Epitope Spreading of Autoantibody Response to PLA2R Associates with Poor Prognosis in Membranous Nephropathy

Abstract

The phospholipase A2 receptor (PLA2R1) is the major autoantigen in idiopathic membranous nephropathy. However, the value of anti-PLA2R1 antibody titers in predicting patient outcomes is unknown. Here, we screened serum samples from 50 patients positive for PLA2R1 for immunoreactivity against a series of PLA2R1 deletion mutants covering the extracellular domains. We identified reactive epitopes in the cysteine-rich (CysR), C-type lectin domain 1 (CTLD1), and C-type lectin domain 7 (CTLD7) domains and confirmed the reactivity with soluble forms of each domain. We then used ELISAs to stratify 69 patients positive for PLA2R1 by serum reactivity to one or more of these domains: CysR (n=23), CysRC1 (n=14), and CysRC1C7 (n=32). Median ELISA titers measured using the full-length PLA2R1 antigens were not statistically different between subgroups. Patients with anti-CysR-restricted activity were younger (P=0.008), had less nephrotic range proteinuria (P=0.02), and exhibited a higher rate of spontaneous remission (P=0.03) and lower rates of renal failure progression (P=0.002) and ESRD (P=0.01) during follow-up. Overall, 31 of 69 patients had poor renal prognosis (urinary protein/creatinine ratio .4 g/g or eGFR,45 ml/min per 1.73 m 2 at end of follow-up). High anti-PLA2R1 activity and epitope spreading beyond the CysR epitope were independent risk factors of poor renal prognosis in multivariable Cox regression analysis. Epitope spreading during follow-up associated with disease worsening (n=3), whereas reverse spreading from a CysRC1C7 profile back to a CysR profile associated with favorable outcome (n=1). We conclude that analysis of the PLA2R1 epitope profile and spreading is a powerful tool for monitoring disease severity and stratifying patients by renal prognosis.

Immune Response against Autoantigen PLA2R Is not Gambling: Implications for Pathophysiology, Prognosis, and Therapy

“…The second important finding of Seitz-Polski et al 10 is the correlation of epitope spreading with clinical presentation and outcome as previously observed in many autoimmune diseases and in rat Heymann nephritis, a faithful animal model of human primary MN. 7,17 They used a soluble form of each domain to develop an epitope-specific ELISA, which allowed characterization of the antibody profile for each patient.…”

“…10 Using nine PLA2R mutants generated by successive deletion of the extracellular domains of the receptor, they confirmed that the CysR region contains the primary dominant epitope, but in addition, they first demonstrated epitope spreading toward the CTLD1 and CTLD7 domains. By using this new approach, they also showed that CysR and CTLD1 are two independent domains recognized by distinct anti-PLA2R autoantibodies.…”

“…Seitz-Polski et al 10 have defined epitope regions of PLA2R autoantigen using recombinant truncated molecules containing approximately 140 amino acid residues. However, such studies do not provide clues on actual contact amino acid residues for antibody paratopes which, in the case of PLA2R, engage conformational structures stabilized by disulfide bonds.…”

“…PLA2R is weakly expressed in podocytes under normal conditions and is not accessible to the circulating T cells, whose function is to activate the antigen-specific B cells. The molecular analysis presented by Seitz-Polski et al 10 should give new impetus to the search for mimicking sequences among antigens of microbial or other environmental origin. Molecular mimicry between these external antigens and PLA2R could lead to production of anti-PLA2R antibodies in patients genetically predisposed to autoimmunity, 16 and might well account for the increasing incidence of primary MN worldwide.…”

“…Although Seitz-Polski et al 10 provide significant advances in the molecular dissection of PLA2R epitopes, several critical questions still need to be addressed. First, they studied circulating antibodies, but the most relevant antibodies are those which are deposited in the glomerulus.…”