A Single-arm, Dose-escalation Trial of Long-acting Recombinant Human IL-7 (NT-I7, Efineptakin Alfa) for Idiopathic CD4 Lymphopenia

Background: Idiopathic CD4 lymphopenia (ICL) is a syndrome characterized by low levels of certain immune cells called CD4 T cells. The low CD4 T cells renders people with ICL prone to many types of severe infections, autoimmune diseases, and cancers. Although these infections and diseases can be treated whenever occur, there is currently no treatment that targeting the underlying deficiency of CD4 T cells can provide a definitive treatment for people with ICL. Objective: To test a new drug (NT-17) in people with ICL which can increase the number of CD4 T cells Eligibility: People aged 18 to 75 years with ICL who are also enrolled in NIH protocol 09-I-0102. Design: Participants will be screened. They will have a physical exam and blood tests. Some participants with high suspicion of central nervous system infection or history of such infections may also undergo a lumbar puncture. A thin needle will be inserted into their lower back to draw out a sample of the fluid around their spinal cord. Participants will receive 3 doses of NT-17, each about 12 weeks apart. NT-17 is injected into the muscle of the upper arm, thigh, or buttock. They will visit the clinic 5 days before each dose and again 2 and 4 weeks after each dose. Blood will be drawn at all visits. Participants will undergo leukapheresis 3 times. Blood will be drawn from a needle in one arm. The blood will pass through a machine that separates out the white blood cells. The remaining blood will be given back through a second needle in the other arm. Some visits will include a rectal swab. Some participants may have additional tests, including a skin exam, skin biopsies, and medical imaging. Participants will have 3 follow-up visits every 3 months after they finish treatment.

* INCLUSION CRITERIA: Individuals must meet all of the following criteria to be eligible for study participation: 1. Aged 18 to 75 years. 2. Able to provide informed consent. 3. Co-enrolled in NIH protocol 09-I-0102, Etiology, Pathogenesis, and Natural History of Idiopathic CD4+ Lymphocytopenia (EPIC) study (NCT0086726). 4. Documented ICL, defined as CD4 T-cell count \=45 years plus no menses for 12 consecutive months without an alternative medical cause). EXCLUSION CRITERIA: Individuals meeting any of the following criteria will be excluded from study participation: 1. Current moderate or severe acute illness (eg, febrile illness, seizure, myocardial infarction, cerebrovascular accident, pulmonary embolism) that in the opinion of the study team would make the individual unsuitable for the study. 2. Clinical or microbiologic evidence of active progressive cryptococcal central nervous system (CNS) disease or nontuberculous mycobacterial (NTM) infections within the last year. History of stable cryptococcal CNS disease or NTM diseases since more than 1 year can be enrolled but will need to have undetectable CSF cryptococcal antigen and initiate/maintain antifungal or antimycobacterial treatment, respectively. 3. Pregnant or breastfeeding. 4. HIV infection, chronic hepatitis B or C infection, and any other recognized congenital or acquired immunodeficiency (eg, SCID IL-2/JAK3/ADA, MAGT1, MHC1 deficiency, CVID, DOCK8). 5. Serum creatinine \>1.5 X ULN, platelets \1.5 X ULN (except if due to Gilbert's syndrome), or immunoglobulin (Ig) G level \