The NLRP3 inflammasome in progressive multiple sclerosis

Abstract

This scientific commentary refers to ‘NLRP3 inflammasome as prognostic factor and therapeutic target in primary progressive multiple sclerosis patients’, by Malhotra et al. (doi:10.1093/brain/awaa084).

This scientific commentary refers to ‘NLRP3 inflammasome as prognostic factor and therapeutic target in primary progressive multiple sclerosis patients’, by Malhotra et al. (doi:10.1093/brain/awaa084).

Multiple sclerosis is a chronic inflammatory disease of the CNS (Baecher-Allan et al., 2018). In most patients, it initially manifests as a relapsing remitting disease (RRMS) featuring discrete, self-limited episodes of neurological dysfunction. However, in about 10–15% of patients, multiple sclerosis manifests from the onset as a progressive and irreversible accumulation of neurological disability (primary progressive multiple sclerosis, PPMS). Compared with RRMS, typical inflammatory lesions are less evident in PPMS, but microglial and astrocyte activation, as well as diffuse axonal damage are detected in the normal-appearing white matter (Miller and Leary, 2007; Baecher-Allan et al., 2018; Faissner et al., 2019). Moreover, diffuse spinal cord lesions and atrophy are prominent in PPMS, consistent with the usual clinical presentation of spastic paraplegia. Most therapies approved for RRMS show little beneficial effect in PPMS, with the exception of ocrelizumab, a humanized monoclonal antibody targeting CD20 on B cells (Montalban et al., 2017). Taken together, these observations suggest that the mechanisms of pathogenesis driving PPMS differ from those in RRMS (Baecher-Allan et al., 2018; Faissner et al., 2019). In this issue of Brain, Malhotra and co-workers identify a potential role for the NLR family pyrin domain-containing 3 (NLRP3) inflammasome in the pathogenesis of PPMS.

Inflammasomes are multiprotein oligomers assembled and activated in response to microbial, inflammation or stress-related stimuli, which catalyse the cleavage and maturation of the pro-inflammatory proteins interleukin-1β (IL-1β) and IL-18, and gasdermin D (Swanson et al., 2019). Dysregulation of the NLPR3 inflammasome has been linked to multiple inflammatory disorders, and is thought to promote CNS inflammation in multiple sclerosis (Beynon et al., 2012; Mascanfroni et al., 2013; Guo et al., 2015; Barclay and Shinohara, 2017).

Malhotra and co-workers, led by Manuel Comabella, first analysed the transcriptional response of peripheral blood mononuclear cells (PBMCs) in patients with multiple sclerosis and controls, and identified an IL1B gene signature that was specifically upregulated in PPMS samples, but not in those from healthy controls or from patients with RRMS, secondary progressive multiple sclerosis (SPMS) or benign multiple sclerosis. Furthermore, the authors found that IL-1β was highly expressed in monocytes in the peripheral blood and CSF of patients with PPMS. In agreement with the known role of the NLPR3 inflammasome in IL-1β maturation (Swanson et al., 2019), the authors detected an increased expression of NLRP3, caspase-1 and the adaptor protein apoptosis-associated speck (ASC) in PPMSmonocytes, concomitant with increased caspase-1 activation and ASC formation. Moreover, Malhotra et al. also detected NLRP3 and IL-1β co-expressed in myeloid cells (monocytes and microglia) in active demyelinated CNS lesions of patients with PPMS. Collectively, these findings support the specific activation of the NLRP3 inflammasome in PPMS monocytes.

The NLRP3 inflammasome is activated by a wide array of stimuli sensed by NLRP3, which include mitochondrial reactive oxygen species, ATP, microbial components and environmental irritants (Guo et al., 2015; Swanson et al., 2019). When activated with ATP and lipopolysaccharide, PPMS monocytes produced higher levels of IL-1β than monocytes from healthy controls or patients with other forms of multiple sclerosis, consistent with an increased NLPR3 inflammasome activation in PPMS.

Blood biomarkers are urgently needed to predict disease course, guide therapeutic choices and monitor response to therapy in patients with multiple sclerosis. Moreover, there is an unmet clinical need for efficacious therapies for progressive multiple sclerosis. Malhotra et al. detected an association between increased IL1B gene expression and poorer prognosis in PPMS, identifying IL-1β as a candidate biomarker for PPMS. In addition, these findings suggest that NLRP3/IL-1β-targeting may provide an efficacious therapeutic approach for PPMS, particularly for patients characterized by a strong IL1B-transcriptional signature. To test this idea, the authors blocked NLRP3 and IL-1β signalling in mouse experimental autoimmune encephalomyelitis (EAE). IL1 receptor signalling blockade with anakinra did not ameliorate EAE, but administration of the NLRP3 inflammasome inhibitor MCC950 did reduce the severity of the disease course. Moreover, NLRP3 inflammasome inhibition with MCC950 also reduced lipopolysaccharide-induced axonal damage in an organotypic cerebellar slice culture system. Overall, these findings identify candidate biomarkers for multiple sclerosis, and suggest that therapeutic inhibition of the NLRP3 inflammasome may benefit certain patients with PPMS.

Figure 1.

NLRP3 inflammasome activation in primary progressive multiple sclerosis (PPMS). Overview of NLRP3 inflammasome activation in monocytes and microglia during PPMS. Inflammasome activation is a two-step process: priming and activation (assembly). NLRP3 inflammasome priming is triggered by pro-inflammatory cytokines and microbial molecules that activate NF-κB, which then drives the transcription of NLRP3 inflammasome components and inactive pro-IL-1β. NLRP3 inflammasome activation is triggered by a wide variety of stimuli such as mitochondrial reactive oxygen species (ROS), ATP and microbial molecules. The activation signal induces oligomerization of NLRP3, adaptor protein apoptosis-associated speck (ASC), and pro-caspase-1 to form the NLRP3 inflammasome complex. Subsequently, pro-caspase-1 is activated to caspase-1, which in turn cleaves pro-IL-1β to active IL-1β which is then secreted from the cell; IL-1β promotes neurotoxicity and inflammation. The activation signal rather than the priming signal is suggested to be PPMS specific, but the activating factor involved is unknown. MCC950 blocks NLRP3 inflammasome activation and could potentially be used to treat PPMS.

This study raises important questions likely to guide future research on the pathogenesis of progressive multiple sclerosis. First, which signal triggers the specific activation of the NLRP3 inflammasome in PPMS, but not in other forms of multiple sclerosis? NLRP3 inflammasome activation occurs via two steps: pro-inflammatory signals that activate the NF-κB transcription factor promote the expression of NLRP3 inflammasome components and pro-IL-1β, but a second signal is required for the activation of the NLRP3 inflammasome protein complex (Swanson et al., 2019). However, Malhotra et al. detected comparable NF-κB activation in monocytes isolated from controls and all of the multiple sclerosis subtypes analysed. This finding suggests that PPMS-specific factors activate the NLRP3 inflammasome. However, the genotyping of patients with PPMS did not detect known single nucleotide polymorphisms (SNPs) associated with an overactive NLRP3 inflammasome (Verma et al., 2012). It is still possible that unknown SNPs in NLRP3 or other genes drive the preferential activation of the NLRP3 inflammasome in PPMS. Alternatively, unidentified viruses, pathogens or environmental factors may drive NLRP3 inflammasome preferential activation in PPMS.

IL-1β promotes inflammation-driven neurotoxicity via excessive glutamate signalling (Mandolesi et al., 2013). Thus IL-1β production in the CNS may contribute to the diffuse neuronal damage detected in PPMS. In addition, IL-1β can also act on astrocytes, triggering pro-inflammatory and neurotoxic responses while interfering with the metabolic support of neurons to further amplify neurodegeneration (Chao et al., 2019; Wheeler et al., 2020). Surprisingly, IL1 receptor blockade did not ameliorate EAE, suggesting that NLRP3 activation contributes to disease pathogenesis through IL-1β-independent mechanisms and/or that anakinra levels in the CNS may not be high enough to block most IL-1β-driven pathogenesis. Future studies should determine the mechanisms through which NLRP3 inflammasome activation in peripheral and CNS-resident cells contributes to PPMS pathogenesis.

Finally, although the study by Malhotra et al. suggests a role for inflammation in PPMS pathogenesis, other mechanisms are also thought to contribute to PPMS pathology (Faissner et al., 2019). Future studies should investigate the heterogeneity of pathogenic mechanisms in PPMS, and in particular the relative contribution of inflammatory and non-inflammatory processes. In combination with biomarkers such as those described by Malhotra and co-workers, these studies may guide the development of efficacious personalized therapies for patients with PPMS.

Funding

Work in the Quintana laboratory is supported by grants NS102807, ES02530, AI126880, and ES029136 from the National Institutes of Health, USA; RG4111A1 from the National Multiple Sclerosis Society and the International Progressive MS Alliance.

Competing interests

The authors report no competing interests.