Inclusion Criteria:

- A diagnosis of dermatomyositis according to 2017 EULAR/ACR Classification Criteria for
Idiopathic Inflammatory Myopathies.

- Adult subjects (18-74 years old).

- Active muscle and skin disease at screening and baseline.

- Current therapy with corticosteroids, hydroxychloroquine, and/or one non-steroid
immunosuppressant.

- Weight > 40 kg to < 130 kg, and with a body mass index (BMI) < 40 kg/m^2.

Exclusion Criteria:

- Dermatomyositis with end-stage organ involvement.

- Dermatomyositis with irreversible muscle involvement.

- History of:

- Any lymphoproliferative disorder;

- Active malignancy;

- Cancer within 5 years prior to screening (exceptions for basal cell
carcinoma, squamous cell carcinoma, ductal carcinoma in situ of the breast,
carcinoma in situ of the uterine cervix, or thyroid carcinoma);

- Cancer-associated dermatomyositis;

- Overlap myositis/connective tissue disease (except for overlap with Sjögren's
syndrome).

- Participants at a risk of thrombosis and cardiovascular disease.

- Participants with a high risk for herpes zoster reactivation.

- Participants with active or recent infections.

Note: Other protocol defined Inclusion/Exclusion Criteria may apply.

Eligibility last updated 7/29/22. Questions regarding updates should be directed to the study team contact.