Target cell CD47 regulates macrophage activation and erythrophagocytosis

Estimated median density identifies donor age and sex differences in red blood cell biological age

BACKGROUND

Donors possess heterogeneous red cell concentrates (RCCs) in terms of the biological age of their red blood cells (RBCs) as a direct result of various donor-dependent factors influencing rates of erythropoiesis. This study aimed to estimate the median biological age of RBCs in RCCs based on donor age and sex to investigate inherent differences in blood products' biological ages over hypothermic storage using estimated median densities (EMDs).

STUDY DESIGN

Sixty RCCs were collected from four donor groups; male and female teenagers (17-19 years old) and seniors (75+ years old). A Percoll density-based separation approach was used to quantify the EMDs indicative of biological age. EMD and mean corpuscular hemoglobin (MCHC) were compared by correlation analyses.

RESULTS

Differences in the median biological age of RCC units were observed with male donors having significantly higher EMDs compared to females (p < .001). Teen male donors possessed the highest EMDs with significantly elevated levels of biologically aged RBCs compared to both female donor groups, regardless of storage duration (p < .05). Throughout most of the 42-day storage period, senior donors, particularly senior females, demonstrated the strongest correlation between EMD and MCHC (R2 > 0.5).

CONCLUSIONS

This study provides further evidence that there are inherent differences between the biological age profiles of RBCs between blood donors of different sex and age. Our findings further highlight that biological age may contribute to RBC quality during storage and that donor characteristics need to be considered when evaluating transfusion safety and efficacy.

Arsenic retention in erythrocytes and excessive erythrophagocytosis is related to low selenium status by impaired redox homeostasis

Arsenic (As) contamination in drinking water is a global public health problem. Epidemiological studies have shown that selenium (Se) deficiency is associated with an increasing risk of arsenism. However, the association between Se status and As retention in erythrocytes and mechanisms underlying this association have not been fully investigated. In the present study, a total of 165 eligible subjects were recruited and As was found to accumulate in blood mainly by retention in erythrocytes. Retention of As in erythrocytes was negatively correlated with Se status, antioxidant parameters related to Se and As methylation capacity, but positively correlated with the protein-binding capacity of As. Additionally, erythrocytes isolated from subjects with low Se status exhibited cellular damage along with lower protein levels of CD47, which could be aggravated by hydrogen peroxide treatment. Consistent with the human study, the erythrocytes from mice with sub-chronic As exposure exhibited similar cellular damage and shown to be phagocytosed by splenic macrophages, and these effects were mitigated by dietary Se supplementation. Furthermore, hydrogen peroxide treatment induced excessive phagocytosis of erythrocytes with As exposure by splenic macrophages, while co-treating erythrocytes with the reducing agent, N-Acetyl-l-cysteine, mitigated this excessive erythrophagocytosis. Hyperactivation of the NFκB pathway was also detected in splenic macrophages after excessive erythrophagocytosis. In conclusion, this study found that low Se status involving impaired redox homeostasis increased As retention in erythrocytes, which were subsequently phagocytosed by splenic macrophages and led to an increased inflammatory status of splenic macrophages. These findings provide insight into physiological features of arsenism related to Se status and redox homeostasis.

Erythrophagocytosis by Microglia/Macrophage in Intracerebral Hemorrhage: From Mechanisms to Translation

Intracerebral hemorrhage (ICH) is a devastating condition characterized by hematoma related mass effect. Microglia/macrophage (M φ) are rapidly recruited in order to remove the red blood cells through erythrophagocytosis. Efficient erythrophagocytosis can detoxify hemolytic products and facilitate neurological recovery after ICH. The underlying mechanisms include modulation of inflammatory response and oxidative stress, among others. It is a dynamic process mediated by a cascade of signal transduction, including “find-me” signals, “eat-me” signals and a set of phagocytotic receptors-ligand pairs that may be exploited as therapeutic targets. This review summarizes mechanistic signaling pathways of erythrophagocytosis and highlights the potential of harnessing M φ-mediated phagocytosis for ICH treatment.

CD47 blocking antibody accelerates hematoma clearance and alleviates hydrocephalus after experimental intraventricular hemorrhage

Background CD47, a glycoprotein on red blood cell membranes, inhibits phagocytosis via interaction with signal regulatory protein α on phagocytes. Our previous research has demonstrated that blocking CD47 accelerates hematoma clearance and reduces brain injury after intracerebral hemorrhage. The current study investigated whether phagocytosis or erythrocyte CD47 impacts hematoma resolution and hydrocephalus development after intraventricular hemorrhage (IVH). Methods Adult (3-month-old) male Fischer 344 rats were intraventricularly injected with 200 μl autologous blood, mixed with either CD47 blocking antibody or isotype IgG, or 200 μl saline as control. In subgroups of CD47 blocking antibody treated rats, clodronate liposomes (to deplete microglia/monocyte-derived macrophages) or control liposomes were co-injected. Magnetic resonance imaging (MRI) was used to evaluate ventricular volume and intraventricular T2* lesion volume (estimating hematoma volume). The brains were harvested after 4 or 72 h for histology to evaluate phagocytosis. Results In adult male rats, CD47 blocking antibody alleviated hydrocephalus development by day 3. In addition, the CD47 blocking antibody reduced intraventricular T2* lesion and T2* non-hypointense lesion size after IVH through day 1 to day 3. Erythrophagocytosis was observed as soon as 4 h after IVH and was enhanced on day 3. Furthermore, intra-hematoma infiltration of CD68, heme oxygenase-1 and ferritin positive phagocytes were upregulated by CD47 blockade by day 3. Clodronate liposomes co-injection caused more severe hydrocephalus and weight loss. Conclusion Blocking CD47 in the hematoma accelerated hematoma clearance and alleviated hemolysis and hydrocephalus development after IVH, suggesting CD47 might be valuable in the future treatment for IVH.

Enhancement of Hematoma Clearance With CD47 Blocking Antibody in Experimental Intracerebral Hemorrhage

Background and Purpose- Our previous studies found that erythrocyte CD47 has a role in regulating hematoma resolution following experimental intracerebral hemorrhage (ICH). The current study examined whether or not a CD47 blocking antibody enhances hematoma clearance in a mouse ICH. Methods- ICH was induced by intracaudate injection of autologous blood in adult C57BL/6 mice. Mice had an ICH or ICH with CD47 blocking antibody or IgG coinjection. In subgroups of CD47 blocking antibody-treated mice, clodronate (to deplete microglia/macrophages) or control liposomes were coinjected. The effects of CD47 blocking antibody on ICH-induced brain injury were also tested in both males and females. Mice had magnetic resonance imaging to examine clot volume, iron deposition, brain swelling, and brain tissue loss. Behavioral tests were performed in all mice, and brains were harvested for brain immunohistochemistry. Results- In male mice, CD47 blocking antibody speeded up hematoma/iron clearance by macrophages/microglia and reduced ICH-induced brain swelling, neuronal loss, and neurological deficits. In contrast, clodronate liposome-induced microglia/macrophage depletion caused more severe brain swelling, neuronal loss, and functional deficits. In addition, similar injury severity in males and females was found in IgG control group and CD47 blocking antibody was also effective in females. Conclusions- Blocking CD47 in the hematoma speeded hematoma clearance and reduced brain injury after ICH suggesting it could be a treatment for ICH patients with surgical clot removal.

Haematoma scavenging in intracerebral haemorrhage: from mechanisms to the clinic

The products of erythrocyte lyses, haemoglobin (Hb) and haem, are recognized as neurotoxins and the main contributors to delayed cerebral oedema and tissue damage after intracerebral haemorrhage (ICH). Finding a means to efficiently promote absorption of the haemolytic products (Hb and haem) around the bleeding area in the brain through stimulating the function of the body's own garbage cleaning system is a novel clinical challenge and critical for functional recovery after ICH. In this review, available information of the brain injury mechanisms underlying ICH and endogenous haematoma scavenging system is provided. Meanwhile, potential intervention strategies are discussed. Intracerebral blood itself has ‘toxic’ effects beyond its volume effect after ICH. Haptoglobin–Hb–CD163 as well as haemopexin–haem–LRP1 is believed to be the most important endogenous scavenging pathway which participates in blood components resolution following ICH. PPARγ–Nrf2 activates the aforementioned clearance pathway and then accelerates haematoma clearance. Meanwhile, the scavenger receptors as novel targets for therapeutic interventions to treat ICH are also highlighted.

Human Erythrocyte Acetylcholinesterase in Health and Disease

The biochemical properties of erythrocyte or human red blood cell (RBC) membrane acetylcholinesterase (AChE) and its applications on laboratory class and on research are reviewed. Evidence of the biochemical and the pathophysiological properties like the association between the RBC AChE enzyme activity and the clinical and biophysical parameters implicated in several diseases are overviewed, and the achievement of RBC AChE as a biomarker and as a prognostic factor are presented. Beyond its function as an enzyme, a special focus is highlighted in this review for a new function of the RBC AChE, namely a component of the signal transduction pathway of nitric oxide.

Modulators of microglial activation and polarization after intracerebral haemorrhage

Intracerebral haemorrhage (ICH) is the most lethal subtype of stroke but currently lacks effective treatment. Microglia are among the first non-neuronal cells on the scene during the innate immune response to ICH. Microglia respond to acute brain injury by becoming activated and developing classic M1-like (proinflammatory) or alternative M2-like (anti-inflammatory) phenotypes. This polarization implies as yet unrecognized actions of microglia in ICH pathology and recovery, perhaps involving microglial production of proinflammatory or anti-inflammatory cytokines and chemokines. Furthermore, alternatively activated M2-like microglia might promote phagocytosis of red blood cells and tissue debris, a major contribution to haematoma clearance. Interactions between microglia and other cells modulate microglial activation and function, and are also important in ICH pathology. This Review summarizes key studies on modulators of microglial activation and polarization after ICH, including M1-like and M2-like microglial phenotype markers, transcription factors and key signalling pathways. Microglial phagocytosis, haematoma resolution, and the potential crosstalk between microglia and T lymphocytes, neurons, astrocytes, and oligodendrocytes in the ICH brain are described. Finally, the clinical and translational implications of microglial polarization in ICH are presented, including the evidence that therapeutic approaches aimed at modulating microglial function might mitigate ICH injury and improve brain repair.

How do red blood cells know when to die?

Human red blood cells (RBCs) are normally phagocytized by macrophages of splenic and hepatic sinusoids at 120 days of age. The destruction of RBCs is ultimately controlled by antagonist effects of phosphatidylserine (PS) and CD47 on the phagocytic activity of macrophages. In this work, we introduce a conceptual model that explains RBC lifespan as a consequence of the dynamics of these molecules. Specifically, we suggest that PS and CD47 define a molecular algorithm that sets the timing of RBC phagocytosis. We show that significant changes in RBC lifespan described in the literature can be explained as alternative outcomes of this algorithm when it is executed in different conditions of oxygen availability. The theoretical model introduced here provides a unified framework to understand a variety of empirical observations regarding RBC biology. It also highlights the role of RBC lifespan as a key element of RBC homeostasis.

MicroRNA-133a suppresses the proliferation, migration, and invasion of laryngeal carcinoma cells by targeting CD47

The study aims to investigate the possible mechanisms of microRNA-133a (miR-133a) targeting CD47 on cell proliferation, apoptosis, migration, and invasion in laryngeal carcinoma. Forty-two laryngeal carcinoma tissue specimens confirmed by pathological examination from laryngeal carcinoma patients as the case group were collected, and 20 chronic laryngitis tissues were gathered as the control group. The human laryngeal carcinoma cell line Hep-2 was marked as the miR-133a mimic, negative control (NC), miR-133a inhibitor, CD47-siRNA, miR-133a inhibitor + CD47-siRNA, and Mock groups. The expression of CD47 protein and miR-133a was detected by immunohistochemistry (IHC) and qRT-PCR. Dual luciferase assay system was used to determine the relationship between CD47 and miR-133a. Western blotting was used to measure the protein expression of CD47 and miR-133a. 5-Ethynyl-2'-deoxyuridine (EDU) method was used to detect the cell proliferation, and flow cytometry and Transwell were used to measure the cell apoptosis and migration and invasion, respectively. The miR-133a expression in laryngeal carcinoma tissues was significantly lower, while the CD47 expression was higher than that in chronic laryngitis tissues (both P < 0.01). The expression of miR-133a in the miR-133a mimic group was significantly higher than that in other groups (P < 0.05), and the messenger RNA (mRNA) and protein expression of CD47 in the CD47-siRNA and miR-133a mimic groups were significantly lower than those in the Mock and NC group (all P < 0.05), while the mRNA and protein expression of CD47 in the miR-133a inhibitor group were higher than in other groups (all P < 0.05). After transfection, the CD47-siRNA group had the strongest inhibitory activity, while the number of living cells in the miR-133a inhibitor group was significantly higher than that in other groups (all P < 0.05). The apoptosis rates in the miR-133a mimic and CD47-siRNA groups were significantly higher than that in the Mock and NC groups (all P < 0.05). The cell numbers that penetrated membrane in the miR-133a mimic and CD47-siRNA groups were less than in the Mock and NC groups (all P < 0.05). Upregulated miR-133a could inhibit proliferation, invasion, and migration and promote cell apoptosis in laryngeal carcinoma by targeting CD47. miR-133a targeting CD47 could be a new direction in the diagnosis and treatment of laryngeal carcinoma.

CD47-SIRPα Interactions Regulate Macrophage Uptake of Plasmodium falciparum-Infected Erythrocytes and Clearance of Malaria In Vivo

CD47 engagement by the macrophage signal regulatory protein alpha (SIRPα) inhibits phagocytic activity and protects red blood cells (RBCs) from erythrophagocytosis. The role of CD47-SIRPα in the innate immune response to Plasmodium falciparum infection is unknown. We hypothesized that disruption of SIRPα signaling may enhance macrophage uptake of malaria parasite-infected RBCs. To test this hypothesis, we examined in vivo clearance in CD47-deficient mice infected with Plasmodium berghei ANKA and in vitro phagocytosis of P. falciparum-infected RBCs by macrophages from SHP-1-deficient (Shp-1(-/-)) mice and NOD.NOR-Idd13.Prkdc(scid) (NS-Idd13) mice, as well as human macrophages, following disruption of CD47-SIRPα interactions with anti-SIRPα antibodies or recombinant SIRPα-Fc fusion protein. Compared to their wild-type counterparts, Cd47(-/-) mice displayed significantly lower parasitemia, decreased endothelial activation, and enhanced survival. Using macrophages from SHP-1-deficient mice or from NS-Idd13 mice, which express a SIRPα variant that does not bind human CD47, we showed that altered SIRPα signaling resulted in enhanced phagocytosis of P. falciparum-infected RBCs. Moreover, disrupting CD47-SIRPα engagement using anti-SIRPα antibodies or SIRPα-Fc fusion protein also increased phagocytosis of P. falciparum-infected RBCs. These results indicate an important role for CD47-SIRPα interactions in innate control of malaria and suggest novel targets for intervention.

CD47 expression in cryopreserved equine cutaneous masses and normal skin

We investigated CD47 expression in cryopreserved sections of equine cutaneous masses and normal skin. CD47 is a cell surface protein expressed on many cell types and overexpressed in some tumors. Interaction of CD47 and signal regulatory protein-alpha (SIRPα) inhibits phagocytosis by macrophages. Formalin-fixed tissues from horses prospectively enrolled in the study were used to establish a histologic diagnosis. Immunohistochemical assays were performed on cryopreserved tissues using anti-CD47 antibodies or IgG control antibodies. CD47 was not expressed on equine normal skin but positivity to CD47 was present in 13 of 24 (54%) masses. Immunotherapy with anti-CD47 antibodies for equine cutaneous tumors that express CD47 warrants further investigation.

Hematoma Changes During Clot Resolution After Experimental Intracerebral Hemorrhage

BACKGROUND AND PURPOSE

Hematoma clearance occurs in the days after intracerebral hemorrhage (ICH) and has not been well studied. In the current study, we examined changes in the hematoma in a piglet ICH model. The effect of deferoxamine on hematoma was also examined.

METHODS

The ICH model was induced by an injection of autologous blood into the right frontal lobe of piglets. First, a natural time course of hematoma changes ≤7 days was determined. Second, the effect of deferoxamine on hematoma changes was examined. Hemoglobin and membrane attack complex levels in the hematoma were examined by enzyme-linked immunosorbent assay. Immunohistochemistry and Western blotting were used to examine CD47 (a regulator of erythrophagocytosis), CD163 (a hemoglobin scavenger receptor), and heme oxygenase-1 (a heme degradation enzyme) in the clot.

RESULTS

After ICH, there was a reduction in red blood cell diameter within the clot with time. This was accompanied by membrane attack complex accumulation and decreased hemoglobin levels. Erythrophagocytosis occurred in the hematoma, and this was associated with reduced clot CD47 levels. Activated macrophages/microglia were CD163 and hemeoxygenase-1 positive, and these accumulated in the clot with time. Deferoxamine treatment attenuated the process of hematoma resolution by reducing member attack complex formation and inhibiting CD47 loss in the clot.

CONCLUSIONS

These results indicate that membrane attack complex and erythrophagocytosis contribute to hematoma clearance after ICH, which can be altered by deferoxamine treatment.

Role of Erythrocyte CD47 in Intracerebral Hematoma Clearance

BACKGROUND AND PURPOSE

Enhancing hematoma clearance through phagocytosis may reduce brain injury after intracerebral hemorrhage. In the current study, we investigated the role of cluster of differentiation 47 (CD47) in regulating erythrophagocytosis and brain injury after intracerebral hemorrhage in nude mice.

METHODS

This study was in 2 parts. First, male adult nude mice had an intracaudate injection of 30 μL saline, blood from male adult wild-type (WT) mice, or blood from CD47 knockout mice. Second, mice had an intracaudate injection of 30 μL CD47 knockout blood with clodronate or control liposomes. Clodronate liposomes were also tested in saline-injected mice. All mice then had magnetic resonance imaging to measure hematoma size and brain swelling. Brains were used for immunohistochemistry and Western blot.

RESULTS

Erythrophagocytosis occurred in and around the hematoma. Injection of CD47 knockout blood resulted in quicker clot resolution, less brain swelling, and less neurological deficits compared with wild-type blood. Higher brain heme oxygenase-1 levels and more microglial activation (mostly M2 polarized microglia) at day 3 were found after CD47 knockout blood injection. Co-injection of clodronate liposomes, to deplete phagocytes, caused more severe brain swelling and less clot resolution.

CONCLUSIONS

These results indicated that CD47 has a key role in hematoma clearance after intracerebral hemorrhage.

Biological validation of bio-engineered red blood cell productions

The generation in vitro of cultured red blood cells (cRBC) could become an alternative to classical transfusion products. However, even when derived from healthy donors, the cRBC generated in vitro from hematopoietic stem cells may display alterations resulting from a poor controlled production process. In this context, we attempted to monitor the quality of the transfusion products arising from new biotechnologies. For that purpose, we developed an in vitro erythrophagocytosis (EP) test with the murine fibroblast cell line MS-5 and human macrophages (reference method). We evaluated 38 batches of cRBC, at the stage of reticulocyte, generated from CD34(+) cells isolated from placental blood or by leukapheresis. We showed that (i) the EP test performed with the MS-5 cell line was sensitive and can replace human macrophages for the evaluation of cultured cells. (ii) The EP tests revealed disparities among the batches of cRBC. (iii) The viability of the cells (determined by calcein-AM test), the expression of CD47 (antiphagocytosis receptor) and the externalization of phosphatidylserine (PS, marker of phagocytosis) were not critical parameters for the validation of the cRBC. (iv) Conversely, the cell deformability determined by ektacytometry was inversely correlated with the intensity of the phagocytic index. Assuming that the culture conditions directly influence the quality of the cell products generated, optimization of the production mode could benefit from the erythrophagocytosis test.

The effect of CD47 modified polymer surfaces on inflammatory cell attachment and activation

CD47 is a transmembrane protein that is a marker of "self". CD47 binding to its cognate receptor in leukocytes and macrophages, signal-regulatory protein alpha (SIRPα), causes inhibition of inflammatory cell attachment. We hypothesized that immobilization of recombinant CD47 on polymeric surfaces would reduce inflammation. Recombinant CD47 was appended to polyvinyl chloride (PVC) or polyurethane (PU) surfaces via photoactivation chemistry. Cell culture studies showed that CD47 immobilization significantly reduced human neutrophil (HL-60) and human monocyte derived macrophage (MDM) (THP-1) attachment to PVC and PU respectively. A neutralizing antibody, directed against SIRPα, inhibited THP-1 and HL-60 binding to PU and PVC surfaces respectively. This antibody also increased the level of SIRPα tyrosine phosphorylation, thereby indicating a direct role for SIRPα mediated signaling in preventing inflammatory cell attachment. Studies using human blood in an ex vivo flow-loop showed that CD47 modified PVC tubing significantly reduced cell binding and neutrophil activation compared to unmodified tubing or poly-2-methoxy-ethylacrylate (PMEA) coated tubing. In ten-week rat subdermal implants, CD47 functionalized PU films showed a significant reduction in markers of MDM mediated oxidative degradation compared to unmodified PU. In conclusion, CD47 functionalized surfaces can resist inflammatory cell interactions both in vitro and in vivo.

The CD47 pathway is deregulated in human immune thrombocytopenia

OBJECTIVE

A novel mechanism of platelet destruction involving the CD47/ signal regulatory protein-α (SIRPα) system has recently been suggested in a mouse model of immune thrombocytopenia (ITP). The CD47 molecule serves as ligand for SIRPα receptor and as receptor for thrombospondin acting as antagonistic to phagocyte activity and a regulator of apoptosis, respectively. In this study, we evaluated if the CD47/SIRPα axis may be involved in the apoptosis and clearance of platelets in human ITP.

MATERIALS AND METHODS

Using flow cytometry, we characterized whether expression of CD47 on fresh and in vitro-aged platelets- and of SIRPα receptor on CD14-derived dendritic cells (DCs), macrophages, circulating DCs, and monocytes is reduced is ITP; whether the in vitro platelet phagocytic capacity of CD14-derived DCs and macrophages is differentially modulated in the presence or absence of antibodies against CD47 and SIRPα in ITP; and whether platelets are more susceptible to the CD47-induced death signal in ITP.

RESULTS

We demonstrated that low platelet count in ITP is not due to increased phagocytosis associated with decreased expression of CD47 on the platelet surface and, despite reduced SIRPα expression, blockage of SIRPα on immature CD14-derived DCs or CD47 on platelets by specific antibodies failed to modify platelet uptake/phagocytosis of DCs. In contrast, targeting platelet CD47 with specific antibody significantly increases platelet phagocytosis of CD14-derived macrophages, and platelets are not healthy because they show increased apoptosis and are resistant to CD47-induced death signal.

CONCLUSIONS

Our results demonstrate that the CD47 pathway in ITP patients abnormally modulates platelet homeostasis.

Markers of platelet activation and apoptosis during storage of apheresis- and buffy coat-derived platelet concentrates for 7 days

BACKGROUND

The different production methods for platelet concentrates (PCs) result in products with variable in vitro quality and in vivo viability. The aim of this study was to compare in vitro variables of PCs produced by apheresis (AP-PC) or the buffy coat (BC-PC) method by applying a number of new and established assays.

STUDY DESIGN AND METHODS

Standard TRIMA Accel (Gambro BCT) AP-PCs (n = 20) and BC-PCs (n = 20) were stored in 100 percent plasma and changes in mitochondrial membrane potential (DeltaPsi(m)) were assessed using 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethlybenzimidazolcarbocyanine iodide (JC-1) dye on Days 1, 3, 5, and 7. The capacity of platelets (PLTs) for oxidative phosphorylation was also monitored by measuring oxygen consumption using a Clark-type electrode. PLT viability was measured using a new assay that utilizes the vital stains calcein-AM and FM4-64. Expression of phosphatidylserine (PS), CD42b, CD47, CD61, and CD62P was also assessed.

RESULTS

Although the JC-1 ratio (FL2/FL1) decreased significantly in both preparations, the percentage of PLTs with depolarized DeltaPsi(m) increased significantly in BC-PCs but not in AP-PCs. However, no significant change was detected in the PLTs' ability to consume oxygen in both preparations. PLTs in BC-PCs also showed significantly lower GPIb, CD47, and CD61 expression than AP-PCs on Day 1. PLTs in both preparations, however, showed a similar increase in CD62P and PS expression during storage, without significant loss of viability.

CONCLUSIONS

PLTs in AP-PCs and BC-PCs undergo different degrees of deterioration in mitochondrial integrity and thus may undergo different degrees of apoptosis. Interventions that maintain mitochondrial integrity may improve PLT viability in vitro.

Hemophagocytic macrophages harbor Salmonella enterica during persistent infection

Salmonella enterica subspecies can establish persistent, systemic infections in mammals, including human typhoid fever. Persistent S. enterica disease is characterized by an initial acute infection that develops into an asymptomatic chronic infection. During both the acute and persistent stages, the bacteria generally reside within professional phagocytes, usually macrophages. It is unclear how salmonellae can survive within macrophages, cells that evolved, in part, to destroy pathogens. Evidence is presented that during the establishment of persistent murine infection, macrophages that contain S. enterica serotype Typhimurium are hemophagocytic. Hemophagocytic macrophages are characterized by the ingestion of non-apoptotic cells of the hematopoietic lineage and are a clinical marker of typhoid fever as well as certain other infectious and genetic diseases. Cell culture assays were developed to evaluate bacterial survival in hemophagocytic macrophages. S. Typhimurium preferentially replicated in macrophages that pre-phagocytosed viable cells, but the bacteria were killed in macrophages that pre-phagocytosed beads or dead cells. These data suggest that during persistent infection hemophagocytic macrophages may provide S. Typhimurium with a survival niche.

Novel structural determinants on SIRP alpha that mediate binding to CD47

Signal regulatory proteins (SIRP-alpha, -beta, and -gamma) are important regulators of several innate immune functions that include leukocyte migration. Membrane distal (D1) domains of SIRPalpha and SIRPgamma, but not SIRPbeta, mediate binding to a cellular ligand termed CD47. Because the extracellular domains of all SIRPs are highly homologous, we hypothesized that some of the 16 residues unique to SIRPalpha.D1 mediate binding to CD47. By site-directed mutagenesis, we determined that SIRPalpha binding to CD47 is independent of N-glycosylation. We also identified three residues critical for CD47 binding by exchanging residues on SIRPalpha with corresponding residues from SIRPbeta. Cumulative substitutions of the critical residues into SIRPbeta resulted in de novo binding of the mutant protein to CD47. Homology modeling of SIRPalpha.D1 revealed topological relationships among critical residues and allowed the identification of critical residues common to SIRPalpha and SIRPbeta. Mapping these critical residues onto the recently reported crystal structure of SIRPalpha.D1 revealed a novel region that is required for CD47 binding and is distinct and lateral to another putative CD47 binding site described on that crystal structure. The importance of this lateral region in mediating SIRPalpha.D1 binding to CD47 was confirmed by epitope mapping analyses of anti-SIRP Abs. These observations highlight a complex nature of the ligand binding requirements for SIRPalpha that appear to be dependent on two distinct but adjacent regions on the membrane distal Ig loop. A better understanding of the structural basis of SIRPalpha/CD47 interactions may provide insights into therapeutics targeting pathologic inflammation.

Non-MHC ligands for inhibitory immune receptors: novel insights and implications for immune regulation

Regulation of cellular responses by inhibitory receptors is crucial for proper function of the immune system. The prototype inhibitory immune receptors are major histocompatibility complex (MHC) class I binding killer-Ig like receptors (KIRs) present on effector cells such as natural killer (NK) cells and effector T cells. However, the recent identification of non-MHC class I ligands for inhibitory immune receptors, such as KLRG1, KLRB1 and LAIR-1, indicates that also MHC class I-independent inhibitory immune receptors play crucial roles in inducing peripheral tolerance. The presence of these receptors on many other immune cell types besides effector cells suggests that tight regulation of cell activation is necessary in all facets of the immune response in both normal and diseased tissue. Here, we review novel insights and implications of non-MHC class I ligand binding to inhibitory immune receptors. We give an overview of the known ligand-receptor pairs by grouping the ligands according to their properties and discuss implications of these interactions for the maintenance of immune balance and for the defense against tumors and pathogens.

Erythrophagocytosis in sickle cell anemia: statistical evidence for a biological phenomenon

The precise role of erythrophagocytosis in sickle cell disease is not known. Using hematological data from three studies and 791 subjects comprising of eight epidemiological groups, we found a strong statistical support for the hypothesis that erythrophagocytosis is increased in sickle cell trait, that neutrophils and lymphocytes are the most likely cells involved in erythrophagocytosis in these subjects and that increased erythrophagocytosis may for a mechanistic explanation for an increased risk of vaso-occlusive crisis in sickle cell trait. Statistically, erythrophagocytosis was not increased in subjects with homozygous sickle cell disease. Our findings offer an interesting mechanistic implication about the presence of a strong autoimmune component of sickle cell trait that can be explained by the well recognized interplay between the receptor molecule signal regulatory protein-alpha (SIRP-alpha) on the phagocyte and its ligand, CD47, on the red blood cell. Our findings also support further and closer evaluation of the other hypothesized mechanisms by which neutrophils and lymphocytes partake in differential degree of erythrophagocytosis in subjects heterozygous for the sickle hemoglobin. Finally, translation of these findings into a clinical realm suggests that the extent of erythrophagocytosis, as measured by peripheral blood hematological indicators, can serve as an important indicator of the likelihood of future vaso-occlusive crisis events in subjects of sickle cell trait.