A 19-year-old female presents with remittent fever for 7 days and symmetric edematous erythema on both cheeks for 2 days, with erythema on fingertips and periungual areas. Laboratory tests: Hb 90 g/L, WBC 3.4×10<sup>9</sup>/L, urine protein (+++), 0-2 casts/HP, ANA(+), LE cells (+). Diagnosed with SLE. What is the first-line treatment?