Current and Future Immunotherapies for Multiple Sclerosis

Abstract

Despite substantial progress in developing new immunotherapies against multiple sclerosis (MS), currently available immunotherapies are only partially effective for this debilitating neurological disease, thus necessitating new therapeutic approaches. Here, we review the immunotherapies already approved for MS as well as relevant clinical trials. Further, we present some experimental approaches that are currently being developed and are focused on modulating the functions of dendritic cells and regulatory T cells.

Introduction

Multiple sclerosis (MS) is a complex autoimmune disease characterized by inflammation of the central nervous system (CNS). It is caused by immune cells that attack components of the myelin sheath surrounding the neuronal axons of the nerves, resulting in demyelination and neurologic dysfunction. Although some patients with MS suffer from the primary-progressive form characterized by its steady progression, most cases of MS are characterized by a relapsing-remitting disease course. However, many MS patients eventually develop secondary-progressive MS, resulting in a steady progression of symptoms.^1,2 While the causes of MS remain elusive, both genetic and environmental factors contribute. It is clear, however, that the key processes leading to MS depend on an aberrant immune response directed against specific antigens derived from components of the CNS.^3–6 This autoimmune process is initiated when specialized antigen presenting cells (APCs) such as conventional dendritic cells (cDCs) activate autoreactive CD4⁺ T lymphocytes (cells). Such neural antigen-specific (encephalitogenic) CD4⁺ T cells can enter the CNS and become additionally re-activated and also recruit other effector cells, such as CD8⁺ T cells, establishing an inflammatory lesion. In addition, CD4⁺ T cells orchestrate functions of autoreactive B cells that, along with natural killer (NK) cells and macrophages, also contribute to MS immunopathology.^1,2 Given the multi-faceted autoimmune nature of MS, multiple FDA-approved disease-modifying therapies (DMTs) that reduce relapses and slow disease progression of the relapsing-remitting and secondary-progressive MS have been developed to modulate the inflammatory immune response that drives MS pathogenesis. Further, intense research involving experimental animal models of MS may result in improved future therapy options for patients. In this article, we discuss the immunological mechanisms of current FDA-approved DMTs and some ongoing clinical trials, as well as provide an overview of the relevant preclinical research focusing on work by our own group affiliated with the Saint Louis University Center for Neuroscience.

FDA-Approved Disease-Modifying Therapies

The available treatments for MS aim to achieve at least one of the two main goals: to restore functions that were lost as a result of specific disease processes and to prevent further tissue damage by blocking the underlying aberrant activity of the immune system. Immunotherapies are crucial for any treatment options because long-term success of MS therapy ultimately depends on limiting the underlying autoimmune process. Beta-inteferon (currently available in three forms: IFN-β-1b, IFN-β-1a, and pegylated-IFN-β-1b) was approved by the FDA in 1993 as the first DMT for MS and remains a first-line treatment.⁷ Although not completely understood in the context of MS, IFN-β exerts broad anti-inflammatory and immunomodulatory effects. By altering several aspects of dendritic cell (DC) biology, a type of APC crucial for antigen presentation and expression of costimulatory molecules and cytokines, IFN-β treatment limits effector CD4⁺ and CD8⁺ T cell responses. It may also enhance development of regulatory T cells (Tregs) that are key to restraining effector T cell functions.^7–9 Further, IFN-β dampens B cell responses by maintaining a population of regulatory B cells and inducing apoptosis of pathogenic memory B cells.^10,11

Another first-line therapy is glatiramer acetate (GA), a mixture of synthetic polypeptides consisting of four amino acids found in myelin basic protein: glutamic acid, lysine, alanine and tyrosine.⁸ Like IFN-β, GA exerts broad immunomodulatory effects that are incompletely understood. GA binds indiscriminately to major histocompatibility complex (MHC) class II molecules on DCs and other APCs, thus competing for binding with myelin-derived peptides.⁸ Although it is unclear how, GA treatment results in the skewing of autoreactive T cells away from the pathogenic effector T cell responses and towards regulatory functions.^12,13 Other broadly immunomodulatory DMTs include dimethyl fumarate (DMF) and its bioequivalent diroximel fumarate, which activate the antioxidant nuclear erythroid 2-related factor 2 (Nrf2) transcriptional pathway.^8,14 The effects of these fumarates include changes in the differentiation of various effector CD4⁺ T cells (specifically a shift from T helper Th1/Th17 to Th2 profile), an expansion of Tregs, and additional inhibition of B and NK cell responses.⁸

There are multiple DMTs that prevent lymphocyte migration, thus preventing effector T cells from crossing the blood-brain barrier. Four of these are sphingosine-1-phosphate (S1P) receptor modulators: fingolimod, siponimod, ozanimod, and ponesimod. S1P forms a gradient in the blood to promote lymphocyte egress from secondary lymphoid tissues through S1P receptor 1 (S1PR1) signaling.¹⁵ Fingolimod (FTY720) is structurally similar to naturally occurring S1P and can be phosphorylated, allowing it to bind to S1PRs. This results in receptor internalization and the eventual proteasomal degradation of S1PR1.^16,17 This inhibits lymphocytes from leaving lymph nodes and entering the central nervous system, thus preventing relapses. Fingolimod was approved as the first oral medication for MS by the FDA in 2010.⁸ Potential side effects of fingolimod include bradycardia due to S1PR3 signaling, so more specific S1P receptor modulators were developed, including siponimod (approved in 2019), ozanimod (approved in 2020), and ponesimod (approved in 2021).^8,18–20 S1P receptor modulators may also have additional therapeutic benefits by reducing neuropathic pain.²¹

Another DMT that prevents the entry of immune cells into the CNS is natalizumab, which was approved in 2006.⁸ Natalizumab is a monoclonal antibody that binds to α₄-integrin. In MS, this prevents interactions between vascular cell adhesion molecule-1 (VCAM-1) and α₄β₁-integrin (very late antigen-4). VCAM-1 is expressed by blood vessel endothelial cells at sites of inflammation and binds to α₄β₁-integrin-expressing leukocytes, allowing for the trans-migration of immune cells across the blood-brain barrier. Natalizumab disrupts this interaction, thus preventing the transmigration of leukocytes, including activated auto-reactive T cells, into the CNS.²²

Another family of DMTs works by depleting lymphocytes that are involved in MS pathology. Ocrelizumab, which was approved in 2017 as the first DMT for primary progressive MS, is a monoclonal antibody that binds to CD20 expressed on immature and mature B cells, thus leading to their depletion.⁸ Alemtuzumab is a monoclonal antibody specific for CD52, which is expressed at high levels on activated B and T cells. Alemtuzumab binding leads to antibody-dependent, cell-mediated cytolysis and complement-dependent cytolysis of the targeted cells.²³

Several DMTs work by suppressing the proliferation of activated T and B cells. Mitoxantrone, a synthetic anthracenedione derivate, was approved for the treatment of MS in 2000 and acts as a type II topoisomerase inhibitor.⁸ By disrupting DNA synthesis and DNA repair, mitoxantrone promotes the death of proliferating T and B cells.²⁴ Cladribine is a purine analogue that is taken up into rapidly proliferating cells, including activated lymphocytes, and incorporated into DNA during synthesis, resulting in DNA strand breakage and apoptosis.⁸ Teriflunomide inhibits de novo pyrimidine synthesis by blocking the enzyme dihydroorotate dehydrogenase, which also leads to the apoptosis of proliferating T and B cells.⁸

Clinical Trials of Interest

While there are several ongoing clinical trials investigating therapies similar to already approved DMTs, there are quite a few therapies in development with novel mechanisms of action. Like many of the approved DMTs, some of these therapies have broad immunomodulatory effects. A phase III clinical trial (NCT03896217) is currently investigating the use of simvastatin in secondary progressive MS (SPMS). Simvastatin is an HMG-CoA reductase inhibitor (statin) commonly used for hyperlipidemia, but statins also exert anti-inflammatory effects such as reducing MHC class II antigen presentation, altering DC maturation and reducing effector CD4⁺ T cell responses. Statins also prevent leukocyte migration across the blood-brain barrier, further making them a promising treatment for MS.²⁵ A phase II trial showed treatment with high-dose simvastatin reduced SPMS progression, thus advancing it to phase III testing.²⁶ Another ongoing phase III trial (NCT04291456) is investigating the use of minocycline which reduces the conversion from clinically isolated syndrome to multiple sclerosis.²⁷ Minocycline is a tetracycline antibiotic with broad immunomodulatory effects that include reducing microglial activation and preventing leukocytes from crossing the blood-brain barrier by inhibiting matrix metalloproteinases (MMPs).²⁸ The recently completed phase II trial of ibudilast, a small molecule phosphodiesterase inhibitor, in progressive MS slowed progression of brain atrophy.²⁹ Ibudilast inhibits macrophage migration inhibitor factor (MIF), reduces IL-1β, TNF-a and IL-6 production, and functions as a TLR4 antagonist. Although it is not completely understood, it is thought that these functions reduce neuroinflammation.²⁹

Other clinical trials are using more targeted antigen-specific approaches to reduce neuroinflammation. Two phase I trials (NCT02618902 and NCT02903537) are investigating autologous myelin peptide-pulsed monocyte-derived dendritic cells (moDCs) tolerized with 1α,25 dihydroxyvitamin D₃ (tolDC-VitD3) in multiple sclerosis patients.³⁰ This approach, which has also been used for rheumatoid arthritis, type 1 diabetes, and Crohn’s disease, aims to induce antigen-specific T cell tolerance that may modulate the MS disease course. A similar approach using autologous moDCs tolerized with dexamethasone for patients with neuromyelitis optica spectrum disorders and MS showed altered immune profiles following treatment, although better moDC-tolerizing conditions may need to be developed.³¹ A different approach to achieve antigen-specific tolerance is being used in the Phase II trial of the TCR peptide vaccine NeuroVax™ (NCT02057159). This trivalent vaccine consists of three peptides (BV5S2, BV6S5, and BV13S1) emulsified in incomplete Freund’s adjuvant and induces IL-10-secreting T regulatory 1 (Tr1) cells and FoxP3⁺ Tregs.³²

Experimental Approaches at the Pre-Clinical Stage

Despite substantial progress in developing new immunotherapies against MS, currently available immunotherapy protocols are only partially effective, and most of these approaches are burdened by side effects. This necessitates new approaches for immunomodulation of MS to specifically alter the functions of offending immune cells without impairing other immune responses such as those against pathogenic microorganisms and tumors.^33,34 There are two main considerations guiding these approaches. First, the neural-derived antigens are specifically recognized only by corresponding encephalitogenic T cells but no other T cells. Second, such specific antigens need to be delivered in a “tolerogenic” context that eventually leads to an elimination or functional inactivation of antigen-activated encephalitogenic T cells. Previous research from various groups showed the efficacy of specific neural antigens in an animal model of MS, experimental autoimmune encephalomyelitis (EAE).³⁵ The characteristic neural inflammation resulting from infiltration of nervous tissues by perivascular CD4⁺ T and other immune cells, eventually leading to clinical symptoms of ascending paralysis, makes EAE a relevant model for MS and a powerful model of immune regulation of Th1 and Th17-dependent autoimmunity.^35–37

Immunization with neural antigens such as myelin oligodendrocyte glycoprotein (MOG), myelin basic protein (MBP) or proteolipid protein (PLP) leads to different variants of EAE resembling the human progressive and relapsing–remitting forms of MS.³⁵ In contrast, the introduction of MOG, MBP and PLP in a non-immunogenic context blocks the corresponding forms of EAE.^35,38,39 However, administration of such antigens without control over which APCs will present them to T cells in vivo may not guarantee the ultimate therapeutic results. Alternatively, some early experimental approaches relied on transfers of autologous DC with tolerogenic functions induced in vitro, and similar approaches are now in clinical trials as discussed above.^33,40 These therapeutic outcomes can be improved by the specific delivery of antigens to APCs with specialized tolerogenic functions in vivo, thus allowing for better control of the ensuing immunological processes such as the de novo induction of antigen-specific peripheral regulatory T cells (pTregs).^34,40 In contrast to the earlier described and well-established roles of Tregs of a thymic origin (tTregs), the functions of pTregs, although crucial, still remain less characterized. In general, Tregs play a pivotal role in protection and recovery from EAE and MS by suppressing autoreactive T cells, and functional defects of Tregs have been implicated in MS pathogenesis.³⁵ However, specific pro-inflammatory autoimmune activation can overwhelm tTreg functions. Resembling the sudden onset of MS in patients, the robust triggering of MS-like EAE in healthy animals indicates that the functions of tTregs are unable to block encephalitogenic T cell activation. In contrast, neuroinflammation can be ameliorated or even completely prevented by antigen-specific pTregs that are formed extrathymically in the peripheral immune system during specific tolerogenic responses to relevant neuronal antigens. Research from our group established that such neural antigen-specific pTregs are induced by a specialized type of cDC and confer a long-lasting tolerance that prevents EAE.^41,42 Although cDCs were initially described as having key roles in priming of effector T cells during immune and autoimmune responses, some cDCs have natural tolerogenic properties that help regulate such effector T cells. These cDCs have been referred to as natural tolerogenic cDCs (ntDCs) to distinguish them from cDCs whose tolerogenic functions can be induced by specific treatments including those administered ex vivo as discussed above and as we reviewed previously.³³ The ntDCs with tolerogenic functions express B and T lymphocyte associated (BTLA) protein, and we defined the specific mechanisms engaged by BTLA that result in enhanced formation of pTregs with anti-autoimmune functions in EAE.^35,41–43 Therefore, further manipulation of these specific BTLA-mediated mechanisms could open new avenues for immunomodulation via manipulating specific formation and functions of relevant pTregs.

Tolerogenic BTLA^hi cDCs also express a lectin receptor DEC-205 that can serve as a specific target in strategies for delivering antigens in vivo for their presentation to T cells. We previously pioneered an approach that depends on engineered recombinant chimeric antibodies specific for DEC-205 that also carry peptides for their subsequent recognition by encephalitogenic T cells (discussed in^33,34,44). This anti-DEC-205 chimeric antibody is comprised of variable (V) regions derived from monoclonal antibodies specific for DEC-205 that are fused with the species-specific heavy and light constant (C) regions derived from separate immunoglobulins. A similar design is found in antibodies used for therapeutic purposes such as the anti-tumor checkpoint blockades.^34,44 Importantly, the constant regions of the DEC-205-specific chimeric antibody are also genetically fused with a defined antigen of choice that is then processed by cDCs and presented on major histocompatibility molecules for its recognition by T cells.^34,44 Such specifically targeted encephalitogenic antigens delivered to tolerogenic BTLA^hiDEC-205⁺ cDCs induce pTregs that block encephalitogenic autoimmune effectors (Figure 1).^34,44 Importantly, pTregs that are induced systemically in the secondary immune organs can suppress local effector responses in the CNS.^35,41 Overall, these approaches allow for highly specific immunomodulation that is focused only on autoreactive encephalitogenic T cells without impairing other immune responses. The importance of well-controlled immunomodulation is further underscored by our most recent results that uncovered a concomitant induction of T cells with regulatory and also effector autoimmune properties under normal homeostatic conditions.⁴⁵ These findings further highlight the importance of therapeutic interventions aimed at APCs with specific tolerogenic properties for novel efficient immunotherapies that can minimize potential risks of overstimulating potentially harmful T cells. Overall, results of this basic research are likely to inform future therapeutic approaches that will be more selective and have fewer side effects.

Figure 1.

Tolerogenic BTLA^hi conventional dendritic cells (cDCs) express the lectin receptor DEC-205, which can serve as a target for specifically delivering antigens for their presentation to T cells with engineered recombinant chimeric antibodies. The constant regions of such DEC-205-specific chimeric antibodies are fused with a defined antigen of choice, such as myelin oligodendrocyte glycoprotein (MOG), that is then processed by BTLA^hiDEC-205⁺ cDCs and presented on major histocompatibility molecules (MHCs) for their subsequent recognition by encephalitogenic T cells. Such specific targeting of encephalitogenic antigens to tolerogenic BTLA^hi cDCs results in the BTLA-dependent induction of peripheral regulatory T cells (pTregs) that block autoimmune responses mediated by encephalitogenic autoimmune effectors.