Systemic lupus erythematosus (SLE) is an autoimmune disease with multifactorial genesis. Recent research suggests a numerical and functional deficit of regulatory T (Treg) cells as an important contributing factor to the pathology seen in SLE. Treg cells play important roles in dampening overt stimulation of effector cells, as seen in many autoimmune diseases. As Treg cells are highly dependent on interleukin-2 (IL-2), application of low doses of IL-2 leads to markedly increased numbers and improved functionality of Treg cells in mice and humans. Several clinical trials investigated the safety of low-dose IL-2 treatment in different autoimmune diseases, including SLE. The trials conducted so far mainly focused on an increase in Treg cells after IL-2 treatment, not evaluating in detail the effects on other immune cells, presumably also playing important roles in the pathogenesis of SLE. For this reason, the investigators of this trial aim to conduct a complete phenotyping of cellular and soluble components in the blood of SLE patients treated with low-dose IL-2. Furthermore, the investigators want to offer this promising treatment to SLE patients in a controlled framework of an investigator initiated clinical trial.