Each vaccine (15x10E6cells in 500 µL solution with 5% human albumin supplemented with 10% DMSO and 4% glucose) will be administered through intradermal injection at 5 sites (100 µL/site) in the posterior cervical region, targeting lymphatic drainage into both superficial and deep cervical lymph nodes (5-10 cm from the cervical lymph nodes). Injection sites will alternate between left and right sides of the neck for each administration timepoint.

In brief, clinical-grade tolDC vaccines will be prepared from leukapheresis starting material of non-mobilized blood and subsequent immunomagnetic selection of CD14+ monocytes using a CliniMACS device. CD14+ monocytes will then be cultured in GMP-grade cell culture medium supplemented with 2% human AB serum, GM-CSF, IL-4 and 1 alpha,25 dihydroxyvitamin D3. At day 4, tolDC will be stimulated using a cytokine cocktail to induce a migratory phenotype. At day 6, tolDC will be harvested, loaded with antigen, resuspended, and cryopreserved. Separate aliquots of the cell product are prepared for final quality control and quality assurance (QC/QA) assessment. This includes cell count, viability, phenotypic analysis using flow cytometry, and induction of T cell hyporesponsiveness in allogeneic mixed leukocyte reaction (allo-MLR).

To evaluate the efficacy of tolDC administration, the change in Expanded Disability Severity Scale (EDSS) will be employed. The participants' disability level well be checked during every visit. The EDSS consists of a 10-point scale of disease severity ranging from 0, i.e. no disability, to 10, i.e. death from MS.

The impact on disability progression will be analyzed by the proportion of participants free from confirmed disability progression, defined by sustained changes in the EDSS score.

Disease progression criteria:

• 1.0 point in participants with a screening EDSS of 0-5.0
• 0.5 point in those with a screening EDSS of 5.5-6.0

Tolerability and safety of tolDC administration will be assessed by recording the incidence, severity, and relationship to study treatment of adverse events throughout the trial.

Participants will be monitored by the treating neurologist for the occurrence and outcomes of AEs, SAEs, relapses and study discontinuations. Full physical examinations, vital signs, and blood and urine samples (only in case of women of childbearing age for pregnancy test) will be recorded at screening, during tolDC administration and during follow-up. An assessment of severity grading of AEs will follow the WHO toxicity grading scale (1= mild, 2= moderate, 3= severe, 4= potentially life threatening).

The 9-HPT is a quantitative assessment of upper limb (arm and hand) function. The test device consists of a container containing nine pegs and a wooden or plastic board with nine empty holes. The participant is required to pick up each peg individually and place them as quickly as possible in the nine holes. Once all the pegs are placed in the holes, the participant must remove them one by one, again as quickly as possible and return them to the container. The total time to complete the task is recorded.

To comprehensively assess therapy-related immunological changes and the induction of antigen-specific tolerance, we will perform high-dimensional immune profiling using cryopreserved peripheral blood mononuclear cells (PBMCs) and matched serum/plasma samples collected at predefined time points throughout the trial. In brief, we will store samples of PBMC in biobank facilities in Antwerp and Spain prior to vaccination and during and after the IMP administrations for future centralized high-dimensional functional immune-profiling.