Interleukin-7 (CYT107) Treatment of Idiopathic CD4 Lymphocytopenia: Expansion of CD4 T Cells (ICICLE)

-INCLUSION CRITERIA: 1. Age greater than or equal to 18 years 2. CD4 T cell count less than 300 cells/microL or less than 20% of total T lymphocytes on 2 occasions at least 6 weeks apart (and at the time of screening) in the absence of any illness accounting for CD4 lymphocytopenia 3. ICL diagnosis that indicates a risk for disease progression, defined as one or both of the following: * CD8 T cell lymphocytopenia (less than 180 cells/microL) * History of opportunistic or otherwise significant infection (e.g., AIDS-defining or highly morbid illnesses, such as Cryptococcus or Mycobacteria infection, severe herpes zoster, JC virus causing progressive multifocal leukoencephalopathy, Kaposi s sarcoma, or severe human papilloma virus condylomata) 4. HIV-1 and HIV-2 seronegativity and below detection of HIV-1 viral load 5. HTLV-1 and HTLV-2 seronegativity 6. Adequate venous access, as determined by the study team, although participants unable to undergo leukapheresis will not be excluded 7. Normal thyroid-stimulating hormone (TSH) 8. Negative serum or urine pregnancy test at time of study enrollment for women of childbearing potential 9. Ability to understand and give informed consent 10. Capacity and willingness to adhere to study procedures, including scheduled follow-up visits 11. Willingness to allow blood and tissue sample storage 12. Established primary care provider EXCLUSION CRITERIA: 1. History of prior cytotoxic, chemotherapeutic, immunosuppressant (e.g., systemic corticosteroids), immunomodulatory (e.g., IL-2, IL-7, interferon-gama), or growth factor therapy within the last 6 months; chemotherapy or immunomodulatory therapy given to treat an underlying disease or condition may be permitted at the protocol team's discretion, provided diagnosis of ICL was established prior to starting this treatment. The treatment has been stable for over 6 months and is being administered at stable doses as maintenance therapy. 2. History of prior participation in another investigational intervention study within the last 6 months. Co-enrollment in other NIAID stem cell mobilization protocols will be permitted at the protocol team s discretion, but CYT107 may be dosed no sooner than 6 months after last dose of that protocol s medications have been given. 3. Active uncontrolled opportunistic infection at the time of enrollment 4. Current or recent history (less than 28 days prior to screening) of a viral, bacterial, parasitic or fungal infection requiring hospitalization and/or systemic treatment, other than long-term maintenance pharmacotherapy 5. Serious illness requiring systemic treatment and/or hospitalization within 56 days (8 weeks) of screening, unless the subject is clinically stable, in the opinion of the Principal Investigator, and has completed therapy or has been on appropriate therapy for greater than 28 days (4 weeks) prior to screening 6. Current or history of hematologic or lymphoid (lymphoma) malignancy 7. Established or planned pregnancies or refusal to use effective birth control (e.g., barrier methods, oral contraceptives, intrauterine devices) for the duration of study involvement, regardless of gender 8. Concurrent breastfeeding 9. Renal insufficiency (e.g., estimated glomerular filtration rate less than 60 mL/min/1.73 m(2)) 10. Any of the following screening laboratory abnormalities: platelets less than 100,000 cells/microL; lipase greater than 1.5 times the ULN; AST, ALT, or alkaline phosphatase greater than 2.5 times the ULN; total bilirubin greater than 1.5 times the ULN 11. History of splenectomy or hematologic disease associated with hypersplenism, such as alpha- or beta-thalassemia, hereditary spherocytosis, Gaucher s disease, or autoimmune hemolytic anemia 12. Cirrhosis of any origin, including alcoholic or non-alcoholic steatohepatitis, either suspected by history or histologically proven 13. History of hepatitis B or C infection, i.e., positive hepatitis B surface antigen, positive anti-hepatitis B core antibody with a detectable hepatitis B DNA viral load, positive anti-hepatitis C antibody and/or detectable hepatitis C RNA viral load (subjects who became negative for hepatitis B DNA or hepatitis C RNA following anti-viral treatment will not qualify for study treatment) 14. Need for anticoagulant medication (e.g., warfarin, heparin), other than aspirin, clopidogrel, or other antiplatelet agent as CYT107 may co-precipitate with heparin if taken together. 15. Active drug or alcohol use or dependence that, in the opinion of the investigator, would interfere with adherence to study requirements (subjects must agree to refrain from substance abuse use during the entire course of the study) 16. Past or current psychiatric illness that, in the opinion of the investigator, would interfere with protocol adherence or the ability and willingness to give written informed consent 17. Current autoimmune conditions requiring systemic (oral, injection, or other parenteral) therapy, as well as psoriasis and optic neuritis regardless of treatment and/or a diagnosis of systemic lupus erythematous based on the screening rheumatologic work up. 18. Cardiac, pulmonary, thyroid, renal, hepatic, neurological (central or peripheral) disease or disorder of hemostasis requiring therapy and considered to be significant by the protocol team 19. History of cardiovascular disease, arrhythmias, or clinically significant ECG abnormalities, including a corrected QT interval (QTc) greater than or equal to 470 milliseconds 20. Family history consistent with an inherited cardiomyopathy or arrhythmia such as the following: * Arrhythmogenic Right Ventricular Dysplasia (ARVD) * Brugada syndrome (BrS) * Congenital Long QT (LQT) * Congenital Short QT intervals (SQT) * Early Repolarization Syndrome * Idiopathic Ventricular Fibrillation (IVF) 21. Uncontrolled hypertension (i.e., resting systolic blood pressure greater than160 mmHg or resting diastolic blood pressure greater than 90 mmHg), despite pharmacologic antihypertensive treatment, confirmed with a second blood pressure measurement done later in the same day. 22. Evidence of circulating neutralizing anti-IL-7 antibodies (prior to initial rhIL-7 administration) To be eligible for rhIL-7 administration, all Exclusion Criteria are prohibited. However, for the purpose of performing baseine pre-IL-7 procedures certain Exclusion Criteria designed to minimize specific complications from rhIL-7 (e.g., autoimmune or lymphoproliferative complications) but that do not increase the risk associated with these procedures are permitted, as baseline procedures will be done prior to IL-7 administration: #6, #11, through #13, #17, #18, #20, #22.