A randomized, double-blind, placebo-controlled, single-ascending dose, phase Ia study to evaluate the safety, tolerability, pharmacokinetic, pharmacodynamics, and immunogenicity of STSA-1301 Subcutaneous Injection in healthy subjects.

Inclusion Criteria:

1. Healthy subjects, male and female, aged between 18 and 65 years, inclusive;
2. Body mass index: 18.0～28.0 kg/m2, inclusive; weight ≥50 kg for males and ≥45kg for females at enrollment;
3. Subjects (including their partners) agree to take highly effective contraceptive measures during the study, and they have no birth plan or sperm donation plan within 3 months after the end of the study;
4. Medical histories, physical examinations, laboratory examinations and study-related examinations and tests of the subjects show normal results or mild abnormalities with no clinical significance before enrollment, and the Investigator judges that they are eligible;
5. Subjects are aware of the risks of the study, and voluntarily participate in the clinical study and sign an informed consent form (ICF).

Exclusion Criteria:

1. Pregnant or lactating women;
2. History of cardiovascular, respiratory, kidney, liver, metabolism, endocrine, gastrointestinal, blood, nerve, skin and mental illness, cancer or other major disease that in the judgment of the Investigator might put the subject as risk on this study.
3. After splenectomy or any major surgery within 6 months prior to screening;
4. Subjects have an active infection or have a serious infection (leading to hospitalization or requiring parenteral antibiotic therapy) within 6 weeks prior to the first dose; subjects with clinically active or chronic uncontrolled bacterial, viral or fungal infections at screening;
5. Total IgG was less than the lower limit of normal at screening. Subjects with absolute neutrophil count <1.5X109/L and/or absolute lymphocyte count <1.0X109/L;
6. Subjects have a history of malignancy;
7. Subjects who are allergic to this product or any of its ingredients, history of eczema, asthma or other allergic diseases;
8. Subjects are TIGRA (T cell interferon gamma release assay) positive at screening. If TIGRA is not available, a PPD skin test may be used instead and chest imaging performed at screening showing evidence of latent/active tuberculosis (TB);
9. Positive screening test results for human immunodeficiency virus (HIV) antibodies, syphilis specific antibody, hepatitis B surface antigen (HBsAg) or hepatitis C antibody (Anti-HCV);
10. Presence of electrocardiogram (ECG) abnormalities during the screening period, as defined by an Investigator;
11. Subjects have any conditions affecting blood collection;
12. Subjects whose daily consumption of coffee, tea and/or cola is more than 750 mL in the last 3 months before enrollment;
13. Subject who have nicotine consumption more than 5 cigarettes or the equivalent amount of tobacco per day within 3 months prior to screening;
14. Subjects whose daily consumption of alcohol at the time of screening or at any time within the prior 6 months is more than 2 standard drinks, where 1 standard drink = 360 mL or 12 oz (1 can) of regular-strength (5%) beer; 150 mL or 5 oz wine; 45 mL or 1.5 oz liquor/spirits (40%); abnormal alcohol test results at screening or baseline;
15. History of drug abuse within 1 year prior to screening; subjects with a positive urine drug abuse screen at screening or baseline;
16. Blood loss or donation>400mL within 3 months prior to screening or history of transfusion or use of any blood products within 3 months prior to enrollment;
17. Participation as a subject in any drug or vaccine or medical device clinical trial within 3 months prior to screening;
18. Vaccination or planned vaccination within 4 weeks prior to screening to 3 months after end of dosing;
19. Subjects who have taken drugs that may affect immune function within 6 months before screening, have received any monoclonal antibody or biological agent for treatment within the previous 3 months, and have previous treatment with any prescribed medications, over-the-counter (OTC) medications, herbal medicines or other supplements within 14 days prior to screening;
20. Subjects with any factors that would, in the Investigator's judgment, preclude them from participating in this study.