Colonisation of the human central nervous system by microglia: the roles of chemokines and vascular adhesion molecules

Establishment of tissue-resident immune populations in the fetus

The immune system establishes during the prenatal period from distinct waves of stem and progenitor cells and continuously adapts to the needs and challenges of early postnatal and adult life. Fetal immune development not only lays the foundation for postnatal immunity but establishes functional populations of tissue-resident immune cells that are instrumental for fetal immune responses amidst organ growth and maturation. This review aims to discuss current knowledge about the development and function of tissue-resident immune populations during fetal life, focusing on the brain, lung, and gastrointestinal tract as sites with distinct developmental trajectories. While recent progress using system-level approaches has shed light on the fetal immune landscape, further work is required to describe precise roles of prenatal immune populations and their migration and adaptation to respective organ environments. Defining points of prenatal susceptibility to environmental challenges will support the search for potential therapeutic targets to positively impact postnatal health.

Sex- and region-specific differences in microglia phenotype and characterization of the peripheral immune response following early-life infection in neonatal male and female rats

Early-life infection has been shown to have profound effects on the brain and behavior across the lifespan, a phenomenon termed "early-life programming". Indeed, many neuropsychiatric disorders begin or have their origins early in life and have been linked to early-life immune activation (e.g. autism, ADHD, and schizophrenia). Furthermore, many of these disorders show a robust sex bias, with males having a higher risk of developing early-onset neurodevelopmental disorders. The concept of early-life programming is now well established, however, it is still unclear how such effects are initiated and then maintained across time to produce such a phenomenon. To begin to address this question, we examined changes in microglia, the immune cells of the brain, and peripheral immune cells in the hours immediately following early-life infection in male and female rats. We found that males showed a significant decrease in BDNF expression and females showed a significant increase in IL-6 expression in the cerebellum following E.coli infection on postnatal day 4; however, for most cytokines examined in the brain and in the periphery we were unable to identify any sex differences in the immune response, at least at the time points examined. Instead, neonatal infection with E.coli increased the expression of a number of cytokines in the brain of both males and females similarly including TNF-α, IL-1β, and CD11b (a marker of microglia activation) in the hippocampus and, in the spleen, TNF-α and IL-1β. We also found that protein levels of GRO-KC, MIP-1a, MCP1, IP-10, TNF-α, and IL-10 were elevated 8-hours postinfection, but this response was resolved by 24-hours. Lastly, we found that males have more thin microglia than females on P5, however, neonatal infection had no effect on any of the microglia morphologies we examined. These data show that sex differences in the acute immune response to neonatal infection are likely gene, region, and even time dependent. Future research should consider these factors in order to develop a comprehensive understanding of the immune response in males and females as these changes are likely the initiating agents that lead to the long-term, and often sex-specific, effects of early-life infection.

An examination of changes in maternal neuroimmune function during pregnancy and the postpartum period

There is strong evidence that the immune system changes dramatically during pregnancy in order to prevent the developing fetus from being "attacked" by the maternal immune system. Due to these alterations in peripheral immune function, many women that suffer from autoimmune disorders actually find significant relief from their symptoms throughout pregnancy; however, these changes can also leave the mother more susceptible to infections that would otherwise be mitigated by the inflammatory response (Robinson and Klein, 2012). Only one other study has looked at changes in microglial number and morphology during pregnancy and the postpartum period (Haim et al., 2016), but no one has yet examined the neuroimmune response following an immune challenge during this time. Therefore, in this study, we investigated the impact of an immune challenge during various time-points throughout pregnancy and the postpartum period on the expression of immune molecules in the brain of the mother and fetus. Our results indicate that similar to the peripheral immune suppression measured during pregnancy, we also see significant suppression of the immune response in the maternal brain, particularly during late gestation. In contrast to the peripheral immune system, immune modulation in the maternal brain extends moderately into the postpartum period. Additionally, we found that the fetal immune response in the brain and placenta is also suppressed just before parturition, suggesting that cytokine production in the fetus and placenta are mirroring the peripheral cytokine response of the mother.

Immunoadolescence: Neuroimmune development and adolescent behavior

The brain is increasingly appreciated to be a constantly rewired organ that yields age-specific behaviors and responses to the environment. Adolescence in particular is a unique period characterized by continued brain maturation, superimposed with transient needs of the organism to traverse a leap from parental dependence to independence. Here we describe how these needs require immune maturation, as well as brain maturation. Our immune system, which protects us from pathogens and regulates inflammation, is in constant communication with our nervous system. Together, neuro-immune signaling regulates our behavioral responses to the environment, making this interaction a likely substrate for adolescent development. We review here the identified as well as understudied components of neuro-immune interactions during adolescence. Synaptic pruning, neurite outgrowth, and neurotransmitter release during adolescence all regulate-and are regulated by-immune signals, which occur via blood-brain barrier dynamics and glial activity. We discuss these processes, as well as how immune signaling during this transitional period of development confers differential effects on behavior and vulnerability to mental illness.

Prenatal stress is a vulnerability factor for altered morphology and biological activity of microglia cells

Several lines of evidence suggest that the dysregulation of the immune system is an important factor in the development of depression. Microglia are the resident macrophages of the central nervous system and a key player in innate immunity of the brain. We hypothesized that prenatal stress (an animal model of depression) as a priming factor could affect microglial cells and might lead to depressive-like disturbances in adult male rat offspring. We investigated the behavioral changes (sucrose preference test, Porsolt test), the expression of C1q and CD40 mRNA and the level of microglia (Iba1 positive) in 3-month-old control and prenatally stressed male offspring rats. In addition, we characterized the morphological and biochemical parameters of potentially harmful (NO, iNOS, IL-1β, IL-18, IL-6, TNF-α, CCL2, CXCL12, CCR2, CXCR4) and beneficial (insulin-like growth factor-1 (IGF-1), brain derived neurotrophic factor (BDNF)) phenotypes in cultures of microglia obtained from the cortices of 1–2 days old control and prenatally stressed pups. The adult prenatally stressed rats showed behavioral (anhedonic- and depression-like) disturbances, enhanced expression of microglial activation markers and an increased number of Iba1-immunopositive cells in the hippocampus and frontal cortex. The morphology of glia was altered in cultures from prenatally stressed rats, as demonstrated by immunofluorescence microscopy. Moreover, in these cultures, we observed enhanced expression of CD40 and MHC II and release of pro-inflammatory cytokines, including IL-1β, IL-18, TNF-α and IL-6. Prenatal stress significantly up-regulated levels of the chemokines CCL2, CXCL12 and altered expression of their receptors, CCR2 and CXCR4 while IGF-1 production was suppressed in cultures of microglia from prenatally stressed rats. Our results suggest that prenatal stress may lead to excessive microglia activation and contribute to the behavioral changes observed in depression in adulthood.

Adaptive phenotype of microglial cells during the normal postnatal development of the somatosensory "Barrel" cortex

Accumulative evidence indicates that microglial cells influence the normal development of central nervous system (CNS) synapses. Yet, the functional properties of microglia in relation with synapse development remain unclear. We recently showed that in layer 4 of the whisker-related barrel field of the mouse somatosensory cortex, microglial cells are recruited only after postnatal day (P)5 in the center of the barrels where thalamo-cortical synapses are concentrated and begin their maturation. In the present study, we analyzed the phenotype of microglia during this developmental process. We show that between P5 and P7 microglial cells acquire a more ramified morphology with a smaller soma, they express classical markers of microglia (Iba1, CD11b, and CD68) but never markers of activation (Mac-2 and MHCII) and rarely the proliferation marker Ki67. Electrophysiological recordings in acute cortical slices showed that at P5 a proportion of layer 4 microglia transiently express voltage-dependant potassium currents of the delayed rectifier family, mostly mediated by Kv1.3 subunits, which are usually expressed by activated microglia under pathological conditions. This proportion of cells with rectifying properties doubles between P5 and P6, in concomitance with the beginning of microglia invasion of the barrel centers. Finally, analysis of the responses mediated by purinergic receptors indicated that a higher percentage of rectifying microglia expressed functional P2Y6 and P2Y12 receptors, as compared with nonrectifying cells, whereas all cells expressed functional P2X7 receptors. Our results indicate that during normal cortical development distinct microglia properties mature differentially, some of them being exquisitely influenced by the local environment of the maturating neuronal network.

Brain development in rodents and humans: Identifying benchmarks of maturation and vulnerability to injury across species

Hypoxic-ischemic and traumatic brain injuries are leading causes of long-term mortality and disability in infants and children. Although several preclinical models using rodents of different ages have been developed, species differences in the timing of key brain maturation events can render comparisons of vulnerability and regenerative capacities difficult to interpret. Traditional models of developmental brain injury have utilized rodents at postnatal day 7-10 as being roughly equivalent to a term human infant, based historically on the measurement of post-mortem brain weights during the 1970s. Here we will examine fundamental brain development processes that occur in both rodents and humans, to delineate a comparable time course of postnatal brain development across species. We consider the timing of neurogenesis, synaptogenesis, gliogenesis, oligodendrocyte maturation and age-dependent behaviors that coincide with developmentally regulated molecular and biochemical changes. In general, while the time scale is considerably different, the sequence of key events in brain maturation is largely consistent between humans and rodents. Further, there are distinct parallels in regional vulnerability as well as functional consequences in response to brain injuries. With a focus on developmental hypoxic-ischemic encephalopathy and traumatic brain injury, this review offers guidelines for researchers when considering the most appropriate rodent age for the developmental stage or process of interest to approximate human brain development.

Quantitating the subtleties of microglial morphology with fractal analysis

It is well established that microglial form and function are inextricably linked. In recent years, the traditional view that microglial form ranges between “ramified resting” and “activated amoeboid” has been emphasized through advancing imaging techniques that point to microglial form being highly dynamic even within the currently accepted morphological categories. Moreover, microglia adopt meaningful intermediate forms between categories, with considerable crossover in function and varying morphologies as they cycle, migrate, wave, phagocytose, and extend and retract fine and gross processes. From a quantitative perspective, it is problematic to measure such variability using traditional methods, but one way of quantitating such detail is through fractal analysis. The techniques of fractal analysis have been used for quantitating microglial morphology, to categorize gross differences but also to differentiate subtle differences (e.g., amongst ramified cells). Multifractal analysis in particular is one technique of fractal analysis that may be useful for identifying intermediate forms. Here we review current trends and methods of fractal analysis, focusing on box counting analysis, including lacunarity and multifractal analysis, as applied to microglial morphology.

Sex, glia, and development: interactions in health and disease

Microglia and astrocytes are the primary immune cells within the central nervous system. Microglia influence processes including neural development, synaptic plasticity and cognition; while their activation and production of immune molecules can induce stereotyped sickness behaviors or pathologies including cognitive dysfunction. Given their role in health and disease, we propose that glia may also be a critical link in understanding the etiology of many neuropsychiatric disorders that present with a strong sex-bias in their symptoms or prevalence. Specifically, males are more likely to be diagnosed with disorders that have distinct developmental origins such as autism or schizophrenia. In contrast, females are more likely to be diagnosed with disorders that present later in life, after the onset of adolescence, such as depression and anxiety disorders. In this review we will summarize the evidence suggesting that sex differences in the colonization and function of glia within the normal developing brain may contribute to distinct windows of vulnerability between males and females. We will also highlight the current gaps in our knowledge as well as the future directions and considerations of research aimed at understanding the link between neuroimmune function and sex differences in mental health disorders.

The immune system and developmental programming of brain and behavior

The brain, endocrine, and immune systems are inextricably linked. Immune molecules have a powerful impact on neuroendocrine function, including hormone-behavior interactions, during health as well as sickness. Similarly, alterations in hormones, such as during stress, can powerfully impact immune function or reactivity. These functional shifts are evolved, adaptive responses that organize changes in behavior and mobilize immune resources, but can also lead to pathology or exacerbate disease if prolonged or exaggerated. The developing brain in particular is exquisitely sensitive to both endogenous and exogenous signals, and increasing evidence suggests the immune system has a critical role in brain development and associated behavioral outcomes for the life of the individual. Indeed, there are associations between many neuropsychiatric disorders and immune dysfunction, with a distinct etiology in neurodevelopment. The goal of this review is to describe the important role of the immune system during brain development, and to discuss some of the many ways in which immune activation during early brain development can affect the later-life outcomes of neural function, immune function, mood and cognition.

Novel nanomaterials for clinical neuroscience

Neurodegenerative disorders including Alzheimer's and Parkinson's diseases, amyotrophic lateral sclerosis, and stroke are rapidly increasing as population ages. The field of nanomedicine is rapidly expanding and promises revolutionary advances to the diagnosis and treatment of devastating human diseases. This paper provides an overview of novel nanomaterials that have potential to improve diagnosis and therapy of neurodegenerative disorders. Examples include liposomes, nanoparticles, polymeric micelles, block ionomer complexes, nanogels, and dendrimers that have been tested clinically or in experimental models for delivery of drugs, genes, and imaging agents. More recently discovered nanotubes and nanofibers are evaluated as promising scaffolds for neuroregeneration. Novel experimental neuroprotective strategies also include nanomaterials, such as fullerenes, which have antioxidant properties to eliminate reactive oxygen species in the brain to mitigate oxidative stress. Novel technologies to enable these materials to cross the blood brain barrier will allow efficient systemic delivery of therapeutic and diagnostic agents to the brain. Furthermore, by combining such nanomaterials with cell-based delivery strategies, the outcomes of neurodegenerative disorders can be greatly improved.

A macrophage-nanozyme delivery system for Parkinson's disease

Selective delivery of antioxidants to the substantia nigra pars compacta (SNpc) during Parkinson's disease (PD) can potentially attenuate oxidative stress and as such increase survival of dopaminergic neurons. To this end, we developed a bone-marrow-derived macrophage (BMM) system to deliver catalase to PD-affected brain regions in an animal model of human disease. To preclude BMM-mediated enzyme degradation, catalase was packaged into a block ionomer complex with a cationic block copolymer, polyethyleneimine-poly(ethylene glycol) (PEI-PEG). The self-assembled catalase/PEI-PEG complexes, "nanozymes", were ca. 60 to 100 nm in size, stable in pH and ionic strength, and retained antioxidant activities. Cytotoxicity was negligible over a range of physiologic nanozyme concentrations. Nanozyme particles were rapidly, 40-60 min, taken up by BMM, retained catalytic activity, and released in active form for greater than 24 h. In contrast, "naked" catalase was rapidly degraded. The released enzyme decomposed microglial hydrogen peroxide following nitrated alpha-synuclein or tumor necrosis factor alpha activation. Following adoptive transfer of nanozyme-loaded BMM to 1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine-intoxicated mice, ca. 0.6% of the injected dose were found in brain. We conclude that cell-mediated delivery of nanozymes can reduce oxidative stress in laboratory and animal models of PD.

Bone marrow-derived microglia contribute to the neuroinflammatory response and express iNOS in the MPTP mouse model of Parkinson's disease

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication in mice results in dopamine neuron degeneration that is alleviated by prevention of microglia cell activation and blockade of iNOS production. However, the role of peripherally derived microglia in this response has not been well characterized. In the present study, we investigated the time course of infiltration and phenotypic differentiation of bone marrow-derived cells (BMDCs) following MPTP treatment in mice, using green fluorescent protein (GFP) bone marrow chimeras. BMDCs were found in the meninges, choroid plexus, blood vessels, and brain parenchyma in both saline and MPTP-treated mice. MPTP stimulated a transient, two-fold increase in the rate of BMDC infiltration into the brain, concomitant with the onset of microglia activation. The majority of BMDCs were microglial in phenotype, as assessed by morphology and expression of the pan-hematopoietic marker CD45 and the microglia marker CD11b. We did not observe BMDCs that expressed neuronal or astroglial markers. Over 90% of bone marrow-derived microglia expressed the inducible form of nitric oxide synthase (iNOS), suggesting that peripherally derived microglia may play a deleterious role in MPTP-induced degeneration.

Albumin stimulates monocyte chemotactic protein-1 expression in rat embryonic mixed brain cells

Albumin, a blood protein absent from the adult brain in physiological situations, can be brought into contact with brain cells during development or, in adult, following breakdown of the blood-brain barrier occurring as a result of local inflammation. In the present study, we show that ovalbumin and albumin induce the release of monocyte chemotactic protein 1 (MCP-1/CCL2) from rat embryonic mixed brain cells. A short-term exposure to ovalbumin during the cell dissociation procedure is sufficient to generate MCP-1 mRNA. A comparable effect is observed when the cells are incubated for 4 hr with ovalbumin or rat albumin, while MCP-1 messengers are barely detectable following bovine albumin exposure. The amount of MCP-1 protein measured in 4 hr-supernatants of albumin-treated cells followed the same albumin-inducing pattern as that of MCP-1 mRNA, while all albumins tested induced MCP-1 protein after a 17 hr-incubation period. The albumin-induced MCP-1 production is significantly inhibited in calphostin C-treated cells, suggesting the implication of a protein kinase C-dependent signaling pathway. This MCP-1-inducing activity is maintained after a lipid extraction procedure but abolished by proteinase K or trypsin treatments of albumin. The MCP-1 secretion following albumin contact with nervous cells could thus interfere, by chemotactic gradient formation, with the brain infiltration program of blood-derived cells during development or brain injury.

Emerging concepts in periventricular white matter injury

Approximately 10% of newborns are born prematurely. Of these children, more than 10% will sustain neurological injuries leading to significant learning disabilities, cerebral palsy, or mental retardation, with very low birth weight infants having an even higher incidence of brain injury. Whereas intraventricular hemorrhage was the most common form of serious neurological injury a decade ago, periventricular white matter injury (PWMI) is now the most common cause of brain injury in preterm infants. The spectrum of chronic PWMI includes focal cystic necrotic lesions (periventricular leukomalacia; PVL) and diffuse myelination disturbances. Recent neuroimaging studies support that the incidence of PVL is declining, whereas diffuse cerebral white matter injury is emerging as the predominant lesion. Factors that predispose to PVL include prematurity, hypoxia, ischemia, and inflammation. It is believed that injury to oligodendrocyte (OL) progenitors contributes to the pathogenesis of myelination disturbances in PWMI by disrupting the maturation of myelin-myelin-forming oligodendrocytes. Other potential mechanisms of injury include activation of microglia and axonal damage. Chemical mediators that may contribute to white matter injury include reactive oxygen (ROS) and nitrogen species (RNS), glutamate, cytokines, and adenosine. As our understanding of the pathogenesis of PWMI improves, it is anticipated that new strategies for directly preventing brain injury in premature infants will evolve.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

Regulation of chemokine receptor expression in human microglia and astrocytes

It has been proposed that the positioning of mobile cells within a tissue is determined by their overall profile of chemokine receptors. This study examines the profiles of chemokine receptors expressed on resting and activated adult human microglial cells, astrocytes and a microglial cell line, CHME3. Microglia express highest levels of CXCR1, CXCR3 and CCR3. Astrocytes also have moderate levels of CXCR1 and CXCR3, and some CCR3, while both cell types also expressed CCR4, CCR5, CCR6, CXCR2, CXCR4 and CXCR5 at lower levels. Activation of the cells with the inflammatory cytokine tumour necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma) increased the expression of some but not all receptors over a period of 24 h. Microglia showed moderate enhancement of receptor expression, while astrocytes responded particularly strongly to TNFalpha with enhanced CXCR3, CCR3 and CXCR1. However, the migratory and proliferative responses of the microglia and astrocytes to the same chemokine were different, with microglia migrating and astrocytes proliferating in response to CXCL10. The data indicates a mechanism by which activated microglia and astrocytes become selectively more sensitive to inflammatory chemokines during CNS disease, and the paper discusses which of the many chemokines present in CNS would have priority of action on microglia and astrocytes.

Peripheral macrophage recruitment in cuprizone-induced CNS demyelination despite an intact blood-brain barrier

The contribution of peripheral macrophage was assessed in cuprizone intoxication, a model of demyelination and remyelination in which the blood-brain barrier remains intact. Flow cytometry of brain cells isolated from cuprizone-treated mice revealed an increase in the percentage of Mac-1(+)/CD45(hi) peripheral macrophage. To confirm these results in situ, C57BL/6 mice were lethally irradiated, transplanted with bone marrow from GFP-transgenic mice, and exposed to cuprizone. GFP(+) peripheral macrophages were seen in the CNS after 2 weeks of treatment, and infiltration continued through 6 weeks. While the peripheral macrophages were far outnumbered by the resident microglia, their recruitment across the blood-brain barrier alludes to a potentially important role.