Phagocytosis of apoptotic cells: a matter of balance

The hepatotoxicity of hexafluoropropylene oxide trimer acid caused by apoptosis via endoplasmic reticulum-mitochondrial crosstalk

As a ubiquitous pollutant in the environment, hexafluoropropylene oxide trimer acid (HFPO-TA) has been proven to have strong hepatotoxicity. However, the underlying mechanism is still unclear. Consequently, in vivo and in vitro models of HFPO-TA exposure were established to investigate the detrimental effects of HFPO-TA on the liver. In vivo, we discovered that HFPO-TA enhanced endoplasmic reticulum (ER)-mitochondrial association, caused mitochondrial oxidative damage, activated ER stress, and induced apoptosis in mouse livers. In vitro experiments confirmed that IP3R overexpression on ER structure increased mitochondrial calcium levels, which led to mitochondrial damage and mitochondria-dependent apoptosis in HepG2 cells exposed to HFPO-TA. Subsequently, damaged mitochondria released a large amount of mitochondrial ROS, which activated ER stress and ER stress-dependent apoptosis. In conclusion, this study demonstrates that HFPO-TA can induce apoptosis by regulating the crosstalk between ER and mitochondria, ultimately leading to liver damage. These findings reveal the significant hepatotoxicity of HFPO-TA and its potential mechanisms.

PD-1 Blockade During Post-partum Involution Reactivates the Anti-tumor Response and Reduces Lymphatic Vessel Density

Post-partum breast cancer patients, or breast cancer patients diagnosed within 10 years of last childbirth, are ~3-5 times more likely to develop metastasis in comparison to non-post-partum, or nulliparous, patients. Additionally, post-partum patients have increased tumor-associated lymphatic vessels and LN involvement, including when controlled for size of the primary tumor. In pre-clinical, immune-competent, mouse mammary tumor models of post-partum breast cancer (PPBC), tumor growth and lymphogenous tumor cell spread occur more rapidly in post-partum hosts. Here we report on PD-L1 expression by lymphatic endothelial cells and CD11b+ cells in the microenvironment of post-partum tumors, which is accompanied by an increase in PD-1 expression by T cells. Additionally, we observed increases in PD-L1 and PD-1 in whole mammary tissues during post-partum mammary gland involution; a known driver of post-partum tumor growth, invasion, and metastasis in pre-clinical models. Importantly, implantation of murine mammary tumor cells during post-partum mammary gland involution elicits a CD8+ T cell population that expresses both the co-inhibitory receptors PD-1 and Lag-3. However, upon anti-PD-1 treatment, during post-partum mammary gland involution, the involution-initiated promotional effects on tumor growth are reversed and the PD-1, Lag-3 double positive population disappears. Consequently, we observed an expansion of poly-functional CD8+ T cells that produced both IFNγ and TNFα. Finally, lymphatic vessel frequency decreased significantly following anti-PD-1 suggesting that anti-PD-1/PD-L1 targeted therapies may have efficacy in reducing tumor growth and dissemination in post-partum breast cancer patients.

Procaspase-activating compound-1 induces apoptosis in Trypanosoma cruzi

Some therapeutics for parasitic, cardiac and neurological diseases activate apoptosis. Therefore, the study of apoptotic proteins in pathogenic organisms is relevant. However, the molecular mechanism of apoptosis in unicellular organisms remain elusive, despite morphological evidence of its occurrence. In Trypanosoma cruzi, the causative agent of Chagas disease, metacaspase 3 (TcMCA3), seems to have a key role in parasite apoptosis. Accordingly, this work provides data concerning TcMCA3 regulation through its interaction with procaspase-activating compound 1 (PAC-1), a procaspase 3 activator. Indeed, PAC-1 reduced T. cruzi epimastigote viability with an IC₅₀ of 14.12 µM and induced loss of mitochondrial potential and exposure of phosphatidylserine, features of the apoptotic process. Notwithstanding, those PAC-1-inducible effects were not conserved in metacyclic trypomastigotes. Moreover, PAC-1 reduced the viability of mammalian cells with a greater IC₅₀ (25.70 µM) compared to T. cruzi epimastigotes, indicating distinct modes of binding between caspases and metacaspases. To shed light on the selectivity of metacaspases and caspases, we determined the structural features related to the PAC-1 binding sites in both types of proteins. These data are important for improving the understanding of the apoptosis pathway in T. cruzi so that TcMCA3 could be better targeted with future pharmaceuticals.

Intestinal macrophages in Peyer's patches, sacculus rotundus and appendix of Angora rabbit

The largest pool of macrophages in the body is harboured by the intestinal mucosa. As the principal phagocytic component of the immune system, macrophages are essential for maintaining mucosal homeostasis as they prevent commensal bacteria from adhering to mucosal epithelial cells. This study provides a RAM11 immunohistochemical and electron microscopic investigation of the existence, localization and distribution of intestinal macrophages in organized gut-associated lymphoid tissue (GALT), including Peyer's patches (PPs), the sacculus rotundus (SR) and the appendix, in the Angora rabbit. Although rabbit intestinal macrophages did not express the tissue macrophage marker macrosialin (CD68), they expressed RAM11. RAM11-positive intestinal macrophages were mostly localized to the subepithelial dome region, interfollicular area and germinal centres (GCs) of the GALT and the lamina propria or submucosa of the ileum and jejunum devoid of PPs and were also observed in the follicle-associated epithelium of PPs, but not in that of the SR and appendix. RAM11-positive macrophages containing engulfed apoptotic bodies were present in the GCs of the lymphoid follicles in the GALT. Electron microscopy further revealed multiple macrophages containing apoptotic bodies within the GCs of the follicles in the GALT. Some macrophage aggregations were observed in the GC and between the GC and the corona region of the follicles in the SR and appendix. Rabbit intestinal macrophages thus undertake both potent phagocytic activity and the efficient scavenging of apoptotic cells. Immunohistochemical data suggest that RAM11 can be reliably used for the determination of intestinal macrophages in the GALT of rabbits.

Generation of blood circulating DNA: the sources, peculiarities of circulation and structure

Extracellular nucleic acids (exNA) were described in blood of both healthy and illness people as early as in 1948, but staied overlooked until middle 60-th. Starting from the beginning of new millennium and mainly in the last 5 years exNA are intensively studied. Main attention is directed to investigation of exNA as the source of diagnostic material whereas the mechanisms of their generation, as well as mechanisms to providing long-term circulation of exNA in the bloodstream are not established unambiguously. According to some authors, the main source of circulating nucleic acids in blood are the processes of apoptosis and necrosis, while others refer to the possible nucleic acid secretion by healthy and tumor cells. Circulating DNA were found to be stable in the blood for a long time, escaping from the action of DNA hydrolyzing enzymes and are apparently packed in different supramolecular complexes. This review presents the opinions of various authors and evidence in favor of all the theories describingappearance of extracellular DNA, the features of the circulation and structure of the extracellular DNA and factors affecting the time of DNA circulation in blood

SIRPα polymorphisms, but not the prion protein, control phagocytosis of apoptotic cells

Prnp(-/-) mice lack the prion protein PrP(C) and are resistant to prion infections, but variable phenotypes have been reported in Prnp(-/-) mice and the physiological function of PrP(C) remains poorly understood. Here we examined a cell-autonomous phenotype, inhibition of macrophage phagocytosis of apoptotic cells, previously reported in Prnp(-/-) mice. Using formal genetic, genomic, and immunological analyses, we found that the regulation of phagocytosis previously ascribed to PrP(C) is instead controlled by a linked locus encoding the signal regulatory protein α (Sirpa). These findings indicate that control of phagocytosis was previously misattributed to the prion protein and illustrate the requirement for stringent approaches to eliminate confounding effects of flanking genes in studies modeling human disease in gene-targeted mice. The plethora of seemingly unrelated functions attributed to PrP(C) suggests that additional phenotypes reported in Prnp(-/-) mice may actually relate to Sirpa or other genetic confounders.

A molecular view on signal transduction by the apoptosome

Apoptosomes are signaling platforms that initiate the dismantling of a cell during apoptosis. In mammals, assembly of the apoptosome is the pivotal point in the mitochondrial pathway of apoptosis, and is prompted by binding of cytochrome c to the apoptotic protease-activating factor 1 (Apaf-1) in the presence of ATP. The resulting wheel-like heptamer of seven molecules Apaf-1 and seven molecules cytochrome c binds and activates the initiator caspase-9, which in turn ignites the downstream caspase cascade. In this review we discuss the molecular determinants for the formation of the mammalian apoptosome and caspase activation and describe the related signaling platforms in flies and nematodes.

Pharmacological modulation of cell death in atherosclerosis: a promising approach towards plaque stabilization?

Despite tremendous advances over the last 15 years in identifying vulnerable atherosclerotic plaques, the incidence of death and disability caused by such lesions still remains the number one health threat in developed countries. Therefore, new systemic or focal therapies aimed at decreasing the overall burden of disease, and a change to a more benign phenotype, are needed. Because cell death is a prominent feature of advanced atherosclerotic plaques with a major impact on plaque destabilization, an increasing number of compounds targeting the apoptotic or autophagic machinery in atherosclerosis are being explored, predominantly at the preclinical level. This review will provide an overview of these compounds, with a focus on both inhibition and stimulation of cell death, to prevent acute coronary syndromes and sudden cardiac death.

Differentiation and glucocorticoid regulated apopto-phagocytic gene expression patterns in human macrophages. Role of Mertk in enhanced phagocytosis

The daily clearance of physiologically dying cells is performed safely mainly by cells in the mononuclear phagocyte system. They can recognize and engulf dying cells utilizing several cooperative mechanisms. In our study we show that the expression of a broad range of apopto-phagocytic genes is strongly up-regulated during differentiation of human monocytes to macrophages with different donor variability. The glucocorticoid dexamethasone has a profound effect on this process by selectively up-regulating six genes and down-regulating several others. The key role of the up-regulated mer tyrosine kinase (Mertk) in dexamethasone induced enhancement of phagocytosis could be demonstrated in human monocyte derived macrophages by gene silencing as well as blocking antibodies, and also in a monocyte-macrophage like cell line. However, the additional role of other glucocorticoid induced elements must be also considered since the presence of autologous serum during phagocytosis could almost completely compensate for the blocked function of Mertk.

Ca(v)2.3 Ca2+ channel interacts with the G1-subunit of V-ATPase

BACKGROUND

Calcium channels are essential in coupling action potential to signal transduction in cells. There are several types of calcium channels, which can be pharmacologically classified as L-, N-, P/Q-, R- and T-type. But molecular basis of R-type channels is less clearly understood compared the other channel types. Therefore the current study aims at understanding the molecular functions of R-type calcium channels by identifying interaction partners of the channel.

METHODS

In order to do so, a yeast two hybrid (Y2H) screen, with carboxy terminus of α1 subunit of the channel, as the bait, was performed. G1 subunit of v-ATPase was identified as a putative interaction partner of human Ca(v)2.3 by using the Y2H screening. The interaction was confirmed by immunoprecipitation. To study the functional importance of the interaction, bafilomycin A(1), a potent and specific inhibitor of v-ATPase was used in patch-clamp recordings in Ca(v)2.3 stably-transfected HEK-293 cells (2C6) as well as in electroretinography of the isolated bovine retina expressing R-type Ca(2+) channels.

RESULTS

G1 subunit of v-ATPase interacts with C-terminal tail of Ca(v)2.3 and bafilomycin A(1) reduces Ca(v)2.3 mediated calcium currents. Additionally peak I(Ca) is inhibited in retinal signal transduction when recorded as ERG b-wave.

CONCLUSIONS

The results suggest that v-ATPase interacts physically and also functionally with Ca(v)2.3. This is the first demonstration of association of Ca(v)2.3 C-terminus with a protein complex which is involved in transmembrane signalling.

Induction of apoptosis in human lymphoblastoid cells by kaempferol 3-O-β-isorhamninoside and rhamnocitrin 3-O-β-isorhamninoside from Rhamnus alaternus L. (Rhamnaceae)

OBJECTIVE

Kaempferol 3-O-β-isorhamninoside (K3O-ir) and rhamnocitrin 3-O-β-isorhamninoside (R3O-ir) from Rhamnus alaternus L leaves are investigated for their ability to induce apoptosis in human lymphoblastoid cells. We have attempted to characterize apoptotic pathway activated by these two flavonoids.

MATERIAL AND METHODS

Apoptosis of the human TK6 lymphoblastoid cell line was detected by DNA fragmentation, PARP cleavage and by evaluating caspase activity.

RESULTS

Apoptosis was observed after 24- and 48-h incubation of the cells with the tested compounds. DNA fragmentation was observed after treatment with flavonoids; this was confirmed by demonstration of PARP cleavage. Caspase-3 and caspase-8 activities were induced by both K3O-ir and R3O-ir flavonoids showing highest activity with compound concentration of 400 μg/ml.

CONCLUSION

We have demonstrated that K3O-ir and R3O-ir induce apoptosis in human lymphoblastoid cells by the extrinsic pathway of apoptosis.

Macrophages from lupus-prone MRL mice have a conditional signaling abnormality that leads to dysregulated expression of numerous genes

Macrophages (mϕ) from pre-diseased mice of the major murine inbred models of spontaneous autoimmunity (AI), including multiple lupus-prone strains and the type I diabetes-prone NOD (non-obese diabetic) strain, have identical apoptotic target-dependent abnormalities. This characteristic feature of mϕ from AI-prone mice suggests that abnormal signaling events induced within mϕ following their interaction with apoptotic targets may predispose to AI. Such signaling abnormalities would affect predominantly the processing and presentation of self-antigen (i.e., derived from apoptotic targets), while sparing the processing and presentation of foreign antigen (i.e., derived from non-apoptotic sources). Here, we used DNA microarrays to test the hypothesis that mϕ from AI-prone mice (MRL/MpJ [MRL/+] or MRL/MpJ-Tnfrsf6 ( lpr ) [MRL/lpr]) differentially express multiple genes in comparison to non-AI mϕ (BALB/c), but do so in a largely apoptotic cell-dependent manner. Mϕ were stimulated with lipopolysaccharide, a potent innate stimulus, in the presence or absence of serum (an experimental surrogate for apoptotic targets). In accord with our hypothesis, the number of genes differentially expressed by MRL mϕ was significantly increased in the presence vs. the absence of serum, the apoptotic target surrogate (n = 401 vs. n = 201). Notably, for genes differentially expressed by MRL mϕ in the presence of serum, serum-free culture normalized their expression to a level statistically indistinguishable from that by non-AI mϕ. Comparisons of mϕ from AI-prone NOD and non-AI C57BL/6 mice corroborated these findings. Together, these data support the hypothesis that mϕ from MRL and other AI-prone mice are characterized by a conditional abnormality elicited by serum lipids or apoptotic targets.

Tumor-associated macrophages as targets for tumor immunotherapy

Restoration of one of the major physiological functions of the body's immune response, the rejection of malignant cells, is a promising yet challenging task for cancer therapy. Prinicipally, immunotherapeutic approaches make use of cells of the adaptive immune system, since antigen-based tumor rejection might be the most specific approach. However, other immune cell populations, such as tumor-associated macrophages (TAMs), contribute significantly to protumor mechanisms elicited by a distorted immune response. In this review, we summarize the current knowledge about the pathology of TAMs and discuss potential therapeutic approaches to overcome TAM-mediated tumor promotion. Hereby, we focus on TAM phenotypes that were observed in the clinically relevant stages of cancer progression. The function of macrophages and other inflammatory cells in the onset of cancer has been discussed elsewhere.

Epithelial cell-directed efferocytosis in the post-partum mammary gland is necessary for tissue homeostasis and future lactation

Background

Mammary glands harbor a profound burden of apoptotic cells (ACs) during post-lactational involution, but little is known regarding mechanisms by which ACs are cleared from the mammary gland, or consequences if this process is interrupted. We investigated AC clearance, also termed efferocytosis, during post-lactational remodeling, using mice deficient for MerTK, Axl, and Tyro3, three related receptor tyrosine kinases (RTKs) regulating macrophage-mediated efferocytosis in monocytes. MerTK expression, apoptosis and the accumulation of apoptotic debris were examined in histological sections of MerTK-deficient, Axl/Tyro3-deficient, and wild-type mammary glands harvested at specific time points during lactation and synchronized involution. The ability of primary mammary epithelial cells (MECs) to engulf ACs was assessed in culture. Transplant of MerTK-deficient mammary epithelium into cleared WT mammary fat pads was used to assess the contribution of WT mammary macrophages to post-lactational efferocytosis.

Results

ACs induced MerTK expression in MECs, resulting in elevated MerTK levels at the earliest stages of involution. Loss of MerTK resulted in AC accumulation in post-lactational MerTK-deficient mammary glands, but not in Axl and Tyro3-deficient mammary glands. Increased vascularization, fibrosis, and epithelial hyperproliferation were observed in MerTK-deficient mammary glands through at least 60 days post-weaning, due to failed efferocytosis after lactation, but did not manifest in nulliparous mice. WT host-derived macrophages failed to rescue efferocytosis in transplanted MerTK-deficient mammary epithelium.

Conclusion

Efferocytosis by MECs through MerTK is crucial for mammary gland homeostasis and function during the post-lactational period. Efferocytosis by MECs thus limits pathologic consequences associated with the apoptotic load following lactation.

Apoptotic volume decrease as a geometric determinant for cell dismantling into apoptotic bodies

Apoptosis is a mode of cell death through which cells are dismantled and cell remains are packed into small, membrane-bound, sealed vesicles called apoptotic bodies, which are easy to erase by phagocytosis by neighbouring and immune system cells. The end point of the process is to cleanly eliminate damaged or unnecessary cells without disrupting the surrounding tissue or eliciting an inflammatory response. The apoptotic process involves a series of specific events including deoxyribonucleic acid and nuclear fragmentation, protease-driven cleavage of specific substrates, which inhibits key survival functions and reorganizes the cell's structure, externalization of molecules involved in phagocytosis, membrane blebbing and cell shrinkage. Apoptotic volume decrease (AVD) leading to cell shrinkage is a core event in the course of apoptosis, the biological meaning of which has not been clearly ascertained. In this article we argue that volume loss is a geometrical requisite for cell dismantling into apoptotic bodies. This is derived from the cell's volume-to-surface ratio. Indeed, package of the original cell volume into smaller membrane-sealed vesicles requires that either cell membrane surface increase or cell volume decrease. In this sense, AVD provides a reservoir of membrane surface for apoptotic body formation. The strategic situation of AVD in the time course of apoptosis is also discussed in the context of apoptotic body formation.

Dendritic cell activation by sensing Mycobacterium tuberculosis-induced apoptotic neutrophils via DC-SIGN

Mycobacterium tuberculosis (Mtb) manipulates cells of the innate immune system to provide the bacteria with a sustainable intracellular niche. Mtb spread through aerosol carrying them deep into the lungs, where they are internalized by phagocytic cells, such as neutrophils (PMNs), dendritic cells (DCs), and macrophages. PMNs undergo accelerated apoptosis after interaction with the bacterium, and apoptotic cells are sequestered by neighboring phagocytes. Removal of aged apoptotic cells because of natural tissue turnover is described as an immunologically silent process facilitating resolution of inflammation and inhibition of DC maturation. Silencing of immune cells could be favorable for intracellular bacteria. The aim of this study was to clarify the interaction between Mtb-induced apoptotic PMNs and DCs, and evaluate whether this interaction follows the proposed anti-inflammatory pathway. In contrast to aged apoptotic cells, Mtb-induced apoptotic PMNs induced functional DC maturation. We found that the cell fraction from Mtb-induced apoptotic PMNs contained almost all stimulatory capacity, suggesting that cell-cell interaction is crucial for DC activation. Inhibitory studies showed that this cell contact-dependent activation required binding of the PMN Mac-1 (CD11b/CD18) to the DC via DC-SIGN and endocytic activity involving the alpha(v)beta(5) but did not involve the scavenger receptor CD36. Taken together, this study demonstrates that the DCs can distinguish between normal and infected apoptotic PMNs via cellular crosstalk, where the DCs can sense the presence of danger on the Mtb-infected PMNs and modulate their response accordingly.

A new model for the transition of APAF-1 from inactive monomer to caspase-activating apoptosome

The cytosolic adaptor protein Apaf-1 is a key player in the intrinsic pathway of apoptosis. Binding of mitochondrially released cytochrome c and of dATP or ATP to Apaf-1 induces the formation of the heptameric apoptosome complex, which in turn activates procaspase-9. We have re-investigated the chain of events leading from monomeric autoinhibited Apaf-1 to the functional apoptosome in vitro. We demonstrate that Apaf-1 does not require energy from nucleotide hydrolysis to eventually form the apoptosome. Despite a low intrinsic hydrolytic activity of the autoinhibited Apaf-1 monomer, nucleotide hydrolysis does not occur at any stage of the process. Rather, mere binding of ATP in concert with the binding of cytochrome c primes Apaf-1 for assembly. Contradicting the current view, there is no strict requirement for an adenine base in the nucleotide. On the basis of our results, we present a new model for the mechanism of apoptosome assembly.

C1q: the perfect complement for a synaptic feast?

The efficient and selective removal of apoptotic cells is an important feature of tissue development, homeostasis and pathology. In the nervous system, synapses and distal axons are selectively eliminated as part of the remodelling that underpins development and pathology, through a process that has some features in common with apoptotic cell removal. Components of the complement cascade are implicated in the efficient removal of apoptotic cells outside the nervous system, and recent evidence suggests that the complement components C1q and C3 have a role in the selective tagging of supernumerary synapses in the developing visual system and in their efficient removal by as yet unidentified cells.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

Beta-sitosterol induces anti-proliferation and apoptosis in human leukemic U937 cells through activation of caspase-3 and induction of Bax/Bcl-2 ratio

Beta-sitosterol is the main dietary phytosterol found in plants and has been shown to inhibit proliferation and induce apoptosis in human solid tumors such as colon and breast cancers. However, the mechanism by which beta-sitosterol induces apoptosis is not completely understood in leukemic cells. This study investigated the mechanism of apoptosis induced by beta-sitosterol in human leukemic U937 cells. beta-Sitosterol induced cytotoxicity and apoptosis in U937 cells in a concentration dependent manner, as measured by hemocytometer counts, fluorescence microscopy, agarose gel electrophoresis, and flow cytometry analysis. The increase in apoptosis induced by beta-sitosterol was associated with down-regulation of Bcl-2, degradation of poly-(ADP-ribose) polymerase (PARP) and phospholipase C (PLC)-gamma1 protein, and activation of caspase-3. beta-Sitosterol induced apoptosis was not associated with changes in the expression of Bcl-xL, Bax, or inhibitor of apoptosis proteins (IAPs). z-DEVD-fmk, a caspase-3 specific inhibitor, blocked caspase-3 activation and PARP degradation, and significantly attenuated beta-sitosterol-induced apoptosis. This suggests that caspase-3 activation is partially essential for beta-sitosterol-induced apoptosis. Bcl-2 overexpression also significantly blocked caspase-3 activation and the decrease in PARP cleavage by beta-sitosterol, and effectively attenuated the apoptotic response to beta-sitosterol. These results show that beta-sitosterol potently induces apoptosis in U937 cells and that beta-sitosterol-induced apoptosis is related to the selective activation of caspase-3 and induction of Bax/Bcl-2 ratio.

Modulation of the immune system by dying cells and the phosphatidylserine-ligand annexin A5

Apoptotic cell death and the efficient clearance of dying cells are essential mechanisms to control tissue homeostasis and to eliminate potential autoantigens. Numerous alterations on the surfaces of dying cells define a highly characteristic membrane signature and enable an unequivocal distinction from vital cells. This way, phagocytosis is initiated and signalling events induced which minimize inflammatory reactions. Therefore, the use of proteins interfering with the clearance process may open up new vistas to improve immunization strategies and may help to understand the mechanisms of autoimmune diseases.

Induction of apoptosis by crambene protects mice against acute pancreatitis via anti-inflammatory pathways

Apoptosis is a teleologically beneficial form of cell death in acute pancreatitis. Our previous work has demonstrated that induction of pancreatic acinar cell apoptosis by crambene protects mice against acute pancreatitis. However, little is known about how the induction of apoptosis reduces the severity of acute pancreatitis. Because the clearance of apoptotic cells might suppress inflammation and critically regulate immune responses, we postulate that clearance of apoptotic cells stimulates an anti-inflammatory response, which has a protective action against acute pancreatitis. To test this hypothesis, induction of apoptosis in acute pancreatitis in vivo and co-cultures of peritoneal resident macrophages with apoptotic acinar cells in vitro were used as experimental systems, testing expression of phagocytic receptors and levels of inflammatory mediators. Moreover, neutralizing anti-interleukin (IL)-10 monoclonal antibody (2.5 mg/kg) was used before the induction of apoptosis in acute pancreatitis, testing whether the protection from apoptosis induction would be removed. Our study showed that clearance of apoptotic acinar cells, which may occur essentially through the CD36-positive macrophage, stimulates the release of anti-inflammatory mediators like IL-10. IL-10 plays an important role in crambene-induced protection in acute pancreatitis. Thus, induction of pancreatic acinar cell apoptosis by crambene protects mice against acute pancreatitis via induction of anti-inflammatory pathways.

Macrophages and dendritic cells use different Axl/Mertk/Tyro3 receptors in clearance of apoptotic cells

The clearance of apoptotic cells is important for regulating tissue homeostasis, inflammation, and autoimmune responses. The absence of receptor tyrosine kinases (Axl, Mertk, and Tyro3) results in widespread accumulation of apoptotic cells and autoantibody production in mice. In this report, we examine the function of the three family members in apoptotic cell clearance by different phagocytic cell types. Mertk elimination nearly abolished macrophage apoptotic cell phagocytosis; elimination of Axl, Tyro3, or both, reduced macrophage phagocytosis by approximately half, indicating that these also play a role. In contrast, apoptotic cell clearance in splenic and bone marrow-derived dendritic cells (DCs) is prolonged compared with macrophages and relied primarily on Axl and Tyro3. The slower ingestion may be due to lower DC expression of Axl and Tyro3 or absence of GAS6 expression, a known ligand for this receptor family. In vivo, phagocytosis of apoptotic material by retinal epithelial cells required Mertk. Unlike macrophages, there did not appear to be any role for Axl or Tyro3 in retinal homeostasis. Likewise, clearance of apoptotic thymocytes in vivo was dramatically reduced in mertk(kd) mice, but was normal in axl/tyro3(-/-) mice. Thus, cell and organ type specificity is clearly delineated, with DCs relying on Axl and Tyro3, retina and thymus requiring Mertk, and macrophages exhibiting an interaction that involves all three family members. Surprisingly, in macrophages, tyrosine phosphorylation of Mertk in response to apoptotic cells is markedly diminished from axl/tyro3(-/-) mice, suggesting that the interactions of these receptors by heterodimerization may be important in some cells.

Clearance of apoptotic and necrotic cells and its immunological consequences

The ultimate and most favorable fate of almost all dying cells is engulfment by neighboring or specialized cells. Efficient clearance of cells undergoing apoptotic death is crucial for normal tissue homeostasis and for the modulation of immune responses. Engulfment of apoptotic cells is finely regulated by a highly redundant system of receptors and bridging molecules on phagocytic cells that detect molecules specific for dying cells. Recognition of necrotic cells by phagocytes is less well understood than recognition of apoptotic cells, but an increasing number of recent studies, which are discussed here, are highlighting its importance. New observations indicate that the interaction of macrophages with dying cells initiates internalization of the apoptotic or necrotic targets, and that internalization can be preceded by "zipper"-like and macropinocytotic mechanisms, respectively. We emphasize that clearance of dying cells is an important fundamental process serving multiple functions in the regulation of normal tissue turnover and homeostasis, and is not just simple anti- or pro-inflammatory responses. Here we review recent findings on genetic pathways participating in apoptotic cell clearance, mechanisms of internalization, and molecules involved in engulfment of apoptotic versus necrotic cells, as well as their immunological consequences and relationships to disease pathogenesis.

Temporal gene expression profiles of target-ablated olfactory epithelium in mice with disrupted expression of scavenger receptor A: impact on macrophages

Target ablation [removal of the olfactory bulb (OBX)] induces apoptotic death of olfactory sensory neurons (OSNs) and an immune response in which activation and recruitment of macrophages (ms) into the olfactory epithelium (OE) occupy a central role. Ms phagocytose apoptotic neurons and secrete cytokines/growth factors that regulate subsequent progenitor cell proliferation and neurogenesis. Scavenger receptor A (SR-A) is a pattern recognition receptor that mediates binding of ms to apoptotic cells and other relevant immune response functions. The aim of this study was to determine the impact of the absence of SR-A on the immune response to OBX. The immune response to OBX was evaluated in mice in which functional expression of the m scavenger receptor (MSR) was eliminated by gene disruption (MSR-/-) and wild-type (wt) mice of the same genetic background. OBX induced significant apoptotic death of mature OSNs in the two strains. However, subsequent m infiltration and activation and progenitor cell proliferation were significantly reduced in MSR-/- vs. wt mice. Gene expression profiling at short intervals after OBX demonstrated significant differences in temporal patterns of expression of several gene categories, including immune response genes. Many immune response genes that showed different temporal patterns of expression are related to m function, including cytokine and chemokine secretion, phagocytosis, and m maturation and activation. These studies suggest that impairment of the immune response to OBX in the OE of MSR-/- mice most likely resulted from decreased m adhesion and subsequent reduced infiltration and activation, with a resultant decrease in neurogenesis.