Mycobacterium tuberculosis is a virulent intracellular pathogen that survives in macrophages even in the presence of an intact adaptive immune response. Type I interferons (IFN) have been shown to exacerbate tuberculosis in mice and to be associated with disease progression in infected humans. Nevertheless, the mechanisms by which type I IFN regulate the host response to M. tuberculosis infection are poorly understood. In this study, we show that M. tuberculosis induces an IFN-related gene expression signature in infected primary human macrophages, which is dependent on host type I IFN signaling as well as the mycobacterial virulence factor, Region of Difference 1. We further demonstrate that type I IFN selectively limits the production of IL-1β, a critical mediator of immunity to M. tuberculosis. This regulation occurs at the level of IL1B mRNA expression, rather than caspase-1 activation or autocrine IL-1 amplification and appears to be preferentially utilized by virulent mycobacteria since avirulent M. bovis bacillus Calmette-Guerin (BCG) fails to trigger significant expression of type I IFN or release of mature IL-1β protein. The latter property is associated with decreased caspase-1-dependent IL-1β maturation in the BCG-infected macrophages. Interestingly, human monocytes in contrast to macrophages produce comparable levels of IL-1β in response to either M. tuberculosis or BCG. Together, these findings demonstrate that virulent and avirulent mycobacteria employ distinct pathways for regulating IL-1β production in human macrophages and reveal that in the case of M. tuberculosis infection the induction of type I IFN is a major mechanism utilized for this purpose.

Recent genome-wide association studies suggest distinct roles for 12 human interferon-alpha (IFN-α) and 3 IFN-λ subtypes that may be elucidated by defining the expression patterns of these sets of genes. To overcome the impediment of high homology among each of the sets, we designed a quantitative real-time PCR assay that incorporates the use of molecular beacon and locked nucleic acid (LNA) probes, and in some instances, LNA oligonucleotide inhibitors. We then measured IFN subtype expression by human peripheral blood mononuclear cells and by purified monocytes, myeloid dendritic cells (mDC), plasmacytoid dendritic cells (pDC), and monocyte-derived macrophages (MDM), and –dendritic cells (MDDC) in response to poly I:C, lipopolysaccharide (LPS), imiquimod and CpG oligonucleotides. We found that in response to poly I:C and LPS, monocytes, MDM and MDDC express a subtype pattern restricted primarily to IFN-β and IFN-λ1. In addition, while CpG elicited expression of all type I IFN subtypes by pDC, imiquimod did not. Furthermore, MDM and mDC highly express IFN-λ, and the subtypes of IFN-λ are expressed hierarchically in the order IFN-λ1 followed by IFN-λ2, and then IFN-λ3. These data support a model of coordinated cell- and ligand-specific expression of types I and III IFN. Defining IFN subtype expression profiles in a variety of contexts may elucidate specific roles for IFN subtypes as protective, therapeutic or pathogenic mediators.

Human I-IFNs include IFN-β and 13 independently regulated subtypes of IFN-α (I-IFNs). TLR7 and -9 induce I-IFNs, but it is unknown whether their subtype repertoire is similar. This study used new PCR arrays that selectively amplify individual I-IFN subtype genes of human and nonhuman primates to characterize the TLR7- and -9-mediated IFN response in vitro and in vivo. We show that in human PBMCs, TLR7 agonists induce a rapid burst of I-IFN transcripts, consisting primarily of IFN-α1/13, -α2, and -α14. In contrast, TLR9 agonists, regardless of the type used (CpG C-, B-, or D-ODN), prompted slower but sustained expression of IFN-α1/13, -α2, -α7, -α8, -α10, -α14, -α16, and -α21. These qualitative differences were translated downstream as differences in the pattern of IFN-inducible genes. In macaque PBMCs, imiquimod produced a short burst of IFN mRNA, dominated by IFN-α8, whereas C- or D-ODN induced a greater than tenfold increase in transcripts for all I-IFN subtypes by 12 h of culture. Differences were more evident in vivo, where TLR7 and -9 agonists induced significantly different levels of I-IFN transcripts in skin. Although the rates of gene transcription differed significantly for individual TLR9 agonists, their IFN-α subtype signature was almost identical, indicating that the type of receptor dictates the quality of the I-IFN response in vitro and in vivo. These results may underlie the differential therapeutic effects of TLR7 and -9 agonists and should inform future clinical studies.