• Age 18 to 55 years, inclusive, at the time of the screening Visit -2.

• Diagnosis of MS according to the 2017 McDonald Criteria139.

• EDSS ≤ 6.0 at the time of randomization (Day 0).

• T2 abnormalities on brain MRI that fulfill the 2017 McDonald MRI criteria for dissemination in space139. A detailed MRI report or MRI images must be available for review by the site neurology investigator.

• Highly active treatment-resistant relapsing MS, defined as ≥ 2 episodes of disease activity in the 36 months prior to the screening visit (Visit -2). The two disease activity episodes will be a clinical MS relapse or MRI evidence of MS disease activity and must meet all the criteria described below:

‣ At least one episode of disease activity must occur following ≥ 1 month of treatment with one of the following: (i) an oral DMT approved by the FDA for the treatment of relapsing MS, or (ii) a monoclonal antibody approved by the FDA for the treatment of relapsing MS, or (iii) rituximab. Qualifying DMTs include: dimethyl fumarate, diroximel fumarate, monomethyl fumarate, teriflunomide, cladribine, daclizumab, ponesimod, siponimod, ozanimod, fingolimod, rituximab, ocrelizumab, natalizumab, alemtuzumab, ublituximab, and ofatumumab, and

⁃ At least one episode of disease activity must have occurred within the 12 months prior to the screening visit (Visit -2), and

⁃ At least one episode of disease activity must be a clinical MS relapse (see item c.i. below). The other episode(s) must occur at least one month before or after the onset of the clinical MS relapse, and must be either another clinical MS relapse or MRI evidence of disease activity (see item c.ii. below):

• i. Clinical MS relapse must be confirmed by a neurologist's assessment and documented contemporaneously in the medical record. If the clinical MS relapse is not documented in the medical record, it must be approved by the study adjudication committee (see Section 3.5), and ii. MRI evidence of disease activity must include ≥ 1 unique active lesion on one or more brain or spinal cord MRIs. Detailed MRI reports or MRI images must be available for review by the site neurology investigator. A unique active lesion is defined as either of the following:

∙ 1\. A gadolinium-enhancing lesion, or 2. A new non-enhancing T2 lesion compared to a reference scan obtained not more than 36 months prior to the screening visit (Visit -2).

∙ 6\. Candidacy for treatment with at least one of the following high efficacy BAT DMTs: cladribine, natalizumab, alemtuzumab, ocrelizumab, ofatumumab, ublituximab and rituximab. Candidacy for treatment for each BAT DMT is defined as meeting all of the following:

• No prior disease activity episode, as defined in Inclusion Criterion #5, with the candidate BAT DMT, and

• No contraindication to the candidate BAT DMT, and

• No treatment with the candidate BAT DMT in the 12 months prior to screening.

• 7\. Completion of COVID-19 vaccination series, according to the current Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) recommendations, ≥ 14 days prior to randomization (Day 0).

• 8\. Positive for VZV antibodies, or completion of at least one dose of the varicella zoster glycoprotein E (gE) Shingrix vaccine at least 4 weeks prior to randomization (Day 0).

• 9\. Insurance approval for MS treatment with at least one candidate BAT DMT (see Inclusion Criterion #6).

• 10\. Ability to comply with study procedures and provide informed consent, in the opinion of the investigator.

• 11\. Females of childbearing potential (defined in Section 5.4.3.1) and males with female partners of childbearing potential are required to adhere to the contraception provisions of Section 5.4.3.1.

• 12\. For participants who use medicinal or recreational marijuana, willingness to substitute MARINOL® if randomized to AHSCT (Section 5.4.2.6).