Efficient Lysis Of Target Cells Research Articles

A novel bispecific antibody platform to direct complement activity for efficient lysis of target cells

Harnessing complement-mediated cytotoxicity by therapeutic antibodies has been limited because of dependency on size and density of antigen, structural constraints resulting from orientation of antibody binding, and blockade of complement activation by inhibitors expressed on target cells. We developed a modular bispecific antibody platform that directs the complement-initiating protein C1q to target cells, increases local complement deposition and induces cytotoxicity against target antigens with a wide-range of expression. The broad utility of this approach to eliminate both prokaryotic and eukaryotic cells was demonstrated by pairing a unique C1q-recruiting arm with multiple targeting arms specific for Staphylococcus aureus, Pseudomonas aeruginosa, B-cells and T-cells, indicating applicability for diverse indications ranging from infectious diseases to cancer. Generation of C1q humanized mice allowed for demonstration of the efficacy of this approach to clear disease-inducing cells in vivo. In summary, we present a novel, broadly applicable, and versatile therapeutic modality for targeted cell depletion.

Targeting AML Using an Fc-Engineered BST1/CD157 Monoclonal Antibody

Antibody-based immunotherapy represents a promising strategy to target and eliminate acute myeloid leukemia (AML) tumor cells. We evaluated BST1/CD157, a novel, slow internalizing surface antigen, for its suitability as immunotherapeutic target in AML. An Fc-optimized antibody against BST1/CD157 was developed (MEN1112/OBT357NF), which binds to the Fc-receptor on natural killer (NK) cells with enhanced affinity. The target antigen expression profile of BST1/CD157 on AML bulk cells as well as on leukemic stem cells (LSC) was analyzed with flow cytometry using a commercially available antibody. BST1/CD157 expression was detected in > 97% of AML patients at primary diagnosis (n=81) and at time of relapse (n=20) using specific fluorescence intensity (SFI, positivity: SFI > 1. 5). Importantly, BST1/CD157 was also expressed on CD34+/CD38-cells (n=20), the compartment with a high percentage of LSCs.Functional titration experiments of MEN1112/OBT357NF showed antibody-mediated lysis of U937 cells in the low picomolar range (EC50 = 30–140 pM) and demonstrated superior ADCC activity compared to its non Fc-engineered parental analogue. To evaluate the additive effect of the complement cascade, healthy donor (HD) monocytes were co-incubated with autologous NK cells and MEN1112/OBT357NF in the presence or absence of 5% donor serum. After 20 hours of incubation, a significantly enhanced lysis efficacy by MEN1112/OBT357NF through the addition of serum compared to control cultures without serum (p=0.02) was demonstrated.For further functional characterization of MEN1112/OBT357NF standard chromium release assays using AML cell lines and NK cells from HDs were performed. In this setting, efficient target cell lysis (30–50%) at various effector cell:target (E:T) ratios (50:1–6:1) was observed. Compared to Rituximab-mediated lysis of RAJI cells, MEN1112/OBT357NF triggered more efficient target cell lysis at much lower effector cell concentrations (33% lysis of OCI-AML3 cells vs. 11% lysis of RAJI cells at E:T 6:1).Furthermore, the cytotoxic potential of NK cells from AML patients after intense double induction chemotherapy on AML cell lines was tested. The results were highly variable with efficient lysis in about one third of samples tested (n=10). As a positive control NK cells from HDs were used which showed superior lysis as compared to patient-derived NK cells. In an autologous set-up using NK cells from either primary diagnosis or at time of remission, we could show efficient lysis of primary AML cells in one of two patient samples (patient in complete remission) in the presence of MEN1112/OBT357NF.In summary, our analysis demonstrates that BST1/CD157 is a promising target antigen for antibody-based therapy in AML. The effective in-vitro ADCC activity supports further development of MEN1112/OBT35NF as an immunotherapy for patients with AML. DisclosuresKrupka:Oxford BioTherapeutics Ltd: Research Funding. Jansen:Oxford BioTherapeutics Ltd: Research Funding. Aud:Oxford BioTherapeutics Ltd: Employment. Dusek:Oxford BioTherapeutics Ltd: Employment. Bisht:Oxford BioTherapeutics Ltd: Employment. Pombo-Villar:Oxford BioTherapeutics Ltd: Employment, Equity Ownership. Rohlff:Oxford BioTherapeutics Ltd: Employment. Subklewe:Oxford BioTherapeutics Ltd: Research Funding.

Characterization of a novel single-chain bispecific antibody for retargeting of T cells to tumor cells via the TCR co-receptor CD8.

There is currently growing interest in retargeting of effector T cells to tumor cells via bispecific antibodies (bsAbs). Usually, bsAbs are directed on the one hand to the CD3 complex of T cells and on the other hand to a molecule expressed on the surface of the target cell. A bsAb-mediated cross-linkage via CD3 leads to an activation of CD8+ T cells and consequently to killing of the target cells. In parallel, CD4+ T cells including TH1, TH2, TH17 cells and even regulatory T cells (Tregs) will be activated as well. Cytokines produced by CD4+ T cells can contribute to severe side effects e. g. life-threatening cytokine storms and, thinking of the immunosupressive function of Tregs, can even be counterproductive. Therefore, we asked whether or not it is feasible to limit retargeting to CD8+ T cells e. g. via targeting of the co-receptor CD8 instead of CD3. In order to test for proof of concept, a novel bsAb with specificity for CD8 and a tumor-associated surface antigen was constructed. Interestingly, we found that pre-activated (but not freshly isolated) CD8+ T cells can be retargeted via CD8-engaging bsAbs leading to an efficient lysis of target cells.

Fratricide Is Essential for Efficient Gene Transfer between Pneumococci in Biofilms

Streptococcus pneumoniae and a number of commensal streptococcal species are competent for natural genetic transformation. The natural habitat of these bacteria is multispecies biofilms in the human oral cavity and nasopharynx. Studies investigating lateral transfer of virulence and antibiotic resistance determinants among streptococci have shown that interspecies as well as intraspecies gene exchange takes place in these environments. We have previously shown that the action of a competence-specific murein hydrolase termed CbpD strongly increases the rate of gene transfer between pneumococci grown in liquid cultures. CbpD is the key component of a bacteriolytic mechanism termed the fratricide mechanism. It is secreted by competent pneumococci and mediates the release of donor DNA from sensitive streptococci present in the same environment. However, in nature, gene exchange between streptococci takes place in biofilms and not in liquid cultures. In the present study, we therefore investigated whether CbpD affects the rate of gene transfer in laboratory-grown biofilms. Our results show that the fratricide mechanism has a strong positive impact on intrabiofilm gene exchange, indicating that it is important for active acquisition of homologous donor DNA under natural conditions. Furthermore, we found that competent biofilm cells of S. pneumoniae acquire a Nov(r) marker much more efficiently from neighboring cells than from the growth medium. Efficient lysis of target cells requires that CbpD act in conjunction with the murein hydrolase LytC. In contrast, the major autolysin LytA does not seem to be important for fratricide-mediated gene exchange in a biofilm environment.

Abstract 5625: In vitro and in vivo pharmacology of MEDI-565 (MT111), a novel CEA/CD3-bispecific single-chain BiTE antibody in development for the treatment of gastrointestinal adenocarcinomas

Abstract MEDI-565 (MT111) is a novel bispecific single-chain antibody of the BiTE (Bispecific T cell engager) class that transiently links carcinoembryonic antigen (CEA) on cancer cells with human CD3 on T cells. MEDI-565 specifically binds to human CEA with an affinity of 8.5 nM but not to any other member of the CEACAM family. In targeted expression studies on cryopreserved and formalin-fixed paraffin-embedded sections, MEDI-565 demonstrated membrane-localized binding specifically to epithelial cells in human cancerous tissues. The highest prevalence of MEDI-565 binding (∼90%) was in adenocarcinomas of gastrointestinal origin. In in vitro killing assays, MEDI-565 recruited T cells via the CD3 antigen (affinity of 310 nM) in a process that required concomitant binding to CEA positive tumor cells for T cell activation. As a consequence, T cells expanded, increased cell surface expression of the activation markers CD69 and CD25, and released perforin and granzymes. The release of cytotoxic granule content led to a Ca2+-dependent activation of pro-caspases and subsequent apoptosis of CEA-expressing tumor cells. Efficient target cell lysis by MEDI-565 was predominantly mediated by CD8+ T cells and occurred within a wide range of effector-to-target ratios (80:1 to 5:1) for T cells derived from various human donors. BiTE mediated killing was not affected by the presence of soluble CEA (≤5 µg/mL). The in vivo activity of MEDI-565 was investigated in subcutaneous xenograft models using immunocompromised SCID mice inoculated with mixtures of human T cells and human cancer cell lines. The antibody was eliminated from the serum with a half-life of a few hours following a single intravenous (IV) or subcutaneous (SC) injection. Despite a relatively short serum half-life, daily IV or SC bolus treatments over 5 days provided sufficient levels of exposure to inhibit the growth of CEA-positive tumors in a dose-dependent manner. Inhibition of growth was observed for tumors of different tissue origins and was dependent on the presence of human T cells and CEA expression by tumor cells. There were no MEDI-565-related in-life observations following treatment. These studies demonstrated that MEDI-565 has potent and selective anti-cancer activity in vitro and in vivo and provides evidence that MEDI-565 may be an effective monotherapy to treat CEA-expressing malignancies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5625.

Fratricide in Streptococcus pneumoniae: contributions and role of the cell wall hydrolases CbpD, LytA and LytC

Pneumococci that have developed the competent state kill and lyse non-competent sister cells and members of closely related species during co-cultivation in vitro. The key component in this process, called fratricide, is the product of the late competence gene cbpD. In addition, the peptidoglycan hydrolases LytA and LytC are required for efficient lysis of target cells. Here, we have investigated the relative contribution and possible role of each of the proteins mentioned above. Previous studies have shown that CbpD is produced exclusively by competent cells, whereas LytA and LytC can be provided by the competent attackers as well as the non-competent target cells. By using an improved assay to compare the effect of cis- versus trans-acting LytA and LytC, we were able to show that target cells are lysed much more efficiently when LytA and LytC are provided in cis, i.e. by the target cells themselves. Western analysis demonstrated that considerable amounts of LytC are present in the growth medium. In contrast, we were not able to detect any extracellular LytA. This finding indicates that LytA- and LytC-mediated fratricide represent different processes. In the absence of LytA and LytC, only a tiny fraction of the target cells were lysed, demonstrating that CbpD does not function efficiently on its own. However, in the presence of 1 mM EDTA, the fraction of target cells lysed directly by CbpD increased dramatically, indicating that divalent cations are involved in the regulation of fratricide under natural conditions.

Protein Kinase Cθ Regulates Stability of the Peripheral Adhesion Ring Junction and Contributes to the Sensitivity of Target Cell Lysis by CTL

Destruction of virus-infected cells by CTL is an extremely sensitive and efficient process. Our previous data suggest that LFA-1-ICAM-1 interactions in the peripheral supramolecular activation cluster (pSMAC) of the immunological synapse mediate formation of a tight adhesion junction that might contribute to the sensitivity of target cell lysis by CTL. Herein, we compared more (CD8(+)) and less (CD4(+)) effective CTL to understand the molecular events that promote efficient target cell lysis. We found that abrogation of the pSMAC formation significantly impaired the ability of CD8(+) but not CD4(+) CTL to lyse target cells despite having no effect of the amount of released granules by both CD8(+) and CD4(+) CTL. Consistent with this, CD4(+) CTL break their synapses more often than do CD8(+) CTL, which leads to the escape of the cytolytic molecules from the interface. CD4(+) CTL treatment with a protein kinase Ctheta inhibitor increases synapse stability and sensitivity of specific target cell lysis. Thus, formation of a stable pSMAC, which is partially controlled by protein kinase Ctheta, functions to confine the released lytic molecules at the synaptic interface and to enhance the effectiveness of target cell lysis.

Critical and Differential Roles of NKp46- and NKp30-Activating Receptors Expressed by Uterine NK Cells in Early Pregnancy

In early human pregnancy, uterine decidual NK cells (dNK) are abundant and considered as cytokine producers but poorly cytotoxic despite their cytolytic granule content, suggesting a negative control of this latter effector function. To investigate the basis of this control, we examined the relative contribution to the cytotoxic function of different activating receptors expressed by dNK. Using a multicolor flow cytometry analysis, we found that freshly isolated dNK exhibit a unique repertoire of activating and inhibitory receptors, identical among all the donors tested. We then demonstrated that in fresh dNK, mAb-specific engagement of NKp46-, and to a lesser extent NKG2C-, but not NKp30-activating receptors induced intracellular calcium mobilization, perforin polarization, granule exocytosis and efficient target cell lysis. NKp46-mediated cytotoxicity is coactivated by CD2 but dramatically blocked by NKG2A coengagement, indicating that the dNK cytotoxic potential could be tightly controlled in vivo. We finally found that in dNK, mAb-specific engagement of NKp30, but not NKp46, triggered the production of IFN-gamma, TNF-alpha, MIP-1alpha, MIP-1beta, and GM-CSF proinflammatory molecules. These data demonstrate a differential, controlled role of NKp46- and NKp30-activating receptors expressed by dNK that could be critical for the outcome of pregnancy and the killing of uterine cells infected by pathogens.

Antigen Sensitivity of CD22-Specific Chimeric TCR Is Modulated by Target Epitope Distance from the Cell Membrane

We have targeted CD22 as a novel tumor-associated Ag for recognition by human CTL genetically modified to express chimeric TCR (cTCR) recognizing this surface molecule. CD22-specific cTCR targeting different epitopes of the CD22 molecule promoted efficient lysis of target cells expressing high levels of CD22 with a maximum lytic potential that appeared to decrease as the distance of the target epitope from the target cell membrane increased. Targeting membrane-distal CD22 epitopes with cTCR(+) CTL revealed defects in both degranulation and lytic granule targeting. CD22-specific cTCR(+) CTL exhibited lower levels of maximum lysis and lower Ag sensitivity than CTL targeting CD20, which has a shorter extracellular domain than CD22. This diminished sensitivity was not a result of reduced avidity of Ag engagement, but instead reflected weaker signaling per triggered cTCR molecule when targeting membrane-distal epitopes of CD22. Both of these parameters were restored by targeting a ligand expressing the same epitope, but constructed as a truncated CD22 molecule to approximate the length of a TCR:peptide-MHC complex. The reduced sensitivity of CD22-specific cTCR(+) CTL for Ag-induced triggering of effector functions has potential therapeutic applications, because such cells selectively lysed B cell lymphoma lines expressing high levels of CD22, but demonstrated minimal activity against autologous normal B cells, which express lower levels of CD22. Thus, our results demonstrate that cTCR signal strength, and consequently Ag sensitivity, can be modulated by differential choice of target epitopes with respect to distance from the cell membrane, allowing discrimination between targets with disparate Ag density.

From a single viral peptide epitope to effective lysis of target cells by CTL (87.50)

Abstract Cytotoxic T lymphocytes (CTL) possess a unique ability to destroy virus-infected cells by releasing cytolytic granules that upon delivery to target cells induce apoptosis. It has been proved that perhaps even a single peptide presented on a target cell renders this cell susceptible for killing by CTL. Using a novel biosensor containing quantum dots (QD) carrying controlled number of cognate and non-cognate peptide-MHC (pMHC) proteins, we have shown that pMHC attached to QD cooperate and that the presence of a single cognate pMHC with all others being non-cognate is sufficient to induce a robust CTL response. To understand how triggering of CTL response leads to delivery of effective lethal hit to target cells, we examined granule polarization and release as well as segregation of immune receptors at interface between CTL and planar bilayer containing cognate pMHC and ICAM-1. We show that exceptionally tight ring junction formed at CTL-bilayer interface functions as a gasket precluding granule escape, which are concentrated in the secretory domain of immunological synapse. We propose that efficient target cell lysis requires 2 essential manipulations of cytolytic granules: release and effective targeting.

T cell avidity determines the level of CTL activation.

To investigate the influence of avidity on T cell activation in vitro and in vivo, we analyzed T cells from St40 and St42 mice, which express the same transgenic TCR specific for an E1a-derived epitope of adenovirus type 5 with different expression levels and therefore different avidities. Splenocytes from both strains showed comparable cytolytic activities and required identical peptide concentrations for efficient target cell lysis and up-regulation of activation markers. However, the kinetics of CD25 up-regulation were strikingly different: whereas the majority of the high-avidity St42 T cells up-regulated the IL-2Ralpha chain within a few hours, low-avidity St40 T cells expressed only 50% of the CD25 of high-avidity T cells after 2 days. In addition, low-avidity T cells proliferated poorly and displayed impaired secretion of IL-2 and IFN-gamma. Similar results were seen with high-avidity St42 T cells stimulated with a partial agonistic peptide. Upon adoptive transfer and subsequent immunization with adenovirus, both high- and low-avidity T cells expanded, but St40 T cells were undetectable 10 days after immunization. Our model system now allows analysis of whether T cells with identical specificities but different avidities influence each other during activation and homeostatic proliferation.

Role of antigen, CD8, and cytotoxic T lymphocyte (CTL) avidity in high dose antigen induction of apoptosis of effector CTL.

Experimental data suggest that negative selection of thymocytes can occur as a result of supraoptimal antigenic stimulation. It is unknown, however, whether such mechanisms are at work in mature CD8+ T lymphocytes. Here, we show that CD8+ effector cytotoxic T lymphocytes (CTL) are susceptible to proliferative inhibition by high dose peptide antigen, leading to apoptotic death mediated by TNF-alpha release. Such inhibition is not reflected in the cytolytic potential of the CTL, since concentrations of antigen that are inhibitory for proliferation promote efficient lysis of target cells. Thus, although CTL have committed to the apoptotic pathway, the kinetics of this process are such that CTL function can occur before death of the CTL. The concentration of antigen required for inhibition is a function of the CTL avidity, in that concentrations of antigen capable of completely inhibiting high avidity CTL maximally stimulate low avidity CTL. Importantly, the inhibition can be detected in both activated and resting CTL. Blocking studies demonstrate that the CD8 molecule contributes significantly to the inhibitory signal as the addition of anti-CD8 antibody restores the proliferative response. Thus, our data support the model that mature CD8+ CTL can accommodate an activation signal of restricted intensity, which, if surpassed, results in deletion of that cell.

HLA restriction of human cytotoxic T lymphocytes specific for influenza virus. Poor recognition of virus associated with HLA A2.

Cytotoxic T lymphocytes (CTL), specific for influenza A/X31 virus, were generated from human peripheral blood lymphocytes. These CTL lysed target cells that were infected with the same virus and that shared HLA A or B locus antigens. Minimal lysis was observed when HLA-D antigens were shared. Not all HLA A and B antigens were equally effective. Efficient lysis of target cells was seen when HLA A1, A3, B7, B8, B27 and BW21 were shared with the CTL, but when HLA A2 was the only shared antigen lysis was usually minimal. This deficiency in CTL function associated with HLA A2 was not absolute. It is suggested that the function of this antigen might be influenced by other surface molecules on the cell and in particular the other HLA products.

Behcet's Syndrome and Vitamin K

Cytotoxicity of human NK cells is activated by receptors that bind ligands on target cells, but the relative contribution of the many different activating and inhibitory NK cell receptors is difficult to assess. In this study, we describe an experimental system that circumvents some of the difficulties. Adhesion through β₂ integrin LFA-1 is a common requirement of CTLs and NK cells for efficient lysis of target cells. However, the contribution of LFA-1 to activation signals for NK cell cytotoxicity, besides its role in adhesion, is unclear. The role of LFA-1 was evaluated by exposing NK cells to human ICAM-1 that was either expressed on a <i>Drosophila</i> insect cell line, or directly coupled to beads. Expression of ICAM-1 on insect cells was sufficient to induce lysis by NK cells through LFA-1. Coexpression of peptide-loaded HLA-C with ICAM-1 on insect cells blocked the LFA-1-dependent cytotoxicity of NK cells that expressed HLA-C-specific inhibitory receptors. Polarization of cytotoxic granules in NK cells toward ICAM-1- and ICAM-2-coated beads showed that engagement of LFA-1 alone is sufficient to initiate activation signals in NK cells. Thus, in contrast to T cells, in which even adhesion through LFA-1 is dependent on signals from other receptors, NK cells receive early activation signals directly through LFA-1.