Real Cause of High Level of Urinary β₂-Microglobulin after Renal Transplantation

Dr. Jürgen Steinhoff, Klinik für Innere Medizin, Medizinische Universität zu Lübeck, Ratzeburger Allee 160, D-23538 Lübeck (FRG) Dear Sir, In their article “What is the true cause of high level urinary ß2-microglobulin after renal transplantation’, Nishi et al. [8] did not mention cytomegalovirus (CMV) infection as a reason for urinary ß2-microglobulin (U-ß2-MG) excretion. As reported by Bäckman et al. [1], CMV infections are known to cause high serum levels of ß2-MG. Because this protein has a low molecular weight (11.7 kD), it should be found in the urine during CMV infections. In order to test this hypothesis, we performed a prospective study in 149 patients undergoing kidney transplantation (table 1). In these patients, we determined ß2-MG, im-munoglobulin G (IgG), transferrin (Tf), albumin (Alb), C-reactive protein (CRP) and α‚-MG in 24-hour urine using a highly sensitive immunoluminometric assay [9]. In all CMV infections (n = 42), an isolated U-ß2-MG excretion was detected [4], while in all rejections, a nonselective glomerular pro-teinuria associated with urinary CRP excretion was observed [3, 6]. In ciclosporin-in-duced nephrotoxicity (n = 10), αpmicroglo-bulinuria was a characteristic feature [5]. We did not detect U-ß2-MG in rejections and cyclosporin nephrotoxicity constantly. U-ß2-MG cannot be accepted as a marker of rejection or tubulotoxicity. This protein is not de-tectabel in the urine constantly under these conditions probably because it is not stable in 24-hour urine. The reason why U-ß2-MG is, indeed, measureable in CMV infection constantly is not known. However, ß2-MG may coat CMV [2] and may become stable and detectable by this mechanism. As demonstrated, detection of isolated U-ß2-MG in 24-hour urine allows to diagnose CMV infections after renal transplantation. What Nishi et al. Í81 described as an unknown Table 1. Changes in the concentration of urinary proteins in patients after renal transplantation with cytomegalovirus infections, rejections or acute ciclosporin nephrotoxicity