A 30-year-old woman presents with bilateral knee joint swelling and pain for 6 months, now with fever and facial edema. Physical exam: temperature 38°C, facial edema, oral ulcers, liver palpable 1 fingerbreadth below the costal margin, spleen palpable 2 fingerbreadths below the costal margin, bilateral knee joint swelling with erythema, swelling, and tenderness, BP 150/90 mmHg (20/12 kPa). Urinalysis: proteinuria (-), ANA (+), anti-Smith antibody (+), diagnosed with SLE. One year later, follow-up shows urinary protein (++), red blood cells 10-15/HPF, BUN 10 mmol/L, Cr 180 μmol/L, and renal biopsy reveals mesangial proliferative lupus nephritis. The first-line treatment at this stage is: