C3-dependent mechanism of microglial priming relevant to multiple sclerosis

Human skin mast cells express complement factors C3 and C5

We examine whether complement factor C3 or C5 is synthesized by human skin-derived mast cells and whether their synthesis is regulated by cytokines. C3 and C5 mRNAs were assessed by RT-PCR, and proteins by flow cytometry, confocal microscopy, Western blotting, and ELISA. C3 and C5 mRNAs were each expressed, and baseline protein levels/10(6) cultured mast cells were 0.9 and 0.8 ng, respectively, and located in the cytoplasm outside of secretory granules. C3 accumulated in mast cell culture medium over time and by 3 d reached a concentration of 9.4 ± 8.0 ng/ml, whereas C5 levels were not detectable (<0.15 ng/ml). Three-day incubations of mast cells with IL-1α, IL-1β, IL-17, IFN-γ, IL-6, or anti-FcεRI did not affect C3 protein levels in culture medium, whereas incubations with PMA, TNF-α, IL-13, or IL-4 enhanced levels of C3 1.7- to 3.3-fold. In contrast with C3, levels of C5 remained undetectable. Importantly, treatment with TNF-α together with either IL-4 or IL-13 synergistically enhanced C3 (but not C5) production in culture medium by 9.8- or 7.1-fold, respectively. This synergy was blocked by attenuating the TNF-α pathway with neutralizing anti-TNF-α Ab, soluble TNFR, or an inhibitor of NF-κB, or by attenuating the IL-4/13 pathway with Jak family or Erk antagonists. Inhibitors of PI3K, Jnk, and p38 MAPK did not affect this synergy. Thus, human mast cells can produce and secrete C3, whereas β-tryptase can act on C3 to generate C3a and C3b, raising the likelihood that mast cells engage complement to modulate immunity and inflammation in vivo.

A shift in microglial β-amyloid binding in Alzheimer's disease is associated with cerebral amyloid angiopathy

Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA) are two common pathologies associated with β-amyloid (Aβ) accumulation and inflammation in the brain; neither is well understood. The objective of this study was to evaluate human post-mortem brains from AD subjects with purely parenchymal pathology, and those with concomitant CAA (and age-matched controls) for differential expression of microglia-associated Aβ ligands thought to mediate Aβ clearance and the association of these receptors with complement activation. Homogenates of brain parenchyma and enriched microvessel fractions from occipital cortex were probed for levels of C3b, membrane attack complex (MAC), CD11b and α-2-macroglobulin and immunoprecipitation was used to immunoprecipitate (IP) CD11b complexed with C3b and Aβ. Both C3b and MAC were significantly increased in CAA compared to AD-only and controls and IP showed significantly increased CD11b/C3b complexes with Aβ in AD/CAA subjects. Confocal microscopy was used to visualize these interactions. MAC was remarkably associated with CAA-affected blood vessels compared to AD-only and control vessels. These findings are consistent with an Aβ clearance mechanism via microglial CD11b that delivers Aβ and C3b to blood vessels in AD/CAA, which leads to Aβ deposition and propagation of complement to the cytolytic MAC, possibly leading to vascular fragility.

Innate immunity and neuroinflammation

Inflammation of central nervous system (CNS) is usually associated with trauma and infection. Neuroinflammation occurs in close relation to trauma, infection, and neurodegenerative diseases. Low-level neuroinflammation is considered to have beneficial effects whereas chronic neuroinflammation can be harmful. Innate immune system consisting of pattern-recognition receptors, macrophages, and complement system plays a key role in CNS homeostasis following injury and infection. Here, we discuss how innate immune components can also contribute to neuroinflammation and neurodegeneration.

Role of microglia in CNS autoimmunity

Multiple sclerosis (MS) is the most common autoimmune disease of the central nervous system (CNS) in the Western world. The disease is characterized histologically by the infiltration of encephalitogenic T_H1/T_H17-polarized CD4⁺ T cells, B cells, and a plethora of myeloid cells, resulting in severe demyelination ultimately leading to a degeneration of neuronal structures. These pathological processes are substantially modulated by microglia, the resident immune competent cells of the CNS. In this overview, we summarize the current knowledge regarding the highly diverse and complex function of microglia during CNS autoimmunity in either promoting tissue injury or tissue repair. Hence, understanding microglia involvement in MS offers new exciting paths for therapeutic intervention.

From development to dysfunction: microglia and the complement cascade in CNS homeostasis

Of the many mysteries that surround the brain, few surpass the awe-inspiring complexity of its development. The intricate wiring of the brain at both the system and molecular level is both spatially and temporally regulated in perfect synchrony. How such a delicate, yet elegant, system arises from an embryo's most basic cells remains at the forefront of neuroscientific research. At the cellular level, the competitive dance between synapses struggling to gain dominance seems to be refereed by both neurons themselves and microglia, the innate immune cells of the nervous system. Additionally, the unexpected complement cascade, a major effecter arm of the innate immune system, is almost certainly involved in synaptic remodeling by tagging destined neurons and synapses for destruction. As suddenly as they appear, the mechanisms of neurogenesis recede entering into adulthood. However, with age and insult, these mechanisms boisterously return, resulting in neurodegeneration. This review describes some of the mechanisms involved in synaptogenesis and wiring of the brain from the point of view of the innate immune system and then covers how similar molecular processes return with age and disease, specifically in the context of Alzheimer's disease.

Neonatal immune challenge exacerbates seizure-induced hippocampus-dependent memory impairment in adult rats

Our aim was to examine whether neonatal lipopolysaccharide (LPS) exposure is associated with changes in microglia and whether these alternations could influence later seizure-induced neurobehavioral outcomes. Male pups were first injected intraperitoneally with either LPS or saline on postnatal day 3 (P3) and postnatal day 5 (P5). Immunohistochemical analysis showed that LPS-treated animals exhibited increased microglia activation that persisted into adolescence. At P45, seizures were induced in rats by intraperitoneal injection of kainic acid (KA). Rats treated with LPS neonatally showed significantly greater proinflammatory responses and performed significantly worse in the Y-maze, Morris water maze, and inhibitory avoidance tasks after KA insult. Treatment with minocycline at the time of neonatal LPS exposure to block LPS-induced microglia alternation attenuated the exaggerated neuroinflammatory responses and alleviated memory impairment associated with the KA insult. Our findings suggest that neonatal immune activation can predispose the brain to exacerbated behavioral deficits following seizures in adulthood, possibly by priming microglia.

Selective upregulation of scavenger receptors in and around demyelinating areas in multiple sclerosis

Autoantibodies and complement opsonization have been implicated in the process of demyelination in the major human CNS demyelinating disease multiple sclerosis (MS), but scavenger receptors (SRs) may also play pathogenetic roles. We characterized SR mRNA and protein expression in postmortem brain tissue from 13 MS patients in relation to active demyelination. CD68, chemokine (C-X-C motif) ligand 16 (CXCL16), class A macrophage SR (SR-AI/II), LOX-1 (lectin-like oxidized low-density lipoprotein receptor 1), FcγRIII, and LRP-1 (low-density lipoprotein receptor-related protein 1) mRNA were upregulated in the rims of chronic active MS lesions. CD68 and CXCL16 mRNA were also upregulated around chronic active MS lesions. By immunohistochemistry, CD68, CXCL16, and SR-AI/II were expressed by foamy macrophages in the rim and by ramified microglia around chronic active MS lesions. CXCL16 and SR-AI/II were also expressed by astrocytes in MS lesions and by primary human microglia and astrocytes in vitro. These data suggest that SRs are involved in myelin uptake in MS, and that upregulation of CD68, CXCL16, and SR-AI/II is one of the initial events in microglia as they initiate myelin phagocytosis. As demyelination continues, additional upregulation of LOX-1, FcγRIII, and LRP-1 may facilitate this process.

Acute phase proteins are major clients for the chaperone action of α₂-macroglobulin in human plasma

Extracellular protein misfolding is implicated in many age-related diseases including Alzheimer's disease, macular degeneration and arthritis. In this study, putative endogenous clients for the chaperone activity of α₂-macroglobulin (α₂M) were identified after human plasma was subjected to physiologically relevant sheer stress at 37 °C for 10 days. Western blot analysis showed that four major acute phase proteins: ceruloplasmin, fibrinogen, α₁-acid glycoprotein and complement component 3, preferentially co-purified with α₂M after plasma was stressed. Furthermore, the formation of complexes between α₂M and these putative chaperone clients, detected by sandwich ELISA, was shown to be enhanced in response to stress. These results support the hypothesis that α₂M plays an important role in extracellular proteostasis by sequestering misfolded proteins and targeting them for disposal, particularly during acute phase reactions.

Menopause leads to elevated expression of macrophage-associated genes in the aging frontal cortex: rat and human studies identify strikingly similar changes

Background

The intricate interactions between the immune, endocrine and central nervous systems shape the innate immune response of the brain. We have previously shown that estradiol suppresses expression of immune genes in the frontal cortex of middle-aged ovariectomized rats, but not in young ones reflecting elevated expression of these genes in middle-aged, ovarian hormone deficient animals. Here, we explored the impact of menopause on the microglia phenotype capitalizing on the differential expression of macrophage-associated genes in quiescent and activated microglia.

Methods

We selected twenty-three genes encoding phagocytic and recognition receptors expressed primarily in microglia, and eleven proinflammatory genes and followed their expression in the rat frontal cortex by real-time PCR. We used young, middle-aged and middle-aged ovariectomized rats to reveal age- and ovariectomy-related alterations. We analyzed the expression of the same set of genes in the postcentral and superior frontal gyrus of pre- and postmenopausal women using raw microarray data from our previous study.

Results

Ovariectomy caused up-regulation of four classic microglia reactivity marker genes including Cd11b, Cd18, Cd45 and Cd86. The change was reversible since estradiol attenuated transcriptional activation of the four marker genes. Expression of genes encoding phagocytic and toll-like receptors such as Cd11b, Cd18, C3, Cd32, Msr2 and Tlr4 increased, whereas scavenger receptor Cd36 decreased following ovariectomy. Ovarian hormone deprivation altered the expression of major components of estrogen and neuronal inhibitory signaling which are involved in the control of microglia reactivity. Strikingly similar changes took place in the postcentral and superior frontal gyrus of postmenopausal women.

Conclusions

Based on the overlapping results of rat and human studies we propose that the microglia phenotype shifts from the resting toward the reactive state which can be characterized by up-regulation of CD11b, CD14, CD18, CD45, CD74, CD86, TLR4, down-regulation of CD36 and unchanged CD40 expression. As a result of this shift, microglial cells have lower threshold for subsequent activation in the forebrain of postmenopausal women.

Deletion of Crry, the murine ortholog of the sporadic Alzheimer's disease risk gene CR1, impacts tau phosphorylation and brain CFH

Large-scale genome-wide SNP association studies have identified an association between variants of CR1, the gene encoding complement component receptor 1, and the sporadic form of Alzheimer's disease. The role of CR1 and the complement system in Alzheimer's disease remains far from clear. In rodents the closest ortholog of CR1 is the Crry gene (Cr1-related protein Y). To begin to explore its role in Alzheimer's disease we examined hippocampal lysates from Crry^−/− mice and age matched controls by immunoblotting. We measured complement factor H, a component of the complement system and biomarker for Alzheimer's disease progression, and tau phosphorylation at the serine 235 site, hyperphosphorylated forms of tau being a defining neuropathological hallmark of the disease. We found that levels of CFH and of tau phosphorylation at serine 235 were strongly and significantly reduced in Crry^−/− samples. These observations provide a starting point for further attempts to determine the role of CR1 in the neuropathological process driving Alzheimer's disease.

Microglia, Alzheimer's disease, and complement

Microglia, the immune cell of the brain, are implicated in cascades leading to neuronal loss and cognitive decline in Alzheimer's disease (AD). Recent genome-wide association studies have indicated a number of risk factors for the development of late-onset AD. Two of these risk factors are an altered immune response and polymorphisms in complement receptor 1. In view of these findings, we discuss how complement signalling in the AD brain and microglial responses in AD intersect. Dysregulation of the complement cascade, either by changes in receptor expression, enhanced activation of different complement pathways or imbalances between complement factor production and complement cascade inhibitors may all contribute to the involvement of complement in AD. Altered complement signalling may reduce the ability of microglia to phagocytose apoptotic cells and clear amyloid beta peptides, modulate the expression by microglia of complement components and receptors, promote complement factor production by plaque-associated cytokines derived from activated microglia and astrocytes, and disrupt complement inhibitor production. The evidence presented here indicates that microglia in AD are influenced by complement factors to adopt protective or harmful phenotypes and the challenge ahead lies in understanding how this can be manipulated to therapeutic advantage to treat late onset AD.

Complement activation in the injured central nervous system: another dual-edged sword?

The complement system, a major component of the innate immune system, is becoming increasingly recognised as a key participant in physiology and disease. The awareness that immunological mediators support various aspects of both normal central nervous system (CNS) function and pathology has led to a renaissance of complement research in neuroscience. Various studies have revealed particularly novel findings on the wide-ranging involvement of complement in neural development, synapse elimination and maturation of neural networks, as well as the progression of pathology in a range of chronic neurodegenerative disorders, and more recently, neurotraumatic events, where rapid disruption of neuronal homeostasis potently triggers complement activation. The purpose of this review is to summarise recent findings on complement activation and acquired brain or spinal cord injury, i.e. ischaemic-reperfusion injury or stroke, traumatic brain injury (TBI) and spinal cord injury (SCI), highlighting the potential for complement-targeted therapeutics to alleviate the devastating consequences of these neurological conditions.

Mast Cells are Important Modifiers of Autoimmune Disease: With so Much Evidence, Why is There Still Controversy?

There is abundant evidence that mast cells are active participants in events that mediate tissue damage in autoimmune disease. Disease-associated increases in mast cell numbers accompanied by mast cell degranulation and elaboration of numerous mast cell mediators at sites of inflammation are commonly observed in many human autoimmune diseases including multiple sclerosis, rheumatoid arthritis, and bullous pemphigoid. In animal models, treatment with mast cell stabilizing drugs or mast cell ablation can result in diminished disease. A variety of receptors including those engaged by antibody, complement, pathogens, and intrinsic danger signals are implicated in mast cell activation in disease. Similar to their role as first responders in infection settings, mast cells likely orchestrate early recruitment of immune cells, including neutrophils, to the sites of autoimmune destruction. This co-localization promotes cellular crosstalk and activation and results in the amplification of the local inflammatory response thereby promoting and sustaining tissue damage. Despite the evidence, there is still a debate regarding the relative role of mast cells in these processes. However, by definition, mast cells can only act as accessory cells to the self-reactive T and/or antibody driven autoimmune responses. Thus, when evaluating mast cell involvement using existing and somewhat imperfect animal models of disease, their importance is sometimes obscured. However, these potent immune cells are undoubtedly major contributors to autoimmunity and should be considered as important targets for therapeutic disease intervention.

Microglia and neurodegeneration: the role of systemic inflammation

It is well accepted that CNS inflammation has a role in the progression of chronic neurodegenerative disease, although the mechanisms through which this occurs are still unclear. The inflammatory response during most chronic neurodegenerative disease is dominated by the microglia and mechanisms by which these cells contribute to neuronal damage and degeneration are the subject of intense study. More recently it has emerged that systemic inflammation has a significant role to play in the progression of these diseases. Well-described adaptive pathways exist to transduce systemic inflammatory signals to the brain, but activation of these pathways appears to be deleterious to the brain if the acute insult is sufficiently robust, as in severe sepsis, or sufficiently prolonged, as in repeated stimulation with robust doses of inflammogens such as lipopolysaccharide (LPS). Significantly, moderate doses of inflammogens produce new pathology in the brain and exacerbate or accelerate features of disease when superimposed upon existing pathology or in the context of genetic predisposition. It is now apparent in multiple chronic disease states, and in ageing, that microglia are primed by prior pathology, or by genetic predisposition, to respond more vigorously to subsequent inflammatory stimulation, thus transforming an adaptive CNS inflammatory response to systemic inflammation, into one that has deleterious consequences for the individual. In this review, the preclinical and clinical evidence supporting a significant role for systemic inflammation in chronic neurodegenerative diseases will be discussed. Mechanisms by which microglia might effect neuronal damage and dysfunction, as a consequence of systemic stimulation, will be highlighted.

The evolving landscape of neurotoxicity by unconjugated bilirubin: role of glial cells and inflammation

Unconjugated hyperbilirubinemia is a common condition in the first week of postnatal life. Although generally harmless, some neonates may develop very high levels of unconjugated bilirubin (UCB), which may surpass the protective mechanisms of the brain in preventing UCB accumulation. In this case, both short-term and long-term neurodevelopmental disabilities, such as acute and chronic UCB encephalopathy, known as kernicterus, or more subtle alterations defined as bilirubin-induced neurological dysfunction (BIND) may be produced. There is a tremendous variability in babies' vulnerability toward UCB for reasons not yet explained, but preterm birth, sepsis, hypoxia, and hemolytic disease are comprised as risk factors. Therefore, UCB levels and neurological abnormalities are not strictly correlated. Even nowadays, the mechanisms of UCB neurotoxicity are still unclear, as are specific biomarkers, and little is known about lasting sequelae attributable to hyperbilirubinemia. On autopsy, UCB was shown to be within neurons, neuronal processes, and microglia, and to produce loss of neurons, demyelination, and gliosis. In isolated cell cultures, UCB was shown to impair neuronal arborization and to induce the release of pro-inflammatory cytokines from microglia and astrocytes. However, cell dependent sensitivity to UCB toxicity and the role of each nerve cell type remains not fully understood. This review provides a comprehensive insight into cell susceptibilities and molecular targets of UCB in neurons, astrocytes, and oligodendrocytes, and on phenotypic and functional responses of microglia to UCB. Interplay among glia elements and cross-talk with neurons, with a special emphasis in the UCB-induced immunostimulation, and the role of sepsis in BIND pathogenesis are highlighted. New and interesting data on the anti-inflammatory and antioxidant activities of different pharmacological agents are also presented, as novel and promising additional therapeutic approaches to BIND.

DAF/CD55 and Protectin/CD59 modulate adaptive immunity and disease outcome in experimental autoimmune myasthenia gravis

The role of regulators of complement activity (RCA) involving CD55 and CD59 in the pathogenesis of experimental autoimmune myasthenia gravis (EAMG) remains unclear. CD55 and CD59 restrict complement activation by inhibiting C3/C5 convertases' activities and membrane attack complex formation, respectively. Actively immunized EAMG mice deficient in either CD55 or CD59 showed significant differences in adaptive immune responses and worsened disease outcome associated with increased levels of serum cytokines, modified production of acetylcholine receptor antibodies, and more complement deposition at the neuromuscular junction. We conclude that modulation of complement activity by RCA represents an alternative in controlling of autoimmune processes in EAMG.