The study is tissue-based, retrospective and prospective, single-centre, non-sponsored.

Primary objective: To examine patient and organ survival and eGFR variation in kidney transplant patients diagnosed histologically with cAMR.

At the kidney transplant follow-up clinic of the UO Nephrology, Dialysis and Transplantation Unit of the IRCCS AOUBO Policlinico di Sant'Orsola, directed by Prof. G. La Manna, all patients with a diagnosis of cAMR made since 01/01/2018 and patients with a new diagnosis of cAMR made after the approval of this protocol by the Ethics Committee will be enrolled.

The study will last 5 years and 6 months.

Inclusion Criteria:

• Age ≥18 years
• Kidney transplant recipients
• eGFR>15 ml/min/1.73 sq m at the time of diagnosis of cAMR
• cAMR diagnosis defined on the basis of the 2018 Banff Classification (17), made after 1/1/2018.
• Obtaining Informed Consent to Participation.

Exclusion Criteria:

• Combined transplant patients (heart-kidney, liver-kidney or kidney-pancreas)
• Histological evidence of concomitant disease (e.g. recurrence of original nephropathy or diagnosis of 'de novo' nephropathy, acute cellular rejection, polyomavirus BK nephropathy, other)

Based on the number of patients attending the Kidney Transplant Follow-up Outpatient Clinic of the Unit of Nephrology, Dialysis and Transplantation Prof. La Manna of the Policlinico di S. Orsola, about 50 patients will be enrolled, of whom about 25 have already been diagnosed and are undergoing treatment and about 25 new diagnoses are expected during the enrolment period in the prospective-only component.