Comparison of M. tuberculosis and M. bovis BCG in diluted whole blood cultures

Despite wide scale vaccination with Mycobacterium bovis BCG, the prevalence of tuberculosis remains high, reflecting the global variable efficacy of this vaccine against adult pulmonary TB. Characterisation of different immune responses to M. tuberculosis and M. bovis BCG would increase understanding of pathology following M. tuberculosis infection or reactivation, and would facilitate the rational design of a new vaccine. Gene expression profiling was conducted on samples from diluted whole blood cultures from three healthy donors following incubation with live mycobacteria for six days. Approximately 8,000 gene entities were at least two-fold up- or down- regulated by the mycobacteria, and both mycobacteria induced similar expression changes in approximately 2,300 genes. Strikingly, many genes exhibited qualitatively different expression patterns, with over 1,000 genes up-regulated in response to M. bovis BCG but not changed by M. tuberculosis. Gene Ontology analysis revealed that the genes which failed to upregulate in M. tuberculosis-infected cultures included a large proportion of genes with lysosomal function. The inhibited up-regulation of expression of IFN-γ-inducible protein 30, acid phosphatase 2, cathepsin B and GM2 ganglioside activator was verified in samples from six biologically independent donors by qRT-PCR. The failure to up-regulate these genes in response to M. tuberculosis may constitute an immune evasion mechanism, preventing intracellular killing and antigen presentation. Blood from three healthy BCG-vaccinated donors was diluted with growth medium and incubated alone or with live M. tuberculosis (H37Rv), M. bovis BCG for 6 days. RNA samples were pooled before hybridisation