Ricardo Chaurio

Molecular and Translational Classifications of DAMPs in Immunogenic Cell Death

The immunogenicity of malignant cells has recently been acknowledged as a critical determinant of efficacy in cancer therapy. Thus, besides developing direct immunostimulatory regimens, including dendritic cell-based vaccines, checkpoint-blocking therapies, and adoptive T-cell transfer, researchers have started to focus on the overall immunobiology of neoplastic cells. It is now clear that cancer cells can succumb to some anticancer therapies by undergoing a peculiar form of cell death that is characterized by an increased immunogenic potential, owing to the emission of the so-called “damage-associated molecular patterns” (DAMPs). The emission of DAMPs and other immunostimulatory factors by cells succumbing to immunogenic cell death (ICD) favors the establishment of a productive interface with the immune system. This results in the elicitation of tumor-targeting immune responses associated with the elimination of residual, treatment-resistant cancer cells, as well as with the establishment of immunological memory. Although ICD has been characterized with increased precision since its discovery, several questions remain to be addressed. Here, we summarize and tabulate the main molecular, immunological, preclinical, and clinical aspects of ICD, in an attempt to capture the essence of this phenomenon, and identify future challenges for this rapidly expanding field of investigation.

Phospholipids: Key Players in Apoptosis and Immune Regulation

Phosphatidylserine (PS), a phospholipid predominantly found in the inner leaflet of eukaryotic cellular membranes, plays important roles in many biological processes. During apoptosis, the asymmetric distribution of phospholipids of the plasma membrane gets lost and PS is translocated to the outer leaflet of the plasma membrane. There, PS acts as one major “eat me” signal that ensures efficient recognition and uptake of apoptotic cells by phagocytes. PS recognition of activated phagocytes induces the secretion of anti-inflammatory cytokines like interleukin-10 and transforming grow factor-beta. Deficiencies in the clearance of apoptotic cells result in the occurrence of secondarily necrotic cells. The latter have lost the membrane integrity and release immune activating danger signals, which may induce inflammatory responses. Accumulation of dead cells containing nuclear autoantigens in sites of immune selection may provide survival signals for autoreactive B-cells. The production of antibodies against nuclear structures determines the initiation of chronic autoimmunity in systemic lupus erythematosus. Since PS on apoptotic cells is an important modulator of the immune response, natural occurring ligands for PS like annexinA5 have profound effects on immune responses against dead and dying cells, including tumour cells. In this review we will focus on the role of PS exposure in the clearance process of dead cells and its implications in clinical situations where apoptosis plays a relevant role, like in cancer, chronic autoimmunity, and infections. Relevance of other phospholipids during the apoptosis process is also discussed.

Monosodium urate crystals induce extracellular DNA traps in neutrophils, eosinophils, and basophils but not in mononuclear cells

Neutrophil extracellular traps (NETs) are fibers of extracellular DNA released from neutrophils due to overwhelming phagocytic stimuli. The function of NETs is to trap and kill microbes to avoid spreading of potential pathogens. NETs are formed after encounter with various gram-positive and -negative bacteria but also in response to mediators causing sterile inflammation like interleukin-8 (IL-8), tumor necrosis factor (TNF), and phorbol myristate acetate (PMA). Here we show the formation of NETs (NETting) in response to monosodium urate (MSU) crystals as further model for sterile inflammation. We identified monocytes, neutrophils, and eosinophils as MSU phagocytosing cells. Basophils did not take up the crystals, instead they upregulated their activation marker CD203c after contact with MSU. Nevertheless, MSU crystals induced extracellular trap formation also in basophils, like in eosinophils and neutrophils, which phagocytose the crystals. In contrast, monocytes do not form NETs despite uptake of the MSU crystals. In contrast to the canonical stimuli like bacteria and PMA, MSU-induced NETosis was not abrogated by plasma. Our data show that MSU crystals induce extracellular DNA trap formation in all three granulocytes lineages (NETs, EETs, and BETs) but not in monocytes, and DNA externalization does not necessitate the uptake of the crystals.

Biochemical insight into physiological effects of H2S: reaction with peroxynitrite and formation of a new nitric oxide donor, sulfinyl nitrite

The reaction of hydrogen sulfide (H2S) with peroxynitrite (a key mediator in numerous pathological states) was studied in vitro and in different cellular models. The results show that H2S can scavenge peroxynitrite with a corresponding second order rate constant of 3.3 ± 0.4 × 10³ M⁻¹·s⁻¹ at 23°C (8 ± 2 × 10³ M⁻¹·s⁻¹ at 37°C). Activation parameters for the reaction (ΔH‡, ΔS‡ and ΔV‡) revealed that the mechanism is rather associative than multi-step free-radical as expected for other thiols. This is in agreement with a primary formation of a new reaction product characterized by spectral and computational studies as HSNO₂ (thionitrate), predominantly present as sulfinyl nitrite, HS(O)NO. This is the first time a thionitrate has been shown to be generated under biologically relevant conditions. The potential of HS(O)NO to serve as a NO donor in a pH-dependent manner and its ability to release NO inside the cells has been demonstrated. Thus sulfide modulates the chemistry and biological effects of peroxynitrite by its scavenging and formation of a new chemical entity (HSNO₂) with the potential to release NO, suppressing the pro-apoptotic, oxidative and nitrative properties of peroxynitrite. Physiological concentrations of H₂S abrogated peroxynitrite-induced cell damage as demonstrated by the: (i) inhibition of apoptosis and necrosis caused by peroxynitrite; (ii) prevention of protein nitration; and (iii) inhibition of PARP-1 [poly(ADP-ribose) polymerase 1] activation in cellular models, implying that a major part of the cytoprotective effects of hydrogen sulfide may be mediated by modulation of peroxynitrite chemistry, in particular under inflammatory conditions.

The role of dead cell clearance in the etiology and pathogenesis of systemic lupus erythematosus: dendritic cells as potential targets

Overwhelming apoptosis combined with a deficiency in clearing apoptotic cells is thought to be an important etiopathogenic event in the autoimmune disease systemic lupus erythematosus (SLE). Lazy macrophages, complement or DNase I deficiency as well as insufficient natural IgM might be important factors leading to such a clearance deficiency. A defective clearance of apoptotic cells leads to the activation and maturation of plasmacytoid and myeloid dendritic cells (DCs) by material derived from secondary necrotic cells carrying modified autoantigens. This results in the presentation of autoantigens to autoreactive T and B cells in an immunogenic manner, thereby leading to autoantibody production, chronic inflammation and severe tissue damage. Since DC activation and IFN-α production by plasmacytoid dendritic cells play a critical role in the course of SLE pathogenesis, therapeutic intervention to end this vicious cycle might be a promising approach for treating the disease.

Magnetic Drug Targeting Reduces the Chemotherapeutic Burden on Circulating Leukocytes

Magnetic drug targeting (MDT) improves the integrity of healthy tissues and cells during treatment with cytotoxic drugs. An anticancer drug is bound to superparamagnetic iron oxide nanoparticles (SPION), injected into the vascular supply of the tumor and directed into the tumor by means of an external magnetic field. In this study, we investigated the impact of SPION, mitoxantrone (MTO) and SPIONMTO on cell viability in vitro and the nonspecific uptake of MTO into circulating leukocytes in vivo. MDT was compared with conventional chemotherapy. MTO uptake and the impact on cell viability were assessed by flow cytometry in a Jurkat cell culture. In order to analyze MTO loading of circulating leukocytes in vivo, we treated tumor-bearing rabbits with MDT and conventional chemotherapy. In vitro experiments showed a dose-dependent MTO uptake and reduction in the viability and proliferation of Jurkat cells. MTO and SPIONMTO showed similar cytotoxic activity. Non-loaded SPION did not have any effect on cell viability in the concentrations tested. Compared with systemic administration in vivo, MDT employing SPIONMTO significantly decreased the chemotherapeutic load in circulating leukocytes. We demonstrated that MDT spares the immune system in comparison with conventional chemotherapy.

Colourful death: Six-parameter classification of cell death by flow cytometry—Dead cells tell tales

The response of the immune system against dying and dead cells strongly depends on the cell death phenotype. Beside other forms of cell death, two clearly distinct populations, early apoptotic and secondary necrotic cells, have been shown to induce anti-inflammation/tolerance and inflammation/immune priming, respectively. Cytofluorometry is a powerful technique to detect morphological and phenotypical changes occurring during cell death. Here, we describe a new technique using AnnexinA5, propidiumiodide, DiIC1(5) and Hoechst 33342 to sub-classify populations of apoptotic and/or necrotic cells. The method allows the fast and reliable identification of several different phases and pathways of cell death by analysing the following cell death associated changes in a single tube: cellular granularity and shrinkage, phosphatidylserine exposure, ion selectivity of the plasma membrane, mitochondrial membrane potential, and DNA content. The clear characterisation of cell death is of major importance for instance in immunization studies, in experimental therapeutic settings, and in the exploration of cell-death associated diseases. It also enables the analysis of immunological properties of distinct populations of dying cells and the pathways involved in this process.

IgG opsonized nuclear remnants from dead cells cause systemic inflammation in SLE

Deficiencies in the recognition and engulfment of apoptotic cells have been reported in patients with systemic lupus erythematosus (SLE). If dying cells are not promptly cleared, they undergo secondary necrosis and release nuclear autoantigens. Secondarily necrotic cell-derived material (SNEC) can be generated in vitro employing various methods. SNEC generated by either methods shows similar DNA content, light scatter characteristics, and binding pattern of dead and dying cell ligands and is readily recognized by autoantibodies (AAb) of many patients with SLE. In whole blood, AAb opsonize SNEC and foster its uptake by blood-borne non-professional phagocytes. We observed a significant secretion of the inflammatory cytokines IL-8 and TNF-α by phagocytes which had engulfed different types of opsonized SNEC. Phagocytosis of SNEC and the subsequent production of inflammatory cytokines were strongly influenced by the presence of DNA in this prey, since DNase I treatment reduced both the uptake of SNEC and the induction of IL-8 and TNF-α production. In conclusion, the proinflammatory phagocytosis by circulating phagocytes of IgG-opsonized cellular remnants fosters systemic inflammation in SLE.

UVB-irradiated apoptotic cells induce accelerated growth of co-implanted viable tumor cells in immune competent mice

The presence of a solid tumor is the result of a complex balance between rejection, tolerance and regeneration in which the interactions of tumor cells with cells of the host immune system contribute strongly to the final outcome. Here we report on a model where lethally UVB-irradiated cells cause accelerated growth of viable tumor cells in vitro and in allogeneic immune competent mice. UVB-irradiated tumor cells alone did not form tumors and failed to induce tolerance for a second challenge with the same allogeneic tumor. Our data show an important role for dying cells in promoting accelerated tumor cell growth of a small number of viable tumor cells in a large inoculum of UVB-irradiated tumor cells. This occurs when viable and dying/dead tumor cells are in close proximity, suggesting that mobile factors contribute to growth promotion. The anti-inflammatory and growth promoting properties of apoptotic cells are based on several independent effects. UVB-irradiated apoptotic cells directly release a growth promoting activity and clearance by macrophages of apoptotic cells is accompanied by the secretion of IL10, TGFß, and PGE2. Growth promotion is even observed with dying heterologous cells implying a conserved mechanism. Future experiments should focus on the effects of dying tumor cells generated in vivo on the outgrowth of surviving tumor cells which is prone to have implications for cancer therapy.

Navigation to the Graveyard-Induction of Various Pathways of Necrosis and Their Classification by Flow Cytometry

Apoptosis and necrosis reflect the program of cell death employed by a dying cell and the final stage of death, respectively. Whereas apoptosis is defined as a physiological, highly organized cell death process, necrosis is commonly considered to be accidental and uncontrolled. Physiological and weak pathological death stimuli preferentially induce apoptosis, while harsh non-physiological insults often immediately instigate (primary) necrosis. If an apoptosing cell transits into a phase of plasma membrane disintegration, this stage of death is referred to as secondary or post-apoptotic necrosis.Here, we present several conditions that stimulate primary and/or secondary necrosis and show that necrosis displays considerably different time courses. For subclassification of necrotic phenotypes we employed a flow cytometric single-tube 4-color staining technique including annexin A5-FITC, propidium iodide, DiIC1(5), and Hoechst 33342.