Monocyte exposure to LPS induces a transient state in which these cells are refractory to further endotoxin stimulation. This phenomenon, termed endotoxin tolerance (ET), is characterized by a decreased production of cytokines in response to the proinflammatory stimulus. We have established a robust model of ET and have determined the time frame and features of LPS unresponsiveness in cultured human monocytes. A large number of genes transcribed in tolerant monocytes were classified as either "tolerizable" or "nontolerizable" depending on their expression levels during the ET phase. Tolerant monocytes exhibit rapid IL-1R-associated kinase-M (IRAK-M) overexpression, high levels of triggering receptor expressed on myeloid cells-1 (TREM-1) and CD64, and a marked down-regulation of MHC molecules and NF-B2. These cells combine potent phagocytic activity with impaired capability for Ag presentation. We also show that circulating monocytes isolated from cystic fibrosis patients share all the determinants that characterize cells locked in an ET state. These findings identify a new mechanism that contributes to impaired inflammation in cystic fibrosis patients despite a high frequency of infections. Our results indicate that a tolerant phenotype interferes with timing, efficiency, and outcome of the innate immune responses against bacterial infections.

Regulation of adaptive immunity by innate immune cells is widely accepted. Conversely, adaptive immune cells can also regulate cells of the innate immune system. Here, we report for the first time the essential role of B cells in regulating macrophage (M/) phenotype. In vitro B cell/M/ co-culture experiments together with experiments in transgenic mice models for B-cell deficiency or overexpression showed B1 cells to polarize M/ to a distinct phenotype. This was characterized by downregulated TNF-a, IL-1b and CCL3, but upregulated IL-10 upon LPS stimulation; constitutive expression of M2 M/ markers (e.g. Ym1, Fizz1) and overexpression of TRIF-dependent cytokines (IFN-b, CCL5). Mechanistically, this phenotype was linked to a defective NF-jB activation, but a functional TRIF/STAT1 pathway. B1-cell-derived IL-10 was found to be instrumental in the polarization of these M/. Finally, in vivo relevance of B1-cell-induced M/ polarization was confirmed using the B16 melanoma tumor model where adoptive transfer of B1 cells induced an M2 polarization of tumor-associated M/. Collectively, our results define a new mechanism of M/ polarization wherein B1 cells play a key role in driving M/ to a unique, but M2-biased phenotype. Future studies along these lines may lead to targeting of B1 cells to regulate M/ response in inflammation and cancer. Supporting Information available online IntroductionMacrophages (Mf) are a heterogeneous cell population involved in diverse physiological processes including anti-microbial defence, wound resolution, inflammation, tissue remodeling, plaque formation in atherosclerosis and promotion of tumor growth [1][2][3][4][5][6][7]. Despite their heterogeneity, Mf can be broadly divided into M1 (classically activated) or M2 (alternatively activated) phenotypes [1,[8][9][10][11][12]. The M1 phenotype is induced in response to microbial products such as LPS or proinflammatory cytokines including IFN-g, IL-1b or TNF-a. M1 Mf produce further proinflammatory cytokines (TNF-a, IL-12 and CCL3) with reactive oxygen and nitrogen intermediates, which combine to give M1 Mf potent antimicrobial, tumoricidal and inflammatory properties [12]. In contrast, the M2 phenotype results from exposure to antiinflammatory molecules such as glucocorticoid hormones, IL-4, Ã These authors contributed equally to this work. 2296IL-13, IL-10 or immune complexes [10][11][12]. M2 Mf are antiinflammatory and immunosuppressive in nature. They are highly phagocytic, preferentially activate the arginase pathway, promote angiogenesis, tissue remodeling and have pro-tumoral activity [8,12]. However, in vivo, the division between M1 and M2 cells may be blurred, with the above phenotypes likely representing two extremes in a continuum of Mf functional states [9,[13][14][15].Transcriptome data demonstrate the existence of distinct polarization phenotypes for Mf associated with specific pathological conditions [4,7,[16][17][18]. Based on this, it is believed that the phenotype of an Mf is a reflection of its immediate microenvironment....

Repeated exposure to low doses of endotoxin results in progressive hyporesponsiveness to subsequent endotoxin challenge, a phenomenon known as endotoxin tolerance. In spite of its clinical significance in sepsis and characterization of the TLR4 signaling pathway as the principal endotoxin detection mechanism, the molecular determinants that induce tolerance remain obscure. We investigated the role of the TRIF/IFN-β pathway in TLR4-induced endotoxin tolerance. Lipid A-induced homotolerance was characterized by the down-regulation of MyD88-dependent proinflammatory cytokines TNF-α and CCL3, but up-regulation of TRIF-dependent cytokine IFN-β. This correlated with a molecular phenotype of defective NF-κB activation but a functional TRIF-dependent STAT1 signaling. Tolerance-induced suppression of TNF-α and CCL3 expression was significantly relieved by TRIF and IFN regulatory factor 3 deficiency, suggesting the involvement of the TRIF pathway in tolerance. Alternatively, selective activation of TRIF by poly(I:C)-induced tolerance to lipid A. Furthermore, pretreatment with rIFN-β also induced tolerance, whereas addition of IFN-β-neutralizing Ab during the tolerization partially alleviated tolerance to lipid A but not TLR2-induced endotoxin homo- or heterotolerance. Furthermore, IFNAR1−/− murine embryonal fibroblast and bone-marrow derived macrophages failed to induce tolerance. Together, these observations constitute evidence for a role of the TRIF/IFN-β pathway in the regulation of lipid A/TLR4-mediated endotoxin homotolerance.