Bystander Cell Death Research Articles

Role of HIV Gp41 mediated fusion/hemifusion in bystander apoptosis.

Mechanisms of HIV-mediated CD4+ T cell loss leading to immunodeficiency are amongst the most extensively studied yet unanswered questions in HIV biology. The level of CD4+ T cell depletion in HIV infected patients far exceeds the number of infected T cells, suggesting an indirect mechanism of HIV pathogenesis termed bystander cell death. Evidence is accumulating that the HIV envelope glycoprotein (Env) is a major determinant of HIV pathogenesis and plays a critical role in bystander cell death. The complex structure and function of HIV Env makes the determination of the mechanism of Env mediated apoptosis more complex than previously thought. This review will examine the complex relationship between HIV Env phenotype, coreceptor expression and immune activation in determining HIV pathogenesis. We review data here corresponding to the role of HIV Env hemifusion activity in HIV pathogenesis and how it interplays with other AIDS associated factors such as chemokine receptor expression and immune activation.

1P-280 放射光X線マイクロビームによる細胞局部照射によって誘導される周辺の非照射細胞の細胞死へのNOの関与(放射線生物/活性酸素,第46回日本生物物理学会年会)

1P-280 放射光X線マイクロビームによる細胞局部照射によって誘導される周辺の非照射細胞の細胞死へのNOの関与(放射線生物/活性酸素,第46回日本生物物理学会年会)

Drug development for treatment of cardiac arrhythmias: targeting the gap junctions

reentrant excitation ([12][1]) is likely the most important mechanism for serious life-threatening arrhythmias. Traditionally, a major focus of antiarrhythmic drug development has been to explore the actions of drugs designed to block conduction in reentrant pathways(s). During the latter part of

Unravelling the neuropathogenesis of Japanese encephalitis

Japanese encephalitis is a devastating disease that causes a frequently fatal encephalitis and is a major health problem throughout much of Asia. Mortality rates can be as high as 30%, with one-third of survivors suffering severe neurological sequelae. Viral cytopathology is important, but the contribution of blood-brain barrier breakdown, the inflammatory response and bystander cell death are unknown. Studies that address the mechanisms producing neuronal dysfunction and damage are required, including the examination of autopsy tissues. A better understanding of these mechanisms may lead to therapeutic strategies to reduce brain injury and neurological sequelae.

Bystander killing of breast cancer MCF‐7 cells by MDA‐MB‐231 cells exposed to 5‐fluorouracil is mediated via Fas

The major drawback with cancer therapy is the development of resistant cells within tumors due to their heterogeneous nature and due to inadequate drug delivery during chemotherapy. Therefore, the propagation of injury ("bystander effect" (BE)) from directly damaged cells to other cells may have great implications in cancer chemotherapy. The general advantage of the bystander cell killing phenomenon is the large therapeutic index that can be achieved. Experiments suggest that this phenomenon is detected in radiation therapy as well as in gene therapy in conjunction with chemotherapy. In the present study, we developed an original in vitro model dedicated to the exploration of bystander cytotoxicity induced during breast carcinoma chemotherapy. In brief, we investigated this perpetuation of injury on untreated bystander MCF-7 breast cancer cells which were coplated with 5-fluorouracil (5-FU)-treated MDA-MB-231 breast cancer cells. To achieve this goal, a specific in vitro coculture model which involved mixing of aggressive MDA-MB-231 breast cancer cells with enhanced green fluorescent protein (EGFP) expressing stable clone of non-metastatic MCF-7 breast cancer cells (MCF-EGFP), was used. A bystander killing effect was observed in MCF-EGFP cells cocultured with MDA-MB-231 cells pretreated with 5-FU. The striking decrease in MCF-EGFP cells, as detected by assaying for total GFP intensity, is mediated by activation of Fas/FasL system. The implication of Fas in MCF-EGFP cell death was confirmed by using antagonistic anti-FasL antibody that reverses bystander cell death by blocking FasL on MDA-MB-231 cells. In addition, inhibition of CD95/Fas receptor on the cell surface of MCF-EGFP cells by treatment with Pifithrin-alpha, a p53 specific transactivation inhibitor, partially abrogated the sensitivity of bystander MCF-EGFP cells. Our data, therefore, demonstrates that the Fas/FasL system could be considered as a new determinant for chemotherapy-induced bystander cell death in breast cancers.

3P263 X線マイクロビームを用いたバイスタンダー細胞死の研究(放射線生物/活性酸素,ポスター発表,第45回日本生物物理学会年会)

3P263 X線マイクロビームを用いたバイスタンダー細胞死の研究(放射線生物/活性酸素,ポスター発表,第45回日本生物物理学会年会)

Mutational analysis of HIV-1 gp41 mediated apoptosis and its correlation with fusion/hemifusion

The loss of CD4+ T cells in HIV-1 infections is hypothesized to be caused by apoptosis of bystander cells mediated via HIV-1 Env glycoprotein. However, the mechanism via which Env mediates this process remains controversial. Specifically the role of HIV-1 gp120 binding to CD4 and CXCR4 versus the fusion process mediated by gp41 remains unresolved. Previously we have demonstrated that Env induced apoptosis in bystander cells is gp41 dependent and correlates with the redistribution of membrane lipids between Env-expressing cells and target cells (hemifusion). We further examined the role of HIV-1 gp41 by performing mutational analysis of HIV-1LAI gp41 mediated apoptosis in bystander cells. A mutation in the fusion domain of gp41 (V513E) resulted in a fusion defective Env that failed to induce apoptosis. Altering fusion capacity of Env via N terminal helix mutation (G547D) or deletion of the cytoplasmic tail (CtDel) of gp41 resulted in reduced and enhanced cell to cell fusion respectively and correlated with apoptosis. Interestingly, mutation in the loop region of gp41 (D589L) exhibited a hemifusion restricted phenotype that induced bystander cell death while being a weak mediator of both cell to cell and virus to cell fusion. However, D589L Env did transfer the lipid dye DiI to the apoptotic bystander cells confirming its ability to induce apoptosis via hemifusion. Hence, we provide the first direct evidence that gp41 mediated hemifusion is sufficient for induction of apoptosis in bystander cells and may explain the mechanism of HIV-1 Env induced T cell depletion.

In vitro generated human memory‐like T cells are CD95 type II cells and resistant towards CD95‐mediated apoptosis

An adaptive immune response implies expansion of activated T cells and subsequent elimination to maintain homeostasis in a process called activation-induced cell death. Some cells, however, differentiate into memory cells and ensure a strong secondary immune response. To analyze the apoptosis phenotype of memory T cells on a cellular and molecular level, we have established an in vitro model of T cell activation and generation of cells phenotypically and functionally similar to memory cells. These long-term cultured T cells show a CD95-resistant phenotype, although they are still sensitive towards TCR/CD3-mediated apoptosis. Biochemical analysis revealed that these cells shift from CD95 type I (direct signaling from the receptor) during the effector phase to CD95 type II cells (dependent on the mitochondrial amplification loop). Moreover, their mitochondria are protected, probably due to high expression levels of Bcl-x(L) and Bcl-2. Thus, our data suggest a mechanism how memory T cells acquire resistance towards bystander cell death via the CD95 system.

Gap junctions remain open during cytochrome c-induced cell death: relationship of conductance to ‘bystander’ cell killing

Previous reports have shown that gap junctions relay cell death in many cell types. However, changes in electrical coupling and their dynamics during cell death are poorly understood. We performed comprehensive studies of electrical coupling following induction of cell death by single-cell cytochrome c (cyC) injection in paired Xenopus oocytes. Cell death was rapidly induced by cyC in injected cells, and cell death was also observed in uninjected bystander cells electrically coupled to the cyC-injected oocytes. Gap junction currents either remained at pre-cyC injection levels or increased dramatically as the injected cell died. Nonjunctional currents increased in injected cells immediately following cyC injection; nonjunctional currents increased slowly in uninjected bystander cells. Bystander cell death occurred only when junctional conductance was approximately 6 muS. Both 1,2-bis-(o-aminophenoxy)-ethane-N,N,-N',N'-tetraacetic acid tetraacetoxy-methyl ester and Xestospongin C inhibited bystander cell death in pairs that had reached the death conductance threshold, suggesting that Ca(2+) and inositol 1,4,5 triphosphate are involved in the process.

Fusion of the Human Immunodeficiency Virus Type 1 Tat Protein Transduction Domain to Thymidine Kinase Increases Bystander Effect and Induces Enhanced Tumor KillingIn Vivo

The clinical success of suicide gene therapy using herpes simplex virus type 1 thymidine kinase (TK) is largely dependent on the capacity of this enzyme to effectively induce the death of bystander cells. We have shown that fusion of TK to an 11-amino acid peptide from the basic domain of the human immunodeficiency virus type 1 Tat protein (Tat11) imparts cell membrane-translocating ability to the enzyme and significantly increases its cytotoxic activity. Here we report on the efficacy of this strategy in two different mouse models of adoptive tumorigenesis, based on the implantation of human Kaposi sarcoma (KS-IMM) cells in nude mice or of B16F10 melanoma cells in syngeneic C57BL/6J mice. Experiments were performed by the subcutaneous injection of mixtures of unmodified tumor cells containing different fractions of TK or Tat11-TK producing cells followed by animal treatment with ganciclovir (GCV). In both systems we consistently found that mice bearing tumors containing Tat11-TK cells displayed significantly retarded tumor growth and prolonged survival as compared with mice inoculated with cells expressing unmodified TK. Collectively, these results demonstrate that fusion of Tat11 to TK imparts remarkable intercellular trafficking capability to the enzyme. This modification of TK might constitute an important step in the optimization of TK suicide gene strategy for gene therapy of cellular proliferation.

Single-cell microinjection of cytochromec can result in gap junction-mediated apoptotic cell death of bystander cells in head and neck cancer

Gap junction intercellular channels are required for metabolic cooperation between cells and regulate normal tissue homeostasis by means of the transfer of small molecules between contacting cells. Not surprisingly, the gap junction phenotype is frequently lost during carcinogenesis in human tissues (including those of the upper aerodigestive tract), freeing individual cancer cells from the growth control signals of normal surrounding tissues and less aggressive adjacent cancer cells. We hypothesized that gap junctional intercellular communication (GJIC) could mediate a bystander effect (apoptotic cell death) in squamous cell carcinoma of the head and neck (SCCHN) cells adjacent to individually targeted SCCHN cells. Single-cell microinjection of cytochrome c was used to induce apoptosis in target SCCHN cells with endogenous GJIC activity and in an SCCHN cell line with exogenously introduced GJIC activity. Apoptosis was followed in target and surrounding bystander cells through light and time course microscopic characterization. All of the preceding experiments were carried out in the absence and presence of 18-beta-glycerretinic acid, a pharmacologic inhibitor of GJIC. When cytochrome c was introduced into SCCHN cells with endogenous GJIC activity through single-cell microinjection, bystander effects (apoptosis of nontarget cells) were observed. When GJIC activity was blocked with the specific pharmacologic inhibitor of gap junctions, 18-beta-glycerretinic acid, a bystander effect was never seen in GJIC active SCCHN cell lines. Gap junction intercellular channels can mediate a bystander effect in SCCHN. Inconsistencies in our data will be discussed in the context of recent advances in this field, as well as our future research directions.

Apoptosis of Bystander T Cells Induced by Human Immunodeficiency Virus Type 1 with Increased Envelope/Receptor Affinity and Coreceptor Binding Site Exposure

Apoptosis of uninfected bystander CD4(+) T cells contributes to T-cell depletion during human immunodeficiency virus type 1 (HIV-1) pathogenesis. The viral and host mechanisms that lead to bystander apoptosis are not well understood. To investigate properties of the viral envelope glycoproteins (Env proteins) that influence the ability of HIV-1 to induce bystander apoptosis, we used molecularly cloned viruses that differ only in specific amino acids in Env. The ability of these strains to induce bystander apoptosis was tested in herpesvirus saimiri-immortalized primary CD4(+) T cells (CD4/HVS), which resemble activated primary T cells. Changes in Env that increase affinity for CD4 or CCR5 or increase coreceptor binding site exposure enhanced the capacity of HIV-1 to induce bystander apoptosis following viral infection or exposure to nonreplicating virions. Apoptosis induced by HIV-1 virions was inhibited by CD4, CXCR4, and CCR5 antibodies or by the CXCR4 inhibitor AMD3100, but not the fusion inhibitor T20. HIV-1 virions with mutant Envs that bind CXCR4 but are defective for CD4 binding or membrane fusion induced apoptosis, whereas CXCR4 binding-defective mutants did not. These results demonstrate that HIV-1 virions induce apoptosis through a CXCR4- or CCR5-dependent pathway that does not require Env/CD4 signaling or membrane fusion and suggest that HIV-1 variants with increased envelope/receptor affinity or coreceptor binding site exposure may promote T-cell depletion in vivo by accelerating bystander cell death.

NFκB Controls the Balance between Fas and Tumor Necrosis Factor Cell Death Pathways during T Cell Receptor-induced Apoptosis Via the Expression of Its Target Gene A20

Activation-induced cell death (AICD), a term originally coined for the anti-CD3-induced apoptosis of T cell hybridomas and thymocytes, is predominantly driven by death receptors and has been involved in the control of autoreactive T cells in the periphery. In the Do-11.10 T cell hybridoma model of AICD, activation of the T cell receptor (TCR) results in Fas-dependent apoptosis. Here, we show that inhibition of the transcription factor nuclear factor kappa B (NF kappa B) in Do-11.10 cells resulted in increased sensitivity to TCR-mediated apoptosis, correlating with defective induction of the anti-apoptotic NF kappa B target gene A20. Stable expression of the zinc finger protein A20 in NF kappa B-negative Do-11.10 cells rescued the phenotype. TCR activation in NF kappa B-deficient Do-11.10 cells resulted predominantly in tumor necrosis factor (TNF) receptor 2 (TNFR2)-dependent bystander cell death rather than classical Fas-dependent AICD. Strikingly, A20 blocked TNF-mediated apoptosis and simultaneously restored TCR-induced Fas-dependent AICD. In addition, NF kappa B downstream of TNFR was required for up-regulation of Fas expression by endogenous TNF secreted in response to TCR stimulation. Together, these results suggest that NF kappa B can play both pro- and anti-apoptotic roles during AICD. We propose that NF kappa B controls the balance between Fas and TNF cell death pathways during AICD via the expression of the zinc finger protein A20.

Gap Junctions Mediate Bystander Cell Death in Developing Retina

During development of the retina, programmed cell death helps to establish the final size and distribution of various cell classes in distinct layers of the tissue. Here we show that dying cells in the developing ganglion and inner nuclear layers are clustered spatially and that gap junction inhibitors decrease the clustering of dying cells. To confirm the role of gap junctions in cell death, we induced targeted cell death via intracellular cytochrome c (Cc) and examined the induced cells and their neighbors for apoptotic morphology or caspase-3 cleavage. These studies indicate that bystander killing extends to coupled cells. Quantitative studies of bystander killing were performed by scrape-loading retinas with Cc in the presence of rhodamine dextran (RD; to identify Cc-loaded cells) and by counting pyknotic cells in cryosections. Although only 1.5% of control scrape-loaded cells (RD alone) showed apoptotic morphology, 97% of Cc scrape-loaded cells were pyknotic. Moreover, bystander killing extended to neighboring cells, not labeled with RD, and was reduced significantly by the gap junction inhibitors octanol and carbenoxolone. We hypothesize that dying cells in the retina generate a gap junction-permeant apoptotic signal that mediates bystander killing. This novel finding of naturally occurring bystander cell death may have important implications in the histogenesis and pathology of the nervous system.

Suicide gene therapy with Herpes simplex virus thymidine kinase and ganciclovir is enhanced with connexins to improve gap junctions and bystander effects.

Connexins are proteins that form gap junctions between cells in various mammalian tissues. Because of their role in intercellular communication, connexins are important in the bystander cell death seen in Herpes simplex virus-thymidine kinase (HSV-TK) gene therapy for brain tumors. A selective review of connexin transduction/transfection studies with particular emphasis to central nervous system tumor cells is presented. In addition, specific references to studies with cell types that demonstrate low gap junction intercellular communication are presented. Data are included with the HT-29 colorectal tumor cell line to support the concept that enhancing gap junction protein expression in otherwise low gap junction communicating HT-29 cells increases bystander cell death and reduces tumor burden beyond what might be expected from HSV-TK and ganciclovir (GCV) treatment alone. Maximum in vitro bystander cell death was always produced when GCV treated co-cultures of TK-transduced and non-TK-transduced HT-29 cell lines were also transduced with connexin-43. When connexin was present in only one group of cells in the co-culture, there was more bystander cell death observed with connexin transduced into the non-TK-transduced cells, rather than the TK-transduced cells. The data presented reinforces conclusions made from earlier findings from cell line mixing experiments in which the non-TK-transduced cell population determined the level of bystander cell death (Burrows et al., 2002).

Expression of MRP4 Confers Resistance to Ganciclovir and Compromises Bystander Cell Killing

The multidrug resistance protein MRP4, a member of the ATP-binding cassette superfamily, confers resistance to purine-based antiretroviral agents. However, the antiviral agent ganciclovir (GCV) has not been shown to be a substrate of MRP4. GCV is important not only in antiviral therapy, but also in the selective killing of tumor cells modified to express herpes simplex virus thymidine kinase (HSV-TK). We therefore tested the effect of MRP4 on the cytotoxicity of GCV, on the ability of GCV to kill cells genetically modified to express HSV-TK, and on the bystander effect in which unmodified target cells are killed by GCV. Cells overexpressing MRP4 had markedly increased resistance to the cytotoxicity of GCV. Although, expression of recombinant HSV-TK increased the intracellular concentration of GCV nucleotide, cells were rescued by the cytoprotective effect of MRP4. In cells that overexpressed MRP4, intracellular accumulation of GCV metabolites was reduced, efflux of these metabolites was increased, and resistance to bystander killing was increased. Therefore, MRP4 can strongly reduce the susceptibility of HSV-TK-expressing cells to GCV, and its overexpression in adjacent cells protects them from bystander cell death. These findings indicate that a nucleotide transporter, such as MRP4, modulates the cellular response to GCV and thus may influence not only the efficacy of antiviral therapy, but also prodrug-based gene therapy, which is critically dependent upon bystander cell killing.

Cell Cycle Dysregulation During Hiv Infection: Perspectives of a Target Based Therapy

Human immunodeficiency virus (HIV) infection is characterized by a severe depletion of both CD4+ and CD8+ T cells, representing the result of virus-mediated killing of infected lymphocytes and the programmed cell death (apoptosis) of the uninfected bystander cells. Since only a small fraction of T lymphocytes are depleted by viral killing, apoptosis represents one of the most important mechanism of T cell death during HIV infection. Several apoptotic pathways can be triggered by the different stimuli: persistent T lymphocyte activation; altered death receptor (Fas, TNF-R, TRAIL R1-R2) membrane expression; viral proteins as well as gp120, Tat, and Nef; host factors such as the unbalance of cytokine synthesis by lymphocyte. Nevertheless, new evidences have demonstrated that the persistent HIV induced T cell activation and proliferation cause a cell cycle dysregulation resulting in a 5-fold increase in apoptotic cells. This perturbation represents a link between HIV infection, T cell activation, accelerated cell turnover and increased apoptosis and may thus represent a new therapeutic target. In fact, Interleukin-2 administration reverts such a cell cycle dysregulation and reduces activation induced T cell apoptosis. Herein we analyze the main HIV-related mechanisms of host cell death, that are dysregulation of the cell cycle and apoptosis induction of T lymphocytes. Finally, the role of cytokines at the site of infection and their association with apoptosis will be discussed to get insights into the immunological perturbations accounting for an accelerated disease progression. Current therapeutic approaches and strategies, like HAART and recombinant cytokines, that may, successfully, improve the immune-system dysregulation, are also discussed.

Anti-HIV-1 Activity of Enfuvirtide (T-20) by Inhibition of Bystander Cell Death

Infection by human immunodeficiency virus type 1 (HIV-1) has been associated with increased cell death of both infected and bystander cells. The envelope glycoprotein complex appears to play an active role in HIV-induced death of bystander cells. We quantified cell-to-cell fusion, single cell death and membrane lipid mixing in cocultures of effector, HIV-1 envelope-expressing cells with peripheral blood mononuclear cells or purified CD4 T lymphocytes from HIV-negative donors, in the presence or the absence of the fusion inhibitor enfuvirtide (T-20, pentafuside, Fuzeon). T-20, which blocks gp41-dependent virus-cell fusion, showed a complete and dose-dependent inhibition of syncytium formation in cocultures of envelope-expressing cells with uninfected cells. Similarly, T-20 totally abrogated death of single bystander CD4 T cells with an IC50 of 0.04 microg/ml. Membrane lipid mixing, as a measure of interaction between envelope-expressing cells and CD4 cells, was also dose-dependently inhibited by T-20. Moreover, effector cells chronically infected with a T-20-resistant virus recovered the ability to induce bystander cell death in the presence of the drug, supporting the role of gp41 in single cell death. In conclusion, T-20 is able to protect CD4 T cells from envelope presentation with a dual effect: inhibition of virus replication and blockade of HIV-1 envelope-induced cell death of bystander CD4 T cells. Protection of cells prior to infection from HIV envelope-dependent bystander effect could lead to a better immune restoration of HIV-1-infected patients that are treated with T-20.

Differential cytotoxicity and bystander effect of the rabbit cytochrome P450 4B1 enzyme gene by two different prodrugs: implications for pharmacogene therapy.

The time course of cytotoxicity induction and the bystander effect of the rabbit cytochrome P450 4B1 (cyp4B1)/4-ipomeanol (4-IM) or 2-aminoanthracene (2-AA) pharmacogene therapy systems were investigated and compared with the herpes simplex virus type 1 thymidine kinase/ganciclovir (HSV-tk/GCV) system. Experiments were performed in rat 9L gliosarcoma cells stably expressing cyp4B1 (9L-4B1), HSV-tk (9L-tk), or their egfp (enhanced green fluorescent protein) fusion genes. Cyp4B1-mediated activation of 2-AA showed a high cell killing efficiency within only 48 hours with an onset after already 15 minutes of prodrug exposure. Residual 9L-4B1 cells were mostly damaged sublethally upon 2-AA treatment showing an S phase arrest by cell cycle analysis. 4-IM treatment of 9L-4B1 cells generated an overall weaker cell killing, especially after prodrug exposure times of less than 48 hours. Residual cells surviving 4-IM treatment showed a G2/M arrest and restarted proliferation after prodrug treatment was stopped. HSV-tk/GCV pharmacogene therapy resulted in a slower cytotoxicity induction than cyp4B1/2-AA treatment with a significantly lower cell killing efficiency after 24 and 48 hours. HSV-tk/GCV-mediated cytotoxicity was widely similar to the cytotoxicity induced by cyp4B1/4-IM with the exception of a continuous 48-hour prodrug exposure where 4-IM treatment showed a significantly higher cell killing rate. Cells surviving HSV-tk/GCV suicide gene therapy were not viable and showed an S-phase arrest. Whereas HSV-tk/GCV induced a strong bystander effect, only moderate bystander cell death depending on cell-to-cell contact was demonstrated in 9L/9L-4B1 cocultures upon 2-AA treatment and was even absent with 4-IM, thereby contrasting with earlier reports. The absence of a strong bystander effect may limit, on one hand, the overall utility of the cyp4B1 systems for cancer gene therapy. On the other hand, the weak bystander effect together with the fast induction of cytotoxicity may provide marked advantages for the use of the cyp4B1 systems as biosafety enhancers for gene marking or replacement studies and donor lymphocyte infusions after allogeneic bone marrow transplantation.

Kiss of death

CD8 cytotoxic T cells are serial killers that slay one infected or cancerous cell after another. During each fatal encounter, an intercellular junction called the immunological synapse forms between killer and victim, according to researchers from Oxford University (Oxford, UK). Their work suggests new functions for the immunological synapse and supports the hypothesis that the immune system governs the number of cytotoxic T cells by “retiring” veterans of multiple battles. Figure Lytic granules (blue) insert between the signaling patch (red) and the adhesion ring (green). In earlier studies immunological synapses were spotted only when an antigen- presenting cell met and activated a CD4 T cell. Although a CD8 cytotoxic T cell may attach to its victim for as little as two minutes, Gillian Griffiths and colleagues observed that synapses also formed during these contacts. The synapses have the characteristic bull's eye structure of a ring of adhesion proteins surrounding a ring of signaling proteins. The authors also showed that the cell-killing granules released by the cytotoxic T lymphocyte emerged from a focused secretory domain within the adhesion domain.The immunological synapse was thought to provide sustained signaling. But the new results suggest that the architecture of the synapse may help guide the secretion of molecules and prevent the death of nearby bystander cells, Griffiths says. She adds that the synapse may also participate in regulating the numbers of cytotoxic T cells. As the two cells part, the T cell picks up some of the victim's proteins, probably by ripping off a hunk of membrane. Thus, the T cells end up looking more and more like their targets, leading them to destroy each other in a phenomenon called fratricide. ▪