus Assay (QFT-G), a commercial interferon-γ (IFNγ) release assay, is used to screen patients for latent tuberculosis infection. Although anti-IFN-γ autoantibodies (Abs) have been recognized to cause indeterminate results, the prevalence and neutralizing capacity of these autoantibodies in general population indeterminate samples are unknown. ObjectivesWe sought to determine whether anti-IFN-γ autoantibodies accounted for indeterminate results in the QFT-G Assay.Design/Methods: Patient plasmas from the nil tube of the QFT-G were screened in a blinded fashion for IFN-γ antibodies using a particle-based approach. Samples were from TB positive, TB negative, and indeterminate samples. Binding activity of IgG to IFNγ was determined as a function of fluorescence intensity. Neutralizing capacity as a measure of antibody function was assessed by intranuclear staining of normal peripheral blood mononuclear cells (PBMCs) for IFNγ-induced pSTAT-1 in the presence of patient plasma. ResultsWe screened 537 patient plasmas (151 positive, 352 negative, 34 indeterminate) for anti-IFN-γ Abs from the nil tubes of QFT-G. Of those screened, 14 samples had binding activity to IFNγ. Three samples with high-titer anti-IFN-γ Abs (> mean plus three standard deviations of healthy control samples) neutralized IFNγ in vitro. Two of these samples were indeterminate, and 1 QFT-G negative. Therefore, of 14 samples with anti- IFNγ binding activity, 3 were neutralizing and 2 of those were indeterminate in QFT-G. ConclusionsAnti-IFN-γ autoantibodies were found in approximately 3% of plasmas submitted to a commercial laboratory. Of the 14 samples with binding activity for IFNγ, only 3 were fully neutralizing, and 2 of those occurred in the indeterminate group, accounting for 5% of all indeterminate samples. Anti-IFN-γ Abs are not rare in the population undergoing QFT-G testing and some of these autoantibodies are neutralizing and can affect the result of the assay.