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- Materials and Methods
Many aspects of calcium oxalate (CaOx) deposition in renal transplant biopsies are not known. Review of all renal transplant biopsies performed in a 7-year period showed that CaOx deposition could be classified into three groups. Group I: Seven biopsies within a month post-transplant displayed rare CaOx foci against a background of acute tubular necrosis or acute cell-mediated rejection. At follow-up, five grafts functioned well and two failed due to chronic allograft nephropathy. CaOx in this context was an incidental finding secondary to a sudden excretion of an end-stage renal disease-induced increased body burden of CaOx. Group II: Two biopsies performed 2 and 10 months post-transplant showed rare CaOx foci against a background of chronic allograft nephropathy, leading to graft loss. CaOx in this context reflected nonspecific parenchymal deposition due to chronic renal failure regardless of causes. Group III: One biopsy with recurrent PH1 characterized by marked CaOx deposition associated with severe tubulointerstitial injury and graft loss 6 months post-transplant. There were two previously reported cases in which CaOx deposition in the renal allografts was due the antihypertensive drug naftidrofuryl oxalate or increased intestinal absorption of CaOx. CaOx deposition in renal allografts can be classified in different categories with distinctive morphologic features and clinical implications.
Materials and Methods
- Top of page
- Materials and Methods
All renal Tx biopsies performed at the Methodist Hospital/Baylor College of Medicine, Houston, Texas, in a 7-year period between 1995 and 2002 were reviewed to identify those with CaOx deposition. A separate case of recurrent primary hyperoxaluria, Type 1 (PH1) from Texas Children's Hospital was also included. The renal Tx biopsies were subjected to light microscopy (LM) using hematoxylin and eosin (H&E), periodic acid-Schiff (PAS), Mason's trichrome (trichome), and Gomori's methenamine silver (silver) stains. Selected biopsies were submitted to von Kossa's, Alizarin red, and Pizzolato's stains, the last of which is known to be specific for CaOx (6). The biopsies performed 2 months post-Tx or later were also submitted to immunofluorescent (IF) study for IgG, IgM, IgA, C3, C4, kappa and lambda light chains, with or without electron microscopic (EM) study. CaOx deposition was identified under polarized light and its density was expressed as foci of CaOx per 20× field of a Nikon microscope Model M048 (Nikon, Houston, TX, USA). The patients' medical records were reviewed with special attention to the conditions, which has been known to be associated with increased intestinal absorption or synthesis of CaOx. The renal biopsy findings were tabulated and correlated with the clinical findings at time of biopsy and during follow-up.
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- Materials and Methods
The current study shows that CaOx crystals in renal allografts are morphologically distinct and similar to those in native kidneys, i.e. colorless but refractive polygonal structures with multicolored birefringence under polarized light (1). These features enable not only easy identification of these crystals but also accurate recognition of their chemical nature. The special stains, i.e. von Kossa, Alizarin red and Pizzolato, performed retrospectively in this study to confirm the chemical nature of the CaOx crystals, are indeed not needed routinely. These crystals, however, are not observed in PAS-, trichome-, or silver-stained sections, even in cases with massive deposition, indicating that CaOx is dissolved during these procedures. CaOx deposition in renal allografts, identified in about 4% of unselected renal Tx biopsies in this study, is rather infrequent. Although higher incidences were reported in studies by Olsen et al. (77%) and Memeo et al. (87%), the latter included only Tx biopsies with ATN and only allograft nephrectomy specimens were studied in the latter (2,3). The current study and the related literature suggest that CaOx deposition in renal allografts may develop in four different clinical contexts, each of which is characterized by distinct morphology and clinical implications (Table 2).
Table 2. Causes of calcium oxalate (CaOx) deposition in renal allografts
|Early post-Tx accumulation of CaOx|
| Immediate post-Tx period|
| Usually few foci of CaOx|
| Background of acute rejection or acute tubular necrosis|
| Renal function back to normal|
|Chronic renal failure|
| Months after Tx|
| Usually few foci of CaOx|
| Background of chronic allograft nephropathy|
| Outcome depending on the background renal injury|
|Recurrent primary hyperoxaluria|
| Variable time after Tx, usually within weeks|
| Widespread renal deposition of CaOx|
| Associated with chronic tubulointerstitial injury|
| Associated with graft loss|
| Due to increased intestinal absorption of Ox or Ox-containing|
| drug (naftidrofuryl oxalate)|
| Pronounced CaOx deposition|
| Multifocal tubular cell necrosis|
| Reversible acute renal failure|
As demonstrated in Group I, CaOx deposition may be found in renal Tx biopsies shortly after Tx against the background of ATN or acute rejection but not acute or chronic tubular cell injury. The deposition is usually scanty and almost exclusively limited to proximal tubules. This is probably the most frequent type of CaOx deposition in renal allografts. Ox is normally filtered through the glomerular capillaries and undergoes bi-directional transport through the proximal tubules (7). A small amount of oxalate may be normally secreted by the enteric route at least in rat, and this excretion may be significantly enhanced in case of renal failure (7). Because kidney is the major organ through which Ox is eliminated and dialysis can only remove a fraction of the daily Ox intake, the serum level of Ox in ESRD patients is progressively increased (8–11). Although this may be associated with increased body burden of Ox (8–11), some studies suggest that these patients may achieve Ox balance, with elevated plasma Ox levels, but without significant tissue accumulation of Ox (12,13). After successful renal Tx, a large amount of Ox is excreted into urine and the serum Ox level returned to normal between 3 d and 3 weeks post-Tx in all patients whose original renal diseases are not primary hyperoxaluria (10,11). These considerations suggest that increased tubular load of Ox is routinely expected in the immediate post-Tx period, and Ox may precipitate in tubular cell or tubular lumen. Because only a small percentage of unselected renal Tx biopsies (around 4% in this study) showed CaOx, additional pathogenetic factors may be involved. ATN may facilitate CaOx deposition, as suggested by the presence of CaOx in 10/13 Tx biopsies with ATN reported by Olsen et al. (2), in four out of five grafts with 'ischemic necrosis' reported by Memeo et al. (3), and in five out of 10 (50%) Tx biopsies in this study. Although the reverse sequence, i.e. CaOx deposition may cause ATN, cannot be ruled out, this is highly unlikely considering the scant CaOx deposition in this context, the lack of necrosis of the tubular cells in contact with CaOx, and the presence in the current study of several Tx biopsies with the type of CaOx deposition similar to that seen in biopsies with ATN but showing acute cell-mediated rejection, rather than ATN. Uremia was present in 27 out of 29 patients whose kidneys at autopsy displayed CaOx deposition, but pathologic changes were noted in only 50% of these kidneys (14). Anuria was present in all seven patients in the Group I of this study. These observations suggest that lack of tubular flow may facilitate CaOx deposition. CaOx may have a direct cytotoxic effect on renal tubular epithelial cells and this may be a factor that can promote further CaOx deposition. Although pathogenetically unsettled, CaOx deposition in this group probably represents an incidental finding of no clinical significance as the poor graft function in each patient was related to ATN or acute cell-mediated rejection and resolved after appropriate therapy.
CaOx can deposit in native kidneys in patients with chronic renal failure regardless of causes (8,14). This can also happen in renal allografts, as shown by the two cases in Group II. Although the morphology of the CaOx per se in these cases is similar to those in Group I, the kidneys in these cases typically display chronic allograft nephropathy characterized by tubular atrophy, interstitial fibrosis, interstitial inflammation associated with irreversible loss of renal function, corresponding to the timing of biopsies usually months or years post-Tx. In this aspect, Memeo et al. reported CaOx deposition in 35 out of 40 (87%) renal allografts removed 2–11 years post-Tx (3). In the same study, CaOx deposition was associated with acute ischemic necrosis in four Tx nephrectomy specimens. Although the mechanism of this association was not discussed, it is possible that renal CaOx deposition in these cases may be due to chronic allograft nephropathy and this developed even before ischemic necrosis. Acute ischemic necrosis was only rarely identified in renal Tx biopsy in our study and CaOx was not seen in any of these biopsies.
CaOx in renal allografts may signify recurrent PH1 (Group III). The recurrence usually develops shortly after Tx with massive CaOx deposition associated with severe acute and chronic renal tissue injury resulting in graft loss (15–19). The high recurrence rate is probably related to a heavy pre-Tx body burden of CaOx and a high plasma level of Ox in PH1 patients, which far exceed those in other ESRD patients (11). Plasma oxalate remains elevated beyond 6 months even with successful renal transplantation if oxalate production is not normalized by hepatic transplantation or, in rare cases, thanks to a full response to pyridoxine (11), whereas plasma oxalate levels returned to normal in others within days (10). In contrast to those in Group I or II, CaOx deposition is most likely the cause of the uniformly observed severe renal tissue injury in recurrent PH1. CaOx crystals may cause damage to tubular or interstitial cells through physical contact or through other mechanisms including release of tubular basement antigen or inflammatory cell/reactive oxygen species-mediated endothelial cell injury, as suggested by in vitro studies (17,20,21). The current case also shows that CaOx deposition in renal Tx may be the first clue for PH1, as recently reported (22). Some PH1 patients may present with ESRD, for which renal Tx is offered without biopsy of the native kidney and PH1 was first suggested by its recurrence in the renal allograft (17,22). It should be emphasized that CaOx in renal allografts of those with PH1 does not necessarily indicate recurrent disease but may represent an incidental finding encountered in the context of those in Group I of this study. This is exemplified by one of our patients and was previously reported (16,18,23). The difference in the post-Tx courses of the two PH1 patients in our study may be related to the fact that in the patient with recurrent disease, the diagnosis of PH1 was not made before transplantation and appropriate pre- and post-Tx care including pyridoxine and intense dialysis to decrease the body burden of CaOx was not attempted. In contrast, the diagnosis of PH1 was definitively made 21 months prior to the renal transplant in the patient with a good outcome. She was treated with pyridoxine, was well prepared for renal transplantation, and also received a liver transplant. The liver transplant provided alanine/glyoxylate aminotransferase activity, which may help normalize Ox metabolism and prevent recurrent oxalosis.
CaOx deposition in native kidneys may be secondary to increased dietary ingestion of CaOx-containing food or medication or poisoning by Ox precursors (1). Intestinal absorption of CaOx occurs mostly in large bowel and is facilitated by fatty acid and bile salts. Any condition associated with fat malabsorption such as Crohn's disease, chronic pancreatitis, or small intestinal bypass surgery may cause increased colonic content of fat and promote Ox absorption potentially leading to renal oxalosis (1). Oxalobacter formigenes, a saprophytic bacterium in colon, was recently found to degrade ingested Ox salts and its loss or decrease may be a factor in increased intestinal absorption of Ox leading to hyperoxalemia (24). These conditions were not observed in any of the patients in the current study, but they may rarely be the causes of CaOx deposition in renal allografts, as evidenced by the reports of extensive CaOx deposition associated with acute graft failure due to chronic pancreatitis-induced steatorrhea or overdose of naftidrofuryl oxalate (Praxilene), an antihypertensive medication with Ox accounting for 19% of total weight (4,5). Pre- or post-Tx urine or serum Ox levels were not available in any patients in this study, except for the two with primary oxalosis. Secondary hyperparathyroidism, indicated by elevated serum parathormone levels, was seen in two patients.
In conclusion, CaOx deposition in renal allografts is rather unusual and can be classified in different categories with distinctive morphologic features and clinical implications.