Cutting edge: FADD is not required for antigen receptor-mediated NF-kappaB activation

Caspase-8 restricts antiviral CD8 T cell hyperaccumulation

The magnitude of CD8 T cell responses against viruses is checked by the balance of proliferation and death. Caspase-8 (CASP8) has the potential to influence response characteristics through initiation of apoptosis, suppression of necroptosis, and modulation of cell death-independent signal transduction. Mice deficient in CASP8 and RIPK3 (Casp8 ^-/- Ripk3 ^-/- ) mount enhanced peak CD8 T cell levels against the natural mouse pathogen murine cytomegalovirus (MCMV) or the human pathogen herpes simplex virus-1 compared with littermate control RIPK3-deficient or WT C57BL/6 mice, suggesting an impact of CASP8 on the magnitude of antiviral CD8 T cell expansion and not on contraction. The higher peak response to MCMV in Casp8 ^-/- Ripk3 ^-/- mice resulted from accumulation of greater numbers of terminally differentiated KLRG1^hi effector CD8 T cell subsets. Antiviral Casp8 ^-/- Ripk3 ^-/- T cells exhibited enhanced proliferation when splenocytes were transferred into WT recipient mice. Thus, cell-autonomous CASP8 normally restricts CD8 T cell proliferation following T cell receptor activation in response to foreign antigen. Memory inflation is a hallmark quality of the T cell response to cytomegalovirus infection. Surprisingly, MCMV-specific memory inflation was not sustained long-term in Casp8 ^-/- Ripk3 ^-/- mice even though these mice retained immunity to secondary challenge. In addition, the accumulation of abnormal B220⁺CD3⁺ T cells in these viable CASP8-deficient mice was reduced by chronic MCMV infection. Combined, these data brings to light the cell death-independent role of CASP8 during CD8 T cell expansion in mice lacking the confounding impact of RIPK3-mediated necroptosis.

Caspase-8 restricts natural killer cell accumulation during MCMV Infection

Natural killer (NK) cells provide important host defense against herpesvirus infections and influence subsequent T cell control of replication and maintenance of latency. NK cells exhibit phases of expansion, contraction and memory formation in response to the natural mouse pathogen murine cytomegalovirus (MCMV). Innate and adaptive immune responses are tightly regulated in mammals to avoid excess tissue damage while preventing acute and chronic viral disease and assuring resistance to reinfection. Caspase (CASP)8 is an autoactivating aspartate-specific cysteine protease that initiates extrinsic apoptosis and prevents receptor interacting protein (RIP) kinase (RIPK)1-RIPK3-driven necroptosis. CASP8 also promotes death-independent signal transduction. All of these activities make contributions to inflammation. Here, we demonstrate that CASP8 restricts NK cell expansion during MCMV infection but does not influence NK memory. Casp8^-/-Ripk3^-/- mice mount higher NK response levels than Casp8^+/-Ripk3^-/- littermate controls or WT C57BL/6 J mice, indicating that RIPK3 deficiency alone does not contribute to NK response patterns. MCMV m157-responsive Ly49H⁺ NK cells support increased expansion of both Ly49H^- NK cells and CD8 T cells in Casp8^-/-Ripk3^-/- mice. Surprisingly, hyperaccumulation of NK cells depends on the pronecrotic kinase RIPK1. Ripk1^-/-Casp8^-/-Ripk3^-/- mice fail to show the enhanced expansion of lymphocytes observed in Casp8^-/-Ripk3^-/- mice even though development and homeostasis are preserved in uninfected Ripk1^-/-Casp8^-/-Ripk3^-/- mice. Thus, CASP8 naturally regulates the magnitude of NK cell responses in response to infection where strong activation signals depend on another key regulator of death signaling, RIPK1. In addition, the strong NK cell response promotes survival of effector CD8 T cells during their expansion. Thus, hyperaccumulation of NK cells and crosstalk with T cells becomes amplified in the absence of extrinsic cell death machinery.

Caspase-8-dependent control of NK- and T cell responses during cytomegalovirus infection

Caspase-8 (CASP8) impacts antiviral immunity in expected as well as unexpected ways. Mice with combined deficiency in CASP8 and RIPK3 cannot support extrinsic apoptosis or RIPK3-dependent programmed necrosis, enabling studies of CASP8 function without complications of unleashed necroptosis. These extrinsic cell death pathways are naturally targeted by murine cytomegalovirus (MCMV)-encoded cell death suppressors, showing they are key to cell-autonomous host defense. Remarkably, Casp8^-/-Ripk3^-/-, Ripk1^-/-Casp8^-/-Ripk3^-/- and Casp8^-/-Ripk3^K51A/K51A mice mount robust antiviral T cell responses to control MCMV infection. Studies in Casp8^-/-Ripk3^-/- mice show that CASP8 restrains expansion of MCMV-specific natural killer (NK) and CD8 T cells without compromising contraction or immune memory. Infected Casp8^-/-Ripk3^-/- or Casp8^-/-Ripk3^K51A/K51A mice have higher levels of virus-specific NK cells and CD8 T cells compared to matched RIPK3-deficient littermates or WT mice. CASP8, likely acting downstream of Fas death receptor, dampens proliferation of CD8 T cells during expansion. Importantly, contraction proceeds unimpaired in the absence of extrinsic death pathways owing to intact Bim-dependent (intrinsic) apoptosis. CD8 T cell memory develops in Casp8^-/-Ripk3^-/- mice, but memory inflation characteristic of MCMV infection is not sustained in the absence of CASP8 function. Despite this, Casp8^-/-Ripk3^-/- mice are immune to secondary challenge. Interferon (IFN)γ is recognized as a key cytokine for adaptive immune control of MCMV. Ifngr^-/-Casp8^-/-Ripk3^-/- mice exhibit increased lifelong persistence in salivary glands as well as lungs compared to Ifngr^-/- and Casp8^-/-Ripk3^-/- mice. Thus, mice deficient in CASP8 and RIPK3 are more dependent on IFNγ mechanisms for sustained T cell immune control of MCMV. Overall, appropriate NK- and T cell immunity to MCMV is dependent on host CASP8 function independent of RIPK3-regulated pathways.

Clinical and genetic signatures of local recurrence in oral squamous cell carcinoma

PURPOSE

Recurrent and metastatic Oral Squamous Cell Carcinoma (OSCC) is often incurable. There are large gaps in the understanding of the clinical course, biology and genetic biomarkers of OSCC which could help us identify patients with high-risk of recurrence who may benefit from intensified therapy or novel targeted therapy trials. The purpose of this study was to identify significant clinical, pathological and genomic risk factors for local recurrence in OSCC.

PATIENTS AND METHODS

Molecular data sets and clinicopathological characteristics of 159 head and neck carcinoma patients were obtained from The Cancer Genome Atlas (TCGA) data portal and analyzed using the Genome Data Analysis Center and cBioPortal to find significant risk factors for tumor recurrence.

RESULTS

The local recurrence rate was 24%. OSCC originating from the buccal mucosa composed 13% of all the tumors in the recurrent group, making it a statistically significant risk of recurrence (P value = 0.03). Likewise, positive surgical margins, pathological T staging, and alcohol consumption were found to be significantly associated with recurrence (P value < 0.05). Genetic profiling revealed the top 5 mutated genes (using the MutSigCV analysis). Only one of these genes, CASP8 was the only gene that was significantly altered only in the recurrent group (Q value = 8.7 × 10^-11). The fingerprint of 5 mutated genes was found in 97% of the patients in the recurrence group. Moreover, copy number alterations in cytoband 5p15.33, which involved amplification in telomerase reverse-transcriptase (TERT) gene, was found to be significant only in the recurrent group.

CONCLUSIONS

In the current study, we found several clinical and genetic characteristics that could define patients with high-risk of OSCC recurrence. This provides a means of identifying patients that may benefit from intensified therapy or novel targeted therapy trials.

Quantitative Detection of Nucleocytoplasmic Transport of Native Proteins in Single Cells

The detection of protein translocation (i.e., the movement of intracellular proteins among various subcellular compartments) conventionally relies on imaging and subcellular-fractionation-based techniques that do not generate information on a large cell population with single-cell resolution. Although special flow cytometric tools such as imaging flow cytometry may generate single-cell data on processes such as nucleocytoplasmic transport, such equipment is expensive (thus has limited accessibility) and has low throughput for examining cells due to the reliance on high-speed imaging. Here we describe a protocol for detecting translocation of native proteins using a common flow cytometer which detects fluorescence intensity without imaging. We conduct chemical release of cytosolic proteins and fluorescence immunostaining of a targeted protein. The detected fluorescence intensity is quantitatively correlated to the cytosolic/nuclear localization of the protein at the single cell level. Our technique provides a simple route for studying nucleocytoplasmic transport with single-cell resolution using common flow cytometers.

Suppressive function of low-dose deguelin on the invasion of oral cancer cells by downregulating tumor necrosis factor alpha-induced nuclear factor-kappa B signaling

BACKGROUND

Deguelin has both antiproliferation and antimetastasis activities. However, high-dose deguelin elicits many undesired side effects. The purpose of this study was to investigate whether the low-dose deguelin can prevent the metastasis of oral cancer.

METHODS

The dose effects of deguelin on metastasis of oral cancer cells were analyzed by in vitro invasion assay and an orthotropic xenograft mouse model. The involvement of tumor necrosis factor alpha (TNF-α)-induced nuclear factor-kappa B (NF-κB) signaling was examined by Western blot and reporter assay.

RESULTS

Low-dose deguelin, which has minimal cytotoxicity, significantly inhibited the invasion and migration of oral cancer cells. These inhibitory effects of low-dose deguelin were mediated by suppressing TNF-α-induced activation of IκB kinase leading to the inhibition of IκB phosphorylation, NF-κB transcriptional activity, and matrix metalloproteinase-2 (MMP2) expression. The low-dose deguelin treatment significantly inhibited tumor growth and invasion without systemic toxicity.

CONCLUSION

The low-dose deguelin suppressed the invasion and migration of oral cancer by downregulating TNF-α-induced NF-κB signaling. © 2015 Wiley Periodicals, Inc. Head Neck 38: E524-E534, 2016.

Functions of caspase 8: the identified and the mysterious

Initially discovered as an initiator protease in apoptosis mediated by death receptors, caspase-8 is now known to have an apparently confounding opposing effect in securing cell survival. It is required to allow mouse embryo survival, and the survival of hematopoietic cells during their development and activation. Classic models in which caspase-8 is depleted or inhibited frequently result in inhibition of apoptosis, and conversion to death through a necrotic pathway. This bewildering switch is now known to be driven by activation of a pathway dependent on protein kinases of the RIP family, which engage a pathway known as necroptosis. If caspase-8 does not control this pathway, necrotic death results. The pro-apoptotic and pro-survival functions of caspase-8 are regulated by a specific interaction with the pseudo-caspase cFLIP, and it is thought that the heterocomplex between these two partners alters the substrate specificity of caspase-8 in favor of inactivating components of the RIP kinase pathway. The description of how caspase-8 and cFLIP coordinate the switch between apoptosis and survival is just beginning. The mechanism is not known, the differential targets are not known, and the reason of why an apoptotic initiator has been co-opted as a critical survival factor is only guessed at. Elucidating these unknowns will be important in understanding mechanisms and possible therapeutic targets in autoimmune, inflammatory, and metastatic diseases.

Programmed necrosis and autophagy in immune function

It has long been known that apoptosis is vital to the generation and maintenance of proper adaptive immune function. An example is the essential requirement for apoptotic signaling during the generation of self-tolerant lymphocytes: the apoptotic death of B and T cells with overt autoreactivity is essential to central tolerance. More recently, the contributions of additional processes including cellular autophagy and programmed necrosis have been implicated in controlling both innate and adaptive immune functions. Evidence has been provided to demonstrate that the death of cells following ligation of death receptors (DRs), a subfamily of cell surface molecules related to tumor necrosis factor receptor 1, is not exclusively the domain of caspase-dependent apoptosis. In cells lacking the capacity to activate caspase-8 following DR ligation, cell death instead occurs via programmed necrosis, or as it has been recently termed, 'necroptosis'. This death process depends on RIP1 and RIP3, serine/threonine kinases that are recruited by DRs, and likely by other cellular signals including DNA damage and antigen receptor ligation. The generation of RIP1/RIP3 containing 'necrosomes' activates downstream necroptotic signaling that ultimately targets cellular energetic metabolism. Also related to cellular metabolic regulation, cellular autophagy has also been found to play unique and important roles in immunity. In this review, we describe the roles of necroptosis and autophagy in innate and adaptive immunity and speculate on the intriguing interplay between these two cellular processes.

Imaging flow cytometry: coping with heterogeneity in biological systems

Imaging flow cytometry (IFC) platforms combine features of flow cytometry and fluorescent microscopy with advances in data-processing algorithms. IFC allows multiparametric fluorescent and morphological analysis of thousands of cellular events and has the unique capability of identifying collected events by their real images. IFC allows the analysis of heterogeneous cell populations, where one of the cellular components has low expression (<0.03%) and can be described by Poisson distribution. With the help of IFC, one can address a critical question of statistical analysis of subcellular distribution of proteins in a cell. Here the authors review advantages of IFC in comparison with more traditional technologies, such as Western blotting and flow cytometry (FC), as well as new high-throughput fluorescent microscopy (HTFM), and discuss further developments of this novel analytical technique.

Mechanisms of necroptosis in T cells

In caspase 8-deficient mouse T cells, necroptosis occurs via a Ripk3- and Ripk1-dependent pathway independent of autophagy and programmed necrosis.

Cell populations are regulated in size by at least two forms of apoptosis. More recently, necroptosis, a parallel, nonapoptotic pathway of cell death, has been described, and this pathway is invoked in the absence of caspase 8. In caspase 8–deficient T cells, necroptosis occurs as the result of antigen receptor–mediated activation. Here, through a genetic analysis, we show that necroptosis in caspase 8–deficient T cells is related neither to the programmed necrosis as defined by the requirement for mitochondrial cyclophilin D nor to autophagy as defined by the requirement for autophagy-related protein 7. Rather, survival of caspase 8–defective T cells can be completely rescued by loss of receptor-interacting serine-threonine kinase (Ripk) 3. Additionally, complementation of a T cell–specific caspase 8 deficiency with a loss of Ripk3 gives rise to lymphoproliferative disease reminiscent of lpr or gld mice. In conjunction with previous work, we conclude that necroptosis in antigen-stimulated caspase 8–deficient T cells is the result of a novel Ripk1- and Ripk3-mediated pathway of cell death.

Laser scanning cytometry: capturing the immune system in situ

Until recently, it has not been possible to image and functionally correlate the key molecular and cellular events underpinning immunity and tolerance in the intact immune system. Certainly, the field has been revolutionized by the advent of tetramers to identify physiologically relevant specificities of T cells, and the introduction of models in which transgenic T-cell receptor and/or B-cell receptor-bearing lymphocytes are adoptively transferred into normal mice and can then be identified by clonotype-specific antibodies using flow cytometry in vitro, or immunohistochemistry ex vivo. However, these approaches do not allow for quantitative analysis of the precise anatomical, phenotypic, signaling, and functional parameters required for dissecting the development of immune responses in health and disease in vivo. Traditionally, assessment of signal transduction pathways has required biochemical or molecular biological analysis of isolated and highly purified subsets of immune system cells. Inevitably, this creates potential artifacts and does not allow identification of the key signaling events for individual cells present in their microenvironment in situ. These difficulties have now been overcome by new methodologies in cell signaling analysis that are sufficiently sensitive to detect signaling events occurring in individual cells in situ and the development of technologies such as laser scanning cytometry that provide the tools to analyze physiologically relevant interactions between molecules and cells of the innate and the adaptive immune system within their natural environmental niche in vivo.

Analytical capabilities of the ImageStream cytometer

Imaging cytometry has recently become an important achievement in development of flow cytometric technologies. The ImageStream cytometer combines the vast features of classical flow cytometry including an impartial analysis of great number of cells in short period of time which results in strong statistical data output, with essential features of fluorescence microscopy such us collecting of real multiparameter images of analyzed objects. In this chapter, we would like to introduce an overview of imaging cytometry platform and emphasize the potential advantages of using this system for several experimental purposes. Moreover, both well established as well as potential applications of imaging cytometry will be described. Eventually, we would like to illustrate the unique use of ImageStream cytometer for identification and characterization of subpopulations of stem/ progenitor cells present in different biological specimens.

Kinetics of NF-κB nucleocytoplasmic transport probed by single-cell screening without imaging

Transport of protein and RNA cargoes between the nucleus and cytoplasm (nucleocytoplasmic transport) is vital for a variety of cellular functions. The studies of kinetics involved in such processes have been hindered by the lack of quantitative tools for measurement of the nuclear and cytosolic fractions of an intracellular protein at the single cell level for a cell population. In this report, we describe using a novel method, microfluidic electroporative flow cytometry, to study kinetics of nucleocytoplasmic transport of an important transcription factor NF-κB. With data collected from single cells, we quantitatively characterize the population-averaged kinetic parameters such as the rate constants and apparent activation barrier for NF-κB transport. Our data demonstrate that NF-κB nucleocytoplasmic transport fits first-order kinetics very well and is a fairly reversible process governed by equilibrium thermodynamics.

Intertwined pathways of programmed cell death in immunity

Programmed cell death (PCD) occurs widely in species from every kingdom of life. It has been shown to be an integral aspect of development in multicellular organisms, and it is an essential component of the immune response to infectious agents. An analysis of the phylogenetic origin of PCD now shows that it evolved independently several times, and it is fundamental to basic cellular physiology. Undoubtedly, PCD pervades all life at every scale of analysis. These considerations provide a backdrop for understanding the complexity of intertwined, but independent, cell death programs that operate within the immune system. In particular, the contributions of apoptosis, autophagy, and necrosis in the resolution of an immune response are considered.

The complex interplay between autophagy, apoptosis, and necrotic signals promotes T-cell homeostasis

Intense research efforts over the last two decades have focused on establishing the significance of apoptotic signaling in adaptive immunity. Without doubt, caspase-dependent apoptosis plays vital roles in many immune processes, including lymphocyte development, positive and negative selection, homeostasis, and self-tolerance. Cell biologists have developed new insights into cell death, establishing that other modes of cell death exist, such as programmed necrosis and type II/autophagic cell death. Additionally, immunologists have identified a number of immunological processes that are highly dependent upon cellular autophagy, including antigen presentation, lymphocyte development and function, pathogen recognition and destruction, and inflammatory regulation. In this review, we provide detailed mechanistic descriptions of cellular autophagy and programmed necrosis induced in response to death receptor ligation, including methods to identify them, and compare and contrast these processes with apoptosis. The crosstalk between these three processes is emphasized as newly formulated evidence suggests that this interplay is vital for efficient T-cell clonal expansion. This new evidence indicates that in addition to apoptosis, autophagy and programmed necrosis play significant roles in the termination of T-cell-dependent immune responses.

Quantitative analysis of protein translocations by microfluidic total internal reflection fluorescence flow cytometry

Protein translocation, or the change in a protein's location between different subcellular compartments, is a critical process by which intracellular proteins carry out their cellular functions. Aberrant translocation events contribute to various diseases ranging from metabolic disorders to cancer. In this study, we demonstrate the use of a newly developed single-cell tool, microfluidic total internal reflection fluorescence flow cytometry (TIRF-FC), for detecting both cytosol to plasma membrane and cytosol to nucleus translocations using the tyrosine kinase Syk and the transcription factor NF-κB as models. This technique detects fluorescent molecules at the plasma membrane and in the membrane-proximal cytosol in single cells. We were able to record quantitatively changes in the fluorescence density in the evanescent field associated with these translocation processes for large cell populations with single cell resolution. We envision that TIRF-FC will provide a new approach to explore the molecular biology and clinical relevance of protein translocations.

Caspase-8 as a therapeutic target in cancer

Caspase-8 is an apical caspase which initiates programmed cell death following death receptor ligation. This central role in apoptosis has prompted significant clinical interest in regulating caspase-8 expression and proteolytic activity. However, caspase-8 has also been found to play a number of non-apoptotic roles in cells, such as promoting activation NF-κB signaling, regulating autophagy and altering endosomal trafficking, and enhancing cellular adhesion and migration. Therefore, depending upon the specific cellular context, caspase-8 may either potentiate or suppress tumor malignancy. Accordingly, a marked heterogeneity exists in the expression patterns of caspase-8 among different tumor types. Therapeutics have been developed which can increase caspase-8 expression, yet it remains unclear whether this approach will be beneficial in all cases. Care is warranted, and the role of caspase-8 should be addressed on a case by case basis.

MHC class II regulation by epigenetic agents and microRNAs

MicroRNAs have been shown to regulate gene expression both transcriptionally and translationally. Here, we examine evidence that various stresses regulate miRNAs which, in turn, regulate immune gene levels. Multiple studies are reviewed showing altered microRNA levels in normal cells under stress and in various disease states, including cancer. Unexpected was the finding that Dicer expression is altered by treatments with several agents, such as interferons and cortisone, employed in the treatment of immune disorders. Potential signal transduction pathways, including JAK/Stat, PI3K and PKR, that may regulate Dicer and microRNA levels in normal and stressed mammalian cells are discussed.

Enzymatically active single chain caspase-8 maintains T-cell survival during clonal expansion

The extrinsic, or death receptor, pathway integrates apoptotic signals through the protease caspase-8 (casp8). Beyond cell death regulation, non-apoptotic functions of casp8 include its essential requirement for hematopoiesis and lymphocyte clonal expansion, and tempering of autophagy in T cells. However, the mechanistic basis for the control of these disparate cellular processes remains elusive. Here, we show that casp8-deficient T-cell survival was rescued by enzymatically active, but not inactive, casp8-expressing retroviruses. The casp8 catalytic induction in proliferating T cell occurred independent of extrinsic and intrinsic apoptotic-signaling cascades and did not induce casp8 proteolytic processing. Using a biotinylated probe selectively targeting enzymatically active caspases, catalytically active full-length casp8 was found in vivo in dividing T cells. A casp8 D387A processing mutant was able to rescue casp8-deficient T-cell proliferation, validating that casp8 self-processing is not required for its non-apoptotic function(s). Finally, casp8 activity was highest in CD8(+) T cells, the most rapidly proliferating subset. These results show that the catalytically competent form of casp8 is required for rapid T-cell proliferation in response to TCR ligation, but that processing of the caspase is only necessary to promote apoptosis.

Retinoic acid induces nuclear accumulation of Raf1 during differentiation of HL-60 cells

All trans-retinoic acid (RA) is a standard therapeutic agent used in differentiation induction therapy treatment of acute promyelocytic leukemia (APL). RA and its metabolites use a diverse set of signal transduction pathways during the differentiation program. In addition to the direct transcriptional targets of the nuclear RAR and RXR receptors, signals derived from membrane receptors and the Raf-MEK-ERK pathway are required. Raf1 phosphorylation and the prolonged activation of Raf1 persisting during the entire differentiation process are required for RA-dependent differentiation of HL-60 cells. Here we identify a nuclear redistribution of Raf1 during the RA-induced differentiation of HL-60 cells. In addition, the nuclear accumulation of Raf1 correlates with an increase in Raf1 phosphorylated at serine 621. The serine 621 phosphorylated Raf1 is predominantly localized in the nucleus. The RA-dependent nuclear accumulation of Raf1 suggests a novel nuclear role for Raf1 during the differentiation process.

Antigen-mediated T cell expansion regulated by parallel pathways of death

T cells enigmatically require caspase-8, an inducer of apoptosis, for antigen-driven expansion and effective antiviral responses, and yet the pathways responsible for this effect have been elusive. A defect in caspase-8 expression does not affect progression through the cell cycle but causes an abnormally high rate of cell death that is distinct from apoptosis and does not involve a loss of NFkappaB activation. Instead, antigen or mitogen activated Casp8-deficient T cells exhibit an alternative type of cell death similar to programmed necrosis that depends on receptor interacting protein (Ripk1). The selective genetic ablation of caspase-8, NFkappaB, and Ripk1, reveals two forms of cell death that can regulate virus-specific T cell expansion.

FADD and caspase-8 control the outcome of autophagic signaling in proliferating T cells

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.

“Decoding the Dots”: The ImageStream system (ISS) as a novel and powerful tool for flow cytometric analysis

The aim of this article is to provide a brief review of the ImageStream system (ISS). The ISS technology was developed as a novel method for multiparameter cell analysis and subsequently as a supportive tool for flow cytometry (FC). ISS integrates the features of FC and fluorescent microscopy collecting images of acquired cells for offline digital image analysis. The article presents an overview of the main characteristics of ISS and a comparison between ISS, FC and the laser scanning cytometer (LSC). We reviewed ISS applications focusing on those involved in cellular phenotyping and provide our own experience with using ISS as a supportive tool to classical FC and demonstrate the compatibility between FC and ISS photometric analysis as well as the advantages of using ISS to confirm FC results.

A Fas-associated death domain protein/caspase-8-signaling axis promotes S-phase entry and maintains S6 kinase activity in T cells responding to IL-2

Fas-associated death domain protein (FADD) constitutes an essential component of TNFR-induced apoptotic signaling. Paradoxically, FADD has also been shown to be crucial for lymphocyte development and activation. In this study, we report that FADD is necessary for long-term maintenance of S6 kinase (S6K) activity. S6 phosphorylation at serines 240 and 244 was only observed after long-term stimulation of wild-type cells, roughly corresponding to the time before S-phase entry, and was poorly induced in T cells expressing a dominantly interfering form of FADD (FADDdd), viral FLIP, or possessing a deficiency in caspase-8. Defects in S6K1 phosphorylation were also observed. However, defective S6K1 phosphorylation was not a consequence of a wholesale defect in mammalian target of rapamycin function, because 4E-BP1 phosphorylation following T cell activation was unaffected by FADDdd expression. Although cyclin D3 up-regulation and retinoblastoma hypophosphorylation occurred normally in FADDdd T cells, cyclin E expression and cyclin-dependent kinase 2 activation were markedly impaired in FADDdd T cells. These results demonstrate that a FADD/caspase-8-signaling axis promotes T cell cycle progression and sustained S6K activity.

The unexpected effect of cyclosporin A on CD56+CD16- and CD56+CD16+ natural killer cell subpopulations

Cyclosporin A (CSA) is commonly used to prevent graft-versus-host disease. The influence of CSA on T-cell function has been extensively investigated; however, the effect of CSA on natural killer (NK) cells is less understood. NK cells were cultured with IL-2 and IL-15 with and without CSA for 1 week. Compared with controls, CSA-treated cultures showed fewer CD56(+)CD16(+)KIR(+) NK cells and a reciprocal increase in CD56(+)CD16(-)KIR(-) cells. These changes were due mainly to a reduced proliferation of the CD56(dim) NK-cell subpopulation and a relative resistance of CD56(bright) NK cells to CSA. Following coculture with K562 targets, CSA-exposed NK cells differed from controls and lacked Ca(2+) oscillations, nuclear factor of activated T cells (NFAT) dephosphorylation, and NFAT nuclear translocation. NK cells cultured in CSA retained cytotoxicity against K562, Raji, and KIR ligand-expressing lymphoblastoid cells. NK cells cultured in CSA showed increases in NKp30 and reductions in NKp44 and NKG2D. Following IL-12 and IL-18 stimulation, CSA-treated NK cells showed more IFN-gamma-producing cells. Using in vitro NK-cell differentiation, progenitor cells gave rise to more CD56(+)KIR(-) NK cells in the presence of CSA than controls. Collectively, these studies show that CSA influences NK-cell function and phenotype, which may have important implications for graft-versus-leukemia effects.

Modulation of DRAK2 autophosphorylation by antigen receptor signaling in primary lymphocytes

Death-associated protein-related apoptotic kinase-2 (DRAK2), a member of the death-associated protein-like family of serine/threonine kinases, is highly expressed in lymphoid organs and is a negative regulator of T cell activation. To investigate the regulation of DRAK2 activity in primary lymphocytes, we employed mass spectrometry to identify sites of autophosphorylation on DRAK2. These studies have revealed a key site of autophosphorylation on serine 12. Using a phospho-specific antibody to detect Ser(12) phosphorylation, we found that autophosphorylation is induced by antigen receptor stimulation in T and B cells. In Jurkat T cells, resting B cells and thymocytes, DRAK2 was hypophosphorylated on Ser(12) but rapidly phosphorylated with antigen receptor ligation. This increase in phosphorylation was dependent on intracellular calcium mobilization, because BAPTA-AM blocked DRAK2 kinase activity, whereas the SERCA inhibitor thapsigargin promoted Ser(12) phosphorylation. Our results show that DRAK2 kinase activity is regulated in a calcium-dependent manner and that Ser(12) phosphorylation is necessary for optimal suppression of T cell activation by this kinase, suggesting a potential feedback loop may act to modulate the activity of this kinase following antigen receptor signaling.

Quantitative analysis of protein co-localization on B cells opsonized with rituximab and complement using the ImageStream multispectral imaging flow cytometer

Binding of the chimeric, humanized anti-CD20 mAb Rituximab (RTX) to B lymphocytes activates complement and promotes covalent deposition of C3 fragments (C3b/iC3b) on cells. Previous fluorescence microscopy studies, based on examination of B cell lines and of blood samples from RTX-treated CLL patients, suggest that C3b/iC3b is closely associated with cell-bound RTX. We examined Raji cells opsonized with serum and RTX with the ImageStream imaging flow cytometer. Cells were stained with fluorescently-labeled RTX and mAbs specific for C3b/iC3b fragments or for human IgG, and then imaged using the ImageStream cytometer and analyzed with an algorithm (Similarity Bright Detail Score, SBDS) which tests for co-localization of fluorescent probes. SBDS, calculated on 10,000 cells, verified that the majority of deposited C3b/iC3b is co-localized with bound RTX. In contrast, when cells were first opsonized in serum alone, washed and then reacted with RTX, SBDS confirmed that RTX and C3b/iC3b are poorly co-localized, thus demonstrating that cell-bound RTX directs deposition of C3b. In addition, a sulfhydryl-specific probe, maleimide conjugated to AF488, exhibited substantial co-localization with an anti-C3b/iC3b mAb on Raji cells opsonized with RTX and serum, thus validating maleimide labeling as an alternative for detecting cell-bound C3b/iC3b. The digital imaging method described should have wide applicability for quantitative analysis of co-localization.

IgE isotype switch and IgE production are enhanced in IL-21-deficient but not IFN-gamma-deficient mice in a Th2-biased response

IgE plays a critical role in the pathogenesis of allergy and asthma. Therefore, suppression of IgE production would provide therapeutic benefits to patients suffering from these diseases. We have reported that the production of IgE is regulated differently in the spleen vs. the draining lymph nodes (LN). IgE isotype switch and IgE producing B cell expansion occur in the draining LN after antigen (Ag) immunization, but do not happen in the spleen. In addition, a population of pre-existing IgE+ cells is observed in the spleen of normal or sham immunized mice, but is not present in the draining LN. To further understand the regulation of IgE production in different lymphoid organs, and the potential inhibitory factors of IgE isotype switch in the spleen, the involvement of IL-21 and IFN-gamma in regulating IgE production was investigated by using the IL-21 and the IFN-gamma deficient mice. We found that in the absence of IL-21 IgE isotype switch and IgE+ cell clonal expansion were dramatically enhanced in the spleen and IgE isotype switch was partially increased in the draining LN. In addition, IgE production of the pre-existing CD19-CD5+B220(low) IgE+ cells in the spleen was also increased in the absence of IL-21 under physiological conditions. In contrast, using the IFN-gamma deficient mice, we did not observe a negative impact of IFN-gamma on either IgE isotype switch or IgE production. Our data suggest that IL-21 appears to be a critical cytokine to keep low IgE levels under physiological and pathological conditions.

Caspases at the crossroads of immune-cell life and death

Caspases are responsible for crucial aspects of inflammation and immune-cell death that are disrupted in a number of genetic autoimmune and autoinflammatory diseases. The caspase family of proteases can be divided into pro-apoptotic and pro-inflammatory members based on their substrate specificity and participation in separate signalling cascades. However, as discussed here, evidence has emerged over the past few years that a number of the caspases thought to be involved solely in apoptosis also contribute to specific aspects of immune-cell development, activation and differentiation, and can even protect cells from some forms of cell death.

Caspases leave the beaten track: caspase-mediated activation of NF-kappaB

The proteolytic activity of the cysteinyl aspartate–specific proteases, named caspases, mainly connotes their central role in apoptosis and inflammation. In this review we report on recent data on the role of caspases in the activation of nuclear factor κB (NF-κB), a transcription factor that fulfils a central role in innate and adaptive immunity, in cellular stress responses and in the induction of anti-apoptotic factors. Two different mechanisms by which caspases activate the NF-κB pathway are discussed.

Sensitivity measurement and compensation in spectral imaging

BACKGROUND

The ImageStream system combines advances in CCD technologies with a novel optical architecture for high sensitivity and multispectral imaging of cells in flow. The sensitivity and dynamic range as well as a methodology for spectral compensation of imagery is presented.

METHODS

Multicolored fluorescent beads were run on the ImageStream and a flow cytometer. Four single color fluorescent control samples of cells were run to quantify spectral overlap. An additional sample, labeled with all colors was run and compensated in six spectral channels.

RESULTS

Analysis of empirical data for sensitivity and dynamic range matched theoretical predictions. The ImageStream system demonstrated fluorescence sensitivity comparable to a PMT-based flow cytometer. A methodology for addressing spectral overlap, individual pixel anomalies, and multiple imaging modalities was demonstrated for spectral compensation of K562 cells. Imagery is shown pre- and post-compensation.

CONCLUSIONS

Unlike intensity measurements made with conventional flow cytometers, object size impacts both dynamic range and fluorescence sensitivity in systems that utilize pixilated detection. Simultaneous imaging of alternate modalities can be employed to increase fluorescent sensitivity. Effective compensation of complex multimode imagery spanning six spectral bands is accomplished in a semi-automated manner.