Routine immunohistochemical staining in membranous nephropathy: in situ detection of phospholipase A2 receptor and thrombospondin type 1 containing 7A domain

Membranous nephropathy (MN) can be idiopathic (iMN) or manifest as a result of systemic underlying conditions as a secondary epiphenomenon. For the prognostic and predictive consequences of this discrimination, the routine use of reliable markers is crucial. This large MN series aimed to evaluate the routine and standardized immunohistochemical (IHC) employment of a panel of 3 biomarkers-phospholipase A2 receptor (PLA2R), thrombospondin type-1 domain-containing 7A (THSD7A), and immunoglobulin (Ig)G4-in the differential diagnosis of MN forms, contributing to the validation of the technique and the correct interpretation of reproducible patterns of reactivity. We classified 95 patients with a biopsy proven diagnosis of MN as primary (n = 72) or secondary (n = 23) cases based on clinical data. After performing an IHC assay directed against PLA2R, THSD7A and IgG4 antigens, samples were interpreted by three different nephropathologists to assess the positivity/negativity of the staining according to new interpretation criteria. Useful interpretation criteria were introduced to exclude false positive patterns of reactivity and to identify only true granular membranous or mesangial deposits in MN. The IHC directed against PLA2R resulted positive in 51 iMN cases and negative in 21, while 4/23 secondary forms were considered positive. Based on these data the technique showed a sensitivity of 71% and specificity of 83%. On the other hand, the IHC analysis for IgG4 resulted positive in 44 cases of iMN and negative in 28 cases, while only 4/23 secondary forms were positive (same cases positive to PLA2R). Finally, THSD7A was found to be positive only in 1 case, which was negative to PLA2R and IgG4. The combination of the results allowed a classification of the series into two major groups: "double-positive" (PLA2R+/IgG4+/THSD7A-) and "triple-negative" (PLA2R-/IgG4-/THSD7A-) cases. Based on these data, the diagnostic performance of the three biomarkers used in a "tandem fashion" can reach 79% sensitivity and 83% specificity, significantly reducing the risk of a false-positive or false-negative result and improving the routine characterization of this frequent glomerulonephritis.