Gonadal hormones impart male-biased behavioral vulnerabilities to immune activation via microglial mitochondrial function

There is a strong male bias in the prevalence of many neurodevelopmental disorders such as autism spectrum disorder. However, the mechanisms underlying this sex bias remain elusive. Infection during the perinatal period is associated with an increased risk of neurodevelopmental disorder development. Here, we used a mouse model of early-life immune activation that reliably induces deficits in social behaviors only in males. We demonstrate that male-biased alterations in social behavior are dependent upon microglial immune signaling and are coupled to alterations in mitochondrial morphology, gene expression, and function specifically within microglia, the innate immune cells of the brain. Additionally, we show that this behavioral and microglial mitochondrial vulnerability to early-life immune activation is programmed by the male-typical perinatal gonadal hormone surge. These findings demonstrate that social behavior in males over the lifespan are regulated by microglia-specific mechanisms that are shaped by events that occur in early development.

Hofbauer cells and fetal brain microglia share transcriptional profiles and responses to maternal diet-induced obesity

Maternal immune activation is associated with adverse offspring neurodevelopmental outcomes, many mediated by in utero microglial programming. As microglia remain inaccessible throughout development, identification of noninvasive biomarkers reflecting fetal brain microglial programming could permit screening and intervention. We used lineage tracing to demonstrate the shared ontogeny between fetal brain macrophages (microglia) and fetal placental macrophages (Hofbauer cells) in a mouse model of maternal diet-induced obesity, and single-cell RNA-seq to demonstrate shared transcriptional programs. Comparison with human datasets demonstrated conservation of placental resident macrophage signatures between mice and humans. Single-cell RNA-seq identified common alterations in fetal microglial and Hofbauer cell gene expression induced by maternal obesity, as well as sex differences in these alterations. We propose that Hofbauer cells, which are easily accessible at birth, provide novel insights into fetal brain microglial programs, and may facilitate the early identification of offspring vulnerable to neurodevelopmental disorders in the setting of maternal exposures.

Microbial modulation via cross-fostering prevents the effects of pervasive environmental stressors on microglia and social behavior, but not the dopamine system

Environmental toxicant exposure, including air pollution, is increasing worldwide. However, toxicant exposures are not equitably distributed. Rather, low-income and minority communities bear the greatest burden, along with higher levels of psychosocial stress. Both air pollution and maternal stress during pregnancy have been linked to neurodevelopmental disorders such as autism, but biological mechanisms and targets for therapeutic intervention remain poorly understood. We demonstrate that combined prenatal exposure to air pollution (diesel exhaust particles, DEP) and maternal stress (MS) in mice induces social behavior deficits only in male offspring, in line with the male bias in autism. These behavioral deficits are accompanied by changes in microglial morphology and gene expression as well as decreased dopamine receptor expression and dopaminergic fiber input in the nucleus accumbens (NAc). Importantly, the gut-brain axis has been implicated in ASD, and both microglia and the dopamine system are sensitive to the composition of the gut microbiome. In line with this, we find that the composition of the gut microbiome and the structure of the intestinal epithelium are significantly shifted in DEP/MS-exposed males. Excitingly, both the DEP/MS-induced social deficits and microglial alterations in males are prevented by shifting the gut microbiome at birth via a cross-fostering procedure. However, while social deficits in DEP/MS males can be reversed by chemogenetic activation of dopamine neurons in the ventral tegmental area, modulation of the gut microbiome does not impact dopamine endpoints. These findings demonstrate male-specific changes in the gut-brain axis following DEP/MS and suggest that the gut microbiome is an important modulator of both social behavior and microglia.

β-Endorphin mediates radiation therapy fatigue

Fatigue is a common adverse effect of external beam radiation therapy in cancer patients. Mechanisms causing radiation fatigue remain unclear, although linkage to skin irradiation has been suggested. β-Endorphin, an endogenous opioid, is synthesized in skin following genotoxic ultraviolet irradiation and acts systemically, producing addiction. Exogenous opiates with the same receptor activity as β-endorphin can cause fatigue. Using rodent models of radiation therapy, exposing tails and sparing vital organs, we tested whether skin-derived β-endorphin contributes to radiation-induced fatigue. Over a 6-week radiation regimen, plasma β-endorphin increased in rats, paralleled by opiate phenotypes (elevated pain thresholds, Straub tail) and fatigue-like behavior, which was reversed in animals treated by the opiate antagonist naloxone. Mechanistically, all these phenotypes were blocked by opiate antagonist treatment and were undetected in either β-endorphin knockout mice or mice lacking keratinocyte p53 expression. These findings implicate skin-derived β-endorphin in systemic effects of radiation therapy. Opioid antagonism may warrant testing in humans as treatment or prevention of radiation-induced fatigue.

Maternal diet disrupts the placenta-brain axis in a sex-specific manner

High maternal weight is associated with detrimental outcomes in offspring, including increased susceptibility to neurological disorders such as anxiety, depression and communicative disorders. Despite widespread acknowledgement of sex biases in the development of these disorders, few studies have investigated potential sex-biased mechanisms underlying disorder susceptibility. Here, we show that a maternal high-fat diet causes endotoxin accumulation in fetal tissue, and subsequent perinatal inflammation contributes to sex-specific behavioural outcomes in offspring. In male offspring exposed to a maternal high-fat diet, increased macrophage Toll-like receptor 4 signalling results in excess microglial phagocytosis of serotonin (5-HT) neurons in the developing dorsal raphe nucleus, decreasing 5-HT bioavailability in the fetal and adult brains. Bulk sequencing from a large cohort of matched first-trimester human samples reveals sex-specific transcriptome-wide changes in placental and brain tissue in response to maternal triglyceride accumulation (a proxy for dietary fat content). Further, fetal brain 5-HT levels decrease as placental triglycerides increase in male mice and male human samples. These findings uncover a microglia-dependent mechanism through which maternal diet can impact offspring susceptibility for neuropsychiatric disorder development in a sex-specific manner.

Protocol to measure endotoxin from opaque tissues in mice using an optimized kinetic limulus amebocyte lysate assay

Endotoxin accumulation has been widely noted in several pathologies ranging from metabolic dysregulation to bacterial infection. Using limulus amebocyte lysate (LAL) assays to detect endotoxin load has been the only reliable way to assess endotoxin accumulation, but assays optimized for detection in opaque tissues are still lacking. We optimized a sensitive Kinetic LAL assay for endotoxin detection from murine tissues. In this protocol, we describe tissue collection and homogenization, followed by the procedure to run the assay and data analysis. For complete details on the use and execution of this protocol, please refer to Ceasrine et al. (2022).

Prenatal opioid exposure inhibits microglial sculpting of the dopamine system selectively in adolescent male offspring

The current opioid epidemic has dramatically increased the number of children who are prenatally exposed to opioids, including oxycodone. A number of social and cognitive abnormalities have been documented in these children as they reach young adulthood. However, little is known about the mechanisms underlying developmental effects of prenatal opioid exposure. Microglia, the resident immune cells of the brain, respond to acute opioid exposure in adulthood. Moreover, microglia are known to sculpt neural circuits during typical development. Indeed, we recently found that microglial phagocytosis of dopamine D1 receptors (D1R) in the nucleus accumbens (NAc) is required for the natural developmental decline in NAc-D1R that occurs between adolescence and adulthood in rats. This microglial pruning occurs only in males, and is required for the normal developmental trajectory of social play behavior. However, virtually nothing is known as to whether this developmental program is altered by prenatal exposure to opioids. Here, we show in rats that maternal oxycodone self-administration during pregnancy leads to reduced adolescent microglial phagocytosis of D1R and subsequently higher D1R density within the NAc in adult male, but not female, offspring. Finally, we show prenatal and adult behavioral deficits in opioid-exposed offspring, including impaired extinction of oxycodone-conditioned place preference in males. This work demonstrates for the first time that microglia play a key role in translating prenatal opioid exposure to changes in neural systems and behavior.

Prenatal environmental stressors impair postnatal microglia function and adult behavior in males

Gestational exposure to environmental toxins and socioeconomic stressors is epidemiologically linked to neurodevelopmental disorders with strong male bias, such as autism. We model these prenatal risk factors in mice by co-exposing pregnant dams to an environmental pollutant and limited-resource stress, which robustly activates the maternal immune system. Only male offspring display long-lasting behavioral abnormalities and alterations in the activity of brain networks encoding social interactions. Cellularly, prenatal stressors diminish microglial function within the anterior cingulate cortex, a central node of the social coding network, in males during early postnatal development. Precise inhibition of microglial phagocytosis within the anterior cingulate cortex (ACC) of wild-type (WT) mice during the same critical period mimics the impact of prenatal stressors on a male-specific behavior, indicating that environmental stressors alter neural circuit formation in males via impairing microglia function during development.

Block et al. show that combined exposure to air pollution and maternal stress during pregnancy activates the maternal immune system and induces male-specific impairments in social behavior and circuit connectivity in offspring. Cellularly, prenatal stressors diminish microglia phagocytic function, and inhibition of microglia phagocytosis phenocopies behavioral deficits from prenatal stressors.

Dietary fat: a potent microglial influencer

Poor nutrition, lack of exercise, and genetic predisposition all contribute to the growing epidemic of obesity. Overweight/obesity create an environment of chronic inflammation that leads to negative physiological and neurological outcomes, such as diabetes, cardiovascular disease, and anxiety/depression. While the whole body contributes to metabolic homeostasis, the neuroimmune system has recently emerged as a key regulator of metabolism. Microglia, the resident immune cells of the brain, respond both directly and indirectly to dietary fat, and the environment in which microglia develop contributes to their responsiveness later in life. Thus, high maternal weight during pregnancy may have consequences for microglial function in offspring. Here, we discuss the most recent findings on microglia signaling in overweight/obesity with a focus on perinatal programming.

Sickness and the social brain: How the immune system regulates behavior across species

Many instances of sickness critically involve the immune system. The immune system talks to the brain in a bi-directional loop. This discourse affords the immune system immense control, such that it can influence behavior and optimize recovery from illness. These behavioral responses to infection are called sickness behaviors and can manifest in many ways, including changes in mood, motivation, or energy. Fascinatingly, most of these changes are conserved across species, and most organisms demonstrate some form of sickness behaviors. One of the most interesting sickness behaviors, and not immediately obvious, is altered sociability. Here, we discuss how the immune system impacts social behavior, by examining the brain regions and immune mediators involved in this process. We first outline how social behavior changes in response to infection in various species. Next, we explore which brain regions control social behavior and their evolutionary origins. Finally, we describe which immune mediators establish the link between illness and social behavior, in the context of both normal development and infection. Overall, we hope to make clear the striking similarities between the mechanisms that facilitate changes in sociability in derived and ancestral vertebrate, as well as invertebrate, species.

Targeting Tau Mitigates Mitochondrial Fragmentation and Oxidative Stress in Amyotrophic Lateral Sclerosis

Understanding the mechanisms underlying amyotrophic lateral sclerosis (ALS) is crucial for the development of new therapies. Previous studies have demonstrated that mitochondrial dysfunction is a key pathogenetic event in ALS. Interestingly, studies in Alzheimer's disease (AD) post-mortem brain and animal models link alterations in mitochondrial function to interactions between hyperphosphorylated tau and dynamin-related protein 1 (DRP1), the GTPase involved in mitochondrial fission. Recent evidence suggest that tau may be involved in ALS pathogenesis, therefore, we sought to determine whether hyperphosphorylated tau may lead to mitochondrial fragmentation and dysfunction in ALS and whether reducing tau may provide a novel therapeutic approach. Our findings demonstrated that pTau-S396 is mis-localized to synapses in post-mortem motor cortex (mCTX) across ALS subtypes. Additionally, the treatment with ALS synaptoneurosomes (SNs), enriched in pTau-S396, increased oxidative stress, induced mitochondrial fragmentation, and altered mitochondrial connectivity without affecting cell survival in vitro. Furthermore, pTau-S396 interacted with DRP1, and similar to pTau-S396, DRP1 accumulated in SNs across ALS subtypes, suggesting increases in mitochondrial fragmentation in ALS. As previously reported, electron microscopy revealed a significant decrease in mitochondria density and length in ALS mCTX. Lastly, reducing tau levels with QC-01-175, a selective tau degrader, prevented ALS SNs-induced mitochondrial fragmentation and oxidative stress in vitro. Collectively, our findings suggest that increases in pTau-S396 may lead to mitochondrial fragmentation and oxidative stress in ALS and decreasing tau may provide a novel strategy to mitigate mitochondrial dysfunction in ALS. pTau-S396 mis-localizes to synapses in ALS. ALS synaptoneurosomes (SNs), enriched in pTau-S396, increase oxidative stress and induce mitochondrial fragmentation in vitro. pTau-S396 interacts with the pro-fission GTPase DRP1 in ALS. Reducing tau with a selective degrader, QC-01-175, mitigates ALS SNs-induced mitochondrial fragmentation and increases in oxidative stress in vitro.

Maternal SARS-CoV-2 infection elicits sexually dimorphic placental immune responses

There is a persistent bias toward higher prevalence and increased severity of coronavirus disease 2019 (COVID-19) in males. Underlying mechanisms accounting for this sex difference remain incompletely understood. Interferon responses have been implicated as a modulator of COVID-19 disease in adults and play a key role in the placental antiviral response. Moreover, the interferon response has been shown to alter Fc receptor expression and therefore may affect placental antibody transfer. Here, we examined the intersection of maternal-fetal antibody transfer, viral-induced placental interferon responses, and fetal sex in pregnant women infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Placental Fc receptor abundance, interferon-stimulated gene (ISG) expression, and SARS-CoV-2 antibody transfer were interrogated in 68 human pregnancies. Sexually dimorphic expression of placental Fc receptors, ISGs and proteins, and interleukin-10 was observed after maternal SARS-CoV-2 infection, with up-regulation of these features in placental tissue of pregnant individuals with male fetuses. Reduced maternal SARS-CoV-2–specific antibody titers and impaired placental antibody transfer were also observed in pregnancies with a male fetus. These results demonstrate fetal sex-specific maternal and placental adaptive and innate immune responses to SARS-CoV-2.

Microglia and Sensitive Periods in Brain Development

From embryonic neuronal migration to adolescent circuit refinement, the immune system plays an essential role throughout central nervous system (CNS) development. Immune signaling molecules serve as a common language between the immune system and CNS, allowing them to work together to modulate brain function both in health and disease. As the resident CNS macrophage, microglia comprise the majority of immune cells in the brain. Much like their peripheral counterparts, microglia survey their environment for pathology, clean up debris, and propagate inflammatory responses when necessary. Beyond this, recent studies have highlighted that microglia perform a number of complex tasks during neural development, from directing neuronal and axonal positioning to pruning synapses, receptors, and even whole cells. In this chapter, we discuss this literature within the framework that immune activation during discrete windows of neural development can profoundly impact brain function long-term, and thus the risk of neurodevelopmental and neuropsychiatric disorders. In this chapter, we review three sensitive developmental periods - embryonic wiring, early postnatal synaptic pruning, and adolescent circuit refinement - in order to highlight the diversity of functions that microglia perform in building a brain. In reviewing this literature, it becomes obvious that timing matters, perhaps more so than the nature of the immune activation itself; largely conserved patterns of microglial response to diverse insults result in different functional impacts depending on the stage of brain maturation at the time of the challenge.

Morphine exposure alters Fos expression in a sex-, age-, and brain region-specific manner during adolescence

Data in both humans and preclinical animal models clearly indicate drug exposure during adolescence, when the "reward" circuitry of the brain develops, increases the risk of substance use and other mental health disorders later in life. Human data indicate that different neural and behavioral sequelae can be observed in early versus late adolescence. However, most studies with rodent models examine a single adolescent age compared to a mature adult age, and often only in males. Herein, we sought to determine whether the acute response to the opioid morphine would also differ across adolescence, and by sex. By quantifying Fos positive cells, a proxy for neural activity, at different stages during adolescence (pre-, early, mid-, and late adolescence) and in multiple reward regions (prefrontal cortex, nucleus accumbens, caudate/putamen), we determined that the neural response to acute morphine is highly dependent on adolescent age, sex, and brain region. These data suggest that heterogeneity in the consequences of adolescent opioid exposure may be due to age- and sex-specific developmental profiles in individual reward processing regions. In future studies, it will be important to add age within adolescence as an independent variable for a holistic view of healthy or abnormal reward-related neural development.

Primetime for microglia: When stress and infection collide

Inflammation during critical windows of development contributes to behavioral affect later in life. In this of Neuron, Cao et al. (2021) demonstrate a novel mechanism through which early life Tlr4-dependent inflammation in microglia permanently alters neuronal function and leaves male mice susceptible to stress-induced depressive-like behaviors.

Associations between maternal obesity, gestational cytokine levels and child obesity in the NEST cohort

BACKGROUND

Although maternal systemic inflammation is hypothesized to link maternal pre-pregnancy obesity to offspring metabolic dysfunction, patient empirical data are limited.

OBJECTIVES

In this study, we hypothesized that pre-pregnancy obesity alters systemic chemo/cytokines concentrations in pregnancy, and this alteration contributes to obesity in children.

METHODS

In a multi-ethnic cohort of 361 mother-child pairs, we measured prenatal concentrations of plasma TNF-α, IL-6, IL-8, IL-1β, IL-4, IFN-γ, IL-12 p70 subunit, and IL-17A using a multiplex ELISA and examined associations of pre-pregnancy obesity on maternal chemo/cytokine levels, and associations of these cytokine levels with offspring body mass index z score (BMI-z) at age 2-6 years using linear regression.

RESULTS

After adjusting for maternal smoking, ethnicity, age, and education, pre-pregnancy obesity was associated with increased concentrations of TNF-α (P = .026) and IFN-γ (P = .06). While we found no evidence for associations between TNF-α concentrations and offspring BMI-z, increased IFN-γ concentrations were associated with decreased BMI-z (P = .0002), primarily in Whites (P = .0011). In addition, increased maternal IL-17A concentrations were associated with increased BMI-z in offspring (P = .0005) with stronger associations in African Americans (P = .0042) than Whites (P = .24).

CONCLUSIONS

Data from this study are consistent with maternal obesity-related inflammation during pregnancy, increasing the risk of childhood obesity in an ethnic-specific manner.

Vitamin D deficiency exacerbates UV/endorphin and opioid addiction

The current opioid epidemic warrants a better understanding of genetic and environmental factors that contribute to opioid addiction. Here we report an increased prevalence of vitamin D (VitD) deficiency in patients diagnosed with opioid use disorder and an inverse and dose-dependent association of VitD levels with self-reported opioid use. We used multiple pharmacologic approaches and genetic mouse models and found that deficiencies in VitD signaling amplify exogenous opioid responses that are normalized upon restoration of VitD signaling. Similarly, physiologic endogenous opioid analgesia and reward responses triggered by ultraviolet (UV) radiation are repressed by VitD signaling, suggesting that a feedback loop exists whereby VitD deficiency produces increased UV/endorphin-seeking behavior until VitD levels are restored by cutaneous VitD synthesis. This feedback may carry the evolutionary advantage of maximizing VitD synthesis. However, unlike UV exposure, exogenous opioid use is not followed by VitD synthesis (and its opioid suppressive effects), contributing to maladaptive addictive behavior.

Sexually dimorphic placental responses to maternal SARS-CoV-2 infection

There is a persistent male bias in the prevalence and severity of COVID-19 disease. Underlying mechanisms accounting for this sex difference remain incompletely understood. Interferon responses have been implicated as a modulator of disease in adults, and play a key role in the placental anti-viral response. Moreover, the interferon response has been shown to alter Fc-receptor expression, and therefore may impact placental antibody transfer. Here we examined the intersection of viral-induced placental interferon responses, maternal-fetal antibody transfer, and fetal sex. Placental interferon stimulated genes (ISGs), Fc-receptor expression, and SARS-CoV-2 antibody transfer were interrogated in 68 pregnancies. Sexually dimorphic placental expression of ISGs, interleukin-10, and Fc receptors was observed following maternal SARS-CoV-2 infection, with upregulation in males. Reduced maternal SARS-CoV-2-specific antibody titers and impaired placental antibody transfer were noted in pregnancies with a male fetus. These results demonstrate fetal sex-specific maternal and placental adaptive and innate immune responses to SARS-CoV-2.

Sickness and the Social Brain: Love in the Time of COVID

As a highly social species, inclusion in social networks and the presence of strong social bonds are critical to our health and well-being. Indeed, impaired social functioning is a component of numerous neuropsychiatric disorders including depression, anxiety, and substance use disorder. During the current COVID-19 pandemic, our social networks are at risk of fracture and many are vulnerable to the negative consequences of social isolation. Importantly, infection itself leads to changes in social behavior as a component of "sickness behavior." Furthermore, as in the case of COVID-19, males and females often differ in their immunological response to infection, and, therefore, in their susceptibility to negative outcomes. In this review, we discuss the many ways in which infection changes social behavior-sometimes to the benefit of the host, and in some instances for the sake of the pathogen-in species ranging from eusocial insects to humans. We also explore the neuroimmune mechanisms by which these changes in social behavior occur. Finally, we touch upon the ways in which the social environment (group living, social isolation, etc.) shapes the immune system and its ability to respond to challenge. Throughout we emphasize how males and females differ in their response to immune activation, both behaviorally and physiologically.

Isolation of Microglia from Mouse or Human Tissue

Microglia are the innate immune cells of the central nervous system. Although numerous methods have been developed to isolate microglia from the brain, the method of dissociation and isolation can have a profound effect on the function of these highly dynamic cells. Here, we present an optimized protocol to isolate CD11b+ cells (microglia) from mouse or human brain tissue using magnetic bead columns. Isolated microglia can be used to model diseases with neuroinflammatory components for potential therapeutic discoveries. For complete details on the use and execution of this protocol, please refer to Hanamsagar et al. (2017), Rivera et al. (2019), and Edlow et al. (2019).

Beyond the looking glass: recent advances in understanding the impact of environmental exposures on neuropsychiatric disease

The etiologic pathways leading to neuropsychiatric diseases remain poorly defined. As genomic technologies have advanced over the past several decades, considerable progress has been made linking neuropsychiatric disorders to genetic underpinnings. Interest and consideration of nongenetic risk factors (e.g., lead exposure and schizophrenia) have, in contrast, lagged behind heritable frameworks of explanation. Thus, the association of neuropsychiatric illness to environmental chemical exposure, and their potential interactions with genetic susceptibility, are largely unexplored. In this review, we describe emerging approaches for considering the impact of chemical risk factors acting alone and in concert with genetic risk, and point to the potential role of epigenetics in mediating exposure effects on transcription of genes implicated in mental disorders. We highlight recent examples of research in nongenetic risk factors in psychiatric disorders that point to potential shared biological mechanisms-synaptic dysfunction, immune alterations, and gut-brain interactions. We outline new tools and resources that can be harnessed for the study of environmental factors in psychiatric disorders. These tools, combined with emerging experimental evidence, suggest that there is a need to broadly incorporate environmental exposures in psychiatric research, with the ultimate goal of identifying modifiable risk factors and informing new treatment strategies for neuropsychiatric disease.

Stressed-Out T Cells Fragment the Mind

The immune system is increasingly recognized to play an integral role in regulating stress responses. In a recent article in Cell, Fan et al. demonstrate a novel mechanism through which stress drives mitochondrial fragmentation-induced xanthine accumulation in mouse CD4⁺ T cells, subsequently acting on oligodendrocytes to induce anxiety-like behaviors.

Sex-dependent neurobiological features of prenatal immune activation via TLR7

Immune activation during pregnancy via infection or autoimmune disease is a risk factor for neuropsychiatric illness. Mouse models of prenatal immune activation often involve maternal administration of agents that activate toll-like receptors (TLRs), a class of pattern recognition receptors that initiate innate immune responses. Such studies have focused primarily on activating the TLR3 or TLR4 subtypes, to mimic immune responses to viral or bacterial infections, respectively. Here, we characterize the effects of prenatal activation of TLR7, which is implicated in the pathogenesis of autoimmune disease. Prenatal TLR7 activation via administration of the selective agonist imiquimod (5.0 mg/kg) induces a phenotype in offspring characterized by reduced anxiety-like behavior, fragmented social behavior, and altered ultrasonic vocalization patterns at 6-12 weeks of age. The characteristics of this phenotype are readily distinguishable from-and in some ways opposite to-those seen following prenatal activation of TLR3 and/or TLR4. Prenatal TLR7-activated mice have normal baseline locomotor activity, but are hyperresponsive to stimuli including social partners, circadian cues, and gonadal hormone fluctuations. These alterations are accompanied by decreases in microglia density but increases in ramifications. RNA-sequencing of dorsal striatum, a region showing profound changes in microglial markers, indicates that prenatal TLR7 activation induces differential expression of hundreds of genes at 13 weeks of age, with virtually no overlap in differentially expressed genes between males and females. Our findings demonstrate that prenatal immune activation can promote a wide range of developmental trajectories, depending on the type and/or pattern of TLR activation and the sex of the offspring.

Microglia and sexual differentiation of the developing brain: A focus on ontogeny and intrinsic factors

Sexual differentiation of the brain during early development likely underlies the strong sex biases prevalent in many neurological conditions. Mounting evidence indicates that microglia, the innate immune cells of the central nervous system, are intricately involved in these sex-specific processes of differentiation. In this review, we synthesize literature demonstrating sex differences in microglial number, morphology, transcriptional state, and functionality throughout spatiotemporal development as well as highlight current literature regarding ontogeny of microglia. Along with vanRyzin et al. in this issue, we explore the idea that differences in microglia imparted by chromosomal or ontogeny-related programming can influence microglial-driven sexual differentiation of the brain, as well as the idea that extrinsic differences in the male and female brain microenvironment may in turn impart sex differences in microglia.

Microglia Sculpt Sex Differences in Social Behavior

Microglia are increasingly recognized as developmental sculptors of neural circuits. In this issue of Neuron, VanRyzin et al. (2019) demonstrate a novel mechanism by which endocannabinoids drive microglia to phagocytose newborn astrocytes in the medial amygdala of male rats, promoting sex differences in social play behavior.

Neuro-Immune Mechanisms Regulating Social Behavior: Dopamine as Mediator?

Social interactions are fundamental to survival and overall health. The mechanisms underlying social behavior are complex, but we now know that immune signaling plays a fundamental role in the regulation of social interactions. Prolonged or exaggerated alterations in social behavior often accompany altered immune signaling and function in pathological states. Thus, unraveling the link between social behavior and immune signaling is a fundamental challenge, not only to advance our understanding of human health and development, but for the design of comprehensive therapeutic approaches for neural disorders. In this review, we synthesize literature demonstrating the bidirectional relationship between social behavior and immune signaling and highlight recent work linking social behavior, immune function, and dopaminergic signaling in adolescent neural and behavioral development.

Removal of microglial-specific MyD88 signaling alters dentate gyrus doublecortin and enhances opioid addiction-like behaviors

Drugs of abuse promote a potent immune response in central nervous system (CNS) via the activation of microglia and astrocytes. However, the molecular mechanisms underlying microglial activation during addiction are not well known. We developed and functionally characterized a novel transgenic mouse (Cx3cr1-CreBT^tg/0:MyD88^f/f [Cre^tg/0]) wherein the immune signaling adaptor gene, MyD88, was specifically deleted in microglia. To test the downstream effects of loss of microglia-specific MyD88 signaling in morphine addiction, Cre^tg/0 and Cre^0/0 mice were tested for reward learning, extinction, and reinstatement using a conditioned place preference (CPP) paradigm. There were no differences in drug acquisition, but Cre^tg/0 mice had prolonged extinction and enhanced reinstatement compared to Cre^0/0 controls. Furthermore, morphine-treated Cre^tg/0 mice showed increased doublecortin (DCX) signal relative to Cre^0/0 control mice in the hippocampus, indicative of increased number of immature neurons. Additionally, there was an increase in colocalization of microglial lysosomal marker CD68 with DCX⁺cells in morphine-treated Cre^tg/0 mice but not in Cre^0/0 or drug-naїve mice, suggesting a specific role for microglial MyD88 signaling in neuronal phagocytosis in the hippocampus. Our results show that MyD88 deletion in microglia may negatively impact maturing neurons within the adult hippocampus and thus reward memories, suggesting a novel protective role for microglia in opioid addiction.

Cell-Type-Specific Interleukin 1 Receptor 1 Signaling in the Brain Regulates Distinct Neuroimmune Activities

Interleukin-1 (IL-1) signaling is important for multiple potentially pathogenic processes in the central nervous system (CNS), but the cell-type-specific roles of IL-1 signaling are unclear. We used a genetic knockin reporter system in mice to track and reciprocally delete or express IL-1 receptor 1 (IL-1R1) in specific cell types, including endothelial cells, ventricular cells, peripheral myeloid cells, microglia, astrocytes, and neurons. We found that endothelial IL-1R1 was necessary and sufficient for mediating sickness behavior and drove leukocyte recruitment to the CNS and impaired neurogenesis, whereas ventricular IL-1R1 was critical for monocyte recruitment to the CNS. Although microglia did not express IL-1R1, IL-1 stimulation of endothelial cells led to the induction of IL-1 in microglia. Together, these findings describe the structure and functions of the brain's IL-1R1-expressing system and lay a foundation for the dissection and identification of IL-1R1 signaling pathways in the pathogenesis of CNS diseases.

Maternal immune activation: reporting guidelines to improve the rigor, reproducibility, and transparency of the model

The 2017 American College of Neuropychopharmacology (ACNP) conference hosted a Study Group on 4 December 2017, Establishing best practice guidelines to improve the rigor, reproducibility, and transparency of the maternal immune activation (MIA) animal model of neurodevelopmental abnormalities. The goals of this session were to (a) evaluate the current literature and establish a consensus on best practices to be implemented in MIA studies, (b) identify remaining research gaps warranting additional data collection and lend to the development of evidence-based best practice design, and (c) inform the MIA research community of these findings. During this session, there was a detailed discussion on the importance of validating immunogen doses and standardizing the general design (e.g., species, immunogenic compound used, housing) of our MIA models both within and across laboratories. The consensus of the study group was that data does not currently exist to support specific evidence-based model selection or methodological recommendations due to lack of consistency in reporting, and that this issue extends to other inflammatory models of neurodevelopmental abnormalities. This launched a call to establish a reporting checklist focusing on validation, implementation, and transparency modeled on the ARRIVE Guidelines and CONSORT (scientific reporting guidelines for animal and clinical research, respectively). Here we provide a summary of the discussions in addition to a suggested checklist of reporting guidelines needed to improve the rigor and reproducibility of this valuable translational model, which can be adapted and applied to other animal models as well.

Microglial dopamine receptor elimination defines sex-specific nucleus accumbens development and social behavior in adolescent rats

Adolescence is a developmental period in which the mesolimbic dopaminergic "reward" circuitry of the brain, including the nucleus accumbens (NAc), undergoes significant plasticity. Dopamine D1 receptors (D1rs) in the NAc are critical for social behavior, but how these receptors are regulated during adolescence is not well understood. In this report, we demonstrate that microglia and complement-mediated phagocytic activity shapes NAc development by eliminating D1rs in male, but not female rats, during adolescence. Moreover, immune-mediated elimination of D1rs is required for natural developmental changes in male social play behavior. These data demonstrate for the first time that microglia and complement-mediated immune signaling (i) participate in adolescent brain development in a sex-specific manner, and (ii) are causally implicated in developmental changes in behavior. These data have broad implications for understanding the adolescent critical period of development, the molecular mechanisms underlying social behavior, and sex differences in brain structure and function.

Associations between maternal cytokine levels during gestation and measures of child cognitive abilities and executive functioning

Preclinical studies demonstrate that environmentally-induced alterations in inflammatory cytokines generated by the maternal and fetal immune system can significantly impact fetal brain development. Yet, the relationship between maternal cytokines during gestation and later cognitive ability and executive function remains understudied. Children (n = 246) were born of mothers enrolled in the Newborn Epigenetic Study - a prospective pre-birth cohort in the Southeastern US. We characterized seven cytokines [IL-1β, IL-4,IL-6, IL-12p70, IL-17A, tumor necrosis factor-α (TNFα), and interferon-γ (IFNγ)] and one chemokine (IL-8) from maternal plasma collected during pregnancy. We assessed children's cognitive abilities and executive functioning at a mean age of 4.5 (SD = 1.1) years. Children's DAS-II and NIH toolbox scores were regressed on cytokines and the chemokine, controlling for maternal age, race, education, body mass index, IQ, parity, smoking status, delivery type, gestational weeks, and child birth weight and sex. Higher IL-12p70 (β_IL-12p70 = 4.26, p = 0.023) and IL-17A (β_IL-17A = 3.70, p = 0.042) levels were related to higher DAS-II GCA score, whereas higher IL-1β (β_IL-1B = -6.07, p = 0.003) was related to lower GCA score. Higher IL-12p70 was related to higher performance on NIH toolbox measures of executive functions related to inhibitory control and attention (β_IL-12p70 = 5.20, p = 0.046) and cognitive flexibility (β_IL-12p70 = 5.10, p = 0.047). Results suggest that dysregulation in gestational immune activity are associated with child cognitive ability and executive functioning.

Gut-immune-brain dysfunction in Autism: Importance of sex

Autism Spectrum Disorder (ASD) is characterized by social behavior deficits, stereotypies, cognitive rigidity, and in some cases severe intellectual impairment and developmental delay. Although ASD is most widely identified by its neurological deficits, gastrointestinal issues are common in ASD. An intimate and complex relationship exists between the gut, the immune system, and the brain, leading to the hypothesis that ASD may be a systems-level disease affecting the gut and immune systems, in addition to the brain. Despite significant advances in understanding the contribution of the gut and immune systems to the etiology of ASD, there is an intriguing commonality among patients that is not well understood: they are predominantly male. Virtually no attention has been given to the potential role of sex-specific regulation of gut, peripheral, and central immune function in ASD, despite the 4:1 male-to-female bias in this disorder. In this review, we discuss recent revelations regarding the impact of gut-immune-brain relationships on social behavior in rodent models and in ASD patients, placing them in the context of known or putative sex specific mechanisms.

Environment matters: microglia function and dysfunction in a changing world

The immune system is our interface with the environment, and immune molecules such as cytokines and chemokines and the cells that produce them within the brain, notably microglia, are critical for normal brain development. This recognition has in recent years led to the working hypothesis that inflammatory events during pregnancy or the early postnatal period, for example, in response to infection, may disrupt the normal developmental trajectory of microglia and consequently their interactions with neurons, thereby contributing to the risk for neurological disorders. The current article outlines recent findings on the impact of diverse, pervasive environmental challenges, beyond infection, including air pollution and maternal stress; and their impact on microglial development and its broad implications for neural pathologies.

Perinatal western-type diet and associated gestational weight gain alter postpartum maternal mood

INTRODUCTION

The role of perinatal diet in postpartum maternal mood disorders, including depression and anxiety, remains unclear. We investigated whether perinatal consumption of a Western-type diet (high in fat and branched-chain amino acids [BCAA]) and associated gestational weight gain (GWG) cause serotonin dysregulation in the central nervous system (CNS), resulting in postpartum depression and anxiety (PPD/A).

METHODS

Mouse dams were fed one of four diets (high-fat/high BCAA, low-fat/high BCAA, high-fat, and low-fat) prior to mating and throughout gestation and lactation. Postpartum behavioral assessments were conducted, and plasma and brain tissues assayed. To evaluate potential clinical utility, we conducted preliminary human studies using data from an extant sample of 17 primiparous women with high GWG, comparing across self-reported postpartum mood symptoms using the Edinburgh Postnatal Depression Scale (EPDS) for percent GWG and plasma amino acid levels.

RESULTS

Mouse dams fed the high-fat/high BCAA diet gained more weight per kcal consumed, and BCAA-supplemented dams lost weight more slowly postpartum. Dams on BCAA-supplemented diets exhibited increased PPD/A-like behavior, decreased dopaminergic function, and decreased plasma tyrosine and histidine levels when assessed on postnatal day (P)8. Preliminary human data showed that GWG accounted for 29% of the variance in EPDS scores. Histidine was also lower in women with higher EPDS scores.

CONCLUSIONS

These findings highlight the role of perinatal diet and excess GWG in the development of postpartum mood disorders.

Beyond infection - Maternal immune activation by environmental factors, microglial development, and relevance for autism spectrum disorders

Immune molecules such as cytokines and chemokines and the cells that produce them within the brain, notably microglia, are critical for normal brain development. This recognition has in recent years led to the working hypothesis that inflammatory events during pregnancy, e.g. in response to infection, may disrupt the normal expression of immune molecules during critical stages of neural development and thereby contribute to the risk for neurodevelopmental disorders such as autism spectrum disorder (ASD). This hypothesis has in large part been shepherded by the work of Dr. Paul Patterson and colleagues, which has elegantly demonstrated that a single viral infection or injection of a viral mimetic to pregnant mice significantly and persistently impacts offspring immune and nervous system function, changes that underlie ASD-like behavioral dysfunction including social and communication deficits. Subsequent studies by many labs - in humans and in non-human animal models - have supported the hypothesis that ongoing disrupted immune molecule expression and/or neuroinflammation contributes to at least a significant subset of ASD. The heterogeneous clinical and biological phenotypes observed in ASD strongly suggest that in genetically susceptible individuals, environmental risk factors combine or synergize to create a tipping or threshold point for dysfunction. Importantly, animal studies showing a link between maternal immune activation (MIA) and ASD-like outcomes in offspring involve different species and diverse environmental factors associated with ASD in humans, beyond infection, including toxin exposures, maternal stress, and maternal obesity, all of which impact inflammatory or immune pathways. The goal of this review is to highlight the broader implications of Dr. Patterson's work for the field of autism, with a focus on the impact that MIA by diverse environmental factors has on fetal brain development, immune system development, and the pathophysiology of ASD.

Maternal inflammatory diet and adverse pregnancy outcomes: Circulating cytokines and genomic imprinting as potential regulators?

Excessive inflammation during pregnancy alters homeostatic mechanisms of the developing fetus and has been linked to adverse pregnancy outcomes. An anti-inflammatory diet could be a promising avenue to combat the pro-inflammatory state of pregnancy, particularly in obese women, but we lack mechanistic data linking this dietary pattern during pregnancy to inflammation and birth outcomes. In an ethnically diverse cohort of 1057 mother-child pairs, we estimated the relationships between dietary inflammatory potential [measured via the energy-adjusted dietary inflammatory index (E-DII™)] and birth outcomes overall, as well as by offspring sex and maternal pre-pregnancy body mass index (BMI). In a subset of women, we also explored associations between E-DII, circulating cytokines (n = 105), and offspring methylation (n = 338) as potential modulators of these relationships using linear regression. Adjusted regression models revealed that women with pro-inflammatory diets had elevated rates of preterm birth among female offspring [β = -0.22, standard error (SE) = 0.07, P<0.01], but not male offspring (β=0.09, SE = 0.06, P<0.12) (P_interaction = 0.003). Similarly, we observed pro-inflammatory diets were associated with higher rates of caesarean delivery among obese women (β = 0.17, SE = 0.08, P = 0.03), but not among women with BMI <25 kg/m² (P_interaction = 0.02). We observed consistent inverse associations between maternal inflammatory cytokine concentrations (IL-12, IL-17, IL-4, IL-6, and TNFα) and lower methylation at the MEG3 regulatory sequence (P<0.05); however, results did not support the link between maternal E-DII and circulating cytokines. We replicate work by others on the association between maternal inflammatory diet and adverse pregnancy outcomes and provide the first empirical evidence supporting the inverse association between circulating cytokine concentrations and offspring methylation.

Generation of a microglial developmental index in mice and in humans reveals a sex difference in maturation and immune reactivity

Evidence suggests many neurological disorders emerge when normal neurodevelopmental trajectories are disrupted, i.e., when circuits or cells do not reach their fully mature state. Microglia play a critical role in normal neurodevelopment and are hypothesized to contribute to brain disease. We used whole transcriptome profiling with Next Generation sequencing of purified developing microglia to identify a microglial developmental gene expression program involving thousands of genes whose expression levels change monotonically (up or down) across development. Importantly, the gene expression program was delayed in males relative to females and exposure of adult male mice to LPS, a potent immune activator, accelerated microglial development in males. Next, a microglial developmental index (MDI) generated from gene expression patterns obtained from purified mouse microglia, was applied to human brain transcriptome datasets to test the hypothesis that variability in microglial development is associated with human diseases such as Alzheimer's and autism where microglia have been suggested to play a role. MDI was significantly increased in both Alzheimer's Disease and in autism, suggesting that accelerated microglial development may contribute to neuropathology. In conclusion, we identified a microglia-specific gene expression program in mice that was used to create a microglia developmental index, which was applied to human datasets containing heterogeneous cell types to reveal differences between healthy and diseased brain samples, and between males and females. This powerful tool has wide ranging applicability to examine microglial development within the context of disease and in response to other variables such as stress and pharmacological treatments.

Gestational Exposure to Air Pollution Alters Cortical Volume, Microglial Morphology, and Microglia-Neuron Interactions in a Sex-Specific Manner

Microglia are the resident immune cells of the brain, important for normal neural development in addition to host defense in response to inflammatory stimuli. Air pollution is one of the most pervasive and harmful environmental toxicants in the modern world, and several large scale epidemiological studies have recently linked prenatal air pollution exposure with an increased risk of neurodevelopmental disorders such as autism spectrum disorder (ASD). Diesel exhaust particles (DEP) are a primary toxic component of air pollution, and markedly activate microglia in vitro and in vivo in adult rodents. We have demonstrated that prenatal exposure to DEP in mice, i.e., to the pregnant dams throughout gestation, results in a persistent vulnerability to behavioral deficits in adult offspring, especially in males, which is intriguing given the greater incidence of ASD in males to females (∼4:1). Moreover, there is a striking upregulation of toll-like receptor (TLR) 4 gene expression within the brains of the same mice, and this expression is primarily in microglia. Here we explored the impact of gestational exposure to DEP or vehicle on microglial morphology in the developing brains of male and female mice. DEP exposure increased inflammatory cytokine protein and altered the morphology of microglia, consistent with activation or a delay in maturation, only within the embryonic brains of male mice; and these effects were dependent on TLR4. DEP exposure also increased cortical volume at embryonic day (E)18, which switched to decreased volume by post-natal day (P)30 in males, suggesting an impact on the developing neural stem cell niche. Consistent with this hypothesis, we found increased microglial-neuronal interactions in male offspring that received DEP compared to all other groups. Taken together, these data suggest a mechanism by which prenatal exposure to environmental toxins may affect microglial development and long-term function, and thereby contribute to the risk of neurodevelopmental disorders.

Optimized solubilization of TRIzol-precipitated protein permits Western blotting analysis to maximize data available from brain tissue

BACKGROUND

Techniques simultaneously assessing multiple levels of molecular processing are appealing because molecular signaling underlying complex neural phenomena occurs at complementary levels. The TRIzol method isolates RNA and DNA, but protein retrieval is difficult due to inefficient solubilization of precipitated protein pellets.

NEW METHOD

We optimized a buffer for the efficient solubilization of protein from TRIzol-precipitated brain tissue for Western blotting analysis, which was also more effective at directly homogenizing brain tissue than RIPA buffer.

RESULTS

Protein yield during solubilization, in addition to protein yield via direct homogenization, is increased by optimizing concentrations of chemicals in a standard lysis buffer. Effective incubation parameters for both total protein yield and the analysis of post-translational modifications is remarkably flexible. Importantly, different neural cell types and protein classes are represented in solubilized protein samples. Moreover, we used dissociated mouse brain tissue to isolate microglia from other cell types and successfully resolved cell type-specific proteins from these small and difficult to attain samples.

COMPARISON WITH EXISTING METHOD(S)

Solubilization buffers to date have been comprised primarily of SDS or urea; the data herein demonstrate that components common to lysis buffers can also enhance protein solubilization both after direct homogenization and after precipitation.

CONCLUSIONS

This method is suitable for assessing gene and protein expression from a single brain sample, allowing for a more comprehensive evaluation of neural phenomena while minimizing the number of subjects.

Sex differences in neurodevelopmental and neurodegenerative disorders: Focus on microglial function and neuroinflammation during development

Several neurological conditions are associated with sex differences in prevalence or outcome. For example, autism predominantly affects boys, depression is more common in women, Parkinson's disease more common in men, and multiple sclerosis in women. In the case of stroke, women have a less favorable outcome and suffer from a more precipitous drop in health status compared to men. As a result, treatment of such diseases is difficult and yields variable results. Despite this, sex is rarely considered when making treatment decisions. The mechanisms underlying sex differences in disease progression are not well understood, however a strong link exists between different inflammation states of men and women and their propensity to develop certain diseases. As neuroinflammation is an important component of pathophysiology in many neurological conditions, it can be speculated that any changes in the state of inflammation in the brain during normal development can potentially lead to an increase in susceptibility to neurological and neurodegenerative diseases. Microglia play a crucial role in onset and modulation of inflammation and thus sex differences in microglial function could explain, at least in part, differences observed in susceptibilities and outcomes of neurological disorders in men and women. Understanding the mechanisms behind sex differences could help develop more targeted therapy with higher success rate, especially in diseases where sex differences are most prominent.

Got worms? Perinatal exposure to helminths prevents persistent immune sensitization and cognitive dysfunction induced by early-life infection

The incidence of autoimmune and inflammatory diseases has risen dramatically in post-industrial societies. "Biome depletion" - loss of commensal microbial and multicellular organisms such as helminths (intestinal worms) that profoundly modulate the immune system - may contribute to these increases. Hyperimmune-associated disorders also affect the brain, especially neurodevelopment, and increasing evidence links early-life infection to cognitive and neurodevelopmental disorders. We have demonstrated previously that rats infected with bacteria as newborns display life-long vulnerabilities to cognitive dysfunction, a vulnerability that is specifically linked to long-term hypersensitivity of microglial cell function, the resident immune cells of the brain. Here, we demonstrate that helminth colonization of pregnant dams attenuated the exaggerated brain cytokine response of their offspring to bacterial infection, and that combined with post-weaning colonization of offspring with helminths (consistent with their mothers treatment) completely prevented enduring microglial sensitization and cognitive dysfunction in adulthood. Importantly, helminths had no overt impact on adaptive immune cell subsets, whereas exaggerated innate inflammatory responses in splenic macrophages were prevented. Finally, helminths altered the effect of neonatal infection on the gut microbiome; neonatal infection with Escherichia coli caused a shift from genera within the Actinobacteria and Tenericutes phyla to genera in the Bacteroidetes phylum in rats not colonized with helminths, but helminths attenuated this effect. In sum, these data point toward an inter-relatedness of various components of the biome, and suggest potential mechanisms by which this helminth might exert therapeutic benefits in the treatment of neuroinflammatory and cognitive disorders.

Examination of sex and minocycline treatment on acute morphine-induced analgesia and inflammatory gene expression along the pain pathway in Sprague-Dawley rats

BACKGROUND

In addition to its classical effects on opioid receptors, morphine can activate glia and stimulate the production of pro-inflammatory immune molecules which in turn counteract the analgesic properties of morphine. We hypothesized that decreased morphine analgesia in females may be the result of exaggerated microglial activation in brain regions critical for analgesia.

METHODS

Male and female rats were treated with morphine and/or minocycline and morphine analgesia was examined using the hot plate. We also examined the expression of microglial and astrocyte markers in the pain pathway.

RESULTS

Males treated with minocycline, a microglial inhibitor, exhibited a significant increase in acute morphine analgesia as previously shown; however, morphine analgesia was not affected by minocycline pretreatment in female rats. Minocycline decreased the expression of glial activation markers in the male spinal cord and periaqueductal gray as expected; however, these same molecules were upregulated in the female.

CONCLUSIONS

These data describe a significant difference between males and females in the behavioral effects following co-administration of morphine and minocycline.

Developmental programming of brain and behavior by perinatal diet: focus on inflammatory mechanisms

Obesity is now epidemic worldwide. Beyond associated diseases such as diabetes, obesity is linked to neuropsychiatric disorders such as depression. Alarmingly maternal obesity and high-fat diet consumption during gestation/lactation may “program” offspring longterm for increased obesity themselves, along with increased vulnerability to mood disorders. We review the evidence that programming of brain and behavior by perinatal diet is propagated by inflammatory mechanisms, as obesity and high-fat diets are independently associated with exaggerated systemic levels of inflammatory mediators. Due to the recognized dual role of these immune molecules (eg, interleukin [IL]-6, 11-1β) in placental function and brain development, any disruption of their delicate balance with growth factors or neurotransmitters (eg, serotonin) by inflammation early in life can permanently alter the trajectory of fetal brain development. Finally, epigenetic regulation of inflammatory pathways is a likely candidate for persistent changes in metabolic and brain function as a consequence of the perinatal environment.

Alteration of the rat cecal microbiome during colonization with the helminth Hymenolepis diminuta

The microbiome is now widely recognized as being important in health and disease, and makes up a substantial subset of the biome within the ecosystem of the vertebrate body. At the same time, multicellular, eukaryotic organisms such as helminths are being recognized as an important component of the biome that shaped the evolution of our genes. The absence of these macroscopic organisms during the early development and life of humans in Western culture probably leads to a wide range of human immunological diseases. However, the interaction between the microbiome and macroscopic components of the biome remains poorly characterized. In this study, the microbiome of the cecum in rats colonized for 2 generations with the small intestinal helminth Hymenolepis diminuta was evaluated. The introduction of this benign helminth, which is of considerable therapeutic interest, led to several changes in the cecal microbiome. Most of the changes were within the Firmicutes phylum, involved about 20% of the total bacteria, and generally entailed a shift from Bacilli to Clostridia species in the presence of the helminth. The results point toward ecological relationships between various components of the biome, with the observed shifts in the microbiome suggesting potential mechanisms by which this helminth might exert therapeutic effects.

A model for the induction of autism in the ecosystem of the human body: the anatomy of a modern pandemic?

Background

The field of autism research is currently divided based on a fundamental question regarding the nature of autism: Some are convinced that autism is a pandemic of modern culture, with environmental factors at the roots. Others are convinced that the disease is not pandemic in nature, but rather that it has been with humanity for millennia, with its biological and neurological underpinnings just now being understood.

Objective

In this review, two lines of reasoning are examined which suggest that autism is indeed a pandemic of modern culture. First, given the widely appreciated derailment of immune function by modern culture, evidence that autism is strongly associated with aberrant immune function is examined. Second, evidence is reviewed indicating that autism is associated with ‘triggers’ that are, for the most part, a construct of modern culture. In light of this reasoning, current epidemiological evidence regarding the incidence of autism, including the role of changing awareness and diagnostic criteria, is examined. Finally, the potential role of the microbial flora (the microbiome) in the pathogenesis of autism is discussed, with the view that the microbial flora is a subset of the life associated with the human body, and that the entire human biome, including both the microbial flora and the fauna, has been radically destabilized by modern culture.

Conclusions

It is suggested that the unequivocal way to resolve the debate regarding the pandemic nature of autism is to perform an experiment: monitor the prevalence of autism after normalizing immune function in a Western population using readily available approaches that address the well-known factors underlying the immune dysfunction in that population.