Archive for May 2010
Conclusions that are either correct, wrong, or somewhat in the middle; does childhood hospitilization for infection increase the risk for autism?
Posted on: May 13, 2010
Hello friends –
The abstract for Association of hospitalization for infection in childhood with diagnosis of autism spectrum disorders: a Danish cohort study hit my inbox the other morning. Here is the abstract
OBJECTIVE: To investigate the association between hospitalization for infection in the perinatal/neonatal period or childhood and the diagnosis of autism spectrum disorders (ASDs). DESIGN: A population-based cohort study. SETTING: Denmark. PARTICIPANTS: All children born in Denmark from January 1, 1980, through December 31, 2002, comprising a total of 1 418 152 children. EXPOSURE: Infection requiring hospitalization. MAIN OUTCOME MEASURE: The adjusted hazard ratio (HR) for ASDs among children hospitalized for infection compared with other children. RESULTS: A total of 7379 children were diagnosed as having ASDs. Children admitted to the hospital for any infectious disease displayed an increased rate of ASD diagnoses (HR, 1.38 [95% confidence interval, 1.31-1.45]). This association was found to be similar for infectious diseases of bacterial and viral origin. Furthermore, children admitted to the hospital for noninfectious disease also displayed an increased rate of ASD diagnoses (HR, 1.76 [95% confidence interval, 1.68-1.86]), and admissions for infection increased the rate of mental retardation (2.18 [2.06-2.31]). CONCLUSIONS: The association between hospitalization for infection and ASDs observed in this study does not suggest causality because a general association is observed across different infection groups. Also, the association is not specific for infection or for ASDs. We discuss a number of noncausal explanatory models
[Emphasis is mine.]
Considering my interest in early life immune activation, and the often difficult to predict, persistent outcomes from a variety of animal models, this study immediately struck me as an interesting one. The authors graciously sent my real world inbox a copy of this paper, as well as a similar one involving maternal infection during pregnancy, which I have yet to read.
Anyways, what strikes me very clearly here is that the authors and I have reached exactly the opposite conclusions towards the potential of a casual link between autism and hospitalization for infection in the perinatal / infancy periods. They apparently feel that the fact that an association is observed across different infectious agents (i.e., bacterial or viral), that this argues against a causal mechanism. But, as I have detailed in A Brief History of Early Life Immune Challenges and Why They (Might) Matter, we have an increasing number of animal studies that indicate that spikes in innate immune system cytokines during critical developmental timeframes can have, perverse and often baffling effects that we are only beginning to understand. Most of this research is brand new, within the past three years, and solely in the realm of animal models. However, the critical component of these studies that the Denmark study fails to take into consideration is that the innate immune response will be initiated regardless if the stimulant is viral or bacterial in nature. That is to say, the evidence from these studies tells us that the fact that we are observing differences across bacterial or viral pathogens is not necessarily an indication of lack of effect, but rather, could instead point towards a global effect, one that happens in both instances; surges in pro-inflammatory cytokines from the innate immune response.
For an example of some of these animal models, we could look to Postnatal Inflammation Increases Seizure Susceptibility in Adult Rats, which observed a tnf-alpha driven, time dependent mechanism that ‘increases seizure susceptibility in adult rats’.
There are critical postnatal periods during which even subtle interventions can have long-lasting effects on adult physiology. We asked whether an immune challenge during early postnatal development can alter neuronal excitability and seizure susceptibility in adults. Postnatal day 14 (P14) male Sprague Dawley rats were injected with the bacterial endotoxin lipopolysaccharide (LPS), and control animals received sterile saline. Three weeks later, extracellular recordings from hippocampal slices revealed enhanced field EPSP slopes after Schaffer collateral stimulation and increased epileptiform burst-firing activity in CA1 after 4-aminopyridine application. Six to 8 weeks after postnatal LPS injection, seizure susceptibility was assessed in response to lithium–pilocarpine, kainic acid, and pentylenetetrazol. Rats treated with LPS showed significantly greater adult seizure susceptibility to all convulsants, as well as increased cytokine release and enhanced neuronal degeneration within the hippocampus after limbic seizures. These persistent increases in seizure susceptibility occurred only when LPS was given during a critical postnatal period (P7 and P14) and not before (P1) or after (P20). This early effect of LPS on adult seizures was blocked by concurrent intracerebroventricular administration of a tumor necrosis factor (TNF) antibody and mimicked by intracerebroventricular injection of rat recombinant TNF. Postnatal LPS injection did not result in permanent changes in microglial (Iba1) activity or hippocampal cytokine [IL-1β (interleukin-1β) and TNF] levels, but caused a slight increase in astrocyte (GFAP) numbers. These novel results indicate that a single LPS injection during a critical postnatal period causes a long-lasting increase in seizure susceptibility that is strongly dependent on TNF.
Another, very similar study, Viral-like brain inflammation during development causes increased seizure susceptibility in adult reports:
Viral infections of the CNS and their accompanying inflammation can cause long-term neurological effects, including increased risk for seizures. To examine the effects of CNS inflammation, we infused polyinosinic:polycytidylic acid, intracerebroventricularly to mimic a viral CNS infection in 14 day-old rats. This caused fever and an increase in the pro-inflammatory cytokine, interleukin (IL)-1beta in the brain. As young adults, these animals were more susceptible to lithium-pilocarpine and pentylenetetrazol-induced seizures and showed memory deficits in fear conditioning. Whereas there was no alteration in adult hippocampal cytokine levels, we found a marked increase in NMDA (NR2A and C) and AMPA (GluR1) glutamate receptor subunit mRNA expression. The increase in seizure susceptibility, glutamate receptor subunits, and hippocampal IL-1beta levels were suppressed by neonatal systemic minocycline. Thus, a novel model of viral CNS inflammation reveals pathophysiological relationships between brain cytokines, glutamate receptors, behaviour and seizures, which can be attenuated by anti-inflammatory agents like minocycline.
If we look closely here, we can see that either viral or bacterial mimics were able to generate similar physiological outcomes, outcomes that have strong correlations to the autism realm, namely increased rates of epilepsy, associations with seizures during infancy, and abnormal EEGs. But importantly for the decision tree in the case of childhood infections in the studies above, taken together, we can see that it didn’t matter if the trigger was bacterial or viral, just that there was an innate immune response at all. This is further evidenced by the fact that in both instances, different anti-inflammatory agents were capable of attenuating the changes. Our mechanism of action does not mandate pathogen specific interactions, in many cases, the cut off is whether or not you generate an innate immune response or not, regardless of the specific trigger. Another way of putting this would be, if an immune response for any pathogen were capable of initiating an cascade responsible for development of autistic behaviors, what would a pattern of hospitalization look like? [Children admitted to the hospital for any infectious disease displayed an increased rate of ASD diagnoses (HR, 1.38 [95% confidence interval, 1.31-1.45]). This association was found to be similar for infectious diseases of bacterial and viral origin.]
If you ask the wrong question even the right answer might not be useful in understanding a mystery.
All that being said, I have begun to see why Denmark makes such an attractive location for this kind of study. They have amassed an impressive set of data that could yield important clues if we can use it wisely.
I also noted that there is a P. Thorsen listed. I, for one, could care less.
– pD
Intriguing Findings – Maternal Obesity, Inflammation, and Consequent Priming of Microglia, Immune Alterations, and Spatial Processing in Offspring (!)
Posted on: May 4, 2010
Hello friends –
I’ve been forced to modify my pubmed alerts so that I don’t miss abstracts like this:
Enduring consequences of maternal obesity for brain inflammation and behavior of offspring
Obesity is well characterized as a systemic inflammatory condition, and is also associated with cognitive disruption, suggesting a link between the two. We assessed whether peripheral inflammation in maternal obesity may be transferred to the offspring brain, in particular, the hippocampus, and thereby result in cognitive dysfunction. Rat dams were fed a high-saturated-fat diet (SFD), a high-trans-fat diet (TFD), or a low-fat diet (LFD) for 4 wk prior to mating, and remained on the diet throughout pregnancy and lactation. SFD/TFD exposure significantly increased body weight in both dams and pups compared to controls. Microglial activation markers were increased in the hippocampus of SFD/TFD pups at birth. At weaning and in adulthood, proinflammatory cytokine expression was strikingly increased in the periphery and hippocampus following a bacterial challenge [lipopolysaccharide (LPS)] in the SFD/TFD groups compared to controls. Microglial activation within the hippocampus was also increased basally in SFD rats, suggesting a chronic priming of the cells. Finally, there were marked changes in anxiety and spatial learning in SFD/TFD groups. These effects were all observed in adulthood, even after the pups were placed on standard chow at weaning, suggesting these outcomes were programmed early in life.
WOW. [All emphasis is mine]
You may note that there isn’t any mention of autism per se here, but we do seem to hit a lot of sweet spots that immediately grabbed my attention for a couple of reasons. While my primary persona as Some Jerk On The Internet is a self appointed autism investigator, somewhere along the line in real life I’ve been trying some relatively strange (for the US) dietary practices; a ‘veganesque’ ingredient selection and the move to a diet based on whole foods, organic when possible. What I’ve noticed during this timeframe is just how fat so many Americans are. The obesity epidemic is real, folks, and doesn’t have the fuzzy nature of ‘increased awareness’ to allow us to (pretend) hope that there isn’t something real happening; we have been getting fatter and fatter for the past few decades. And here we have evidence that obesity can create physiological and behavioral changes in offspring through our mediator de jour, inflammation.
So, why am I blogging about this paper on an autism blog? Creepily enough, a lot of the differences listed here (well, all of them, actually), have similarities to findings in the autism realm.
At weaning and in adulthood, proinflammatory cytokine expression was strikingly increased in the periphery and hippocampus following a bacterial challenge [lipopolysaccharide (LPS)] in the SFD/TFD groups compared to controls.
With human subjects, it is a bit problematic to determine if there are ‘striking increases’ in proinflammatory cytokine expression in the hippocampus following bacterial challenge, but in vitro, we have scads of evidence that the autism population creates an exaggerated innate immune response when compared to ‘normal’. The most recent example of this is, Differential monocyte responses to TLR ligands in children with autism spectrum disorders, by Enstrom, which I also blogged about. We also have Ashwood, and several by Jyonouchi showing similar findings; increased production of proinflammatory cytokines TNF-alpha, IL-6, and IL1-B to some TLR agonists, including TLR4.
Microglial activation within the hippocampus was also increased basally in SFD rats, suggesting a chronic priming of the cells.
Of course, the seminal paper in this regard was Vargas, Neuroglial Activation and Neuroinflammation in the Brain of Patients with Autism, which found increased levels of microglial activation in an autism cohort; their focus seemed to be areas other than the hippocampus. Similar findings of an ongoing immune response within the CNS in autism population can be found in Elevated immune response in the brain of autistic patients, and Immune transcriptome alterations in the temporal cortex of subjects with autism. The concept of ‘primed microglia’ is touched on in another paper by Bilbo, Early-life programming of later-life brain and behavior: a critical role for the immune system, which I’d like to get to eventually, but haven’t had the time for yet, but essentially suggests that there are time critical periods during which the microglia are vulnerable to persistent immunological modification, changing their resting state and response to future immune challenges.
Finally, there were marked changes in anxiety and spatial learning in SFD/TFD groups.
Oh yeah. Everyone has heard the story about the kid with autism who can paint New York City after a helicopter ride, or seeing colors in sound, or whatever, but there does also seem to be a lot of applied research involving specific kinds of visual tests that people with autism seem to do better at than people without. Curiously, we even have a knock out model in rodents that show superior spatial processing skills. Anyone who knows a couple of kids with autism knows one that has anxiety problems; there are some evaluations of this, but honestly, this type of thing suffers a bit from my mind because in order to ask the right question, ‘Are you anxious?’, you’ve eliminated a chunk of the autism population. If we break down to the chemical level and start looking at known biomarkers for the stress response, the HPA-Axis, we’ve got tons of evidence that something is amiss.
Here are some of the juicier parts of the paper. From the introduction:
Obesity and insulin resistance are also strongly linked to cognitive dysfunction, including Alzheimer’s disease (13, 14). Neuroinflammation is independently linked to cognitive disruption (15); brain IL-1 expression, in particular, is implicated in Alzheimer’s disease pathogenesis (16). However, a direct mechanism linking these diverse factors is lacking; that is, whether peripheral inflammation in obesity contributes directly to inflammation/ cytokine production in cognitive regions of the brain, and thus cognitive disruption, remains unclear. Moreover, whether maternal obesity may program inflammation within the brains of offspring long term, particularly in regions important for cognition, such as the hippocampus, is virtually unknown. Tozuka et al. (17) recently reported long-term impairments in neurogenesis within the dentate gyrus as a consequence of being born to obese mouse dams, although the researchers did not explore a potential role for inflammation. Notably, White et al. (18) reported increased glial activation and oxidative stress in the cortex of high-fat-diet-fed rats that were also born to high-fat dams. Glia are the primary immunocompetent cells of brain; thus, long-term changes in their activity as a consequence of diet could be critical in long-term programming of neural function.
The whole ‘long term programming of neural function’ theory is beautiful and terrifying. From the discussion section:
A central question in this study was to identify whether systemic inflammation is transferred to cognitive regions of the brain. The answer to that question is clearly yes, especially in the SFD rats. These animals exhibited increased peripheral cytokines (in liver, fat,and serum) and hippocampal IL-1 responses to an LPS challenge. At P20 and in adults, rats from SFD dams exhibited a very large increase in hippocampal IL-1 following a moderate dose of LPS (Fig. 4). Males exhibited a larger response than females in adulthood, although the diet effect was significant in both sexes. The exaggerated response in adults was particularly striking, given that these animals had been fed a low-fat diet since weaning, a period of time longer in duration (9 wk) than the total time they were exposed to the high-fat diets during development (6 wk). Notable as well was the increase in basal levels of IL-1 in high-fat diet groups in adulthood. These data indicate a basal shift in the expression of this cytokine.
It is almost as if being male provides risks every time they bother to look. Oh well. This would seem to be an illustration of ‘long term programming’, what happened during development was more important than what happened afterwards. There were some differences found in the transfat group compared to the saturated fat group, primarily observed in differences in neural and peripheral response to LPS; the authors theorize that this is related to the deposition locations of the different kinds of fats; i.e., saturated fats make it into the brain more easily than trans fats, which are stored in the liver (where they measured perfipheral inflammation).
We explored the influence of a trans-fat-rich vs. saturated-fat-rich diet independently in this study. This comparison yielded surprising findings as well, as the SFD appeared to be much more deleterious for body weight, leptin, and IL-1 compared to the TFD, especially in males. Conversely, CRP expression in the liver, a reliable risk factor for heart disease in humans, was significantly increased in the TFD groups following LPS compared to the SFD and LFD groups. It should be noted, however, that the dams consumed less of the TFD than of the SFD, suggesting it may have been less palatable, and therefore, induced fewer changes. Furthermore, the role of CRP as an acute-phase protein in rodents is controversial (52); support for this idea is the lack of increase in response to LPS in every diet group. Thus, the role of CRP in this experimental model, if any, remains to be further explored.
Remarkably, the authors found that low fat diet rats performed better in some tasks, and the authors speculate that a high fat diet may produce some cognitive gains.
A very intriguing possibility as well is that increased basal IL-1 in the high-fat-diet groups facilitated cognition. A growing body of evidence suggests a role for IL-1 in normal, nonpathological, synaptic plasticity mechanisms within the brain, including memory (44). IL-1 is critical for long-term potentiation (LTP) maintenance during learning (45, 46). However, exaggerated IL-1 within the brain is also strongly associated with memory impairment, providing support for an inverted-U function for optimal IL-1 and cognition(46, 47).
The inveterd U function is, I believe, similar to the concept of hormesis, wherein exposure to an agent and physiological response does not necessarily follow a straight linear response. A good example of this that may be the Pessah studies, which found that for some types of PCBs, low level exposure caused more problems than high levels of exposure. [Note: Beware of anyone who wants to use the ‘poison is in the dose’ cannard, which might be meaningful if the measurement endpoints are mortality, but increasingly less worthwhile if you want to measure subtle effects.]
Here is the closing paragraph:
In closing, it is clear that maternal high-fat diet has a profound influence on the innate immune response of the offspring, in both the periphery and the brain, and that this has enduring consequences for cognition and affect in both males and females. Future studies are needed to assess whether peripheral signals such as leptin vs. central targets such as microglia may be driving the responses in brain, and whether immune targeting (e.g., TLR4 signaling) may be sufficient to prevent exaggerated CNS inflammation in high-fatdiet-exposed pups.
Great idea at the end! I’d love to see a paper where they replicated these groups, but one group of rats also got fish oil or other anti-inflammatories to see if the effect of the inflammation was attenuated. TLR4 knockout mice might also be neat to see. This is a cool study that tells us just how much we still have to learn about how our choices can have very difficult to predict effects.
One of the authors of this paper, Staci Bilbo, has been on a bit of a tear lately regarding the effect of early life immune challenges and subsequent immune and behavioral differences in the treatment animals, including recent hits like Early-life infection is a vulnerability factor for aging-related glial alterations and cognitive decline, Enduring consequences of early-life infection on glial and neural cell genesis within cognitive regions of the brain, and Early-life programming of later-life brain and behavior: a critical role for the immune system, all of which may have implications for everyone’s favorite environmental agent. I’ll be tackling those papers, and several others that have come out with similar methodologies soon enough, but the entire Frontline debacle has left me a little exhausted on the issue.
– pD
passionless Droning about autism 