• Participant must be 18 to 70 years of age inclusive, at the time of signing the informed consent.

• Diagnosis of SLE:

‣ Diagnosis of SLE according to the 2019 EULAR/ACR classification criteria for SLE

⁃ Positive for one or more of: anti-nuclear antibodies (titre ≥ 1:80), anti-dsDNA or anti-Sm at screening.

⁃ Active, moderate-severe disease at screening, defined as clinical SLEDAI-2K ≥ 4.

⁃ Intolerance or inadequate response to ≥ 3 available SOC treatments, used for at least 3 months each, for which at least one must be a biologic SoC or cyclophosphamide.

• Diagnosis of IIM:

‣ Must have probable or definite diagnosis of PM or DM (excluding IBM and cancer associated myositis) according to the 2017 EULAR/ACR classification criteria for adult myositis.

⁃ Positive for ≥ 1 disease-specific autoantibody at screening.

⁃ MMT-8 score of ≤ 142/150 and/or CDASI-A \> 6

⁃ Fulfill at least one of the following criteria of active disease at screening:

• (i) One or more muscle enzyme elevation (CK, AST, ALT, aldolase, LDH) ≥ 1.3 × ULN (ii) If criterion 3(d)(i) is not met, then at least one of the following criteria must be met:

⁃ Report from MRI performed within 3 months prior to screening with evidence of muscle inflammation

⁃ Report from muscle biopsy performed within 3 months prior to screening that demonstrates active inflammation

⁃ Report from electromyography performed within 3 months prior to screening that exhibits irritable myopathic pattern.

• (e) Intolerance or inadequate response to corticosteroids and ≥2 other SoC treatments, used for at least 3 months each, for which at least one must be a biologic SoC, immunoglobulin or cyclophosphamide.

• For participants receiving any of the following therapies at screening:

‣ Oral prednisone (or equivalent): Daily dose ≤ 20 mg/day and must be stable for ≥ 1 week prior to Day 1.

⁃ Immunosuppressive treatment: for ≥ 3 months prior to Day 1 and at a stable dose for ≥ 4 weeks prior to Day 1.