⁃ ALL COHORTS:

⁃ Ability of the participant or legally authorized representative (LAR) to understand and be willing to sign a written informed consent document

⁃ Age 18 or older

⁃ For females of childbearing potential, stated willingness to use an accepted method of contraception for the duration of the study. Accepted methods of contraception are:

∙ Total abstinence

‣ Use of an implanted or intrauterine hormonal device for at least 30 consecutive days before study drug administration

‣ Use of oral, patch or injectable contraceptives or a vaginal hormonal device for at least 30 consecutive days before study drug infusion

‣ Use of a non-hormonal intrauterine device for at least 30 consecutive days before study drug administration

‣ Two barrier methods such as a diaphragm with spermicide or a condom with spermicide

⁃ For sexually active males with a female partner of childbearing potential, stated willingness to agree to use a condom with spermicide for the duration of the study.

⁃ Diagnosis of immune bone marrow failure (see specific cohort)

⁃ COHORT 1: RELAPSED/REFRACTORY SAA:

⁃ Meet all 3 criteria below:

⁃ Severe aplastic anemia\*:

• Bone marrow cellularity \<30% excluding lymphocytes

⁃ AND

⁃ At least two of the following:

⁃ Absolute neutrophil count \< 0.5 x 10\^9/L

⁃ Platelet count \< 20 x 10\^9/L

⁃ Absolute Reticulocyte count \< 60 x 10\^9/L

∙ Relapsed or refractory disease as evidenced by a course of at least 1 prior therapy.

‣ Not suitable for transplant due to age, co-morbidities, lack of suitable donor, or participant choice.

• Patients who have a documented historic diagnosis of SAA and have received an ATG-based therapy in the past and are now relapsed / refractory may be included in this cohort even if documentation of original CBC and bone marrow are unavailable.

⁃ COHORT 2: RELAPSED/REFRACTORY MODERATE AA:

⁃ Moderate AA:

⁃ Aplastic anemia (hypocellular bone marrow for age) with no evidence for other disease processes causing marrow failure, and depression of at least two out of three blood counts below the normal values but not fulfilling the criteria for SAA:

∙ Absolute neutrophil count \<= l.2 x 10\^9/L

‣ Platelet count \<= 70 x 10\^9/L

‣ Anemia with hemoglobin \<= 9 g/dL and absolute reticulocyte count \< 60 x 10\^9/L or transfusion dependence

⁃ Relapsed or refractory disease as evidenced by a course of at least 1 prior therapy.

⁃ COHORT 3: RELAPSED/REFRACTORY UNILINEAGE BONE MARROW FAILURE DISORDERS:

⁃ Cytopenia in lineage as below:

• Erythroid lineage: Hemoglobin \<= 9 g/dL and reticulocyte count \< 60 x 10\^9/L or red cell transfusion dependence and hypocellular bone marrow for age with absent or reduced red cell precursors

⁃ OR

⁃ Platelet lineage: Thrombocytopenia \<= 30 x 10\^9/L or platelet transfusion dependence and hypocellular bone marrow for age with absent or reduced megakaryocytes

⁃ OR

⁃ Granulocyte lineage: Neutropenia \<= 0.5 x 10\^9/L and hypocellular bone marrow for age with absent or reduced granulopoiesis

⁃ No evidence of viral or drug suppression of the marrow, T-LGL, dysplasia, or underproduction anemias secondary to B12, folate, iron or other reversible causes.

⁃ Relapsed or refractory disease as evidenced by a course of at least 1 prior therapy.

⁃ COHORT 4: RELAPSED/REFRACTORY T-LGL WITH CYTOPENIAS:

⁃ Clinical history supportive of the diagnosis of T-LGL leukemia (i.e., a history of cytopenias with peripheral blood morphologic evidence of LGLs).

⁃ Immunophenotypic studies of peripheral blood showing an increased population of T-LGLs (suggested by staining with CD3+, CD8+ and CD16+ or CD57+) or gamma-delta T cells.

⁃ Restricted or clonal rearrangement of the T-cell receptor by PCR

⁃ AND cytopenia as follows:

⁃ Severe neutropenia (\< 0.5 x 10\^9/L);

⁃ OR

⁃ Severe thrombocytopenia (\<= 20 x 10\^9/L), or moderate thrombocytopenia (\<= 50 x 10\^9/L) with active bleeding;

⁃ OR

⁃ Symptomatic anemia with a hemoglobin \<= 9 g/dL or red blood cell transfusion dependence

• Relapsed or refractory disease as evidenced by a course of at least 1 prior therapy.

⁃ COHORT 5: HYPOPLASTIC MDS:

• A diagnosis of hypoplastic MDS by WHO 2016, WHO 2022, or ICC criteria with significant cytopenias defined as:

⁃ Bone marrow hypocellular for age

⁃ AND

⁃ Either morphologic dysplasia or cytogenetic abnormality

⁃ AND

⁃ At least one of the following:

⁃ Neutropenia: Absolute neutrophil count \< 0.5 x 10\^9/L

⁃ Thrombocytopenia: Platelet count \< 30 x 10\^9/L or platelet transfusion dependence

⁃ Anemia: Hemoglobin \< 9g/dL or red cell transfusion-dependence or absolute reticulocyte count \<60 x 10\^9/L

∙ Relapsed or refractory disease as evidenced by a course of at least 1 prior therapy