The purpose of this study is to determine the safety, pharmacokinetics, and pharmacodynamics of NKTT120 in adult patients with stable sickle cell disease.

Inclusion Criteria:

• Age 18 to 50 years
• Subject has a confirmed diagnosis of HbSS or HbSβ0thal
• Subject has stable SCD defined as not having acute VOC, ACS, or other major SCD associated event during the month prior to enrollment

Exclusion Criteria:

• Subject had an SCD-related VOC or ACS that required hospitalization or treatment in acute care outpatient setting in the month prior to enrollment
• Subject requires a program of prescheduled regularly administered packed red blood cell (pRBC) transfusions or Subject received a pRBC transfusion in the month prior to enrollment
• Subject has evidence of latent or active tuberculosis
• Subject has a major concurrent illness or medical condition
• Subject is pregnant or nursing