Immune-Monitoring Disease Activity in Primary Membranous Nephropathy

Abstract

Primary membranous nephropathy (MN) is a glomerular disease mediated by autoreactive antibodies, being the main cause of nephrotic syndrome among adult patients. While the pathogenesis of MN is still controversial, the detection of autoantibodies against two specific glomerular antigens, phospholipase A2 receptor (PLA₂R) and thrombospondin type 1 domain containing 7A (THSD7A), together with the beneficial effect of therapies targeting B cells, have highlighted the main role of autoreactive B cells driving this renal disease. In fact, the detection of PLA₂R-specific IgG4 antibodies has resulted in a paradigm shift regarding the diagnosis as well as a better prediction of the progression and recurrence of primary MN. Nevertheless, some patients do not show remission of the nephrotic syndrome or do rapidly recur after immunosuppression withdrawal, regardless the absence of detectable anti-PLA₂R antibodies, thus highlighting the need of other immune biomarkers for MN risk-stratification. Notably, the exclusive evaluation of circulating antibodies may significantly underestimate the magnitude of the global humoral memory immune response since it may exclude the role of antigen-specific memory B cells. Therefore, the assessment of PLA₂R-specific B-cell immune responses using novel technologies in a functional manner may provide novel insight on the pathogenic mechanisms of B cells triggering MN as well as refine current immune-risk stratification solely based on circulating autoantibodies.

Keywords: PLA2R; THSD7A; autoreactive B cells; glomerulonephritis recurrence; membranous nephropathy.

Figure 1

Activation and inhibition of autoimmune B cell responses and the influence of different B cell subsets in the fluctuation of circulating anti-PLA₂R antibodies. (A) After failure in self-tolerance mechanisms, autoreactive naïve B cells may encounter the self-antigen and can be activated in the secondary lymphoid organ by helper signals from T follicular helper cells. Then, B cells can differentiate into short-lived plasma cells, which secrete mainly IgM antibodies or differentiate into memory B cells or long-lived plasma cells after somatic hypermutation and immunoglobulin isotype class switching in the germinal center. After a re-encounter with the self-antigen, memory B cells can rapidly differentiate into antibody-secreting cells, sustaining long-lasting humoral immunity. Memory B cells may occupy empty bone marrow niches after secondary activation replenishing plasma cell pool. Anti-CD20 monoclonal antibodies (Rituximab) mainly target naïve B cells and memory B cells but not long-lived plasma cells. (B) Levels of anti-PLA₂R autoantibodies may fluctuate over time and may become undetectable without indicating MN remission. Memory B cells can be detected in the absence of antibody levels in serum and its rapid differentiation and production of antibodies can be of great importance for a subsequent humoral response. Such effective and rapid response of the memory-B cell population indicates that although anti-PLA₂R autoantibodies may not be detected in serum, PLA₂R-specific memory B cells can be a target indicator of MN relapse.

Figure 2

Measuring anti-PLA₂R reactive memory B cells. (A) Peripheral blood mononuclear cells are first polyclonally activated for 6 days to expand the pool of memory B cells and antibody secreting cells (ASC). Expanded cells are next used for an enzyme-linked immune absorbent spot (ELISPOT) assay to detect cells producing antibodies against PLA₂R. (B) Two representative patients with membranous nephropathy and similar levels of proteinuria and circulating anti-PLA₂R antibodies. Patient #1 has a positive ELISPOT, indicating the presence of autoreactive memory B cells (sign of active disease), while Patient #2 has no detectable autoreactive memory B cells (indicative of a remission phase). Adapted from Luque et al. (112).