Immunity in amyotrophic lateral sclerosis: blurred lines between excessive inflammation and inefficient immune responses

Abstract

Despite wide genetic, environmental and clinical heterogeneity in amyotrophic lateral sclerosis, a rapidly fatal neurodegenerative disease targeting motoneurons, neuroinflammation is a common finding. It is marked by local glial activation, T cell infiltration and systemic immune system activation. The immune system has a prominent role in the pathogenesis of various chronic diseases, hence some of them, including some types of cancer, are successfully targeted by immunotherapeutic approaches. However, various anti-inflammatory or immunosuppressive therapies in amyotrophic lateral sclerosis have failed. This prompted increased scrutiny over the immune-mediated processes underlying amyotrophic lateral sclerosis. Perhaps the biggest conundrum is that amyotrophic lateral sclerosis pathogenesis exhibits features of three otherwise distinct immune dysfunctions-excessive inflammation, autoimmunity and inefficient immune responses. Epidemiological and genome-wide association studies show only minimal overlap between amyotrophic lateral sclerosis and autoimmune diseases, so excessive inflammation is usually thought to be secondary to protein aggregation, mitochondrial damage or other stresses. In contrast, several recently characterized amyotrophic lateral sclerosis-linked mutations, including those in TBK1, OPTN, CYLD and C9orf72, could lead to inefficient immune responses and/or damage pile-up, suggesting that an innate immunodeficiency may also be a trigger and/or modifier of this disease. In such cases, non-selective immunosuppression would further restrict neuroprotective immune responses. Here we discuss multiple layers of immune-mediated neuroprotection and neurotoxicity in amyotrophic lateral sclerosis. Particular focus is placed on individual patient mutations that directly or indirectly affect the immune system, and the mechanisms by which these mutations influence disease progression. The topic of immunity in amyotrophic lateral sclerosis is timely and relevant, because it is one of the few common and potentially malleable denominators in this heterogenous disease. Importantly, amyotrophic lateral sclerosis progression has recently been intricately linked to patient T cell and monocyte profiles, as well as polymorphisms in cytokine and chemokine receptors. For this reason, precise patient stratification based on immunophenotyping will be crucial for efficient therapies.

Keywords: amyotrophic lateral sclerosis; immunodeficiency; neuroimmunity neurodegeneration; neuroinflammation.

Graphical Abstract

Figure 1

All roads lead to inflammation: genetic evidence. Mutations in a small subset of genes (OPTN, TBK1, CYLD and C9orf72) are directly linked with dysfunctional inflammatory responses. Most genes mutated in ALS disrupt proteostasis by increased protein aggregation, decreased proteasomal degradation or impaired autophagy. Aggregated proteins can in turn trigger inflammation. Inflammation can also be triggered by mutations in VAPB that causes endoplasmic reticulum stress or mutations in SOD1 and SQSTM1 that cause mitochondrial damage and oxidative stress. The link between inflammation and other proposed ALS-causing mechanisms is bidirectional: enhanced inflammation amplifies other pathogenic mechanism. Remark: genes marked in red are proposed act by more than one pathogenic mechanism.

Figure 2

Acute neuroinflammation.  (A) Microglia maintain homeostasis in the CNS by secreting neurotrophic factors (BDNF, GDNF, IGF-1). Microglial activation is suppressed by healthy neurons that provide negative co-stimulation via CD200 and CX3CL1, and astrocytes that secrete TGF-β. (B) Various PAMPs/DAMPs from dead/dying neurons and/or pathogens switch microglia from resting to proinflammatory state by binding to various pathogen and scavenger receptors. Such microglia clear cellular debris, produce proinflammatory factors (TNF, IL-1β, IL-6, ROS/RNS) and upregulate various receptors. (C) Upon damage resolution, the remaining neurons restore negative CD200 and CX3CL1 co-stimulation so microglia shift to an anti-inflammatory phenotype. Such microglia secrete anti-inflammatory cytokines IL-10 and IL-4, growth factors, and upregulate arginase 1, YM-1 and CD163, which leads to the resolution of inflammation and restoration of the resting state.

Figure 3

Chronic neuroinflammation.  (A) The initial prolonged or repetitive damage in presymptomatic microglia elicits an anti-inflammatory phenotype. Neuronal negative co-stimulation with CD200 molecule and CX3CL1 is still active, and microglia show blunted proinflammatory response and neuroprotective properties: decreased TLR activity, increased production of anti-inflammatory cytokine IL-10, neurotrophic factors (BDNF, IGF-1) and scavenger receptors YM-1 and CD163. (B) Upon long-term chronic stimulation and/or repetitive hits microglia switch to highly proinflammatory state. Neurons in the symptomatic phase lose their ability to restrain microglial activation by negative co-stimulation, and various DAMPs (protein aggregates, debris, etc.) bind to their respective receptors (TLRs, TREM2, etc.) to activate downstream proinflammatory cascade, which results in profound changes in microglial transcriptional profiles (homeostatic genes are downregulated; proinflammatory genes are upregulated). Finally, proinflammatory microglia induce collateral neuronal damage, creating a vicious cycle that further amplifies inflammation.

Figure 4

The timeline of ALS progression.  (A) The progressive loss of neuroprotective immune responses over the course of ALS is depicted. (B) The potential prolonged and/or repetitive hits that affect ALS pathogenesis are shown. Inherited or sporadic mutations in ALS-associated genes could serve as primary hits that negatively affect to the initial anti-inflammatory phase, whereas secondary hit/s would set off the rapidly progressive stage marked with uncontrolled inflammation. (C) The disease progression is shaped by the immune factors. Various polymorphisms or other factors that result in lower-binding capacity of CX3CL1, higher levels of IL-6 and IL-6R, or reduced Treg numbers and function, are linked to rapid disease progression.

Figure 5

Hyperinflammation and immunodeficiency as primary triggers of motoneuron death in ALS.  (A) Microglial skewing towards uncontrolled inflammation could be a repercussion of inadequate shut down of NF-κB signalling exerted by constitutive activation of IKKβ (*clinical correlate not yet reported), or mutations in proposed negative regulators OPTN and TNIP1. Overactivated NF-κB in reactive astrocytes elicits similar effects. Such excessive proinflammatory factor production from chronically activated glia ultimately causes motor neuron death. (B) The inability of microglia to optimally respond to damage is a potential trigger for neurodegeneration in ALS. ALS-linked loss-of-function mutations in OPTN and TBK1 lead to decreased production of IFN-β, dysregulated inflammatory responses, reduced phagocytic capacity of microglia and decreased neuronal autophagy. ALS-linked mutant CYLD^M719V was proposed to impair autophagy flux and reduce NF-kB activation. Such inadequate response to damage finally leads to accumulation of various DAMPs (protein aggregates, ATP, HMGB1, etc.), thus sparking neurotoxic chronic inflammation. Loss of immunosurveillance by the adaptive immunity, in particular T cells, could contribute to dysregulated responses and neurotoxicity.