Key Inclusion Criteria:
- Reproductive status-related eligibility and contraception requirements:
- Participants must agree to use protocol-specified method(s) of contraception where applicable
Inclusion Criteria for multiple sclerosis (MS):
MS (Relapsing and progressive forms):
- Diagnosed with MS according to the 2017 revision of the McDonald diagnostic criteria
Relapsing forms of MS (relapsing-remitting multiple sclerosis (RRMS), active secondary-progressive multiple sclerosis (aSPMS)):
- Inadequate response to previous therapies is defined as evidence of breakthrough disease activity within 12 months prior to screening while on high efficacy disease-modifying therapy (DMT) OR Inadequate response to previous therapies defined as intolerance to ≥ 2 DMTs due to side effects prohibiting the chronic use of the DMT.
- Expanded Disability Status Scale (EDSS) 0 to 5.5
Progressive forms of MS (primary-progressive multiple sclerosis (PPMS) and non-active secondary-progressive multiple sclerosis (naSPMS)):
- Inadequate response to previous therapies is defined as evidence of disease progression within 12 months prior to screening despite standard of care therapy for naSPMS or despite ocrelizumab, where available, for PPMS
- Absence of clinical relapses for at least 24 months
- No evidence of Gadolinium enhancing (GadE+) on magnetic resonance imaging (MRI) brain at screening or baseline
- EDSS of 3 to 6.5 who are ambulatory
Inclusion Criteria for myasthenia gravis (MG):
- Documentation of autoantibodies against acetylcholine receptor (AChR), muscle-specific kinase (MuSK), or low-density lipoprotein receptor-related protein 4 (LRP4)
- Diagnosis of MG with generalized weakness meeting criteria as defined by the Myasthenia Gravis Foundation of American (MGFA) classification of II- IV at screening
- Myasthenia Gravis Activities of Daily Living (MG-ADL) score ≥ 6 (> 50% of the total score due to non-ocular symptoms)
- Quantitative Myasthenia Gravis (QMG) score ≥ 10
- Inadequate response to previous therapies while taking at least 2 classes of immunosuppressants (ie, steroids, azathioprine (AZA), mycophenolate mofetil (MMF), intravenous immunoglobulin (IVIg), biologics (eg, rituximab, anti-neonatal fragment crystallizable (Fc) receptor (FcRN) class, and anti-complement class))
- Thymectomy allowed if completed ≥ 12 months prior to screening
Inclusion Criteria for chronic inflammatory demyelinating polyneuropathy (CIDP):
- Probable or definite CIDP as defined by the 2010 European Federation of Neurological Societies/Peripheral Nerve Society (EFNS/PNS) criteria, relapsing or progressive forms
- CIDP Disease Activity Status (CDAS) score ≥ 3 at screening
- Inflammatory neuropathy cause and treatment (INCAT) score ≥ 3
- Inadequate response to previous therapies despite standard of care therapy (ie, steroids, IVIg, subcutaneous immunoglobulin (SCIg), plasmapheresis exchange (PLEX), rituximab, or anti FcRN) OR Unable to tolerate standard of care due to side effects with ongoing disease activity
- Except for nodal/paranodal CIDP, historical documentation of objective improvement in the past 24 months while on IVIg, SCIg, PLEX, or anti-FcRN OR Historical documentation of objective disease worsening in the past 24 months when IVIg, SCIg, PLEX, or anti-FcRN has been reduced or interrupted
Key Exclusion Criteria:
- History or presence of central nervous system (CNS) or peripheral nervous system disorders before enrollment that may impact cognition, strength, or cause weakness
- History of autologous or allogeneic stem cell transplant and/or organ transplant
Exclusion Criteria for MS:
- Cohort 1 or 2; inability to complete 9-hole Peg Test (9-HPT) in < 240 seconds and Timed 25 foot Walk (T25FW) < 150 seconds
- History of hypersensitivity to parenteral administration of gadolinium-based contrast agents
- Any renal condition that would preclude the administration of gadolinium (for the relapsing forms of MS and progressive forms of MS)
- Any contraindication to lumbar puncture (LP) (for the relapsing forms of MS and progressive forms of MS)
Exclusion Criteria for MG:
- Current myasthenic crisis not effectively controlled within 2 weeks before enrollment
- Thymectomy performed within 12 months of baseline
Exclusion Criteria for CIDP:
- Pure sensory CIDP and focal CIDP
- Polyneuropathy of other causes
Note: Other protocol defined Inclusion/Exclusion criteria may apply.