2.1 Peptide-Based Tolerance-Inducing Vaccination
To date, induction of in vivo antigen-specific tolerance by subcutaneous or oral administration of peptides (i.e., peptide vaccination) has proven to be a well-tolerated and successful therapy for allergies [
16‐
18]. Given the success in allergies, the possibility of treating MS with peptide vaccination is being investigated. Some of the most promising results were described by Jurynczyk et al. [
6] and Walczak et al. [
7] in two phase I/II studies in which the investigators transdermally applied peptides derived from MOG, MBP and PLP. They demonstrated induction of immunological tolerance by activation of Langerhans cells and subsequent induction of interleukin (IL)-10-secreting T cells [
7]. Moreover, the immunological effect was clinically translated in a placebo-controlled trial that demonstrated a significant reduction in annualized relapse rate and magnetic resonance imaging (MRI)-defined measurements of the disease [
7]. Interestingly, lower peptide concentrations following intramuscular [
13] and transdermal [
7] administration achieved an even better clinical outcome, underscoring the importance of dosage to achieve tolerance. Nevertheless, whereas peptide vaccination is known to induce tolerance in steady-state conditions, unexpected adverse events can be anticipated following administration in a pro-inflammatory environment. Indeed, three patients in a phase II clinical trial investigating vaccination with a MBP-derived APL, in which amino-acid substitutions were incorporated at T-cell receptor (TCR) contact positions, demonstrated disease exacerbations following treatment. A clear association with the vaccination strategy was demonstrated in two of the patients, even after the dose was lowered, and the trial was halted [
8,
19].
Alternatively, soluble synthetic peptides were designed to mimic the naturally processed epitopes. These so-called apitopes induce antigen-specific expansion of regulatory T cells, capable of “switching-off” pathogenic T cells, which produce pro-inflammatory cytokines and are responsible for myelin damage in the CNS. In this context, two clinical trials have recently completed evaluation of the safety and biological disease parameters of ATX-MS-1467, a mixture of four short peptides derived from MBP, i.e., ATX-MS1 (MBP
30-44), ATX-MS4 (MBP
131-145), ATX-MS6 (MBP
140-154), and ATX-MS7 (MBP
83-99). ATX-MS-1467 is administered intradermally every 2 weeks for 20 weeks. Patients initially receive a dose titration of 50 and 200 μg for 4 weeks, then a dose of 800 μg every 2 weeks for 16 weeks. A phase I open-label dose-escalating study demonstrated that ATX-MS-1467 was safe and well-tolerated in a group of six patients with secondary-progressive MS (SPMS), up to a dose of 800 μg [
20]. A recent multicenter, open-label, single-arm, baseline-controlled phase IIa clinical trial (NCT01973491) evaluated the clinical and biological effects of ATX-MS-1467 in 19 patients with relapsing MS (RMS). No treatment-related serious adverse events were observed, and the adverse event profile was mild, with < 50% of patients experiencing local injection site reactions. Although there was no placebo group with which to compare results, a review of MRI data showed that treatment with ATX-MS-1467 led to a 78% decrease in new T1 Gadolinium-enhancing lesions as compared with baseline [
21].
To engage T cells specific for the naturally processed antigen and to serve as a tolerogen, peptides must reach the resident antigen-presenting cells in vivo. This process can be facilitated by targeting specific markers expressed on the surface of antigen-presenting cells. For instance, the mannose receptor cluster of differentiation (CD)-206 is a C-type lectin primarily present on the membrane of macrophages and immature DCs. In this context, encapsulation of selected immunodominant MBP peptides into mannosylated liposomes significantly enhanced the uptake of the peptides by DCs via the CD206 receptor. This resulted in immune tolerance towards the myelin-derived antigens. CD206-targeted liposomal delivery of co-encapsulated immunodominant MBP sequences MBP
46–62, MBP
124–139 and MBP
147–170 (Xemys™, JSC Pharmsynthez, Moscow, Russia) was investigated in a phase I, multicenter, open-label, dose-escalating safety and proof-of-concept study in patients with RRMS or SPMS with relapses for whom first-line DMTs had failed. Patients received six weekly subcutaneous injections with incremental doses from 50 to 900 μg. After the last injection, patients were followed-up for 12 weeks. No dose-limiting toxicities were observed during treatment. Local injection site reactions were the most common adverse event [
22]. Interestingly, a statistically significant decrease compared with baseline was observed in serum CCL2, CCL4, IL-7, and IL-2 levels at study completion (week 18) [
23].
A completely different approach is effectuated by TCR peptide vaccination. Hereto, short amino acid sequences derived from the TCR of pathogenic T cell clones are administered in an attempt to induce T-cell-mediated immunoregulation directed at T cells expressing those TCRs. The repertoire of TCR peptide-reactive T cells is positively selected in the thymus after depletion of negatively selected clonotypes, and it has been hypothesized that TCR-specific T cells might represent a subset of the naturally induced regulatory T cells. In patients with MS, the Vβ repertoire of activated T cells has been reported to be derived predominantly from the Vβ5.2 and Vβ6.1 families [
24]. Hence, several clinical trials have investigated the administration of incremental doses of TCR Vβ5.2 and Vβ6.1 peptides. Intradermal injection of synthetic TCR Vβ5.2 peptides resulted in clinical improvement paralleled by beneficial immunological effects, such as the generation of TCR peptide-specific T cells and reduction of MBP-specific T cells, in a double-blind placebo-controlled trial in 22 patients with progressive MS [
25]. Repeated intramuscular injections of TCRVβ6 peptide also resulted in immunoregulatory effects, warranting further exploration of this approach in the treatment of MS [
26]. Administration of both peptides was safe and did not worsen the disease course following both administration routes [
27]. Moreover, a peptide-specific immune response was induced in 50–60% of patients with MS following intradermal injection of TCR Vβ5.2 peptides, whereas 90% of patients with MS demonstrated measurable T-cell immunity towards the Vβ6 peptides upon intramuscular injection in inactivated Freund’s adjuvant (IFA). For this, it was hypothesized that a vaccine consisting of three TCR peptides (BV5S2, BV6S5, and BV13S1) emulsified in IFA would be more immunogenic than the three peptides in saline alone. The trivalent peptide TCR vaccine, now called Neurovax (Immune Response BioPharma, Atlantic City, NJ, USA), was investigated in several clinical trials and found to be safe and to induce a surge of proliferating IL-1-secreting TCR peptide-specific T cells [
28‐
31]. A phase IIb study in patients with SPMS (clinical trials.gov identifier NCT02057159) to investigate the efficacy and safety of the vaccine is yet to start.
2.2 DNA Vaccination
BHT-3009 is a DNA vaccine that is made of genetically engineered DNA that encodes the full-length human MBP [
32,
33]. The plasmid backbone has been modified in such a way that it could lead to favorable immunological changes in patients with MS (reduction in the number of immunostimulatory CpG motifs and increase in the number of immunoinhibitory GpG motifs). Its purpose is to restore tolerance to self, leaving protective immunity against infectious and tumor antigens intact. BHT-3009 was first investigated in a randomized placebo-controlled phase I/II trial in patients with RRMS or SPMS and was shown to be safe and well tolerated. Moreover, a reduction in contrast-enhancing lesions on MRI was accompanied by reduced proliferation of interferon-γ-producing myelin-reactive T cells and decreased titers of myelin-specific autoantibodies in the cerebrospinal fluid [
34]. Next, a phase II randomized placebo-controlled trial comparing two doses of BHT-3009 was conducted in 289 patients with RRMS. Remarkably, the high dose of 1.5 mg was ineffective, but the low dose of 0.5 mg showed a trend towards a 50–61% decrease in the number of new enhancing lesions as compared with placebo (
p = 0.07). In addition, a profound reduction in myelin-specific auto-antibody titers was seen, indicative of induction of antigen-specific immune tolerance. Nevertheless, no beneficial effects on disease course were observed [
13], and whether the vaccine will enter phase III clinical trials remains to be seen.