Low-dose endotoxemia is prevalent in humans with adverse health conditions, and it correlates with the pathogenesis of chronic inflammatory diseases such as atherosclerosis, diabetes, and neurologic inflammation. However, the underlying molecular mechanisms are poorly understood. In this study, we demonstrate that subclinical low-dose LPS skews macrophages into a mild proinflammatory state, through cell surface TLR4, IL-1R–associated kinase-1, and the Toll-interacting protein. Unlike high-dose LPS, low-dose LPS does not induce robust activation of NF-κB, MAPKs, PI3K, or anti-inflammatory mediators. Instead, low-dose LPS induces activating transcription factor 2 through Toll-interacting protein–mediated generation of mitochondrial reactive oxygen species, allowing mild induction of proinflammatory mediators. Low-dose LPS also suppresses PI3K and related negative regulators of inflammatory genes. Our data reveal novel mechanisms responsible for skewed and persistent low-grade inflammation, a cardinal feature of chronic inflammatory diseases.

IL-4 was identified as the first cytokine to be produced by mast cells and is responsible for promoting mast cell IL-13 production. IL-4 and IL-13 play a prominent role in stimulating and maintaining the allergic response. As closely related genes, IL-4 and IL-13 share a common receptor subunit, IL-4Rα, necessary for signaling. Here we summarize the literature on mast cell activation associated with IL-4 and IL-13 production, including downstream signaling. We also describe the positive and negative roles each cytokine plays in mast cell immunity and detail the differences that exist between mouse and human mast cell responses to IL-4 and IL-13.

Background: Super-low-dose endotoxemia contributes to cell stress and low-grade inflammation. Results: Super-low-dose LPS removed Tollip from late endosomes/lysosomes and blocked lysosome fusion with endosomes or autophagosomes. Tollip knock-out mice had impaired wound healing. Conclusion: Super-low-dose LPS leads to cell stress through clearing Tollip and blocking lysosome fusion. Significance: Our data reveal molecular dynamics of innate immunity regulation.

Lactic acid (LA) is present in tumors, asthma, and wound healing, environments with elevated IL-33 and mast cell infiltration. While IL-33 is a potent mast cell activator, how LA affects IL-33-mediated mast cell function is unknown. To investigate this, mouse bone marrow-derived mast cells (BMMC) were cultured with or without LA and activated with IL-33. LA reduced IL-33-mediated cytokine and chemokine production. Using inhibitors for monocarboxylate transporters (MCT) or replacing LA with sodium lactate revealed that LA effects are MCT-1- and pH-dependent. LA selectively altered IL-33 signaling, suppressing TAK1, JNK, ERK, and NFκB phosphorylation, but not p38 phosphorylation. LA effects in other contexts have been linked to HIF-1α, which was enhanced in BMMC treated with LA. Since HIF-1α has been shown to regulate the microRNA miR-155 in other systems, LA effects on miR-155-5p and -3p species were measured. In fact, LA selectively suppressed miR-155-5p in a HIF-1α-dependent manner. Moreover, overexpressing miR-155-5p, but not miR-155-3p, abolished LA effects on IL-33-induced cytokine production. These in vitro effects of reducing cytokines were consistent in vivo, since LA injected intraperitoneally into C57BL/6 mice suppressed IL-33-induced plasma cytokine levels. Lastly, IL-33 effects on primary human mast cells were suppressed by LA in an MCT-dependent manner. Our data demonstrate that LA, present in inflammatory and malignant microenvironments, can alter mast cell behavior to suppress inflammation.

TGFβ1 is involved in many pathological conditions, including autoimmune disorders, cancer, cardiovascular and allergic diseases. We have previously found that TGFβ1 can suppress IgE-mediated mast cell activation of human and mouse mast cells. IL-33 is a member of the IL-1 family capable of inducing mast cell responses and enhancing IgE-mediated activation. In this study, we investigated the effects of TGFβ on IL-33-mediated mast cell activation. Bone marrow-derived mast cells cultured in TGFβ −1, −2, or −3 showed reduced IL-33-mediated production of TNF, IL-6, IL-13 and MCP-1, in a concentration-dependent manner. TGFβ1 inhibited IL-33-mediated Akt and ERK phosphorylation as well as NFκB- and AP-1-mediated transcription. These effects were functionally important, as TGFβ1 injection suppressed IL-33-induced systemic cytokines in vivo and inhibited IL-33-mediated cytokine release from human mast cells. TGFβ1 also suppressed the combined effects of IL-33 and IgE-mediated activation on mouse and human mast cells. The role of IL-33 in the pathogenesis of allergic diseases is incompletely understood. These findings, consistent with our previously reported effects of TGFβ1 on IgE-mediated activation, demonstrate that TGFβ1 can provide broad inhibitory signals to activated mast cells.