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There is an urgent need to identify additional diagnostic biomarkers for bovine TB to complement existing read-out systems such as interferon-gamma and for predictive markers of vaccine efficacy to accelerate vaccine development. To evaluate the potential of miRNAs as such biomarkers, we have analysed their expression in bovine PPD stimulated PBMC isolated from unvaccinated and BCG vaccinated cattle before and following Mycobacterium bovis ( M. bovis ) infection. Using a bovine microRNA microarray, miR-155 was found to show a significant up-regulation in expression in early (week 2) and late (week 11) M. bovis post-infection samples from unvaccinated cattle, while in BCG vaccinated cattle up-regulation was observed only in late post-infection samples. No differential expression of miR-155 was observed in pre-infection samples from unvaccinated and vaccinated cattle. These observations suggest that miR-155 could be exploited as a marker distinguishing vaccinated from infected animals (DIVA). Analysis by TaqMan RT-PCR, verified the up-regulation of miR-155 in unvaccinated cattle post-infection. Significant correlation was found between the degree of pathology and miR-155 induction in the experimentally infected cattle, suggesting miR-155 is a biomarker of disease development and/or severity. Induction of miR155 expression in cattle sourced from farms with confirmed bTB that tested positive in the tuberculin skin or interferon-gamma blood test was found to be significantly higher in cattle presenting with more advanced pathology (defined by the presence of visible TB lesions) compared to infected cattle without visible pathology and thus likely to be of lower infectivity than those with more advanced disease. In conclusion, our data indicate that miR-155 has potential both as a diagnostic and prognostic biomarker that could be used to identify animals with advanced pathology and as a DIVA test read-out. Its role in the immune biology of bovine TB will also be discussed. [ABSTRACT FROM AUTHOR]