Study to Evaluate Safety and Efficacy of Benralizumab in Subjects With Hypereosinophilic Syndrome

* INCLUSION CRITERIA: A subject will be eligible for participation in the study only if all of the following criteria apply: 1. Male or female subject greater than or equal to18 and less than or equal to 75 years of age at screening. 2. A female subject is eligible for this study only if she is not pregnant or breast-feeding and one of the following: 1. Of childbearing potential but agrees to practice effective contraception, as determined by the PI, or abstinence throughout the study and for 3 months after administration of the last dose of investigational study drug 2. Of non-child-bearing potential Females of non-child-bearing potential are defined as females with functioning ovaries with a documented history of tubal ligation or hysterectomy or females who are post-menopausal, as defined by 12 months of spontaneous amenorrhea with an appropriate clinical profile, e.g. age appropriate, \>45 years, in the absence of hormone replacement therapy. In questionable cases, a blood sample for follicle stimulating hormone and estradiol will be obtained to confirm child-bearing potential. Acceptable methods of contraception may include one or more of the following: 1\) male partner who is sterile prior to the female subject s entry into the study and is the sole sexual partner for the female subject; 2) implants of levonorgestrel; 3) injectable progestogen, 4) an intrauterine device with a documented failure rate of \<1%; and 5) double barrier methods including diaphragm or condom with a spermicide. 3\) A male subject is eligible for this study only if he is one of the following: 1. Surgically sterile 2. Agrees to practice effective contraception (see above) or abstinence throughout the study and for 3 months after the last administration of the investigational study drug 4\) Documented diagnosis of HES (history of persistent eosinophilia \>1500/(micro) L without secondary cause and evidence of end organ manifestations attributable to the eosinophilia) 5\) Signs or symptoms of HES and AEC \>1000/(micro) L on stable HES therapy for greater than or equal to 1 month at the time of enrollment 6\) Participation in protocol 94-I-0079 (Activation and function of eosinophils in conditions with blood or tissue eosinophilia) 7\) Agrees to storage of samples for study Participation of Women: Contraception: The effects of benralizumab on the developing human fetus are unknown. For this reason, men and women of childbearing potential must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry and for the duration of study participation. Females of childbearing-age must have a negative pregnancy test result prior to receiving benralizumab. During the course of the study, if a woman becomes pregnant or suspects she is pregnant, she should inform the study staff and her primary care physician immediately. If a subject becomes pregnant, the investigational drug will be discontinued immediately, and the subject will be counselled as to how to resume approved therapeutic options in consultation with an obstetric provider. EXCLUSION CRITERIA: A subject will be excluded from participation in the study if any of the following criteria apply at the time of enrollment: 1. Subjects with life-threatening or other serious illness or clinical manifestation of HES deemed inappropriate for inclusion in study per the principal investigator, including but not restricted to severe cardiac involvement and prior thromboembolic disease. 2. Human immunodeficiency virus (HIV) or other known immunodeficiency 3. Positive hepatitis B surface antigen, or hepatitis C virus antibody serology, or a positive medical history for hepatitis B or C. Patients with a history of hepatitis B vaccination without history of hepatitis B are allowed to enroll 4. Presence of FIP1L1/PDGFRA or another known imatinib-sensitive mutation 5. Diagnosis of systemic mastocytosis or serum tryptase level \>40 ng/mL 6. Known lymphoma, hematological malignancy, advanced and metastatic solid tumors and/or subjects who are under chemotherapy, radiotherapy or interleukin 2 treatment 7. Known history of allergic or anaphylactic reaction to previous antibody therapy, including intravenous immunoglobulin and licensed or experimental monoclonal antibodies. 8. A helminth parasitic infection diagnosed within 24 weeks prior to the date informed consent is obtained 9. Acute bacterial or viral infection (subjects may be enrolled once the acute infection has resolved). 10. Receipt of intravenous immunoglobulin (IVIG) within 30 days prior to the date informed consent is obtained 11. Receipt of any marketed (eg omalizumab) or investigational biologic within 4 months or 5 half-lives prior to the date informed consent is obtained, whichever is longer 12. Receipt of live attenuated vaccines 30 days prior to the date of randomization 13. Receipt of inactive/killed vaccinations (e.g. inactive influenza) are allowed provided they are not administered within 1 week before/after any study visit 14. Receipt of any investigational nonbiologic within 30 days or 5 half-lives prior to the date informed consent is obtained, whichever is longer 15. History of alcohol or drug abuse within 12 months prior to the date informed consent is obtained 16. Previous treatment with benralizumab (MEDI-563). Co-enrollment Guidelines: Co-enrollment in other trials is restricted, other than enrollment on observational studies or those evaluating the use of a licensed medication. Study staff should be notified of co-enrollment as it may require the approval of the Investigator. Justification for Exclusion of Children: Because there are insufficient data regarding dosing or adverse events available in adults with HES to judge the potential risk in children, children are excluded from this study.