Early Progression of Xanthogranulomatous Pyelonephritis in Children Might Be Dependent on Vimentin Expression

Background

Xanthogranulomatous pyelonephritis (XP) is an extremely rare, severe, atypical form of chronic renal parenchymal inflammation accompanied by hydronephrosis and/or urolithiasis [1]. It accounts for approximately 6–10/1,000 surgically proven cases of chronic pyelonephritis. It is more common in women, with a peak incidence in the sixth and seventh decades, and usually follows chronic diseases such diabetes or leukemia. Its manifestations mimic those of neoplastic and other inflammatory renal parenchymal diseases and, consequently, it is often misdiagnosed clinically [2,3]. The clinical manifestation of XP is not specific. Patients often appear chronically ill. The most common symptoms include fever, chills, anorexia-related weight loss, and flank lumbar pain that is usually dull and persistent.

In children, XP is diagnosed sporadically. Only single case reports have been described so far, and limited cohort studies (involving up to 30 children) have been designed and performed. In the majority of cases, however, XP occurs in girls [4,5]. Nephrectomy is a treatment of choice in all cases, and if done, the future prognosis for the affected child is excellent.

The pathomechanism of XP is not yet fully understood [3]. Occasionally, it may mimic tuberculosis, sarcoidosis, or Wegener granulomatosis [2,6–8]. In most cases, an ineffectively treated pyelonephritis (secondary to calculus or non-calculus urinary obstruction), chronic renal ischemia, lipid metabolism disorders, and, finally, an altered immune response are believed to destroy renal parenchyma and develop areas with the distinctly yellow tinge, which gave the name of the disease [9,10]. A developing failure of one kidney leads to the compensatory overgrowth of the opposite kidney. Such a hypertrophy is regarded as a late XP symptom.

Microscopically, XP is indicated by the presence of multinucleated giant cells and lipid-laden macrophages, as well as inflammatory infiltration, intensive fibrosis, and glomerular sclerosis [11]. The lipid accumulation in kidney parenchyma is one of the most mysterious symptoms. Most likely, it is secondary to the altered flow of low-density lipoprotein (LDL)-derived cholesterol particles inside the affected cells. Physiologically, the process of LDL-derived cholesterol transport from lysosomes to the sites of its esterification is dependent on vimentin, which is a molecule comprising the skeleton in mesenchymal cells [12]. Moreover, the process of kidney fibrosis is initiated by epithelial-mesenchymal transition which can be demonstrated by vimentin presence in the switched cells [13].

Is it then possible that vimentin is a key factor provoking lipid accumulation and renal fibrosis in XP patients? An attempt to answer this question was made by the careful study of the case history of a 7-year-old girl who was diagnosed with an early stage of XP.

Case Report

HISTOLOGY:

The explanted kidney was cut longitudinally in two pieces. One half was placed in Bouin solution for approximately 24 hours, embedded in paraffin and cut at 4 μm sections. The sections were subsequently deparaffinized and incubated overnight in a humidified chamber with the following specific antibodies: anti-vimentin (Dako, M7020, clone Vim 3B4, diluted 1: 25), anti-CD68 (Dako, M0814, clone KP1, diluted 1: 50) and anti-Ki-67 (Dako, M0814, clone MIB-1, diluted 1: 75).

Immunohistochemical reactions were performed by employing Dako EnVision Detection System, Peroxidase/DAB+, Rabbit/ Mouse (K5007). Finally, the slides were counterstained with hematoxylin. Negative controls were performed with the use of normal mouse IgG at the same concentrations as the primary antibodies (Dako, X0931).

Grossly, the renal parenchyma has performed a yellowish tinge with areas of diffuse scarring (Figure 4). Microscopic examination revealed numerous foci of granuloma formations with no evident exponents of dysplastic or neoplastic abnormalities (Ki67 negative immunostaining). Significant immune cell infiltrations without atypical features and scattered foam cells arranging the numerous foci of granuloma inflammation were noticed. These areas were intensively infiltrated by CD68-positive cells (macrophages), and accompanied by glomerular fibrosis (Figure 5) as well as atrophic proximal tubules (Figure 6).

Interestingly, vimentin expression in the renal parenchyma adjacent to granuloma lesions has presented a classic topography involving endothelia and tunica media of renal blood vessels (interlobular arteries and veins as well as glomerular capillaries, Figure 7).

The areas of immune cells infiltration (with high proportion of CD68-positive cells) revealed the immunohistochemical presence of vimentin not only within blood vessels but also in the meshwork of leukocytes penetration (Figures 8, 9) forming a fiber-like structures in the closest proximity of involved glomeruli. What is more, inflammatory glomeruli did not express vimentin in the restricted and atrophic vascular bed, but they presented it within glomerular mesangium (Figure 10).

Both cortical and medullar territories which were already fibrous did not expressed vimentin within ground substance and cells of dense connective tissue (Figure 11). Vimentin was again found within endothelia, and, to some extent, in the network forming tunica media of blood vessels. These fibrous areas were still accompanied by CD68-positive cells infiltration, however, their concentration was smaller and they were not accompanied by extensive infiltration of immune cells (Figure 12).

Discussion

Xanthogranulomatous pyelonephritis (XP) is usually diagnosed in the late stages of the disease [1,9,10]. First of all, it is an unusual form of chronic pyelonephritis and, as was already mentioned in introduction, it is an extremely rare cause of severe kidney destruction. Most affected individuals develop recurrent fevers, anemia, and a painful kidney mass. Moreover, the disease course is long enough that compensatory hypertrophy of the healthy kidney is observed [14].

In our study, a 7-year old girl diagnosed with XP did not present with abdominal pain. No compensatory hypertrophy of the left kidney was observed. The laboratory values were within physiological ranges. Also a microscopic evaluation of kidney parenchyma revealed the areas which still were not inflammatory infiltrated or rearranged by fibrous components. Based on these observations, we assumed that the patient was diagnosed at a relatively early stage of the disease. The case was not complicated by the coexistence of other diseases. From this point of view, this particular case might help us in understanding some basic elements leading to kidney destruction as observed in a majority of XP patients.

As was already mentioned, the exact etiology of XP is still unknown. Generally, it is accepted that the disease process requires long-term renal obstruction and infection [1,4,9,10]. However, there are only a small proportion of patients who develop XP during Proteus or Escherichia coli infections. Similarly, only in single patients with a long history of kidney stones has XP been recognized. For this reason, a number of other trigger mechanisms are considered in XP pathophysiology [15]. One of them is abnormal lipid metabolism [15,16].

Interestingly, disorders in lipid metabolism in XP patients do not necessarily have to be reflected in standard laboratory tests [10,15]. These abnormalities could be expressed at the tissue level, leading to abnormal activation of macrophages [15]. All agree that the primary factor promoting morphological changes in the kidney of XP patients is uncontrolled macrophage activity [15,17].

In line with the aforementioned, we decided to examine a tissue expression of vimentin, which is a factor that attracts macrophages into areas of inflammation [18], increases their invasiveness [19] and, finally, stimulates phagocytosis [20]. In addition, during chronic inflammation, amino acid residues can be enzymatically transformed into citrulline protein residues such as vimentin by a process called citrullination [21]. If their shapes are significantly altered to normal proteins, the proteins may be recognized by the immune system as antigens, thus exaggerating an immune response [21].

Finally, vimentin is a well-established protein involved in tissue cholesterol homeostasis and epithelial-mesenchymal transition, which are also observed at the tissue level in XP patients [22].

Taking this information together, is it possible that XP development can be induced by vimentin overexpression in renal parenchyma?

The immunohistochemical pattern of vimentin expression revealed its transient presence within leukocyte infiltration mesh-work of the XP kidney. The areas which were not yet involved in XP devastation, as well as already fibrous territories, have presented vimentin expression exclusively within renal vasculature (interlobular blood vessels, glomerular tufts and peritubular capillaries). Vimentin, as studied in infiltrated areas, was expressed both in glomerular and tubular spaces; and it was accompanied by intensive macrophage presence. With the progress of fibrosis, vimentin expression was systematically decreasing and the number of CD68-positive cells was restricted.

Such a vimentin time-related and area-related topography might suggest not only its direct effect on macrophages but also profibrotic function. This observation is not unique. Several reports indicated vimentin as a promising marker of tubulointerstitial fibrosis in the early stages of chronic renal failure. Others suggest that anti-vimentin auto-antibodies in renal transplant recipients have been correlated with interstitial fibrosis and tubular atrophy [23].

We do realize that the process of kidney reconstruction in XP patients probably is not possible to stop or reverse by employing contemporary therapeutic tools. The only treatment is nephrectomy. However, in some patients with bilateral XP, saving part of the intact kidney must be taken under consideration (we have no reports describing patients who underwent a kidney transplantation after bilateral XP). Perhaps these patients would benefit more if such an operation is followed by biological treatment, including anti-vimentin antibodies [4,24].

Conclusions

Vimentin expression in XP patients confirms a focal character of chronic granuloma formation and may suggest the complexity of XP pathogenesis involves not only macrophages and fibroblasts activation but also local lipid deregulation and fibrosis.