Role of glycosphingolipid-enriched microdomains in innate immunity: microdomain-dependent phagocytic cell functions

The innate immune system is the first line of defense against pathogenic microorganisms, such as bacteria, fungi, and viruses. Phagocytes, such as neutrophils and macrophages, play an important role in the innate immune system by recognizing, engulfing, and eliminating pathogens. It has been suggested that lipid membrane microdomains/rafts of phagocytes are involved in these innate immune responses, including superoxide generation, cell migration, and phagocytosis. Lactosylceramide (LacCer), a neutral glycosphingolipid, forms glycosphingolipid-enriched microdomains together with the Src family kinase, Lyn, on the neutrophil plasma membrane. LacCer-enriched microdomains have been suggested to play important roles in innate immune function of neutrophils. However, the molecular mechanisms underlying these phenomena remain largely unknown. Recent proteomic analyses of microdomains from phagocytes have provided insight into membrane microdomain-mediated functions in the processes of phagocytosis. In this review, we discuss the membrane microdomain-associated immune functions of phagocytes, focusing on those functions of LacCer-enriched microdomains and recent proteomic approaches to determine the molecular mechanisms underlying these functions.     

Genetic analysis of innate immunity: TIR adapter proteins in innate and adaptive immune responses

The innate immune system senses pathogens largely through signals initiated by a collection of phylogenetically related proteins known as "Toll-like receptors" (TLRs), of which 10 representatives are encoded in the human genome. Our understanding of the sensing role played by the TLRs began with the positional cloning of a spontaneous mutation (Lps(d)) in the gene encoding the mammalian lipopolysaccharide (LPS) receptor. Other key innate immunity proteins have been disclosed by germline mutagenesis, and are discussed in the present review.     

Membrane Fas ligand activates innate immunity and terminates ocular immune privilege

It has been proposed that the constitutive expression of Fas ligand (FasL) in the eye maintains immune privilege, in part through inducing apoptosis of infiltrating Fas(+) T cells. However, the role of FasL in immune privilege remains controversial due to studies that indicate FasL is both pro- and anti-inflammatory. To elucidate the mechanism(s) by which FasL regulates immune privilege, we used an ocular tumor model and examined the individual roles of the membrane-bound and soluble form of FasL in regulating ocular inflammation. Following injection into the privileged eye, tumors expressing only soluble FasL failed to trigger inflammation and grew progressively. By contrast, tumors expressing only membrane FasL 1) initiated vigorous neutrophil-mediated inflammation, 2) terminated immune privilege, and 3) were completely rejected. Moreover, the rejection coincided with activation of both innate and adaptive immunity. Interestingly, a higher threshold level of membrane FasL on tumors is required to initiate inflammation within the immune privileged eye, as compared with nonprivileged sites. The higher threshold is due to the suppressive microenvironment found within aqueous humor that blocks membrane FasL activation of neutrophils. However, aqueous humor is unable to completely block the proinflammatory effects of tumor cells that express high levels of membrane FasL. In conclusion, our data indicate that the function of FasL on intraocular tumors is determined by the microenvironment in conjunction with the form and level of FasL expressed.     

Mitochondria in innate immune responses

The innate immune system has a key role in the mammalian immune response. Recent research has demonstrated that mitochondria participate in a broad range of innate immune pathways, functioning as signalling platforms and contributing to effector responses. In addition to regulating antiviral signalling, mounting evidence suggests that mitochondria facilitate antibacterial immunity by generating reactive oxygen species and contribute to innate immune activation following cellular damage and stress. Therefore, in addition to their well-appreciated roles in cellular metabolism and programmed cell death, mitochondria appear to function as centrally positioned hubs in the innate immune system. Here, we review the emerging knowledge about the roles of mitochondria in innate immunity.     

Leukotrienes: underappreciated mediators of innate immune responses

Leukotrienes are bronchoconstrictor and vasoactive lipid mediators that are targets in the treatment of asthma. Although they are increasingly recognized to exert broad proinflammatory effects, their role in innate immune responses is less well appreciated. These molecules are indeed synthesized by resident and recruited leukocytes during infection. Acting via cell surface G protein-coupled receptors and subsequent intracellular signaling events, they enhance leukocyte accumulation, phagocyte capacity for microbial ingestion and killing, and generation of other proinflammatory mediators. Interestingly, a variety of acquired states of immunodeficiency, such as HIV infection and malnutrition, are characterized by a relative deficiency of leukotriene synthesis. The data reviewed herein point to leukotrienes as underappreciated yet highly relevant mediators of innate immunity.     

Bacterial adaptation to host innate immunity responses

Several recent reports show that different bacterial components trigger innate and inflammatory responses in host organisms. In parallel, selected bacterial virulence factors have been identified that interfere with corresponding responses. In many cases, this involves interference with host proinflammatory signal transduction pathways, whereas in selected cases bacterial virulence factors interfere with host antibacterial mechanisms. This indicates that bacteria, besides activating cellular responses, also have the capacity to directly interact with branches of the innate defence.     

Factors influencing innate immunity and vaccine responses in infancy

Despite significant progress in reducing the burden of mortality in children under the age of five, reducing mortality in newborns remains a major challenge. Infection plays a significant role in infant deaths and interventions such as early vaccination or antenatal immunization could make a significant contribution to prevention of such deaths. In the last few years, we have gained new insights into immune ontogeny and are now beginning to understand the impact of vaccines, nutrition and environmental factors on ‘training′ of the immune response in early life. This review article sets out to explain why vaccine responses can be heterogeneous between populations and individuals by providing examples chosen to illustrate the impact of host, pathogen and environmental factors on shaping the immune ‘interactome′ in young children.     

Phosphoprotein associated with glycosphingolipid-enriched microdomains differentially modulates SRC kinase activity in brain maturation

Src family kinases (SFK) control multiple processes during brain development and function. We show here that the phosphoprotein associated with glycosphigolipid-enriched microdomains (PAG)/Csk binding protein (Cbp) modulates SFK activity in the brain. The timing and localization of PAG expression overlap with Fyn and Src, both of which we find associated to PAG. We demonstrate in newborn (P1) mice that PAG negatively regulates Src family kinases (SFK). P1 Pag1 ^-/- mouse brains show decreased recruitment of Csk into lipid rafts, reduced phosphorylation of the inhibitory tyrosines within SFKs, and an increase in SFK activity of >/ = 50%. While in brain of P1 mice, PAG and Csk are highly and ubiquitously expressed, little Csk is found in adult brain suggesting altered modes of SFK regulation. In adult brain Pag1-deficiency has no effect upon Csk-distribution or inhibitory tyrosine phosphorylation, but kinase activity is now reduced (−20–30%), pointing to the development of a compensatory mechanism that may involve PSD93. The distribution of the Csk-homologous kinase CHK is not altered. Importantly, since the activities of Fyn and Src are decreased in adult Pag1 ^-/- mice, thus presenting the reversed phenotype of P1, this provides the first in vivo evidence for a Csk-independent positive regulatory function for PAG in the brain.     

Legionella secreted effectors and innate immune responses

Legionella pneumophila is a facultative intracellular pathogen capable of replicating in a wide spectrum of cells. Successful infection by Legionella requires the Dot/Icm type IV secretion system, which translocates a large number of effector proteins into infected cells. By co-opting numerous host cellular processes, these proteins function to establish a specialized organelle that allows bacterial survival and proliferation. Even within the vacuole, L. pneumophila triggers robust immune responses. Recent studies reveal that a subset of Legionella effectors directly target some basic components of the host innate immunity systems such as phagosome maturation. Others play essential roles in engaging the host innate immune surveillance system. This review will highlight recent progress in our understanding of these interactions and discuss implications for the study of the immune detection mechanisms.     

Forward genetic dissection of afferent immunity: the role of TIR adapter proteins in innate and adaptive immune responses

The innate immune system senses pathogens largely through signals initiated by proteins known as 'Toll-like receptors' (TLRs), of which ten representatives are known to be encoded in the human genome. The understanding of the biochemical circuitry that maintains the innate capacity for immune recognition and response has loomed as a major hurdle in immunology. A total of five adapter proteins with cytoplasmic domain homology to the TLRs are known to exist in mammals. These proteins show preferential association with individual TLR family members, giving a particular character to the signals that distinct microorganisms initiate, and also initiate the adaptive immune response. The adaptive immune response is dependent upon upregulation of costimulatory molecules (UCM) such as CD80 and CD86. Forward genetic analysis has revealed that this upregulation depends upon an adapter encoded by a locus known as Lps2, and upon type I interferon receptor signaling.     

Adaptive immune cells temper initial innate responses

Toll-like receptors (TLRs) recognize conserved microbial structures called pathogen-associated molecular patterns. Signaling from TLRs leads to upregulation of co-stimulatory molecules for better priming of T cells and secretion of inflammatory cytokines by innate immune cells. Lymphocyte-deficient hosts often die of acute infection, presumably owing to their lack of an adaptive immune response to effectively clear pathogens. However, we show here that an unleashed innate immune response due to the absence of residential T cells can also be a direct cause of death. Viral infection or administration of poly(I:C), a ligand for TLR3, led to cytokine storm in T-cell- or lymphocyte-deficient mice in a fashion dependent on NK cells and tumor necrosis factor. We have further shown, through the depletion of CD4+ and CD8+ cells in wild-type mice and the transfer of T lymphocytes into Rag-1-deficient mice, respectively, that T cells are both necessary and sufficient to temper the early innate response. In addition to the effects of natural regulatory T cells, close contact of resting CD4+CD25-Foxp3- or CD8+ T cells with innate cells could also suppress the cytokine surge by various innate cells in an antigen-independent fashion. Therefore, adaptive immune cells have an unexpected role in tempering initial innate responses.     

Dendritic cells in innate immune responses against HIV

Dendritic cells (DCs) were recently found to be innate immunity effectors against tumoral cells and viruses. (i) In response to most viruses, including HIV, plasmacytoid DCs are responsible for most of the type I IFN secretion, which is strongly anti-viral and induces TH1 type responses. Myeloid DCs secrete IL-12, which is also important for TH1-type and cytotoxic responses. In HIV patient blood, both DC population numbers decrease as early as the primary stage. Plasmacytoid DC numbers correlate with type I IFN secretion, which is a prognosis predictor, particularly under treatment. IL-12 secretion is also defective. Immunotherapies to replace the defective cytokines or to restore a potentially defective DC-T lymphocyte feed-back might help patients restore their immune responses under antiviral therapy. (ii) After measles and other viral infections, or incubation with dsRNA, DCs become cytotoxic and consequently exhibit natural killer function, through upregulation of type I IFN secretion which enhances TRAIL expression. In HIV infection, this mechanism was not demonstrated yet, but it might a) be responsible for the massive apoptosis of uninfected lymphocytes, and b) increase specific immunity through cross-presentation of antigens from infected cells killed by DCs. (iii) DCs direct expansion and effector functions of NK cells in the absence of adaptive-type cytokines and modulate NKT cell IFN-gamma production. Reciprocally, NK activation triggers DC maturation. HIV-1 Tat inhibits NK cell cytotoxicity directly and probably through inhibition of IL-12 secretion by DC. Therefore, understanding the functions of DCs in innate immune responses and in pathogenesis will help obtain better HIV replication control.     

Chitin modulates innate immune responses of keratinocytes

Background

Chitin, after cellulose the second most abundant polysaccharide in nature, is an essential component of exoskeletons of crabs, shrimps and insects and protects these organisms from harsh conditions in their environment. Unexpectedly, chitin has been found to activate innate immune cells and to elicit murine airway inflammation. The skin represents the outer barrier of the human host defense and is in frequent contact with chitin-bearing organisms, such as house-dust mites or flies. The effects of chitin on keratinocytes, however, are poorly understood.

Methodology/Principal Findings

We hypothesized that chitin stimulates keratinocytes and thereby modulates the innate immune response of the skin. Here we show that chitin is bioactive on primary and immortalized keratinocytes by triggering production of pro-inflammatory cytokines and chemokines. Chitin stimulation further induced the expression of the Toll-like receptor (TLR) TLR4 on keratinocytes at mRNA and protein level. Chitin-induced effects were mainly abrogated when TLR2 was blocked, suggesting that TLR2 senses chitin on keratinocytes.

Conclusions/Significance

We speculate that chitin-bearing organisms modulate the innate immune response towards pathogens by upregulating secretion of cytokines and chemokines and expression of MyD88-associated TLRs, two major components of innate immunity. The clinical relevance of this mechanism remains to be defined.     

Hiding in plain sight: how HIV evades innate immune responses

Two groups have identified SAMHD1, a protein encoded by an Aicardi-Goutières Syndrome susceptibility gene, as the factor that restricts infection of macrophages and dendritic cells with HIV-1. Here we discuss implications of this discovery for induction of antiviral protective immunity.     

Membrane microdomains in immunoreceptor signaling

Membrane microdomains denoted commonly as lipid rafts (or membrane rafts) have been implicated in T-cell receptor (TCR), and more generally immunoreceptor, signaling for over 25 years. However, this area of research has been complicated by doubts about the real nature (and even existence) of these membrane entities, especially because of methodological problems connected with possible detergent artefacts. Recent progress in biophysical approaches and functional studies of raft resident proteins apparently clarified many controversial aspects in this area. At present, the prevailing view is that these membrane microdomains are indeed involved in many aspects of cell biology, including immunoreceptor signaling. Moreover, several other types of raft-like microdomains (perhaps better termed nanodomains) have been described, which apparently also play important biological roles.     

Stress responses and innate immunity: aging as a contributory factor

Evolutionary pressure has selected individuals with traits that allow them to survive to reproduction, without consideration of the consequences for the post-child rearing years and old age. In the 21st century, society is populated increasingly by the elderly and with the falling birth rate and improved health care this trend is set to continue for the foreseeable future. To minimize the potential burden on health services one would hope that 'growing old gracefully' should also mean 'growing old healthily'. However, for too many the aging process is accompanied by increasing physical and mental frailty producing an elevated risk of physical and psychological stress in old age. Stress is a potent modulator of immune function, which in youth can be compensated for by the presence of an optimal immune response. In the elderly the immune response is blunted as a result of the decline in several components of the immune system (immune senescence) and a shifting to a chronic pro-inflammatory status (the so-called 'inflamm-aging' effect). We discuss here what is known of the effects of both stress and aging upon the innate immune system, focusing in particular upon the age-related alterations in the hypopituitary-adrenal axis. We propose a double hit model for age and stress in which the age-related increase in the cortisol/sulphated dehydroepiandrosterone ratio synergizes with elevated cortisol during stress to reduce immunity in the elderly significantly.     

Toll-like receptors are key participants in innate immune responses

During an infection, one of the principal challenges for the host is to detect the pathogen and activate a rapid defensive response. The Toll-like family of receptors (TLRs), among other pattern recognition receptors (PRR), performs this detection process in vertebrate and invertebrate organisms. These type I transmembrane receptors identify microbial conserved structures or pathogen-associated molecular patterns (PAMPs). Recognition of microbial components by TLRs initiates signaling transduction pathways that induce gene expression. These gene products regulate innate immune responses and further develop an antigen-specific acquired immunity. TLR signaling pathways are regulated by intracellular adaptor molecules, such as MyD88, TIRAP/Mal, between others that provide specificity of individual TLR- mediated signaling pathways. TLR-mediated activation of innate immunity is involved not only in host defense against pathogens but also in immune disorders. The involvement of TLR-mediated pathways in auto-immune and inflammatory diseases is described in this review article.     

The X-files in immunity: sex-based differences predispose immune responses

Despite accumulating evidence in support of sex-based differences in innate and adaptive immune responses, in the susceptibility to infectious diseases and in the prevalence of autoimmune diseases, health research and clinical practice do not address these distinctions, and most research studies of immune responses do not stratify by sex. X-linked genes, hormones and societal context are among the many factors that contribute to disparate immune responses in males and females. It is crucial to address sex-based differences in disease pathogenesis and in the pharmacokinetics and pharmacodynamics of therapeutic medications to provide optimal disease management for both sexes.