Development and evaluation of a real-time reverse transcription-PCR assay for quantification of gamma interferon mRNA to diagnose tuberculosis in multiple animal species.

Tuberculosis of free-ranging and captive wildlife, including species implicated in the maintenance and transmission of Mycobacterium bovis, is a difficult disease to diagnose and control. Historically, diagnosis of tuberculosis has relied largely upon assays of cell-mediated immunity (CMI), such as tuberculin skin testing. This approach, however, is problematic or impractical for use with many wildlife species. Increasingly, in vitro diagnostic tests, including gamma interferon (IFN-gamma)-based assays, are replacing or complementing skin testing of cattle and humans. Analogous assays are unavailable for most wildlife because of a lack of species-specific immunological reagents. This report describes the development and validation of a whole-blood assay to quantify antigen-specific IFN-gamma mRNA expression by quantitative real-time reverse transcription-PCR. Oligonucleotide primers and probes were designed and tested for reactivity towards several susceptible species of interest with respect to tuberculosis infection. The assay was subsequently optimized to quantify the IFN-gamma mRNA expression in elk and red deer (Cervus elaphus) and was evaluated for its ability to detect mycobacterial antigen-specific responses of experimentally tuberculosis-infected animals. The assay was a simple, rapid, and sensitive measure of antigen-specific CMI. The IFN-gamma mRNA responses correlated well with IFN-gamma protein production and showed performance in determining an animal's infection status superior to that of either lymphocyte proliferation or IFN-gamma protein enzyme-linked immunosorbent assay methods. An additional advantage is the ease with which the assay can be modified to reliably quantify IFN-gamma expression by using consensus sequences of closely related species or of other species for which IFN-gamma sequence information is available.