Cellular‐FLIP, Raji isoform (c‐FLIP_R) modulates cell death induction upon T‐cell activation and infection

Abstract

Dysregulation of apoptosis caused by an imbalance of pro-and anti-apoptotic proteinKeywords: Apoptosis r CD95 r c-FLIP r Listeria monocytogenes r T cells Introduction CD95 (Fas/APO-1)-induced apoptosis is an essential control mechanism of the immune system that protects the host against cancer and autoimmunity [1]. CD95 is a transmembrane receptor Correspondence: Dr. Ingo Schmitz e-mail: ingo.schmitz@helmholtz-hzi.de belonging to the tumor necrosis factor (TNF) receptor superfamily [2]. Binding of the cognate ligand known as CD95L or FasL [3] leads to oligomerization of the CD95 receptor and recruitment of the adapter molecule Fas-associated death domain (FADD), which in turn recruits the initiator caspase procaspase-8 via its death * These authors contributed equally to this work. Eur. J. Immunol. 2013Immunol. . 43: 1499Immunol. -1510 effector domains [4]. This complex is called the death-inducing signaling complex [5], at which procaspase-8 is activated and cleaved. Since cellular FLICE-inhibitory (c-FLIP) proteins contain death effector domains as well, these proteins compete with caspase-8 for FADD binding [6]. Recruitment of c-FLIP results in altered death-inducing signaling complex composition and apoptosis inhibition.The Cflar gene encodes different c-FLIP protein isoforms, which can have opposing functions [7,8]. For instance, c-FLIP L contains a caspase-like domain lacking proteolytic activity and acts in a proapoptotic manner when expressed in low amounts but in an antiapoptotic manner when highly expressed [7]. In addition, humans generate two solely anti-apoptotic acting short isoforms called c-FLIP S and c-FLIP R [6,9], whose expression is regulated by a single nucleotide polymorphism [10]. Strikingly, c-FLIP R expression in humans is associated with an increased risk of follicular lymphoma [10]. The role of c-FLIP S in the human immune response has been extensively analyzed. For instance, c-FLIP S is highly induced in recently activated human T cells and contributes to resistance to CD95-induced apoptosis during the early phase of an immune response [11][12][13][14]. In contrast, the function of the c-FLIP R isoform remains enigmatic.Mouse models established so far for analysis of the physiological functions of short c-FLIP isoforms express human c-FLIP S in a T-cell-specific manner [15,16]. Both studies reported decreased CD95-mediated apoptosis in transgenic T cells, normal lymphocyte cellularity, and decreased proliferation of T cells upon activation [15,16]. However, since the murine Cflar gene (encoding c-FLIP) allows expression of c-FLIP R as the short isoform next to c-FLIP L but not c-FLIP S expression [17], the murine system seems to be a more suitable model for gaining a better understanding of the function of c-FLIP R in vivo. Of note, it is currently not known whether murine c-FLIP R is expressed endogenously at the protein level and whether it modulates the immune response during infection. To address these questions, we analyzed endogenous c-FLIP protein expression and show that m...

Constitutive expression of murine c-FLIPR causes autoimmunity in aged mice

“…We previously reported that these transgenic mice have a better clearance of the Gram-positive bacteria Listeria monocytogenes compared with infected wild-type (WT) mice. 28 Here we show that aged mice constitutively expressing murine c-FLIP R have altered lymphocyte populations with higher levels of activated B and T cells compared with WT littermates. Moreover, vavFLIP R animals spontaneously develop autoimmunity with age, showing features of systemic lupus erythematosus (SLE).…”

“…7, 30 c-FLIP R inhibits apoptosis by competing with caspase-8 for binding sites at the DISC. 17, 21 The mouse model vavFLIP R , in which murine c-FLIP R is under control of the vav -promoter to ensure expression in all hematopoietic compartments, 28 was used to study the functional role of murine c-FLIP R in the immune system. As thymocytes as well as B and T cells from vavFLIP R mice were protected against CD95-induced apoptosis, 28 we investigated if the impaired lymphocyte apoptosis in vavFLIP R mice would lead to elevated numbers of lymphocytes and altered lymphocyte populations with age.…”

“…28 We analysed aged WT and vavFLIP R mice to investigate if the constitutive expression of c-FLIP R alters lymphocyte populations over time. Dendritic cells (DCs), macrophages and granulocytes have been reported to be regulated through death receptors and c-FLIP proteins.…”

Attenuation of Immune-Mediated Influenza Pneumonia by Targeting the Inducible Co-Stimulator (ICOS) Molecule on T Cells

“…Intracellular FoxP3 expression was determined using the FoxP3 staining buffer kit (FJK-16s, eBioscience) according to the manufacturer’s recommendations. For apoptosis assay, Annexin/7AAD staining was performed as described previously [35]. Flow cytometric analysis was performed using a FACSCanto or LSRFortessa (BD Biosciences) running with THE FACSDiva software (version 6.1.3) (BD Biosciences).…”

“…All the samples were run in triplicates. The results were obtained with FACS acquisition as described [35].…”