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

Abstract

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.

Keywords: Air pollution; Developmental programming; Environmental toxins; Neurodevelopmental disorders.

Figure 1

(A) Microglia transition from a round/amoeboid morphology early in development (∼E10, when they first begin to colonize the CNS) to a ramified morphology with small cell bodies and thin processes by P30. (B) DEP increases the number of round microglia in cortex at E18, and the number of thick microglia at P30, in males only, suggesting DEP delays their maturation and/or leads to their activation. There are similar effects in other brain regions including hippocampus (not shown) (C) All of the effects of DEP in males are dependent on TLR4. *p<0.05 for all; NS = not significantly different.

Figure 2

Toll-like receptors (TLR)s are the primary innate immune receptors which recognize pathogens in the body and brain. TLR4 recognizes bacteria, such as E. coli, and lipopolysaccharide (LPS), which is the cell wall component. TLR3 recognizes double-stranded RNA, the genetic code for viruses, as well as viral mimetics, such as Poly IC. Though TLR4 and TLR3 activation induces very different intracellular signaling pathways, activation of either receptor results in the transcription and translation of many similar inflammatory cytokines. The role of these receptors has recently expanded to include the recognition of more general “danger” associated molecular patterns (DAMPs). TLRs, including both TLR4 and TLR2, can identify and respond to DAMPs that are produced and released by nearby cells undergoing cell death or distress. To date, proteins that have been identified as DAMPs include hyaluronic acid, Heat Shock Proteins, and high mobility group box (HMGB) 1. CD14 = cluster of differentiation 14, TRIF = Toll-like receptor adaptor molecule, TIRAP = Toll-interleukin 1 receptor (TIR) domain containing adaptor protein, MyD88 = Myeloid differentiation primary response gene (88), IRAK = interleukin-1 receptor-associated kinase, TRAF = TNF receptor-associated factor, JNK = c-Jun N-terminal kinase, TKB1 = a tyrosine kinase, AP1 = activator protein 1 transcription factor, NFκB = nuclear factor kappa-light-chain-enhancer of activated B cells transcription factor, IRF3 = interferon regulatory factor 3 transcription factor. Adapted from (Schwarz and Bilbo, 2011).

Figure 3

(A) Experimental timeline, treatments, and outcomes assessed. (B) USVs were collected during a 1 min separation from dam and littermates. (C) P15 pups were placed into the center of a novel arena with equal portions of home cage vs. unfamiliar nest material, and total activity, latency to enter and time spent in each portion were assessed. (D) Cognitive and anxiety testing in adulthood, see text for details. (E) Inflammatory gene expression and (F) cytokine protein in whole brain at P30; *p<0.05. VEH = vehicle control, DEP = diesel exhaust particles, MS = maternal stress.

Figure 4

The process of cleaving pro-IL-1β into the mature, active protein often requires the formation of an inflammasome, an intracellular multiprotein complex that mediates processing and maturation of IL-1β via activation of caspase-1. This signaling cascade requires both a priming step (SIGNAL 1, acting on the TLR), and an activation step (SIGNAL 2, acting on, e.g. a purinergic receptor such as P2X₇ or a variety of other scavenger receptors, see Hornung and Latz, 2010). SIGNAL 1 candidates include both exogenous ligands/PAMPs such as LPS, and endogenous “danger” signals/DAMPs that are released in response to toxins such as DEP (or cellular damage induced by DEP). Ligation of the TLR by SIGNAL 1 initiates the intracellular signaling cascade to activate the transcription factor NFkB and the subsequent production of pro-IL-1β, as well as Nod-Like-Receptor family Pyrin domain containing 3 (NLRP3). NLRP3 then translocates to the cytosol to form a complex with the proteins ASC and pro-caspase1, a critical step in the formation of the inflammasome. SIGNAL 2 candidates include ATP, reactive oxygen species (ROS), heat shock proteins (HSP), HMGB1, and many others, released in response to cellular distress or tissue damage. This second signal activates the inflammasome complex for cleavage of pro-IL-1β into IL-1β and release.