In situ detection of Mycobacterium tuberculosis transcripts in human lung granulomas reveals differential gene expression in necrotic lesions

We have used RNA-RNA in situ hybridization to detect the expression of several Mycobacterium. tuberculosis genes in tuberculous granulomas in lung tissue sections from tuberculosis patients. The M. tuberculosis genes chosen fall into two classes. Four genes (icl, narX, and Rv2557 and Rv2558) have been implicated in the persistence of the bacterium in the host, and two genes (iniB and kasA) are upregulated in response to isoniazid exposure. Both necrotic and nonnecrotic granulomas were identified in all of the patients. Necrotic granulomas were divided into three zones: an outer lymphocyte cuff containing lymphocytes and macrophages, a transition zone consisting of necrotic material interspersed with macrophages, and a central acellular necrotic region. Transcripts of all of the genes studied were found in nonnecrotic granulomas and in the lymphocyte cuff of necrotic granulomas. Mycobacterial gene expression was associated with CD68-positive myeloid cells. Rv2557 and/or its homologue Rv2558, kasA, and iniB were expressed within the transition zone of necrotic granulomas, whereas icl and narX transcripts were absent from this area. There was no evidence of transcription of any of the genes examined in the central necrotic region, although mycobacterial DNA was present. The differential expression of genes within granulomas demonstrates that M. tuberculosis exists in a variety of metabolic states and may be indicative of the response to different microenvironments. These observations confirm that genes identified in models of persistence or in response to drug treatment in vitro are expressed in the human host.