This study investigates the efficacy and safety of belimumab compared to placebo, in addition to standard therapy, for the treatment of participants with systemic sclerosis associated interstitial lung disease (SSc-ILD). The study will evaluate the effect of belimumab treatment on lung function as well as on extra-pulmonary disease manifestations, including skin thickening and general symptoms, such as fatigue, that impact quality of life (QoL).

Inclusion Criteria:

1. Participant is 18 years of age inclusive, or older at the time of signing the informed consent.

2. Documented diagnosis of SSc as defined by the American College of Rheumatology / European League Against Rheumatism 2013 SSc classification criteria.

3. Diffuse cutaneous disease, defined as presence of thickened skin with mRSS >0 over at least one skin area proximal to elbows and/or knees in addition to distal areas involvement on Day 1.

4. Total mRSS ≥15 on Day 1.

5. Evidence of interstitial lung disease on centrally read screening HRCT.

6. Anticentromere antibody negative on central test at screening.

7. Evidence for active or progressive disease

8. Participant has an area of uninvolved or mildly thickened skin that, in the opinion of the investigator, would allow SC injection at the abdomen or the front, middle region of the thigh.

9. Participant is capable and willing to self-administer the study medication or has a caregiver who is capable and willing to administer the study medication throughout the study.

10. A female participant is eligible to participate if she is not pregnant or breastfeeding, and one of the following conditions applies:

    Is a Woman of Non-Childbearing Potential (WONCBP) OR Is a Woman of Childbearing Potential (WOCBP) and using a contraceptive method that is highly effective.

11. Capable of giving signed informed consent.

Exclusion Criteria:

1. Systemic sclerosis-like illness, including but not limited to localized scleroderma (morphoea), eosinophilic fasciitis, sclerodermoid graft-versus-host disease, fibro mucinous conditions (scleroedema, scleromyxoedema), scleroderma-like conditions that are associated with environmental chemical and drug exposure (e.g., toxic rapeseed oil, vinyl chloride, bleomycin, gadolinium-based contrast agents [nephrogenic systemic fibrosis], or due to metabolic disease).
2. Primary diagnosis of a rheumatic autoimmune disease other than dcSSc, including but not limited to rheumatoid arthritis, systemic lupus erythematosus, polymyositis, dermatomyositis, systemic vasculitis, Sjogren's syndrome, antisynthetase syndrome, or mixed connective tissue disease, as determined by the investigator.
3. FVC ≤45% of predicted, or a DLco (corrected for hemoglobin) ≤40% of predicted or requiring supplemental oxygen at screening.
4. Pulmonary arterial hypertension, as determined by the investigator at, or prior to first day of dosing (Day 1).
5. SSc renal crisis within 6 months prior to the first day of dosing (Day 1).
6. History or presence of cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrine, hematologic, or neurological disorders capable of significantly altering the absorption, metabolism, or elimination of drugs; constituting a risk when taking the study intervention or interfering with the interpretation of data.
7. Obstructive pulmonary disease (pre-bronchodilator FEV1/FVC <0.7).
8. Significant emphysema on screening HRCT (extent of emphysema exceeds extent of ILD).
9. Previous or planned major organ transplant (e.g., heart, lung, kidney, liver) or bone marrow transplant (e.g., autologous stem cell transplant).
10. Treatment with biologic agents, such as intravenous immunoglobulin or monoclonal antibodies, including marketed drugs, within 3 months or 5 half-lives (whichever is longer) prior to dosing.
11. Treatment with rituximab within 6 months prior to Day 1.
12. Treatment with non-biologic systemic immunosuppressive medication, other than mycophenolate, methotrexate or azathioprine (including, but not limited to cyclosporine A, tacrolimus, leflunomide, oral or parenteral gold, Janus kinase (JAK) inhibitors) within 3 months prior to Day 1.
13. Treatment with cyclophosphamide (oral or intravenous) within 6 months prior to Day 1.
14. Use of anti-fibrotic agents including colchicine, D-penicillamine, pirfenidone or tyrosine kinase inhibitors (e.g., nintedanib, nilotinib, imatinib, dasatinib) within 4 weeks prior to Day 1.
15. Cytotoxic drugs such as, chlorambucil, nitrogen mustard, or other alkylating agents within 6 months of Day 1.
16. Treatment with IM or IV corticosteroids within 1 month prior to Day 1.

Qualified researchers may request access to anonymized individual patient-level data (IPD) and related study documents of the eligible studies via the Data Sharing Portal. Details on GSK's data sharing criteria can be found at: https://www.gsk.com/en-gb/innovation/trials/data-transparency/

Anonymized IPD will be made available within 6 months of publication of primary, key secondary and safety results for studies in product with approved indication(s) or terminated asset(s) across all indications.

Anonymized IPD is shared with researchers whose proposals are approved by an Independent Review Panel and after a Data Sharing Agreement is in place. Access is provided for an initial period of 12 months but an extension may be granted, when justified, for up to 6 months.

This is a double-blind study where participant and investigator are masked.