Interstitial lung disease (ILD) is a common pulmonary manifestation in chronic tissue diseases (CTD), significantly affecting patient's prognosis.

The main purpose of this study is to evaluate the efficacy of telitacicept compared with placebo in slowing down the decline in lung volume in patients with interstitial lung disease associated with connective tissue disease (CTD-ILD) on the basis of standard treatment.

Inclusion Criteria:

1. Voluntary informed consent provided;
2. Male or female aged ≥ 18 years old;
3. Documented diagnosis of rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), idiopathic inflammatory myopathy (IIM), Sjogren's syndrome (SjD) , Systemic sclerosis（SSc）,or mixed connective tissue disease (MCTD) in accordance with internationally recognized classification criteria;
4. Diagnosis of ILD on High Resolution Computed Tomography (HRCT) with disease extent of greater than or equal to (≥) 10% of the whole lung (WL-ILD);
5. During screening, FVC%Pred ≥ 45%;
6. During screening, DLCO%Pred(corrected for hemoglobin) ≥ 30%;
7. Stable standard treatment was received before randomization to control ILD and/or connective tissue disease;

Exclusion Criteria:

1. Diagnosis of Interstitial lung disease other than CTD-ILD;

2. ILD progresses rapidly within 12 weeks before screening or during screening;

3. During screening, HRCT showed severe emphysema (the degree of emphysema exceeded that of ILD);

4. Obstructive pulmonary disease (pre-bronchodilator Forced Expiratory Volume (FEV1) /FVC <0.7);

5. Pulmonary arterial hypertension requiring therapy, as determined by the investigator;

6. Having diffuse alveolar hemorrhage (DAH) or other pulmonary conditions that may have confounding effects, as well as related signs or symptoms;

7. Unable to complete the pulmonary function test, or requiring supplementary oxygen supply;

8. Clinically significant laboratory abnormalities;

9. QTc interval prolongation on ECG;

10. Allergy to human or mouse-derived biological products, or history of other drug allergies, and the investigator deems that the patient is not eligible to participate.

11. Previous treatments received:

    • Previous or planned organ transplantation or bone marrow transplantation;
    • Plasma exchange or extracorporeal light separation exchange was performed within 6 months before randomization, or a plasma filtration device was used;
    • Any targeted BLyS or APRIL drug treatment was received within 12 weeks before randomization;
    • Biologic therapy was received within 12 weeks or within 5 half-lives of the corresponding drug (whichever is longer) before randomization;
    • B-cell depletion drugs were used within 6 months before randomization;
    • Non-biological systemic immunosuppressive drugs other than standard treatment were used within 4 weeks before randomization;
    • Anti-fibrotic drugs were used within 4 weeks before randomization;
    • Cyclophosphamide treatment was received within 6 months before randomization;
    • Cytotoxic drugs were used within 6 months before randomization;
    • Intravenous or intramuscular glucocorticoids were used within 4 weeks before randomization;

12. Major surgery within 12 weeks prior to screening or planned during the duration of the study；

13. Received or plan to receive any live vaccine within 28 days prior to randomization;

14. Participation in any clinical trial 28 days prior to randomization or within 5 times the half-life of an investigational drug (whichever is longer);

15. has active hepatitis or a history of severe liver disease；

16. Acute or chronic infection requiring treatment;

17. suffered from symptomatic herpes zoster within 12 weeks prior to screening;

18. Active tuberculosis；

19. HIV infection;

20. History of malignant tumors；

21. Significant cardiovascular disease, liver, kidney, respiratory, endocrine or hematologic disease, or other medical conditions that, in the opinion of the investigator, would preclude the subject's participation in the study or require hospitalization during the trial;

22. History of drug or alcohol abuse or dependence；

23. Pregnant or lactating women, and those intending to become pregnant during the trial;

24. Patients considered unsuitable by the investigator to participate in the trial ;