Inclusion Criteria:
* Age ≥ 18 years old
* Women of childbearing potential and males must agree to use 2 methods of effective contraception, with at least 1 method being highly effective.
Inclusion Criteria for Severe Aplastic Anemia (SAA) (Cohorts A and B):
* Participants with SAA must have a current or prior diagnosis of SAA or very SAA.
Inclusion Criteria for Refractory Severe Aplastic Anemia (Cohort A) Participants with refractory SAA must have:
* Received one ≥ 3-month course of ATG and/or CSA-based IST.
* Refractory SAA, defined as failure to achieve CR or PR ≥ 3 months after starting ATG and/or CSA-based IST.
Inclusion Criteria for Relapsed Severe Aplastic Anemia (Cohort B):
* Relapsed SAA, defined as relapse following a CR or PR that was achieved ≥ 3 months after starting ATG- and/or cyclosporine A (CSA)-based IST.
Inclusion Criteria for Relapsed or Refractory Transfusion-Dependent Non-Severe Aplastic Anemia (Cohort C):
* Current or prior diagnosis of NSAA
* No current or prior diagnosis of SAA.
* Received at least 1 prior course of IST such as ATG- or CSA, with or without a TPO-R agonist (lasting ≥ 3 months).
* Meets criteria for transfusion dependence (either RBC or platelet):
* RBC transfusion dependence: transfusion of ≥ 2 units of RBCs in the past 56 days
* Platelet transfusion dependence: transfusion of ≥ 1 unit of apheresis platelets in the past 28 days
Participants entering the Extension Treatment Period must meet the following criteria:
* Signed informed consent form (ICF) for the Extension Treatment Period.
* CR or PR by Week 24 during the Main Treatment Period
Exclusion Criteria:
* Diagnosis of Fanconi anemia, dyskeratosis congenita, or other congenital BMF syndrome.
* Prior HCT.
* Planning to receive HCT as treatment for AA.
* Evidence of a clonal disorder with poor risk cytogenetics per Revised International Prognostic Scoring System (IPSS-R) for MDS.
* Diagnosis of PNH or a clonal hematologic bone marrow disorder such as LGLL. Existence of PNH clones, LGLL cells, or clonal hematopoiesis of indeterminate potential (CHIP) clones without a clinical diagnosis is not exclusionary.
* Use of a T-cell depleting agent (e.g., ATG, alemtuzumab, thymoglobulin) within 3 months prior to Day 1.
* Use of a B-cell depleting agent (e.g., rituximab, ocrelizumab, ofatumumab, ublituximab) within 28 days prior to Day 1.
* Use of any of the following within 14 days of Day 1, unless used as an established therapy at screening and there is evidence of either progressive cytopenia or lack of count improvement over the 3 months before screening:
* Calcineurin inhibitor (e.g., cyclosporine), TPO-R agonist (e.g., eltrombopag), Androgen (e.g., danazol), Oral Janus kinase (JAK) inhibitor
* Regardless of their use as an established therapy at screening, use of the above medications is prohibited for all participants from 3 months after Day 1.
* Current infection not adequately responding to appropriate therapy or requiring hospitalization.
* Current uncontrolled or invasive infection with cytomegalovirus (CMV), Epstein Barr virus (EBV), varicella zoster virus (VZV), herpes simplex virus (HSV), or human T-lymphotropic virus-1 (HTLV-1), defined by polymerase chain reaction (PCR) at screening. Note that low level CMV, EBV, or VZV viremia is not exclusionary.
* Human immunodeficiency virus (HIV) infection.
* Current or prior infection with hepatitis B virus (HBV)
* Current hepatitis C virus (HCV) infection
* Latent tuberculosis (TB) infection as indicated by IFN-γ release assay without documentation of appropriate treatment (appropriate therapy as defined by the World Health Organization \[WHO\] and/or the United States Centers for Disease Control and Prevention).
* Estimated glomerular filtration rate \< 30 mL/min/1.73 m2 at screening (using the Chronic Kidney Disease Epidemiology Collaboration formula; Levey 2009).
* Total bilirubin \> 1.5 × upper limit of normal (ULN) (\> 3 × ULN if known Gilbert's disease).
* Aspartate aminotransferase or alanine aminotransferase \> 2.5 × ULN (except in participants with known iron overload).
