Anti-Phospholipase A₂ Receptor Antibodies Correlate with Clinical Status in Idiopathic Membranous Nephropathy

Discussion

We found that 77.8% of iMN patients in our European cohort had antibodies reactive with human PLA₂R, similar to the proportion of anti-PLA₂R-positive primary MN patients in the United States (5) and other international iMN-patient cohorts (L.H.B., unpublished observations). Although the exact mechanisms and the role of PLA₂R in pathogenesis of iMN are currently still unknown, these data strengthen the role of PLA₂R as the major target antigen in iMN. Other recently described antigens may play a role in our anti-PLA₂R-negative patients (6).

Our blinded analysis of samples collected at various stages of clinical disease activity revealed a clear association of anti-PLA₂R antibodies with clinical status as defined by proteinuria. Anti-PLA₂R levels were high when patients had nephrotic levels of proteinuria, were substantially decreased or absent in the setting of spontaneous or treatment-induced remission, and were again increased in association with disease recurrence. Remarkably, we observed that the level of anti-PLA₂R as semiquantitatively assessed by immunoblotting was positively correlated with the amount of proteinuria. This was not only true for baseline samples but also when combining all data obtained during follow-up. Although a clear causal relationship cannot be established by these data, it is tempting to speculate that higher titers of anti-PLA₂R autoantibodies may lead to more subepithelial antibody-antigen immune deposits, greater podocyte damage, and thus increased levels of urinary protein. This hypothesis is further strengthened by the observation that anti-PLA₂R-autoantibody levels at baseline also strongly correlated with other markers of renal damage such as urinary excretion of β2m and IgG and serum-creatinine levels. Future work is needed to determine whether these antibodies are directly pathogenic to the human podocyte.

In two patients, findings were seemingly inconsistent. In one patient, anti-PLA₂R autoantibodies were absent at first presentation but clearly positive 4 months later. In the second patient, anti-PLA₂R autoantibodies remained present during remission and were absent with increasing proteinuria during follow-up.

We have no biologically plausible explanation for the findings in the first patient. Autoantibodies were absent in the sample taken at presentation, shortly after a renal biopsy that exhibited all of the features typical of an idiopathic membranous nephropathy and at a time when the patient was severely nephrotic (proteinuria, 12.5 g/d; serum albumin, 2.4 g/dl). Because the repeated sample was strongly positive, we have to acknowledge the possibility of an inadvertent mislabeling of the first specimen.

We have proposed that the immunologically active phase of MN (defined by the presence of circulating anti-PLA₂R antibodies) must end before a corresponding improvement is seen in proteinuria or other clinical features (8). This makes sense conceptually, because some degree of recovery of both the glomerular basement membrane and podocyte structural damage need to occur before the proteinuria can completely resolve. Residual proteinuria may at times remain indefinitely in the absence of immunologic activity, because of tubulointerstitial damage or secondary glomerular changes from prolonged disease. For this reason, a definition of partial remission based only on urinary protein may be flawed, because similar levels of proteinuria can occur with or without the presence of immunological activity. It is thus possible that our patient, in whom there was no anti-PLA₂R detectable in the sample at the end of follow-up, although there was still a persistent proteinuria of 4.8 g/d, had actually already remitted immunologically but had residual proteinuria on the basis of secondary focal segmental glomerulosclerosis lesions. The serum-albumin level at that moment had slightly increased to 3.3 g/dl, which in our opinion supports the diagnosis of secondary focal segmental glomerulosclerosis rather than active membranous nephropathy.

Variability in proteinuria caused by dosing and compliance with inhibitors of the renin-angiotensin system and/or declining renal function may also lead to spurious conclusions about remission status that are based only on amounts of urinary protein. Thus, in the patient in whom anti-PLA₂R antibody levels were detected despite a clinically-defined remission, the lower level of urinary protein may not have been reflective of a true immunological remission. Indeed, the patient's eventual progressive loss of renal function is consistent with persistent immunological disease activity throughout this apparent remission. This point may become relevant in patients with advanced renal failure from iMN who are eligible for transplantation, because the presence or absence of anti-PLA₂R auto antibodies may influence the potential for recurrent disease in the allograft (9). However, further studies are warranted to establish the risk of recurrent iMN in anti-PLA₂R-positive patients.

In conclusion, the role of PLA₂R as a major antigen in iMN was confirmed in an independent, European patient cohort, and levels of circulating anti-PLA₂R revealed a strong correlation with clinical disease activity. Furthermore, although anti-PLA₂R-autoantibody levels correlated strongly with proteinuria, we propose that both can also provide independent information, especially when they are discordant. In those occasions when low or absent anti-PLA₂R levels indicate the remission of immunologic activity, “residual” proteinuria may reflect as-yet unresolved structural changes in the glomerulus or interstitial/tubular damage.

We propose that detection and measurement of these autoantibodies may play an important role in the monitoring of disease activity and treatment efficacy. Larger studies are clearly needed to confirm our data and to evaluate the sensitivity and specificity of change in anti-PLA₂R-autoantibody level for predicting a change in disease activity in the individual patient.