Transmembrane Mutations to FcγRIIA Alter Its Association with Lipid Rafts: Implications for Receptor Signaling

Potential Association of Maker Expression of Low-Density Neutrophils and Their Phenotypes in Patients with Periodontitis: Control Study

Background

Neutrophils play an important role in maintaining periodontal status in conditions of healthy homeostasis. They achieve their surveillance function by continuously migrating to the gingival sulcus and eradicating periodontal pathogens. In addition, neutrophils are considered an integral element in the pathogenesis of periodontal diseases. Among several neutrophil subsets, low-density neutrophils (LDN) have recently received attention and are linked with cancer, immunological, inflammatory, and infectious diseases. However, the presence, phenotypes, and potential role of LDN in the pathogenesis of periodontitis have not yet been investigated.

Objectives

To investigate the presence, subsets (normal, band, suppressive, and active), and phenotypes via marker expression surface protein known as the cluster of differentiation (CD) (CD16b, CD14, CD15, and CD62L) of LDN in patients with periodontitis.

Materials and Methods

The observational case-control study was conducted to estimate the potential role of LDNs in periodontitis. Venous blood and periodontal indices were obtained from 40 healthy control individuals and 60 periodontitis patients. Subsequently, CD16b, CD62L, CD14, and CD15 expression on the surface of LDN was examined by multicolor flow cytometry, and their subsets were classified as "normal" (CD16brightCD62Lbright), "bands" (CD16dimCD62Lbright), "suppressive" (CD16brightCD62Ldim), and "active" (CD16brightCD62Lnegative).

Results

There was a significant difference in the expression of LDN markers for active and suppressive phenotypes, respectively, favoring periodontitis over the control group. In contrast, there were significantly higher levels of CD16b, CD62L, and CD15 ("normal") in the control group when compared with the periodontitis group.

Conclusion

LDN was associated with periodontitis as it was significantly increased in the periodontitis group in comparison with the control group and was positively correlated with all periodontal parameters. Cells from both groups of patients (periodontitis and control) expressed a normal mature phenotype (CD16b + High, CD62L + High, CD15+, and CD14-). Regarding subsets, the normal LDN (CD16brightCD62Lbright) was the most predominant phenotype in both periodontitis and control groups. However, the active subset increased in periodontitis compared to normal, indicating their destructive role in periodontitis.

Low-Density Neutrophils in Healthy Individuals Display a Mature Primed Phenotype

Neutrophils are the most abundant leukocytes in human peripheral blood, comprising about 70% of all leukocytes. They are regarded as the first line of defense of the innate immune system, but neutrophils have also the ability of regulating the adaptive immune response. Recently, However, multiple phenotypes and functional states of neutrophils have been reported, particularly in inflammation, autoimmunity, and cancer. One possible subtype of neutrophils, the so-called low-density neutrophils (LDN) is found among mononuclear cells (MNC), monocytes and lymphocytes, after separating the leukocytes from blood by density gradient centrifugation. LDN increase in numbers during several pathological conditions. However, LDN present in healthy conditions have not been investigated further. Therefore, in order to confirm the presence of LDN in blood of healthy individuals and to explore some of their cellular functions, neutrophils and MNC were isolated by density gradient centrifugation. Purified neutrophils were further characterized by multicolor flow cytometry (FACS) and then, using the same FACS parameters cells in the MNC fraction were analyzed. Within the MNC, LDN were consistently found. These LDN had a normal mature neutrophil morphology and displayed a CD10+, CD11b+, CD14low, CD15high, CD16bhigh, CD62L+, CD66b+, and CXCR4+ phenotype. These LDN had an enhanced reactive oxygen species (ROS) production and increased phagocytic capacity and were able to produce neutrophil extracellular traps (NET) similarly to neutrophils. These data confirm the presence of a small number of LDN is blood of healthy individuals and suggest that these LDN represent mature cells with a primed phenotype.

Promoters and Antagonists of Phagocytosis: A Plastic and Tunable Response

Recent observations indicate that, rather than being an all-or-none response, phagocytosis is finely tuned by a host of developmental and environmental factors. The expression of key phagocytic determinants is regulated via transcriptional and epigenetic means that confer memory on the process. Membrane traffic, the cytoskeleton, and inside-out signaling control the activation of phagocytic receptors and their ability to access their targets. An exquisite extra layer of complexity is introduced by the coexistence of distinct "eat-me" and "don't-eat-me" signals on targets and of corresponding "eat" and "don't-eat" receptors on the phagocyte surface. Moreover, assorted physical barriers constitute "don't-come-close-to-me" hurdles that obstruct the engagement of ligands by receptors. The expression, mobility, and accessibility of all these determinants can be modulated, conferring extreme plasticity on phagocytosis and providing attractive targets for therapeutic intervention in cancer, atherosclerosis, and dementia. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Examining the Underappreciated Role of S-Acylated Proteins as Critical Regulators of Phagocytosis and Phagosome Maturation in Macrophages

Phagocytosis is a receptor-mediated process used by cells to engulf a wide variety of particulates, including microorganisms and apoptotic cells. Many of the proteins involved in this highly orchestrated process are post-translationally modified with lipids as a means of regulating signal transduction, membrane remodeling, phagosome maturation and other immunomodulatory functions of phagocytes. S-acylation, generally referred to as S-palmitoylation, is the post-translational attachment of fatty acids to a cysteine residue exposed topologically to the cytosol. This modification is reversible due to the intrinsically labile thioester bond between the lipid and sulfur atom of cysteine, and thus lends itself to a variety of regulatory scenarios. Here we present an overview of a growing number of S-acylated proteins known to regulate phagocytosis and phagosome biology in macrophages.

Impact of Plasma Membrane Domains on IgG Fc Receptor Function

Lipid cell membranes not only represent the physical boundaries of cells. They also actively participate in many cellular processes. This contribution is facilitated by highly complex mixtures of different lipids and incorporation of various membrane proteins. One group of membrane-associated receptors are Fc receptors (FcRs). These cell-surface receptors are crucial for the activity of most immune cells as they bind immunoglobulins such as immunoglobulin G (IgG). Based on distinct mechanisms of IgG binding, two classes of Fc receptors are now recognized: the canonical type I FcγRs and select C-type lectin receptors newly referred to as type II FcRs. Upon IgG immune complex induced cross-linking, these receptors are known to induce a multitude of cellular effector responses in a cell-type dependent manner, including internalization, antigen processing, and presentation as well as production of cytokines. The response is also determined by specific intracellular signaling domains, allowing FcRs to either positively or negatively modulate immune cell activity. Expression of cell-type specific combinations and numbers of receptors therefore ultimately sets a threshold for induction of effector responses. Mechanistically, receptor cross-linking and localization to lipid rafts, i.e., organized membrane microdomains enriched in intracellular signaling proteins, were proposed as major determinants of initial FcR activation. Given that immune cell membranes might also vary in their lipid compositions, it is reasonable to speculate, that the cell membrane and especially lipid rafts serve as an additional regulator of FcR activity. In this article, we aim to summarize the current knowledge on the interplay of lipid rafts and IgG binding FcRs with a focus on the plasma membrane composition and receptor localization in immune cells, the proposed mechanisms underlying this localization and consequences for FcR function with respect to their immunoregulatory capacity.

Selective recruitment of Nck and Syk contribute to distinct leukocyte immune-type receptor-initiated target interactions

The ability of phagocytes to recognize, immobilize, and engulf extracellular targets are fundamental immune cell processes that allow for the destruction of a variety of microbial intruders. The phagocytic process depends onsignalling events that initiate dynamic changes in the plasma membrane architecture that are required to accommodate the internalization of large particulate targets. To better understand fundamental molecular mechanisms responsible for facilitating phagocytic receptor-mediated regulation of cytoskeletal networks, our research has focused on investigating representative immunoregulatory proteins from the channel catfish (Ictalurus punctatus) leukocyte immune-type receptor family (IpLITRs). Specifically, we have shown that a specific IpLITR-type can regulate the constitutive deployment of filopodial-like structures to actively capture and secure targets to the phagocyte surface, which is followed by F-actin mediated membrane dynamics that are associated with the formation of phagocytic cup-like structures that precede target engulfment. In the present study, we use confocal imaging to examine the recruitment of mediators of the F-actin cytoskeleton during IpLITR-mediated regulation of membrane dynamics. Our results provide novel details regarding the dynamic recruitment of the signaling effectors Nck and Syk during classical as well as atypical IpLITR-induced phagocytic processes.

The desmosome is a mesoscale lipid raft-like membrane domain

Desmogleins (Dsgs) are cadherin family adhesion molecules essential for epidermal integrity. Previous studies have shown that desmogleins associate with lipid rafts, but the significance of this association was not clear. Here, we report that the desmoglein transmembrane domain (TMD) is the primary determinant of raft association. Further, we identify a novel mutation in the DSG1 TMD (G562R) that causes severe dermatitis, multiple allergies, and metabolic wasting syndrome. Molecular modeling predicts that this G-to-R mutation shortens the DSG1 TMD, and experiments directly demonstrate that this mutation compromises both lipid raft association and desmosome incorporation. Finally, cryo-electron tomography indicates that the lipid bilayer within the desmosome is ∼10% thicker than adjacent regions of the plasma membrane. These findings suggest that differences in bilayer thickness influence the organization of adhesion molecules within the epithelial plasma membrane, with cadherin TMDs recruited to the desmosome via the establishment of a specialized mesoscale lipid raft-like membrane domain.

Protein Disulfide Isomerase Inhibitor Suppresses Viral Replication and Production during Antibody-Dependent Enhancement of Dengue Virus Infection in Human Monocytic Cells

One of several mechanisms that leads to the development of dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) is called antibody-dependent enhancement (ADE). Monocytes can be infected by the ADE phenomenon, which occurs in dengue secondary infection. This study aimed to investigate the proteins involved in ADE of DENV infection in the human monocytic cell line U937. The phosphoproteins were used to perform and analyze for protein expression using mass spectrometry (GeLC-MS/MS). The differential phosphoproteins revealed 1131 altered proteins compared between isotype- and DENV-specific antibody-treated monocytes. The altered proteins revealed 558 upregulated proteins and 573 downregulated proteins. Protein disulfide isomerase (PDI), which is an enzyme that had a high-ranking fold change and that catalyzes the formation, breakage, and rearrangement of disulfide bonds within a protein molecule, was selected for further study. PDI was found to be important for dengue virus infectivity during the ADE model. The effect of PDI inhibition was also shown to be involved in the early stage of life cycle by time-of-drug-addition assay. These results suggest that PDI is important for protein translation and virion assembly of dengue virus during infection in human monocytes, and it may play a significant role as a chaperone to stabilize dengue protein synthesis.

Entamoeba histolytica Induce Signaling via Raf/MEK/ERK for Neutrophil Extracellular Trap (NET) Formation

Amoebiasis, the disease caused by Entamoeba histolytica is the third leading cause of human deaths among parasite infections. E. histolytica was reported associated with around 100 million cases of amoebic dysentery, colitis and amoebic liver abscess that lead to almost 50,000 fatalities worldwide in 2010. E. histolytica infection is associated with the induction of inflammation characterized by a large number of infiltrating neutrophils. These neutrophils have been implicated in defense against this parasite, by mechanisms not completely described. The neutrophil antimicrobial mechanisms include phagocytosis, degranulation, and formation of neutrophil extracellular traps (NETs). Recently, our group reported that NETs are also produced in response to E. histolytica trophozoites. But, the mechanism for NETs induction remains unknown. In this report we explored the possibility that E. histolytica leads to NETs formation via a signaling pathway similar to the pathways activated by PMA or the Fc receptor FcγRIIIb. Neutrophils were stimulated by E. histolytica trophozoites and the effect of various pharmacological inhibitors on amoeba-induced NETs formation was assessed. Selective inhibitors of Raf, MEK, and NF-κB prevented E. histolytica-induced NET formation. In contrast, inhibitors of PKC, TAK1, and NADPH-oxidase did not block E. histolytica-induced NETs formation. E. histolytica induced phosphorylation of ERK in a Raf and MEK dependent manner. These data show that E. histolytica activates a signaling pathway to induce NETs formation, that involves Raf/MEK/ERK, but it is independent of PKC, TAK1, and reactive oxygen species (ROS). Thus, amoebas activate neutrophils via a different pathway from the pathways activated by PMA or the IgG receptor FcγRIIIb.

Molecular Mechanisms of Phagosome Formation

Phagocytosis refers to the active process that allows cells to take up large particulate material upon binding to surface receptors. The discovery of phagocytosis in 1883 by Elie Metchnikoff, leading to the concept that specialized cells are implicated in the defense against microbes, was one of the starting points of the field of immunology. After more than a century of research, phagocytosis is now appreciated to be a widely used process that enables the cellular uptake of a remarkable variety of particles, including bacteria, fungi, parasites, viruses, dead cells, and assorted debris and solid materials. Uptake of foreign particles is performed almost exclusively by specialized myeloid cells, commonly termed "professional phagocytes": neutrophils, monocytes, macrophages, and dendritic cells. Phagocytosis of microbes not only stops or at least restricts the spread of infection but also plays an important role in regulating the innate and adaptive immune responses. Activation of the myeloid cells upon phagocytosis leads to the secretion of cytokines and chemokines that convey signals to a variety of immune cells. Moreover, foreign antigens generated by the degradation of microbes following phagocytosis are loaded onto the major histocompatibility complex for presentation to specific T lymphocytes. However, phagocytosis is not restricted to professional myeloid phagocytes; an expanding diversity of cell types appear capable of engulfing apoptotic bodies and debris, playing a critical role in tissue remodeling and in the clearance of billions of effete cells every day.

Phagocytosis: A Fundamental Process in Immunity

One hundred years have passed since the death of Élie Metchnikoff (1845-1916). He was the first to observe the uptake of particles by cells and realized the importance of this process for the host response to injury and infection. He also was a strong advocate of the role of phagocytosis in cellular immunity, and with this he gave us the basis for our modern understanding of inflammation and the innate and acquired immune responses. Phagocytosis is an elegant but complex process for the ingestion and elimination of pathogens, but it is also important for the elimination of apoptotic cells and hence fundamental for tissue homeostasis. Phagocytosis can be divided into four main steps: (i) recognition of the target particle, (ii) signaling to activate the internalization machinery, (iii) phagosome formation, and (iv) phagolysosome maturation. In recent years, the use of new tools of molecular biology and microscopy has provided new insights into the cellular mechanisms of phagocytosis. In this review, we present a general view of our current knowledge on phagocytosis. We emphasize novel molecular findings, particularly on phagosome formation and maturation, and discuss aspects that remain incompletely understood.

DENV up-regulates the HMG-CoA reductase activity through the impairment of AMPK phosphorylation: A potential antiviral target

Dengue is the most common mosquito-borne viral disease in humans. Changes of lipid-related metabolites in endoplasmic reticulum of dengue virus (DENV) infected cells have been associated with replicative complexes formation. Previously, we reported that DENV infection inhibits HMGCR phosphorylation generating a cholesterol-enriched cellular environment in order to favor viral replication. In this work, using enzymatic assays, ELISA, and WB we found a significant higher activity of HMGCR in DENV infected cells, associated with the inactivation of AMPK. AMPK activation by metformin declined the HMGCR activity suggesting that AMPK inactivation mediates the enhanced activity of HMGCR. A reduction on AMPK phosphorylation activity was observed in DENV infected cells at 12 and 24 hpi. HMGCR and cholesterol co-localized with viral proteins NS3, NS4A and E, suggesting a role for HMGCR and AMPK activity in the formation of DENV replicative complexes. Furthermore, metformin and lovastatin (HMGCR inhibitor) altered this co-localization as well as replicative complexes formation supporting that active HMGCR is required for replicative complexes formation. In agreement, metformin prompted a significant dose-dependent antiviral effect in DENV infected cells, while compound C (AMPK inhibitor) augmented the viral genome copies and the percentage of infected cells. The PP2A activity, the main modulating phosphatase of HMGCR, was not affected by DENV infection. These data demonstrate that the elevated activity of HMGCR observed in DENV infected cells is mediated through AMPK inhibition and not by increase in PP2A activity. Interestingly, the inhibition of this phosphatase showed an antiviral effect in an HMGCR-independent manner. These results suggest that DENV infection increases HMGCR activity through AMPK inactivation leading to higher cholesterol levels in endoplasmic reticulum necessary for replicative complexes formation. This work provides new information about the mechanisms involved in host lipid metabolism during DENV replicative cycle and identifies new potential antiviral targets for DENV replication.

DENV replicative complexes formation is associated with changes of lipid-related metabolites in endoplasmic reticulum, such as an increase in cholesterol synthesis. This increase correlates with a significant augment in the activity of HMGCoA reductase (the limiting enzyme in cholesterol synthesis), favoring a cholesterol-enriched cellular environment. The augment in the activity of the HMGCR observed in infected cells is caused by a decrease in the phosphorylation level of the HMGCR, associated with the inactivation of AMPK. In agreement, AMPK activation by metformin reduces HMGCR activity and affects viral replication. The role HMGCR and AMPK activity in DENV replicative complexes formation was confirmed by the co-localization of HMGCR and cholesterol with the viral proteins NS3, NS4A and E. Furthermore, metformin and lovastatin (HMGCR inhibitor) treatments altered this co-localization as well as replicative complexes formation supporting that active HMGCR is required for replicative complexes formation. The results show that during DENV infection, an increase in the HMGCR activity occurs through AMPK inactivation, leading to higher cholesterol levels in endoplasmic reticulum necessary for replicative complexes formation. This work provides new information about the mechanisms involved in host lipid metabolism during DENV replicative cycle and identifies potential new antiviral targets for DENV replication.

Fcγ-receptor IIa-mediated Src Signaling Pathway Is Essential for the Antibody-Dependent Enhancement of Ebola Virus Infection

Antibody-dependent enhancement (ADE) of Ebola virus (EBOV) infection has been demonstrated in vitro, raising concerns about the detrimental potential of some anti-EBOV antibodies. ADE has been described for many viruses and mostly depends on the cross-linking of virus-antibody complexes to cell surface Fc receptors, leading to enhanced infection. However, little is known about the molecular mechanisms underlying this phenomenon. Here we show that Fcγ-receptor IIa (FcγRIIa)-mediated intracellular signaling through Src family protein tyrosine kinases (PTKs) is required for ADE of EBOV infection. We found that deletion of the FcγRIIa cytoplasmic tail abolished EBOV ADE due to decreased virus uptake into cellular endosomes. Furthermore, EBOV ADE, but not non-ADE infection, was significantly reduced by inhibition of the Src family protein PTK pathway, which was also found to be important to promote phagocytosis/macropinocytosis for viral uptake into endosomes. We further confirmed a significant increase of the Src phosphorylation mediated by ADE. These data suggest that antibody-EBOV complexes bound to the cell surface FcγRIIa activate the Src signaling pathway that leads to enhanced viral entry into cells, providing a novel perspective for the general understanding of ADE of virus infection.

Characterization of NF-κB Reporter U937 Cells and Their Application for the Detection of Inflammatory Immune-Complexes

Our study tested the hypothesis that immunoglobulins differ in their ability to activate the nuclear factor-κB pathway mediated cellular responses. These responses are modulated by several properties of the immune complex, including the ratio of antibody isotypes binding to antigen. Immunoassays allow the measurement of antigen specific antibodies belonging to distinct immunoglobulin classes and subclasses but not the net biological effect of the combination of these antibodies. We set out to develop a biosensor that is suitable for the detection and characterization of antigen specific serum antibodies. We genetically modified the monocytoid U937 cell line carrying Fc receptors with a plasmid encoding NF-κB promoter-driven GFP. This clone, U937-NF-κB, was characterized with respect to FcR expression and response to solid-phase immunoglobulins. Human IgG3, IgG4 and IgG1 induced GFP production in a time- and dose-dependent manner, in this order of efficacy, while IgG2 triggered no activation at the concentrations tested. IgA elicited no response alone but showed significant synergism with IgG3 and IgG4. We confirmed the importance of activation via FcγRI by direct stimulation with monoclonal antibody and by competition assays. We used citrullinated peptides and serum from rheumatoid arthritis patients to generate immune complexes and to study the activation of U937-NF-κB, observing again a synergistic effect between IgG and IgA. Our results show that immunoglobulins have distinct pro-inflammatory potential, and that U937-NF-κB is suitable for the estimation of biological effects of immune-complexes, offering insight into monocyte activation and pathogenesis of antibody mediated diseases.

Validation of research trajectory 1 of an Exposome framework: Exposure to benzo(a)pyrene confers enhanced susceptibility to bacterial infection

The exposome provides a framework for understanding elucidation of an uncharacterized molecular mechanism conferring enhanced susceptibility of macrophage membranes to bacterial infection after exposure to the environmental contaminant benzo(a)pyrene, [B(a)P]. The fundamental requirement in activation of macrophage effector functions is the binding of immunoglobulins to Fc receptors. FcγRIIa (CD32a), a member of the Fc family of immunoreceptors with low affinity for immunoglobulin G, has been reported to bind preferentially to IgG within lipid rafts. Previous research suggested that exposure to B(a)P suppressed macrophage effector functions but the molecular mechanisms remain elusive. The goal of this study was to elucidate the mechanism(s) of B(a)P-exposure induced suppression of macrophage function by examining the resultant effects of exposure-induced insult on CD32-lipid raft interactions in the regulation of IgG binding to CD32. The results demonstrate that exposure of macrophages to B(a)P alters lipid raft integrity by decreasing membrane cholesterol 25% while increasing CD32 into non-lipid raft fractions. This robust diminution in membrane cholesterol and 30% exclusion of CD32 from lipid rafts causes a significant reduction in CD32-mediated IgG binding to suppress essential macrophage effector functions. Such exposures across the lifespan would have the potential to induce immunosuppressive endophenotypes in vulnerable populations.

Differential Use of Human Neutrophil Fcγ Receptors for Inducing Neutrophil Extracellular Trap Formation

Neutrophils (PMN) are the most abundant leukocytes in the blood. PMN migrate from the circulation to sites of infection, where they are responsible for antimicrobial functions. PMN use phagocytosis, degranulation, and formation of neutrophil extracellular traps (NETs) to kill microbes. NETs are fibers composed of chromatin and neutrophil-granule proteins. Several pathogens, including bacteria, fungi, and parasites, and also some pharmacological stimuli such as phorbol 12-myristate 13-acetate (PMA) are efficient inducers of NETs. Antigen-antibody complexes are also capable of inducing NET formation. However the particular Fcγ receptor involved in triggering this function is a matter of controversy. In order to provide some insight into what Fcγ receptor is responsible for NET formation, each of the two human Fcγ receptors was stimulated individually by specific monoclonal antibodies and NET formation was evaluated. FcγRIIa cross-linking did not promote NET formation. Cross-linking other receptors such as integrins also did not promote NET formation. In contrast FcγRIIIb cross-linking induced NET formation similarly to PMA stimulation. NET formation was dependent on NADPH-oxidase, PKC, and ERK activation. These data show that cross-linking FcγRIIIb is responsible for NET formation by the human neutrophil.

Antibody modulation: Limiting the efficacy of therapeutic antibodies

Monoclonal antibodies (mAb) have revolutionised the way in which we treat disease. From cancer to autoimmunity, antibody therapy has been responsible for some of the most impressive clinical responses observed in the last 2 decades. A key component of this success has been their generally low levels of toxicity, and unique mechanisms of action. These two facets have allowed them to (a) be integrated rapidly into clinical practice in combination with conventional radio- and chemo-therapies and (b) to avoid the resistance mechanisms typically observed with classical small molecule drugs, such as upregulation of drug efflux transporters, dysregulation of apoptosis and mutations in key target enzymes/pathways. Although success with mAb therapies has been impressive, they are also subject to their own resistance mechanisms. In this perspective we discuss the various ways in which mAb therapeutics can be inhibited, concentrating mainly on the ways in which they can be removed from the target cell surface-a process called modulation. This can be achieved either in a cis-fashion on a single cell or in trans, precipitated by engagement with a second phagocytic cell. The evidence for each of these processes will be discussed, in addition to possible therapeutic strategies that might be employed to inhibit or reverse them.

Group V secreted phospholipase A2 is upregulated by IL-4 in human macrophages and mediates phagocytosis via hydrolysis of ethanolamine phospholipids

Studies on the heterogeneity and plasticity of macrophage populations led to the identification of two major polarization states: classically activated macrophages or M1, induced by IFN-γ plus LPS, and alternatively activated macrophages, induced by IL-4. We studied the expression of multiple phospholipase A2 enzymes in human macrophages and the effect that polarization of the cells has on their levels. At least 11 phospholipase A2 genes were found at significant levels in human macrophages, as detected by quantitative PCR. None of these exhibited marked changes after treating the cells with IFN-γ plus LPS. However, macrophage treatment with IL-4 led to strong upregulation of the secreted group V phospholipase A2 (sPLA2-V), both at the mRNA and protein levels. In parallel with increasing sPLA2-V expression levels, IL-4-treated macrophages exhibited increased phagocytosis of yeast-derived zymosan and bacteria, and we show that both events are causally related, because cells deficient in sPLA2-V exhibited decreased phagocytosis, and cells overexpressing the enzyme manifested higher rates of phagocytosis. Mass spectrometry analyses of lipid changes in the IL-4-treated macrophages suggest that ethanolamine lysophospholipid (LPE) is an sPLA2-V-derived product that may be involved in regulating phagocytosis. Cellular levels of LPE are selectively maintained by sPLA2-V. By supplementing sPLA2-V-deficient cells with LPE, phagocytosis of zymosan or bacteria was fully restored in IL-4-treated cells. Collectively, our results show that sPLA2-V is required for efficient phagocytosis by IL-4-treated human macrophages and provide evidence that sPLA2-V-derived LPE is involved in the process.

Activatory and inhibitory Fcγ receptors augment rituximab-mediated internalization of CD20 independent of signaling via the cytoplasmic domain

Type I anti-CD20 mAb such as rituximab and ofatumumab engage with the inhibitory FcγR, FcγRIIb on the surface of B cells, resulting in immunoreceptor tyrosine-based inhibitory motif (ITIM) phosphorylation. Internalization of the CD20·mAb·FcγRIIb complex follows, the rate of which correlates with FcγRIIb expression. In contrast, although type II anti-CD20 mAb such as tositumomab and obinutuzumab also interact with and activate FcγRIIb, this interaction fails to augment the rate of CD20·mAb internalization, raising the question of whether ITIM phosphorylation plays any role in this process. We have assessed the molecular requirements for the internalization process and demonstrate that in contrast to internalization of IgG immune complexes, FcγRIIb-augmented internalization of rituximab-ligated CD20 occurs independently of the FcγRIIb ITIM, indicating that signaling downstream of FcγRIIb is not required. In transfected cells, activatory FcγRI, FcγRIIa, and FcγRIIIa augmented internalization of rituximab-ligated CD20 in a similar manner. However, FcγRIIa mediated a slower rate of internalization than cells expressing equivalent levels of the highly homologous FcγRIIb. The difference was maintained in cells expressing FcγRIIa and FcγRIIb lacking cytoplasmic domains and in which the transmembrane domains had been exchanged. This difference may be due to increased degradation of FcγRIIa, which traffics to lysosomes independently of rituximab. We conclude that the cytoplasmic domain of FcγR is not required for promoting internalization of rituximab-ligated CD20. Instead, we propose that FcγR provides a structural role in augmenting endocytosis that differs from that employed during the endocytosis of immune complexes.

A 175 million year history of T cell regulatory molecules reveals widespread selection, with adaptive evolution of disease alleles

T cell activation plays a central role in immune response and in the maintenance of self-tolerance. We analyzed the evolutionary history of T cell regulatory molecules. Nine genes involved in triggering T cell activation or in regulating the ensuing response evolved adaptively in mammals. Several positively selected sites overlap with positions interacting with the binding partner or with cellular components. Population genetic analysis in humans revealed a complex scenario of local (FASLG, CD40LG, HAVCR2) and worldwide (FAS, ICOSLG) adaptation and H. sapiens-to-Neandertal gene flow (gene transfer between populations). Disease variants in these genes are preferential targets of pathogen-driven selection, and a Crohn's disease risk polymorphism targeted by bacterial-driven selection modulates the expression of ICOSLG in response to a bacterial superantigen. Therefore, we used evolutionary information to generate experimentally testable hypotheses concerning the function of specific genetic variants and indicate that adaptation to infection underlies the maintenance of autoimmune risk alleles.

FcγRIIa requires lipid rafts, but not co-localization into rafts, for effector function

OBJECTIVE

To determine if receptor localization into lipid rafts, or the lipid rafts themselves, are important for FcγRIIa effector functions.

MATERIAL

Wild-type FcγRIIa or mutant FcγRIIa(C208A) that does not translocate to lipid rafts were transfected into Chinese hamster ovary (CHO) cells which have been shown to be reliable cells for studying FcγR function.

TREATMENT

Cells were treated with buffer or methyl-β-cyclodextrin (MβCD) to deplete cholesterol and dissolve the structure of lipid rafts.

METHODS

To evaluate lipid raft association, transfected CHO cells were lysed and centrifuged over a sucrose gradient. Fractions were run on SDS-PAGE and blotted for FcγRIIa or sphingolipid GM1 to illustrate the lipid raft fractions. Lateral mobility of GFP-tagged wild-type or mutant FcγRIIa was assessed using fluorescence recovery after photobleaching (FRAP) microscopy. Internalization of IgG-opsonized erythrocytes was assessed by fluorescence microscopy and uptake of heat-aggregated IgG (haIgG) was measured using flow cytometry.

RESULTS

We observed that FcγRIIa(C208A) did not localize into lipid rafts. However, the mutant FcγRIIa retained lateral mobility and effector function similar to wild-type FcγRIIa. However, mutant FcγRIIa function was abolished upon treatment with MβCD.

CONCLUSIONS

Lipid rafts provide an essential component required for effector activities independent of receptor localization.

Evidence for the presence of functional lipid rafts in immune cells of ectothermic organisms

The role of lipid rafts in non-mammalian leukocytes has been scarcely investigated. We performed biochemical and functional analysis of lipid rafts in fish leukocytes. Fish Flotillin-1 and a fish GM1-like molecule (fGM1-L) were found in low density detergent-resistant membranes (LD-DRM) in goldfish macrophages and catfish B lymphocytes, similarly to mammals. The presence of flotillin-1 and fGM1-L in LD-DRM was sensitive to increased detergent concentrations, and cholesterol extraction. Confocal microscopy analysis of flotillin-1 and fGM1-L in fish leukocytes showed a distinctive punctuated staining pattern, suggestive of pre-existing rafts. Confocal microscopy analysis of macrophages showed that the membrane of phagosomes containing serum-opsonized zymosan was enriched in fGM1-L, and zymosan phagocytosis was reduced after cholesterol extraction. The presence of flotillin-1 and fGM1-L in LD-DRM, the microscopic evidence of flotillin-1 and fGM1-L on fish macrophages and B-cells, and the sensitivity of phagocytosis to cholesterol extraction, indicate that lipid rafts are biochemically and functionally similar in leukocytes from fish and mammals.

Examination of the stimulatory signaling potential of a channel catfish leukocyte immune-type receptor and associated adaptor

Expressed by various subsets of myeloid and lymphoid immune cells, channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are predicted to play a key role in the initiation and termination of teleost cellular effector responses. These type I transmembrane proteins belong to the immunoglobulin superfamily and display features of immunoregulatory receptors with inhibitory and/or stimulatory signaling potential. Expanding on our previous work, which demonstrated that putative stimulatory IpLITR-types associated with the catfish adaptor proteins IpFcRγ and FcRγ-L, this study focuses on the functional significance of this immune receptor-adaptor signaling complex. Specifically, we generated an epitope-tagged chimeric receptor construct by fusing the extracellular domain of IpLITR 2.6b with the transmembrane region and cytoplasmic tail of IpFcRγ-L. This chimera was stably expressed in a rat basophilic leukemia (RBL) cell line, RBL-2H3, and following cross-linking of the surface receptor with an anti-hemagglutinin monoclonal antibody or opsonized microspheres, the chimeric teleost receptor induced cellular degranulation and phagocytic responses, respectively. Site-directed mutagenesis of the immunoreceptor tyrosine-based activation motif encoded within the cytoplasmic tail of the chimera confirmed that these functional responses were dependent on the phosphorylated tyrosines within this motif. Using a combination of phospho-specific antibodies and pharmacological inhibitors, we also demonstrate that the IpLITR/IpFcRγ-L-induced degranulation response requires the activity of Src homology 2 domain containing protein tyrosine phosphatases, phosphatidylinositol 3-kinase, protein kinase C, and mitogen-activated protein kinases but appears independent of the c-Jun N-terminal kinase and p38 MAP kinase pathways. In addition to this first look at stimulatory IpLITR-mediated signaling and its influence on cellular effector responses, the advantage of generating RBL-2H3 cells stably expressing a functional IpLITR-adaptor chimera will be discussed.

Commercial naphthenic acids and the organic fraction of oil sands process water downregulate pro-inflammatory gene expression and macrophage antimicrobial responses

This is the first report showing that the organic fraction of oil sands process water (OSPW-OF), and commercial naphthenic acids (C-NAs), cause immunotoxicity. The exposure of mouse bone marrow-derived macrophages (BMDM) to different amounts of C-NAs or OSPW-OF, did not affect cell viability in vitro. We examined whether exposure of BMDM to C-NAs or OSPW-OF affected various antimicrobial responses of these cells. A dose-dependent decrease in nitric oxide response was observed after treatment of BMDM with OSPW-OF, but not with C-NAs. Although OSPW-OF and C-NAs both down-regulated the respiratory burst response of BMDM, the suppression of the production of reactive oxygen intermediates was more pronounced in cells treated with OSPW-OF. Treatment with OSPW-OF or C-NAs reduced BMDM phagocytosis of zymosan and latex beads. The decrease of BMDM antimicrobial response after exposure to OSPW-OF or C-NAs, was accompanied by decreased pro-inflammatory cytokine gene expression. Oral exposure of mice to OSPW-OF caused down-regulation in the expression of genes encoding pro-inflammatory cytokines IFNγ, IL-1β and CSF-1. Our findings indicated that OSPW causes immunotoxic effects that may impair the ability of an exposed host to defend against infectious disease. Furthermore, given the differences between the effects of OSPW-OF and C-NAs, C-NAs should not be assumed to be a direct surrogate for the immunotoxic chemical species in OSPW.

Fc gammaRIIIb triggers raft-dependent calcium influx in IgG-mediated responses in human neutrophils

Human neutrophils constitutively express a unique combination of FcγRs, namely FcγRIIa and FcγRIIIb. Numerous lines of evidence support the concept that these FcγRs generate only partially characterized intracellular signals. However, despite the fact that both receptors are likely to be engaged simultaneously in a physiological setting, no recent publications have investigated the distinct, although partially convergent, results of their joint activation in IgG-dependent responses. To examine the significance of the co-expression of FcγRIIa and FcγRIIIb on human neutrophils, we analyzed the neutrophil responses to stimuli that engage these FcγRs, namely the phagocytosis of human IgG-opsonized zymosan and the responses to heat-aggregated IgGs. Blocking antibodies to either FcγR significantly decreased the phagocytic index and the stimulated production of superoxide anions. Both receptors are required for optimal IgG-dependent responses by human neutrophils. On the other hand, only blocking antibodies to FcγRIIIb, but not to FcγRIIa, inhibited the mobilization of calcium in response to heat-aggregated IgGs. Furthermore, phagocytosis of IgG-opsonized zymosan by human neutrophils required an extracellular influx of calcium that was blocked only by antibodies against FcγRIIIb. We also observed that this calcium influx as well as the IgG-dependent phagocytosis were dependent on the integrity of the plasma membrane detergent-resistant microdomains to which both isoforms were recruited following stimulation by heat-aggregated IgGs. These data clarify the mechanisms that regulate the FcγRs constitutively expressed on human neutrophils, describe a specific contribution of FcγRIIIb at the level of the mobilization of calcium, and provide evidence for a crucial role of detergent-resistant microdomains in this process.

Divergent intracellular sorting of Fc{gamma}RIIA and Fc{gamma}RIIB2

The human low affinity FcγRII family includes both the activating receptor FcγRIIA and the inhibitory receptor FcγRIIB2. These receptors have opposing signaling functions but are both capable of internalizing IgG-containing immune complexes through clathrin-mediated endocytosis. We demonstrate that upon engagement by multivalent aggregated human IgG, FcγRIIA expressed in ts20 Chinese hamster fibroblasts is delivered along with its ligand to lysosomal compartments for degradation, while FcγRIIB2 dissociates from the ligand and is routed separately into the recycling pathway. FcγRIIA sorting to lysosomes requires receptor multimerization, but does not require either Src family kinase activity or ubiquitylation of receptor lysine residues. The sorting of FcγRIIB2 away from a degradative fate is not due to its lower affinity for IgG and occurs even upon persistent receptor aggregation. Upon co-engagement of FcγRIIA and FcγRIIB2, the receptors are sorted independently to distinct final fates after dissociation of co-clustering ligand. These results reveal fundamental differences in the trafficking behavior of different Fcγ receptors.

Differential requirement of lipid rafts for FcγRIIA mediated effector activities

Immunoglobulin G (IgG) dependent activities are important in host defense and autoimmune diseases. Various cell types including macrophages and neutrophils contribute to pathogen destruction and tissue damage through binding of IgG to Fcγ receptors (FcγR). One member of this family, FcγRIIA, is a transmembrane glycoprotein known to mediate binding and internalization of IgG-containing targets. FcγRIIA has been observed to translocate into lipids rafts upon binding IgG-containing targets. We hypothesize that lipid rafts participate to different extents in binding and internalizing targets of different sizes. We demonstrate that disruption of lipid rafts with 8mM methyl-β-cyclodextrin (MβCD) nearly abolishes binding (91% reduction) and phagocytosis (60% reduction) of large IgG-coated targets. Conversely, binding and internalization of small IgG-complexes is less dependent on lipid rafts (49% and 17% inhibition at 8mM MβCD, respectively). These observations suggest that differences between phagocytosis and endocytosis may arise as early as the initial stages of ligand recognition.

Fcgamma receptors exhibit different phagocytosis potential in human neutrophils

In neutrophils, two receptors for IgG antibodies, namely FcgammaRIIA and FcgammaRIIIB are constitutively expressed, and a third one, FcgammaRI, can be upregulated by interferon-gamma. Whether FcgammaRIIIB is capable of triggering phagocytosis by itself is still controversial. The main role of FcgammaRI has not been clearly established in these cells. To address this problem, neutrophils were treated with interferon-gamma, and then phagocytosis mediated by each type of Fcgamma receptor was evaluated by flow cytometry. FcgammaRIIA was the most efficient receptor for phagocytosis. FcgammaRIIIB could mediate phagocytosis but much less efficiently than FcgammaRIIA. Both FcgammaRIIA- and FcgammaRIIIB-mediated phagocytosis were blocked by inhibitors of Src family kinases, Syk, PI 3-K, and ERK. In contrast, interferon-gamma-induced FcgammaRI was not able to mediate phagocytosis. Also, FcgammaRI did not activate ERK in the nucleus, but was however able to stimulate an efficient calcium rise. These data show that different neutrophil Fcgamma receptors possess different phagocytosis capabilities: FcgammaRIIA and FcgammaRIIIB, but not FcgammaRI, promote phagocytosis.

Association of FcgammaRIIa (CD32a) with lipid rafts regulates ligand binding activity

Binding of Igs to myeloid cells via FcR is a key event in the control of innate and acquired immunity. FcgammaRIIa (CD32a) is a receptor for multivalent IgG expressed predominantly by myeloid cells, and its association with microdomains rich in cholesterol and sphingolipids, termed as lipid rafts, has been reported to be essential for efficient signaling. However, for many myeloid cell types, ligand binding to CD32a is suppressed by as yet undefined mechanisms. In this study, we have examined the role of CD32a-lipid raft interactions in the regulation of IgG binding to CD32a. Disruption of lipid raft structure following depletion or sequestration of membrane cholesterol greatly inhibited CD32a-mediated IgG binding. Furthermore, specific CD32a mutants, which show reduced association with lipid rafts (A224S and C241A), displayed decreased levels of IgG binding compared with wild-type CD32a. In contrast, constitutively lipid raft-associated CD32a (GPI-anchored CD32a) exhibited increased capacity for IgG binding compared with the full-length transmembrane CD32a. Our findings clearly suggest a major role for lipid rafts in the regulation of IgG binding and, more specifically, that suppression of CD32a-mediated IgG binding in myeloid cells is achieved by receptor exclusion from lipid raft membrane microdomains.

An fMLP receptor is involved in activation of phagocytosis by hemocytes from specific insect species

In mammalian phagocytes, the bacterial formylated peptide fMLP functions both as a potent enhancer of phagocytosis and chemoattractant. fMLP has been reported to be chemotactic for hemocytes of two marine invertebrates, and of the insect Manduca sexta (Lepidoptera). Whether fMLP is also able to activate phagocytosis has not been explored in hemocytes of any invertebrate. To determine the effect of fMLP on insect hemocyte phagocytosis, in vitro phagocytosis assays were performed with hemocytes from the insects: Gromphadorhina portentosa (Blattodea), Acheta domesticus (Orthoptera), Zophobas morio (Coleoptera), and Galleria mellonella (Lepidoptera). Phagocytosis of latex, zymosan (yeast), Gram-positive and Gram-negative bacteria was measured by flow cytometry, in the presence of increasing fMLP concentrations. G. portentosa hemocytes showed no enhancement of phagocytosis by fMLP. A. domesticus hemocytes had increased phagocytosis of latex and Gram-negative bacteria in the presence of fMLP. Z. morio hemocytes increased phagocytosis of latex, yeast, and Gram-negative bacteria after fMLP stimulation. Galleria mellonella hemocytes increased phagocytosis of latex after fMLP stimulation. Treating hemocytes with Pertussis toxin, a known inhibitor of the signaling pathway initiated by the mammalian fMLP receptor, returned phagocytosis to basal levels. Also, hemocytes from all insect species tested presented a similar chemotactic response to fMLP. These data suggest that, whereas the ability of hemocytes to chemotactically-respond to fMLP is conserved in insects ranging from Blattodea to Lepidoptera, the ability to respond to fMLP by activating phagocytosis is restricted to specific insect species.

Essential and unique roles of PIP5K-gamma and -alpha in Fcgamma receptor-mediated phagocytosis

The actin cytoskeleton is dynamically remodeled during Fcγ receptor (FcγR)-mediated phagocytosis in a phosphatidylinositol (4,5)-bisphosphate (PIP₂)-dependent manner. We investigated the role of type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K) γ and α isoforms, which synthesize PIP₂, during phagocytosis. PIP5K-γ−/− bone marrow–derived macrophages (BMM) have a highly polymerized actin cytoskeleton and are defective in attachment to IgG-opsonized particles and FcγR clustering. Delivery of exogenous PIP₂ rescued these defects. PIP5K-γ knockout BMM also have more RhoA and less Rac1 activation, and pharmacological manipulations establish that they contribute to the abnormal phenotype. Likewise, depletion of PIP5K-γ by RNA interference inhibits particle attachment. In contrast, PIP5K-α knockout or silencing has no effect on attachment but inhibits ingestion by decreasing Wiskott-Aldrich syndrome protein activation, and hence actin polymerization, in the nascent phagocytic cup. In addition, PIP5K-γ but not PIP5K-α is transiently activated by spleen tyrosine kinase–mediated phosphorylation. We propose that PIP5K-γ acts upstream of Rac/Rho and that the differential regulation of PIP5K-γ and -α allows them to work in tandem to modulate the actin cytoskeleton during the attachment and ingestion phases of phagocytosis.

Reovirus FAST protein transmembrane domains function in a modular, primary sequence-independent manner to mediate cell-cell membrane fusion

The FAST proteins are a unique family of virus-encoded cell-cell membrane fusion proteins. In the absence of a cleavable N-terminal signal peptide, a single-pass transmembrane domain (TMD) functions as a reverse signal-anchor to direct the FAST proteins into the plasma membrane in an N(exo)/C(cyt) topology. There is little information available on the role of the FAST protein TMD in the cell-cell membrane fusion reaction. We show that in the absence of conservation in the length or primary amino acid sequence, the p14 TMD can be functionally exchanged with the TMDs of the p10 and p15 FAST proteins. This is not the case for chimeric p14 proteins containing the TMDs of two different enveloped viral fusion proteins or a cellular membrane protein; such chimeric proteins were defective for both pore formation and syncytiogenesis. TMD structural features that are conserved within members of the FAST protein family presumably play direct roles in the fusion reaction. Molecular modeling suggests that the funnel-shaped architecture of the FAST protein TMDs may represent such a conserved structural and functional motif. Interestingly, although heterologous TMDs exert diverse influences on the trafficking of the p14 FAST protein, these TMDs are capable of functioning as reverse signal-anchor sequences to direct p14 into lipid rafts in the correct membrane topology. The FAST protein TMDs are therefore not primary determinants of type III protein topology, but they do play a direct, sequence-independent role in the membrane fusion reaction.

Cholesterol accumulation by macrophages impairs phagosome maturation

Macrophages are key to the pathogenesis of atherosclerosis. They take up and store excessive amounts of cholesterol associated with modified low density lipoprotein, eventually becoming foam cells that display altered immune responsiveness. We studied the effects of cholesterol accumulation on phagosome formation and maturation, using lipid transport antagonists and cholesterol transport-deficient mutants. In macrophages treated with U18666A, a transport antagonist that prevents cholesterol exit from late endosomes/lysosomes, the early stages of maturation proceeded normally; phagosomes acquired Rab5, phosphatidylinositol 3-phosphate, and EEA1 and merged with LAMP-containing vesicles. However, fusion with lysosomes was impaired. Rab7, which is required for phagolysosome formation, was acquired by phagosomes but remained inactive. Maturation was also studied in fibroblasts from Niemann-Pick type C individuals that have defective cholesterol transport. Transfection of FcgammaIIA receptors was used to confer phagocytic capability to these fibroblasts. Niemann-Pick type C phagosomes failed to fuse with lysosomes, whereas wild type fibroblasts formed normal phagolysosomes. These findings indicate that cholesterol accumulation can have a detrimental effect on phagosome maturation by impairing the activation of Rab7, sequestering it and its effectors in cholesterol-enriched multilamellar compartments.

How Mycobacterium tuberculosis subverts host immune responses

Mycobacterium tuberculosis is the causative agent of pulmonary tuberculosis which has infected one third of the mankind and causes 2-3 million deaths worldwide each year. The persistence of the infection ensues from the ability of M. tuberculosis to subvert host immune responses in favor of survival and growth of mycobacteria in macrophages. The mechanisms by which M. tuberculosis manipulates the host immune system have only recently come to light. These activities are attributed to lipoarabinomannans (LAM) and their precursors lipomannans (LM), two predominant glycolipids of M. tuberculosis cell wall. LM are able to skew anti-mycobacterial immune responses into un-protective ones, while LAM evoke immunosupression upon binding to macrophage and dendritic cell receptors specialized in binding to "self" host components. A newly emerging idea implicates plasma membrane rafts in LM and LAM signaling. Depending on acylation patterns, the glycolipids may either directly incorporate into the raft membrane via mannosylphosphatidylinositol anchors or interact with raft-associated proteins to affect the assembly of receptor signaling complexes.

Pathogen destruction versus intracellular survival: the role of lipids as phagosomal fate determinants

Phagocytosis is a key component of the innate immune response and of the clearance of apoptotic bodies. Phagosome formation and subsequent maturation require extensive cytoskeletal rearrangement and precisely choreographed vesicular fusion and fission events. The objectives of this review are to highlight the functional importance of lipids in the phagocytic process, to discuss how pathogenic microorganisms can in some cases manipulate host lipid metabolism to either co-opt or disrupt phagosome maturation and promote their own survival, and to describe how defective phagosomal lipid metabolism can result in disease.

Targeting of pseudorabies virus structural proteins to axons requires association of the viral Us9 protein with lipid rafts

The pseudorabies virus (PRV) Us9 protein plays a central role in targeting viral capsids and glycoproteins to axons of dissociated sympathetic neurons. As a result, Us9 null mutants are defective in anterograde transmission of infection in vivo. However, it is unclear how Us9 promotes axonal sorting of so many viral proteins. It is known that the glycoproteins gB, gC, gD and gE are associated with lipid raft microdomains on the surface of infected swine kidney cells and monocytes, and are directed into the axon in a Us9-dependent manner. In this report, we determined that Us9 is associated with lipid rafts, and that this association is critical to Us9-mediated sorting of viral structural proteins. We used infected non-polarized and polarized PC12 cells, a rat pheochromocytoma cell line that acquires many of the characteristics of sympathetic neurons in the presence of nerve growth factor (NGF). In these cells, Us9 is highly enriched in detergent-resistant membranes (DRMs). Moreover, reducing the affinity of Us9 for lipid rafts inhibited anterograde transmission of infection from sympathetic neurons to epithelial cells in vitro. We conclude that association of Us9 with lipid rafts is key for efficient targeting of structural proteins to axons and, as a consequence, for directional spread of PRV from pre-synaptic to post-synaptic neurons and cells of the mammalian nervous system.

Lipid signaling and the modulation of surface charge during phagocytosis

Phagocytosis is an important component of innate and adaptive immunity. The formation of phagosomes and the subsequent maturation that capacitates them for pathogen elimination and antigen presentation are complex processes that involve signal transduction, cytoskeletal reorganization, and membrane remodeling. Lipids are increasingly appreciated to play a crucial role in these events. Sphingolipids, cholesterol, and glycerophospholipids, notably the phosphoinositides, are required for the segregation of signaling microdomains and for the generation of second messengers. They are also instrumental in the remodeling of the actin cytoskeleton and in directing membrane traffic. They accomplish these feats by congregating into liquid-ordered domains, by generating active metabolites that activate receptors, and by recruiting and anchoring specific protein ligands to the membrane, often altering their conformation and catalytic activity. A less appreciated role of acidic phospholipids is their contribution to the negative surface charge of the inner leaflet of the plasmalemma. The unique negativity of the inner aspect of the plasma membrane serves to attract and anchor key signaling and effector molecules that are required to initiate phagosome formation. Conversely, the loss of charge that accompanies phospholipid metabolism as phagosomes seal facilitates the dissociation of proteins and the termination of signaling and cytoskeleton assembly. In this manner, lipids provide a binary electrostatic switch to control phagocytosis.