Antibody targeting of phosphatidylserine for the detection and immunotherapy of cancer

Phosphatidylserine and Tyro3-Axl-Mertk Receptor Tyrosine Kinase level detection in plasma and on plasma-derived extracellular vesicle surface in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is a chronic lymphoproliferative disorder characterized by monoclonal B cell proliferation. Studies carried out in recent years suggest that extracellular vesicles (EVs) may be a potential biomarker in cancer. Tyro3-Axl-Mertk (TAM) Receptor Tyrosine Kinases (RTKs) and Phosphatidylserine (PS) have crucial roles in macrophage-mediated immune response under normal conditions. In the tumor microenvironment, these molecules contribute to immunosuppressive signals and prevent the formation of local and systemic antitumor immune responses. Based on this, we aimed to evaluate the amount of PS and TAM RTK in plasma and on the surface of EVs in CLL patients and healthy volunteers in this study. In this study, 25 CLL (11 F/14 M) patients in the Rai (O-I) stage, newly diagnosed or followed up without treatment, and 15 healthy volunteers (11 F/4 M) as a control group were included. For all samples, PS and TAM RTK levels were examined first in the plasma and then in the EVs obtained from the plasma. We detected a significant decrease in plasma PS, and TAM RTK levels in CLL patients compared to the control. Besides, we determined a significant increase in TAM RTK levels on the EV surface in CLL, except for PS. In conclusion, these receptor levels measured by ELISA in plasma may not be effective for the preliminary detection of CLL. However, especially TAM RTKs on the surface of EVs may be good biomarkers and potential targets for CLL therapies.

Discovery of lipid profiles in plasma-derived extracellular vesicles as biomarkers for breast cancer diagnosis

Lipids are a major component of extracellular vesicles; however, their significance in tumorigenesis and progression has not been well elucidated. As we previously found that lipid profiles drastically changed in breast tumors upon progression, we hypothesized that lipid profiles of plasma-derived extracellular vesicles could be utilized as breast cancer biomarkers. Here, we adopted modified sucrose cushion ultracentrifugation to isolate plasma-derived extracellular vesicles from breast cancer (n = 105), benign (n = 11), and healthy individuals (n = 43) in two independent cohorts (n = 126 and n = 33) and conducted targeted lipidomic analysis. We established a breast cancer diagnostic model comprising three lipids that showed favorable performance with the area under the receiver operating characteristic curve of 0.759, 0.743, and 0.804 in the training, internal validation, and external test sets, respectively. Moreover, we identified several lipids that could effectively discriminate breast cancer progression and subtypes: phosphatidylethanolamines and phosphatidylserines were relatively higher in Stage III, whereas phosphatidylcholines and sphingomyelins were higher in Stage IV; phosphatidylcholines and ceramides were correspondingly concentrated in HER2-positive patients, while lysophosphatidylcholines and polyunsaturated triglycerides were concentrated in the triple-negative breast cancer subtype. Lipid profiling of plasma-derived extracellular vesicles is a non-invasive and promising approach for diagnosing, staging, and subtyping breast cancer.

Gla-domain mediated targeting of externalized phosphatidylserine for intracellular delivery

Phosphatidylserine (PS) is a negatively charged phospholipid normally localized to the inner leaflet of the plasma membrane of cells but is externalized onto the cell surface during apoptosis as well as in malignant and infected cells. Consequently, PS may comprise an important molecular target in diagnostics, imaging, and targeted delivery of therapeutic agents. While an array of PS-binding molecules exist, their utility has been limited by their inability to internalize diagnostic or therapeutic payloads. We describe the generation, isolation, characterization, and utility of a PS-binding motif comprised of a carboxylated glutamic acid (GLA) residue domain that both recognizes and binds cell surface-exposed PS, and then unlike other PS-binding molecules is internalized into these cells. Internalization is independent of the traditional endosomal-lysosomal pathway, directly entering the cytosol of the target cell rapidly. We demonstrate that this PS recognition extends to stem cells and that GLA-domain-conjugated probes can be detected upon intravenous administration in animal models of infectious disease and cancer. GLA domain binding and internalization offer new opportunities for specifically targeting cells with surface-exposed PS for imaging and delivery of therapeutics.

Design, Synthesis and Anti-Melanoma Activity of Novel Annexin V Derivative with β3-Integrin Affinity

Tumor tissues often exhibit unique integrin receptor presentation during development, such as high exposures of α_vβ₃ and α_IIbβ₃ integrins. These features are not present in normal tissues. The induction of selective thrombosis and infarction in the tumor-feeding vessels, as well as specific antagonism of α_vβ₃ integrin on the surface of tumor endothelial cells, is a potential novel antitumor strategy. The Echistatin-Annexin V (EAV) fusion protein is a novel Annexin V (ANV) derivative that possesses a high degree of α_vβ₃ and α_IIbβ₃ integrin receptor recognition and binding characteristics while retaining the specific binding ability of the natural ANV molecule for phosphatidylserine (PS). We systematically investigated the biological effects of this novel molecule with superimposed functions on mouse melanoma. We found that EAV inhibited the viability and migration of B16F10 murine melanoma cells in a dose-dependent manner, exhibited good tumor suppressive effects in a xenograft mouse melanoma model, strongly induced tumor tissue necrosis in mice, and targeted the inhibition of angiogenesis in mouse melanoma tumor tissue. EAV exhibited stronger biological effects than natural ANV molecules in inhibiting melanoma in mice. The unique biological effects of EAV are based on its high β₃-type integrin receptor-specific recognition and binding ability, as well as its highly selective binding to PS molecules. Based on these findings, we propose that EAV-mediated tumor suppression is a novel and promising antitumor strategy that targets both PS- and integrin β₃-positive tumor neovascularization and the tumor cells themselves, thus providing a possible mechanism for the treatment of melanoma.

Mobilizing phospholipids on tumor plasma membrane implicates phosphatidylserine externalization blockade for cancer immunotherapy

In "healthy" tumor cells, phosphatidylserine (PS) is predominately localized in the inner plasma membrane leaflet. During apoptosis, PS relocates to the outer leaflet. Herein, we established PSout tumor models with tumor cells lacking PS flippase component CDC50A, constantly exposing PS but alive. PSout tumors developed bigger than wild-type (WT) tumors, featuring M2 polarized tumor-associated macrophages (TAMs) and fewer tumor-antigen-specific T cells. The PS receptor TIM-3 is responsible for PS recognition. Employing an opposite tumor model, PSin, with tumor cells lacking the PS scramblase Xkr8 and unable to expose PS during otherwise normal apoptosis, we find that the accumulated apoptotic tumor cells produce and release cyclic GAMP (cGAMP) to immune cells to activate the STING pathway, leading to TAM M1 polarization, suppressed interleukin (IL)-10 secretion, and natural killer (NK) cell cytotoxicity. Silencing Xkr8 in vivo by either short hairpin RNA (shRNA) or small interfering RNA (siRNA) to achieve a PS externalization blockade provides robust therapeutic anti-tumor efficiency.

Lipidomic and Membrane Mechanical Signatures in Triple-Negative Breast Cancer: Scope for Membrane-Based Theranostics

Triple-negative breast cancer (TNBC) is a highly aggressive form of breast cancer associated with poor prognosis, higher grade, and a high rate of metastatic occurrence. Limited therapeutic interventions and the compounding issue of drug resistance in triple-negative breast cancer warrants the discovery of novel therapeutic targets and diagnostic modules. To this view, in addition to proteins, lipids also regulate cellular functions via the formation of membranes that modulate membrane protein function, diffusion, and their localization; thus, orchestrating signaling hot spots enriched in specific lipids/proteins on cell membranes. Lipid deregulation in cancer leads to reprogramming of the membrane dynamics and functions impacting cell proliferation, metabolism, and metastasis, providing exciting starting points for developing lipid-based approaches for treating TNBC. In this review, we provide a detailed account of specific lipidic changes in breast cancer, link the altered lipidome with membrane structure and mechanical properties, and describe how these are linked to subsequent downstream functions implicit in cancer progression, metastasis, and chemoresistance. At the fundamental level, we discuss how the lipid-centric findings in TNBC are providing cues for developing lipid-inspired theranostic strategies while bridging existing gaps in our understanding of the functional involvement of lipid membranes in cancer.

Bilayer Forming Phospholipids as Targets for Cancer Therapy

Phospholipids represent a crucial component for the structure of cell membranes. Phosphatidylcholine and phosphatidylethanolamine are two phospholipids that comprise the majority of cell membranes. De novo biosynthesis of phosphatidylcholine and phosphatidylethanolamine occurs via the Kennedy pathway, and perturbations in the regulation of this pathway are linked to a variety of human diseases, including cancer. Altered phosphatidylcholine and phosphatidylethanolamine membrane content, phospholipid metabolite levels, and fatty acid profiles are frequently identified as hallmarks of cancer development and progression. This review summarizes the research on how phospholipid metabolism changes over oncogenic transformation, and how phospholipid profiling can differentiate between human cancer and healthy tissues, with a focus on colorectal cancer, breast cancer, and non-small cell lung cancer. The potential for phospholipids to serve as biomarkers for diagnostics, or as anticancer therapy targets, is also discussed.

Importance of Efferocytosis in COVID-19 Mortality

COVID-19 is a generally benign coronavirus disease that can spread rapidly, except for those with a group of risk factors. Since the pathogenesis responsible for the severity of the disease has not been clearly revealed, effective treatment alternatives has not been developed. The hallmark of the SARS-CoV-2-infected cells is apoptosis. Apoptotic cells are cleared through a sterile process defined as efferocytosis by professional and nonprofessional phagocytic cells. The disease would be rapidly brought under control in the organism that can achieve effective efferocytosis, which is also a kind of innate immune response. In the risk group, the efferocytic process is defective. With the addition of the apoptotic cell load associated with SARS-COV-2 infection, failure to achieve efferocytosis of dying cells can initiate secondary necrosis, which is a highly destructive process. Uncontrolled inflammation and coagulation abnormalities caused by secondary necrosis reason in various organ failures, lung in particular, which are responsible for the poor prognosis. Following the short and simplified information, this opinion paper aims to present possible treatment options that can control the severity of COVID-19 by detailing the mechanisms that can cause defective efferocytosis.

Characterization and modulation of surface charges to enhance extracellular vesicle isolation in plasma

Extracellular vesicles (EVs) carry information inherited from parental cells, having significant potential for disease diagnosis. In blood, however, EVs are outnumbered >104-fold by low density lipoproteins (LDLs), yet similar in size and density. These fundamental disadvantages often cause LDL spillover into EV isolates, thus confounding assay results. We hypothesized that EVs can be further separated from LDLs based on electric charge: EVs and LDLs have different lipid composition, which can lead to differential surface charge densities. To test this hypothesis, we modeled and quantified the surface charge of EVs and LDLs, and used the information to optimally separate EVs from LDLs via ion-exchange chromatography. Methods: We built an enhanced dual-mode chromatography (eDMC) device which performed i) size-exclusion to remove particles smaller than EVs and LDLs and ii) cation-exchange in an acidic elution to retain LDLs longer than EVs. The performance of the eDMC, in comparison to size-exclusion only, was evaluated by analyzing the yield and purity of the isolated EVs. Results: By measuring and modeling zeta potentials at different buffer pH, we estimated surface charge densities of EVs (-6.2 mC/m2) and LDLs (-3.6 mC/m2), revealing that EVs are more negatively charged than LDLs. Furthermore, the charge difference between EVs and LDLs was maximal at a weak acidic condition (pH = 6.4). By applying these findings, we optimized eDMC operation to enrich EVs directly from plasma, depleting >99.8% of LPPs within 30 min. Minimizing LDL contamination improved analytical signals in EV molecular assays, including single vesicle imaging, bulk protein measurements, and mRNA detection. Conclusions: These developments will promote the translational value of the dual-mode separation - a fast, equipment-free, and non-biased way for EV isolation from plasma samples.

Blocking antibody-mediated phosphatidylserine enhances cancer immunotherapy

Cancer immunotherapy is a major breakthrough in tumor therapy and has been used in monotherapy or combination therapy. However, it has been associated with poor immune tolerance in some patients or immune-related adverse events. Therefore, ideal and reliable tumor elimination strategies are urgently needed to overcome these shortcomings. Phosphatidylserine (PS) is a negatively charged phospholipid, usually present in the inner lobules of eukaryotic cell membranes. Under certain physiological or pathological conditions, PS may be exposed on the outer leaflets of apoptotic cells serving as recognition signals by phagocytes and modulating the immune response. On the contrary, increased exposure of PS in the tumor microenvironment can significantly antagonize the body's anti-tumor immunity, thereby promoting tumor growth and metastasis. During radiotherapy and chemotherapy, PS-mediated immunosuppression increases the PS levels in necrotic tissue in the tumor microenvironment, further suppressing tumor immunity. PS-targeted therapy is a promising strategy in cancer immunotherapy. It inhibits tumor growth and improves the anti-tumor activity of immune checkpoint inhibitors. A comprehensive understanding of the mechanism of PS-targeted therapy opens up a new perspective for future cancer immunotherapies.

Mistiming Death: Modeling the Time-Domain Variability of Tumor Apoptosis and Implications for Molecular Imaging of Cell Death

PURPOSE

Apoptosis, in the context of cancer, is a form of programmed cell death induced by chemotherapy, radiotherapy, and immunotherapy. As this is a central pathway in treatment response, considerable effort has been expended on the development of molecular imaging agents to non-invasively measure tumor apoptosis prior to quantitative changes in tumor dimensions. Despite these efforts, clinical trials directed at imaging apoptosis by PET, SPECT, and MRI have failed to robustly predict response to treatment with high sensitivity and specificity. Although these shortcomings may be linked to probe design, we propose that the combination of variability in the timing of maximal in vivo tumor apoptosis and sub-optimal sampling times fundamentally limits the predictive power of PET/SPECT apoptosis imaging.

PROCEDURES

Herein, we surveyed the literature describing the time course of therapy-induced tumor apoptosis in vivo and used these data to construct a mathematical model describing the onset, duration, amplitude, and variability of the apoptotic response. Uncertainty in the underlying time of initiation of tumor apoptosis was simulated by Gaussian, uniform, and Landau distributions centered at the median time-to-maximum apoptotic rate derived from the literature. We then computationally sampled these models for various durations to simulate PET/SPECT imaging agents with variable effective half-lives.

RESULTS

Models with a narrow Gaussian distribution of initiation times for tumor apoptosis predicted high contrast ratios and strong predictive values for all effective tracer half-lives. However, when uncertainty in apoptosis initiation times were simulated with uniform and Landau distributions, high contrast ratios and predictive values were only obtained with extremely long imaging windows (days). The imaging contrast ratios predicted in these models were consistent with those seen in pre-clinical apoptosis PET/SPECT imaging studies and suggest that uncertainty in the timing of tumor cell death plays a significant role in the maximal contrast obtainable. Moreover, when uncertainty in both apoptosis initiation and imaging start times were simulated, the predicted contrast ratios were dramatically reduced for all tracer half-lives.

CONCLUSIONS

These studies illustrate the effect of uncertainty of apoptosis initiation on the predictive power of PET/SPECT apoptosis imaging agents and suggest that long integration times are required to surmount uncertainty in the time domain of this biological process.

The Evolution of Safe and Effective Coaguligands for Vascular Targeting and Precision Thrombosis of Solid Tumors and Vascular Malformations

In cardiovascular and cerebrovascular biology, control of thrombosis and the coagulation cascade in ischemic stroke, myocardial infarction, and other coagulopathies is the focus of significant research around the world. Ischemic stroke remains one of the largest causes of death and disability in developed countries. Preventing thrombosis and protecting vessel patency is the primary goal. However, utilization of the body’s natural coagulation cascades as an approach for targeted destruction of abnormal, disease-associated vessels and tissues has been increasing over the last 30 years. This vascular targeting approach, often termed “vascular infarction”, describes the deliberate, targeted delivery of a thrombogenic effector to diseased blood vessels with the aim to induce localized activation of the coagulation cascade and stable thrombus formation, leading to vessel occlusion and ablation. As systemic delivery of pro-thrombotic agents may cause consternation amongst traditional stroke researchers, proponents of the approach must suitably establish both efficacy and safety to take this field forward. In this review, we describe the evolution of this field and, with a focus on thrombogenic effectors, summarize the current literature with respect to emerging trends in “coaguligand” development, in targeted tumor vessel destruction, and in expansion of the approach to the treatment of brain vascular malformations.

Phosphatidylserine-Targeting Monoclonal Antibodies Exhibit Distinct Biochemical and Cellular Effects on Anti-CD3/CD28-Stimulated T Cell IFN-γ and TNF-α Production

Phosphatidylserine (PS)-targeting monoclonal Abs (mAbs) that directly target PS and target PS via β2-gp1 (β2GP1) have been in preclinical and clinical development for over 10 y for the treatment of infectious diseases and cancer. Although the intended targets of PS-binding mAbs have traditionally included pathogens as well as stressed tumor cells and its associated vasculature in oncology, the effects of PS-targeting mAbs on activated immune cells, notably T cells, which externalize PS upon Ag stimulation, is not well understood. Using human T cells from healthy donor PBMCs activated with an anti-CD3 + anti-CD28 Ab mixture (anti-CD3/CD28) as a model for TCR-mediated PS externalization and T cell stimulation, we investigated effects of two different PS-targeting mAbs, 11.31 and bavituximab (Bavi), on TCR activation and TCR-mediated cytokine production in an ex vivo paradigm. Although 11.31 and Bavi bind selectivity to anti-CD3/28 activated T cells in a PS-dependent manner, surprisingly, they display distinct functional activities in their effect on IFN-γ and TNF-ɑ production, whereby 11.31, but not Bavi, suppressed cytokine production. This inhibitory effect on anti-CD3/28 activated T cells was observed on both CD4⁺ and CD8⁺ cells and independently of monocytes, suggesting the effects of 11.31 were directly mediated by binding to externalized PS on activated T cells. Imaging showed 11.31 and Bavi bind at distinct focal depots on the cell membrane. Collectively, our findings indicate that PS-targeting mAb 11.31 suppresses cytokine production by anti-CD3/28 activated T cells.

Combination of a T cell activating therapy and anti-phosphatidylserine enhances anti-tumour immune responses in a HPV16 E7-expressing C3 tumour model

DPX is a novel delivery platform that generates targeted CD8 + T cells and drives antigen-specific cytotoxic T cells into tumours. Cancer cells upregulate phosphatidylserine (PS) on the cell surface as a mechanism to induce an immunosuppressive microenvironment. Development of anti-PS targeting antibodies have highlighted the ability of a PS-blockade to enhance tumour control by T cells by releasing immunosuppression. Here, C57BL/6 mice were implanted with HPV16 E7 target-expressing C3 tumours and subjected to low dose intermittent cyclophosphamide (CPA) in combination with DPX-R9F treatment targeting an E7 antigen with and without anti-PS and/or anti-PD-1 targeting antibodies. Immune responses were assessed via IFN-γ ELISPOT assay and the tumour microenvironment was further analyzed using RT-qPCR. We show that the combination of DPX-R9F and PS-targeting antibodies with and without anti-PD-1 demonstrated increased efficacy compared to untreated controls. All treatments containing DPX-R9F led to T cell activation as assessed by IFN-γ ELISPOT. Furthermore, DPX-R9F/anti-PS treatment significantly elevated cytotoxic T cells, macrophages and dendritic cells based on RT-qPCR analysis. Overall, our data indicates that anti-tumour responses are driven through a variety of immune cells within this model and highlights the need to investigate combination therapies which increase tumour immune infiltration, such as anti-phosphotidylserine.

Targeting Phosphatidylserine Enhances the Anti-tumor Response to Tumor-Directed Radiation Therapy in a Preclinical Model of Melanoma

Phosphatidylserine (PS) is exposed on the surface of apoptotic cells and is known to promote immunosuppressive signals in the tumor microenvironment (TME). Antibodies that block PS interaction with its receptors have been shown to repolarize the TME into a proinflammatory state. Radiation therapy (RT) is an effective focal treatment of isolated solid tumors but is less effective at controlling metastatic cancers. We found that tumor-directed RT caused an increase in expression of PS on the surface of viable immune infiltrates in mouse B16 melanoma. We hypothesize that PS expression on immune cells may provide negative feedback to immune cells in the TME. Treatment with an antibody that targets PS (mch1N11) enhanced the anti-tumor efficacy of tumor-directed RT and improved overall survival. This combination led to an increase in proinflammatory tumor-associated macrophages. The addition of anti-PD-1 to RT and mch1N11 led to even greater anti-tumor efficacy and overall survival. We found increased PS expression on several immune subsets in the blood of patients with metastatic melanoma after receiving tumor-directed RT. These findings highlight the potential of combining PS targeting with RT and PD-1 pathway blockade to improve outcomes in patients with advanced-stage cancers.

Budhu et al. show that tumor-directed irradiation of murine B16 melanoma causes an increase in PS on the surface of infiltrating immune cells. Blocking PS and RT improves the anti-tumor efficacy and overall survival, which can be further improved with the addition of anti-PD-1. Melanoma patients exhibit increased PS on their PBMCs after RT.

Advances in understanding and in multi-disciplinary methodology used to assess lipid regulation of signalling cascades from the cancer cell plasma membrane

The lipid bilayer is a functional component of cells, forming a stable platform for the initiation of key biological processes, including cell signalling. There are distinct changes in the lipid composition of cell membranes during oncogenic transformation resulting in aberrant activation and inactivation of signalling transduction pathways. Studying the role of the cell membrane in cell signalling is challenging, since techniques are often limited to by timescale, resolution, sensitivity, and averaging. To overcome these limitations, combining 'computational', 'wet-lab' and 'semi-dry' approaches offers the best opportunity to resolving complex biological processes involved in membrane organisation. In this review, we highlight analytical tools that have been applied for the study of cell signalling initiation from the cancer cell membranes through computational microscopy, biological assays, and membrane biophysics. The cancer therapeutic potential of extracellular membrane-modulating agents, such as cholesterol-reducing agents is also discussed, as is the need for future collaborative inter-disciplinary research for studying the role of the cell membrane and its components in cancer therapy.

BPRDP056, a novel small molecule drug conjugate specifically targeting phosphatidylserine for cancer therapy

Zinc(II)-dipicolylamine (Zn-DPA) has been shown to specifically identify and bind to phosphatidylserine (PS), which exists in bulk in the tumor microenvironment. BPRDP056, a Zn-DPA-SN38 conjugate was designed to provide PS-targeted drug delivery of a cytotoxic SN38 to the tumor microenvironment, thereby allowing a lower dosage of SN38 that induces apoptosis in cancer cells. Micro-Western assay showed that BPRDP056 exhibited apoptotic signal levels similar to those of CPT-11 in the treated tumors growing in mice. Pharmacokinetic study showed that BPRDP056 has excellent systemic stability in circulation in mice and rats. BPRDP056 is accumulated in tumors and thus increases the cytotoxic effects of SN38. The in vivo antitumor activities of BPRDP056 have been shown to be significant in subcutaneous pancreas, prostate, colon, liver, breast, and glioblastoma tumors, included an orthotopic pancreatic tumor, in mice. BPRDP056 shrunk tumors at a lower (~20% only) dosing intensity of SN38 compared to that of SN38 conjugated in CPT-11 in all tumor models tested. A wide spectrum of antitumor activities is expected to treat all cancer types of PS-rich tumor microenvironments. BPRDP056 is a first-in-class small molecule drug conjugate for cancer therapy.

Lipid composition of the cancer cell membrane

Cancer cell possesses numerous adaptations to resist the immune system response and chemotherapy. One of the most significant properties of the neoplastic cells is the altered lipid metabolism, and consequently, the abnormal cell membrane composition. Like in the case of phosphatidylcholine, these changes result in the modulation of certain enzymes and accumulation of energetic material, which could be used for a higher proliferation rate. The changes are so prominent, that some lipids, such as phosphatidylserines, could even be considered as the cancer biomarkers. Additionally, some changes of biophysical properties of cell membranes lead to the higher resistance to chemotherapy, and finally to the disturbances in signalling pathways. Namely, the increased levels of certain lipids, like for instance phosphatidylserine, lead to the attenuation of the immune system response. Also, changes in lipid saturation prevent the cells from demanding conditions of the microenvironment. Particularly interesting is the significance of cell membrane cholesterol content in the modulation of metastasis. This review paper discusses the roles of each lipid type in cancer physiology. The review combined theoretical data with clinical studies to show novel therapeutic options concerning the modulation of cell membranes in oncology.

Cell Death in the Tumor Microenvironment: Implications for Cancer Immunotherapy

The physiological fate of cells that die by apoptosis is their prompt and efficient removal by efferocytosis. During these processes, apoptotic cells release intracellular constituents that include purine nucleotides, lysophosphatidylcholine (LPC), and Sphingosine-1-phosphate (S1P) that induce migration and chemo-attraction of phagocytes as well as mitogens and extracellular membrane-bound vesicles that contribute to apoptosis-induced compensatory proliferation and alteration of the extracellular matrix and the vascular network. Additionally, during efferocytosis, phagocytic cells produce a number of anti-inflammatory and resolving factors, and, together with apoptotic cells, efferocytic events have a homeostatic function that regulates tissue repair. These homeostatic functions are dysregulated in cancers, where, aforementioned events, if not properly controlled, can lead to cancer progression and immune escape. Here, we summarize evidence that apoptosis and efferocytosis are exploited in cancer, as well as discuss current translation and clinical efforts to harness signals from dying cells into therapeutic strategies.

Biological Function and Immunotherapy Utilizing Phosphatidylserine-based Nanoparticles

Phosphatidylserine (PS) is a naturally occurring anionic phospholipid that is primarily located in the inner leaflet of eukaryotic cell membranes. The role of PS during apoptosis is one of the most studied biological functions of PS. Externalization of PS during apoptosis mediates an "eat me" signal for phagocytic uptake, leading to clearance of apoptotic cells and thus maintain self-tolerance by immunological ignorance. However, an emerging view is that PS exposure-mediated cellular uptake is not an immunologically silent event, but rather promoting an active tolerance towards self and foreign proteins. This biological property of PS has been exploited by parasites and viruses in order to evade immune surveillance of the host immune system. Further, this novel immune regulatory property of PS that results in tolerance induction can be harnessed for clinical applications, such as to treat autoimmune conditions and to reduce immunogenicity of therapeutic proteins. This review attempts to provide an overview of the biological functions of PS in the immune response and its potential therapeutic applications.

TIMs, TAMs, and PS- antibody targeting: implications for cancer immunotherapy

Immunotherapy for cancer is making impressive strides at improving survival of a subset of cancer patients. To increase the breadth of patients that benefit from immunotherapy, new strategies that combat the immunosuppressive microenvironment of tumors are needed. Phosphatidylserine (PS) signaling is exploited by tumors to enhance tumor immune evasion and thus strategies to inhibit PS-mediated immune suppression have potential to increase the efficacy of immunotherapy. PS is a membrane lipid that flips to the outer surface of the cell membrane during apoptosis and/or cell stress. Externalized PS can drive efferocytosis or engage PS receptors (PSRs) to promote local immune suppression. In the tumor microenvironment (TME) PS-mediated immune suppression is often termed apoptotic mimicry. Monoclonal antibodies (mAbs) targeting PS or PSRs have been developed and are in preclinical and clinical testing. The TIM (T-cell/transmembrane, immunoglobulin, and mucin) and TAM (Tyro3, AXL, and MerTK) family of receptors are PSRs that have been shown to drive PS-mediated immune suppression in tumors. This review will highlight the development of mAbs targeting PS, TIM-3 and the TAM receptors.

Occlusion of Animal Model Arteriovenous Malformations Using Vascular Targeting

Brain arteriovenous malformations (AVMs) are a significant cause of intracerebral hemorrhage in children and young adults. Currently, one third of patients have no viable treatment options. Vascular targeting agents (VTAs) are being designed to deliver pro-thrombotic molecules to the abnormal AVM vessels for rapid occlusion and cure. This study assessed the efficacy of a pro-thrombotic VTA targeting phosphatidylserine (PS) in a radiation-primed AVM animal model. The model AVM was surgically created in rats by anastomosis of the left external jugular vein to the adjacent common carotid artery. After 6 weeks, the AVM was irradiated (20 Gy) using gamma knife surgery (GKS). A PS-targeting VTA was created by conjugation of annexin V with human thrombin and administered intravenously 3 weeks post-GKS or sham. Unconjugated thrombin was used as a non-targeting control. AVM thrombosis and occlusion was monitored 3 weeks later by angiography and histology. Preliminary experiments established a safe dose of active thrombin for systemic administration. Subsequently, a single dose of annexin V-thrombin conjugate (0.77 mg/kg) resulted in angiographic AVM occlusion in sham (75%) and irradiated (63%) animals, while non-targeted thrombin did not. Lowering the conjugate dose (0.38 mg/kg) decreased angiographic AVM occlusion in sham (13%) relative to irradiated (80%) animals (p = 0.03) as did delivery of two consecutive doses of 0.38 mg/kg, 2 days apart (sham (0%); irradiated (78%); p = 0.003). These findings demonstrate efficacy of the PS-targeting VTA and the feasibility of a vascular targeting approach for occlusion of high-flow AVMs. Targeting specificity can be enhanced by radiation-sensitization and VTA dose modification.

Synthesis, characterization, anti-proliferative properties and DNA binding of benzochromene derivatives: Increased Bax/Bcl-2 ratio and caspase-dependent apoptosis in colorectal cancer cell line

3-Amino-1-aryl-1H-benzo[f]chromene-2-carbonitrile derivatives were synthesized from three-component reaction of arylaldehyde, malononitrile and 2-naphthol in the presence of 1, 4-bis(4-ferrocenylbutyl)piperazine as a new catalyst. Cytotoxic potencies of the compounds were tested on HT-29 cells. 3-Amino-1-(4-fluorophenyl)-1H-benzo[f]chromene-2-carbonitrile (4c) was more active among these compounds and was selected for further studies. Apoptosis was investigated by acridine orange/ethidium bromide (AO/EtBr) double staining and flow cytometry. The qRT-PCR was used to analyze the expression of pro- and anti-apoptotic genes. The binding attributes of 4c with calf thymus DNA (ctDNA) was examined using multi-spectroscopic measurements. We found that 4c had potent cytotoxic activity against HT-29 cells with an IC₅₀ value of 60 µM through induction of cell cycle arrest in the sub-G1 phase and apoptosis. RT-PCR analysis demonstrated down-regulation of Bcl-2 expression, while the expression of Bax, caspase-3, -8 and -9 genes was up-regulated in HT-29 cells incubated with 4c compared with control cells. These studies revealed that 4c interacts with DNA through groove binding mode with the intrinsic binding constant (K_b) of 3 × 10² M^-1. Thus, 4c is a valuable candidate for further evaluation as a new series of potent chemotherapeutic family in colon cancer treatment.

Apoptotic cell clearance in the tumor microenvironment: a potential cancer therapeutic target

Millions of cells in the human body undergo apoptosis not only under normal physiological conditions but also under pathological conditions such as infection or other diseases related to acute tissue injury. Swift apoptotic cell clearance is essential for tissue homeostasis. Defective clearance of dead cells is linked to pathogenesis of diseases such as inflammatory diseases, atherosclerosis, neurological disease, and cancer. Significance of apoptotic cell clearance has been emerging as an interesting field for disease treatment. Efficient apoptotic cell clearance plays an important role in reducing inflammation through the suppression of inappropriate inflammatory responses under healthy and diseased conditions. However, apoptotic cell clearance related to cancer pathogenesis is more complex in tumor microenvironments. Chronic inflammation resulting from the failure of apoptotic cell clearance can contribute to tumor progression. Conversely, tumor cells can exploit the anti-inflammatory effect of apoptotic cell clearance to generate an immunosuppressive tumor microenvironment. In this review, focus is on the current understanding of apoptotic cell clearance in the tumor microenvironment. Furthermore, we discuss how signaling molecules (PtdSer and PtdSer recognition receptor) mediating apoptotic cell clearance are aberrantly expressed in the tumor microenvironment and their current development state as potential therapeutic targets for clinical cancer therapy.

Phosphatidylserine receptor-targeting therapies for the treatment of cancer

Asymmetric distribution of phospholipids across the plasma membrane is a unique characteristic of eukaryotic cells. Phosphatidylcholine and sphingomyelin are exposed in the outer leaflet, and phosphatidylserine (PS) is predominantly located in the inner leaflet. Redistribution of PS to the cell surface can be observed in several physiological conditions, such as apoptosis and platelet activation, or in pathological conditions, such as the release of microvesicles/exosomes from tumor tissues. PS binding to the phosphatidylserine receptor (PSR) on immune cells initiates immunosuppressive pathways that can lead to immune evasion by cancer cells. Conversely, PSR activation of cancer cells plays an important role in their survival, proliferation and metastasis. Herein, we briefly summarize both recent advances in our understanding of the pathological roles of PS and its receptor in cancer biology, as well as relevant pharmacological approaches.

Subtypes of tumour cell-derived small extracellular vesicles having differently externalized phosphatidylserine

Phosphatidylserine (PS) has skewed distributions in the plasma membrane and is preferentially located in the inner leaflet of normal cells. Tumour cells, however, expose PS at the outer leaflet of cell surfaces, thereby potentially modulating the bio-signalling of cells. Interestingly, exosomes - or, more properly, small extracellular vesicles (sEVs) - which are secreted from tumour cells, are enriched with externalized PS, have been proposed as being involved in the progression of cancers, and could be used as a marker for tumour diagnostics. However, the sEV fractions prepared from various methods are composed of different subtypes of vesicles, and knowledge about the subtypes enriched with exposed PS is still limited. Here, we differentiated sEVs from cancer cell lines by density gradient centrifugation and characterized the separated fractions by using gold-labelling of PS in atomic force microscopy, thrombin generation assay, size and zeta potential measurements, and western blot analysis. These analyses revealed a previously unreported PS⁺-enriched sEV subtype, which is characterized by a lower density than that of canonical exosomes (1.06 g/ml vs. 1.08 g/ml), larger size (122 nm vs. 105 nm), more negative zeta potential (-28 mV vs. -21 mV), and lower abundance of canonical exosomal markers. The identification of the PS-exposed subtype of sEVs will provide deeper insight into the role of EVs in tumour biology and enhance the development of EV-based tumour diagnosis and therapy.