International Journal of Nephrology 3. Membranous Nephropathy Membranous nephropathy (MN) is one of the most common causes of the nephrotic syndromes in adults, corresponding to 20 percent of the cases [66–68]. The main pathogenic mechanism involved in MN is the deposition of immune complexes, in subepithelial regions that leads to a progressive thinning of the glomerular capillary [68, 69]. For more than 50 years [66, 67, 69–71], the Heymann nephritis model, was induced in rats immunized with a crude kidney-cortex preparation. The data collected from this model suggested that subepithelial glomerular depositions occur due to the circulating immune complexes, caused by membrane fractions from rat renal brush border [66–69, 71–74]. Additionally, with the advent of the passive Heymann nephritis model, it was observed that rats treated with antibodies directed against brush-border proteins also had the same subepithelial depositions, suggesting that circulating immune complexes are not necessary for this event [75, 76]. Afterwards, megalin was found to be the rat antigen involved in this process. Megalin is expressed in the basal surface of the podocyte foot process, the same subepithelial space where immune complexes are formed. That was the ﬁrst evidence that podocytes could be engaged with the formation of immune complexes. After the development of genome sequencing techniques, speciﬁc megalin epitopes were discovered and were associated with the formation of immune complexes [66, 69, 71–74]. Concomitant with megalin discovery, a new experimental model for MN was established by rabbit immunization with cationic fetal bovine serum. This procedure led to the observation that exclusively immunized animals were capable of having subepithelial IgG and C3 deposits, while the animals that received anionic or neutral serum had complex deposits in the mesangium. This experimental model of Heymann nephritis has been reproduced in dogs, cats, rabbits, rats, and mice [77]. Likewise, proteinuria was more intense in animals that received the cationic serum, showing that podocytes might not play a key role in the formation of immune com- plexes [72–74, 78]. Renal cortex analysis of mice that devel- oped MN after immunization with fetal calf serum by cDNA microarrays, showed that 175 genes had altered expression in relation to normal kidneys, and metallothionein-1—Mt- 1, cathepsin D—CtsD, and laminin receptor LAMR-1-1, previously associated with injury, inﬂammation, and cell- matrix interactions, were overexpressed. This increase in expression was conﬁrmed by Western blotting, and CtsD and Mt-1 were expressed predominantly in tubulointerstitial compartment and LAMR-1 in glomeruli, distribution evi- denced by immunohistochemistry [74]. Other enzymes such as DPP IV, NEP, or aminopeptidase A were also recognized as target antigens for circulating antibodies in animal models. NEP enzyme is located in the Bowman’s capsule and proximal tubule in both human and rabbit kidneys. The DPP IV enzyme, however, is also found in podocytes, indicating that these two enzymatic antigens may participate in the pathogenesis of membranous nephropathy [67, 68]. 3 The ﬁrst human antigen to be linked with an autoim- mune cause of the disease was the phospholipase A2 receptor (PLA2R). This antigen is expressed in podocytes and it is a member of the mannose receptor family acting as a transmembrane receptor for secreted phospholipases. The interaction between PLA2R and its speciﬁc antibody might potentiate the activation of the complement cascade, which, in turn, might damage the ﬁltration membrane and induce proteinuria [71, 78]. Recent studies have shown an association between the presence of phospholi- pase A2 receptor (PLA2R) in 70% of patients with MN [79]. The role played by the complement system seems to be essential for the development of the disease. That would be reasonable given the fact that IgG immunoglobulin binding to complement fraction C1q induces proteinuria [66, 69, 71]. Analysis of the glomerular capillary area in kidney biopsies collected from membranous nephropathies demonstrates the presence of C5b-9 membrane attack complexes of the complement system. Animal models demonstrated that podocyte lesions might be mediated by reactive oxygen species (ROS) produced in response to the glomerular membrane damage and the deposi- tion of the immune complexes. These ROS might have their damaging eﬀect on the matrix proteins enhanced by lipid peroxidation. The C5b-9 complex might dam- age the podocyte DNA directly or through the induction of ROS production. Podocyte lesion might increase the expression of matrix metalloproteinase-9 (MMP-9) in these cells, which might induce collagen IV degradation and alterations in nephrin expression. Thus, lipid peroxida- tion, complement system activation, and ROS production would provide future therapeutic targets for membranous nephropathy [66, 68, 69, 71, 72]. Another molecule of the complement system which has recently been related to the membranous nephropathy is C4d [80]. C4d is generated by the classical or lectin complement pathway. This fragment is highly stable and covalently binds to cell surfaces. Patients with MN show deposition of C4d in situ and it is believed that this molecule is involved in the pathogenesis of this disease. However, interest- ingly, such as this deposit is not seen in cases of min- imal lesion disease, this molecule has great potential as a tool in the diﬀerential diagnosis between these two entities [80]. Immunoglobulin subclasses IgG1 and IgG4 are regularly found in MN, being identically deposited in the glomeruli, but presenting a conﬂicting expression in patients with antenatal form of the disease. Evaluation of the production proﬁle of IgG1, IgG4, and anti-NEP antibodies have shown that IgG4 alone is not enough to produce nephropathy. Children born from mothers who produced decreased anti- NEP antibody levels, but sustained IgG4 subclass, did not present any renal alteration. On the other hand, children whose mother produced all classes of antibodies did present renal failure at birth. One suitable explanation would be that the Fc portion of the IgG classes would diﬀerentially interact with complement system and induce variable cellular lesions [66–68, 70–72, 78].