Gene expression and cytokine profile correlate with mycobacterial growth in a human BCG challenge model

Bacille Calmette Guerin (BCG) is the most widely administered vaccine in the world yet its mechanism of action remains unclear. In this study, we have used samples from a human mycobacterial challenge study in which previously BCG-vaccinated or BCG-naïve UK adults were challenged intradermally with a standard dose of BCG vaccine. BCG remaining at the site of vaccination was quantified by polymerase chain reaction from a skin biopsy sample taken two weeks post-challenge and was used as a measure of BCG growth and functional anti-mycobacterial immunity. Using gene expression microarrays and flow cytometric analysis of intracellular cytokine production, we show that the magnitude of the immune response is greater in previously vaccinated volunteers and that this correlates with reduced mycobacterial growth but increased scarring at the vaccination site. In particular the IFNγ and IL-17 pathways are strongly induced in previously vaccinated volunteers and correlate with reduced mycobacterial growth in this population. This study supports the use of BCG challenge as a tool for evaluating vaccine efficacy and identifying mechanisms of anti-mycobacterial immunity Two groups of volunteers were recruited: BCG naïve (group A, n=11) and those with had previously been vaccinated with BCG (group C, n=13). Volunteers were challenged with a standard dose of SSI BCG on day of challenge (0) and a punch biopsy taken from the site 14 days later (day 14). BCG remaining at the site (present in the biopsy) was quantified by qPCR. BCG was quantified once for each volunteer, at day 14. PBMC were collected from volunteers on days 0, 2, 7 and 14. Gene expression was determined in unstimulated (U) and BCG-stimulated (B) PBMC for the volunteers at each timepoint.