Nephritis, Lupus

The goal of this clinical trial is to learn if cenerimod, on top of regular treatment, works to treat active lupus nephritis in adults with systemic lupus erythematosus and active lupus nephritis. It will also learn about the safety of cenerimod. The main questions it aims to answer are: * Does cenerimod improve kidney function in participants? * What medical problems do participants have when taking cenerimod? Researchers will compare cenerimod to a placebo (a look-alike substance that contains no drug) to see how well cenerimod works when it is added to regular treatment. Participants will: * Take cenerimod or a placebo every day for 76 weeks (approximately 1.5 years), on top of regular treatment. * Visit the clinic every 1 to 3 months for checkups and tests.