Potential of the dual IFN-γ/IL-2 fluorescence-immunospot assay to distinguish different stages in bovine tuberculosis

Human studies have identified the potential of measuring Mycobacterium tuberculosis specific IFN-γ and/or IL-2 secreting T cell subsets to distinguish different clinical stages of human tuberculosis (TB). To assess these functional T cell subsets in different states of bovine TB we have established a bovine dual IFN-γ/IL-2 fluorescence-immunospot (FluoroSpot) assay and analysed the frequencies of Mycobacterium bovis (M. bovis) specific IL-2 and/or IFN-γ producing cells in PBMC from 30 cattle naturally infected with M. bovis. Depending on their post mortem results the animals were grouped in 22 cattle with visible lesions (VL) and 8 cattle without visible lesions (NVL). In response to bovine tuberculin purified protein derivative (PPD-B) the frequencies of cytokine producing cells and proportions of IL-2 single producers were significantly higher in VL compared to NVL while PWM-induced cytokine responses were similar between the two groups. Dual IL-2⁺IFN-γ⁺ T cells could be identified as the largest PPD-B responsive T cell subset in both cattle groups. In conclusion, our FluoroSpot is a valid method to enumerate individual antigen-specific IFN-γ⁺ and IL-2⁺ T cell subsets ex vivo. The greater levels of single IL-2 producing T cells associated with the presence of pathology could be a potential biomarker for active TB in cattle.