Systemic lupus erythematosus (SLE) is a chronic disorder characterized by profound immune and metabolic disturbances. It emerges due to a complex interplay of genetic and environmental factors. Alteration in immune cell metabolism is another feature of SLE. Mitochondria, serving as the main regulator of cell metabolism, exhibits pronounced dysfunction in SLE patients.

Kidneys are among the most frequently affected organs in SLE, with 30-40% of patients developing lupus nephritis (LN) over the course of their disease. LN patients exhibit varying degrees of renal injury, significantly reducing their survival rate. Usually, LN originates from abnormal immune responses, resulting in the formation and deposition of immune complexes in the kidneys, triggering the release of pro-inflammatory cytokines, cell adhesion molecules, and chemokines, thereby inducing inflammation. Extensive glomerular mitochondrial damage has been reported in kidney biopsies obtained from patients with LN. So, identifying peripheral immune markers of mitochondrial dysfunction may be beneficial in assessing SLE activity and the extent of organ involvement.

Among these markers, fibroblast growth factor 21 (FGF-21) is one of the circulating immune markers which is expressed in response to mitochondrial stress. FGF-21 is known to be primarily produced in the liver, but under stress conditions, it can also be produced by the kidneys. In addition, it correlates with the degree of renal impairment in various kidney diseases.

Inclusion Criteria:

- 1- Patients who will be diagnosed as SLE according to the 2019 EULAR/ACR Classification criteria for Systemic Lupus Erythematosus.

2-SLE Patients > 18 years old.

Exclusion Criteria:

• 1-SLE Patients < 18 years old. 2- Patients with other rheumatic diseases or overlap syndromes. 3- Patients with malignancy. 4- Patients with liver diseases.

• 50 SLE patients with lupus nephritis
• 25 SLE patients without lupus nephritis
• 20 healthy controls