Negative regulation of phagocytosis in murine macrophages by the Src kinase family member, Fgr

Aging-induced Alternation in the Gut Microbiota Impairs Host Antibacterial Defense

Older individuals experience increased susceptibility and mortality to bacterial infections, but the underlying etiology remains unclear. Herein, it is shown that aging-associated reduction of commensal Parabacteroides goldsteinii (P. goldsteinii) in both aged mice and humans critically contributes to worse outcomes of bacterial infection. The colonization of live P. goldsteinii conferred protection against aging-associated bacterial infections. Metabolomic profiling reveals a protective compound, apigenin, generated by P. goldsteinii, antagonizes bacterial clearance defects in aged mice. AMP-binding protein (ampB) is identified as a key gene involved in apigenin synthesis in P. goldsteinii using homologous recombination in bacteria. Mechanistically, apigenin binds directly to the potential sites on Fgr (M341 and D404), preventing its inhibitory role on Vav1 phosphorylation, and therefore promoting the activation of Cdc42/Rac1, Arp2/3 expression and subsequent actin reorganization, which contributes to the enhanced phagocytosis of macrophages to bacteria. Collectively, the findings suggest that dysbiosis of the gut microbiota may impair host defense mechanisms and increase susceptibility to bacterial infections in older adults and highlight the microbiota-apigenin-Fgr axis as a possible route to ameliorate aging-associated antibacterial defects.

Spatial transcriptomic analysis identifies epithelium-macrophage crosstalk in endometriotic lesions

The mechanisms underlying the pathophysiology of endometriosis, characterized by the presence of endometrium-like tissue outside the uterus, remain poorly understood. This study aimed to identify cell type-specific gene expression changes in superficial peritoneal endometriotic lesions and elucidate the crosstalk among the stroma, epithelium, and macrophages compared to patient-matched eutopic endometrium. Surprisingly, comparison between lesions and eutopic endometrium revealed transcriptional similarities, indicating minimal alterations in the sub-epithelial stroma and epithelium of lesions. Spatial transcriptomics highlighted increased signaling between the lesion epithelium and macrophages, emphasizing the role of the epithelium in driving lesion inflammation. We propose that the superficial endometriotic lesion epithelium orchestrates inflammatory signaling and promotes a pro-repair phenotype in macrophages, providing a new role for complement 3 in lesion pathobiology. This study underscores the significance of considering spatial context and cellular interactions in uncovering mechanisms governing disease in endometriotic lesions.

Nanolevel Immunomodulators in Sepsis: Novel Roles, Current Perspectives, and Future Directions

Sepsis represents a profound challenge in critical care, characterized by a severe systemic inflammatory response which can lead to multi-organ failure and death. The intricate pathophysiology of sepsis involves an overwhelming immune reaction that disrupts normal host defense mechanisms, necessitating innovative approaches to modulation. Nanoscale immunomodulators, with their precision targeting and controlled release capabilities, have emerged as a potent solution to recalibrate immune responses in sepsis. This review explores the recent advancements in nanotechnology for sepsis management, emphasizing the integration of nanoparticulate systems to modulate immune function and inflammatory pathways. Discussions detail the development of the immune system, the distinct inflammatory responses triggered by sepsis, and the scientific principles underpinning nanoscale immunomodulation, including specific targeting mechanisms and delivery systems. The review highlights nanoformulation designs aimed at enhancing bioavailability, stability, and therapeutic efficacy, which shows promise in clinical settings by modulating key inflammatory pathways. Ultimately, this review synthesizes the current state of knowledge and projects future directions for research, underscoring the transformative potential of nanolevel immunomodulators for sepsis treatment through innovative technologies and therapeutic strategies.

A pan-allelic human SIRPα-blocking antibody, ES004-B5, promotes tumor killing by enhancing macrophage phagocytosis and subsequently inducing an effective T-cell response

As a major immune cell type in the tumor microenvironment, tumor-associated macrophages secrete suppressive factors that can inhibit antitumor immunity and promote tumor progression. One approach trying to utilize macrophages for immunotherapy has been to block the CD47-SIRPα axis, which mediates inhibitory signaling, to promote phagocytosis of tumor cells. Many CD47-targeted agents, namely, anti-CD47 antibodies and SIRPα fusion proteins, were associated with a diverse spectrum of toxicities that limit their use in clinical settings. Universal expression of CD47 also leads to a severe "antigen sink" effect of CD47-targeted agents. Given that the CD47 receptor, SIRPα, has a more restricted expression profile and may have CD47-independent functions, targeting SIRPα is considered to have distinct advantages in improving clinical efficacy with a better safety profile. We have developed ES004-B5, a potentially best-in-class pan-allelic human SIRPα-blocking antibody using hybridoma technology. ES004-B5 binds to major human SIRPα variants through a unique epitope with high affinity. By blocking CD47-induced inhibitory "don't-eat-me" signaling, ES004-B5 exerts superior antitumor activity in combination with anti-tumor-associated antigen antibodies in vitro and in vivo. Unlike CD47-targeted agents, ES004-B5 exhibits an excellent safety profile in nonhuman primates. ES004-B5 has potential to be an important backbone for SIRPα-based combination therapy and/or bispecific antibodies, which will likely overcome the limitations of CD47-targeted agents encountered in clinical settings.

Spatial Transcriptomic Analysis Identifies Epithelium-Macrophage Crosstalk in Endometriotic Lesions

The mechanisms underlying the pathophysiology of endometriosis, characterized by the presence of endometrium-like tissue outside the uterus, remain poorly understood. This study aimed to identify cell type-specific gene expression changes in superficial peritoneal endometriotic lesions and elucidate the crosstalk among the stroma, epithelium, and macrophages compared to patient-matched eutopic endometrium. Surprisingly, comparison between lesions and eutopic endometrium revealed transcriptional similarities, indicating minimal alterations in the sub-epithelial stroma and epithelium of lesions. Spatial transcriptomics highlighted increased signaling between the lesion epithelium and macrophages, emphasizing the role of the epithelium in driving lesion inflammation. We propose that the superficial endometriotic lesion epithelium orchestrates inflammatory signaling and promotes a pro-repair phenotype in macrophages, providing a new role for Complement 3 in lesion pathobiology. This study underscores the significance of considering spatial context and cellular interactions in uncovering mechanisms governing disease in endometriotic lesions.

MARCKS Inhibition Alters Bovine Neutrophil Responses to Salmonella Typhimurium

Neutrophils are innate immune cells that respond quickly to sites of bacterial infection and play an essential role in host defense. Interestingly, some bacterial pathogens benefit from exuberant neutrophil inflammation. Salmonella is one such pathogen that can utilize the toxic mediators released by neutrophils to colonize the intestine and cause enterocolitis. Because neutrophils can aid gut colonization during Salmonella infection, neutrophils represent a potential host-directed therapeutic target. Myristoylated alanine-rich C-kinase substrate (MARCKS) is an actin-binding protein that plays an essential role in many neutrophil effector responses. We hypothesized that inhibition of MARCKS protein would alter bovine neutrophil responses to Salmonella Typhimurium (STm) ex vivo. We used a MARCKS inhibitor peptide to investigate the role of MARCKS in neutrophil responses to STm. This study demonstrates that MARCKS inhibition attenuated STm-induced neutrophil adhesion and chemotaxis. Interestingly, MARCKS inhibition also enhanced neutrophil phagocytosis and respiratory burst in response to STm. This is the first report describing the role of MARCKS protein in neutrophil antibacterial responses.

A mAb to SIRPα downregulates the priming of naive CD4 + T cell in Primary immune thrombocytopenia

SIRPα is a transmembrane protein that binds the protein tyrosine phosphatases SHP-1 and SHP-2 through its cytoplasmic region and is abundantly expressed on monocytes, dendritic cells, and macrophages. Studies recently showed that SIRPα is essential for priming of CD4 + T cells by DCs and for development of Th17 cell-mediated autoimmune diseases. We have now further evaluated the importance of SIRPα and that of its ligand CD47 in primary immune thrombocytopenia (ITP). In this study, we show that there was a low expression state of SIRPα on the surface of monocytes. Treatment of cells culture from ITP patients with a mAb to SIRPα that blocks the binding of SIRPα to CD47 downregulated the ITP response. The abilities of monocytes from ITP patients to stimulate an allogenic MLR were reduced. The proliferation of, and production of IL-2, by CD4 + T cells from ITP patients were inhibited, the Treg cell numbers and the production of IL-10 pairs were upregulated, and the production of TGF-β not was inhibited, by a mAb to SIRPα. Moreover, a mAb to SIRPα, the expression of HLA-DR and CD86 were markedly inhibited and the expression of CD80 was slightly upregulated, on the surface of CD14 + monocytes from ITP patients as compared with healthy subjects. However, blockade of SIRPα increased the secretion of TLR-dependent cytokines TNF-α, IL-6 and IL-1β by PBMCs, which may be considered as a reserve in response to danger signals. These results suggest that SIRPα on monocytes is essential for the priming of naive T cells and the development of ITP. Therefore, SIRPα is a potential therapeutic target for ITP and other autoimmune diseases.

Integrated Single-Cell and Transcriptome Sequencing Analyses Identify Dipeptidase 2 as an Immune-Associated Prognostic Biomarker for Lung Adenocarcinoma

Dipeptidase 2 (DPEP2) is a dipeptidyl peptidase that plays an important role in the hydrolysis of leukotriene D4 (LTD4) to leukotriene E4 (LTE4). Previous studies have suggested that LTD4 promotes tumor progression and survival in non-small cell lung cancer (NSCLC). Therefore, we hypothesized that DPEP2 may play a pivotal role in this tumor. Given that lung adenocarcinoma (LUAD) is the most common subtype of NSCLC, our study aimed to examine the expression and function of DPEP2 in LUAD. Based on bioinformatics and the analysis of clinical samples, our findings revealed that DPEP2 is highly expressed in normal lung tissues, but downregulated in LUAD tissues, and its expression levels were significantly associated with clinical indicators of tumor grade and prognosis. Pathway enrichment analysis showed that DPEP2 is involved in biological processes such as chemokine signaling pathways, leukocyte trans-endothelial migration, and humoral immune responses in LUAD. In addition, DPEP2 expression was significantly associated with various immune cells, especially monocytes-macrophages. Single-cell transcriptome data further confirmed the expression of DPEP2 dominantly in macrophages from normal lung tissues. Analysis of the TCIA database revealed that high DPEP2 expression is associated with a stronger response to immune checkpoint inhibitors such as CTLA4 and PD1, and determines sensitivity to LUAD therapeutic agents. Furthermore, we found that DPEP2 inhibits the migration and invasion of LUAD cells. Therefore, DPEP2 may serve as a potential immune biomarker and therapeutic target for LUAD, providing novel therapeutic approaches for this disease.

Activation of immune signals during organ transplantation

The activation of host's innate and adaptive immune systems can lead to acute and chronic graft rejection, which seriously impacts graft survival. Thus, it is particularly significant to clarify the immune signals, which are critical to the initiation and maintenance of rejection generated after transplantation. The initiation of response to graft is dependent on sensing of danger and stranger molecules. The ischemia and reperfusion of grafts lead to cell stress or death, followed by releasing a variety of damage-associated molecular patterns (DAMPs), which are recognized by pattern recognition receptors (PRRs) of host immune cells to activate intracellular immune signals and induce sterile inflammation. In addition to DAMPs, the graft exposed to 'non-self' antigens (stranger molecules) are recognized by the host immune system, stimulating a more intense immune response and further aggravating the graft damage. The polymorphism of MHC genes between different individuals is the key for host or donor immune cells to identify heterologous 'non-self' components in allogeneic and xenogeneic organ transplantation. The recognition of 'non-self' antigen by immune cells mediates the activation of immune signals between donor and host, resulting in adaptive memory immunity and innate trained immunity to the graft, which poses a challenge to the long-term survival of the graft. This review focuses on innate and adaptive immune cells receptor recognition of damage-associated molecular patterns, alloantigens and xenoantigens, which is described as danger model and stranger model. In this review, we also discuss the innate trained immunity in organ transplantation.

Nanomaterials targeting macrophages in sepsis: A promising approach for sepsis management

Sepsis is a life-threatening organ dysfunction resulting from dysregulated host responses to infection. Macrophages play significant roles in host against pathogens and the immunopathogenesis of sepsis, such as phagocytosis of pathogens, secretion of cytokines, and phenotype reprogramming. However, the rapid progression of sepsis impairs macrophage function, and conventional antimicrobial and supportive treatment are not sufficient to restore dysregulated macrophages roles. Nanoparticles own unique physicochemical properties, surface functions, localized surface plasmon resonance phenomenon, passive targeting in vivo, good biocompatibility and biodegradability, are accessible for biomedical applications. Once into the body, NPs are recognized by host immune system. Macrophages are phagocytes in innate immunity dedicated to the recognition of foreign substances, including nanoparticles, with which an immune response subsequently occurs. Various design strategies, such as surface functionalization, have been implemented to manipulate the recognition of nanoparticles by monocytes/macrophages, and engulfed by them to regulate their function in sepsis, compensating for the shortcomings of sepsis traditional methods. The review summarizes the mechanism of nanomaterials targeting macrophages and recent advances in nanomedicine targeting macrophages in sepsis, which provides good insight for exploring macrophage-based nano-management in sepsis.

Pro-efferocytic macrophage membrane biomimetic nanoparticles for the synergistic treatment of atherosclerosis via competition effect

Macrophages participate in many links in the pathological process of atherosclerosis (AS) and the regulation of influence of macrophages at the molecular level might be a new avenue for AS treatment. For this aim, the macrophage membrane biomimetic nanoparticles, derived from macrophage membrane coated SHP1i-loaded liposome NPs (MM@Lips-SHP1i) was designed. Due to the reservation of intrinsic membrane proteins and function from macrophages, the biomimic nanoparticles could effectively evade clearance by the immune system, prolong blood circulation time and actively tend and aggregate to atherosclerotic plaques. More importantly, in the plaque area, MM@Lips-SHP1i nanoparticles could compete with macrophages in vivo to bind with oxidized low-density lipoprotein (oxLDL) and lipopolysaccharide (LPS), reduce uptake of new lipids by macrophages, reduce foam cell formation, and inhibit the expression of pro-inflammatory cytokines. In addition, small molecule inhibitor of SHP-1, the downstream effector molecule of CD47 loaded in macrophage membrane biomimetic nanoparticles could interrupt CD47-SIRPα signal transduction in monocytes and macrophages, thereby enhancing the efferocytosis of macrophages, inhibiting the progression of plaque, achieving synergistic treatment of atherosclerosis. This work focuses on the key process in the formation of AS, macrophage foaming and chronic inflammation, and is based on the fact that macrophage membrane biomimetic nanoparticles can preserve the key surface proteins of macrophages closely related to the formation of AS, providing a new avenue to inhibit the progression of AS by utilizing the biological characteristics of macrophage membrane in macrophage membrane biomimetic nanoparticles.

Upregulation of programmed cell death 1 by interferon gamma and its biological functions in human monocytes

Programmed cell death 1 (PD-1) is a co-inhibitory checkpoint receptor expressed in various immune cells, especially in activated T cells. Engagement of PD-1 with its ligand leads to the exhausted T cells and impaired antitumor immunity. To date, PD-1 expression and its roles have been widely reported in T cells but not well defined in innate immune cells including monocytes. In this study, expression of PD-1 was investigated in human monocytes. Here we observed that among cytokines tested, IFN-γ significantly upregulated the PD-1 expression in both THP-1 cell line and human primary monocytes in a dose- and time-dependent manner. This effect was reduced by PI3K inhibitor, suggesting that the involvement of PI3K/AKT pathway. Furthermore, enrichment of active histone mark H3K4me3 in the Pdcd1 promotor was also observed in IFN-γ-induced THP-1, indicating that epigenetic regulation also plays a role in IFN-γ-induced PD-1 expression. To investigate the biological functions of PD-1, Pdcd1 was deleted in THP-1 cell line by CRISPR/Cas9 system and the phagocytic ability was investigated. The results showed that the PD-1 deficiency in THP-1 cell line resulted in significantly poor phagocytic potency against carboxylated-modified latex beads. Moreover, the PD-1 deficiency or blocking PD-1/PD-L1 interaction by immune checkpoint inhibitor resulted in an impaired induction of IL-4-induced CD163 expression in THP-1 cell line. Taken together, these results highlighted the importance of PD-1 expression in some of key monocyte functions.

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Interferon gamma treatment induces PD-1 upregulation in human monocytes.

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PI3K/AKT pathway is crucial for IFN-γ-induced PD-1 expression.

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Active histone mark H3K4me3 in Pdcd1 promoter accompanies IFN-γ treatment.

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PD-1 knockout in THP-1 cell line impairs phagocytosis and M2 polarization.

The Impaired Mechanism and Facilitated Therapies of Efferocytosis in Atherosclerosis

ABSTRACT

Cardiovascular disease is responsible for the largest number of deaths worldwide, and atherosclerosis is the primary cause. Apoptotic cell accumulation in atherosclerotic plaques leads to necrotic core formation and plaque rupture. Emerging findings show that the progression of atherosclerosis appears to suppress the elimination of apoptotic cells. Mechanistically, the reduced edibility of apoptotic cells, insufficient phagocytic capacity of phagocytes, downregulation of bridging molecules, and dysfunction in the polarization of macrophages lead to impaired efferocytosis in atherosclerotic plaques. This review focuses on the characteristics of efferocytosis in plaques and the therapeutic strategies aimed at promoting efferocytosis in atherosclerosis, which would provide novel insights for the development of antiatherosclerotic drugs based on efferocytosis.

Inflammasomes: a preclinical assessment of targeting in atherosclerosis

INTRODUCTION

Inflammasomes are central to atherosclerotic vascular dysfunction with regulatory effects on inflammation, immune modulation, and lipid metabolism. The NLRP3 inflammasome is a critical catalyst for atherogenesis thus highlighting its importance in understanding the pathophysiology of atherosclerosis and for the identification of novel therapeutic targets and biomarkers for the treatment of cardiovascular disease.

AREAS COVERED

This review includes an overview of macrophage lipid metabolism and the role of NLRP3 inflammasome activity in cardiovascular inflammation and atherosclerosis. We highlight key activators, signal transducers and major regulatory components that are being considered as putative therapeutic targets for inhibition of NLRP3-mediated cardiovascular inflammation and atherosclerosis.

EXPERT OPINION

NLRP3 inflammasome activity lies at the nexus between inflammation and cholesterol metabolism; it offers unique opportunities for understanding atherosclerotic pathophysiology and identifying novel modes of treatment. As such, a host of NLRP3 signaling cascade components have been identified as putative targets for drug development. We catalog these current discoveries in therapeutic targeting of the NLRP3 inflammasome and, utilizing the CANTOS trial as the translational (bench-to-bedside) archetype, we examine the complexities, challenges, and ultimate goals facing the field of atherosclerosis research.

Pro-efferocytic nanoparticles are specifically taken up by lesional macrophages and prevent atherosclerosis

Atherosclerosis is the process that underlies heart attack and stroke. A characteristic feature of the atherosclerotic plaque is the accumulation of apoptotic cells in the necrotic core. Prophagocytic antibody-based therapies are currently being explored to stimulate the phagocytic clearance of apoptotic cells; however, these therapies can cause off-target clearance of healthy tissues, which leads to toxicities such as anaemia. Here we developed a macrophage-specific nanotherapy based on single-walled carbon nanotubes loaded with a chemical inhibitor of the antiphagocytic CD47-SIRPα signalling axis. We demonstrate that these single-walled carbon nanotubes accumulate within the atherosclerotic plaque, reactivate lesional phagocytosis and reduce the plaque burden in atheroprone apolipoprotein-E-deficient mice without compromising safety, and thereby overcome a key translational barrier for this class of drugs. Single-cell RNA sequencing analysis reveals that prophagocytic single-walled carbon nanotubes decrease the expression of inflammatory genes linked to cytokine and chemokine pathways in lesional macrophages, which demonstrates the potential of 'Trojan horse' nanoparticles to prevent atherosclerotic cardiovascular disease.

The Role of Macrophages in Neuroinflammatory and Neurodegenerative Pathways of Alzheimer's Disease, Amyotrophic Lateral Sclerosis, and Multiple Sclerosis: Pathogenetic Cellular Effectors and Potential Therapeutic Targets

In physiological conditions, different types of macrophages can be found within the central nervous system (CNS), i.e., microglia, meningeal macrophages, and perivascular (blood-brain barrier) and choroid plexus (blood-cerebrospinal fluid barrier) macrophages. Microglia and tissue-resident macrophages, as well as blood-borne monocytes, have different origins, as the former derive from yolk sac erythromyeloid precursors and the latter from the fetal liver or bone marrow. Accordingly, specific phenotypic patterns characterize each population. These cells function to maintain homeostasis and are directly involved in the development and resolution of neuroinflammatory processes. Also, following inflammation, circulating monocytes can be recruited and enter the CNS, therefore contributing to brain pathology. These cell populations have now been identified as key players in CNS pathology, including autoimmune diseases, such as multiple sclerosis, and degenerative diseases, such as Amyotrophic Lateral Sclerosis and Alzheimer’s disease. Here, we review the evidence on the involvement of CNS macrophages in neuroinflammation and the advantages, pitfalls, and translational opportunities of pharmacological interventions targeting these heterogeneous cellular populations for the treatment of brain diseases.

CNS inflammation and neurodegeneration

There is an increasing recognition that inflammation plays a critical role in neurodegenerative diseases of the CNS, including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and the prototypic neuroinflammatory disease multiple sclerosis (MS). Differential immune responses involving the adaptive versus the innate immune system are observed at various stages of neurodegenerative diseases, and may not only drive disease processes but could serve as therapeutic targets. Ongoing investigations into the specific inflammatory mechanisms that play roles in disease causation and progression have revealed lessons about inflammation-driven neurodegeneration that can be applied to other neurodegenerative diseases. An increasing number of immunotherapeutic strategies that have been successful in MS are now being applied to other neurodegenerative diseases. Some approaches suppress CNS immune mechanisms, while others harness the immune system to clear deleterious products and cells. This Review focuses on the mechanisms by which inflammation, mediated either by the peripheral immune response or by endogenous CNS immune mechanisms, can affect CNS neurodegeneration.

CD47 Plays a Role as a Negative Regulator in Inducing Protective Immune Responses to Vaccination against Influenza Virus

An integrin-associated protein CD47, which is a ligand for the inhibitory receptor signal regulatory protein α, is expressed on B and T cells, as well as on most innate immune cells. However, the roles of CD47 in the immune responses to viral infection or vaccination remain unknown. We investigated the role of CD47 in inducing humoral immune responses after intranasal infection with virus or immunization with influenza virus-like particles (VLPs). Virus infection or vaccination with VLPs containing hemagglutinin from A/PR8/34 influenza virus induced higher levels of antigen-specific IgG2c isotype dominant antibodies in CD47-deficient (CD47KO) mice than in wild-type (WT) mice. CD47KO mice with vaccination showed greater protective efficacy against lethal challenge, as evidenced by no loss in body weight and reduced lung viral titers compared to WT mice. In addition, inflammatory responses which include cytokine production, leukocyte infiltrates, and gamma interferon-producing CD4(+) T cells, as well as an anti-inflammatory cytokine (interleukin-10), were reduced in the lungs of vaccinated CD47KO mice after challenge with influenza virus. Analysis of lymphocytes indicated that GL7(+) germinal center B cells were induced at higher levels in the draining lymph nodes of CD47KO mice compared to those in WT mice. Notably, CD47KO mice exhibited significant increases in the numbers of antigen-specific memory B cells in spleens and plasma cells in bone marrow despite their lower levels of background IgG antibodies. These results suggest that CD47 plays a role as a negative regulator in inducing protective immune responses to influenza vaccination.

IMPORTANCE

Molecular mechanisms that control B cell activation to produce protective antibodies upon viral vaccination remain poorly understood. The CD47 molecule is known to be a ligand for the inhibitory receptor signal regulatory protein α and expressed on the surfaces of most immune cell types. CD47 was previously demonstrated to play an important role in modulating the migration of monocytes, neutrophils, polymorphonuclear neutrophils, and dendritic cells into the inflamed tissues. The results of this study demonstrate new roles of CD47 in negatively regulating the induction of protective IgG antibodies, germinal center B cells, and plasma cells secreting antigen-specific antibodies, as well as macrophages, upon influenza vaccination and challenge. As a consequence, vaccinated CD47-deficient mice demonstrated better control of influenza viral infection and enhanced protection. This study provides insights into understanding the regulatory functions of CD47 in inducing adaptive immunity to vaccination.

Innate sensing of oxidation-specific epitopes in health and disease

Ageing, infections and inflammation result in oxidative stress that can irreversibly damage cellular structures. The oxidative damage of lipids in membranes or lipoproteins is one of these deleterious consequences that not only alters lipid function but also leads to the formation of neo-self epitopes - oxidation-specific epitopes (OSEs) - which are present on dying cells and damaged proteins. OSEs represent endogenous damage-associated molecular patterns that are recognized by pattern recognition receptors and the proteins of the innate immune system, and thereby enable the host to sense and remove dangerous biological waste and to maintain homeostasis. If this system is dysfunctional or overwhelmed, the accumulation of OSEs can trigger chronic inflammation and the development of diseases, such as atherosclerosis and age-related macular degeneration. Understanding the molecular components and mechanisms that are involved in this process will help to identify individuals with an increased risk of developing chronic inflammation, and will also help to indicate novel modes of therapeutic intervention.

SIRP/CD47 signaling in neurological disorders

Microglia play important roles in the process of neuronal injury and recovery. Numeous surface receptors have been described to regulate microglial activation. These receptors tightly mediate normal microglial functions including cell mobility, phagocytosis, and production of inflammatory mediators or trophic factors. In recent years, significant progresses have been achieved for understanding the signaling mechanisms underlying these receptors. Their specific roles in neurological diseases have been documented. This review will focus on the signal regulatory protein (SIRP) and its ligand CD47, two surface receptors expressed on microglia and other cells in the central nervous system (CNS) such as neurons. We will discuss the involvement of SIRP/CD47 signaling in microglial activation and in the interplay between microglia and other CNS cells. Current studies reveal the importance of CD47 and SIRPα in the process of neuroinflammation in the CNS disorders. The dual and contradictory role of CD47 suggests that targeting the SIRPα/CD47 signaling may achieve different effects depending on disease stage. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

Neurobiology of microglial action in CNS injuries: receptor-mediated signaling mechanisms and functional roles

Microglia are the first line of immune defense against central nervous system (CNS) injuries and disorders. These highly plastic cells play dualistic roles in neuronal injury and recovery and are known for their ability to assume diverse phenotypes. A broad range of surface receptors are expressed on microglia and mediate microglial 'On' or 'Off' responses to signals from other host cells as well as invading microorganisms. The integrated actions of these receptors result in tightly regulated biological functions, including cell mobility, phagocytosis, the induction of acquired immunity, and trophic factor/inflammatory mediator release. Over the last few years, significant advances have been made toward deciphering the signaling mechanisms related to these receptors and their specific cellular functions. In this review, we describe the current state of knowledge of the surface receptors involved in microglial activation, with an emphasis on their engagement of distinct functional programs and their roles in CNS injuries. It will become evident from this review that microglial homeostasis is carefully maintained by multiple counterbalanced strategies, including, but not limited to, 'On' and 'Off' receptor signaling. Specific regulation of theses microglial receptors may be a promising therapeutic strategy against CNS injuries.

The cellular prion protein negatively regulates phagocytosis and cytokine expression in murine bone marrow-derived macrophages

The cellular prion protein (PrP^C) is a glycosylphosphatidylinositol (GPI)-anchored glycoprotein on the cell surface. Previous studies have demonstrated contradictory roles for PrP^C in connection with the phagocytic ability of macrophages. In the present work, we investigated the function of PrP^C in phagocytosis and cytokine expression in bone marrow-derived macrophages infected with Escherichia coli. E. coli infection induced an increase in the PRNP mRNA level. Knockout of PrP^C promoted bacterial uptake; upregulated Rab5, Rab7, and Eea1 mRNA expression; and increased the recruitment of lysosomal-associated membrane protein-2 to phagosomes, suggesting enhanced microbicidal activity. Remarkably, knockout of PrP^C suppressed the proliferation of internalized bacteria and increased the expression of cytokines such as interleukin-1β. Collectively, our data reveal an important role of PrP^C as a negative regulator for phagocytosis, phagosome maturation, cytokine expression, and macrophage microbicidal activity.

The involvement of MiR-1-clathrin pathway in the regulation of phagocytosis

Phagocytosis, one of the most powerful immune responses, is a complicated process regulated by many factors. However the regulation of phagocytosis mediated by microRNAs has not been extensively investigated. To address this issue, the regulation of phagocytosis by miR-1 was characterized in this study. The results showed that miR-1 played an important role in the phagocytosis regulation in shrimp in vivo. The sequence analysis indicated that miR-1 was highly conserved from invertebrates to mammals, suggesting that miR-1 might share the similar or same functions in phagocytosis of shrimp hemocytes and mammalian macrophages. The data presented that miR-1 was significantly downregulated in cancerous macrophage RAW264.7 cells compared with those in the isolated murine macrophage and in the immortalized macrophage ANA-1. The findings showed that miR-1 had a great effect on the regulation of phagocytosis in cancerous macrophage by the inhibition of clathrin heavy chain 1 (CLTC1) gene. Therefore our study presented a novel miR-1-mediated regulation of phagocytosis both in invertebrate and in vertebrate.

Thrombospondin-1 activation of signal-regulatory protein-α stimulates reactive oxygen species production and promotes renal ischemia reperfusion injury

Ischemia reperfusion injury (IRI) causes tissue and organ injury, in part, through alterations in tissue blood flow and the production of reactive oxygen species. The cell surface receptor signal-regulatory protein-α (SIRP-α) is expressed on inflammatory cells and suppresses phagocytosis, but the function of SIRP-α in IRI has not been determined. We reported previously that the matricellular protein thrombospondin-1 is upregulated in IRI. Here, we report a novel interaction between thrombospondin-1 and SIRP-α on nonphagocytic cells. In cell-free experiments, thrombospondin-1 bound SIRP-α. In vascular smooth muscle cells and renal tubular epithelial cells, treatment with thrombospondin-1 led to phosphorylation of SIRP-α and downstream activation of Src homology domain 2-containing phosphatase-1. Thrombospondin-1 also stimulated phosphorylation of p47(phox) (an organizer subunit for nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 1/2) and increased production of superoxide, both of which were abrogated by knockdown or antibody blockade of SIRP-α. In rodent aortic rings, treatment with thrombospondin-1 increased the production of superoxide and inhibited nitric oxide-mediated vasodilation in a SIRP-α-dependent manner. Renal IRI upregulated the thrombospondin-1-SIRP-α signaling axis and was associated with increased superoxide production and cell death. A SIRP-α antibody that blocks thrombospondin-1 activation of SIRP-α mitigated the effects of renal IRI, increasing blood flow, suppressing production of reactive oxygen species, and preserving cellular architecture. A role for CD47 in SIRP-α activation in these pathways is also described. Overall, these results suggest that thrombospondin-1 binding to SIRP-α on nonphagocytic cells activates NADPH oxidase, limits vasodilation, and promotes renal IRI.

Human complement Factor H modulates C1q-mediated phagocytosis of apoptotic cells

Complement is implicated in the clearance of apoptotic cells by phagocytes. Deficiencies in early complement components, particularly C1q, are associated with an increased risk of the development of systemic lupus erythematosus. C1q is considered to be important in this process through interaction with apoptotic cells and phagocytes. In the present study, we confirm that apoptotic cells are recognized not only by C1q but also by the complement regulatory protein Factor H. Both C1q and Factor H bind to apoptotic cells in a dose-dependent and saturable manner. We further examined the role of C1q and Factor H in the clearance of apoptotic cells by monocytes. C1q enhanced uptake/adhesion of apoptotic cells by monocytes whereas Factor H alone had no effect on this process. However, when both C1q and Factor H were present on the apoptotic cell surface, C1q-mediated enhancement of uptake/adhesion of the apoptotic cells by monocytes was reduced. This effect of Factor H also occurred if monocytes were pre-treated with Factor H, and then exposed to C1q-coated apoptotic cells. The results were consistent with Factor H interacting with monocytes through the integrin CD11b/CD18. We conclude that under physiological conditions, Factor H may be important in controlling the inflammation which might arise from C1q deposition on apoptotic cells.

Factor H as a regulator of the classical pathway activation

C1q, the first subcomponent of the classical pathway, is a charge pattern recognition molecule that binds a diverse range of self, non-self and altered self ligands, leading to pro-inflammatory complement activation. Although complement is required for tissue homeostasis as well as defence against pathogens, exaggerated complement activation can be damaging to the tissue. Therefore, a fine balance between complement activation and inhibition is necessary. We have recently found that factor H, a polyanion recognition molecule and soluble regulator of alternative pathway activation in blood and on cell surfaces, can directly compete with C1q in binding to anionic phospholipids (cardiolipin), lipid A and Escherichia coli (three known activators of the classical pathway) and acts as a direct down regulator of the complement classical pathway. This ability of factor H to dampen classical pathway activation is distinct from its role as an alternative pathway down-regulator. Thus, by directly competing for specific C1q ligands (exogenous as well as endogenous), factor H is likely to be involved in fine-tuning and balancing the C1q-driven inflammatory processes in autoimmunity and infection. However, in the case of apoptotic cells, C1q-mediated enhancement of uptake/adhesion of the apoptotic cells by monocytes was reduced by factor H. Thus, factor H may be important in controlling the inflammation, which might arise from C1q deposition on apoptotic cells.

Regulation of tyrosine phosphorylation in macrophage phagocytosis and chemotaxis

Macrophages display a large variety of surface receptors that are critical for their normal cellular functions in host defense, including finding sites of infection (chemotaxis) and removing foreign particles (phagocytosis). However, inappropriate regulation of these processes can lead to human diseases. Many of these receptors utilize tyrosine phosphorylation cascades to initiate and terminate signals leading to cell migration and clearance of infection. Actin remodeling dominates these processes and many regulators have been identified. This review focuses on how tyrosine kinases and phosphatases regulate actin dynamics leading to macrophage chemotaxis and phagocytosis.

Functional roles of Src and Fgr in ovarian carcinoma

PURPOSE

Src is an attractive target because it is overexpressed in a number of malignancies, including ovarian cancer. However, the effect of Src silencing on other Src family kinases (SFKs) is not known. We hypothesized that other SFK members could compensate for the lack of Src activity.

EXPERIMENTAL DESIGN

Cell viability after either Src or Fgr silencing was examined in ovarian cancer cell lines by MTT assay. Expression of SFKs after Src silencing in ovarian cancer cells was examined by real-time reverse transcriptase (RT)-PCR. Therapeutic effect of in vivo Src and/or Fgr silencing was examined using siRNA incorporated into chitosan nanoparticles (siRNA/CH-NP). Microvessel density, cell proliferation, and apoptosis markers were determined by immunohistochemical staining in ovarian tumor tissues.

RESULTS

Src silencing enhanced cytotoxicity of docetaxel in both SKOV3ip1 and HeyA8 cells. In addition, Src silencing using siRNA/CH-NP in combination with docetaxel resulted in significant inhibition of tumor growth compared with control siRNA/CH-NP (81.8% reduction in SKOV3ip1, P = 0.017; 84.3% reduction in HeyA8, P < 0.005). These effects were mediated by decreased tumor cell proliferation and angiogenesis, and increased tumor cell apoptosis. Next, we assessed the effects of Src silencing on other SFK members in ovarian cancer cell lines. Src silencing resulted in significantly increased Fgr levels. Dual Src and Fgr silencing in vitro resulted in increased apoptosis that was mediated by increased caspase and AKT activity. In addition, dual silencing of Src and Fgr in vivo using siRNA/CH-NP resulted in the greatest reduction in tumor growth compared with silencing of either Src or Fgr alone in the HeyA8 model (68.8%, P < 0.05).

CONCLUSIONS

This study demonstrates that, in addition to Src, Fgr plays a biologically significant role in ovarian cancer growth and might represent an important target.

Integrin alphaMbeta2 clustering triggers phosphorylation and activation of protein kinase C delta that regulates transcription factor Foxp1 expression in monocytes

Integrins are type I membrane and heterodimeric (alphabeta) cell adhesion receptors. Intracellular signals triggered by ligand-bound integrins are important for cell growth, differentiation, and migration. Integrin alpha(M)beta(2) plays key roles in myeloid cell adhesion, phagocytosis, and degranulation. In this study, we show that protein kinase C (PKC) delta is involved in alpha(M)beta(2) signaling. In human monocytic U937 cells and peripheral blood monocytes, alpha(M)beta(2) clustering induced PKCdelta translocation to the plasma membrane, followed by Tyr(311) phosphorylation and activation of PKCdelta by the src family kinases Hck and Lyn. Interestingly, alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation was not mediated by the tyrosine kinase Syk, which is a well reported kinase in beta(2) integrin signaling. Analysis of the beta(2) cytoplasmic tail showed that the sequence Asn(727)-Ser(734) is important in alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation. It has been shown that alpha(M)beta(2) clustering regulates the expression the transcription factor Foxp1 that has a role in monocyte differentiation. We show that Foxp1 expression was reduced in monocytes that were allowed to adhere to human microvascular endothelial cells. However, the expression of Foxp1 was not affected in monocytes that were treated with PKCdelta-targeting small interfering RNA, suggesting that PKCdelta regulates Foxp1 expression. These results demonstrate a role of PKCdelta in alpha(M)beta(2)-mediated Foxp1 regulation in monocytes.

Deletion of activating Fcgamma receptors does not confer protection in murine cryoglobulinemia-associated membranoproliferative glomerulonephritis

Many types of glomerulonephritis are initiated by the deposition of immune complexes, which induce tissue injury via either engagement of Fc receptors on effector cells or via complement activation. Four murine Fcgamma receptors (FcgammaRs) have been identified at present. Ligand binding to FcgammaRI, III, and IV induces cell activation via the immunoreceptor tyrosine-based activation motif on the common gamma chain (FcRgamma). In this study, FcRgamma chain knockout (FcRgamma(-/-)) mice were crossed with thymic stromal lymphopoietin transgenic (TSLPtg) mice, which develop cryoglobulinemic membranoproliferative glomerulonephritis (MPGN). Female mice were studied at 30 and 50 days of age, when MPGN is in early and fully developed stages, respectively. Both TSLPtg and TSLPtg/FcRgamma(-/-) mice developed MPGN with massive glomerular immune deposits, mesangial cell proliferation, extensive mesangial matrix accumulation, and macrophage influx. TSLPtg/FcRgamma(-/-) mice had more glomerular immune complex deposits and higher levels of circulating cryoglobulins, IgG2a, IgG2b, and IgM, compared with TSLPtg mice. TSLPtg and TSLPtg/FcRgamma(-/-) mice developed similar levels of proteinuria. These results demonstrated that deletion of activating FcgammaRs does not confer protection in this model of immune complex-mediated MPGN. The findings contradict accepted paradigms on the role of activating FcgammaRs in promoting features of glomerulonephritis as seen in other model systems. We speculate engagement of FcgammaRs on cells such as monocytes/macrophages may be important for the clearance of deposited immune complexes and extracellular matrix proteins.

Identification of novel SHPS-1-associated proteins and their roles in regulation of insulin-like growth factor-dependent responses in vascular smooth muscle cells

Tyrosine phosphatase non-receptor type substrate-1 (SHPS-1), a transmembrane protein, plays a vital role in cell migration and proliferation. Our previous studies have shown that insulin-like growth factor-I (IGF-I) stimulates SHPS-1 phosphorylation, leading to recruitment of SHP-2, c-Src, Shc, and Grb2.p85 to phosphorylated SHPS-1. Assembly of this signaling complex is required for optimal stimulation of both mitogen-activated protein and phosphatidylinositol 3-kinase pathways. The main aim of the present study was to identify novel proteins that interacted with the cytoplasmic domain of SHPS-1 (SHPS-1/CD) in response to IGF-I stimulation and define the role of these interactions in mediating specific biological functions. We performed a functional proteomic screening to identify SHPS-1 binding partners using combination of mRNA display and the tandem affinity purification-tag methods. Screening identified a number of proteins not previously known to interact with phosphorylated SHPS-1/CD. These novel SHPS-1 binding partners represent several functional categories including heat shock proteins, protein kinases and phosphatases, and proteins that regulate transcription or translation. In Vivo and in vitro studies suggested that most of the proteins bound to SHPS-1 via binding to one of the four SH2 domain containing proteins, SHP-2, CTK, SUPT6H, and STAT1, that directly bound to SHPS-1. Although the binding of most of these proteins to SHPS-1 was positively regulated by IGF-I, a few were negatively regulated, suggesting differential regulation of protein complexes assembled on SHPS-1/CD in response to IGF-I. Further studies showed that truncation of SHPS-1/CD significantly impaired IGF-I-dependent AKT signal transduction and subsequent biological functions including cell survival, protein synthesis, protein aggregation, and prevention of apoptosis. The results emphasize the importance of formation of SHPS-1 signaling complex induced by IGF-I and provide novel insights into our knowledge of the role of this molecular scaffold in regulation of IGF-I-stimulated signal transduction and biological actions.

p66shc negatively regulates insulin-like growth factor I signal transduction via inhibition of p52shc binding to Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 leading to impaired growth factor receptor-bound protein-2 membrane recruitment

Our previous studies have indicated an essential role of p52shc in mediating IGF-I activation of MAPK in smooth muscle cells (SMC). However, the role of the p66 isoform of shc in IGF-I signal transduction is unclear. In the current study, two approaches were employed to investigate the role of p66shc in mediating IGF-I signaling. Knockdown p66shc by small interfering RNA enhanced IGF-I-stimulated p52shc tyrosine phosphorylation and growth factor receptor-bound protein-2 (Grb2) association, resulting in increased IGF-I-dependent MAPK activation. This was associated with enhanced IGF-I-stimulated cell proliferation. In contrast, knockdown of p66shc did not affect IGF-I-stimulated IGF-I receptor tyrosine phosphorylation. Overexpression of p66shc impaired IGF-I-stimulated p52shc tyrosine phosphorylation and p52shc-Grb2 association. In addition, IGF-I-dependent MAPK activation was also impaired, and SMC proliferation in response to IGF-I was inhibited. IGF-I-dependent cell migration was enhanced by p66shc knockdown and attenuated by p66shc overexpression. Mechanistic studies indicated that p66shc inhibited IGF-I signal transduction via competitively inhibiting the binding of Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2) to SHP substrate-1 (SHPS-1), leading to the disruption of SHPS-1/SHP-2/Src/p52shc complex formation, an event that has been shown previously to be essential for p52shc phosphorylation and Grb2 recruitment. These findings indicate that p66shc functions to negatively regulate the formation of a signaling complex that is required for p52shc activation in response to IGF-I, thus leading to attenuation of IGF-I-stimulated cell proliferation and migration.

The diverse functions of Src family kinases in macrophages

Macrophages are key components of the innate immune response. These cells possess a diverse repertoire of receptors that allow them to respond to a host of external stimuli including cytokines, chemokines, and pathogen-associated molecules. Signals resulting from these stimuli activate a number of macrophage functional responses such as adhesion, migration, phagocytosis, proliferation, survival, cytokine release and production of reactive oxygen and nitrogen species. The cytoplasmic tyrosine kinase Src and its family members (SFKs) have been implicated in many intracellular signaling pathways in macrophages, initiated by a diverse set of receptors ranging from integrins to Toll-like receptors. However, it has been difficult to implicate any given member of the family in any specific pathway. SFKs appear to have overlapping and complementary functions in many pathways. Perhaps the function of these enzymes is to modulate the overall intracellular signaling network in macrophages, rather than operating as exclusive signaling switches for defined pathways. In general, SFKs may function more like rheostats, influencing the amplitude of many pathways.

Expression of the human beta3 integrin subunit in mouse smooth muscle cells enhances IGF-I-stimulated signaling and proliferation

Optimal stimulation of signal transduction and biological functions by IGF-I in porcine smooth muscle cells (pSMC) requires ligand occupancy of the alphaVbeta3 integrin. Binding of heparin-binding domain (HBD) of vitronectin (VN) to the cysteine loop (C-loop) region of beta3 is required for pSMC to respond optimally to IGF-I stimulation. Mouse smooth muscle cells (mSMC), which express a form of beta3 whose sequence within the C-loop region is different than porcine or human beta3, do not respond optimally to IGF-I, and IGF-I stimulated beta3 and SHPS-1 phosphorylation which are necessary for optimal IGF-I signaling were undetectable. VN also had no effect on IGF-I stimulated the cell proliferation. In contrast, when human beta3 (hbeta3) was introduced into mSMC, there was an enhanced VN binding in spite of an equivalent amount of total beta3 expression, and IGF-I-dependent beta3, and SHPS-1 phosphorylation were detected. In addition, there was enhanced IGF-I-stimulated Shc association with SHPS-1, Shc tyrosine phosphorylation, Shc and Grb2 association, and MAP kinase activation leading to increased cell proliferation. These enhancements could be further augmented by adding a peptide containing the HBD of VN. To determine if these changes were mediated by the C-loop region of beta3, an antibody that reacts with that region of beta3 was utilized. The addition of the hbeta3 C-loop antibody abolished VN-induced enhancement of IGF-I signaling and IGF-I-stimulated cell proliferation. These results strongly support the conclusion that optimal SMC responsiveness to IGF-I requires ligand interaction with the C-loop domain of hbeta3.

Dual regulation of SIRPalpha phosphorylation by integrins and CD47

Signal regulatory protein alpha (SIRPalpha, SHPS-1) is a plasma membrane receptor for CD47 and a key regulator of phagocytosis, growth factor signaling, and migration. Phosphorylation of immunoreceptor tyrosine-based inhibition motifs in its cytoplasmic tail is essential for the functional effects of SIRPalpha, at least in part, because the phosphorylated immunoreceptor tyrosine-based inhibition motifs recruit Src homology 2 domain-containing tyrosine phosphatases. Ligation by CD47 and integrin engagement both have been thought to regulate SIRPalpha phosphorylation. However, their distinct contributions have not been distinguished. Here, we show that the importance of CD47 varies with cell type, since ligation of CD47 is not necessary for SIRPalpha phosphorylation in myeloid cells, whereas it is required in endothelial cells. In contrast, integrin-mediated adhesion is required for SIRPalpha phosphorylation in both cell types. This shows that SIRPalpha phosphorylation is dually regulated and demonstrates a new mechanism for functional cooperation between integrins and the integrin-associated protein CD47.

Burying the dead: the impact of failed apoptotic cell removal (efferocytosis) on chronic inflammatory lung disease

Apoptosis and the removal of apoptotic cells (termed efferocytosis) are tightly coupled with the regulation of normal lung structure, both in the developing and adult organism. Processes that disrupt or uncouple this balance have the potential to alter normal cell turnover, ultimately resulting in the induction of lung pathology and disease. Apoptotic cells are increased in several chronic inflammatory lung diseases, including cystic fibrosis (CF), non-CF bronchiectasis, COPD, and asthma. While this may well be due to the enhanced induction of apoptosis, increasing data suggest that the clearance of dying cells is also impaired. Because efferocytosis appears to be a key regulatory checkpoint for the innate immune system, the adaptive immune system, and cell proliferation, the failure of this highly conserved process may contribute to disease pathogenesis by impeding both the resolution of inflammation and the maintenance of alveolar integrity. The recognition of impaired efferocytosis as a contributor to chronic inflammation may ultimately direct us toward the identification of new disease biomarkers, as well as novel therapeutic approaches.

The duplicitous nature of the Lyn tyrosine kinase in growth factor signaling

The Lyn tyrosine kinase is a unique member of the Src family of non-receptor protein tyrosine kinases whose principal role is to regulate signals through inhibitory receptors thereby promoting signal attenuation. Lyn is renowned for its role in B cell antigen receptor and FcepsilonRI signaling; however, it is becoming increasingly apparent that Lyn also functions in signal transduction from growth factor receptors including the receptors for GM-CSF, IL-3, IL-5, SCF, erythropoietin, CSF-1, G-CSF, thrombopoietin and Flt3 ligand. Numerous studies have implicated Lyn in growth factor receptor signal amplification, while a number also suggest that Lyn participates in negative regulation of growth factor signaling. Indeed Lyn-deficient mice are hyper-responsive to myeloid growth factors and develop a myeloproliferative disorder that predisposes the mice to macrophage tumours, with loss of negative regulation through SHP-1 and SHIP-1 thought to be the major contributing factor to this phenotype. Developing a clear understanding of Lyn's role in establishing signaling thresholds in growth factor receptor signal amplification and signal inhibition may have important implications in the management of leukemias that may depend on Lyn activity.

Haemophilus ducreyi targets Src family protein tyrosine kinases to inhibit phagocytic signaling

Haemophilus ducreyi, the etiologic agent of the sexually transmitted disease chancroid, has been shown to inhibit phagocytosis of both itself and secondary targets in vitro. Immunodepletion of LspA proteins from H. ducreyi culture supernatant fluid abolished this inhibitory effect, indicating that the LspA proteins are necessary for the inhibition of phagocytosis by H. ducreyi. Fluorescence microscopy revealed that macrophages incubated with wild-type H. ducreyi, but not with a lspA1 lspA2 mutant, were unable to complete development of the phagocytic cup around immunoglobulin G-opsonized targets. Examination of the phosphotyrosine protein profiles of these two sets of macrophages showed that those incubated with wild-type H. ducreyi had greatly reduced phosphorylation levels of proteins in the 50-to-60-kDa range. Subsequent experiments revealed reductions in the catalytic activities of both Lyn and Hck, two members of the Src family of protein tyrosine kinases that are known to be involved in the proximal signaling steps of Fcgamma receptor-mediated phagocytosis. Additional experiments confirmed reductions in the levels of both active Lyn and active Hck in three different immune cell lines, but not in HeLa cells, exposed to wild-type H. ducreyi. This is the first example of a bacterial pathogen that suppresses Src family protein tyrosine kinase activity to subvert phagocytic signaling in hostcells.

Src and Syk kinases: key regulators of phagocytic cell activation

Src-family kinases and Syk tyrosine kinases have crucial roles in multiple leukocyte intracellular signaling pathways. In immunoreceptor-related pathways, these enzymes work together sequentially, with Src-family kinases phosphorylating specific protein substrates, which in turn recruit and activate Syk. Recent evidence indicates that several non-immunoreceptors also use Src-family kinases and Syk in this same fashion. In leukocyte integrin signaling, the interaction between the kinases is more complex, where they appear to act in a sequential manner but the mechanisms by which they are activated remain poorly defined. Elucidating the regulation of these tyrosine kinase-based signaling pathways in leukocytes remains an important goal in understanding how immune cells respond to the multitude of activating agents they encounter.

Role of SHPS-1 in the regulation of insulin-like growth factor I-stimulated Shc and mitogen-activated protein kinase activation in vascular smooth muscle cells

Insulin-like growth factor I (IGF-I) stimulates smooth muscle cell (SMC) proliferation, and the mitogen-activated protein kinase (MAPK) pathway plays an important role in mediating IGF-I-induced mitogenic signaling. Our prior studies have shown that recruitment of Src homology 2 domain tyrosine phosphatase (SHP-2) to the membrane scaffolding protein Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1) is required for IGF-I-dependent MAPK activation. The current studies were undertaken to define the upstream signaling components that are required for IGF-I-stimulated MAPK activation and the role of SHPS-1 in regulating this process. The results show that IGF-I-induced Shc phosphorylation and its subsequent binding to Grb2 is required for sustained phosphorylation of MAPK and increased cell proliferation in SMCs. Furthermore, for Shc to be phosphorylated in response to IGF-I requires that Shc must associate with SHPS-1 and this association is mediated in part by SHP-2. Preincubation of cells with a peptide that contains a phospho-tyrosine binding motif sequence derived from SHPS-1 inhibited IGF-I-stimulated SHP-2 transfer to SHPS-1, the association of Shc with SHPS-1, and IGF-I-dependent Shc phosphorylation. Expression of an SHPS-1 mutant that did not bind to Shc or SHP-2 resulted in decreased Shc and MAPK phosphorylation in response to IGF-I. In addition, SMCs expressing a mutant form of the beta3 subunit of the alphaVbeta3, which results in impairment of SHP-2 transfer to SHPS-1, also showed attenuated IGF-I-dependent Shc and MAPK phosphorylation. Further analysis showed that Shc and SHP-2 can be coimmunoprecipitated after IGF-I stimulation. A cell-permeable peptide that contained a polyproline sequence from Shc selectively inhibited Shc/SHP-2 association and impaired Shc but not SHP-2 binding to SHPS-1. Exposure to this peptide also inhibited IGF-I-stimulated Shc and MAPK phosphorylation. Cells expressing a mutant form of Shc with the four prolines substituted with alanines showed no Shc/SHPS-1 association in response to IGF-I. We conclude that SHPS-1 functions as an anchor protein that recruits both Shc and SHP-2 and that their recruitment is necessary for IGF-I-dependent Shc phosphorylation, which is required for an optimal mitogenic response in SMCs.

The Src family kinases Hck and Fgr negatively regulate neutrophil and dendritic cell chemokine signaling via PIR-B

In classical descriptions of leukocyte chemokine signaling, Src family kinases are thought to function in a positive fashion by coupling receptor associated Galpha subunits to downstream mitogen activated protein (MAP) kinase activation. However, neutrophils derived from hck-/-fgr-/- mice and dendritic cells (DCs) from fgr-/- animals manifested significantly higher intracellular signaling (Ca2+ flux, MAP kinase activation, actin polymerization) and functional responses (chemotaxis in vitro and migration in vivo) to a number of different chemokines. These kinases may mediate their effect through the inhibitory receptor PIR-B since neutrophils and DCs from pir-b-/- mice were also hyperresponsive to chemokine stimulation. In wild-type (wt) cells dephosphorylation of PIR-B was associated with maximal chemokine signaling, whereas in hck-/-fgr-/- cells PIR-B was unphosphorylated. These data support a model in which the Src family kinases Hck and Fgr function as negative regulators of myeloid cell chemokine signaling by maintaining the tonic phosphorylation of PIR-B.

The coordination of signaling during Fc receptor-mediated phagocytosis

Phagocytosis by macrophages can be initiated by Fcgamma receptors (FcR) in membranes that bind to Fc regions of immunoglobulin G (IgG). Activated FcR transduce signals to cytoplasm, which regulate the internalization of IgG-coated particles into plasma membrane-derived vacuoles, phagosomes. Particles internalized by phagocytosis are much larger than FcR, which prompts questions of if and how the receptors are coordinated with each other. FcR-mediated signal transduction entails recruitment of proteins from cytoplasm to the receptor, largely via protein phosphorylation. These FcR signaling complexes then activate proteins that regulate actin, myosin, membrane fusion, and the production of reactive oxygen intermediates. Recent fluorescence microscopic studies of phagocytosis in macrophages indicate that signaling by FcR occurs as a sequence of distinct stages, evident in the spatial and temporal patterns of phosphoinositides, protein kinase C, and Rho-family GTPase activation on forming phagosomes. The coordination of these stages may be regulated by lipids or lipid-anchored proteins, which diffuse away from FcR complexes. Lateral diffusion of FcR-derived signals could integrate FcR-dependent responses over large areas of membrane in the forming phagosome.

Rac2-deficient murine macrophages have selective defects in superoxide production and phagocytosis of opsonized particles

The Rho family GTPase Rac is a crucial participant in numerous cellular functions and acts as a molecular switch for signal transduction. Mice deficient in hemopoietic-specific Rac2 exhibited agonist-specific defects in neutrophil functions including chemoattractant-stimulated filamentous actin polymerization and chemotaxis, and superoxide production elicited by phorbol ester, fMLP, or IgG-coated particles, despite expression of the highly homologous Rac1 isoform. In this study, functional responses of Rac2-null murine macrophages were characterized to examine whether Rac2 also has nonredundant functions in this phagocytic lineage. In contrast to murine neutrophils, in which Rac1 and Rac2 are present in similar amounts, Rac1 was approximately 4-fold more abundant than Rac2 in both bone marrow-derived and peritoneal exudate macrophages, and macrophage Rac1 levels were unchanged by the absence of Rac2. Accumulation of exudate macrophages during peritoneal inflammation was reduced in rac2(-/-) mice. FcgammaR-mediated phagocytosis of IgG-coated SRBC was also significantly decreased in Rac2-null macrophages, as was NADPH oxidase activity in response to phorbol ester or FcgammaR stimulation. However, phagocytosis and oxidant production stimulated by serum-opsonized zymosan was normal in rac2(-/-) macrophages. Macrophage morphology was also similar in wild-type and Rac2-null cells, as was actin polymerization induced by FcgammaR-mediated phagocytosis or M-CSF. Hence, Rac2-null macrophages have selective defects paralleling many of the observed functional defects in Rac2-null neutrophils. These results provide genetic evidence that although Rac2 is a relatively minor isoform in murine macrophages, it plays a nonoverlapping role with Rac1 to regulate host defense functions in this phagocyte lineage.

The cellular prion protein modulates phagocytosis and inflammatory response

The cellular prion protein (PrPc) is a glycoprotein anchored by glycosylphosphatidylinositol (GPI) to the cell surface and is abundantly expressed in the central nervous system. It is also expressed in a variety of cell types of the immune system. We investigated the role of PrPc in the phagocytosis of apoptotic cells and other particles. Macrophages from mice with deletion of the Prnp gene showed higher rates of phagocytosis than wild-type macrophages in in vitro assays. The elimination of GPI-anchored proteins from the cell surface of macrophages from wild-type mice rendered these cells as efficient as macrophages derived from knockout mice. In situ detection of phagocytosis of apoptotic bodies within the retina indicated augmented phagocytotic activity in knockout mice. In an in vivo assay of acute peritonitis, knockout mice showed more efficient phagocytosis of zymosan particles than wild-type mice. In addition, leukocyte recruitment was altered in knockout mice, as compared with wild type. The data show that PrPc modulates phagocytosis in vitro and in vivo. This activity is described for the first time and may be important for normal macrophage functions as well as for the pathogenesis of prion diseases.

Activation of RBL-2H3 mast cells is dependent on tyrosine phosphorylation of phospholipase D2 by Fyn and Fgr

Both phospholipase D1 (PLD1) and PLD2 regulate degranulation when RBL-2H3 cells are stimulated via the immunoglobulin E receptor, Fc epsilon RI. However, the activation mechanism for PLD2 is unclear. As reported here, PLD2 but not PLD1 is phosphorylated through the Src kinases, Fyn and Fgr, and this phosphorylation appears to regulate PLD2 activation and degranulation. For example, only hemagglutinin-tagged PLD2 was tyrosine phosphorylated in antigen-stimulated cells that had been made to express HA-PLD1 and HA-PLD2. This phosphorylation was blocked by a Src kinase inhibitor or by small interfering RNAs directed against Fyn and Fgr and was enhanced by overexpression of Fyn and Fgr but not by other Src kinases. The phosphorylation and activity of PLD2 were further enhanced by the tyrosine phosphatase inhibitor, Na(3)VO(4). Mutation of PLD2 at tyrosines 11, 14, 165, or 470 partially impaired, and mutation of all tyrosines blocked, PLD2 phosphorylation and activation, although two of these mutations were detrimental to PLD2 function. PLD2 phosphorylation preceded degranulation, both events were equally sensitive to inhibition of Src kinase activity, and both were enhanced by coexpression of PLD2 and the Src kinases. The findings provide the first description of a mechanism for activation of PLD2 in a physiological setting and of a role for Fgr in Fc epsilon RI-mediated signaling.

Reduced phagocytosis of colloidal carriers using soluble CD47

PURPOSE

This study was designed to illustrate the feasibility of using soluble CD47 protein to antagonize phagocytosis of colloidal drug carriers by macrophages.

METHODS

Expression of CD47-streptavidin (CD47-SA) fusion protein was achieved in B21CodonPlus host cells following IPTG induction. Murine macrophage cell line J774A.1, expressing high levels of SIRPalpha, was selected as the biologic model system for phagocytosis. FITC-labeled perfluorocarbon (PFC) emulsions were used as the colloidal carriers to trigger phagocytosis. Microscopy (inverted light and UV-fluorescence) and flow cytometry were used to qualitatively and quantitatively determine the degree of phagocytosis, respectively.

RESULTS

The bacterially expressed, purified CD47-SA had neither cytotoxic nor cytostatic effects when incubated with J774A.1 cells up to a concentration of 400 nM for 24 h. Phagocytosis of FITC-labeled PFC emulsions was significantly diminished when macrophages were pretreated with 100 nM CD47-SA for 1 h.

CONCLUSIONS

We demonstrated that soluble CD47-SA antagonized phagocytosis of colloidal carriers to a significant degree by interaction with macrophage SIRPalpha.