Toll-like receptor (TLR) proteins mediate cellular activation by microbes and microbial products. To delineate the role of TLR proteins in the development of host immune responses against mycobacteria, wild-type and TLR-deficient mice were infected with nonpathogenic Mycobacterium bovis bacillus Calmette-Guerin (BCG). Two weeks after intraperitoneal challenge with BCG, few bacilli were present in the lungs of wild-type and TLR4(-/-) mice, whereas bacterial loads were tenfold higher in the lungs of infected TLR2(-/-) mice. BCG challenge in vitro strongly induced proinflammatory cytokine secretion by macrophages from wild-type and TLR4(-/-) mice but not by TLR2(-/-) macrophages. In contrast, intracellular uptake, intracellular bacterial growth, and suppression of intracellular bacterial growth in vitro by interferon-gamma (IFN-gamma) were similar in macrophages from all three mouse strains, suggesting that BCG growth in the lungs of TLR2(-/-) mice was a consequence of defective adaptive immunity. Antigenic stimulation of splenocytes from infected wild-type and TLR4(-/-) mice induced T cell proliferation in vitro, whereas T cells from TLR2(-/-) mice failed to proliferate. Unexpectedly, activated CD4(+) T cells from both TLR-deficient mouse strains secreted little IFN-gamma in vitro compared with control T cells. A role for TLR4 in the control of bacterial growth and IFN-gamma production in vivo was observed only when mice were infected with higher numbers of BCG. Thus, TLR2 and TLR4 appear to regulate distinct aspects of the host immune response against BCG.

Both lymphoid and myeloid cells express two related members of the IFN regulatory factor (IRF) family of transcription factors, specifically IRF-4 and IFN consensus binding protein (ICSBP or IRF-8). We previously reported that macrophages express IRF-4 and in combination with the ETS-like protein PU.1 can synergistically activate a human IL-1β reporter gene. Here we report that this synergy is mediated by a composite PU.1/IRF element located within an upstream enhancer known to confer cytokine- and LPS-inducible expression. In macrophages, synergistic activation of IL-1β reporter gene expression was preferentially mediated by IRF-4, whereas IRF-4 and ICSBP were equally capable of synergizing with PU.1 when coexpressed in fibroblasts. Furthermore, coexpression of IRF-1 and IRF-2 dramatically increased the capacity of both PU.1/IRF-4 and PU.1/ICSBP to induce IL-1β reporter gene expression in fibroblasts. The additional synergy observed with IRF-1 and IRF-2 coexpression is mediated by a region of DNA distinct from either the IL-1β enhancer or promoter. We also assessed the capacity of these transcription factors to activate endogenous IL-1β gene when overexpressed in human embryonic kidney 293 cells. Although ectopic expression of PU.1 alone was sufficient to activate modest levels of IL-1β transcripts, endogenous IL-1β expression was markedly increased following coexpression of additional IRF proteins. Thus, maximal expression of both a human IL-1β reporter gene and the endogenous IL-1β gene was observed in cells that coexpressed PU.1, IRF-4 (or ICSBP), IRF1, and IRF2. Together, our observations suggest that these factors may function together as an enhanceosome.

Cytokine transcription is usually regulated by transcription factor binding and chromatin remodeling following an inducing signal. By contrast, these data showed the interleukin (IL)-1␤ promoter assembles into a "poised" structure, as evidenced by nuclease accessibility and loss of core histones immediately surrounding the transcription start site. Strikingly, these properties do not change upon transcriptional activation by lipopolysaccharide. Furthermore, association of two key transcriptional activators, PU.1 and C/EBP␤, is robust pre-and post-stimulation indicating the IL-1␤ promoter is packaged into a nontranscribed but poised promoter architecture in cells capable of rapidly inducing IL-1␤. Monocyte stimulation causes recruitment of a third factor, IRF-4, to the IL-1␤ enhancer. PU.1 phosphorylation at a CK2 kinase consensus element is required for this recruitment. We showed that CK2 phosphorylates PU.1, CK2 inhibitors abrogate IL-1␤ induction, and CK2 inducibly associates with the IL-1␤ enhancer. Taken together, these data indicate a novel two-step mechanism for IL-1␤ transcription: 1) formation of a poised chromatin architecture, and 2) phosphorylation of an enhancer-bound factor that recruits other activators. We propose that this poised structure may generally characterize rapidly activated genes.

Sepsis is initiated by interactions between microbial products and host inflammatory cells. Toll-like receptors (TLRs) are central innate immune mediators of sepsis that recognize different components of microorganisms. Peptidoglycan-associated lipoprotein (PAL) is a ubiquitous gram-negative bacterial outer-membrane protein that is shed by bacteria into the circulation of septic animals. We explored the inflammatory effects of purified PAL and of a naturally occurring form of PAL that is shed into serum. PAL is released into human serum by Escherichia coli bacteria in a form that induces cytokine production by macrophages and is tightly associated with lipopolysaccharide (LPS). PAL activates inflammation through TLR2. PAL and LPS synergistically activate macrophages. These data suggest that PAL may play an important role in the pathogenesis of sepsis and imply that physiologically relevant PAL and LPS are shed into serum and act in concert to initiate inflammation in sepsis.