In diagnosing medical kidney disease, the renal pathologist evaluates the different “compartments” of the kidney: glomeruli, tubules, interstitium, and vessels. The pathologist describes both the pattern of injury in these compartments, and tries to determine the cause of the injury, incorporating information from the patient's medical history and clinical exam findings, radiographic features, and laboratory results. IgG4-related kidney disease (IgG4-RKD) was first recognized as a tubulointerstitial pattern of injury – tubulointerstitial nephritis – that formed fibroinflammatory mass lesion(s) in the context of autoimmune pancreatitis (type I), now termed IgG4-related pancreatitis, which also presented with fibroinflammatory mass lesion(s).1 This type of tubulointerstitial nephritis showed histologic similarity to the lesions of autoimmune pancreatitis and other organs involved that were being described at that time as extrapancreatic involvement by autoimmune pancreatitis.2,3 Our understanding of the histology and immunophenotype of IgG4-related tubulointerstitial nephritis (IgG4-TIN) evolved with time and with increased recognition of cases. We learned that patients could present with functional impairment of the kidney (increased serum creatinine) in addition to mass forming lesions, and that IgG4-TIN could show a range of histologic patterns of injury.

We have also recognized involvement by IgG4-related disease (IgG4-RD) of the other renal compartments. Within glomeruli, the most common and characteristic pattern of injury is membranous glomerulonephritis, which can occur with or without concurrent IgG4-TIN.4,5 The clinical presentation with predominant glomerular disease is different from IgG4-TIN. Other glomerular diseases have been seen in patients with IgG4-RD. Arteries in the kidney can show plasma cell arteritis, so far recognized only in the context of tubulointerstitial nephritis.6

The first International Symposium on IgG4-RD was held in Boston in October 2011, with two resulting publications on recommendations for the nomenclature of IgG4-RD and a consensus statement on the pathology of IgG4-RD.7,8 Once IgG4-RD was more clearly defined, pathologists and other physicians were eager to make the diagnosis of this “new” disease, which had actually been described by Mikulicz at a meeting presentation in 1888 and published in 1892.9,10 However, particularly in the kidney, there are many different causes of the tubulointerstitial, glomerular, and arterial patterns of injury that may also be seen as part of IgG4-RKD. Limited experience with and knowledge of the differential diagnosis of IgG4-RKD can lead to misdiagnosis. In this review, we will focus on the more recently recognized patterns of injury in IgG4-RKD as an update to the previously published review11, as well as cover the mimics of IgG4-RKD.