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The innate immune system has developed to acquire a wide variety of pattern-recognition receptors (PRRs) to identify potential pathogens, whereas pathogens have also developed to escape host innate immune responses. ITIM-bearing receptors are attractive targets for pathogens to attenuate immune responses against them; however, the in vivo role of the inhibitory PRRs in host–bacteria interactions remains unknown. We demonstrate in this article that Staphylococcus aureus, a major Gram-positive bacteria, exploits inhibitory PRR paired Ig-like receptor (PIR)-B on macrophages to suppress ERK1/2 and inflammasome activation, and subsequent IL-6 and IL-1β secretion. Consequently, Pirb−/− mice infected with S. aureus showed enhanced inflammation and more effective bacterial clearance, resulting in resistance to the sepsis. Screening of S. aureus mutants identified lipoteichoic acid (LTA) as an essential bacterial cell wall component required for binding to PIR-B and modulating inflammatory responses. In vivo, however, an LTA-deficient S. aureus mutant was highly virulent and poorly recognized by macrophages in both wild-type and Pirb−/− mice, demonstrating that LTA recognition by PRRs other than PIR-B mediates effective bacterial elimination. These results provide direct evidence that bacteria exploit the inhibitory receptor for virulence, and host immune system counterbalances the infection.

Macrophages recognize the presence of infection by using the Toll-like receptor (TLR) family of proteins that detect ligands on bacterial, viral, and fungal pathogens. We show that murine macrophages stimulated with pathogen products known to signal through TLRs express ligands for the NKG2D receptor, found on NK cells, activated CD8+ T cells and activated macrophages. TLR signaling, through the MyD88 adaptor, up-regulates transcription of the retinoic acid early inducible-1 (RAE-1) family of NKG2D ligands, but not H-60 or murine UL16-binding protein-like transcript-1. RAE-1 proteins are found on the surface of activated, but not resting, macrophages and can be detected by NKG2D on NK cells resulting in down-regulation of this receptor both in vitro and in vivo. RAE-1-NKG2D interactions provide a mechanism by which NK cells and infected macrophages communicate directly during an innate immune response to infection.

IL-12 (or LPS) priming and subsequent challenge by LPS produces the generalized Shwartzman reaction. IFN-γ induced by IL-12 is a crucial cytokine in the priming phase. In vivo depletion of both NK cells and NK1+ αβ T cells of mice by anti-NK1.1 Ab greatly reduced the elevation of serum IFN-γ induced by IL-12 and significantly reduced mortality after subsequent injection of LPS, whereas depletion of NK cells alone by anti-asialo GM1 Ab only partially decreased serum IFN-γ, and lethality was not changed. Cell sorting and culture experiments confirmed that liver NK1+ αβ T cells of IL-12-injected mice produced greater amounts of IFN-γ than did liver NK cells. MHC class I-deficient mice of C57BL/6 background, which lack a majority of NK1+ αβ T cells, produced low amounts of IFN-γ by IL-12; no mortality was observed after the LPS challenge. However, production of TNF-α in the second phase (after LPS challenge) was not inhibited by depletion of NK cells alone or both subsets. IL-12 and subsequent LPS challenge activated NK1+ αβ T cells in the liver and induced strong cytotoxicity of these cells not only against tumor cells (including Fas-negative tumors) but also against a syngeneic hepatocyte cell line. Our findings show that IFN-γ produced by NK1+ αβ T cells is essential for the IL-12 priming of the Shwartzman reaction, and the autoreactivity of NK1+ αβ T cells in the liver is involved in the hepatic disorders that are sometimes caused by IL-12, LPS, or the generalized Shwartzman reaction.

NKG2D is an activating receptor on NK cells and activated CD8 T cells that has been implicated in tumor immunity, viral immunity and autoimmunity. The ligands of NKG2D are tightly regulated stress-inducible molecules, whose expression is triggered by transformation or infection. In mice, these NKG2D ligands include the retinoic acid early inducible-1 (RAE-1) proteins. However, the mechanisms of transcriptional regulation in RAE-1 genes are poorly understood. We examined what transcription factors regulate RAE-1 expression. We found that RAE-1 delta has six exons and RAE-1 epsilon has seven exons. We isolated genomic DNAs of upstream from exon one which are thought of RAE-1 promoters in RAE-1 delta or RAE-1 epsilon. We found that consensus sequences of several transcription factors are present in RAE promoters. Therefore, these transcription factors may regulate RAE-1 expressions. This work is supported by Grants-in Aid for Science Research from the Ministries of Education, Culture, Sports, Science, and Technology of Japan, Grants-in Aid for Science Research from the Ministriesof Health, Labor, and Welfare of Japan, Human Frontier Science Program CDA, The Mitsubishi Foundation, The Sumitomo Foundation, Takeda Science Foundation and Uehara Memorial Foundation.