Divisione di Nefrologia e Dialisi, Ospedale Maggiore, IRCCS, Milano, Italy
Keywords: decoy cells; phase-contrast microscopy; polyomavirus BK infection; tubulitis; viral inclusion bodies
Renal allograft recipients are at risk of reactivation of polyomavirus BK (PVBK). When reactivated, the virus proliferates within the nuclei of the cells of the uroepithelium and of renal tubules with a possible severe derangement of the renal function and allograft frequent loss [15].
The most frequent disorder caused by PVBK is a nephropathy characterized by tubular changes often associated with interstitial inflammation and tubulitis, the distinguishing lesion being tubular cells whose nuclei are heavily altered by viral inclusion bodies [15].
The hallmark of the PVBK infection is the presence in the urine of so-called decoy cells, which are infected cells shed into the urine from the renal tubules (especially the distal segments and the collecting ducts) and uroepithelium (especially the superficial layers) [5].
In the experience of some authors, the higher the number of decoy cells in the urine, the greater the probability of finding the PVBK nephropathy [2]. In contrast, other investigators have reported that even when the decoy cells are abundant, the positive predictive value is as low as 27% [5], which indicates that decoy cells may be present without any clinical disease. Patients at higher risk of developing PVBK nephropathy are those who have decoy cells in the urine in association with impaired renal function and repeated episodes of acute rejection, and are under treatment with tacrolimus and/or mycophenolate mofetil [5].
Usually decoy cells are identified by Papanicolaou stain on fixed urine that has been either smeared or cytocentrifuged [2,5]. Other techniques are identification of specific PVBK antigens by immunocytochemistry [4,5], or electron microscopy, which shows typical viral particles with a diameter of 45 nm within the nuclear inclusions [25].
By Papanicolaou stain, most decoy cells show a much enlarged nucleus, which is occupied by a basophilic inclusion surrounded by chromatin that confers a ground-glass or gelatinous appearance. Sometimes the nuclear inclusion has a vesicular aspect, or it may be surrounded by a halo, and the chromatin may be clumped [2,5]. When decoy cells derive from the uroepithelium, the heavily enlarged and altered nuclei as well as the irregular shape of the cell body can mimic the changes observed in neoplastic cells [2,5].
We show here how decoy cells can be easily identified by phase-contrast microscopy. The images presented here (Figure 1) were observed in October 2000 in the urine of a 31-year-old Italian man (with congenital obstructive uropathy), who in June 1999 had received a kidney from his mother. Four months later, while receiving tacrolimus and mycophenolate mofetil, the patient developed a progressive renal dysfunction, with serum creatinine rising slowly to 4.3 mg/dl. The presence of PVBK was suspected after the finding of many decoy cells (more than one per high-power field, at x400) by phase-contrast microscopy, and was later confirmed by the renal biopsy findings (focal acute tubular necrosis, many tubular cells with typical nuclear inclusion bodies, interstitial inflammation, and tubulitis), and by the finding, by polymerase chain reaction, of PVBK DNA in the patient's plasma (courtesy of the Institute of Virology, University of Pavia).
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Notes
Correspondence and offprint requests to: Giovanni B. Fogazzi, Divisione di Nefrologia e Dialisi, Ospedale Maggiore, IRCCS, Via Commenda 15, I-20122 Milano, Italy.
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