Fifty million new infections with Mycobacterium tuberculosis occur annually, claiming 2-3 million lives from tuberculosis worldwide. Despite the apparent lack of significant genetic heterogeneity between strains of M. tuberculosis, there is mounting evidence that considerable heterogeneity exists in molecules important in disease pathogenesis. These differences may manifest in the ability of some isolates to modify the host cellular immune response, thereby contributing to the observed diversity of clinical outcomes. Here we describe the identification and functional relevance of a highly biologically active lipid species-a polyketide synthase-derived phenolic glycolipid (PGL) produced by a subset of M. tuberculosis isolates belonging to the W-Beijing family that show 'hyperlethality' in murine disease models. Disruption of PGL synthesis results in loss of this hypervirulent phenotype without significantly affecting bacterial load during disease. Loss of PGL was found to correlate with an increase in the release of the pro-inflammatory cytokines tumour-necrosis factor-alpha and interleukins 6 and 12 in vitro. Furthermore, the overproduction of PGL by M. tuberculosis or the addition of purified PGL to monocyte-derived macrophages was found to inhibit the release of these pro-inflammatory mediators in a dose-dependent manner.

To understand how virulent mycobacteria subvert host immunity and establish disease, we examined the differential response of mice to infection with various human outbreak Mycobacterium tuberculosis clinical isolates. One clinical isolate, HN878, was found to be hypervirulent, as demonstrated by unusually early death of infected immune-competent mice, compared with infection with other clinical isolates. The differential effect on survival required lymphocyte function because severe combined immunodeficiency (SCID) mice infected with HN878 or other clinical isolates all died at the same rate. The hypervirulence of HN878 was associated with failure to induce M. tuberculosis-specific proliferation and IFN-␥ production by spleen and lymph node cells from infected mice. In addition, 2-to 4-fold lower levels of tumor necrosis factor-␣ (TNF-␣), IL-6, IL-12, and IFN-␥ mRNAs were observed in lungs of HN878-infected mice. IL-10, IL-4, and IL-5 mRNA levels were not significantly elevated in lungs of HN878 infected mice. In contrast, IFN-␣ mRNA levels were significantly higher in lungs of these mice. To further investigate the role of Type 1 IFNs, mice infected with HN878 were treated intranasally with purified IFN-␣͞␤. The treatment resulted in increased lung bacillary loads and even further reduced survival. These results suggest that the hypervirulence of HN878 may be due to failure of this strain to stimulate Th1 type immunity. In addition, the lack of development of Th1 immunity in response to HN878 appears to be associated with increased induction of Type 1 IFNs.

The role of type I interferons (IFNs) in the host response to bacterial infections is controversial. Here, we examined the role of IFN-alpha/beta in the murine response to infection with Mycobacterium tuberculosis, using wildtype mice, mice with impaired signaling through the type I IFN receptor (IFNAR), and mice treated to reduce levels of type I IFNs. In this study, we used virulent clinical isolates of M. tuberculosis, including HN878, W4, and CDC1551. Our results indicate that higher levels of type I IFNs are induced by the HN878 and W4 strains. Induction of type I IFNs was associated with lower levels of tumor necrosis factor-alpha (TNF-alpha) and interleukin- 12 (IL-12) and reduced T cell activation, and associated with decreased survival of the mice infected with HN878 or W4 relative to infection with CDC1551. Infection of mice with HN878 and W4 was also associated with relatively higher levels of mRNA for a number of negative regulators of the Jak-Stat signaling pathway, such as suppressors of cytokine signaling (SOCS) 1, 4, and 5, CD45, protein inhibitor of activated Stat1 (PIAS1), protein tyrosine phosphatase nonreceptor type 1 (Ptpn1), and protein tyrosine phosphatase nonreceptor type substrate 1 (Ptpns1). Taken together, these results suggest that increased type I IFNs may be deleterious for survival of M. tuberculosis-infected mice in association with reduced Th1 immunity.

In vitro infection of monocytes with Mycobacterium tuberculosis HN878 and related W/Beijing isolates preferentially induced interleukin-4 (IL-4) and IL-13, which characterize Th2 polarized immunity. In contrast, CDC1551 induced more IL-12 and other molecules associated with phagocyte activation and Th1 protective immunity. The differential cytokine-chemokine response was mediated by extracted lipids, suggesting that these molecules regulate host responses to infection.