Phase 1b/2a in SLE With Budoputug

Inclusion Criteria:. * Diagnosis of SLE fulfilling SLICC criteria or 2019 European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) criteria at least 24 weeks prior to study entry, with one of the following at Screening * If taking a corticosteroid regimen prior to Screening, currently receiving ≤ 20 mg prednisone or equivalent by Day -28. Exceptions for patients with active LN are detailed in Section XX (where we talk about CS use). * Female participants of non-childbearing potential must be either surgically sterile (hysterectomy, bilateral tubal ligation, salpingectomy, and/or bilateral oophorectomy at least 26 weeks before the Screening Visit) or postmenopausal, defined as spontaneous amenorrhea for at least 1 year, with follicle-stimulating hormone in the postmenopausal range at Screening, as per central laboratory reference range. Additional criteria for Phase 2a participants only: * Hybrid SLEDAI ≥ 6 at Screening AND Day 1. Note: only the clinical aspects of hybrid SLEDAI must be confirmed on Day 1; laboratory values that contribute to hybrid SLEDAI do not need to be re-measured prior to dosing on Day 1. Additional criteria for contributions to hybrid SLEDAI are as follows: * Participants with neurological system contribution to hybrid SLEDAI will not be enrolled. * At least 6 points from the hybrid SLEDAI score must be derived from organ system involvement excluding points from oral ulcers, alopecia, or anti-dsDNA auto-antibododies Additional criteria for Phase 2a participants with LN: * Diagnosis of International Society of Nephrology/Renal Pathology Society (ISN/RPS) 2003 Class III or IV LN as evidenced by renal biopsy performed within 2 years prior to or during Screening, either with or without the presence of Class V LN. * Proteinuria (UPCR \> 1.0 gram per gram \[g/g\]), based on a sample from a 24 hour urine collection during Screening. * Estimated glomerular filtration rate (eGFR) ≥ 30 mL/min/1.73 m2 (as calculated by the Chronic Kidney Disease Epidemiology Collaboration formula). * Currently receiving 1 or more immunosuppressive agents that has been stable for dose and route of administration for ≥ 8 weeks prior to baseline. Exclusion Criteria: * Use of IV, intramuscular, intra-articular, or high-potency intralesional corticosteroids within 6 weeks prior to Screening or expectation of requiring parenteral corticosteroids during the study. Exceptions include protocol required pre-medication prior to infusion of budoprutug. * Use of high-potency topical corticosteroid and/or topical agents (immunosuppressant) for skin lesions within 7 days prior to Screening * The following laboratory values: * Absolute neutrophil count \< 1.0 × 109/L. * White blood cell count \< 2.0 × 109/L. * Cluster of differentiation (CD) 19 B-cell count \<80 cells/µL. * Aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) \> 2 × ULN. * Total bilirubin \> 1.5 × ULN, unless due to Gilbert's syndrome. * History of serious or significant infusion reaction associated with rituximab, belimumab, anifrolumab, or other monoclonal antibody therapy. * History of or current diagnosis of active tuberculosis, untreated latent tuberculosis infection (LTBI) or undergoing current treatment for untreated LTBI * History of chronic (i.e., chronic urinary tract infection), recurrent (3 or more of the same type of infection in a 52-week period), latent, or recent serious infection * Any previous use of CD19 targeted therapy (e.g., inebilizumab or cell therapy).