This will be a combined retrospective and prospective cohort study, that will evaluate the incidence of de novo Systemic Lupus Erythematosus major organ manifestations (defined as BILAG A flares) in patients receiving belimumab (Arm A) and compare it to 2 standard-of-care groups (SoC) (Arm B: patients on SoC; Arm C: patients on SoC followed-up up to May 1st 2014, the first date where belimumab was available in Greece).

The investigators will utilize survival analysis methods (Kaplan-Meier survival curves and Cox regression) and mixed effects longitudinal analyses. Additionally, the investigators will employ propensity score matching and/or inverse probability of treatment weighting, to create balanced cohorts and reduce bias.

Belimumab (BEL) has been used for the treatment of SLE for over 10 years, with clear evidence for a beneficial effect in reduction of disease activity and frequency of flares, as well as prevention of damage accrual.

Whether BEL may also result in prevention of incident, major organ involvement in patients with SLE is less clear. A recent post-hoc analysis of the BLISS trials suggested that BEL (although at the low, rather than standard-dose) reduced the rate of de novo renal flares (defined by BILAG) compared to standard-of-care but real-life data are scarce. Importantly, data on the ability of belimumab to decrease major neuropsychiatric events in SLE such as strokes, seizures and cognitive dysfunction or other severe manifestations such as severe hematologic or cardiopulmonary disease is not known.

To this end, the investigators propose a propensity score-matched, real-world, with the use of longitudinal data from large patient cohorts and time-to-event analyses. Given the non-experimental setting of this study, the investigators will also employ propensity score matching (PSM), to create balanced cohorts of BEL-treated and non-BEL-treated patients and reduce bias in estimating treatment efficacy.

The study will consist of 3 arms, as follows: Arm A: patients on treatment with belimumab; Arm B: patients on standard-of-care (SoC) treatment from May 1st 2014 onwards; Arm C: a historical cohort of patients on SoC followed-up up to May 1st 2014, the first date where belimumab was available in Greece.

Inclusion Criteria:

• Systemic lupus erythematosus classification according to the Systemic Lupus International Collaborating Clinics (SLICC) criteria
• Age ≥ 18 years old
• Time of follow-up ≥ 6 months
• Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K) ≥ 4
• Physician Global Assessment (PGA) visual analogue score > 1

Exclusion Criteria:

• Patients with incomplete medical records or missing key variables
• Patients with concomitant autoimmune disorders (excluding thyroid disease)

Adult systemic lupus erythematosus patients, receiving belimumab (Arm A) or standard-of-care treatment (Arms B and C).

The investigators are not currently planning on sharing Individual participant data (IPD). This decision is based on the fact the investigators have not made specific agreements with our hospital's IRB about individual participant data sharing. Study results will be disseminated through peer-reviewed publications, which will provide comprehensive findings while maintaining participant confidentiality. However, upon reasonable requests from qualified researchers for legitimate scientific purposes, the investigators may consider sharing de-identified IPD on a case-by-case basis, after approval by our IRB.