IL-17 is a cytokine produced by T cells in response to IL-23. Recent data support a new subset of CD4 Th cells distinct from Th1 or Th2 cells that produce IL-17 and may contribute to inflammation. In this study, we demonstrate that, in naive mice, as well as during Mycobacterium tuberculosis infection, IL-17 production is primarily from γδ T cells and other non-CD4+CD8+ cells, rather than CD4 T cells. The production of IL-17 by these cells is stimulated by IL-23 alone, and strongly induced by the cytokines, including IL-23, produced by M. tuberculosis-infected dendritic cells. IL-23 is present in the lungs early in infection and the IL-17-producing cells, such as γδ T cells, may represent a central innate protective response to pulmonary infection.

Transcription factors that regulate monocyte subset differentiation in bone marrow have not yet been identified. Here we show that the orphan nuclear receptor NR4A1 controls Ly6C− monocyte differentiation. Ly6C− monocytes , which function in a surveillance role in circulation, were absent in Nr4a1−/− mice. Normal numbers of myeloid progenitor cells were present in Nr4a1−/− mice, indicating that the defect occurs during later stages of monocyte development. The defect is cell-intrinsic, as wild-type mice receiving bone marrow from Nr4a1−/− mice developed reduced numbers of patrolling monocytes. Ly6C− monocytes remaining in the bone marrow of Nr4a1−/− mice were arrested in the S phase of the cell cycle and underwent apoptosis. Thus, NR4A1 functions as a master regulator of differentiation and survival of ‘patrolling’ Ly6C− monocytes.

IFN-γ is necessary in both humans and mice for control of Mycobacterium tuberculosis (M. tuberculosis). CD4 T cells are a significant source of IFN-γ during acute infection in mice and are required for control of bacterial growth and host survival. However, several other types of cells can and do produce IFN-γ during the course of the infection. We sought to determine whether IFN-γ from sources other than CD4 T cells was sufficient to control M. tuberculosis infection and whether CD4 T cells had a role in addition to IFN-γ production. To investigate the role of IFN-γ from CD4 T cells, a murine adoptive transfer model was developed in which all cells were capable of producing IFN-γ, with the exception of CD4 T cells. Our data in this system support that CD4 T cells are essential for control of infection but also that IFN-γ from CD4 T cells is necessary for host survival and optimal long-term control of bacterial burden. In addition, IFN-γ from CD4 T cells was required for a robust CD8 T cell response. IFN-γ from T cells inhibited intracellular replication of M. tuberculosis in macrophages, suggesting IFN-γ may be necessary for intracellular bactericidal activity. Thus, although CD4 T cells play additional roles in the control of M. tuberculosis infection, IFN-γ is a major function by which these cells participate in resistance to tuberculosis.

The liver is an important site of host-microbe interaction. Although hepatocytes have been reported to be responsive to lipopolysaccharide (LPS), the global gene expression changes by LPS and mechanism(s) by which LPS stimulates cultured hepatocytes remain uncertain. Cultures of primary mouse hepatocytes were incubated with LPS to assess its effects on the global gene expression, hepatic transcription factors, and mitogen-activated protein (MAP) kinase activation. DNA microarray analysis indicated that LPS modulates the selective expression of more than 80 genes and expressed sequence tags. We have shown previously that hepatocytes express CD14, which is required both for uptake and responsiveness to LPS. In other cells, responsiveness to microbial products requires expression of Toll-like receptors (TLR) and their associated accessory molecules. Hepatocytes expressed TLR1 through TLR9 as well as MyD88 and MD-2 transcripts, as shown by reverse transcriptase PCR analysis, indicating that hepatocytes express all known microbe recognition molecules. The MAP kinase extracellular signal-regulated kinase 1/2 was phosphorylated in response to LPS in mouse hepatocytes, and the levels of phosphorylation were lower in hepatocytes from TLR4-null mice. NF-B activation was reduced in TLR4-mutant or -null hepatocytes compared to control hepatocytes, and this defect was partially restored by adenoviral transduction of mouse TLR4. Thus, hepatocytes respond to nanogram concentrations of LPS through a TLR4 response pathway.Lipopolysaccharide (LPS), a glycolipid constituent of the outer membrane of gram-negative bacteria, initiates signaling cascades in cells such as macrophages and endothelial cells, leading to the release of cytokines and other inflammatory mediators during sepsis. Excessive production of these mediators can cause septic shock and multiple organ failure (55).A decade ago, CD14, a 55-kDa glycoprotein and monocyte differentiation antigen, was identified as an important LPS recognition molecule (60). CD14 alone, however, is unable to transduce the intracellular LPS signal, since CD14 is only tethered to the cytoplasmic membrane by a glycosyl phosphatidylinositol anchor and lacks a membrane-spanning domain (17). Members of a family of proteins, the mammalian homologues of the Drosophila Toll protein, were found to act as transmembrane coreceptors to CD14 in the cellular response to LPS (34). These Toll-like receptors (TLR) contain ectodomains with leucine-rich repeats, and their intracellular motifs are highly homologous to intracellular signaling domains of interleukin-1 receptor type I (IL-1RI) and IL-1RI accessory protein (reviewed in reference 5). Following dimerization of the TLR, these domains attract the adapter protein MyD88, which in turn recruits the IL-1R-associated kinase. Following this association, IL-1R-associated kinase phosphorylates tumor necrosis factor receptor-associated factor 6, which in turn attracts two more protein tyrosine kinases, transforming growth factor beta-activated kinase 1 (TAK-1)...

CD4 T cells are believed to be important in protection against Mycobacterium tuberculosis, but the relative contribution to control of initial or latent infection is not known. Antibody-mediated depletion of CD4 T cells in M. tuberculosis-infected cynomolgus macaques was used to study the role of CD4 T cells during acute and latent infection. Anti-CD4 antibody severely reduced levels of CD4 T cells in blood, airways, and lymph nodes. Increased pathology and bacterial burden were observed in CD4-depleted monkeys during the first 8 weeks of infection compared to controls. CD4-depleted monkeys had greater interferon (IFN)-γ expression and altered expression of CD8 T cell activation markers. During latent infection, CD4 depletion resulted in clinical reactivation in only three of six monkeys. Reactivation was associated with lower CD4 T cells in the hilar lymph nodes. During both acute and latent infection, CD4 depletion was associated with reduced percentages of CXCR3(+) expressing CD8 T cells, reported to be involved in T cell recruitment, regulatory function, and effector and memory T cell maturation. CXCR3(+) CD8 T cells from hilar lymph nodes had more mycobacteria-specific cytokine expression and greater coexpression of multiple cytokines compared to CXCR3(-) CD8 T cells. CD4 T cells are required for protection against acute infection but reactivation from latent infection is dependent on the severity of depletion in the draining lymph nodes. CD4 depletion influences CD8 T cell function. This study has important implications for human HIV-M. tuberculosis coinfection.