Safety of Belimumab in People With Idiopathic CD4 Lymphopenia and Autoantibodies (Phoebe)

Background: People with Idiopathic CD4 lymphopenia (ICL) have lower numbers of a type of white blood cell called CD4 cells. White blood cells fight against infections. Low levels of CD4 cells may make a person more likely to get sick. There are no approved treatments for ICL. Researchers think a drug called belimumab may be able to help in specific situations. Objective: To see if belimumab is safe for people with ICL. Eligibility: People ages 18-70 who have ICL and are participating in NIH protocol 09-I-0102 (EPIC) Design: Participants will be screened with: Medical and medication history Physical exam Questionnaire about mental health and depression Blood and urine tests Participants will have a baseline visit. This will include some repeats of the screening tests. They may also have leukapheresis: Blood will be taken from a needle in one arm and passed through a machine that separates out the white blood cells. The rest of the blood will be returned through a needle in the other arm. Participants will receive 8 doses of belimumab through IV: A needle will insert a thin plastic tube into an arm vein. Belimumab will be given through the IV line. The first 3 doses will be given every 2 weeks. The other 5 will be given once every 4 weeks. Participants will have a physical exam and blood and urine tests at each dosing visit. They will be monitored for up to 4 hours after the infusion. Participants will have 3 follow-up visits, at around 8, 16, and 24 weeks after the last dose of belimumab. They will have a physical exam and blood and urine tests. Once they finish this protocol and they will continue to be followed under 09-I-0102 (EPIC study).