Apoptosis was induced rapidly in HeLa cells after exposure to bacterial Shiga toxin (Stx1 and Stx2; 10 ng/ml). Approximately 60% of HeLa cells became apoptotic within 4 h as detected by DNA fragmentation, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) assay, and electron microscopy. Stx1-induced apoptosis required enzymatic activity of the Stx1A subunit, and apoptosis was not induced by the Stx2B subunit alone or by the anti-globotriaosylceramide antibody. This activity was also inhibited by brefeldin A, indicating the need for toxin processing through the Golgi apparatus. The intracellular pathway leading to apoptosis was further defined. Exposure of HeLa cells to Stx1 activated caspases 3, 6, 8, and 9, as measured both by an enzymatic assay with synthetic substrates and by detection of proteolytically activated forms of these caspases by Western immunoblotting. Preincubation of HeLa cells with substrate inhibitors of caspases 3, 6, and 8 protected the cells against Stx1-dependent apoptosis. These results led to a more detailed examination of the mitochondrial pathway of apoptosis. Apoptosis induced by Stx1 was accompanied by damage to mitochondrial membranes, measured as a reduced mitochondrial membrane potential, and increased release of cytochrome c from mitochondria at 3 to 4 h. Bid, an endogenous protein known to permeabilize mitochondrial membranes, was activated in a Stx1-dependent manner. Caspase-8 is known to activate Bid, and a specific inhibitor of caspase-8 prevented the mitochondrial damage. Although these data suggested that caspase-8-mediated cleavage of Bid with release of cytochrome c from mitochondria and activation of caspase-9 were responsible for the apoptosis, preincubation of HeLa cells with a specific inhibitor of caspase-9 did not protect against apoptosis. These results were explained by the discovery of a simultaneous Stx1-dependent increase in endogenous XIAP, a direct inhibitor of caspase-9. We conclude that the primary pathway of Stx1-induced apoptosis and DNA fragmentation in HeLa cells is unique and includes caspases 8, 6, and 3 but is independent of events in the mitochondrial pathway.
The physiological relevance of naturally occurring IgM-ALA remains to be elucidated. These autoantibodies are present from birth and increase in diverse inflammatory states that are both infectious and noninfectious. Clinical observations showing significantly less acute allograft rejections in recipients having high IgM-ALA levels, led us to investigate whether IgM-ALA could have a functional role in attenuating T cell mediated inflammatory responses. In pursuit of this hypothesis, we did studies using IgM purified from the serum of normal individuals, patients with end stage renal disease, and HIV-1 infection. All preparations of IgM immunoprecipitated certain receptors e.g., CD3, CD4, CCR5, and CXCR4 from whole cell lysates but failed to immunoprecipitate IL-2R and HLA Ags. In physiological doses IgM down-regulated CD4, CD2 and CD86 but not CD8 and CD28, inhibited T cell proliferation, decreased production of certain proinflammatory cytokines e.g., TNF-α, IL-13 and IL-2, but not IFN- γ, IL-1β, GM-CSF, IL-6 and IL-8 and inhibited leukocyte chemotaxis. These inhibitory effects were more pronounced when using IgM from patients with high levels of IgM-ALA and these inhibitory effects were significantly reduced after using IgM preabsorbed with leukocytes. IgM-ALA binding to leukocytes was found to be highly specific, as <10% of IgM secreting B cell clones had IgM-ALA specificity with some clones having specificity for either T cells or monocytes. These findings support the concept that IgM-ALA provides an innate mechanism to regulate T cell mediated inflammatory responses.
Little is known about the function of natural IgM auto-antibodies and especially IgM with anti-leukocyte reactivity (IgM-ALA). Natural IgM-ALA auto-antibodies are present at birth and characteristically increase during inflammatory and infective conditions. Our prior clinical observations and those of others showing less rejections in renal and cardiac allografts transplanted into recipients with high levels of IgM-ALA, led us to investigate if IgM-ALA regulate the inflammatory response. Here we show that IgM, in physiologic doses, inhibit pro-inflammatory cells from proliferating and producing IFN-γ and IL-17 in response to alloantigens (MLR), anti-CD3 and the glycolipid alpha-gal ceramide. We show in an IgMko murine model, with intact B cells and Tregs, that there is more severe inflammation and loss of function in absence of IgM after renal ischemia reperfusion injury (IRI) and cardiac allograft rejection. Replenishing IgM in IgMko or increasing the levels of IgM-ALA in WT-B6 mice significantly attenuated the inflammation in both these inflammatory models which involve IFN-γ and IL-17. The protective effect on renal IRI wasnot observed using IgM pre-adsorbed with leukocytes to remove IgM-ALA. We provide data to show that the anti-inflammatory effect of IgM is in part mediated by inhibiting TLR4 induced NF-kB translocation into the nucleus and inhibiting differentiation of activated T cells into TH-1 and TH-17 cells. These observations highlight the importance of IgM-ALA in regulating excess inflammation mediated by both innate and adaptive immune mechanisms and where the inflammatory response involves TH-17 cells that are not effectively regulated by Tregs.
We have previously shown that polyclonal natural IgM protects mice from renal IRI by inhibiting the reperfusion inflammatory response. We hypothesized that a potential mechanism involved IgM modulation of dendritic cells as we observed high IgM binding to splenic DC. To test this hypothesis, we pre-treated BMDC with polyclonal murine or human IgM prior to LPS activation and demonstrate that 0.5 × 106 IgM/LPS pretreated BMDC, when injected into WT-B6 mice, 24 hours before renal ischemia, protect mice from developing renal IRI. We show that this switching of LPS activated BMDC to a regulatory phenotype requires modulation of BMDC function that is mediated by IgM binding to non-apoptotic BMDC receptors. Regulatory BMDC require IL-10 and PD1 as well as downregulation of CD40 and p65NF-κB phosphorylation to protect in renal IRI. Blocking the PD1 ligand binding site just before intravenous injection of IgM/LPS pretreated BMDC or using IL-10ko BMDC fails to induce protection. Similarly, IgM/LPS pretreated BMDC are rendered non-protective by increasing CD40 expression and phosphorylation of p65NF-κB. How IgM/LPS regulatory BMDC suppress in-vivo ischemia induced innate inflammation remains to be determined. However, we show that suppression is dependent on other in-vivo regulatory mechanisms in the host i.e. CD25+ T cells, B cells, IL10 and circulating IgM. There was no increase in Foxp3+ Tregs in the spleen either before or after renal IRI. Collectively, these findings show that natural IgM anti-leucocyte antibodies can switch BMDC to a regulatory phenotype despite the presence of LPS that ordinarily induces BMDC maturation.
Dendritic cells (DCs) are central in regulating immune responses of kidney ischemia-reperfusion injury (IRI), and strategies to alter DC function may provide new therapeutic opportunities. Sphingosine 1-phosphate (S1P) modulates immunity through binding to its receptors (S1P1-5), and protection from kidney IRI occurs in mice treated with S1PR agonist, FTY720 (FTY). We tested if ex vivo propagation of DCs with FTY could be used as cellular therapy to limit the off-target effects associated with systemic FTY administration in kidney IRI. DCs have the ability of regulate innate and adaptive responses and we posited that treatment of DC with FTY may underlie improvements in kidney IRI. Herein, it was observed that treatment of bone marrow derived dendritic cells (BMDCs) with FTY induced mitochondrial biogenesis, FTY-treated BMDCs (FTY-DCs) showed significantly higher oxygen consumption rate and ATP production compared to vehicle treated BMDCs (Veh-DCs). Adoptive transfer of FTY-DCs to mice 24 h before or 4 h after IRI significantly protected the kidneys from injury compared to mice treated with Veh-DCs. Additionally, allogeneic adoptive transfer of C57BL/6J FTY-DCs into BALB/c mice equally protected the kidneys from IRI. FTY-DCs propagated from S1pr1-deficient DCs derived from CD11cCreS1pr1 fl/fl mice as well as blunting mitochondrial oxidation in wildtype (WT) FTY-DCs prior to transfer abrogated the protection observed by FTY-DCs. We queried if DC mitochondrial content alters kidney responses after IRI, a novel but little studied phenomenon shown to be integral to regulation of the immune response. Transfer of mitochondria rich FTY-DCs protects kidneys from IRI as transferred FTY-DCs donated their mitochondria to recipient splenocytes (i.e., macrophages) and prior splenectomy abrogated this protection. Adoptive transfer of FTY-DCs either prior to or after ischemic injury protects kidneys from IRI demonstrating a potent role for donor DC-mitochondria in FTY's efficacy. This is the first evidence, to our knowledge, that DCs have the potential to protect against kidney injury by donating mitochondria to splenic macrophages to alter their bioenergetics thus making them anti-inflammatory. In conclusion, the results support Rousselle et al. Transfer of FTY-DC Prevents Kidney IRI that ex vivo FTY720-induction of the regulatory DC phenotype could have therapeutic relevance that can be preventively infused to reduce acute kidney injury.
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