A 45-year-old man has been suffering from ankylosing spondylitis for 15 years. For the last 3 years he has been noticing facial swelling and edemas of the limbs. Objectively, he assumes a "beggar's" position. X-ray shows "bamboo spine" changes in the thoracic and lumbar segnients. Heart ultrasound shows aortic regurgitation. Complete blood count: Hb-106 g/L; leukocytes - 8.9 × 109/L; ESR - 40 mm/hour. Daily proteinuria 9.6 grams per 24 hours. Blood creatinine -230 mcmol/L. What is the cause of kidney failure in this case?