Fas-associated death domain protein (FADD) and caspase-8 (casp8) are vital intermediaries in apoptotic signaling induced by tumor necrosis factor family ligands. Paradoxically, lymphocytes lacking FADD or casp8 fail to undergo normal clonal expansion following antigen receptor cross-linking and succumb to caspase-independent cell death upon activation. Here we show that T cells lacking FADD or casp8 activity are subject to hyperactive autophagic signaling and subvert a cellular survival mechanism into a potent death process. T cell autophagy, enhanced by mitogenic signaling, recruits casp8 through interaction with FADD:Atg5-Atg12 complexes. Inhibition of autophagic signaling with 3-methyladenine, dominant-negative Vps34, or Atg7 shRNA rescued T cells expressing a dominant-negative FADD protein. The necroptosis inhibitor Nec-1, which blocks receptor interacting protein kinase 1 (RIP kinase 1), also completely rescued T cells lacking FADD or casp8 activity. Thus, while autophagy is necessary for rapid T cell proliferation, our findings suggest that FADD and casp8 form a feedback loop to limit autophagy and prevent this salvage pathway from inducing RIPK1-dependent necroptotic cell death. Thus, linkage of FADD and casp8 to autophagic signaling intermediates is essential for rapid T cell clonal expansion and may normally serve to promote caspase-dependent apoptosis under hyperautophagic conditions, thereby averting necrosis and inflammation in vivo.apoptosis ͉ autophagy ͉ caspases ͉ FADD ͉ necroptosis
Caspase-8 (casp8) is required for extrinsic apoptosis, and mice deficient in casp8 fail to develop and die in utero while ultimately failing to maintain the proliferation of T cells, B cells, and a host of other cell types. Paradoxically, these failures are not caused by a defect in apoptosis, but by a presumed proliferative function of this protease. Indeed, following mitogenic stimulation, T cells lacking casp8 or its adaptor protein FADD (Fas-associated death domain protein) develop a hyperautophagic morphology, and die a programmed necrosis-like death process termed necroptosis. Recent studies have demonstrated that receptor-interacting protein kinases (RIPKs) RIPK1 and RIPK3 together facilitate TNFinduced necroptosis, but the precise role of RIPKs in the demise of T cells lacking FADD or casp8 activity is unknown. Here we demonstrate that RIPK3 and FADD have opposing and complementary roles in promoting T-cell clonal expansion and homeostasis. We show that the defective proliferation of T cells bearing an interfering form of FADD (FADDdd) is rescued by crossing with RIPK3 −/− mice, although such rescue ultimately leads to lymphadenopathy. Enhanced recovery of these double-mutant T cells following stimulation demonstrates that FADD, casp8, and RIPK3 are all essential for clonal expansion, contraction, and antiviral responses. Finally, we demonstrate that caspase-mediated cleavage of RIPK1-containing necrosis inducing complexes (necrosomes) is sufficient to prevent necroptosis in the face of death receptor signaling. These studies highlight the "two-faced" nature of casp8 activity, promoting clonal expansion in some situations and apoptotic demise in others.
Although they are one of the oldest family of proteins known (first described in 1884 by Kossel), histones continue to surprise researchers with their ever expanding roles in biology. In the past 25 years, the view of core histone octamers as a simple spool around which DNA in the nucleus is wound and linker histones as mere fasteners clipping it all together has transformed into the realization that histones play a vital role in transcriptional regulation. Through post-translational modifications, histones control the accessibility of transcription factors and a host of other proteins to multiple, conceivably thousands of, genes at once. While researchers have spent decades deciphering the role of histones in the overall structure of chromatin, it might surprise some to find that an entirely separate faction of scientists have focused on the role of histones beyond the confines of the nuclear envelope. In the past decade, there has been an accumulation of observations that suggest that histones can be found at the mitochondrion during the onset of apoptotic signaling and even at the cell surface, acting as a receptor for bacterial and viral proteins. More provocatively, immunologists are becoming convinced that they can also be found in the lumen of several tissues, acting as antimicrobial agents--critical components of an ancient innate immune system. Perhaps nowhere is this observation as dramatic as in the ability of neutrophils to entrap bacterial pathogens by casting out "nets" of DNA and histones that not only act as a physical barrier, but also display bactericidal activity. As our views regarding the role of histones inside and outside the cell evolve, some have begun to develop therapies that either utilize or target histones in the fight against cancer, microbial infection, and autoimmune disease. It is our goal here to begin the process of merging the dichotomous lives of histones both within and without the nuclear membrane.
PURPOSE. DARPin molecules are a novel class of small proteins that contain engineered ankyrin repeat domain(s) and bind to target proteins with high specificity and affinity. Abicipar-pegol (abicipar), a DARPin molecule targeting vascular endothelial growth factor-A (VEGF-A), is currently under evaluation in patients with age-related macular degeneration. The pharmacodynamic properties of abicipar were characterized using in vivo and in vitro assays. METHODS. The binding affinity of abicipar was assessed using a kinetic exclusion assay (KinExA). In vitro assays evaluated abicipar effects on VEGF-A 165-induced calcium mobilization and tube formation in human umbilical vein endothelial cells. Abicipar was tested in vivo in a mouse model of corneal neovascularization and a rabbit model of chronic retinal neovascularization. The efficacies of abicipar and ranibizumab were compared in a rabbit model of VEGF-A 165-induced retinal vasculopathy. RESULTS. Abicipar has a high affinity for the soluble isoforms of VEGF-A; binding affinities for human VEGF-A 165 are approximately 100-fold greater than those of ranibizumab and bevacizumab and are similar for rat VEGF-A 164 but approximately 20-fold lower for rabbit VEGF-A 165. Abicipar was effective in cell-based and in vivo models of angiogenesis and vascular leak, blocking neovascularization in a mouse model of corneal neovascularization and vascular permeability in a rabbit model of chronic neovascularization. In a rabbit model of VEGF-A 165-induced vasculopathy, the duration of effect of abicipar was longer than ranibizumab when the two compounds were administered at molar-equivalent doses. CONCLUSIONS. These data support the testing of abicipar as a treatment for retinal diseases characterized by neovascularization and vascular leak.
Sera and peripheral blood mononuclear cells (PBMC) from patients displaying different clinical symptoms as well as from normal uninfected individuals (NI) were used to evaluate the humoral and cellular responses of Chagas' disease patients to Trypanosoma cruzi-derived paraflagellar rod proteins (PFR). Our results show that sera from both asymptomatic Chagas' disease patients (ACP) and cardiac Chagas' disease patients (CCP) have higher levels of antibodies to PFR than sera from NI. Immunoglobulin G1 (IgG1) and IgG3 were the main Ig isotypes that recognized PFR. We also tested three recombinant forms of PFR, named rPAR-1, rPAR-2, and rPAR-3, by Western blot analysis. Sera from seven out of eight patients with Chagas' disease recognized one of the three rPAR forms. Sera from 75, 50, and 37.5% of Chagas' disease patients tested recognized rPAR-3, rPAR-2, and rPAR-1, respectively. PFR induced proliferation of 100 and 70% of PBMC from ACP and CCP, respectively.
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