Central fibrous area in the glomerular vascular pole consists of fibrous collagens and is associated with advanced age: a cross-sectional study

Aging inevitably alters renal structure and function [1‐5]. Histologically, the interlobular arteries of the kidney develop progressive arteriosclerosis, characterized by intimal and medial hypertrophy. This eventually leads to ischemic glomerular collapse, glomerulosclerosis, tubular atrophy, and interstitial fibrosis [1‐6]. The glomerular filtration rate (GFR) declines with advancing age [1, 2, 4, 5]. In addition, aging increases susceptibility to renal injury, resulting in incomplete recovery and progressive kidney dysfunction [7‐10]. Living-related kidney donors tend to be older, with an associated increase in delayed graft function and risk of acute rejection. These have the potential to adversely affect graft survival [5, 11‐13]. Thus, recognizing the histological features associated with older age is helpful for the optimal management of patients with both native kidney diseases and allograft kidneys.

Recently, small, hyalinotic, and monotonous nodular areas in glomerular vascular poles lesions, called central fibrous areas (CFAs), have been detected as a valuable indicator of kidney aging [14]. CFAs have been observed in healthy kidneys and IgA nephropathy [14]. However, the components of CFAs and whether other kidney diseases conspire with CFAs remain unclear. To clarify these points, this study examined the histological features, epidemiology, and clinicopathological features of CFAs.

A CFA is a monotonous, fibril-rich lesion located in the glomerular vascular pole. Its formation is associated with older age. Aging kidneys show various histological findings, including glomerulosclerosis, tubular atrophy, interstitial fibrosis, arteriosclerosis, and nephron loss [1‐5, 17]. Although these structural changes cause GFR decline, they are non-specific and thus difficult to distinguish from secondary scar lesions induced by hypertension, diabetes mellitus, and primary kidney diseases. In the present study, although CFAs were observed in various kidney diseases and their formation was univariately associated with hypertension, hyperlipidemia, sclerotic vascular changes of the interlobular artery and arteriole, and membranous nephropathy, multivariate analysis revealed that higher age was the only independent factor associated with CFA formation. Our results confirmed that CFA could serve as a histological marker of older age.

CFA is observed in various kidney diseases and its frequency increases with age. Inomata first described this pathological finding as vascular pole deposits (VPD) [15]. VPD was observed in 8.9% of 2963 cases of various kidney diseases, and its number per specimen was 1 in most cases [15]. Inomata concluded that VPD was not disease-specific and had no bearing on age, sex, or any clinical signs and symptoms, including nephrotic syndrome and hypertension [15]; however, statistical analysis was not performed to support this conclusion. Recently, Kanetsuna et al. Showed that CFA significantly increased in older kidneys compared to younger kidneys, using zero-hour biopsy specimens from living renal donors, and IgA nephropathy biopsy specimens [14]. Although the frequency of CFA was not shown, the average number of CFAs per specimen was 3.0 of 49.7 glomeruli (6.0%) in the zero-hour biopsy group from living kidney donors and 1.2 of 19.9 glomeruli (6.0%) in the IgA nephropathy group [14]. In the present study, we detected CFA in 55.4% of cases, greater than the frequency in Inomata’s research, whereas the number of CFA per specimen was 2 and the CFA-positive rate was 5.3%, similar to the 2 previous studies [14, 15]. Using various kidney diseases, including IgA nephropathy, we also confirmed that older age was the only predictor associated with CFA formation, supporting the results of Kanetsuna et al. [14]. The difference in CFA frequency could be due to two reasons. First, the age distribution of the enrolled subjects differed among studies. Although Inomata’s study did not provide the average age, most participants were under 40 [15]. In Kanetsuna’s analysis, the mean age was 58.3 ± 11.2 years old in the zero-hour group and 33.1 ± 13.9 years old in the IgA nephropathy group [14]. Considering that the median age in the CFA (+) group in our study was 62.5 years old, the fact that the frequency in our study was more significant than those of Inomata’s research and the IgA nephropathy group in Kanetsuna’s study, and was similar to that of the zero-hour biopsy group in Kanetsuna’s study, was not unusual. Second, the definition of CFA differed slightly among studies. Inomata only included hyaline deposits as VPDs [15], whereas Kanetsuna et al. [14] included both hyaline and fibrous lesions. This study and Inomata’s included cylindrical shapes in addition to nodular forms [15], whereas Kanetsuna et al. included only nodular lesions [14]. Given the different age populations among these studies, these differences may be due to the stage of CFA formation. VPD may be a precursor of CFA and may be present in younger populations, and then CFA may form in advanced age. In summary, CFA is observed in various kidney diseases and is a helpful pathological finding for detecting kidney aging.

The morphogenesis of CFAs remains unresolved in the context of advanced age. Neal et al. Termed the glomerular vessels embedded in the mesangium of the vascular pole as the vascular chamber and showed that the vascular chamber widened in correlation with glomerular tuft size in the adult kidney [18]. Using multiphoton imaging and electron microscopy, they elegantly revealed that collagen bundles containing fibrous collagens appeared at the wall of the afferent vascular chamber [18], suggesting that the collagen bundles are formed by the hemodynamic effect that regulates glomerular pressure. In our study, immunohistochemistry revealed that CFA consisted of fibrillar collagens (collagen I and III), in addition to nonfibrillar collagens (collagen IV and VI). Electron microscopy also confirmed that the presence of fibrillar collagens in CFA is distinct from the extracellular matrix components of the mesangial matrix, which consist mainly of non-fibrillar collagens [19], supporting the findings of Neal’s study. Because vascular chambers are associated with large adult glomeruli [18], we speculate that longstanding high hemodynamic pressure on the walls of vascular chambers accelerates CFA formation in aged kidneys, as discussed by Kanetsuna et al. [14]. In addition, intrinsic glomerular cells, such as mesangial cells, vascular smooth muscle cells, pericytes, and endothelial cells of the afferent vascular chamber, might produce fibrillar collagens in the context of advanced age and kidney diseases. Under disease conditions, mesangial cells acquire a myofibroblast-like phenotype that produces interstitial collagens in addition to normal matrix constituents [20]. Potassium ion channels K (Ca)3.1 on mesangial cells, promote transforming growth factor β1-induced premature senescence and myofibroblast phenotype transition of mesangial cells [21]. These findings suggest a connection between cellular senescence and myofibroblast-like phenotypic changes in mesangial cells. Likewise, vascular smooth muscle cells, pericytes, and endothelial cells of the afferent vascular chamber may be associated with interstitial collagen production due to cellular senescence induced by aging, hemodynamic pressure, and kidney diseases. In our study, α-SMA-positive cells, a major marker of myofibroblasts [22], were observed in or adjacent to the CFA, adding weight to our speculation. Although the frequency of α-SMA-positive cell inclusion was relatively low, and other underlying mechanisms may also be implicated in the morphogenesis of CFA, the accumulation of fibrillar collagens produced by myofibroblasts may be one mechanism underlying CFA formation at the glomerular vascular pole with increasing age, hemodynamic pressure, and diseases. Further studies are needed to elucidate the underlying mechanisms.

Another interesting question is whether CFA progresses to glomerulosclerosis and causes kidney dysfunction. Uesugi et al. Showed 3D imaging of the vascular tree of aging kidneys and determined that arteriosclerosis of the distal interlobular artery contributed to glomerulosclerosis [23]. In our study, CFA formation was univariately associated with vascular sclerosis of both the interlobular artery and arteriole, but not with glomerular tuft size, glomerulosclerosis, or kidney dysfunction. In addition, none of these histological findings were significantly associated with CFA formation on multivariate analysis. Although CFA often encircles afferent arterioles or vascular chambers, the vascular lumens do not become narrow, and the glomerular tuft does not shrink or enlarge. Furthermore, membranous nephropathy was more frequent in the CFA (+) group than in the CFA (−) group, but no clinicopathological difference was observed between the two groups of membranous nephropathy. Similarly, Kanetsuna et al. did not find any correlation between CFA formation and serum creatinine, estimated GFR, or glomerular sclerosis rate in either the IgA nephropathy group or zero-hour biopsy group [14]. Thus, CFA does not yet appear to directly cause glomerulosclerosis or kidney dysfunction and this point should be elucidated by a large cohort study in the future given the cross-sectional designs of both our study and the previous study [14].

CFA could be a useful histological marker for kidney aging. Detrimental effects of kidney aging have been reported increasingly. Kidneys from aged patients after acute kidney injury exhibit tertiary lymphoid tissue, which accelerates local inflammation and leads to kidney dysfunction [13]. In addition, kidney allografts from aged patients are associated with delayed graft function and risk of acute rejection, which could be linked to poor graft survival [5, 11‐13]. Although, future studies are clearly needed to clarify the relationship between CFA formation and phenomena which are related to kidney aging, such as tertiary lymphoid tissue, chronic kidney disease progression, and allograft survival, recognizing CFA formation in the kidney might lead to appropriate clinical management of elderly patients with chronic kidney diseases.

Our study has some limitations. First, the patients had heterogeneous background kidney diseases, leading to small sample sizes given the variety of diseases. Advanced age and various kidney diseases that induce renal structural damage can promote the accumulation of senescent cells [5, 24]. In Kanetsuna’s study, the effect of higher age on CFA formation was more pronounced in IgA nephropathy specimens than in zero-hour biopsy specimens [14]. Thus, the heterogeneity of kidney specimens may have affected CFA formation. Second, CFA is formed only in the glomerular vascular pole; therefore, the prevalence of CFA may also depend on the sample size. Although the correlation between CFA numbers and glomerular numbers was low, the effect of kidney biopsy specimens could not be eliminated and large samples, such as wedge biopsy and nephrectomy specimens would be more helpful to assess the precise estimation of CFA frequency. Nonetheless, each specimen contained a sufficient number of glomeruli, and the number did not differ significantly between the CFA (+) and CFA (−) groups. Furthermore, the number of CFAs per specimen did not differ significantly from that of other studies [14, 15], thus minimizing the possibility of sample error in this study. Third, we could not exclude selection bias. Although we collected all consecutive patients who did not meet exclusion criteria from multiple facilities, kidney specimens had been collected from seven hospitals in the Hokuriku region of Japan for 1 year. In addition, all patients enrolled in the present study were Japanese adults. Aging-related kidney scarring is much less marked in Japanese patients than in Caucasian and African-American patients [25]. Although research related to CFA was conducted in the United Kingdom [18], it remains unknown whether CFA formation occurs in all races. Finally, the cross-sectional design did not permit us to assess the causal effect of CFA formation on such factors as chronic kidney disease progression. Similarly, although it was not observed whether CFA formation had any clinicopathological effect on membranous nephropathy, the result could be due to the small number sample of patients. Large cohort studies are needed to clarify whether CFA has a detrimental effect on various kidney diseases and works as a useful histological marker for aging kidneys.

A CFA is a monotonous, fibrous, collagen-rich area in the glomerular vascular pole and is associated with older age. CFAs are observed in various kidney diseases and could serve as a histological finding in aging kidneys. A prospective large cohort study using homogenous kidney specimens is needed to clarify the clinical significance and morphogenesis of CFA formation.