Autocrine TNF is critical for the survival of human dendritic cells by regulating BAK, BCL-2, and FLIPL.

The life span of dendritic cells (DCs) is determined by the balance of pro- and antiapoptotic proteins. In this study, we report that serum-free cultured human monocyte-derived DCs after TLR stimulation with polyinosinic acid-polycytidylic acid or LPS underwent apoptosis, which was correlated with low TNF production. Apoptosis was prevented by the addition of exogenous TNF or by concomitant stimulation with R-848, which strongly amplified endogenous TNF production. Neutralization of TNF confirmed that DC survival was mediated by autocrine TNF induced either by stimulation with R-848 or by ligation of CD40. DCs stimulated by polyinosinic acid-polycytidylic acid or IFN-ß, another known inducer of DC apoptosis, were characterized by high levels and activation of the proapoptotic protein BAK. The ratio of antiapoptotic BCL-2 to BAK correlated best with the survival of activated DCs. Addition of TNF increased this ratio but had little effect on BAX and XIAP. Knockdown experiments using small interfering RNAs confirmed that the survival of activated and also of immature DCs was regulated by BAK and showed that TNF was protective only in the presence of FLIP(L). Together, our data demonstrate that the survival of DCs during differentiation and activation depends on autocrine TNF and that the inhibition of BAK plays an important role in this process.

Premonitory epidermolysis bullosa acquisita mimicking eyelid dermatitis: successful treatment with rituximab and protein A immunoapheresis.

We report a unique case of a 71-year-old female patient with epidermolysis bullosa acquisita. The patient initially presented with the clinical symptoms of bilateral eyelid dermatitis that occurred several months prior to the development of oral and pharyngeal erosions and blisters. While no contact allergy was found by patch testing, direct immunofluorescence microscopy demonstrated linear deposits of IgG at the basement membrane zone. By indirect immunofluorescence microscopy on human salt-split skin, IgG antibodies bound to the dermal side of the split. Immunoblot analysis showed predominant IgG4 reactivity of the patient's serum with the recombinant non-collagenous-1 domain of type VII collagen. Because of treatment resistance to systemic corticosteroids, dapsone, and colchicine, we initiated a combination treatment of protein A immunoapheresis and rituximab. With this treatment, complete remission was achieved within 4 months. Our case highlights that epidermolysis bullosa acquisita may initially mimic an eyelid dermatitis. Consequently, dermatologists should be aware of this rare differential diagnosis of eyelid dermatitis where a contact allergy or atopic dermatitis is absent.

FLIP ing the coin? Death receptor-mediated signals during skin tumorigenesis.

Keratinocyte skin cancer is a multi-step process, during which a number of obstacles have to be overcome by the tumor cell to allow the development of a manifest tumor. Beside proliferation and immortality, apoptosis resistance is one additional and critical step during skin carcinogenesis. Over the past two decades, much has been learned about the prototypical membrane-bound inducers of apoptosis, namely the death receptors and their ligands, and the apoptosis signalling pathways activated by death receptors have been elucidated in great detail. In contrast, much less is known about the tissue-specific role of the death receptor/ligands systems during the development of skin cancer. Here, we summarize and discuss the role of this intriguing receptor family and the potential mechanistical impact of the intracellular caspase-8 inhibitor cFLIP for keratinocyte skin cancer. Given more recent data about cFLIP and its isoforms, a more complex regulatory role of cFLIP can be suspected. Indeed, cFLIP may not solely interfere with death receptor-mediated apoptosis signalling pathways, but may positively or negatively influence other, potential harmful signalling pathways such as the production of inflammatory cytokines, tumor cell migration or the activation of transcription factors such as NF-kappaB, considered crucial during skin tumorigenesis. In this respect, cFLIP may act to 'FLIP the coin' during the development of keratinocyte skin cancer.

NF-kappaB inhibition reveals differential mechanisms of TNF versus TRAIL-induced apoptosis upstream or at the level of caspase-8 activation independent of cIAP2.

Death ligands not only activate a death program but also regulate inflammatory signalling pathways, for example, through NF-kappaB induction. Although tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and TNF both activate NF-kappaB in human keratinocytes, only TRAIL potently induces apoptosis. However, when induction of NF-kappaB was inhibited with a kinase dead IKK2 mutant (IKK2-KD), TNF- but not TRAIL-induced apoptosis was dramatically enhanced. Acquired susceptibility to TNF-induced apoptosis was due to increased caspase-8 activation. To investigate the mechanism of resistance of HaCaT keratinocytes to TNF-induced apoptosis, we analyzed a panel of NF-kappaB-regulated effector molecules. Interestingly, the inhibitor of apoptosis protein (IAP) family member cIAP2, but not cIAP1, X-linked inhibitor of apoptosis, TNF receptor-associated factor (TRAF)-1, or TRAF2, was downregulated in sensitive but not in resistant HaCaT keratinocytes. Surprisingly, however, stable inducible expression of cIAP2 was not sufficient to render IKK2-KD-sensitized keratinocytes resistant to TNF, and reduction of cIAP2 alone did not increase the sensitivity of HaCaT keratinocytes to TNF. In conclusion, we demonstrate that inhibition of NF-kappaB dramatically sensitizes human keratinocytes to TNF- but not to TRAIL-induced apoptosis and that this sensitization for TNF was largely independent of cIAP2. Our data thus clearly exclude the candidates proposed to date to confer TNF apoptosis resistance and suggest the function of an unanticipated effector of NF-kappaB critical for the survival of HaCaT keratinocytes upstream or at the level of caspase-8 activation.

Up-regulation of the T cell quiescence factor KLF2 in a leukaemic T-cell line after expression of the inositol 5'-phosphatase SHIP-1.

Inositol 5'-phosphatase SHIP-1 (SHIP) is a negative regulator of signal transduction in haematopoietic cells. SHIP inactivation may be involved in the pathogenesis of leukaemia. An inducible expression system was combined with microarray analysis to identify target genes regulated by SHIP in the human T-cell leukaemia cell line Jurkat. One gene identified was Krüppel-like factor 2 (KLF2), which was up-regulated two to threefold at the RNA and protein level after the induced expression of SHIP. KLF2, a negative regulator of T cell proliferation, has been implicated in T cell quiescence. KLF2 or SHIP expression in Jurkat cells caused 45% or 60% reduction of proliferation, respectively. SHIP can up-regulate KLF2 expression, implicating KLF2 in the SHIP-mediated growth inhibition of a human leukaemic T-cell line.

Probing the expression and function of the P2X7 purinoceptor with antibodies raised by genetic immunization.

The cytolytic P2X7 purinoceptor is widely expressed on leucocytes and has sparked interest because of its peculiar ability to induce a large nonselective membrane pore following treatment of cells with ecto-ATP. Antibodies raised against synthetic P2X7 peptides generally work well in Western-Blot analyses but fail to recognize the native protein on the cell surface. Genetic immunization is a useful technique to raise antibodies directed against proteins in native conformation. Using this technique we have generated highly specific polyclonal (rabbit) and monoclonal (rat) anti-P2X7 antibodies that readily detect mouse P2X7 on the surface of living cells by immunofluorescence analyses and flow cytometry. Binding of these antibodies to P2X7 is reduced within seconds after treatment of cells with ATP, suggesting that ligand binding induces a conformational shift and/or the shedding of P2X7. By site directed mutagenesis we have mutated three conserved arginine residues (R294A, R307A, R316A) in the extracellular loop of P2X7 near the second transmembrane region. Each of these mutations results in loss of ATP response. FACS and immunoblot analyses reveal that the R294A mutant is expressed at higher levels than wild-type P2X7 in transfected cells, whereas the R307A and R316A mutants are barely detectable because there is no or very little protein synthesis of these constructs. In accord with its resistance to ATP-induced activation the R294A mutant is not down-modulated from the cell surface by ATP-treatment.

T-cell survival regulator LKLF is not involved in inappropriate apoptosis of diabetes-prone BBDP rat T cells.

Diabetes-prone BB (dpBB) rats develop autoimmune insulin-dependent diabetes mellitus (IDDM) at high frequency as a consequence of a defect in T cell development, caused by a mutation in a single gene locus on rat chromosome 4 (lyp) which has recently been identified as immune-associated nucleotide 4 (ian4). A phenotype similar to dpBB rat lymphopenia has recently been described in the mouse as the result of the targeted inactivation of the gene for the transcription factor LKLF (Lung Krüppel-like factor, KLF2) in the immune system. We cloned the LKLF gene of the rat and screened a panel of rat/hamster radiation hybrid cell lines to determine its chromosomal localization. We conclude that the LKLF gene is not defective in dpBB rats and that its expression is not compromised by the lyp mutation.