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

Background: * Idiopathic CD4 lymphocytopenia (ICL) is a condition in which patients have low levels of T cells, a type of white blood cell that helps fight infection. Animal studies have shown that an experimental drug Interleukin 7 (IL-7), which is named CYT107, can increase the number and function of T cells. CYT107, however, has not been used in people with ICL. Objectives: * To determine the safety of CYT107 in people with ICL. * To determine whether CYT107 will increase the number and function of T cells in people with ICL. Eligibility: * Patients 18 years of age and older diagnosed with ICL and who are at risk of becoming sick because of this condition are eligible for this study. In addition, patients must not be pregnant, or have other illnesses that would cause low CD4 T cell counts, such as human immunodeficiency virus (HIV) or human T-lymphotrophic virus (HTLV) infection. Design: * The initial screening visit will include the following examinations and tests: * A complete physical exam and medical history * Blood analysis, including CD4 T cell count; complete blood count and additional blood tests to determine clotting ability and blood composition; thyroid, liver, kidney, and pancreatic function tests; HIV and HTLV tests; and tests for anti-IL-7 antibodies that block normal IL-7 activity * Routine urine test * Urine or blood pregnancy test for women * Chest X-ray * Electrocardiogram * Spleen ultrasound. * The baseline visit will include blood tests to determine levels of each of the major types of antibodies, a test of genetic background, and more detailed CD4 and protein analysis. In addition, leukapheresis (a procedure to collect large numbers of immune cells without red blood cells) will be done. Participants will also have the option of having colon and lymph node biopsies. * The schedule will be as follows: * Weeks 1, 2, and 3 (Cycle 1): Three weekly IL-7 dosing visits. * Weeks 5, 8, and 12: Follow-up visits. * Weeks 24, 25, and 26 (Cycle 2):