This study will have two Phases: Phase 1a and Phase 1b. The goal of this clinical study is to learn more about the study drug KITE-363, to establish dosing, tolerability, safety, and preliminary efficacy of KITE-363 in participants with refractory autoimmune diseases.

The primary objectives of this study are:

Phase 1a: To evaluate the safety and tolerability of KITE-363 in participants with autoimmune disease. To determine the recommended dose for Phase 1b.

Phase 1b: To evaluate the safety and efficacy of KITE-363 in participants with autoimmune disease.

Key Inclusion Criteria:

Inclusion Criteria for systemic lupus erythematosus (SLE) and lupus nephritis (LN):

• Age ≥ 18 years
• Meet the European Alliance of Associations for Rheumatology (EULAR)- American College of Rheumatology (ACR) 2019 classification criteria for SLE
• Presence of either double-stranded deoxyribonucleic acid (DNA) anti- double-stranded DNA (anti-dsDNA) and/or anti-Smith antibodies at screening per local laboratory.
• Moderate to severe, active disease defined as at least one British Isles Lupus Assessment Group (BILAG-A) score or 2 BILAG B (excluding constitutional and/or neuropsychiatric organ system).
• Refractory to steroids and inadequate response or intolerance to at least 2 of the following treatments, used for at least 3 months each: cyclophosphamide, mycophenolate mofetil or its derivatives, belimumab, anifrolumab, rituximab, obinutuzumab, methotrexate, azathioprine, cyclosporin, tacrolimus, or voclosporin.
• For LN: Refractory to steroids and inadequate response or intolerance to at least 2 of the following treatments, used for at least 3 months each: cyclophosphamide, mycophenolate mofetil or its derivatives, belimumab, rituximab, obinutuzumab, azathioprine, cyclosporin, tacrolimus, or voclosporin

Inclusion Criteria for LN:

• Renal biopsy-proven Class III or intravenous (IV) ± V LN according to the revised International Society of Nephrology and Renal Pathology Society (ISN/RPS) criteria within 6 months prior to or during screening
• Evidence of active LN at screening

Inclusion Criteria for systemic sclerosis (SSc):

• Age ≥ 18 years
• Diffuse Systemic Sclerosis (SSc) according to ACR/EULAR 2013 classification criteria with active skin disease and/or progressive SSc-interstitial lung disease (ILD) OR limited SSc with progressive ILD.
• Refractory or intolerance to 1 of the following for a minimum of 3 months and/or contraindication: mycophenolate mofetil or its derivatives, methotrexate, tocilizumab (or other IL-6 inhibitor), rituximab (or other B-cell depleting agent), nintedanib (or other antifibrotic agents), cyclophosphamide.
• High-resolution computer tomography (HRCT) scan and pulmonary function test (PFT) within 3 months prior to screening.

Inclusion Criteria for idiopathic inflammatory myopathy (IIM):

• Age ≥ 18 years
• Probable or definite IIM based on EULAR/ACR 2017 classification (excluding inclusion body myositis).
• Active disease demonstrated by electromyography (EMG), magnetic resonance imaging (MRI) or muscle enzymes
• Moderate to severe disease activity
• Positive for myositis specific antibodies for patients with non-dermatomyostitis IIM
• HRCT scan and PFT within 3 months prior to screening.
• Refractory or intolerance to at least 1 month of glucocorticoids and standardized use of at least 2 immunosuppressant/modulator (eg, intravenous gamma globulins, methotrexate, mycophenolate mofetil and its derivatives, azathioprine, cyclophosphamide, calcineurin inhibitors, Janus kinase (JAK) inhibitors, rituximab or other B-cell depleting agent).

Inclusion Criteria for all Cohorts:

• Adequate hepatic, renal, pulmonary, and cardiac function.

Key Exclusion Criteria:

Exclusion Criteria for all Cohorts:

• Females of childbearing potential who are pregnant or breast feeding.
• Dialysis within the past year.
• History of malignancy, within the last 5 years.
• Hypogammaglobulinemia requiring immunoglobulin replacement.
• History of autologous or allogeneic stem cell transplant and/or organ transplant.
• Prior treatment with cellular therapy, gene therapy and/or T-cell engager therapy.
• Known history of HIV infection, or hepatitis B or C virus infections.
• Active or untreated latent tuberculosis (TB).
• Active or uncontrolled infections.
• Nonspecific, overlap, mixed autoimmune diseases not clearly identified into any of the studied cohorts.

Exclusion Criteria for LN:

• Significant pre-existing damage or rapidly progressive glomerulonephritis (GN).

Exclusion Criteria for SLE:

• Drug-induced SLE.
• Catastrophic antiphospholipid syndrome.
• Thrombotic thrombocytopenic purpura.
• Active or unstable lupus neuropsychiatric manifestations within last 6 months.

Exclusion Criteria for SSc:

• Infected digital ulceration or necrosis with signs of infection.
• Severe pulmonary hypertension.
• History of systemic sclerosis renal crisis within 12 months prior to enrollment.
• History of active bleeding related to gastric antral vascular ectasia.

Exclusion Criteria for IIM:

• Other inflammatory and noninflammatory myopathies.
• Severe, irreversible muscle damage.

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