Two distinct papillary phenotypes are commonly seen during endoscopic examination of calcium oxalate (CaOx) stone formers. For example, the patients depicted in Fig. 1 were examined on the same day. Both patients presented with calcium oxalate stones: Micro CT analysis revealed that in the patient on the left, the stones consisted of 91% CaOx and 9% apatite, while in the patient on the right, they were composed of 76% CaOx and 24% apatite. Despite similar stone compositions, the appearance of their papillae differed significantly. The patient on the left exhibited mostly normal-looking papillae, except for the presence of Randall’s plaque. During the operation, seven papillae from this patient were assessed [17], with mean scores of 0.4 for plugging/dilated ducts, 0.7 for loss of papillary contour, and 0.9 for Randall’s plaque. Conversely, the patient on the right displayed ductal plugging in every papilla, with four papillae scoring means of 1.75 for plugging/dilated ducts, 1.5 for loss of contour, and 0.5 for Randall’s plaque. An average score of 1.75 for plugging means that all papillae had plugs or dilated ducts, with three of the four papillae having more than five yellow plaque deposits (plugs) or dilated ducts each [17].
These two phenotypes were seen in our previous work [4], and we have now extended these observations to include 101 idiopathic CaOx patients. Papillary scoring revealed an inverse correlation between the amount of Randall’s plaque and the score for plugging/dilated ducts (Fig. 2, r2 = 0.34, P < 0.0001). Scores for plugging were positively correlated with loss of papilla mass (loss of contour, r2 = 0.27, P < 0.0001). Conversely, scores for Randall’s plaque were inversely correlated with loss of papilla mass (contour, r2 = 0.13, P = 0.0002).
Classifying these 101 patients into high-plaque, high-plugging, or neither, showed no distinctions in age, serum chemistries or 24-hour urine values (Table 1). There were more females in the high-plugging group (P = 0.04), and papillary pitting was more prevalent in both the high-plaque and high-plugging groups as compared with the patients who had low mineral scores. Overall, though, these patients were remarkably similar by most clinical measures.
The severe nature of ductal mineral plugs
It is crucial to clarify that the plugging scored in this study represents grossly dilated collecting ducts so filled with mineral that they show up as yellow regions on the papilla tip [17, 19]. This plugging score also includes dilated collecting duct openings, indicating a ductal mineral plug that had since fallen out [17]. These dilated ducts and ductal plugs at the papilla tip are quite large (as much as 20 times larger in diameter than normal collecting ducts) [19], and are very different from the occasional plugged tubule that can be seen in normal kidneys higher up in the medulla, and which measure around 40 µm in diameter [20, 21]. Those mineral plugs seen in normal kidneys are smaller in diameter than a human hair, and so even if they were at the surface of the papilla, they would not be discernable by endoscope. The ductal plugs scored in these CaOx stone formers thus represent a severe pathology.
Ductal plugging and the progressive loss of papilla tissue
The loss of papillary tissue with ductal plugging—measured by loss of papillary contour scores—is suggested by a hypothesis from 2005 in which the presence of a ductal mineral plug would incite destructive activity in nearby cells, so that the injury would progress through the papillary tissue [19]. Details of this published hypothesis included the injury of collecting duct cells by luminal crystals, death of those cells, expansion of the mineral plug so that it presses into the nearby tissue, and the resulting provocation of inflammation in tissues surrounding the plugged collecting duct [19]. Evidence for the last stages of this hypothesis is showing up in some newer work, as shown in Fig. 3, which shows a collecting duct adjacent to a ductal mineral plug. The collecting duct appears normal in structure, with the exception of the presence of T-cells migrating up between the principal cells (arrows in Fig. 3d). When lymphocytes are seen to invade renal tubular epithelium—a phenomenon called tubulitis—it is viewed as a sign of impending necrosis of the tubule [22].
Panels a and b in Fig. 3 show a healthy collecting duct (CD) lying close to a collecting duct that is filled with mineral and which shows very little staining for collecting duct cells (CD injured). In panels c and d, the healthy collecting duct is seen to have T-cells within its epithelium. (Note that this phenomenon of tubulitis in collecting ducts is difficult to detect without special stains.) There are also a few T-cells next to the mineral plug. Panels e and f add visualization of neutrophils, which are seen to completely surround the mineral plug. Panels g and h add visualization of macrophages, which are also surrounding the mineral plug, so that the mineral plug appears to be thoroughly enveloped by immune cells.
This focal investment of immune cells against the mineral plug is quite dramatic and points to a general increase in immune cell activity in the tissue. Note that the ductal plug in this case has not yet grown to a size that would press against neighboring tissues, but still there is T-cell activity attacking an adjacent collecting duct. This concept, in which a ductal plug would lead to destruction of nearby tissues, was first proposed for the profound papillary injury seen in papillary biopsies from brushite stone formers [19], but has also been proposed for other kinds of stone former in which ductal mineral plugs are also found [23]. These investigators also proposed that injury of a collecting duct epithelium might reduce its ability to acidify the tubular fluid and thus lead to the formation of a new calcium phosphate plug [19]. Thus, an initial ductal mineral plug would lead to injury to surrounding tubules which would then also become plugged with mineral, and the progression of tissue injury could spread through the papilla. There may also be a role for reactive oxygen species in all of this, as crystals interacting with cells can induce the production of reactive oxygen species and so contribute to the inflammatory state [16, 24].
Randall’s plaque and mild inflammation of the papilla tissue
Contrast this picture with that shown in Fig. 4, in which a biopsy from a CaOx stone former shows Randall’s plaque and no mineral plugs. The plaque regions—which are interstitial, initiating in the basement membranes of thin limbs [25]—have macrophages associated with them, but no tubulitis seen in any surrounding tubules. The association of Randall’s plaque with macrophages has been reported before [18]. An absence of tubular injury has been reported repeatedly for CaOx biopsies containing only Randall’s plaque [23, 25, 26].
Moreover, the inverse relationship of Randall’s plaque scores with scores for loss of papilla mass suggests that the presence of plaque is consistent with the overall health of the papillary tissue. Whereas the presence of ductal plugs of mineral is associated with loss of papilla tissue, an abundance of Randall’s plaque correlates with a healthier papilla appearance. When we see a patient with Randall’s plaque, the kidney often looks very healthy and the papillae show only Randall’s plaque and the very shallow erosion pits left from previous stone events [27].
Hypothesis
Putting these data together, we see that there are two phenotypes of CaOx stone former and it seems likely that long-term prognosis of these phenotypes may not be the same (Fig. 5). We hypothesize that the typical CaOx stone former making stones on Randall’s plaque has what is in the long term a disease that does not lead to destruction of papillary tissue, and which is unlikely to lead to loss of many nephrons. While this condition does result in a large number of stones [4] which are especially recurrent [2], the size of stones in this phenotype is relatively small [4]. The Randall’s plaque lesion is interstitial and the collecting ducts are free from luminal obstruction. The loss of calcified papillary tissue (Randall’s plaque) with a stone [4] must cause the breakage of loops of Henle, but with each stone lost only a few nephrons will be damaged. The immune cell infiltrate appears to consist mainly of macrophages collected near the region of interstitial mineral, with minimal T-cell activity [18]. All of this points to a form of stone disease that will not result in long-term degradation of renal function.
In contrast, CaOx stone formers who produce luminal mineral plugs in the papillae exhibit several features that could lead to renal function loss over time. The plugging of collecting ducts indicates profound obstruction of urine flow, with plugs visible at the surface of the papilla being significantly larger than tubular mineral plugs described in normal kidneys. Since each terminal collecting duct serves thousands of glomeruli, blockage of these could lead to loss of many nephrons and the resultant decline in renal function. Additionally, if active inflammation accompanies these massive tubular mineral plugs, further damage may occur to papillary tissue. This additional damage could include injury to surrounding collecting ducts, injury which can make them also susceptible to the precipitation of apatite in the lumen to plug up additional trees of nephrons. The visual observation of loss of papillary mass in kidneys with plugged collecting ducts aligns with this depiction of plugging being detrimental to papillary tissue. If papillary tissue is damaged in a way that destroys collecting ducts, renal cortical damage may also follow.
Currently, the only way to distinguish CaOx stone formers as being Randall’s plaque or plugging types is via endoscopic observation. Thus, testing these hypotheses of progression to chronic kidney disease will initially be limited to patient populations undergoing endoscopic stone removal. We predict that CaOx patients who show extensive ductal plugging in their papillae will also have, over time, an increased rate of decline in renal filtration function [28]. In contrast, CaOx patients without ductal plugging should show a decline in renal filtration function with age that is similar to non-stone formers.