Inclusion Criteria - Diagnosis of Systemic Lupus Erythematosus (SLE) defined as follows:. i) Fulfilling the 2019 European League Against Rheumatism (EULAR) / American College of Rheumatology (ACR) classification criteria of SLE. ii) Presence of anti-dsDNA, anti-histone, anti-chromatin, anti-Ro (anti-SS-A), anti-La (anti-SS-B), or anti-Sm antibodies at screening. - SLE disease activity. i) Active disease at screening, with recent ≥ 1 major organ system with a BILAG A score (excluding musculoskeletal, mucocutaneous, and/or constitutional organ system). ii) Inadequate response to glucocorticoids and to at least 2 of the following treatments, used for at least 3 months each: cyclophosphamide, mycophenolic acid or its derivatives, belimumab, azathioprine, anifrolumab, methotrexate, rituximab, obinutuzumab, cyclosporin, tacrolimus or voclosporin. - Diagnosis of Idiopathic Inflammatory Myopathy (IIM) defined as follows:. i) Fulfilling the 2017 EULAR/ACR classification criteria for probable or definite IIM. ii) Participant diagnosed with the following IIM subgroups: dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM), anti-synthetase syndrome (ASyS), and polymyositis (PM). iii) Presence of at least 1 myositis specific (MSA), associated antibody (MAA), or ANA at screening or prior to screening. - IIM disease activity. i) Severe muscle AND/OR skin involvement. ii) Proof of activity as documented by:. A. An active myositis-associated rash OR. B. A recent muscle biopsy OR. C. An elevated CK > 3 times the upper limit of normal OR. D. Participants diagnosed IIM AND progressive Interstitial Lung Disease (ILD) on high-resolution computed tomography (HRCT) iii) Inadequate response to glucocorticoids and at least 2 of the following treatments used for at least 3 months: azathioprine, methotrexate, cyclosporin A, tacrolimus, MMF, cyclophosphamide, leflunomide, IVIG, and rituximab. - Diagnosis of Systemic Sclerosis (SSc) defined as follows:. i) Fulfilling 2013 EULAR/ACR classification criteria for SSc. ii) Antinuclear Antibody (ANA) positive at screening or prior to screening. - SSc disease activity. i) Participants diagnosed with diffuse cutaneous SSc OR diffuse or limited cutaneous SSc AND progressive ILD, AND. ii) Inadequate response to at least 1 of the following treatments used for at least 3 months: mycophenolate, cyclophosphamide, rituximab, nintedanib, azathioprine, tocilizumab, or intravenous immunoglobulins (IVIG). Exclusion Criteria - Diagnosis of drug-induced SLE rather than idiopathic SLE. - Other systemic autoimmune diseases (eg, multiple sclerosis, psoriasis, inflammatory bowel disease, etc) are excluded. Participants with type I autoimmune diabetes mellitus, thyroid autoimmune disease, Celiac disease, or secondary Sjögren's syndrome are not excluded. - SLE overlap syndromes including, but not limited to, rheumatoid arthritis, scleroderma, and mixed connective tissue disease, are excluded. - Present or recent clinically significant CNS pathology, within 12 months. - IIM disease activity. i) Other forms of IIM: Inclusion Body Myositis, Amyopathic DM, any form of juvenile myositis. ii) Myositis other than IIM, eg, drug-induced myositis and PM associated with HIV. iii) Participants with severe muscle damage (Physician VAS for muscle damage in Myositis Damage Index > 7 cm on a 10 cm scale), permanent weakness due to a non-IIM cause (eg, stroke), or myositis with cardiac involvement. - SSc disease activity. i) SSc related PAH requiring active treatment. ii) Rapidly progressive SSc related lower GI (small and large intestines) involvement (requiring parenteral nutrition); active gastric antral vascular ectasia. iii) Prior scleroderma renal crisis. - Other protocol-defined Inclusion/Exclusion criteria apply.