Histopathological and clinical correlation in polyomavirus nephropathy in renal transplantation: An application of BANFF 2019 classification update

Aim: Polyomavirus nephropathy (PVN) develops in 1-10% of kidney transplant patients in the first two years after transplantation. Graft loss rates can approach 50% in patients with PVN. Although significant improvements have been made in graft survival through the knowledge gained in kidney transplantation in the last two decades, PVN still remains an important cause of graft loss. On the other hand, determining the prognosis of the disease can be a guide in clinical follow-up and treatment selection. There are various classifications proposed in the literature for histological grading of PVN and predicting the clinical progression. The previous classifications could not be applied in multicenter studies and were not widely accepted. In line with the results of the large-scale study of the Banff Working Group published in 2018, a new PVN classification was presented at the 2019 Banff Conference and recommended for use. In this study, our aim is to reveal the histomorphological findings and clinical characteristics of PVN, to investigate the relationships between various histological parameters and clinical progression, and to evaluate the success of the 2019 Banff PVN Classification to predict the prognosis of the disease. Material and Methods: Ninety-nine biopsies of 60 consecutive patients who were diagnosed with PVN via biopsy between 2002 and 2020 in the Department of Pathology of Istanbul Faculty of Medicine were evaluated. All lesions were scored according to the current Banff 2019 Classification and all cases were classified according to the 2019 Banff PVN Classification. In accordance with the classification criteria, PVN classes were created by combining Banff interstitial fibrosis score (ci) with the viral load (pvl) which determined by evaluating the proportion of morphological viral cytopathic changes and immunhistochemical SV40 positivity. In addition, a classification model was presented by us by combining pvl score with inflammation score in the areas of interstitial fibrosis/tubular atrophy (i-IFTA). Proportion in the biopsy and frequency in the study group of the other morphological findings were also revealed. Intensity and pattern of SV40 staining were evaluated. Histological findings were compared with clinical parameters obtained from patients' files. Banff PVN classes and PVN classes created according to our model were compared in terms of clinical presentation, prognosis and graft survival. Conclusion: PVN, not uncommonly, may develop in the first 6 months after transplantation. Since a significant number of cases do not have morphological viral cytopathic changes, SV40 immunohistochemical examination should be routinely applied to each biopsy. In our study, the rate of graft loss was high as 50%. Histological parameters can predict graft loss. Banff 2019 PVN Classification was successful in determining clinical presentation, prognosis and graft survival in our study. In addition to the current classification, evaluation of i-IFTA score for graft survival in future studies may contribute to the PVN grading.