A Phase 1/2 Study of NKX019 in Subjects With Immune-Mediated Diseases (Ntrust-2)

General Inclusion Criteria: * Age ≥18 and ≤75 years * Signed informed consent and ability to adhere to the study visit schedule and comply with other protocol requirements * Women of childbearing potential must have negative pregnancy tests at screening and baseline and agree to abstinence or acceptable birth control from 2 weeks before the first dose through 1 year after the last dose * Participants taking corticosteroids must be at a stable prednisone (or equivalent) dose ≤20 mg/day for ≥14 days before Screening * Participants on immunosuppressives or immunomodulators (other than corticosteroids) must have stable dosing for ≥4 weeks before Screening * eGFR (2021 CKD-EPI equation) ≥45 mL/min/1.73 m² at screening For Subjects with Systemic Sclerosis (SSc): * Meets the 2013 ACR/EULAR classification criteria for SSc * Severe skin involvement defined as modified Rodnan skin score (mRSS ≥30) or active skin disease (mRSS ≥15 with, within the prior 6 months, an mRSS increase ≥3 units, 1 new body area with ≥2 mRSS units, or 2 new body areas with ≥1 mRSS unit each), OR moderate-to-severe interstitial lung disease (ILD) on HRCT with FVC \<70% predicted or DLCO \<70% predicted (or ILD on HRCT with progression per protocol-defined criteria) * 10 years or less since the first non-Raynaud's sign or symptom * Inadequate response or intolerance to at least one prior treatment, including cyclophosphamide, methotrexate, MMF/mycophenolic acid, nintedanib, rituximab, or tocilizumab For Subjects with Idiopathic Inflammatory Myopathies (IIM): * Diagnosis of IIM per the 2017 ACR/EULAR Classification Criteria * One positive myositis antibody (historical) * Disease activity defined as manual muscle testing (MMT-8) score \<136/150 * Creatine kinase or aldolase ≥1.5×ULN and Clinician Global Assessment ≥2 cm, with evidence of active myositis (MRI, EMG, or biopsy within 6 months) or MDAAT Global Extramuscular Activity Score ≥2 cm; participants with DM, JDM, or ASyS may qualify with CK/aldolase \<1.5×ULN if Clinician Global Assessment ≥2 cm and either of the above is met or CDASI ≥15, pending discussion with the medical monitor * Inadequate response to ≥3 months of at least 2 immunosuppressive therapies (including glucocorticoids) For Subjects with ANCA-Associated Vasculitis (AAV): * Meets the 2022 ACR/EULAR classification criteria for Granulomatosis with Polyangiitis (GPA) or Microscopic Polyangiitis (MPA) * Relapsed or refractory AAV despite repeated immunosuppressive treatment, or requiring prolonged/repeated unacceptable glucocorticoid doses to maintain disease control * Positive anti-proteinase-3 (PR3-ANCA) or anti-myeloperoxidase (MPO-ANCA) at screening * Meets BVAS version 3 activity threshold (≥1 major item, ≥3 other items, or ≥2 renal items) For Subjects with Rheumatoid Arthritis (RA): * Documented diagnosis of RA, meeting the 2010 ACR/EULAR classification criteria * Rheumatoid Factor (RF) or Anti-Citrullinated Protein Antibody (ACPA) positive * CRP \>3 mg/L * Inadequate response, defined as failure to achieve a clinically meaningful improvement (eg, ACR50 response or DAS28 remaining \>3.2) after ≥12 weeks of therapy with: ≥1 conventional synthetic DMARD (eg, methotrexate, leflunomide, sulfasalazine, hydroxychloroquine) plus either: ≥2 biologic DMARDs with distinct mechanisms of action (eg, TNF inhibitors, abatacept, anti-IL-6/IL-6R, rituximab) or ≥1 biologic DMARD and ≥1 targeted synthetic DMARD (eg, a JAK inhibitor), having failed no more than 4 biologics or targeted synthetic DMARDs with unique mechanisms of action * Minimum of 6 swollen joint counts (based on the 66-joint count) and 6 tender joint counts (based on the 68-joint count) at Screening (28-joint counts used at Baseline and thereafter) General Exclusion Criteria: * Currently requiring or expected to require renal dialysis during the study * Previous solid organ or hematopoietic cell transplant, or planned transplant during the study treatment period * Congenital or acquired immunodeficiency resulting in severe infection, or receiving chronic immunoglobulin replacement therapy * Liver disease or dysfunction, including cirrhosis and/or bilirubin ≥3× the upper limit of normal * Pulmonary comorbidity (eg, COPD or asthma requiring daily oral steroids), resting hypoxemia (\<92% oxygen saturation on room air), or significant smoking history (\>10 pack-years) with active pulmonary disease * ILD requiring supplemental oxygen therapy, FVC \<45% predicted, or DLCO ≤40% predicted at screening * White blood cell count \<3,000/mm³, hemoglobin levels ≤9 g/dL, absolute neutrophil count ≤1,500/mm³, platelets ≤100,000/mm³, or blood transfusion within 60 days before lymphodepletion (LD) * Major cardiac disease, abnormalities, or interventions (eg, uncontrolled angina, myocardial infarction within 12 weeks, prior coronary artery bypass graft surgery, ≥Class III congestive heart failure (CHF), significantly decreased ejection fraction (EF) ≤40%, prolongation of the QT interval corrected for heart rate (QTc) \>480 msec, or a ≥Grade 2 thrombotic/embolic event within 12 weeks of the first dose * Active bleeding disorder * Overlapping autoimmune condition for which the condition or its treatment may affect study assessments or outcomes * Pregnancy, breastfeeding, or, if of childbearing potential, not using adequate contraception * Current infection requiring active systemic anti-infective therapy, or a recent acute infection treated systemically within 30 days of planned LD; participants who are asymptomatic and being treated for a non-systemic infection (eg, urinary tract infection) are not excluded on this basis and may be eligible after discussion with the Sponsor Medical Monitor * History of positive human immunodeficiency virus (HIV) antibody or PCR test, or a positive test at screening, Hepatitis B or C positive at screening, or active or latent tuberculosis requiring suppressive therapy * Major surgery within 28 days before the first dose of NKX019,