CD27low CD4 T lymphocytes that accumulate in the mouse lungs during mycobacterial infection differentiate from CD27high precursors in situ, produce IFN-gamma, and protect the host against tuberculosis infection.

The generation of effector, IFN-gamma producing T lymphocytes and their accumulation at sites of infection are critical for host protection against various infectious diseases. The activation and differentiation of naive T lymphocytes into effector memory cells starts in lymphoid tissues, but it is not clear whether the Ag-experienced cells that leave lymph nodes (LN) are mature or if they undergo further changes in the periphery. We have previously shown that CD44(high)CD62L(low) effector CD4 T lymphocytes generated during the course of mycobacterial infection can be segregated into two subsets on the basis of CD27 receptor expression. Only the CD27(low) subset exhibited a high capacity for IFN-gamma secretion, indicating that low CD27 expression is characteristic of fully differentiated effector CD4 T lymphocytes. We demonstrate now that CD27(low) IFN-gamma-producing CD4 T lymphocytes accumulate in the lungs but are rare in LNs. Several factors contribute to their preferential accumulation. First, CD27(low) CD4 T lymphocytes present in the LN are highly susceptible to apoptosis. Second, circulating CD27(low) CD4 T cells do not enter the LN but efficiently migrate to the lungs. Third, CD27(high) effector CD4 T cells that enter the lungs down-regulate CD27 expression in situ. In genetically heterogeneous mice that exhibit varying susceptibility to tuberculosis, the accumulation of mature CD27(low) CD4 T cells in the lungs correlates with the degree of protection against infection. Thus, we propose that terminal maturation of effector CD4 T lymphocytes in the periphery provides the host with efficient local defense and avoids potentially harmful actions of inflammatory cytokines in lymphoid organs.

Mutants of Mycobacterium tuberculosis lacking three of the five rpf-like genes are defective for growth in vivo and for resuscitation in vitro.

Mycobacterium tuberculosis contains five genes, rpfA through rpfE, that bear significant homology to the resuscitation-promoting factor (rpf) gene of Micrococcus luteus, whose product is required to resuscitate the growth of dormant cultures of M. luteus and is essential for the growth of this organism. Previous studies have shown that deletion of any one of the five rpf-like genes did not affect the growth or survival of M. tuberculosis in vitro. In conjunction with the results of whole-genome expression profiling, this finding was indicative of their functional redundancy. In this study, we demonstrate that the single deletion mutants are phenotypically similar to wild-type M. tuberculosis H37Rv in vivo. The deletion of individual rpf-like genes had no discernible effect on the growth or long-term survival of M. tuberculosis in liquid culture, and the ability to resuscitate spontaneously from a nonculturable state in a most probable number assay was also unaffected for the three strains tested (the DeltarpfB, DeltarpfD, and DeltarpfE strains). In contrast, two multiple strains, KDT8 (DeltarpfA-mutation DeltarpfC DeltarpfB) and KDT9 (DeltarpfA DeltarpfC DeltarpfD), which lack three of the five rpf-like genes, were significantly yet differentially attenuated in a mouse infection model. These mutants were also unable to resuscitate spontaneously in vitro, demonstrating the importance of the Rpf-like proteins of M. tuberculosis in resuscitation from the nonculturable state. These results strongly suggest that the biological functions of the five rpf-like genes of M. tuberculosis are not wholly redundant and underscore the potential utility of these proteins as targets for therapeutic intervention.

The OtsAB pathway is essential for trehalose biosynthesis in Mycobacterium tuberculosis.

The disaccharide trehalose is the major free sugar in the cytoplasm of mycobacteria; it is a constituent of cell wall glycolipids, and it plays a role in mycolic acid transport during cell wall biogenesis. The pleiotropic role of trehalose in the biology of Mycobacterium tuberculosis and its absence from mammalian cells suggests that its biosynthesis may provide a useful target for novel drugs. However, there are three potential pathways for trehalose biosynthesis in M. tuberculosis, and the aim of the present study was to introduce mutations into each of the pathways to determine whether or not they are functionally redundant. The results show that the OtsAB pathway, which generates trehalose from glucose and glucose-6-phosphate, is the dominant pathway required for M. tuberculosis growth in laboratory culture and for virulence in a mouse model. Of the two otsB homologues annotated in the genome sequence of M. tuberculosis, only OtsB2 (Rv3372) has a functional role in the pathway. OtsB2, trehalose-6-phosphate phosphatase, is strictly essential for growth and provides a tractable target for high throughput screening. Inactivation of the TreYZ pathway, which can generate trehalose from alpha-1,4-linked glucose polymers, had no effect on the growth of M. tuberculosis in vitro or in mice. Deletion of the treS gene altered the late stages of pathogenesis of M. tuberculosis in mice, significantly increasing the time to death in a chronic infection model. Because the TreS enzyme catalyzes the interconversion of trehalose and maltose, the mouse phenotype could reflect either a requirement for synthesis of additional trehalose or, conversely, a requirement for breakdown of stored trehalose to liberate free glucose.

Mycobacterial dissemination and cellular responses after 1-lobe restricted tuberculosis infection of genetically susceptible and resistant mice.

BACKGROUND AND METHODS: To study mycobacterial dissemination and immune-cell trafficking in tuberculosis, we developed a mouse model in which we introduced 1 microL of Mycobacterium tuberculosis directly into the middle lobe of the right lung. We investigated the kinetics of both mycobacterial spread to different anatomical sites and recruitment of phagocytes and activated lymphocytes. RESULTS: Mycobacterial dissemination was independent of susceptibility to infection and was identical in H-2-congenic mouse strains with high and low resistance to tuberculosis. In resistant mice, recruitment of phagocytic cells to the uninfected lung occurred before the appearance of mycobacteria and decreased shortly thereafter. In susceptible mice, this recruitment was delayed in both lungs but increased during a 10-week period. Recruitment of CD4+ and CD8+ lymphocytes to the contralateral lung was observed before mycobacterial dissemination in both strains, so mycobacterial seeding of secondary tissues occurred in the presence of immune lymphocytes. In resistant mice, more T cells expressed the CD44hi CD62lo activation phenotype, and higher levels of interferon- gamma were produced. CONCLUSIONS: Mycobacterial spread to lymphoid organs preceded spread to the initially uninfected contralateral lung. Genetic differences in susceptibility to tuberculosis are associated with differences in dynamics of the immune response, rather than differences in mycobacterial trafficking.

Different innate ability of I/St and A/Sn mice to combat virulent Mycobacterium tuberculosis: phenotypes expressed in lung and extrapulmonary macrophages.

Mice of the I/St and A/Sn inbred strains display a severe and moderate course, respectively, of disease caused by Mycobacterium tuberculosis. Earlier, we showed that the response to mycobacterial antigens in I/St mice compared to that in A/Sn mice is shifted toward Th2-like reactivity and a higher proliferative activity and turnover of T cells. However, the physiologic basis for different expressions of tuberculosis severity in these mice remains largely unknown. Here, we extend our previous observations with evidence that I/St interstitial lung macrophages are defective in the ability to inhibit mycobacterial growth and to survive following in vitro infection with M. tuberculosis H37Rv. A unique feature of this phenotype is its exclusive expression in freshly isolated lung macrophages. The defect is not displayed in ex vivo macrophages obtained from the peritoneal cavity nor in macrophages developed in vitro from progenitors extracted from various organs, including the lung itself. In addition, we show that, in sharp contrast to peritoneal macrophages, the mycobactericidal capacity of lung macrophages is not elevated in the presence of exogenous gamma interferon. Our data suggest that the in vivo differentiation in a particular anatomical microenvironment determines the pattern of macrophage-mycobacterium interaction. Thus, caution should be exercised when conclusions based upon the results obtained in a particular in vitro system are generalized to the functions of all phagocytes during M. tuberculosis infection.