AB-101 (also known as AlloNK) is an off-the shelf, allogeneic cell product made of natural killer cells, also called NK cells. White blood cells are part of the immune system and NK cells are a type of white blood cell that is known to enhance the effect of monoclonal antibody therapies. This clinical trial will enroll adult patients with lupus nephritis Class III or IV either with or without the presence of Class V who relapsed or did not respond to previous standard of care treatment approaches, or other forms of refractory systemic lupus erythematosus. The primary objective is to assess the safety, tolerability and preliminary activity of AB-101 plus a B-cell depleting mAb (e.g., rituximab, obinutuzumab) after cyclophosphamide and fludarabine in adult subjects with relapsed/refractory lupus nephritis Class III or IV, with or without the presence of Class V, or other forms of refractory systemic lupus erythematosus. Patients will be assigned to receive either AB-101 alone as monotherapy or in combination with a B-cell depleting mAb (e.g., rituximab, obinutuzumab). All patients will receive at least 1 treatment cycle of AB-101, followed by scheduled assessments of overall health and response status. Patients may receive up to 2 cycles of treatment spaced 24 weeks apart.