Remyelination is a process which may be distinguished into repair of damaged myelin or de novo synthesis. It is thought to be operated in the CNS either by activation of terminally differentiated oligodendrocytes or by recruitment and migration of staminal precursors known as oligodendrocyte progenitor cells (OPCs) [53,54]. OPCs are also thought to be critical for tuning inflammation and angiogenesis; furthermore, they possess complex electrophysiological properties and are thought to form synapses with neurons [55,56].

It is well accepted that remyelination constitutes a continuous and ubiquitous process occurring within the CNS, although it is unclear whether it might be sufficient in restoring myelin function in lesioned areas. Some lesions, in fact, undergo an incomplete repair, characterized by formation of thin sheaths surrounding axons, especially at the lesion border [57]. These areas are defined as “shadow plaques” and are thought to be areas where the remyelination process has come to a halt [58]. It is not well known whether they are the result of single or repeated demyelinating processes, whether they are more prone to subsequent remyelination or whether they might harbor a quiescent recovery potential, but intriguingly, these areas are devoid of OPC elements and, therefore, myelin restoration is thought to be operated only by mature oligodendrocytes [59].

Remyelination declines with aging; it is regulated by synaptic activity but is also highly influenced by the secretory and signaling activity of astroglial elements, as well as by iron transport and phagocytic activity of macrophages and microglial cells [60,61]. Chronic inflammation might impair remyelination dynamics, yielding incomplete repair of damaged sheaths. In accordance with this hypothesis, it has been reported that in addition to shadow plaques, slowly expanding lesions, characteristic of progressive disease, possess a lower remyelination potential [62].

It is currently under debate whether in humans de novo myelination might be more effective in repairing injured structures in comparison to activation of differentiated oligodendrocytes. It has, however, been esteemed that only 0.3% of oligodendroglial elements are regenerated per year; therefore, activation of differentiated elements appears of crucial importance, as well as the mechanisms that might render this process more efficient [59]. Remyelination has also been shown to reverse the alteration in mitochondrial numbers observed in demyelinated axons, suggesting a potential in counteracting neuroaxonal loss [63]. On the whole, remyelination appears as a fundamental process in MS with the potential to preserve functioning of sensory and motor systems and, therefore, delaying and limiting disability.

Several biochemical cascades, involving lipid metabolism, cholesterol efflux, retinoid-X-receptor α dependent pathways, phagocytosis, but also epigenetic regulation through histone deacetylases, have been implied as potential mechanistic targets [64,65,66,67]. Among these, leucine-rich repeat and Ig domain-containing 1 (LINGO-1), a glycoprotein expressed by neurons, and OPCs, whose blockade has been shown to improve myelination in animal models of the disease, has been proposed as a promising target for remyelination [68]. Opicinumab, a monoclonal antibody targeting LINGO-1, has failed in trials to reach its efficacy endpoints, despite showing at the highest doses and in younger patients a small improvement in disability worthy of further research [69].

Pathogenesis has been mostly studied through animal models either involving immunization against CNS antigens, infection with neurotropic viruses or administration of neurotoxic/myelinotoxic compounds, such as lysolecithin or cuprizone [70].

The most commonly adopted models derive from parenteral administration of myelin-derived peptides, such as myelin basic protein (MBP), myelin oligodendrocytic glycoprotein (MOG) and proteolipid protein (PLP) complexed with adjuvants, which results in an inflammatory demyelinating disease of the CNS, termed experimental allergic encephalomyelitis (EAE). These models have allowed researchers to characterize in detail some pathogenetic aspects useful to extrapolate data for therapy development, although they do not allow reproduction of every aspect of the human disease, especially concerning its multiphasic clinical course [71]. Other animal models involve infection with neurotropic viruses, such as Theiler’s encephalomyelitis virus (TMEV) [72].

Evidence from genomic studies suggests a critical role of loci involved in antigen presentation, such as HLA DRB1*15:0, in conferring susceptibility to the disease, while other HLA haplotypes, such as the A*02 and B*44, have been associated with a protective effect [5].

HLA genes can be distinguished into three classes: class I and class II HLA encode for major histocompatibility complex (MHC) proteins, which are crucial for antigen presentation, while class III HLA loci encode for molecules involved in the inflammatory cascade, such as complement proteins, tumor necrosis factors (TNFs), 21-hydroxylase and heat shock proteins [73,74].

MHC class I molecules (encoded by HLA-A, HLA-B and HLA-C loci), present intracellular self- or non-self-antigens to CD8⁺ cytotoxic T cell receptors and killer cell immunoglobulin-like receptors (KIR) [75]. On the other hand, class II molecules (encoded by HLA-DP, HLA-DQ and HLA-DR) are expressed on the membrane of antigen-presenting cells (such as macrophages, B cells and dendritic cells) and serve the function of displaying short antigen peptides to CD4⁺ helper T cells [76].

In addition to HLA loci, more than 200 non-MHC-coding genomic variants have been reported to confer susceptibility to MS, albeit with different effect sizes. In fact, many of these variants affect genes involved in immune system pathways, such as interleukin 2 receptor subunit α (IL-2RA), but also intronic and intragenic sequences related to splicing and quantitative gene expression [77]. Daclizumab, which inhibits IL2RA, has shown high clinical efficacy in preventing MS relapses, although it has been withdrawn for hyperacute hepatotoxicity [78].

As for effector mechanisms, in accordance with autoptic data, pathogenesis shows great similarities to T-cell-mediated diseases; therefore, a central role has been theorized for CD4⁺ and CD8⁺ lymphocytes [79,80]. The former, when primed towards their proinflammatory T_h1 and T_h17 phenotypes, are thought to be important directors of the immune response towards the CNS [38], while the latter, primed to their cytotoxic phenotypes, are the predominant cell type, under a quantitative perspective, surrounding demyelinated axons [33,81,82].

Many currently approved therapies for MS modulate various aspects of T cell function, including response to activating stimuli, functional polarization, egress from lymph nodes, migration and CNS entrance [83]. Glatiramer acetate is thought to modulate T helper cell polarization toward a Th₂ phenotype, dampening CNS-directed inflammation [84].

In addition, recent studies on distinct immune system cell subtypes in MS highlight the role of several regulatory subpopulations of the innate and adaptive immune system in balancing disease severity, such as forkhead box protein 3 (FOXP3) positive CD4⁺ cells, Tr1-positive CD4 cells [5], CD56^bright NK cells [85]. Subsets of anti-inflammatory CD8⁺ cells have been described, but to date a single surface antigen combination conferring this functional phenotype has not been defined [85,86,87].

In recent decades, evidence from animal and clinical studies supported an important role for B cells, given their role in tuning T cell function, antigen presentation, autoantibody production and also in leptomeningeal lymphoid follicle formation [40]. A similar role is shared with dendritic cells, which orchestrate T cell activation through similar processes [88]. B cells comprise a heterogeneous host of naïve, memory and effector subpopulations also including tolerogenic and anti-inflammatory subsets, collectively termed as B_regs, characterized by production of IL-10, IL-35 and TGF-β [89].

In clinical studies, B cell-directed anti-CD20 antibodies (especially ocrelizumab) have shown significant benefits and have been approved in both RRMS and PPMS, where they possess lesser efficacy [90]. Their long-lived therapeutic effects might derive from their ability to blunt proliferation of proinflammatory clones such as mature naïve B cells and memory B cells with a parallel stimulation of regulatory populations, such as IL-10-producing B cells, including autoreactive regulatory clones [5,91]. Another very important target for B cell physiology, closely related to MS, is constituted by Bruton tyrosine kinase (BTK), a master regulator of B-cell activation, whose expression is not exclusive to B cells since it has also been detected in myeloid T cells and osteoclasts. It is considered as a “rheostat” of proinflammatory signaling and a regulator of autoreactive cells [92]. Besides tuning B cell receptor (BCR) activation, BTK takes part in signaling of toll-like and Fc receptors, modulating the inflammatory response; therefore, its excessive activity has been related to autoimmunity [93,94]. In addition to its effects on immune activation and inflammation cascades, a recent in vivo study on cultured cerebellar slices interestingly shows that BTK activity is upregulated after lysophosphatidylcholine and metronidazole-induced demyelination, while its inhibition might hasten myelin repair, suggesting complex effects on the CNS [93].

The observed effectiveness, in both animal models and human subjects, of BTK inhibitors in several autoimmune diseases further strengthens the hypothesis that regulation of this cascade might be of therapeutic value [95]. Currently, two brain-penetrant BTK inhibitors, evobrutinib and tolebrutnib, are being assessed with regard to their effectiveness in preventing MS relapses and in reducing disease activity [94].

Several studies have observed a shift in energy metabolism affecting lymphocytes, macrophages and dendritic cells towards aerobic glycolysis (the so-called Warburg effect) in association with dysfunctional oxidative phosphorylation in T lymphocytes [96,97]. It is not known whether this process, termed “metabolic reprogramming”, might be the cause or a consequence of aberrant immune activation.

Given the predominance of immune-mediated mechanisms in animal models and the clinical responses associated with immune modulators, several efforts have been made to elucidate the relationship between specific cytokines and disease phenotypes, but also between effector mechanisms of the innate immune system, such as the kinin or complement cascade, and MS pathogenesis. Among cytokines, Th₁ and Th₁₇ cytokines are considered pivotal proinflammatory signals, while IL-10, IL-27, IL-35 and especially type I interferons have been associated with disease amelioration [5,98].

TNF-α blockade has been shown to trigger significant disease exacerbation [99,100], while immunomodulation via interferon β (IFN-β) has shown significant clinical efficacy in preventing disease relapses [101]. IFN-β is thought to target antigen presentation processes, modulate cytokine secretion, T cell polarization and MHC molecule expression, although recent studies have also suggested that the activity of the cGAS-STING pathway, a critical regulator of endogenous type I IFN production, might constitute an important determinant of the effectiveness of interferon therapy [98,102,103].

The cGAS-STING pathway, in brief, is considered an intracellular “damage-sensing” cascade involved in innate immunity that is primarily activated by binding of cGAS (cyclic GMP-AMP synthase) to exogenous/endogenous double-stranded DNA fragments outside of the cell nucleus [104]. It has been suggested that its activity might be altered during infections but also in several brain inflammatory disorders [98]. cGAS activation produces 2′5’-cyclic adenosine monophosphate guanosine monophosphate (2′5′-cGAMP), which activates tank-binding kinase 1 (TBK1) and IκB kinase (IKK), inducing STING (stimulator of interferon genes) oligomerization [105,106,107]. STING activation leads to phosphorylation and activation of interferon regulatory factor 3 (IRF3) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), which upregulate the type I interferon response, upregulating expression of interferon regulated genes, in turn regulating the synthesis of TNF-α, IL-1β and IL-6, but also of the STING protein [106]. It has been recently observed that RRMS patients might exhibit in peripheral blood mononuclear cells a downregulated activity of the cGAS-STING/IFN-β-axis, while also displaying a reduced expression of interferon regulated genes [103]. It has been therefore suggested that interferon therapy might be mostly effective in patients with a downregulated endogenous response, perhaps in addition to pharmacological modulation of STING activity [105].

Among other soluble signals of innate immunity, studies in EAE models have highlighted a role for bradykinin (BK) in modulating cytokine secretion and CNS lesion development [108]. BK type 1 receptor (B1) activation is thought to mediate BBB breakdown and increased vascular permeability, favoring inflammation [109]. In EAE models, enalapril administration has been shown to increase plasma BK concentration and reduce clinical and pathological severity, while B1 receptor blockade counteracted the protective effects of enalapril [110]. In human studies, increased B1 receptor expression has been detected in T lymphocytes isolated from peripheral blood of MS patients with respect to control subjects, suggesting a potential role in CNS inflammation [111]. As for the complement system, autoptic studies have shown involvement in myelin phagocytosis within acute lesions, but also persistent deposition in chronic lesions in PPMS as well as in gray matter lesions [112]. The complement system might play a complex role in disease pathogenesis since its effects are not limited to debris clearance, but also to processes related to survival cascades. Evidence from cell models has shown that astrocytes secrete complement proteins when stimulated by proinflammatory stimuli as TNF α, IL-1β and IL-8 [113], while sublytic levels of C5b-C9 proteins might drive antiapoptotic responses in oligodendroglial elements [114]. The complement system is also thought to play an important role in removal and maintenance of synaptic structures [115,116]. Evidence from plasma and CSF biomarker studies shows a trend towards increased concentration of complement components, such as C1q, C3 and C4 in RRMS, SPMS and PPMS, as well as an increase in endogenous inhibitors, such as factor H, suggesting heightened complement activity in all forms of the disease [117,118].

These observations might therefore constitute a rationale for assessing complement regulation as a therapeutic target; in a small series of patients, eculizumab, a C5 inhibitor, has recently shown a discrete tolerability in a small series of MS patients, with no severe adverse drug reactions nor disease relapses, supporting further clinical assessment [119].

Data from experimental models suggest that no single antigen or effector cell type might be sufficient to summarize every pathogenetic aspect of MS, whose immunopathogenesis is multifactorial and underlies an interaction, in the periphery and in the CNS, between proinflammatory stimuli, specific subtypes of immune cells and host-specific factors. Among host-specific factors, polygenic susceptibility, hormonal influences, epigenomic regulation, gut microbiome signals and environmental factors (including pollutants and smoking) might shape disease activity [120].

It has been proposed that the gut microbiome may alter the MS immunopathological framework at least by a dual mechanism. In fact, gut dysbiosis, i.e., imbalance between tolerogenic and proinflammatory commensals, might promote inflammation in remote sites, while molecular mimicry between gut antigens and CNS epitopes might select autoreactive cell clones. Gut microbes produce metabolites that also directly target the CNS. It was observed that dietary tryptophan may be metabolized through the serotonin, indole and kynurenine pathways into components that act as aryl hydrocarbon receptor agonists [121] and exert anti-inflammatory actions mediated by astrocytes [122]. The effectiveness, in preclinical models, of the immunomodulatory drug laquinimod, which however failed to reach significant clinical endpoints [123], is thought to derive from its effect on glial aryl hydrocarbon receptors [124]. In addition, dimethylfumarate, a drug approved to treat RRMS, has been found to reduce bacterial production of neurotoxic phenol and indole catabolites of phenylalanine and tryptophan [125].

Under this perspective, the efficacy of the so-called “immune reconstituting” therapies, such as cladribine, alemtuzumab and bone marrow transplantation, might depend not only on quenching of acute inflammation, but also on reconfiguring the immune repertoire to a point that allows previously suppressed cells to emerge and affect immunologic processing in distant sites [126,127]. Ocrelizumab, despite requiring maintenance therapy, could be considered, due to its long-lived effects, as a drug with a profile of action closely comparable to immune reconstituting therapies [128].

On the whole, acute inflammation, which is preponderant in RRMS, is considered highly dependent on the entrance in the CNS of pathogenic autoreactive cells from the periphery and might lead over time to formation of persistent meningeal tertiary lymphoid structures [129]. In accordance with this hypothesis, it has been observed that therapies targeting infiltration of autoreactive cells in the CNS, such as natalizumab, an anti-very late antigen 4 (VLA4) monoclonal antibody, are far more effective in RRMS [130,131], while they might induce devastating disease rebounds after a prolonged discontinuation [132].

In addition, another recently discovered layer of regulation of inflammatory activity is represented by endogenous transposable elements, such as human endogenous retroviruses, whose activation has been linked to both disease relapses and progression [21,23]. Recently, temelimab, a monoclonal antibody directed against the HERV-W envelope protein, has shown discrete tolerability in small MS cohorts and promising effects on radiological markers [133].

Despite the abundance of data highlighting the predominance of mechanisms pertaining to the innate and adaptive immune system in sustaining acute attacks, several signaling pathways pertaining to astrocytic, oligodendrocytic and microglial elements appear of primary importance in poising disease activity and determining neuron survival from the earliest phases [134].