Disparities between IgG4-related kidney disease and extrarenal IgG4-related disease in a case–control study based on 450 patients

We aimed to compare the demographic, clinical and laboratory characteristics between IgG4-related kidney disease (IgG4-RKD+) and extrarenal IgG4-related disease (IgG4-RKD−) in a large Chinese cohort, as well as describing the radiological and pathological features of IgG4-RKD+. We retrospectively analyzed the medical records of 470 IgG4-related disease (IgG4-RD) patients at Peking University People’s Hospital from January 2004 to January 2020. The demographic, clinical, laboratory, radiological and pathological characteristics between IgG4-RKD+ and IgG4-RKD− were compared. Twenty IgG4-RD patients who had definite etiology of renal impairment including diabetes, hypertension and etc. were excluded. Among the remained 450 IgG4-RD patients, 53 were diagnosed with IgG4-RKD+ . IgG4-RKD+ patients had older age at onset and at diagnosis. Male to female ratio of IgG4-RKD+ patients is significantly higher. In the IgG4-RKD+ group, the most commonly involved organs were salivary gland, lymph nodes and pancreas. It was found that renal function was impaired in approximately 40% of IgG4-RKD+ patients. The most common imaging finding is multiple, often bilateral, hypodense lesions. Male sex, more than three organs involved, and low serum C3 level were risk factors for IgG4-RKD+ in IgG4-RD patients. These findings indicate potential differences in pathogenesis of these two phenotypes.

All patients underwent radiology examinations consisting of Computed Tomography (CT), or Magnetic resonance imaging (MRI), and some patients also received 18F-fluorodeoxyglucose PET-CT.
We fixed all tissue biopsy samples in formalin and embedded in paraffin, then stained them with hematoxylin and eosin (H&E) and immunocytochemistry (IHC). IHC was performed using the avidin-biotin complexperoxidase method with monoclonal antibody to human IgG4 (Zymed, Carlsbad, CA; dilution 1:50 or 1:100 depending on staining laboratory) on sections from paraffin-embedded tissue.
Pathological findings. In total, 6 patients with IgG4-RKD+ underwent renal biopsy; all of the 6 (100%) patients had TIN. TIN in 1 case (16.6%) was associated with glomerular disease. Membranous nephropathy was the cause of glomerular disease in this case.
All the patients with TIN had a lymphoplasmocytic (LPC) infiltrate with fibrosis. The LPC infiltration was diffuse in 3 patients and patchy in the other 3 patients. In all of the 6 patients with TIN, IgG4 staining demonstrated > 10 IgG4+ plasma cells per high-power field (in the most concentrated area), and all these patients fulfilled the Raissian criteria for IgG4-TIN 14 . www.nature.com/scientificreports/ Radiological findings. We used contrast-enhanced CT to identify radiological abnormalities in IgG4-RKD+ patients except those with renal dysfunction. Fifty of the 53 IgG4-RKD+ patients exhibited characteristic findings of the kidney radiology. Among them, 14 patients were presented with more than one kind of lesion. The most common finding is multiple, often bilateral, hypodense lesions in 31 (58.5%) IgG4-RKD+ patients, which are called small cortical hypodense nodules (Fig. 4A), followed by thickening of the renal pelvic wall in 18 (34.0%) IgG4-RKD+ patients (Fig. 4B), and ureteric obstruction and hydronephrosis related to RPF in 9 (17.0%) patients who had also other specific kidney lesions of IgG4-RKD+ (Fig. 4C). Besides, diffuse patchy involvement, tumor-like less-enhanced mass and rim-like lesion were observed in 8 (15.1%), 2 (3.8%) and 1(1.9%) patient, respectively.

Discussion
In this study, we compared the demographic, clinical, and laboratory characteristics of 53 IgG4-RKD+ patients and 397 IgG4-RKD− patients, as well as describing the radiological and pathological findings in patients with IgG4-RKD+. To our best knowledge, this is the largest case-control study of IgG4-RKD+ and IgG4-RKD− phenotypes. IgG4-RD is manifested by typical clinical features, including tumor-like lesions, dense infiltration with IgG4positive plasma cells, and extensive fibrosis of multiple organs. There is a great variability of disease manifestations for IgG4-RD, and the identification of different IgG4-RD subgroups is crucial, as a consequence of significant disparities in the characteristics of IgG4-RD regarding different organs 6,15 . In our study, the frequency of kidney involvement in IgG4-RD patients was 11.8%, which is lower than that of Japan (23.7%) and Mexico (24.6%) 6,16 , but similar to that of UK study 11 . The heterogeneity of IgG4-RKD+ definition and ethnicity in various studies may explain for this diversity. The following organs were found more commonly involved in IgG4-RKD+ patients, including salivary gland, lymph nodes and pancreas. In addition, multi-organ involvement was common in IgG4-RKD+ patients. Therefore, it is necessary to perform a general checkup to obtain a comprehensive view of the patients especially for patients with IgG4-RKD+.
Male sex, involvement of three or more organs, and low serum C3 level were risk factors for IgG4-RKD+ in IgG4-RD patients. IgG4-RD is a multi-organ immune-mediated condition that could influence almost any organ system in the body. More involved organs may suggest the higher activity of disease. In most cases, IgG4-RKD+ is diagnosed in the context of known extrarenal IgG4-RD or active status of IgG4-RD. With progressive renal decline or detection of characteristic radiological features when evaluating extrarenal IgG4-RD, kidney involvement became evident 14,17 , which may explain for the association between involvement of 3 or more organs and IgG4-RKD+. Renal involvement may appear as an intrinsic kidney disease (IgG4-RKD+) or as a consequence of ureteric obstruction from retroperitoneal fibrosis (IgG4-RPF). IgG4-RPF often concentrated in the periaortic region and ureters can be entrapped, leading to hydronephrosis and renal injury. Therefore, it is necessary to distinguish between the kidney lesions from IgG4-RKD+ and IgG4-RPF. Similar to previous studies 18 , male predominance in IgG4-RKD+ may be explained by the viewpoint that female patients were more likely to present with superficial organ involvement, while male patients with internal organ involvement. www.nature.com/scientificreports/ There are some interpretations for the association of low serum C3 levels and IgG4-RKD+. The first descriptions of IgG4 TIN was previously described as "idiopathic hypocomplementemic tubulointerstitial nephritis" with extensive tubulointerstitial deposits 19 . Only about 16-34% of IgG4-RD patients have low serum complement levels, despite that hypocomplementemia is a feature of IgG4-RD. Nevertheless, more than 50% of patients with active IgG4-TIN have low concentration of complement 14,17 . Therefore, hypocomplementemia is considered an  20 . Therefore, this study added insights into the hypocomplementemia in IgG4-RKD+ patients. Low serum C3 level was found a risk factor for the development of IgG4-RKD+. Hypocomplementaemia is not characteristic feature of most IgG4-RD patients, which often suggests the existence of IgG4-RKD+, thus scrutiny is necessary.
In previously reported studies, kidney function in IgG4-RKD+ patients varies from normal to renal failure, and the development of renal dysfunction also varies from relatively acute to slowly progressive 13,14,16,17,21 . In our cohort, the renal function was impaired manifesting as the reduced eGFR, elevated blood urea nitrogen, elevated serum creatine and abnormal specific renal tubular function test. It could be attributed to IgG4-related TIN in the patients, or to the glomerular disease. IgG4-related TIN occurred in the vast majority of IgG4-RKD+ patients, and MGN was reported less than that. Consistent with the rates in previous studies, IgG4-related TIN occurred in all the 6 IgG4-RKD+ patients who had received renal biopsy. In the urinalysis in IgG4-related TIN, we could find typically mild to moderate proteinuria, as well as occasionally the presence of white blood cells 17 , which was also accordant with our result. For IgG4-RD patients, it is necessary to carry out urine routine test and renal function test (both glomerular and tubular function), in order to timely detect glomerular and renal tubular lesions.
Main abnormalities on renal imaging were revealed in a total of 42 (79.2%) of IgG4-RKD+ patients: multiple low-density nodules, hydronephrosis and thickening of renal pelvic wall. Similar with previous studies, there were also some other imaging manifestations in our cohort, including diffuse patchy involvement of the bilateral kidneys and rim-like lesion of the kidney 11,22 . CT was the most common mode of renal imaging, including PET-CT, which is increasingly used. PET-CT could contribute to excluding malignancy with little radiative damage. Moreover, PET-CT is helpful for discovering the involvement of some silent lesions, however, its cost should also be taken into account.
One of the limitations of this study is its retrospective nature, indicating that some affected organs may be neglected, although most patients have undergone general examinations, including FDG-PET. Moreover, only a small number of IgG4-RKD+ cases were diagnosed based on renal biopsy. Compared to biopsy from superficial tissue, there would be higher risk of iatrogenic trauma when patients receive deep kidney biopsy, thus some patients would not accept it. In addition, although our study has the relatively largest sample size yet, the number of patients is still small. We should cautiously interpret the results.

Conclusion
In summary, we have specified demographic, clinical, and laboratory differences between IgG4-RKD+ patients and IgG4-RKD− patients. IgG4-RKD+ patients had older age at onset and older age at diagnosis. The male to female ratio of IgG4-RKD+ patients is significantly higher. The most commonly involved organs of IgG4-RKD+ patients were salivary gland, lymph nodes and pancreas. Male sex, involvement of three or more organs, and low